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diff --git a/42602-0.txt b/42602-0.txt index 606707d..75c8521 100644 --- a/42602-0.txt +++ b/42602-0.txt @@ -1,40 +1,4 @@ -The Project Gutenberg eBook, The Steam Engine Explained and Illustrated -(Seventh Edition), by Dionysius Lardner - - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - - - - -Title: The Steam Engine Explained and Illustrated (Seventh Edition) - With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt - - -Author: Dionysius Lardner - - - -Release Date: April 26, 2013 [eBook #42602] - -Language: English - -Character set encoding: UTF-8 - - -***START OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND -ILLUSTRATED (SEVENTH EDITION)*** - - -E-text prepared by Chris Curnow, RichardW, and the Online Distributed -Proofreading Team (http://www.pgdp.net) from page images generously made -available by Internet Archive (https://archive.org) - - +*** START OF THE PROJECT GUTENBERG EBOOK 42602 *** Note: Project Gutenberg also has an HTML version of this file which includes the original illustrations. @@ -21256,363 +21220,4 @@ Transcriber's endnote: tables will not look good unless viewed with a monospace font, such as Courier New or Lucida Console. - - -***END OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND -ILLUSTRATED (SEVENTH EDITION)*** - - -******* This file should be named 42602-0.txt or 42602-0.zip ******* - - -This and all associated files of various formats will be found in: -http://www.gutenberg.org/dirs/4/2/6/0/42602 - - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - - - - -Title: The Steam Engine Explained and Illustrated (Seventh Edition) - With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt - - -Author: Dionysius Lardner - - - -Release Date: April 26, 2013 [eBook #42602] - -Language: English - -Character set encoding: ISO-8859-1 - - -***START OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND -ILLUSTRATED (SEVENTH EDITION)*** - - -E-text prepared by Chris Curnow, RichardW, and the Online Distributed -Proofreading Team (http://www.pgdp.net) from page images generously made -available by Internet Archive (https://archive.org) - - - -Note: Project Gutenberg also has an HTML version of this - file which includes the original illustrations. - See 42602-h.htm or 42602-h.zip: - (http://www.gutenberg.org/files/42602/42602-h/42602-h.htm) - or - (http://www.gutenberg.org/files/42602/42602-h.zip) - Some characters might not display in the html version. - If so, the reader should consult the iso-8859-1 (Latin-1) - text file 42602-8.txt or 42602.zip: - (http://www.gutenberg.org/files/42602/42602-8.txt) - or - (http://www.gutenberg.org/files/42602/42602-8.zip) - - - Images of the original pages are available through - Internet Archive. See - https://archive.org/details/steamengineexpla00lardrich - - -Transcriber's note: - - Text enclosed by underscores is in italics (_italics_). - - The notation "^{n}" means that n is superscript. The notation - "_{n}" means that n is subscript. - - Text that was in small capitals has been converted to all - capitals. - - The original page numbers are embedded in square brackets, - e.g. "[Pg135]". - - More detailed notes are provided in the Transcriber's endnote. - - - - - -[Illustration: WATT. - -Engraved by H. Adlard, from a Drawing by H. Corbould, taken with -the permission of James Watt, Esq. - -FROM THE STATUE BY CHANTREY. - -London: Taylor & Walton, Upper Gower Street.] - - -THE STEAM ENGINE EXPLAINED AND ILLUSTRATED; - -With an Account of Its Invention and Progressive Improvement, -and Its Application to Navigation and Railways; - -Including also a Memoir of Watt. - -by - -DIONYSIUS LARDNER, D.C.L. F.R.S. -&c. &c. - -SEVENTH EDITION, - -Illustrated by Engravings on Wood. - - - - - - - -London: -Printed for Taylor and Walton, -28. Upper Gower Street. - -MDCCCXL. - -London: -Printed by A. Spottiswoode, -New-Street-Square. - - - - - TO - - THE RIGHT HONOURABLE - - HENRY LORD BROUGHAM AND VAUX, - - FELLOW OF THE ROYAL SOCIETY, - - AND - - MEMBER OF THE NATIONAL INSTITUTE OF FRANCE, - - AS A MARK OF PUBLIC RESPECT - - AND - - A TESTIMONY OF PRIVATE REGARD, - - THIS WORK - - INSCRIBED, BY HIS ATTACHED FRIEND, - - THE AUTHOR. - - - - -ADVERTISEMENT. - - -The Drawings for several of the Cuts in this Volume have been -taken, by the permission of Mr. Weale, from the admirable Plates -annexed to the last edition of Tredgold on the Steam Engine and on -Steam Navigation. This acknowledgment is especially due for the -Illustrations which abound in this Volume. - -_London, June, 1840._ - - - - -[Illustration: LONDON ENTRANCE TO THE BIRMINGHAM RAIL-ROAD.] - - - - - CONTENTS. - - - CHAPTER I. - - PRELIMINARY MATTER. - - Page - - The Steam Engine, a Subject of popular Interest 4 - - Effects of Steam 5 - - Great Power of Steam 7 - - Object of this Work 9 - - Disputes respecting the Invention 11 - - Hero of Alexandria's Machine 13 - - Blasco De Garay's Proposition to propel Vessels - by a Machine 16 - - Solomon De Caus 17 - - Giovanni Branca proposes to work Mills by Steam 22 - - Marquis of Worcester 23 - - Mechanical Properties of Fluids 25 - - Elastic and Inelastic Fluids 25 - - Elasticity of Gases 28 - - Effects of Heat 29 - - Application of these Principles to the Engines - of Hero, De Caus, and Lord Worcester 30 - - Sir Samuel Morland 34 - - Denis Papin 36 - - Atmospheric Pressure 38 - - Weight of Air 39 - - Pressure of Air 41 - - Barometer 41 - - Elastic Force of Air and Gases 42 - - Force obtained by a Vacuum 43 - - Common Pump 43 - - Rarefaction by Heat 44 - - Process of filling Thermometers 44 - - Papin's Method of producing a Vacuum 44 - - His Discovery of the Condensation of Steam 45 - - Thomas Savery 47 - - - CHAP. II. - - ENGINES OF SAVERY AND NEWCOMEN. - - Savery's Engine 49 - - Boilers and their Appendages 50 - - Working Apparatus described 51 - - Mode of Operation 52 - - Defects of Savery's Engine 58 - - Newcomen's Engine described by Papin 62 - - Newcomen and Cawley obtain a Patent for - Atmospheric Engine 65 - - Accidental Discovery of Condensation by Injection 69 - - Potter's Discovery of the Method of working - the Valves 71 - - His Contrivance improved by the Substitution - of a Plug Frame 72 - - Advantages of the Atmospheric Engine over that - of Savery 72 - - The Power of Savery's Engine restricted 73 - - It contained no new Principle 73 - - Its practical Superiority 73 - - - CHAP. III. - - EARLY CAREER AND DISCOVERIES OF JAMES WATT. - - Atmospheric Engine improved by Beighton 75 - - Smeaton's Improvements in the Atmospheric Engine 76 - - Brindley obtains a Patent for Improvement in 76 - - Invents the Self-regulating Feeder 76 - - Infancy of James Watt 77 - - His Descent and Parentage 77 - - Anecdotes of his Boyhood 78 - - His early Acquirements 79 - - Goes to London 80 - - Returns to Glasgow 80 - - Appointed Instrument-maker to the University 81 - - Opens a Shop in Glasgow 81 - - His Friends and Patrons 81 - - Professor Robison's Remarks on Watt's personal - Character 82 - - His industrious and studious Habits 82 - - His Attention first directed to Steam 83 - - Experiments on High-pressure Engine 83 - - Repairs an Atmospheric Model 84 - - Experimental Inquiry consequent on this 84 - - Its Results 84 - - Observes great Defects in the Atmospheric Engine 85 - - His first Attempt to improve it 85 - - His early Experiments on Steam 87 - - Discovery by Experiment of the Expansion which - Water undergoes in Evaporation 90 - - Discovers the latent Heat of Steam 91 - - Informed by Dr. Black of the Theory of latent Heat 93 - - His Improvement not due to Black 93 - - - CHAP. IV. - - EXPOSITION OF PHYSICAL PRINCIPLES. - - Construction of Thermometer 98 - - Method of graduating it 99 - - Freezing and boiling Points 99 - - Latent Heat of Water 101 - - Quantity of Heat necessary to convert Ice into - Water, first noticed by Dr. Black 101 - - Examination of the analogous Effects produced - by the continued Application of Heat to Water - in the liquid State 102 - - Process of Boiling 104 - - Reconversion of Steam into Water 104 - - Conversion of Water into Steam 105 - - Latent Heat of Steam 107 - - Boiling Point varies 108 - - Different in different Places 109 - - Inquiry whether a Diminution of Pressure will - produce a corresponding Effect on the boiling - Point 112 - - Table showing the Temperature at which Water - will boil under different Pressures of the - Atmosphere 113 - - Mechanical Force of Steam 115 - - Facts to be observed in 117 - - - CHAP. V. - - FURTHER DISCOVERIES OF WATT. - - Watt finds that Condensation in the Cylinder - is incompatible with a due Economy of Fuel 119 - - Conceives the Notion of condensing out of the - Cylinder 120 - - Discovers separate Condensation 121 - - Invents the Air Pump 123 - - Substitutes Steam Pressure for Atmospheric - Pressure 123 - - Invents the Steam Case, or Jacket 124 - - His first Experiments to realise these - Inventions 125 - - His experimental Apparatus 125 - - His experimental Models fitted up at Delft - House, in Glasgow 128 - - Difficulties of bringing the improved Engines - into Use 129 - - Watt first employed by Roebuck as a Civil - Engineer 130 - - His Partnership with Roebuck 130 - - His first Patent 130 - - Experimental Engine at Kinneal 131 - - Abstract of the Act of Parliament for the - Extension of his Patent 132 - - Description of his single-acting Steam Engine 133 - - - CHAP. VI. - - WATT'S ENGINES. - - Correspondence of Watt with Smeaton 145 - - Failure of Condensation by Surface 146 - - Improvements in Construction of Piston 147 - - Method of Packing 148 - - Improvements in boring the Cylinder 149 - - Disadvantages of the new compared with the old - Engines 150 - - Greatly increased Economy of Fuel 150 - - Economy of the Engine 151 - - Expedients to force the new Engines into Use 151 - - Correspondence of Boulton 153 - - Correspondence with Smeaton 155 - - Efficiency of Fuel in the new Engines 156 - - Discovery of the expansive Action of Steam 157 - - Watt states it in a Letter to Dr. Small 157 - - Its Principle explained 158 - - Mechanical Effects resulting from it 162 - - Computed Effect of cutting off Steam at - different Portions of the Stroke 163 - - Produces a variable Power 163 - - Expedients for equalising the Power 164 - - Expansive Principle in Watt's Engines limited 165 - - Its more extensive Application in the Cornish - Engines 165 - - - CHAP. VII. - - DOUBLE-ACTING ENGINE. - - Common Steam 168 - - Superheated Steam 170 - - Laws of Dalton and Gay Lussac 171 - - Law of Mariotte 171 - - Relation between Temperature and Pressure of - common Steam 171 - - Effects of the Expansion of common Steam 173 - - Mechanical Effects of Steam 173 - - Method of equalising the expansive Force 174 - - Hornblower's Engine 175 - - Woolf's Engine 176 - - Watt's Attempts to extend the Steam Engine to - Manufactures 178 - - Papin's projected Applications of the Steam - Engine 178 - - Savery's Application of the Engine to move - Machinery 180 - - Jonathan Hull's Application to Water Wheels 180 - - Champion of Bristol applies the Atmospheric - Engine to raise Water 181 - - Stewart's Application of the Engine to - Mill-work 182 - - Wasbrough's Application of the Fly-wheel and - Crank 183 - - Reasons why Watt's single-acting Steam Engine - was not adapted to produce continuous uniform - Motion of Rotation 184 - - Watt's Second Patent 186 - - Sun-and-Planet Wheels 187 - - Valves of double-acting Engine 189 - - - CHAP. VIII. - - DOUBLE-ACTING ENGINE. - - Methods of connecting the Piston-rod and Beam - in the double-acting Engine 193 - - Rack and Sector 194 - - Parallel Motion 195 - - Connection of Piston-rod and Beam 195 - - Connecting Rod and Crank 203 - - Fly-wheel 205 - - Throttle-valve 207 - - Governor 209 - - Construction and Operation of the - double-acting Engine 216 - - Eccentric 225 - - Cocks and Valves 227 - - Single-clack Valves 227 - - Double-clack Valves 228 - - Conical Valves 228 - - Slide Valves 229 - - Murray's Slides 229 - - D Valves 230 - - Seaward's Slides 235 - - Single Cock 238 - - Two-way Cock 239 - - Four-way Cock 239 - - Pistons 242 - - Common hemp-packed Piston 242 - - Woolf's Piston 243 - - Metallic Pistons 244 - - Cartwright's Engine 245 - - Cartwright's Piston 247 - - Barton's Piston 248 - - - CHAP. IX. - - BOILERS AND FURNACES. - - Analysis of Coal 252 - - Process of Combustion 253 - - Heat evolved in it 254 - - Form and Structure of Boiler 255 - - Waggon Boiler 255 - - Furnace 256 - - Method of feeding it 257 - - Combustion of Gas in Flues 260 - - Williams's Patent for Method of consuming - unburned Gases 260 - - Construction of Grate and Ash-pit 261 - - Magnitude of heating Surface of Boiler 262 - - Steam-space and Water-space in Boiler 263 - - Position of Flues 264 - - Method of feeding Boiler 265 - - Method of indicating the Level of Water in - Boiler 266 - - Level Gauges 266 - - Self-regulating Feeders 267 - - Steam Gauge 270 - - Barometer Gauge 272 - - Watt's Invention of the Indicator 274 - - Counter 278 - - Safety-valve 279 - - Fusible Plugs 280 - - Self-regulating Damper 281 - - Brunton's Self-regulating Furnace 283 - - Gross and useful Effect of an Engine 285 - - Power and Duty of Engines 287 - - Horse-power of Steam Engines 289 - - Evaporation proportional to Horse-power 290 - - Sources of Loss of Power 292 - - Absence of good practical Rules for Power 292 - - Common Rules followed by Engine-makers 292 - - Duty distinguished from Power 294 - - Duty of Boilers 294 - - Proportion of Stroke to Diameter of Cylinder 295 - - Duty of Engines 296 - - Cornish System of Inspection 297 - - Table showing the Improvement of Cornish Engines 298 - - Beneficial Effects of Cornish Inspection 299 - - Successive Improvements on which the increased - Duty of Engines depends, traced by John Taylor - in his "Records of Mining" 299 - - - CHAP. X. - - LIFE OF WATT. - - Watt's Friends and Associates at Birmingham 302 - - His Invention of the Copying Press 302 - - Heating Apartments and Buildings by Steam 303 - - Watt's Machine for drying Linen 303 - - His Share in the Discovery of the Composition - of Water 303 - - The Merit of this Discovery shared between - Cavendish, Lavoisier, and Watt 305 - - Anecdote of Watt's Activity of Mind 309 - - His Introduction of the Use of Chlorine in - Bleaching 310 - - One of the Founders of the Pneumatic - Institution at Clifton 310 - - His first Marriage 311 - - Death of his first Wife 311 - - His second Marriage 311 - - Death of his younger Son 311 - - Extracts from his Letters 312 - - Character of Watt by Lord Brougham 313 - - Extract from Sir Walter Scott's "Monastery" on - the Character of Watt 314 - - Sketch of the Life of Watt from the "Scotsman" - by Lord Jeffrey 315 - - Occupation of his old Age 318 - - His Invention of Machine for copying Sculpture 318 - - His last Days 318 - - Monuments to his Memory 319 - - Inscription by Lord Brougham on the Pedestal - of the Monument in Westminster Abbey 320 - - - CHAP. XI. - - LOCOMOTIVE ENGINES ON RAILWAYS. - - Page - High-pressure Engines 322 - - One of the earliest Forms of the Steam Engine 322 - - Description of Leupold's Engine 323 - - Non-condensing Engine of Messrs. Trevethick - and Vivian 324 - - Construction of a Machine for moving Carriages - on Railroads 328 - - Effects of Railway Transport 329 - - Moral and political Consequences of 334 - - History of the Locomotive Engine 337 - - Construction of Locomotive Engine by Blinkensop 337 - - Messrs. Chapman's Contrivance 337 - - Walking Engine 337 - - Mr. Stephenson's Engines at Killingworth 339 - - Liverpool and Manchester Railway 342 - - The Directors offer a Prize for the best - Locomotive Engine 344 - - Experimental Trial 344 - - The "Rocket," "Sanspareil," and "Novelty" 344 - - Admirable Arrangement in the Rocket 345 - - Description of the "Sanspareil" 347 - - Description of the "Novelty" 349 - - The Superiority of the "Rocket" 350 - - Method of subdividing the Flue into Tubes 353 - - Progressive Improvement of Locomotive Engines 354 - - Dr. Lardner's Experiments in 1832 357 - - Adoption of Brass Tubes 360 - - Great Expense of Locomotive Power 361 - - Mr. Booth's Report 362 - - Detailed Description of the most improved - Locomotive Engines 365 - - Substitution of Brass for Copper Tubes - ascribed to Mr. Dixon 370 - - Power of Locomotive Engines 379 - - Position of the Eccentrics 379 - - Pressure of Steam in the Boiler 401 - - Dr. Lardner's Experiments in 1838 406 - - Resistance to Railway Trains 407 - - Dr. Lardner's Experiments on the Great Western - Railway 408 - - Experiments on Resistance 409 - - Restrictions on Gradients 410 - - Compensating Effect of Gradients 412 - - Experiment with the "Hecla" 412 - - Disposition of Gradients should be uniform 415 - - Methods of surmounting steep Inclinations 415 - - - CHAP. XII. - - LOCOMOTIVE ENGINES ON TURNPIKE ROADS. - - Railways and Stone Roads compared 420 - - Gurney's Steam Carriage 423 - - The Boiler of Gurney's Engine 423 - - His Method of cleansing Boilers 428 - - Convenience and Safety of Steam Carriages 432 - - Two Methods of applying Locomotive Engines - upon common Roads 434 - - Horse Carriages compared with Steam 435 - - Extract from Mr. Farey's Evidence before the - House of Commons 435 - - Hancock's Steam Carriage 436 - - How it differs from that of Mr. Gurney 437 - - Ogle's Locomotive Carriage 438 - - Dr. Church's Steam Engine 439 - - - CHAP. XIII. - - STEAM NAVIGATION. - - Form and Arrangement of Marine Engines 441 - - Arrangement of the Engine-room 446 - - Marine Boilers 448 - - Effects of Sea Water in Boilers 450 - - Remedies for them 451 - - Blow-off Cocks 452 - - Indicators of Saltness 453 - - Seaward's Indicator 454 - - His Method of blowing out 454 - - Field's Brine Pumps 456 - - Tubular Condensers applied by Mr. Watt 457 - - Hall's Condensers 458 - - Substitution of Copper for Iron Boilers 460 - - Process of Stoking 462 - - Watt's Expedient of attaching Felt to the - Boiler Surface 463 - - Means of economising Fuel 463 - - Number and Arrangement of Furnaces and Flues 463 - - Howard's Marine Engine 464 - - Application of the expansive Principle in - Marine Engines 466 - - Recent Improvements of Messrs. Maudslay and - Field 467 - - Humphrey's Marine Engine 470 - - Common Paddle-wheel 472 - - Feathering Paddles 474 - - Galloway's Patent for a Paddle-wheel with - movable Paddles 476 - - Split Paddle 478 - - Proportion of Power to Tonnage 480 - - Improved Efficiency of Marine Engines 482 - - Iron Steam Vessels 483 - - Steam Navigation to India 484 - - - CHAP. XIV. - - AMERICAN STEAM NAVIGATION. - - Steam Navigation first established in America 487 - - Circumstances which led to it 488 - - Attempts of Fitch and Rumsey to apply the - single-acting Engine to the Propulsion of - Vessels 489 - - Stevens of Hoboken commences Experiments in - Steam Navigation 489 - - Experiments of Livingstone and Fulton 489 - - Fulton's first Boat 490 - - The Hudson navigated by Steam 491 - - Extension and Improvement of River Navigation 492 - - American Steamers 494 - - Difference between them and European Steamers 494 - - Steamers on the Hudson 494 - - American Paddle-wheels 495 - - Sea-going American Steamers 496 - - Speed attained by American Steamers 497 - - Lake Steamers 499 - - The Mississippi and its Tributaries 499 - - Steam-boats navigating it 500 - - Their Structure and Machinery 500 - - New Orleans Harbour 503 - - Steam Tugs 503 - - - APPENDIX. - - _On the Relation between the Temperature, Pressure, and_ - _Density of Common Steam._ - - Empirical Formula of Biot, showing the - Relation between the Pressure and Temperature 505 - - Empirical formula of Southern 506 - Tredgold 506 - Mellet 506 - De Pambour 506 - MM. Dulong and Arago 506 - - Law of the Expansion of elastic Fluids, - discovered by Dalton and Gay Lussac 506 - - Formula for the Relation between the Volumes - and Temperatures 507 - - Law of Mariotte 507 - - Table of Pressures, Temperatures, Volumes, and - Mechanical Effects of Steam 509 - - Empirical Formulæ for the Relation between the - Volume of Water and that of the Steam produced - by its Evaporation under given Pressures 511 - - Formula of Navier 511 - - Modified by De Pambour 511 - - _On the Expansive Action of Steam._ - - Mechanical Effect produced during a given - Extent of Expansion 511 - - Mechanical Effect produced during Evaporation - and subsequent Expansion 512 - - Application to double-acting Engines 513 - - Formula for Pressure of Steam in Cylinders 514 - - Formula for total Mechanical Effect per Minute - of Steam when cut off at any proposed Part of - the Stroke 514 - - Formulæ exhibiting the Relation between the - Resistance of the Load, the Resistances of the - Engine, the Evaporation, the Speed of the - Piston, and the Magnitude of the Cylinder 515 - - Formulæ showing the Relation between the Power - of the Engine, the Evaporation, and the useful - Load 516 - - Formulæ for the _useful Effect_ and the _Duty_ 517 - - Estimates of the several Sources of - Resistances 518 - - Tables to facilitate the Computation of the - Effects of Expansive Engines 519 - - Table of the Areas of Pistons 520 - - EXAMPLES of the Application of these Formulæ 521 - - - INDEX. 523 - - - - -[Illustration: VIADUCT, NEAR WATFORD, BIRMINGHAM RAIL-ROAD.] - -[Pg001] - - - - -THE STEAM ENGINE. - - - - -[Pg003] - - - - -[Illustration: HERO OF ALEXANDRIA.] - -CHAPTER I. - - THE STEAM ENGINE, A SUBJECT OF POPULAR INTEREST. -- THE OBJECT - OF THIS WORK. -- DISPUTES RESPECTING THE INVENTION. -- HERO. -- DE - GARAY. -- DE CAUS. -- BRANCA. -- MARQUIS OF WORCESTER. -- PHYSICAL - PRINCIPLES. -- ELASTIC AND INELASTIC FLUIDS. -- THEIR - PROPERTIES. -- APPLICATION OF THESE PRINCIPLES TO THE ENGINES - OF HERO, DE CAUS, AND LORD WORCESTER. -- SIR SAMUEL MORLAND. -- - PAPIN. -- ATMOSPHERIC PRESSURE. -- THE WEIGHT OF AIR. -- LESS AT - GREATER HEIGHTS. -- BAROMETER. -- PRESSURE OF AIR. -- ELASTIC - FORCE OF AIR AND GASES. -- FORCE PRODUCED BY A VACUUM. -- COMMON - PUMP. -- RAREFACTION BY HEAT. -- PAPIN'S METHODS OF PRODUCING A - VACUUM. -- HIS DISCOVERY OF THE CONDENSATION OF STEAM. -- - SAVERY. - - -(1.) That the history of the invention of a piece of mechanism, -and the description of its structure, operation, and [Pg004] -uses, should be capable of being rendered the subject matter of a -volume, destined not alone for the instruction of engineers or -machinists, but for the information and amusement of the public in -general, is a statement which at no very remote period would have -been deemed extravagant and incredible. - -Advanced as we are in the art of rendering knowledge popular, and -cultivated as the public taste is in the appreciation of the -expedients by which science ministers to the uses of life, there -is still perhaps but one machine of which such a proposition can -be truly predicated: it is needless to say that that machine is -the STEAM ENGINE. There are many circumstances attending this -extraordinary piece of mechanism which impart to it an interest so -universally felt. Whether we regard the details of its structure -and operation, the physical principles which it calls into play, -and the beautiful contrivances by which these physical principles -are rendered available;--or, passing over these _means_, we direct -our attention to the _ends_ which they attain, we are equally -filled with astonishment and admiration. The history of the steam -engine offers to our notice a series of contrivances which, for -exquisite and refined ingenuity, stand without any parallel in the -annals of mechanical science. These admirable inventions, unlike -other results of scientific inquiry, have also this peculiarity, -that, to understand their excellence and to perceive their beauty, -no previous or subsidiary knowledge is necessary, save what may be -imparted with facility and clearness in the progress of the -explanation and development of the machine itself. A simple and -clear exposition, divested of needless technicalities and aided by -well-selected diagrams, is all that is necessary to render the -construction and operation of the steam engine, in all its forms, -intelligible to persons of plain understanding and moderate -information. - -But if the contrivances by which this vast power is brought to bear -on the arts and manufactures, be rendered attractive by their great -mechanical beauty, how much more imposing will the subject become -when the effects which the steam engine has produced upon the -well-being of the human race are considered. It has penetrated the -crust of the earth, and drawn from beneath it boundless treasures -[Pg005] of mineral wealth, which, without its aid, would have been -rendered inaccessible; it has drawn up, in measureless quantity, -the fuel on which its own life and activity depend; it has relieved -men from their most slavish toils, and reduced labour in a great -degree to light and easy superintendence. To enumerate its present -effects, would be to count almost every comfort and every luxury of -life. It has increased the sum of human happiness, not only by -calling new pleasures into existence, but by so cheapening former -enjoyments as to render them attainable by those who before could -never have hoped to share them: the surface of the land, and the -face of the waters, are traversed with equal facility by its power; -and by thus stimulating and facilitating the intercourse of nation -with nation, and the commerce of people with people, it has knit -together remote countries by bonds of amity not likely to be -broken. Streams of knowledge and information are kept flowing -between distant centres of population, those more advanced -diffusing civilisation and improvement among those that are more -backward. The press itself, to which mankind owes in so large a -degree the rapidity of their improvement in modern times, has had -its power and influence increased in a manifold ratio by its union -with the steam engine. It is thus that literature is cheapened, -and, by being cheapened, diffused; it is thus that Reason has taken -the place of Force, and the pen has superseded the sword; it is -thus that war has almost ceased upon the earth, and that the -differences which inevitably arise between people and people are -for the most part adjusted by peaceful negotiation. - -Deep as the interest must be with which the steam engine will be -regarded in every civilised country, it presents peculiar claims -upon the attention of the people of Great Britain. Its invention -and progressive improvement are the work of our own time and our -own country; it has been produced and matured almost within the -last century, and is the exclusive offspring of British genius, -fostered and sustained by British enterprise and British capital. - -The steam engine is a mechanical contrivance, by which coal, wood, -or other fuel is rendered capable of executing any [Pg006] kind -of labour. COALS are by it made to spin, weave, dye, print and -dress silks, cottons, woollens, and other cloths; to make paper, -and print books upon it when made; to convert corn into flour; to -express oil from the olive, and wine from the grape; to draw up -metal from the bowels of the earth; to pound and smelt it, to melt -and mould it; to forge it; to roll it, and to fashion it into -every desirable form; to transport these manifold products of its -own labour to the doors of those for whose convenience they are -produced; to carry persons and goods over the waters of rivers, -lakes, seas, and oceans, in opposition alike to the natural -difficulties of wind and water; to carry the wind-bound ship out -of port; to place her on the open deep ready to commence her -voyage; to throw its arms around the ship of war, and place her -side by side with the enemy; to transport over the surface of the -deep persons and information, from town to town, and from country -to country, with a speed as much exceeding that of the ordinary -wind, as the ordinary wind exceeds that of a common pedestrian. - -Such are the virtues, such the powers, which the steam engine has -conferred upon COALS. The means of calling these powers into -activity are supplied by a substance which nature has happily -provided in unbounded quantity in every part of the earth; and -though it has no price, it has inestimable value: this substance -is WATER. - -A pint of water may be evaporated by two ounces of coals. In its -evaporation it swells into two hundred and sixteen gallons of -steam, with a mechanical force sufficient to raise a weight of -thirty-seven tons a foot high. The steam thus produced has a -pressure equal to that of common atmospheric air; and by allowing -it to expand, by virtue of its elasticity, a further mechanical -force may be obtained, at least equal in amount to the former. A -pint of water, therefore, and two ounces of common coal, are thus -rendered capable of doing as much work as is equivalent to -seventy-four tons raised a foot high. - -The circumstances under which the steam engine is worked on a -railway are not favourable to the economy of fuel. Nevertheless -a pound of coke burned in a locomotive engine [Pg007] will -evaporate about five pints of water. In their evaporation they -will exert a mechanical force sufficient to draw two tons weight -on the railway a distance of one mile in two minutes. Four horses -working in a stage-coach on a common road are necessary to draw -the same weight the same distance in six minutes. - -A train of coaches weighing about eighty tons, and transporting -two hundred and forty passengers with their luggage, has been -taken from Liverpool to Birmingham, and back from Birmingham to -Liverpool, the trip each way taking about four hours and a -quarter, stoppages included. The distance between these places by -the railway is ninety-five miles. This double journey of one -hundred and ninety miles is effected by the mechanical force -produced in the combustion of four tons of coke, the value of -which is about five pounds. To carry the same number of passengers -daily between the same places by stage-coaches on a common road, -would require twenty coaches and an establishment of three -thousand eight hundred horses, with which the journey in each -direction would be performed in about twelve hours, stoppages -included. - -The circumference of the earth measures twenty-five thousand -miles; and if it were begirt with an iron railway, such a train as -above described, carrying two hundred and forty passengers, would -be drawn round it by the combustion of about thirty tons of coke, -and the circuit would be accomplished in five weeks. - -In the drainage of the Cornish mines the economy of fuel is much -attended to, and coals are there made to do more work than -elsewhere. A bushel of coals usually raises forty thousand tons of -water a foot high; but it has on some occasions raised sixty -thousand tons the same height. Let us take its labour at fifty -thousand tons raised one foot high. A horse worked in a fast -stage-coach pulls against an average resistance of about a quarter -of a hundred weight. Against this he is able to work at the usual -speed through about eight miles daily: his work is therefore -equivalent to one thousand tons raised one foot. A bushel of coals -consequently, as used in Cornwall, performs as much labour as a -day's work of one hundred such horses. [Pg008] - -The great pyramid of Egypt stands upon a base measuring seven -hundred feet each way, and is five hundred feet high, its weight -being twelve thousand seven hundred and sixty millions of pounds. -Herodotus states, that in constructing it one hundred thousand men -were constantly employed for twenty years. The materials of this -pyramid would be raised from the ground to their present position -by the combustion of about four hundred and eighty tons of coals. - -The Menai Bridge consists of about two thousand tons of iron, and -its height above the level of the water is one hundred and twenty -feet. Its mass might be lifted from the level of the water to its -present position by the combustion of four bushels of coal. - -The enormous consumption of coals produced by the application of -the steam engine in the arts and manufactures, as well as to -railways and navigation, has of late years excited the fears of -many as to the possibility of the exhaustion of our coal-mines. -Such apprehensions are, however, altogether groundless. If the -present consumption of coal be estimated at sixteen millions of -tons annually, it is demonstrable that the coal-fields of this -country would not be exhausted for many centuries. - -But in speculations like these, the probable, if not certain -progress of improvement and discovery ought not be overlooked; and -we may safely pronounce that, long before such a period of time -shall have rolled away, other and more powerful mechanical agents -will supersede the use of coal. Philosophy already directs her -finger at sources of inexhaustible power in the phenomena of -electricity and magnetism. The alternate decomposition and -recomposition of water, by magnetism and electricity, has too -close an analogy to the alternate processes of vaporisation and -condensation, not to occur at once to every mind: the development -of the gases from solid matter by the operation of the chemical -affinities, and their subsequent condensation into the liquid -form, has already been essayed as a source of power. In a word, -the general state of physical science at the present moment, the -vigour, activity, and sagacity with which researches in it are -prosecuted in every civilised [Pg009] country, the increasing -consideration in which scientific men are held, and the personal -honours and rewards which begin to be conferred upon them, all -justify the expectation that we are on the eve of mechanical -discoveries still greater than any which have yet appeared; and -that the steam engine itself, with the gigantic powers conferred -upon it by the immortal Watt, will dwindle into insignificance in -comparison with the energies of nature which are still to be -revealed; and that the day will come when that machine, which is -now extending the blessings of civilisation to the most remote -skirts of the globe, will cease to have existence except in the -page of history. - - -(2.) The object of the present volume will be to deliver, in an -easy and familiar style, an historical view of the invention of the -steam engine, and an exposition of its structure and operation in -the various forms in which it is now used, and of its most -important applications in the arts of life, especially in transport -by land and water. It is hoped that the details of these subjects -may be rendered easily intelligible to all persons of ordinary -information, whether urged by that natural and laudable spirit of -inquiry awakened by contemplating effects on the material and -social condition of our species, so rapid and so memorable as those -which have followed the invention of the steam engine, and by the -pleasure which results from the perception of the numerous -instances of successful contrivances and beautiful applications of -science to art which it unfolds,--or impelled by the exigencies of -trade or profession to acquire an acquaintance with a machine on -which, more than any other, the prosperity of our commercial and -manufacturing interests depends. It will be our aim to afford to -the former class all the information which they can require; and, -if this work be not as comprehensive in its scope, and as minute in -its details, as some of the latter may wish, it will at least serve -as an easy and convenient introduction to other works more -voluminous, costly, and detailed, but less elementary in their -matter, and less familiar in their style. - -In explaining the different forms of steam engine which have been -proposed in the course of the progressive improvement [Pg010] of -that machine from its early rude and imperfect state to its -present comparatively perfect form, it will be necessary to advert -to various physical phenomena and mechanical principles, which, -however obvious to those who are conversant with matters of -science, must necessarily be at least imperfectly known by the -great majority of our readers. To refer for information on such -topics to other works on Mechanics and general Physics, would be -with most readers ineffectual, and with all unsatisfactory. In -former editions of the present work, we consigned these necessary -general principles of physics and mechanics to a preliminary -chapter; but it appears, on the whole, more convenient not to -remove the exposition of the principle from the place where its -application is required. We shall therefore pause as we proceed, -where these difficulties occur, to give such explanation and -illustration as may seem best suited to render them intelligible -and interesting to the unscientific reader. - -The history of the arts and manufactures affords no example of any -invention the credit for which has been claimed by so many -different nations and individuals as that of the steam engine. The -advocates of the competitors for this honour have urged their -pretensions, and pressed their claims, with a zeal which has -occasionally outstripped the bounds of discretion, and the contest -has not unfrequently been tinged with prejudices, national and -personal, and characterised by a degree of asperity altogether -unworthy of so noble a cause, and beneath the dignity of science. - -"When a question is clearly proposed, it is already half resolved." -Let us see whether a careful attention to this maxim will aid us -in the investigation of the origin of the steam engine. The source -of the power of that machine is found in the following natural -phenomena. - -_First._ When fire is applied to water, the liquid swells into -vapour, and in undergoing this change exerts, as has been already -stated, a considerable amount of mechanical force. This force may, -by proper means, be rendered applicable to any purpose for which -labour or power is needful. - -_Second._ The vapour so produced is endowed with the property of -elasticity, in virtue of which it is capable of [Pg011] swelling -or expanding into increased dimensions, exerting, as it expands, a -force, the energy of which is gradually diminished as the -dimensions of the vapour are increased. This mechanical force is -likewise capable of being applied to any useful purpose for which -labour or power is necessary. - -_Third._ This vapour is capable, by proper means, of being -reconverted into water; and when so reconverted, it shrinks into -its original dimensions, deserting the large space which it -occupied as vapour, and leaving that space a vacuum. It is known -in physics, that when a vacuum is produced, surrounding bodies -have a tendency to rush into it with a definite amount of force. -Consequently any agent which produces a vacuum, becomes a source -of a considerable amount of mechanical power. By its reconversion -into water, therefore, steam again becomes a mechanical agent. - -Such are the natural phenomena in which are found the original -sources of all steam power. In some forms of steam engine one of -these is used, and in some another, and in some the application of -all of them is combined; but in no existing form of steam engine -whatever is there any other source of mechanical power. - -Neither these nor any other natural forces can be applied -_immediately_ to any useful purpose. The interposition of -mechanism is indispensable; on the invention and contrivance of -that mechanism depends altogether the useful application of these -natural forces. - -The world owes the steam engine then partly to _discovery_, and -partly to _invention_. - -He that discovered the fact, that mechanical force was produced in -the conversion of water into steam, must be justly held to be a -sharer in the merit of the steam engine, even though he should -never have practically applied his discovery. The like may be said -of him who first discovered the source of the mechanical power -arising from the expansion of steam. - -The discoverer of the fact, that steam being reconverted into -water greatly contracted its dimensions, and thereby produced a -vacuum, is likewise entitled to a share of the credit. [Pg012] - -The mechanism by which these natural forces have been rendered so -universally available as a moving power, is very various and -complicated, and cannot be traced to one inventor. "If a -watchmaker," says M. Arago, "well instructed in the history of his -art, were required to give a categorical answer to the question, -Who has invented watches? he would remain mute; but the question -would be divested of much of its difficulty if he were required -separately to declare who discovered the use of the main spring, -the different forms of escapement, or the balance wheel." So it is -with the steam engine. It is a combination of a great variety of -contrivances, distinct from each other, which are the production -of several inventors. If, however, one name more than the rest be -entitled to special notice; if _he_ is entitled to the chief -credit of the invention who by the powers of his mechanical genius -has imparted to the steam engine that form, and conferred upon it -those qualities, on which mainly depends its present extensive -utility, and by which it has become an agent of transcendant -power, spreading its beneficial effects throughout every part of -the civilised globe, then the universal voice will, as it were by -acclamation, award the honour to one individual, whose pre-eminent -genius places him far above all other competitors, and from the -application of whose mental energies to this machine may be dated -those grand effects which render it a topic of interest to all for -whom the progress of civilisation has any attractions. Before the -era rendered memorable by the discoveries of JAMES WATT, the steam -engine, which has since become an object of such universal -interest, was a machine of extremely limited power, inferior in -importance and usefulness to most other mechanical agents used as -prime movers; but, from that epoch, it is scarcely necessary here -to state, that it became a subject not of British interest only, -but one having an important connection with the progress of the -human race. - - -HERO OF ALEXANDRIA, 120 B. C. - -[Illustration: _Fig._ 1.] - - -(3.) The discovery of the fact, that a mechanical force is -produced when water is evaporated by the application of heat, -[Pg013] must be considered as the first capital step in the -invention of the steam engine. It is recorded in a work entitled -_Spiritalia seu Pneumatica_, that Hero of Alexandria contrived a -machine, 120 years before the Christian era, which was moved by -the mechanical force of the vapour of water. The principle of this -machine admits of easy explanation: When a fluid issues from any -vessel in which it is confined, that vessel suffers a force equal -to that with which the fluid escapes from it, and in the opposite -direction. If water issues from an orifice, a pressure is produced -behind the orifice corresponding to the force with which the water -escapes. If a man discharge a gun, the gases produced by the -explosion of the powder issue with a certain force from the -muzzle, and his shoulder is driven backwards by the recoil with a -corresponding force. If the muzzle, instead of being presented -forwards, were turned at right angles to the length of the gun, -then, as the gases of explosion would escape sideways, the recoil -would likewise take place sideways, and the shooter, instead of -being driven backward, would be made to spin round as a dancer -pirouettes. This was the principle of Hero's steam engine. A small -globe or ball was placed on pivots at A and B (_fig._ 1.), on -which it was capable of revolving: steam was supplied through one -of these pivots from one of the tubes D C E F, which communicated -with the boiler. This steam filled the globe A I B K, and also the -arms I H and K G. A lateral orifice, represented at G, near the -end of these arms, allowed the steam to escape in a jet, and the -reaction, producing a recoil, had a tendency to drive the arm -round. A small orifice at H, on the other side of the tube, -produced a like effect. In the same manner, any convenient number -of arms might be provided, surrounding the globe and communicating -with its interior like the spokes of a wheel. Thus these arms, -having lateral orifices for the escape of the steam, all placed so -that the recoil may [Pg014] tend to turn the globe in the same -direction, a rotatory motion might be communicated to any -machinery which it was desired to move. - -After having been allowed to slumber for nearly two thousand -years, this machine has recently been revived, and engines -constructed similar to it are now working in these countries. In -the proper place we shall describe Avery's Rotatory Engine, which -it will be seen is, not only in its principle, but almost in its -details, the machine of HERO OF ALEXANDRIA. - -Although the elastic force of steam was not reduced to numerical -measure by the ancients, nor brought under control, nor applied to -any useful purpose, yet it appears to have been recognised in -vague and general terms. Aristotle, Seneca, and other ancient -writers, accounted for earthquakes by the sudden conversion of -water into steam within the earth. This change, according to them, -was effected by subterranean heat. Such tremendous effects being -ascribed to steam, it can scarcely be doubted that the Greeks and -Romans were acquainted with the fact, that water in passing into -vapour exercises considerable mechanical power. They were aware -that the earthquakes, which they ascribed to this cause, exerted -forces sufficiently powerful to extend the natural limits of the -ocean; to overturn from their foundations the most massive -monuments of human labour; to raise islands in the midst of seas; -and to heave up the surface of the land of level continents so as -to form lofty mountains. - -Such notions, however, resulted not as consequences of any exact -or scientific principles, but from vague analogies derived from -effects which could not fail to have been manifested in the arts, -such as those which commonly occurred in the process of casting in -metal the splendid statues which adorned the temples, gardens, and -public places of Rome and Athens. The artisan was liable to the -same accidents to which modern founders are exposed, produced by -the casual presence of a little water in the mould into which the -molten metal is poured. Under such circumstances, the sudden -formation of steam of an extreme pressure produces, as is well -known, explosions attended with destructive effects. The Grecian -[Pg015] and Roman artisans were subject to such accidents; and the -philosopher, generalising such a fact, would arrive at a solution -of the grander class of phenomena of earthquakes and volcanoes. - -Before natural phenomena are rendered subservient to purposes of -utility, they are often made to minister to the objects of -superstition. The power of steam is not an exception to this rule. -It is recorded in the Chronicles, that upon the banks of the Weser -the ancient Teutonic gods sometimes marked their displeasure by a -sort of thunderbolt, which was immediately succeeded by a cloud -that filled the temple. An image of the god _Busterich_, which was -found in some excavations, clearly explains the manner in which -this prodigy was accomplished by the priests. The head of the -metal god was hollow, and contained within it a pot of water: the -mouth, and another hole, above the forehead, were stopped by -wooden plugs; a small stove, adroitly placed in a cavity of the -head under the pot, contained charcoal, which, being lighted, -gradually heated the liquid contained in the head. The vapour -produced from the water, having acquired sufficient pressure, -forced out the wooden plugs with a loud report, and they were -immediately followed by two jets of steam, which formed a dense -cloud round the god, and concealed him from his astonished -worshippers.[1] - -Among other amusing anecdotes showing the knowledge which the -ancients had of the mechanical force of steam, it is related that -Anthemius, the architect of Saint Sophia, occupied a house next -door to that of Zeno, between whom and Anthemius there existed a -feud. To annoy his neighbour, Anthemius placed on the ground floor -of his own house several close digesters, or boilers, containing -water. A flexible tube proceeded from the top of each of these, -which was conducted through a hole made in the wall between the -houses, and which communicated with the space under the floors of -the rooms in the house of Zeno. When Anthemius desired to annoy -his neighbour, he lighted fires under his boilers, and the steam -produced by them rushed in such quantity and with [Pg016] such -force under Zeno's floors, that they were made to heave with all -the usual symptoms of an earthquake.[2] - - -BLASCO DE GARAY, A. D. 1543. - - -(4.) In the year 1826, M. de Navarrete published, in Zach's -Astronomical Correspondence, a communication from Thomas Gonzales, -Director of the royal archives of Simancas, giving an account of -an experiment reported to have been made in the year 1543, in -which a vessel was propelled by a machine having the appearance of -a steam engine. - -Blasco de Garay, a sea captain, proposed in that year to the Emperor -Charles V. to propel vessels by a machine which he had invented, -even in time of calm, without oars or sails. Notwithstanding the -apparent improbability attending this project, the Emperor ordered -the experiment to be made in the port of Barcelona, and the 17th of -June, 1543, was the day appointed for its trial. The commissioners -appointed by Charles V. to attend and witness the experiment were -Don Henry of Toledo, Don Pedro of Cardona, the treasurer Ravago, the -vice chancellor and intendant of Catalonia, and others. The vessel -on which the experiment was made was the Trinity, 200 tons burthen, -which had just discharged a cargo of corn at Barcelona. Garay -concealed the nature of his machinery, even from the commissioners. -All that could be discovered during the trial was, that it consisted -of a large boiler containing water, and that wheels were attached to -each side of the vessel, by the revolution of which it was -propelled. The commissioners having witnessed the experiment, made a -report to the king, approving generally of the invention, -particularly on account of the ease and promptitude with which the -vessel could be put about by it. - -The treasurer Ravago, who was himself hostile to the project, -reported that the machine was capable of propelling a vessel at the -rate of two leagues in three hours; but the other commissioners -stated that it made a league an hour at the least, and that it put -the vessel about as speedily as would be accomplished with a galley -worked according to the common [Pg017] method. Ravago reported that -the machinery was too complicated and expensive, and that it was -subject to the danger of the boiler bursting. - -After the experiment was made, Garay took away all the machinery, -leaving nothing but the framing of wood in the arsenals of -Barcelona. - -Notwithstanding the opposition of Ravago, the invention was -approved, and the inventor was promoted and received a pecuniary -reward, besides having all his expenses paid. - -From the circumstance of the nature of the machinery having been -concealed, it is impossible to say in what this machine consisted; -but as a boiler was used, it is probable, though not certain, that -steam was the agent. There have been various machines proposed, of -which a furnace and boiler form a part, and in which the agency of -steam is not used. The machine of Amontons furnishes an example of -this. It is most probable that the contrivance of Garay was -identical with that of Hero. The low state of the arts in Spain in -the sixteenth century would be incompatible with the construction -of any machine requiring great precision of execution. But the -simplicity of Hero's contrivance would have rendered its -construction and operation quite practicable. As to the claims to -the invention of the steam engine advanced by the advocates of De -Garay, founded on the above document, a refutation is supplied by -the admission, that though he was rewarded and promoted by the -government of the day, in consequence of the experiment, and -although the great usefulness of the contrivance in towing ships -out of port, &c., was admitted, yet it does not appear that a -second experiment was ever tried, much less that the machine was -ever brought into practical use. - - -SOLOMON DE CAUS, 1615. - - -(5.) Solomon De Caus was engineer and architect to Louis XIII., -king of France, before the year 1612. In that year he entered the -service of the Elector Palatine, who married the daughter of King -James I., with whom he came to England. He was there employed by -the Prince of Wales in ornamenting the gardens of his house at -Richmond. [Pg018] During his sojourn in England he composed and -published at London, in the same year, a Treatise on Perspective. -This person was the author of a work entitled, "_Les Raisons des -Forces Mouvantes, avec diverses Machines tant utiles que -plaisantes_," which was apparently composed at Heidelburg, but -published at Franckfort, in 1615. The same work was subsequently -republished in Paris in 1623. - -The treatise commences with definitions of what were then -considered the four elements: earth, air, fire, and water. Air is -defined to be a cold, dry, and light element, capable of -compression, by which it may be rendered very violent. He says, -"The violence will be great when water exhales in air by means of -fire, and that the said air is enclosed: as, for example, take a -ball of copper of one or two feet diameter, and one inch thick, -which being filled with water by a small hole, which shall be -strongly stopped with a peg, so that neither air nor water can -escape, it is certain that if we put the said ball upon a great -fire, so that it will become very hot, that it will cause a -compression so violent, that the ball will burst in pieces, with a -noise like a petard." - -The effect which is here described is due to the combined pressure -of the heated air contained in the ball and the high pressure -steam raised from the water, but much more to the latter than to -the former. It is evident, however, from the language of De Caus, -that he ascribes the force entirely to the air, and seems to -consider that the force of the air proceeded from the water which -exhaled in it. - -The first theorem is, "that the parts of the elements mix together -for a time, and then each returns to its place" (the elements here -referred to being apparently air and water). Upon this subject the -following is an example: "Take a round vessel of copper, soldered -close on every side, and with a tube, whereof one end approaches -nearly to the bottom of the vessel, and the other end, which -projects on the outside of the vessel, has a stop-cock; there is -also a hole in the top of the vessel, with a plug to stop it. If -this vessel will contain three pots of water, then pour in one pot -of water, and place the vessel on the fire about three or four -minutes, leaving the hole open; then take the vessel off the -[Pg019] fire, and a little after pour out the water at the hole, -and it will be found that a part of the said water has been -evaporated by the heat of the fire. Then pour in one pot of water -as before, and stop up the hole and the cock, and put the vessel -on the fire for the same time as before; then take it off, and let -it cool of itself, without opening the plug, and after it is quite -cold pour out the water, and it will be found exactly the same -quantity as was put in. Thus we see that the water which was -evaporated (the first time that the vessel was put on the fire) is -returned into water the second time when that vapour has been shut -up in the vessel, and cooled of itself." - -In the description of these experiments, the processes of -evaporation and condensation are obscurely indicated; but there is -no intimation that the author possessed any knowledge of the -elastic force of steam. His theorem is, that the parts of the -element water mix for a time with the parts of the element air; -that fire causes this mixture, and that on removing the fire, and -dissipating the heat, then the parts of the water mixed with air -return to their proper place, forming again part of the water. -There is no indication of a _change of property_ of the water in -passing into vapour. It is difficult to conceive, if De Caus had -been aware that the vapour of water possessed the same violent -force which he distinctly and in terms ascribes to air, or if he -had been aware that in effect the vapour of the water produced by -the fire was a fluid, possessing exactly the same mechanical -qualities, and producing the same mechanical effects as air, that -he would not have expressed himself clearly on the subject. - -He proceeds to give another demonstration that heat will cause the -particles of water to mix with those of air. - -"After having put the measure of water into the vessel, and shut -the vent-hole, and opened the cock, put the vessel on the fire, -and put the pot under the cock; then the water of the vessel, -raising itself by the heat of the fire, will run out through the -cock; but about one sixth or one eighth part of the water will not -run out, because the violence of the vapour which causes the water -to rise proceeds from the [Pg020] said water; which vapour goes -out through the cock after the water with great violence. There is -also another example in quicksilver, or mercury, which is a fluid -mineral, but being heated by fire, exhales in vapour, and mixes -with the air for a time; but after the said vapour is cooled, it -returns to its first nature of quicksilver. The vapour of water is -much lighter, and therefore it rises higher," &c. &c. - -In this second demonstration there appears to be some obscure -indication of the force of steam in the words "because of the -violence of the vapour which causes the water to rise," &c. - -The fifth theorem is the following:-- - -"_Water will mount by the help of fire higher than its level_," -which is explained and proved in the following terms:-- - -[Illustration: _Fig._ 2.] - -"The third method of raising water is by the aid of fire. On this -principle may be constructed various machines: I shall here -describe one. Let a ball of copper marked A; well soldered in -every part, to which is attached a tube and stop-cock marked D, by -which water may be introduced; and also another tube marked B C, -which will be soldered into the top of the ball, and the lower end -C of which shall descend nearly to the bottom of the ball without -touching it. Let the said ball be filled with water through the -tube D, then shutting the stop-cock D, and opening the stop-cock -in the vertical tube B C, let the ball be placed upon a fire the -heat acting upon the said ball will cause the water to rise in the -tube B C." - -In the apparatus as here described, the space enclosed in the -boiler above the surface of the water is filled with air. By the -action of the fire, two effects are produced: first, the air -enclosed above the water, being heated, acquires increased -elasticity, and presses with a corresponding force on the surface -of the water. By this means a column of water will be driven up -the tube A B at such a height as will balance the elasticity of -the heated air confined in the boiler; but besides [Pg021] this -the water contained in the boiler being heated, will produce -steam, which being mixed with air contained in the boiler, will -likewise press with its proper elasticity on the surface of the -water, and will combine with the air in raising a column of water -in the tube A B. In the above description of the machine, the -force which raises the water in the tube A B is ascribed to the -fire, no mention being made of the water, or of the vapour or -steam produced from it having any agency in raising the water in -the tube A B. - -Antecedently to the date of this invention, the effect of heat in -increasing the elastic force of air was known, and so far as the -above description goes, the whole operation might be ascribed to -the air by a person having no knowledge whatever of the elasticity -of steam. M. Arago, however, who, on the grounds of this passage -in the work of De Caus, claims for him a share of the honour of -the invention of the steam engine, contends that the agency of -steam in this apparatus was perfectly known to De Caus, although -no mention is made of steam in the above description, because in -the second demonstration above quoted he uses the words, "the -violence of the vapour which causes the water to rise proceeds -from the said water; which vapour goes out from the cock after the -water with great violence." By these words M. Arago considers that -De Caus expresses the quality of elasticity proper to the vapour, -and that the context justifies the inference, that to this -elasticity he ascribed the elevation of the water in the tube C B. - -There appears to be some uncertainty attending the birthplace of -De Caus. In the _Biographie Universelle_ he is said to have been -born and to have died in Normandy. M. Arago assigns Dieppe, or its -neighbourhood, as his birthplace. - -There was another engineer and architect, Isaac De Caus, a native -of Dieppe, who published a work in folio, entitled "_Nouvelle -Invention de Lever l'Eau plus haut que sa Source, avec quelque -Machines mouvantes, par le Moyen de l'Eau, et un Discours de la -Conduite d'Icelle_." This volume is without a date, but from the -nature of its contents it would appear to have been published -before the work of Solomon De Caus already cited. The drawings and -machines described in both [Pg022] are exactly the same; but the -definitions and theorems quoted above on raising water by fire are -not given in the work of Isaac. It seems, therefore, that Solomon -De Caus re-published, with additions, the work of Isaac De Caus. -From the same birthplace being assigned to both these authors, as -well as from the similarity of their pursuits, it is likely they -were members of the same family, and from their christian names -they were probably Jews. - -The work cited above, was dedicated to Louis XIII., and in the -dedication Solomon De Caus calls himself the subject of that -monarch; and in the privilege prefixed to the work he is -designated, "Our well-beloved Solomon De Caus, master engineer, -being at present in the service of our dear and well-beloved -cousin, the Prince Elector Palatine, has made known to us," -&c.--"we, desiring to gratify the said De Caus, he being our -subject," &c. - -It is therefore certain, whatever may have been the birthplace of De -Caus, that he was at least a subject of France. The circumstance of -his work being written in French, though published beyond the Rhine, -is also an argument in favour of his being a native of that country. - - -GIOVANNI BRANCA, 1629. - - -(6.) Giovanni Branca of Loretto in Italy, an engineer and -architect, proposed to work mills of different kinds by steam -issuing from a large æolopile, and blowing against the vanes of a -wheel. Branca was the author of many ingenious mechanical -inventions, a collection of which he dedicated to M. Cenci, the -governor of Loretto. These were published in a work printed at -Rome in 1629. It is a thin quarto, entitled "_Le Machine volume -nuovo, et di molto artificio da fare effetti maravigliosi tanto -Spiritali quanto di Animale Operatione, arichito di bellissime -figure. Del Sig. Giovanni Branca, Cittadino Romano. In Roma, -1629._" The work contains sixty-three engravings, accompanied by -descriptions in Italian and Latin. Branca's steam engine, -represented in the twenty-fifth plate, consists of a wheel -furnished with flat vanes upon its rim, like the boards of a -paddle wheel. The steam is produced in a close vessel, and made to -issue with violence from the extremity [Pg023] of a pipe directed -against the vanes, and causes the wheel to revolve. This motion -being imparted by the usual mechanical contrivances, any machinery -may be impelled by it. Different useful applications of this power -are contained in the work, viz. pestles and mortars for pounding -materials to make gunpowder, and rolling stones for grinding the -same; machines for raising water by buckets, for sawing timbers, -for driving piles, &c. &c. - -This method of applying the force of steam has no analogy to any -application of steam in modern engines. - - -EDWARD SOMERSET, MARQUIS OF WORCESTER, 1663. - - -(7.) Of all the names which figure in the early annals of steam, -by far the most remarkable is that of the Marquis of Worcester, -who has left a description of a machine in a work, entitled "The -Scantling of One Hundred Inventions," which has been generally in -this country considered as giving him a right to the honour of -having been the inventor of the steam engine. - -Lord Worcester having been engaged on the side of the Royalists in -the civil wars of the revolution, lost his fortune, and went to -Ireland, where he was imprisoned. He escaped from thence, and -reached France; from that country he ventured to London, as a -secret agent of Charles II., but was detected, and imprisoned in -the Tower, where he remained until the restoration, when he was -set at liberty. Tradition has connected the invention of the steam -engine with the following anecdote:--One day, during his -imprisonment, Lord Worcester observed the lid of the pot in which -his dinner was being cooked, suddenly forced upwards by the vapour -of the water which was boiling in it. Reflecting on this, it -occurred to him that the same force which raised the cover of the -pot might be rendered, when properly applied, a useful and -convenient moving power. After he recovered his liberty, he -accordingly proceeded to carry into effect this conception. The -contrivance to which he was ultimately led is described in the -following terms in the sixty-eighth invention, in the work above -named:-- - -"I have invented an admirable and forcible way to drive [Pg024] -up water by fire; not by drawing or sucking it upwards, for that -must be, as the philosopher terms it, _infra sphoerum activitatis_, -which is but at such a distance. But this way hath no bounder if -the vessels be strong enough. For I have taken a piece of whole -cannon whereof the end was burst, and filled it three quarters -full of water, stopping and screwing up the broken end, as also -the touch-hole, and making a constant fire under it; within -twenty-four hours, it burst and made a great crack. So that, -having a way to make my vessels so that they are strengthened by -the force within them, and the one to fill after the other, I have -seen the water run like a constant fountain stream forty feet -high. One vessel of water rarefied by fire driveth up forty of -cold water, and a man that tends the work has but to turn two -cocks; that one vessel of water being consumed, another begins to -force and refill with cold water, and so successively; the fire -being tended and kept constant, which the self-same person may -likewise abundantly perform in the interim between the necessity -of turning the said cocks." - -Since the date of the publication of the "Century of Inventions" -was the year 1663, the experiments here mentioned must have been -made before that year. The description of the machine here given, -as well as others in the same work, was intended by the author, -not to convey a knowledge of the nature of the mechanism which he -used, but only to express the effects produced, and to indicate -the physical principle on which they depended. It should also be -observed, that an air of mystery was thrown by Worcester over the -accounts of all the machines which he described; and therefore any -obscurity in the above description ought not to be regarded as an -evidence against his claim to the discovery of the mechanical -agency of steam, so far as that agency is indicated by the effects -said by him to be produced. The above account is, however, -sufficiently distinct and explicit to enable any one possessing a -knowledge of the mechanical qualities of steam to perceive the -general nature of the machine described. To render this machine, -and that of De Caus, previously described, intelligible to those -who are not familiar with physical science, we must here explain -some general principles on which their agency depends. [Pg025] - - -(8.) Fluid bodies are of two kinds, inelastic fluids, or liquids, -and elastic fluids, or gases. Of the former of these classes, -water is the most familiar example, and of the latter, air. - -These two species of fluids are each distinguished by peculiar -mechanical properties. - -[Illustration: _Fig._ 3.] - - -(9.) The constituent particles of a liquid are distinguished from -those of solids by having little or no coherence; so that unless -the mass be confined by the sides of the vessel which contains it, -the particles will fall asunder by their gravity. A mass of -liquid, therefore, unlike a solid, can never retain any particular -form, but will accommodate itself to the form of the vessel in -which it is placed. It will press against the bottom of the vessel -which contains it with the whole force of its weight, and it will -press against the sides with a force proportional to the depth of -the particles in contact with the sides measured from the surface -of the liquid above. This lateral pressure also distinguishes -liquids from solids. Let us take for illustration the case of a -square or a cubical vessel, A B C D, _fig._ 3. If a solid body, -such as a piece of lead, be cut to the shape of this vessel, so as -to fit in it without pressing with any force against its sides, -the mechanical effect which would be produced by it when placed in -the vessel, would be merely a pressure upon the bottom, B C, the -amount of which would be equal to the weight of the metallic mass. -No pressure would be exerted against the sides; for the coherence -of the particles of the solid maintaining them in their position, -the removal of the sides would not subject the solid body -contained in the vessel to any change. - -Now let us suppose this solid mass of lead to be rendered liquid -by being melted. The constituent particles will then be deprived -of that cohesion by which they were held together; they will -accordingly have a tendency to separate, and fall asunder by their -gravity, and will only be prevented from actually doing so by the -support afforded to them by the sides, [Pg026] A B, D C, of the -vessel. They will therefore produce a pressure against the sides, -which was not produced by the lead in its solid state. This -pressure will vary at different depths: thus a part of the side of -the vessel at P will receive a pressure proportional to the depth -of the point P below the surface of the lead. If, for example, we -take a square inch of the inner surface of the side of the vessel -at P, it will sustain an outward pressure equal to the weight of a -column of lead having a square inch for its base, and a height -equal to P A. And, in like manner, every square inch of the sides -of the vessel will sustain an outward pressure equal to the weight -of a column of lead having a square inch for its base, and a -height equal to the depth of the point below the surface of the -lead. - - -(10.) We have here proceeded upon the supposition that no force -acts on the upper surface A D of the lead. If any force presses A -D downwards, that force would be transferred to the bottom by the -lead, and would produce a pressure on the bottom B C equal to its -own amount in addition to the weight of the lead; and if the lead -were solid, this would be the only additional mechanical effect -which such a force acting on the surface A D of the lead would -produce. But if, on the other hand, the lead were liquified, then -the force now adverted to, acting on the surface A D, would not -only produce a pressure on the bottom B C, equal to its own amount -in addition to the weight of the lead, but it would also produce a -pressure against every part of the sides of the vessel, equal to -that which it would produce upon an equal magnitude of the surface -A D. - -Thus if we suppose any mechanical cause producing a pressure on -the surface A D amounting to ten pounds on each square inch, the -effect which would be produced, if the lead were solid, would be -an additional pressure on the base B C amounting to ten pounds per -square inch. But if the lead were liquid, besides this pressure on -each square inch of the base B C, there would likewise be a -pressure of ten pounds on every square inch of the sides of the -vessel. - -All that has been here stated with respect to a square or a -cubical vessel will be equally applicable to a vessel of any other -form. [Pg027] - - -(11.) The second class of fluids are distinguished from liquids by -the particles not merely being destitute of cohesion, but having a -tendency directly the reverse, to repel each other, and fly -asunder with more or less force. Thus if a vessel, such as that -represented in _fig._ 3., were filled with a fluid of this kind, -being open at the top, and not being restrained by any pressure -incumbent upon it, the particles of the fluid would not rest in -the vessel by their gravity, as those of the liquid would do; but -they would, by their mutual repulsion, fly asunder, and rise out -of the vessel, as smoke is seen to rise from a chimney, or steam -from the spout of a kettle. Let us suppose, then, that the vessel -in which an elastic fluid is contained is closed on every side by -solid surfaces. In fact, let us imagine that the square or cubical -vessel represented in _fig._ 3. is closed by a square lid at the -top A D, having contained in it an elastic fluid, such as -atmospheric air. - -If such a cover, or lid, had been placed upon a liquid, the cover -would sustain no pressure from the fluid, nor would any mechanical -effect be produced, save those already described in the case of -the open vessel; but when the fluid contained in the vessel is -elastic, as is the case with air, then the elasticity (by which -name is expressed the tendency of the particles of the fluid to -fly asunder) will produce peculiar mechanical effects, which have -no existence whatever in the case of a liquid. - -It is true that, supposing the fluid to be air or any other gas or -vapour, a pressure will be produced upon the bottom B C of the -vessel equivalent to the weight of such fluid, and lateral -pressures will be produced on the different points of the sides by -the weight of that part of the fluid which is above these points; -but gases and vapours are bodies of such extreme levity, that -these effects due to their weight are neglected in practice. - -Putting, then, the weight of the air contained in the vessel out -of the question, let us consider the effect of its elasticity. If -the vessel, as already described, be supposed to contain -atmospheric air in its ordinary state, the tendency of the -constituent particles to fly asunder will be such as to produce on -every square inch of the inner surface of the vessel [Pg028] a -pressure amounting to fifteen pounds; this pressure being, as -already stated, quite independent of the weight of the air. In -fact, this pressure would continue to exist if the air contained -in the vessel actually ceased to have weight by being removed from -the neighbourhood of the earth, which is the cause of its gravity. - - -(12.) Different gases are endowed with different degrees of -elasticity, and the same gas may have its elasticity increased or -diminished, either by varying the space within which it is -confined, or by altering the temperature to which it is exposed. - -If the space within which an elastic fluid is enclosed be -enlarged, its elasticity is found to diminish in the same -proportion. Thus if the air contained in the vessel A B C D -(_fig._ 3.) be allowed to pass into a vessel of twice the -magnitude, the elasticity of the particles will cause them to -repel each other, so that the same quantity of air shall diffuse -itself throughout the larger vessel, assuming double its former -bulk. Under such circumstances, the pressure which it would exert -upon the sides of the larger vessel would be only half that which -it had exerted on the sides of the smaller vessel. If, on the -other hand, it were forced into a vessel of half the magnitude of -A B C D, as it might be, then its elasticity would be double, and -it would press on the inner surface of that vessel with twice the -force with which it pressed on that of the vessel A B C D. - -This power of swelling and contracting its dimensions according to -the dimensions of the vessel in which it is confined, or to the -force compressing it, is a quality which results immediately from -elasticity, and is consequently one which is peculiar to the gases -or elastic fluids, and does not at all appertain to liquids. If -the liquid contained in the vessel A B C D were transferred to a -vessel of twice the magnitude, it would only occupy half the -capacity of that vessel, and it could not by any means be -transferred, as we have supposed the air or gas to be, to a vessel -of half the dimensions, since it is inelastic and incompressible. - - -(13.) The elasticity of gases is likewise varied by varying the -temperature to which they are exposed; thus, in general, [Pg029] -if air or any other gas be augmented in temperature, it will -likewise be increased in elasticity; and if, on the other hand, it -be diminished in temperature, it will be likewise diminished in -its elastic force. The more heated, therefore, any air or gas -confined in a vessel becomes, the greater will be the force with -which it will press on the inner surface of that vessel, and tend -to burst it. - - -(14.) The same body may, by the agency of heat, be made to pass -successively through the different states of solid, liquid, and -gas, or vapour. The most familiar and obvious example of these -successive transitions is presented by water. Exposed to a certain -temperature, water can only exist as a solid; as the temperature -is increased, the ice, or solid water, is liquefied; and by the -continued application of heat, this water again undergoes a -change, and assumes the form, and acquires the mechanical -qualities, of air or gas: in such a state it is called STEAM. - -This is a common property of all liquids. If they be exposed for a -sufficient length of time to a sufficient degree of heat, they -will always be converted into elastic fluids. These are usually -distinguished from air and other permanent gases, which never are -known to exist in the liquid form, by the term _vapour_, by which, -therefore, must be understood an elastic fluid which at common -temperatures exists in the liquid or solid state; by _steam_ is -expressed the vapour of water; and by _gases_, those elastic -fluids which like air are never known--at least, under ordinary -circumstances--to exist in any other but the elastic form. - - -(15.) When a liquid is caused, by the application of heat, to take -the form of an elastic fluid, or is evaporated, besides acquiring -the property of elasticity, it always undergoes a considerable -change of bulk. The amount of this change is different with -different liquids, and even with the same liquid it varies with -the circumstances under which the change is produced. - - -(16.) When water is evaporated under ordinary circumstances,--that -is, when exposed to no other external pressure than that of the -atmosphere,--it increases its volume about seventeen-hundred-fold. -Thus a cubic inch of liquid [Pg030] water would form about -seventeen hundred cubic inches of common steam. If, however, the -water be confined by a greater pressure than that produced by the -common atmosphere, then the increase of volume which takes place -in its evaporation would be less in proportion. - -These important physical circumstances are now only indicated in a -general way. As we proceed with our account of the invention and -improvement of the steam engine, they will be developed more fully -and accurately. - - -(17.) After duly considering what has been just explained, no -difficulty will be found in comprehending the principles on which -the first rude attempts at the mechanical application of steam -already stated depend. In the apparatus ascribed to _Hero_ of -Alexandria, the elasticity of the vapour contained in the arms of -the revolving ball causes that vapour to issue from the lateral -orifices in the arms, such as that of G, _fig._ 1. As these -orifices, however, are exposed to the common atmosphere pressing -inwards with a force, the mean amount of which has been stated to -be about fifteen pounds per square inch, it follows that the steam -cannot escape from these orifices until its pressure or elasticity -exceeds this amount, and that when it does, the force with which -it will so escape will be the excess of its elasticity above that -of the atmosphere; and it is the reaction produced by this -difference of pressure, causing the arms to recoil, which will -give motion to the machine. - -In the case of the apparatus of _De Caus_ (5.), the heat of the -fire acting on the vessel D C (_fig._ 2.) will raise the -temperature of the water contained in it, and also of the air -confined within it above the surface of that water. This air, as -it is increased in temperature, will also increase in elasticity; -it will therefore press on the surface of the water with increased -force, and will gradually force the water upwards in the tube; and -this effect would continue until all the water in the vessel would -be forced up the tube. - -But at the same time that the heat acting on the vessel increases -the temperature of the air above the water, it also produces a -partial evaporation of the water, so that more or less steam is -mixed with the air in the vessel above the surface [Pg031] of the -water; and this steam possessing elasticity, unites with the air -in pressing on the surface of the water, and in raising it in the -tube. - -[Illustration: _Figs._ 4, 5, and 6.] - -Let us now revert to the brief account of the engine of the -Marquis of Worcester, described in "The Century of Inventions." We -collect from that description that the vessel in which the water -was evaporated was separate from those which contained the water -to be elevated; also that there were two vessels of the like -description, the contents of which were alternately elevated by -the pressure of the "water rarefied by the fire;" in other words -by steam; and that the water was raised in an uninterrupted -stream, by the management of two cocks communicating with these -vessels and with the boiler. The following is such an apparatus as -would answer this description. Let E (_fig._ 4.) be the vessel -containing the water to be evaporated, placed over a proper -furnace A; let S be a pipe to allow the steam produced from the -boiling water in E to pass into the vessels where its mechanical -action is required. Let R represent a cock or regulator, having in -it a curved passage, leading from S to the tube T, when the lever -or handle L is in the position represented by the cut; but leading -to the tube T', when the lever L is turned one quarter of a -revolution to the right, as represented in _fig._ 5. By the -shifting of this lever, therefore, the steam pipe S may be made to -communicate alternately with the tubes T and T'. The tubes T and -T' are carried respectively to two vessels V and V', which are -filled with the water required to be raised. In these [Pg032] -vessels tubes enter at C and C', descending nearly to the bottom: -these tubes have valves at B and B', opening upwards, by which -water will be allowed to pass into the vertical tube F, but which -will not allow it to return downwards, the valves B and B' being -then closed by the weight of the water above them. - -Let G G' be a pipe entering the sides of the vessels V and V', for -the purpose of filling them with the water to be raised: let K be -a cock having a curved passage similar to the cock R, and leading -to a tube by which water is supplied from the reservoir or other -source from which the water to be raised is drawn. When the cock K -is placed as represented in _fig._ 4., the water from the -reservoir will flow through the curved passage in the cock K into -the tube G', and thence into the vessel V'; but when this cock is -turned one quarter round, by shifting the lever to the left, it -will take the position represented in _fig._ 6., and the water -will flow through the curved passage into the tube G, and thence -into the vessel V. Let us now suppose the vessel V already filled -with water to be elevated, and the vessel V' to have discharged -its contents. The cock R is turned, so as to allow the steam -generated in the boiler E to pass into the tube T, and thence into -the upper part of the vessel V, while the cock K is turned so as -to allow the water from the reservoir to pass into the tube G', -and thence into the vessel V'. The steam collecting in the upper -part of the vessel V' presses with its elastic force on the -surface of the water therein, and forces the water upwards in the -tube C; it passes through the valve B, which it opens by the -upward pressure received from the action of the steam, and thence -into the tube F, its descent into the tube C' being prevented by -the valve V', which can only be opened upwards. As the steam is -gradually supplied from the boiler E, the water in the vessel V is -forced up the tube C, through the valve B, and into the tube F, -until all the contents of the vessel V above the lower end of the -tube C have been raised. In the meanwhile, the vessel V' has been -filled with water, through the cock K: when this has been -accomplished, the man who attends the machine shifts the cocks R -and K, so as to give them the position represented in _fig._ 5. -and _fig._ 6. [Pg033] In this position, the steam from the -boiler, being excluded from the tube T, will be conducted to the -tube T', and thence to the vessel V', while the water from the -reservoir will be excluded from the tube G', and conducted through -the tube G to the vessel V. The vessel V will thus be replenished -and, by a process similar to that already described, the contents -of the vessel V' will be forced up the tube C', through the valve -B', and into the tube F; its descent into the tube C being -prevented by the valve B, which will then be closed. After the -contents of the vessel V' have thus been raised, and the vessel V -replenished, the two cocks R and K are once more shifted, and the -contents of V raised while V' is replenished, and so on. - -[Illustration: _Fig._ 4, 5, and 6.] - -If, having comprehended the apparatus here described, the reader -refers to the description of the Marquis of Worcester's machine, -he will find that all the conditions therein laid down are -fulfilled by it. One vessel (E) of "water rarefied by fire" may by -such means "drive up forty (or more) of cold water; and the man -that tends the work has but to turn two cocks, that one vessel (V) -of water being consumed, another (V') begins to force and refill -with cold water, and so on successively, the fire being tended and -kept constant; which the self-same person may likewise abundantly -perform, in the interim between the necessity of turning the said -cocks." - -On comparing this with the contrivance previously suggested by De -Caus, it will be observed, that even if De Caus [Pg034] knew the -physical agent by which the water was driven upwards in the -apparatus described by him, still it was only a method of causing -a vessel of boiling water to empty itself; and before a repetition -of the process could be made, the vessel should be refilled, and -again boiled. In the contrivance of Lord Worcester, on the other -hand, the agency of the steam was employed in the same manner as -it is in the steam engines of the present day, being generated in -one vessel, and used for mechanical purposes in another. Nor must -this distinction be regarded as trifling or insignificant, because -on it depends the whole practicability of using steam as a -mechanical agent. Had its action been confined to the vessel in -which it was produced, it never could have been employed for any -useful purpose. - -Although many of the projects contained in Lord Worcester's work -were in the highest degree extravagant and absurd, yet the engine -above described is far from being the only practicable and useful -invention proposed in it. On the contrary, many of his inventions -have been reproduced, and some brought into general use since his -time. Among these may be mentioned, stenography, telegraphs, -floating baths, speaking statues, carriages from which horses can -be disengaged if unruly, combination locks, secret escutcheons for -locks, candle moulds, the rasping mill, the gravel engine, &c. - - -SIR SAMUEL MORLAND, 1683. - - -(18.) Sir Samuel Morland was the son of a baronet of the same name, -who had received his title at the restoration for some services to -the royalist party, performed by him during the wars of the -Commonwealth. He appears to have devoted much attention to -mechanics, in which he attained some celebrity. He was the reputed -inventor of several ingenious contrivances, such as the drum capstan -for ships, the plunger pump, &c. He also investigated various -questions in acoustics, and among others, the determination of the -best form for the speaking-trumpet. - -In 1680, Sir Samuel Morland was appointed Master [Pg035] of the -Works to Charles II., and in the following year was sent to -France, to execute some waterworks for Louis XIV. In 1683, while -in France, he wrote in the French language, a work entitled -"_Elevation des Eaux par toute sorte de Machines, reduite à la -Mesure, au Poids et à la Balance. Presentée à sa Majesté très -Chrestienne, par le Chevalier Morland, Gentilhomme Ordinaire de la -Chambre Privée, et Maistre des Méchaniques du Roi de la Grande -Brétagne, 1683._" This book is preserved in manuscript in the -Harleian Collection in the British Museum. It is written on -vellum, and consists of only thirty-eight pages. It contains -tables of measures and weights, theorems for the calculation of -the volumes of cylinders, the weights of columns of water, the -thickness of lead for pipes, and is concluded by a chapter on -steam, consisting of four pages, of which the following is a -translation:-- - -"_The principles of the new force of fire invented by Chevalier -Morland in 1682, and presented to His Most Christian Majesty in -1683_:-- - -"'Water being converted into vapour by the force of fire, these -vapours shortly require a greater space (about 2000 times) than the -water before occupied, and sooner than be constantly confined would -split a piece of cannon. But being duly regulated according to the -rules of statics, and by science reduced to measure, weight, and -balance, then they bear their load peaceably (like good horses), and -thus become of great use to mankind, particularly for raising water, -according to the following table, which shows the number of pounds -that may be raised 1800 times per hour to a height of six inches by -cylinders half filled with water, as well as the different diameters -and depths of the said cylinders.'" - -There is nothing in the description here given which can indicate -the form of the machine by which Morland proposed to render the -force of steam a useful mover. It is, however, remarkable, that at -this early period, before experiments had been made on the -expansion which water undergoes in evaporation, he should have -given so near an approximation to [Pg036] the actual amount of -that expansion. It is scarcely supposable that such an estimate -could be obtained by him otherwise than by experiment. - -The work containing the above description was not printed; but a -work bearing nearly the same title, containing, however, no -mention of the force of steam, was published by him in Paris in -the year 1685. In this he describes various experiments made by -him at St. Germains on the weight of the water of the Seine, and -gives weights of the columns of water, the contents of cylinders, -&c. - -Soon after the publication of this work, Morland returned to -England, and resided near the court till his death. The celebrated -John Evelyn mentioned having paid a visit to him at his house at -Hammersmith, in 1695, when he had become aged and blind, but was -still remarkable for his mechanical ingenuity. "On the 25th of -October," says Evelyn, "the Archbishop and myself went to -Hammersmith to visit Sir Samuel Morland, who was entirely blind; a -very mortifying sight. He showed us his invention of writing -(short-hand), which was very ingenious; also his wooden kalendar, -which instructed him all by feeling; and other pretty and useful -inventions of mills, pumps, &c.; and the pump he had erected, that -serves water to his garden and to passengers, with an inscription, -and brings from a filthy part of the Thames near it a most perfect -and pure water."[3] - -He died at Hammersmith, in January 1696; and before his death, as -a penance for his past life, was guilty of the eccentricity of -burying in the ground six feet deep a great collection of music -which he possessed.[4] - - -DENIS PAPIN, 1688. - - -(19.) Denis Papin, a native of Blois in France, and professor of -mathematics at Marbourg, is the name which stands next recorded in -the progressive invention of the steam engine. To this philosopher -is due the discovery of one of the qualities of steam, to the -proper management of which is owing much of the efficacy of the -modern steam engine. [Pg037] - -Papin was born at Blois in France. He devoted his youth to the -study of medicine, in which he took a degree at Paris. The -revocation of the Edict of Nantes having driven him into exile, he -went to England, where the celebrated Boyle associated him in -several of his experiments with the air-pump, and caused him to be -elected a fellow of the Royal Society in 1681. Having been invited -to Germany by the Landgrave of Hesse, he discharged during several -years the duties of professor of mathematics at the university of -Marbourg, where he died in 1710. Notwithstanding his discoveries -respecting the agency of steam, he never received any mark of -distinction in his own country. The truth is, the importance and -value of these investigations were not apparent until long -afterwards. - -This philosopher conceived the idea of producing a moving power by -means of a piston working in a cylinder, in the manner which we -shall now briefly explain. - -[Illustration: _Fig._ 7.] - -Let A B (_fig._ 7.) be a cylinder open at the top, and let a -piston P be fitted into it, so as to move in it air tight. At the -bottom of the cylinder suppose an opening provided, which can be -closed at pleasure, by a stop-cock, or otherwise, so that the -communication may be opened and closed at will between the -interior of the cylinder and the external air. This stop-cock -being opened, let the piston be drawn upwards till it reach the -top of the cylinder. Let the stop-cock at the bottom be then -removed, and imagine that some means can be supplied by which the -air within the cylinder can be suddenly annihilated. The piston, -now at the top, will have above it the pressure of the atmosphere; -and having no air below, it will be resisted in its descent by no -force save that arising from its friction with the cylinder. If, -then, the force of the air above the piston be greater than the -resistance arising from this friction, the piston will descend -with the excess of this force, and will continue so to descend -until it reach the bottom of the cylinder. Having attained that -position, let us [Pg038] suppose the stop-cock in the bottom -opened, so as to allow the external air to pass freely below the -piston. The piston may now be drawn to the top of the cylinder -again, offering no resistance save that of its weight, and its -friction with the cylinder. Having reached the top of the cylinder -once more, let the stop-cock be closed, and the air included -within the cylinder once more annihilated. A second descent of the -piston will take place, with the same force as before, and in like -manner the process may be continued indefinitely. - -Now, if it should appear that means could be provided suddenly and -repeatedly to annihilate the air within the cylinder, and that the -pressure of the atmosphere above the piston should exert a force -compared with which the weight of the piston and its friction are -trifling, it is evident that a moving power would be obtained -which would be capable, by proper mechanism, of being applied to -any useful purpose, but which would more especially be applicable -to the working of pumps, the motion of which corresponds with that -which has been just ascribed to the piston in the cylinder. Such -were the first ideas of Papin. But in order to enable those who -are not conversant with physical science fully to appreciate their -importance, it will be necessary here to explain some of the -mechanical properties of atmospheric air. - - -(20.) The atmosphere is the thin, transparent, colourless, and -therefore invisible, fluid in which we live and move, which by -respiration sustains animal life, and is otherwise connected with -various important functions of organised matter. This fluid is so -light and attenuated, that it might at first be doubted whether it -be really a body at all; and, indeed, the name expressing -incorporeal beings, _spirit_, is a word in its origin signifying -_air_.[5] The air, however, is light only as compared with other -material substances, which exist in a more condensed state: it -possesses the quality of weight as absolutely as the most solid -and massive bodies in nature, and to render this quality manifest, -it is only necessary to submit a sufficient quantity of air to any -of the usual tests of gravitation. [Pg039] - -A direct demonstration of this may be given by the following -experiment:--On the mouth of a flask let a stop-cock be fastened so -as to be air-tight. The interior of the flask may then be put into -free communication with the external air, or that communication may -be cut off at pleasure, by opening or closing the stop-cock. If a -syringe be applied to the mouth of the flask, the stop-cock being -open a part of the air contained in it may be drawn out. After this, -the stop-cock being closed, and the syringe detached, let the -flask be placed in the dish of a good balance, and accurately -counterpoised by weights in the other dish. This counterpoise will -then represent the weight of the flask, and of the air which has -remained in it. If the stop-cock be now opened, air will immediately -rush in, and replace that which the syringe had withdrawn from the -flask; and immediately the dish of the balance containing the flask -will sink by the effect of the weight of the air thus admitted into -the flask. - -If the weight of quantity of air so small as to be capable of -being withdrawn by a syringe from an ordinary flask be thus of -sensible amount, it may be easily imagined that the vast mass of -atmosphere extending from the surface of the earth upwards, to a -height not ascertained with precision, but certainly not being -less than thirty miles, must be very considerable. Such a force, -pressing as it must constantly do, upon the surfaces of all -bodies, whether solid or fluid, and resisting and modifying their -movements, would play an important part in all mechanical -phenomena; and it is, therefore, not sufficient merely to have -recognised its existence, but it is most needful to measure its -amount with that degree of certainty and precision, which will -enable us to estimate its effects on those phenomena which we -shall have to investigate. - - -(21.) The amount of the pressure of the atmosphere on each square -inch of horizontal surface on which it rests, is obviously the -weight of the column of air extending from that square inch of -surface upwards to the top of the atmosphere. This force is -measured by the following means:-- - -[Illustration: _Fig._ 8.] - -[Illustration: _Fig._ 9.] - -Take a glass tube, A B (_fig._ 8.), above 32 inches long, open at -one end A, and closed at the other end B, and let it [Pg040] be -filled with mercury (quicksilver). Let a glass vessel or cistern -C, containing a quantity of mercury, be also provided. Applying -the finger at A, so as to prevent the mercury in the tube from -falling out, let the tube be inverted, and the end, stopped by the -finger, plunged into the mercury in C. When the end of the tube is -below the surface of the mercury in C (_fig._ 9.), let the finger -be removed. It will be found that the mercury in the tube will -not, as might be expected, fall to the level of the mercury in the -cistern C, which it would do were the end B open, so as to admit -the air into the upper part of the tube. On the other hand, the -level D of the mercury in the tube will be nearly 30 inches above -the level C of the mercury in the cistern. - -The cause of this effect is, that the weight of the atmosphere -rests on the surface C of the mercury in the cistern, and tends -thereby to press it up, or rather to resist its fall in the tube; -and as the fall is not assisted by the weight of the atmosphere on -the surface D (since B is closed), it follows, that as much -mercury remains suspended in the tube above the level C, as the -weight of the atmosphere is able to support. - -If the section of the tube were equal to the magnitude of a square -inch, the weight of the column of mercury in the tube above the -level C would be exactly equal to the weight of the atmosphere on -each square inch of the surface C. - - -(22.) If such an apparatus be observed from time to time, it will -be found that the column of mercury sustained in the tube will be -subject to variation between certain limits, never falling below -twenty-eight inches, and never rising above thirty-one inches. -This variation of the mercurial column is produced by a -corresponding variation in the weight of the atmosphere. - -If the apparatus be transported to any height above its ordinary -position, it will have a less quantity of atmosphere above it, and -therefore the surface of the mercury in the cistern will be -pressed by a less weight, and consequently the [Pg041] column of -mercury will fall proportionally. In virtue of this effect, such -an instrument has been rendered a means of measuring heights, such -as the heights of mountains, the ascents of balloons, &c. &c. - - -(23.) If a proper scale be attached to the tube containing the -mercurial column, showing the absolute height of the column -sustained at any time, and indicating its changes of height, the -instrument becomes a BAROMETER. - -Two cubic inches of mercury weigh very nearly one pound -avoirdupois.[6] Hence, when the barometric column measures thirty -inches, the weight of the atmosphere resting on each square inch -of surface is about fifteen pounds. - - -(24.) It is an established property of fluids, that they press -equally in all directions; and air, like every other fluid, -participates in this quality. Hence, it follows, that when the -downward pressure or weight of the atmosphere is fifteen pounds on -the square inch, the lateral, upward, and oblique pressures are of -the same amount. But, independently of the general principle, it -may be satisfactory to give experimental proof of this. - -[Illustration: _Fig._ 10.] - -Let four glass tubes, A, B, C, D (_fig._ 10.), be constructed of -sufficient length, closed at one end, A, B, C, D, and open at the -other. Let the open ends of three of them be bent, as represented -in the tubes B, C, D. Being previously filled with mercury, let -them all be gently inverted, so as to have their closed ends up, -as here represented. It will be found that the mercury will be -sustained in all, and that the difference of the levels in all -will be the same.[7] Thus, the mercury is sustained in A by the -upward pressure of the atmosphere; in B, by its horizontal or -lateral pressure; in C, by its downward pressure; [Pg042] and in -D, by its oblique pressure: and, as the difference of the levels -is the same in all, these pressures are exactly equal. - - -(25.) The same arrangement by which the pressure of the atmosphere -is measured by a mercurial column of equivalent weight, also -supplies the means of measuring the pressure or elasticity of -atmospheric air, or any other gas or vapour, whether in a more or -less compressed or rarefied state; and as instruments constructed -on this principle are of considerable use in steam engines, we -shall take this occasion to explain their principle and -application. - -In the experiments described in (21), the space D B in the top of -the barometer-tube, from which the mercury descended, is a vacuum. -If, however, it were occupied by a quantity of air in a rarefied -state, or any other gas or vapour, such gas or vapour would press -on the surface of the mercury at D, with a force determined by its -elasticity. In that case, the atmospheric pressure acting on the -surface of the mercury C in the cistern, would be balanced by the -combined forces of the weight of the mercurial column sustained in -the tube, and the elasticity of the gas or vapour in the upper -part of it. Now if we know the actual amount of the atmospheric -pressure,--that is to say, the height of the column of mercury -which it would be capable of sustaining,--we should then be able -to determine the pressure of the rarefied air in the space C D. - -For example, let us suppose that the barometric column, when B D -(_fig._ 9.) is a vacuum, measures thirty inches: the atmospheric -pressure, therefore, would be equal to the weight of a column of -mercury of that height. Let us suppose that the elasticity of the -gas or vapour occupying the upper part of the tube D B causes the -column to fall to the height of twenty-six inches: it is evident, -then, that the pressure of the air in the top of the tube would be -equal to the weight of a column of mercury of four inches. In -fine, to determine the pressure of the rarefied gas or vapour in -the top of the tube, it is only necessary to observe the -difference between the height of the column of mercury actually -sustained in the tube, and the column sustained at the same time -and [Pg043] place in a common barometer: the difference of the -two will be the column of mercury whose weight will represent the -pressure of the vapour or gas in the top of the tube. - - -(26.) Whenever the air contained in any vessel or other enclosed -space has by any means had its pressure reduced so as to be -rendered less than that of the external air, the external air will -have a tendency to rush into such vessel or enclosed space with a -force proportionate to the excess of the pressure of such external -air over that of the air within; and if any communication be -opened between the interior of such vessel or enclosed space, and -the external air, the latter will rush in until an equilibrium be -established between the pressures within and without. It is -evident that the force thus obtained by diminishing the pressure -of air within a vessel may be applied to any mechanical purpose. - -It is by such means that water is raised in an ordinary pump. A -portion of the air contained between the piston of the pump and -the surface of the water below, is withdrawn by the action of the -piston, and the pressure of the air remaining under the piston is -thereby diminished. The superior pressure of the atmosphere upon -the external surface of the water in the well then forces up a -column of water in the pump-barrel, and this is continued as the -air is more and more rarefied by the action of the piston. By -whatever means, therefore, the air can be wholly or partially -withdrawn from any space, a mechanical power will be thereby -developed, proportional in its amount and efficacy to the quantity -of air so withdrawn. If, however, such air be withdrawn by any -mechanical process, such as by a syringe, by a common pump, or by -an air-pump, the quantity of force expended in withdrawing it is -always equivalent to the amount of mechanical power obtained by -the vacuum or partial vacuum so produced. Indeed the power -expended is greater than the power so obtained, inasmuch as the -friction, leakage, &c. of the exhausting apparatus must be allowed -for. - - -(27.) There are, however, various other means by which air may be -partially expelled from a vessel besides the direct application of -mechanical force. Thus if heat be applied to [Pg044] the vessel, -the air, as has been already explained, will acquire increased -elasticity, and will rush from the vessel with a force proportionate -to the excess of its elasticity above that of the external air, and -this process may be continued by increasing the heat to which the -vessel is exposed, until a very considerable portion of the air has -been expelled. If the orifice by which the air has escaped be then -closed, and the vessel be allowed to cool, the air within, by having -its temperature reduced to that of the external air, will lose all -the elasticity which it had gained from the heat, and will be in the -same condition as if an equivalent quantity of air had been -withdrawn by any mechanical agent. The external air, therefore, will -have a tendency to rush in with a force corresponding to the -difference of pressures. - -The process of filling thermometers with mercury shows one use of -producing a high degree of rarefaction by heat. To construct the -instrument it is necessary to fill the bulb and a part of the tube -with mercury; but the bore of the tube is so small that the -mercury cannot be introduced by any ordinary means. It is -therefore held over flame until heated to a high temperature. The -air within it gradually increasing in pressure as its temperature -is raised, is forced through the small bore of the tube, until the -pressure of the air within becomes no more than equal to the -pressure of the external atmosphere; this air being so rarefied -that quantity in the bulb bears a very small proportion to its -contents at common temperatures. The mouth of the tube is then -plunged into mercury, and as the bulb cools, the air within it -loses its elasticity, and the superior pressure upon the external -surface forces the mercury into the tube. This continues until the -air remaining within the bulb has been so contracted, that its -pressure combined with the weight of the mercury, shall balance -the atmospheric pressure. The tube is then reversed, and the air -which remained rises in a bubble to the surface, and escapes. - - -(28.) Let us now return to the proceedings of Papin. How great a -power would result from such a machine as he conceived, will be -apparent, if it be considered that the unresisted atmosphere -exercises a pressure of about fifteen pounds on [Pg045] each -square inch of surface exposed to it, and that if the piston in -the cylinder imagined by Papin, had a diameter of only one foot, -its superficial magnitude would be about 114 square inches. The -pressure of the atmosphere upon it, therefore, would be 114 times -fifteen pounds, or 1710 pounds. Papin first proposed to produce -the vacuum under the piston by means of common air pumps, worked -by a water-wheel; and by such means he conceived that the power of -a river, stream, or waterfall might be conveyed by pipes to a -distance. While he was in England, in 1687, he laid his -contrivance before the Royal Society of London, but was met by -objections and difficulties, the nature of which he does not -explain. - -It is, however, apparent, from what has been already explained, -that such a method of proceeding would amount to a mere transfer -of power, and would not, properly speaking, be itself a moving -force: the moving power would, in reality, be the force of the -water by which the water-wheel would be driven; and the air-pumps, -tubes, together with the piston and cylinder, would be merely -means of conveying the power of the water-wheel to the objects to -be moved, or the machinery to be driven. Papin states, that, long -before this, he had attempted to expel the air from his cylinder -by means of gunpowder; but, notwithstanding all the precautions -which he could take, there always remained a considerable -quantity; so much, indeed, as to deprive the vacuum of more than -half its proper force. At length he adopted an expedient for the -production of a vacuum which forms a most important step in the -progressive invention of the steam engine, and which gives to -Papin's name a high place in the history of that machine. This -method is explained in the following paragraph of a work published -by Papin in 1695, at Cassel, entitled "_Recueil de diverses Pièces -touchant quelques nouvelles Machines_", p. 53. - -"I have endeavoured," says he, "to attain this end (viz. the -production of a vacuum in the cylinder) in another way. As water -has the property of elasticity, when converted into steam by heat, -and afterwards of being so completely recondensed by cold, that -there does not remain the least [Pg046] appearance of this -elasticity, I have thought that it would not be difficult to work -machines in which, by means of a moderate heat and at a small -cost, water might produce that perfect vacuum which has vainly -been sought by means of gunpowder." - -This remarkable passage is given in the work just cited, as an -extract from the "Leipsic Acts," of August, 1690. - -Let us pause here to explain more fully this important discovery. - - -(29.) We have explained that, in its conversion into vapour, by -the application of heat, water, besides acquiring the property of -elasticity, undergoes a vast enlargement of bulk, filling, under -ordinary circumstances, about 1700 times more space than it -occupied in the liquid form. This fact was known generally, though -not with numerical accuracy, by Papin, having been the foundation -of the machines previously invented and published by De Caus and -Lord Worcester; the happy idea of reversing the process occurred -to him. If water in its conversion into steam swelled into many -hundred times its original bulk, it would necessarily follow, that -steam, being reconverted into water, would shrink into its -primitive dimensions. Papin therefore saw, that if he could by any -means expel the air from his cylinder under the piston, and -replace it by the pure vapour of water, he could cause that vapour -to be reconverted into a comparatively minute quantity of water by -depriving it of the heat which sustained it in the state of steam, -and that by accomplishing this, the space in the cylinder under -the piston would become a vacuum; that by such means, the pressure -of the atmosphere above the piston would take full effect, and -would urge the piston down; that by introducing more steam under -the piston, it might be again raised by the elastic force of the -steam, the destruction of which by cold water would again produce -the descent of the piston with the same mechanical force; and that -in this way the alternate ascent and descent of the piston might -be continued indefinitely. - -In accordance with these ideas, Papin constructed a model -consisting of a small cylinder, in which was placed a solid -piston; [Pg047] and in the bottom of the cylinder under the -piston was contained in a small quantity of water. The piston -being in immediate contact with this water, so as to exclude the -atmospheric air, on applying fire to the bottom of the cylinder, -steam was produced, the elastic force of which raised the piston -to the top of the cylinder; the fire being then removed, and the -cylinder being cooled by the surrounding air, the steam was -condensed and reconverted into water, leaving a vacuum in the -cylinder into which the piston was pressed by the force of the -atmosphere. The fire being applied and subsequently removed, -another ascent and descent were accomplished; and in the same -manner the alternate motion of the piston might be continued. -Papin described no other form of machine by which this property -could be rendered available in practice; but he states generally, -that the same end may be attained by various forms of machines -easy to be imagined.[8] - - -THOMAS SAVERY, 1698. - - -(30.) The discovery of the method of making a vacuum by the -condensation of steam was reproduced, before 1698, by Captain -Thomas Savery, to whom a patent was granted in that year for a -steam engine to be applied to the raising of water, &c. Savery -proposed to combine the machine described by the Marquis of -Worcester with an apparatus for raising water by suction into a -vacuum produced by the condensation of steam. - -Savery appears to have been ignorant of the publication of Papin, -and stated that his discovery of the condensing principle arose -from the following circumstance:-- - -Having drunk a flask of Florence at a tavern, and flung the empty -flask on the fire, he called for a basin of water to wash his -hands. A small quantity which remained in the flask began to boil, -and steam issued from its mouth. It occurred to him to try what -effect would be produced by inverting the flask and plunging its -mouth in the cold water. Putting on a thick glove to defend his -hand from the heat, he seized the [Pg048] flask, and the moment -he plunged its mouth in the water the liquid immediately rushed up -into the flask and filled it. - -Savery stated that this circumstance immediately suggested to him -the possibility of giving effect to the atmospheric pressure by -creating a vacuum in this manner. He thought that if, instead of -exhausting the barrel of a pump by the usual laborious method of a -piston and sucker, it was exhausted by first filling it with -steam, and then condensing the same steam, the atmospheric -pressure would force the water from the well into the pump-barrel, -and into any vessel connected with it, provided that vessel were -not more than about thirty-four feet above the elevation of the -water in the well. He perceived also, that, having lifted the -water to this height, he might use the elastic force of steam in -the manner described by the Marquis of Worcester to raise the same -water to a still greater elevation, and that the same steam which -accomplished this mechanical effect would serve, by its subsequent -condensation, to reproduce the vacuum, and draw up more water. It -was on this principle that Savery constructed the first engine in -which steam was ever brought into practical operation. - -[Illustration: BRANCA'S ENGINE.] - - FOOTNOTES: - - [1] Arago, Eloge historique de James Watt; p. 22. - - [2] Ibid., p. 21. note. - - [3] Farey, Treatise on the Steam Engine, p. 93. - - [4] Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165 - - [5] SPIRITUS, _breath_ or _air_. - - [6] Exactly 15·68 oz. = 0·98 lb. - - [7] This experiment with the tube A requires to be very - carefully executed, and the tube should be one of small bore. - - [8] Recueil de diverses Pièces touchant quelques nouvelles - Machines, p. 38. - -[Pg049] - - - - -[Illustration: SAVERY'S ENGINE.] - -CHAP. II. - -ENGINES OF SAVERY AND NEWCOMEN. - - SAVERY'S ENGINE. -- BOILERS AND THEIR APPENDAGES. -- WORKING - APPARATUS. -- MODE OF OPERATION. -- DEFECTS OF THE ENGINE. -- - NEWCOMEN AND CAWLEY. -- ATMOSPHERIC ENGINE. -- ACCIDENTAL - DISCOVERY OF CONDENSATION BY INJECTION. -- HUMPHREY POTTER - MAKES THE ENGINE WORK ITSELF. -- ADVANTAGES OF THE ATMOSPHERIC - ENGINE OVER THAT OF SAVERY. -- IT CONTAINED NO NEW PRINCIPLE. -- - ITS PRACTICAL SUPERIORITY. - - -(31.) The steam engine contrived by Savery, like every other which -has since been constructed, consists of two parts, essentially -distinct. The first is that which is employed to [Pg050] generate -the steam, which is called the boiler; and the second, that in -which the steam is applied as a moving power. - -[Illustration: _Fig._ 11.] - -The former apparatus in Savery's engine consists of two strong -boilers, sections of which are represented at D and E in _fig._ -11.; D the greater boiler, and E the less. The tubes T and T' -communicate with the working apparatus, which we shall presently -describe. A thin plate of metal R, is applied closely to the top -of the great boiler D, turning on a centre C, so that by moving a -lever applied to the axis C on the outside of the top, the sliding -plate R can be brought from the mouth of the one tube to the mouth -of the other alternately. This sliding valve is called the -_regulator_, since it is by it that the communications between the -boiler and two steam vessels (hereafter described) are alternately -opened and closed, the lever which effects this being moved at -intervals by the hand of the attendant. - -Two _gauge cocks_ are represented at G, G', the use of which is to -determine the depth of water in the boiler. One, G, has its lower -aperture a little above the proper depth; and the other, G', a -little below it. Cocks are attached to the upper ends G, G', which -can be opened or closed at pleasure. The steam collected in the -top of the boiler pressing on the surface of the water, forces it -up in the tubes G, G', if their lower ends be immersed. Upon -opening the cocks G, G', if water be forced from both, there is -too much water in the boiler, since the mouth of G is _below_ its -level. If steam issue from both, there is too little water in the -boiler, since the mouth of G' is _above_ its level. But if steam -issue from G, and water from G', the water in the boiler is at its -proper level. This ingenious contrivance for determining the level -of the water in the boiler is the invention of Savery, and is used -in many instances at the present day. - -The mouth of the pipe G should be at a level of a little less -[Pg051] than one third of the whole depth, and the mouth of G' at -a level little lower than one third; for it is requisite that -about two thirds of the boiler should be kept filled with water. -The tube I forms a communication between the greater boiler D and -the lesser or feeding boiler E, descending nearly to the bottom of -it. This communication can be opened and closed at pleasure by the -cock K. A gauge pipe is inserted similar to G, G', but extending -nearly to the bottom. From this boiler a tube F extends, which is -continued to a cistern C (_fig._ 12.), and a cock is placed at M, -which, when opened, allows the water from the cistern to flow into -the feeding boiler E, and which is closed when that boiler is -filled. The manner in which this cistern is supplied will be -described hereafter. - -Let us now suppose that the principal boiler is filled to the -level between the gauge pipes, and that the subsidiary boiler is -nearly full of water, the cock K and the gauge cocks G G' being -all closed. The fire being lighted beneath D, and the water -boiled, steam is produced, and is transmitted through one or other -of the tubes T, T', to the working apparatus. When evaporation has -reduced the water in D below the level of G', it will be necessary -to replenish the boiler D. This is effected thus:--A fire being -lighted beneath the feeding boiler E, steam is produced in it -above the surface of the water, which, having no escape, presses -on the surface so as to force it up in the pipe I. The cock K -being then opened, the boiling water is forced into the principal -boiler D, into which it is allowed to flow until water issues from -the gauge cock G'. When this takes place, the cock K is closed, -and the fire removed from E until the great boiler again wants -replenishing. When the feeding boiler E has been exhausted, it is -replenished from the cistern C (_fig._ 12.), through the pipe F, -by opening the cock M. - - -(32.) We shall now describe the working apparatus in which the -steam is used as a moving power. - -Let V V' (_fig._ 12.) be two steam vessels communicating by the -tubes T T' (marked by the same letters in _fig._ 11.) with the -greater boiler D. - -[Illustration: _Fig._ 12.] - -Let S be a pipe, called the _suction pipe_, descending into -[Pg052] the well or reservoir from which the water is to be -raised, and communicating with each of the steam vessels through -tubes D D', by valves A A', which open upwards. Let F be a pipe -continued from the level of the engine to whatever higher level it -is intended to elevate the water. The steam vessels V V' -communicate with the _force-pipe_ F by valves B B', which open -upwards, through the tubes E E'. Over the steam vessels and on the -force-pipe is placed a small cistern C, already mentioned, which -is kept filled with cold water from the force-pipe, and from the -bottom of which proceeds a pipe terminated with a cock G. This is -called the _condensing pipe_, and can be brought alternately over -each steam vessel. From this cistern another pipe communicates -with the feeding boiler (_fig._ 11.), by the cock M.[9] - -The communication of the pipes T T' with the boiler can be opened -and closed alternately, by the regulator R (_fig._ 11.), already -described. - -Now suppose the steam vessels and tubes to be all filled with -common atmospheric air, and that the regulator be placed so that -the communication between the tube T and the boiler be opened, the -communication between the other tube T' and the boiler being -closed, steam will flow into V through T. At first, while the -vessel V is cold, the steam will be condensed, and will fall in -drops of water on the bottom and sides of the vessel. The -continued supply of steam from the boiler will at length impart -such a degree of heat to the vessel V, that it will cease to -condense it. Mixed with the heated air [Pg053] contained in the -vessel V, it will have an elastic force greater than the -atmospheric pressure, and will therefore force open the valve B, -through which a mixture of air and steam will be driven until all -the air in the vessel V will have passed out, and it will contain -nothing but the pure vapour of water. - -When this has taken place, suppose the regulator be moved so as to -close the communication between the tube T and the boiler, and to -stop the further supply of steam to the vessel V; and at the same -time let the condensing pipe G be brought over the vessel V, and -the cock opened so as to let a stream of cold water flow upon it. -This will cool the vessel V, and the steam with which it is filled -will be condensed and fall in a few drops of water, leaving the -interior of the vessel a vacuum. The valve B will be kept closed -by the atmospheric pressure. But the elastic force of the air -between the valve A and the surface of the water in the well, or -reservoir, will open A, so that a part of this air will rush in, -and occupy the vessel V. The air in the suction pipe S, being thus -allowed an increased space, will be proportionally diminished in -its elastic force, and its pressure will no longer balance that of -the atmosphere acting on the external surface of the water in the -reservoir. This pressure will, therefore, force water up in the -tube S until its weight, together with the elastic force of the -air above it, balances the atmospheric pressure. When this has -taken place, the water will cease to ascend. - -Let us now suppose that, by shifting the regulator, the -communication is opened between T and the boiler, so that steam -flows again into V. The condensing cock G being removed, the -vessel will be again heated as before, the air expelled, and its -place filled by the steam. The condensing pipe being again allowed -to play upon the vessel V, and the further supply of steam being -stopped, a vacuum will be produced in V, and the atmospheric -pressure will force the water through the valve A into the vessel -V, which it will nearly fill, a small quantity of air, however, -remaining above it. - -Thus far the mechanical agency employed in elevating the water is -the atmospheric pressure; and the power of steam is no further -employed than in the production of a vacuum. [Pg054] But, in -order to continue the elevation of the water through the force -pipe F, above the level of the steam vessel, it will be necessary -to use the elastic pressure of the steam. The vessel V is now -nearly filled by the water which has been forced into it by the -atmosphere. Let us suppose that, the regulator being shifted -again, the communication between the tube T and the boiler is -opened, the condensing cock removed, and that steam flows into V. -At first, coming in contact with the cold surface of the water and -that of the vessel, it is condensed; but the vessel is soon -heated, and the water formed by the condensed steam collects in a -sheet or film upon the surface of the water in V, so as to form a -surface as hot as boiling water.[10] The steam then being no -longer condensed, presses on the surface of the water with its -elastic force; and when that pressure becomes greater than the -atmospheric pressure, the valve B is forced open, and the water -issuing through it, passes through E into the force-pipe F; and -this is continued until the steam has forced all the water from V, -and occupies its place. - -The further admission of steam through T is once more stopped by -moving the regulator; and the condensing pipe being again allowed -to play on V, so as to condense the steam which fills it, produces -a vacuum. Into this vacuum, as before, the atmospheric pressure -will force the water, and fill the vessel V. The condensing pipe -being then closed, and steam admitted through T, the water in V -will be forced by its pressure through the valve B and tube E into -F, and so the process is continued. - -We have not yet noticed the other steam vessel V', which, as far -as we have described, would have remained filled with common -atmospheric air, the pressure of which on the valve A' would have -prevented the water raised in the suction pipe S from passing -through it. However, this is not the case; for, during the entire -process which has been described in V, similar effects have been -produced in V', which we have only omitted to notice to avoid the -confusion which the two processes might produce. It will be -remembered, that after the steam, in the first instance, having -flowed from the boiler [Pg055] through T, has blown the air out -of V through B, the communication between T and the boiler is -closed. Now the same motion of the regulator which closes this, -opens the communication between T' and the boiler; for the sliding -plate R (_fig._ 11.) is moved from the one tube to the other, and -at the same time, as we have already stated, the condensing pipe -is brought to play on V. While, therefore, a vacuum is being -formed in V by condensation, the steam, flowing through T', blows -out the air through B', as already described in the other vessel -V; and while the air in S is rushing up through A into V, followed -by the water raised in S by the atmospheric pressure, the vessel -V' is being filled with steam, and the air is completely expelled -from it. - -The communication between T and the boiler is now again opened, -and the communication between T' and the boiler closed by moving -the regulator R (_fig._ 11.) from the tube T to T'; at the same -time the condensing pipe is removed from over V, and brought to -play upon V'. While the steam once more expels the air from V -through B, a vacuum is formed by condensation in V', into which -the water in S rushes through the valve A'. In the mean time V is -again filled with steam. The communication between T and the -boiler is now closed, and that between T' and the boiler is -opened, and the condensing pipe removed from V', and brought to -play on V. While the steam from the boiler forces the water in V' -through B' into the force-pipe F, a vacuum is being produced in V, -into which water is raised by the atmospheric pressure. - -Thus each of the vessels V V' is alternately filled from S, and -the water thence forced into F. The same steam which forces the -water from the vessels into F, having done its duty, is condensed, -and brings up the water from S, by giving effect to the -atmospheric pressure. - -During this process, two alternate motions or adjustments must be -constantly made; the communication between T and the boiler must -be opened, and that between T' and the boiler closed, which is -done by one motion of the regulator. The condensing pipe at the -same time must be brought from V to play on V', which is done by -the lever placed upon it. Again [Pg056] the communication between -T' and the boiler is to be opened, and that between T and the -boiler closed; this is done by moving back the regulator. The -condensing pipe is brought from V' to V by moving back the other -lever, and so on alternately. - -For the clearness and convenience of description, some slight and -otherwise unimportant changes have been made in the position of -the parts. A perspective view of this engine is represented at the -head of this chapter. The different parts already described will -easily be recognised. - -The engine of Savery was very clearly described in a small work -published in London in 1702, entitled, _The Miner's Friend, or an -Engine to raise Water by Fire described, and the Manner of Fixing -it in Mines; with an Account of the several Uses it is applicable -unto, and an Answer to the Objection made against it; by Thomas -Savery, Gentleman_. This volume was dedicated to William III. (to -whom the engine had been exhibited at Hampton Court palace), to -the Royal Society, and to the mining adventurers of England. The -following are the uses to which Savery proposed the engine should -be applied: _First_, to raise water for turning all sorts of -mills; _second_, supplying palaces and houses with water, and -supplying means of extinguishing fire therein by the water so -raised; _third_, the supplying cities and towns with water; -_fourth_, draining fens or marshes; _fifth_, for ships; _sixth_, -the drainage of mines. - -Dr. Harris, in his _Lexicon Technicum, or Dictionary of Arts and -Sciences_, mentions a machine of Savery's for propelling a vessel -in a calm, by paddle-wheels placed at the side; but it does not -appear that Savery contemplated the application of a steam engine -to work these wheels. - -It is only from scattered passages in publications of the day that -it can be ascertained to what extent the engines of Savery were -practically applied. In his address to the Royal Society, he -speaks of the "difficulties and expense which he encountered in -instructing artisans to make engines according to his wish; but -that after much experience the workmen had become such masters of -the thing, that they bound themselves to deliver the engines -'exactly tight and fit for [Pg057] service, and such as he -(Savery) dare warrant them to every one that has occasion for -them.'" - -In his address to the miners of England he also says, "that the -frequent disorders and cumbersomeness of water engines then in use -encouraged him to invent engines to work by this new force; that -though they were obliged to encounter the oddest and almost -insuperable difficulties, yet he spared neither time, pains, nor -money, till he had conquered them." - -In Bradley's _Improvements of Planting and Gardening_, 1718, the -author thus speaks of an engine erected by Savery:-- - -"Supposing the situation of a house or garden to be a considerable -height above any pond, river, or spring, and that it has at -present no other conveniency of water than what is brought -continually by men or horses to it. In this case, the wonderful -invention of the late Mr. Savery, F.R.S., for raising water by -fire, will not only supply the defect, by flinging up as much -water as may be desired, but may be maintained with very little -trouble and very small expense. - -"It is now about six years since Mr. Savery set up one of them for -that curious gentleman Mr. Balle, at Cambden House, Kensington, -near London, which has succeeded so well that there has not been -any want of water since it has been built; and, with the -improvements since made to it, I am apt to believe will be less -subject to be out of order than any engine whatever." - -It is remarkable that, notwithstanding the high pressure steam -necessary for the operation of Savery's engine, he does not appear -to have adopted the obvious expedient of a safety valve. The -safety valve had been previously known, having been invented about -the year 1681, by Papin, for his digester, which was a close -boiler, contrived by him for stewing meat and digesting bones, by -submitting them to a higher temperature than that of water boiling -in an open vessel. - -The safety valve which has ever since been used for steam boilers -of every kind is a valve which opens outwards, and is fitted to an -aperture in the boiler, so as to be steam tight. It is pressed -down by a weight, the amount of which is regulated by the maximum -pressure to which it is intended the steam [Pg058] shall be -limited. Thus, if the magnitude of the valve be a square inch, and -the pressure of the steam be limited to 10 lbs. per square inch -above the pressure of the atmosphere, then the valve would be -loaded with a weight of 10 lbs.; but as it was found necessary to -vary from time to time the limiting pressure of the steam, or the -load of the safety valve, these valves were usually constructed so -as to be held down by the pressure of a lever having a sliding -weight upon it. By moving the weight on the arm of the lever, the -pressure on the valve could be increased or diminished at the -discretion of the engineer. This contrivance was first applied to -Savery's engines, by Desaguliers, about the year 1717, before -which year Savery died. - -It is justly observed by Mr. Farey, in his treatise on the steam -engine, that, "when a comparison is made between Captain[11] -Savery's engine and those of his predecessors, the result will be -in every respect favourable to his character as an inventor, and -as a practical engineer; all the details of his invention are made -out in a masterly style, and accidents and contingencies are -provided for, so as to render it a real working engine; whereas De -Caus, the Marquis of Worcester, Sir Samuel Morland, and Papin, -though ingenious philosophers, only produced mere outlines, which -required great labour and skill of subsequent inventors to fill -up, and make them sufficiently complete to be put in execution." - -About the year 1718 further improvements were made in the -construction of Savery's engine, by Dr. Desaguliers; but it is -probable that some of these were suggested by the proceedings of -the inventors of the atmospheric engine, which shall presently -describe. - - -(33.) In order duly to appreciate the value of improvements, it is -necessary first to perceive the defects which these improvements -are designed to remove. Savery's steam engine, considering how -little was known of the value and properties of steam, and how low -the general standard of mechanical knowledge was in his day, is -certainly highly [Pg059] creditable to his genius. Nevertheless -it had very considerable defects, and was finally found to be -inefficient for the most important purposes to which he proposed -applying it. - -At the time of this invention, the mines in England had greatly -increased in depth, and the process of draining them had become -both expensive and difficult; so much so, that it was found in -many instances that their produce did not cover the cost of -working them. The drainage of these mines was the most important -purpose to which Savery proposed to apply his steam engine. - -It has been already stated that the pressure of the atmosphere -amounts to about fifteen pounds on every square inch. Now, a -column of water, whose base is one square inch, and whose height -is thirty-four feet, weighs about fifteen pounds. If we suppose -that a perfect vacuum were produced in the steam vessels V V' -(_fig._ 12.) by condensation, the atmospheric pressure would fail -to force up the water, if the height of the top of these vessels -above the water to be raised exceeded thirty-four feet. It is -plain, therefore, that the engine cannot be more than thirty-four -feet above the water which it is intended to elevate. But in fact -it cannot be so much; for the vacuum produced in the steam vessels -V V' is never perfect. Water, when not submitted to the pressure -of the atmosphere, will vaporise at a very low temperature, as we -shall hereafter explain; and it was found that a vapour possessing -a considerable elasticity would, notwithstanding the condensation, -remain in the vessels V V' and the pipe S, and would oppose the -ascent of the water. In consequence of this, the engine could -never be placed with practical advantage at a greater height than -twenty-six feet above the level of the water to be raised. - - -(34.) When the water is elevated to the engine, and the steam -vessels filled, if steam be introduced above the water in V, it -must first balance the atmospheric pressure, before it can force -the water through the valve B. Here, then, is a mechanical -pressure of fifteen pounds per square inch expended, without any -water being raised by it. If steam of twice that elastic force be -used, it will elevate a column in F of thirty-four feet in height; -and if steam of triple the force be used, it will raise a column -of sixty-eight feet high, [Pg060] which, added to twenty-six feet -raised by the atmosphere, gives a total lift of ninety-four feet. - -In effecting this, steam of a pressure equal to three times that -of the atmosphere acts on the inner surface of the vessels V V'. -One third of this bursting pressure is balanced by the pressure of -the atmosphere on the external surface of the vessels; but an -effective pressure of thirty pounds per square inch still remains, -tending to burst the vessels. It was found that the apparatus -could not be constructed to bear more than this with safety; and, -therefore, in practice, the lift of such an engine was limited to -about ninety perpendicular feet. In order to raise the water from -the bottom of the mine by these engines, therefore, it was -necessary to place one at every ninety feet of the depth; so that -the water raised by one through the first ninety feet should be -received in a reservoir, from which it was to be elevated the next -ninety feet by another, and so on. - -Besides this, it was found that sufficient strength could not be -given to those engines, if constructed upon a large scale. - -They were, therefore, necessarily very limited in their -dimensions, and were incapable of raising the water with -sufficient speed. Hence arose a necessity for several engines at -each level, which greatly increased the expense. - - -(35.) These, however, were not the only defects of Savery's -engines. The consumption of fuel was enormous, the proportion of -heat wasted being much more than what was used in either forcing -up the water, or producing a _vacuum_. This will be very easily -understood by attending to the process of working the engine -already described. - -When the steam is first introduced from the boiler into the steam -vessels V V', preparatory to the formation of a vacuum, it is -necessary that it should heat these vessels up to the temperature -of the steam itself; for until then the steam will be condensed -the moment it enters the vessel by the cold surface. All this -heat, therefore, spent in raising the temperature of the steam -vessels is wasted. Again, when the water has ascended and filled -the vessels V V', and steam is introduced to force this water -through B B' into F, it is immediately condensed by the cold -surface in V V', and does not [Pg061] begin to act until a -quantity of hot water, formed by condensed steam, is collected on -the surface of the cold water which fills these vessels. Hence -another source of the waste of heat arises. - -When the steam begins to act upon the surface of the water in V -V', and to force it down, the cold surface of the vessels is -gradually exposed to the steam, and must be heated while the steam -continues its action; and when the water has been forced out of -the vessel, the vessel itself has been heated to the temperature -of the steam which fills it, all which heat is dissipated by the -subsequent process of condensation. It must thus be evident that -the steam used in forcing up the the water in F, and in producing -a vacuum, bears a very small proportion indeed to what is consumed -in heating the apparatus after condensation. - - -(36.) There is also another circumstance which increases the -consumption of fuel. The water must be forced through B, not only -against the atmospheric pressure, but also against a column of -sixty-eight feet of water. Steam is therefore required of a -pressure of forty-five pounds on the square inch. Consequently the -water in the boiler must be boiled under this pressure. That this -should take place, it is necessary that the water should be raised -to a temperature considerably above 212°, even so high as 275°; -and thus an increased heat must be given to the boiler. -Independently of the other defects, this intense heat weakened and -gradually destroyed the apparatus. - -Savery was the first who suggested the method of expressing the -power of an engine with reference to that of horses. In this -comparison, however, he supposed each horse to work but eight -hours a day, while the engine works for twenty-four hours. This -method of expressing the power of steam engines will be explained -hereafter. - - -(37.) The failure of the engines proposed by Captain Savery in the -work of drainage, from the causes which have been just mentioned, -and the increasing necessity for effecting this object, arising -from the large property in mines which became every year -unproductive by being flooded, stimulated the ingenuity [Pg062] -of mechanics to contrive some means of rendering those powers of -steam exhibited in Savery's engine available. - -Thomas Newcomen, the reputed inventor of the atmospheric engine, was -an ironmonger, or, according to some, a blacksmith, in the town of -Dartmouth in Devonshire. From his personal acquaintance and -intercourse with Dr. Hooke, the celebrated natural philosopher, it -is probable that he was a person of some education, and therefore -likely to be above the position of a blacksmith. Being in the habit -of visiting the tin mines in Cornwall, Newcomen became acquainted -with the engine invented by Savery, and with the causes which led to -its inefficiency for the purposes of drainage. - -It has been stated that Papin, about the year 1690, proposed the -construction of an engine working by the atmospheric pressure -acting on one side of a piston against a vacuum produced by the -condensation of steam on the other side. Papin was not conscious -of the importance of this principle; for, so far from ever having -attempted to apply it to practical purposes, he probably never -constructed, even on a small scale, any machine illustrating it. -On the contrary, he abandoned the project the moment he was -informed of the principle and structure of the steam engine of -Savery; and he then proposed an engine for raising water, acting -by the expansive force of steam similar to Savery's, but -abandoning the method of working by a vacuum. - -This engine is described by Papin in a work published in 1707. - -[Illustration: _Fig._ 13.] - -A (_fig._ 13.) is an oval boiler, having a safety-valve B, which -limits the pressure of the steam. It is connected with a cylinder -C, by a curved pipe having a stop-cock at D. A pipe with a -stop-cock G opens from the top of the cylinder into the -atmosphere, and a safety-valve F is placed upon the cylinder. A -hollow copper piston H moves freely in the cylinder, and floats -upon the water. O is a funnel with a valve L in the bottom, -opening downwards, through which the cylinder C may be filled with -water to the level of the top of the funnel. A close air-vessel -communicates with the cylinder C by the curved tube, and has a -valve K opening upwards. The force-pipe through which the water is -raised communicates [Pg063] with the air-vessel I. If the cock D -be shut, and the cock G opened, water poured into the funnel O -will rise into the cylinder C, the air which fills the cylinder -escaping through the open pipe G. When the cylinder is thus filled -with water, let the cock G be closed, and the cock D opened. The -steam from the boiler, after heating the metal of the cylinder, -will force the piston downwards, and drive the water through the -curved tube into the vessel I, from which its return is prevented -by the valve K, which is closed by its weight. The air which -filled the vessel I will then be compressed, and by its elasticity -will drive a column of water up the pipe N. After the contents of -the cylinder have been thus discharged it may be refilled in the -same manner, and the process repeated. - -It will be perceived that this project is nothing more than a -reproduction of the engine of the Marquis of Worcester. In the -preface to the work containing this description, Papin gives an -extract from a letter addressed by him to Leibnitz in 1698, from -which it appears that he had abandoned his idea of working the -piston by the atmospheric pressure acting against a vacuum, -considering it to be a contrivance inferior [Pg064] to the engine -now described. "We now raise water," he says, "by the force of -fire, _in a more advantageous manner than that which I had -published some years before_; for besides the suction, we now also -use the pressure which the water exerts upon other bodies in -dilating itself by heat; instead of which I before employed the -suction only, the effects of which are more limited." - -From documents which have been preserved in the Royal Society, it -appears that Newcomen was acquainted with Papin's writings, and -therefore probably first derived from them the suggestion which he -subsequently realised in the atmospheric engine. Among some papers -of Dr. Hooke's have been found notes for the use of Newcomen, on -Papin's method of transmitting the force of a stream or fall of -water to a distance by pipes. Hooke dissuaded Newcomen from -attempting any machine on this principle, which, as first proposed -by Papin, was impracticable. He exposed the fallacy of Papin's -first project in several discourses before the Royal Society, and -considered his improved edition of it, though free from fallacy, -as impracticable. - -Papin's project for producing a vacuum under a piston by -condensing the steam having been published in the _Actæ -Eruditorum_, in Latin, in 1690, and in French, at Cassel, in 1695, -and subsequently, in the _Philosophical Transactions_, in England -in 1697, cannot be supposed to be unknown to Dr. Hooke; and if -known to him, would probably have been communicated to Newcomen. -Dr. Hooke died in 1703, some years before the date of Newcomen's -invention. - -John Cawley, who was the associate of Newcomen in his experiments -and inquiries, was a plumber and glazier of the same town. -Newcomen and Cawley obtained a patent for the atmospheric engine -in 1705, in which Savery was associated, he having previously -obtained a patent for the method of producing a vacuum by the -condensation of steam, which was essential to Newcomen's -contrivance. It was not, however, until about the year 1711 that -any engine had been constructed under this patent. - -In the latter end of that year, according to Desaguliers, the -patentees "made proposals to drain a colliery at Griff, in -[Pg065] Warwickshire, in which work five hundred horses were -constantly employed. This proposal not being accepted, they -contracted, in the following March, to drain water for Mr. Back of -Wolverhampton, where, after many laborious attempts, they -succeeded in making their engine work; but not being either -philosophers to understand the reason, or mathematicians enough to -calculate the power and proportions of the parts, they very -luckily, by accident, found what they sought for." - -[Illustration: _Fig._ 14.] - -Newcomen resumed the old method of raising the water from the -mines by ordinary pumps, but conceived the idea of working these -pumps by some moving power less expensive than that of horses. The -means whereby he proposed effecting this, was by connecting the -end of the pump-rod D (_fig._ 14.) by a chain with the arch head A -of a [Pg066] working-beam A B, playing on an axis C. The other -arch head B of this beam was connected by a chain with the rod E -of a solid piston P, which moved air-tight in a cylinder F. If a -vacuum be created beneath the piston P, the atmospheric pressure -acting upon it will press it down with a force of fifteen pounds -per square inch; and the end A of the beam being thus raised, the -pump-rod D will be drawn up. If a pressure equivalent to the -atmosphere be then introduced below the piston, so as to -neutralise the downward pressure, the piston will be in a state of -indifference as to the rising or falling; and if in this case the -rod D be made heavier than the piston and its rod, so as to -overcome the friction, it will descend, and elevate the piston -again to the top of the cylinder. The vacuum being again produced, -another descent of the piston, and consequent elevation of the -pump-rod, will take place; and so the process may be continued. - -Such was Newcomen's first conception of the _atmospheric engine_; -and the contrivance had much, even at the first view, to recommend -it. The power of such a machine would depend entirely on the -magnitude of the piston; and being independent of highly elastic -steam, would not expose the materials to the destructive heat -which was necessary for working Savery's engine. Supposing a -perfect vacuum to be produced under the piston in the cylinder, an -effective downward pressure would be obtained, amounting to -fifteen times as many pounds as there are square inches in the -section of the piston.[12] Thus, if the base of the piston were -100 square inches, a pressure equal to 1500 pounds would be -obtained. - - -(38.) In order to accomplish this, two things were necessary: 1. -To make a speedy and effectual vacuum below the [Pg067] piston in -the descent; and, 2. To contrive a counterpoise for the atmosphere -in the ascent. - -The condensation of steam immediately presented itself as the most -effectual means of accomplishing the former; and the elastic force -of the same steam previous to condensation an obvious method of -effecting the latter. Nothing now remained to carry the design -into execution, but the contrivance of means for the alternate -introduction and condensation of the steam; and Newcomen and -Cawley were accordingly granted a patent in 1707, in which Savery -was united, in consequence of the principle of condensation for -which he had previously received a patent being necessary to the -projected machine. We shall now describe the _atmospheric engine_, -as first constructed by Newcomen:-- - -The boiler K (_fig._ 14.) is placed over a furnace I, the flue of -which winds round it, so as to communicate heat to every part of -the bottom of it. In the top, which is hemispherical, two -gauge-cocks G G' are placed, as in Savery's engine, and a _puppet -valve_ V, which opens upward, and is loaded at one pound per -square inch; so that when the steam produced in the boiler exceeds -the pressure of the atmosphere by more than one pound on the -square inch, the valve V is lifted, and the steam escapes through -it, and continues to escape until its pressure is sufficiently -diminished, when the valve V again falls into its seat. This valve -performs the office of the safety-valve in modern engines. - -The great steam-tube is represented at S, which conducts steam -from the boiler to the cylinder; and a feeding pipe T, furnished -with a cock, which is opened and closed at pleasure, proceeds from -a cistern L to the boiler. By this pipe the boiler may be -replenished from the cistern, when the gauge cock G' indicates -that the level has fallen below it. The cistern L is supplied with -hot water, by means which we shall presently explain. - - -(39.) To understand the mechanism necessary to work the piston, -let us consider how the supply and condensation of steam must be -regulated. When the piston has been forced to the bottom of the -cylinder by the atmospheric pressure acting against a vacuum, in -order to balance that pressure, [Pg068] and enable it to be drawn -up by the weight of the pump-rod, it is necessary to introduce -steam from the boiler. This is accomplished by opening the cock R -in the steam pipe S. The steam being thus introduced from the -boiler, its pressure balances the action of the atmosphere upon -the piston, which is immediately drawn to the top of the cylinder -by the weight of the pump-rod D. It then becomes necessary to -condense this steam, in order to produce a vacuum. To accomplish -this, the further supply of steam must be cut off, which is done -by closing the cock R. The supply of steam from the boiler being -thus suspended, the application of cold water on the external -surface of the cylinder becomes necessary to condense the steam -within it. This was done by enclosing the cylinder within another, -leaving a space between them.[13] Into this space cold water was -allowed to flow from a cock M placed over it, supplied by a pipe -from the cistern N. This cistern is supplied with water by a pump -O, which is worked by the engine. - -The cold water supplied from M, having filled the space between -the two cylinders, abstracts the heat from the inner one; and -condensing the steam, produces a vacuum, into which the piston is -forced by the atmospheric pressure. Preparatory to the next -descent, the water which thus fills the space between the -cylinders, and which is warmed by the heat abstracted from the -steam, must be discharged, in order to give room for a fresh -supply of cold water from M. An aperture, furnished with a cock, -is accordingly provided in the bottom of the cylinder, through -which the water is discharged into the cistern L; and being warm, -is adapted for the supply of the boiler through T, as already -mentioned. - -The cock R being now again opened, steam is admitted below the -piston, which, as before, ascends, and the descent is again -accomplished by closing the cock R, and opening the cock M, -admitting cold water between the cylinders, and thereby condensing -the steam below the piston. - -The condensed steam, thus reduced to water, will collect [Pg069] -in the bottom of the cylinder, and resist the descent of the -piston. It is therefore necessary to provide an exit for it, which -is done by a valve opening _outwards_ into a tube which leads to -the feeding cistern L, into which the condensed steam is driven. - -That the piston should continue to be air-tight, it was necessary -to keep a constant supply of water over it; this was done by a -cock similar to M, which allowed water to flow from the pipe M on -the piston. - - -(40.) Soon after the first construction of these engines, an -accidental circumstance suggested to Newcomen a much better method -of condensation than the application of cold water on the external -surface of the cylinder. An engine was observed to work several -strokes with unusual rapidity, and without the regular supply of -the condensing water. Upon examining the piston, a hole was found -in it, through which the water, which was poured on to keep it -air-tight, flowed, and instantly condensed the steam under it. - -On this suggestion Newcomen abandoned the external cylinder, and -introduced a pipe H, furnished with a cock Q, into the bottom of -the cylinder, so that, on turning the cock, the pressure of the -water in the pipe H, from the level of the water in the cistern N, -would force the water to rise as a jet into the cylinder, and -would instantly condense the steam. This method of condensing by -injection formed a very important improvement in the engine, and -is still used. - - -(41.) Having taken a general view of the parts of the atmospheric -engine, let us now consider more particularly its operation. - -When the engine is not working, the weight of the pump-rod D -(_fig._ 14.) draws down the beam A, and draws the piston to the -top of the cylinder, where it rests. Let us suppose all the cocks -and valves closed, and the boiler filled to the proper depth. The -fire being lighted beneath it, the water is boiled until the steam -acquires sufficient force to lift the valve V. When this takes -place, the engine may be started. For this purpose the regulating -valve R is opened. The steam rushes in, and is first condensed by -the cold cylinder. After a short time the cylinder acquires the -temperature of the steam, which then [Pg070] ceases to be -condensed, and mixes with the air which filled the cylinder. The -steam and heated air, having a greater force than the atmospheric -pressure, will open a valve placed at the end X of a small tube in -the bottom of the cylinder, and which opens outwards. From this -(which is called the _blowing valve_[14]) the steam and air rush -in a constant stream, until all the air has been expelled, and the -cylinder is filled with the pure vapour of water. This process is -called _blowing_ the engine preparatory to starting it. - -When it is about to be started, the engine-man closes the -regulator R, and thereby suspends the supply of steam from the -boiler. At the same time he opens the _condensing valve_ H[15]; -and thereby throws up a jet of cold water into the cylinder. This -immediately condenses the steam contained in the cylinder, and -produces the vacuum. (The atmosphere cannot enter the _blowing_ -valve, because it opens _outwards_, so that no air can enter to -vitiate the vacuum.) The atmospheric pressure above the piston now -takes effect, and forces it down in the cylinder. The descent -being completed, the engine-man closes the condensing valve H, and -opens the regulator, R. By this means he stops the play of the jet -within the cylinder, and admits the steam from the boiler. The -first effect of the steam is to expel the condensing water and -condensed steam which are collected in the bottom of the cylinder, -through the tube Y, containing a valve which opens _outwards_ -(called the _eduction valve_), which leads to the hot cistern L, -into which this water is therefore discharged. - -When the steam admitted through R ceases to be condensed, it -balances the atmospheric pressure above the piston, and thus -permits it to be drawn to the top of the cylinder by the weight of -the rod D. This ascent of the piston is also assisted by the -circumstance of the steam being somewhat stronger than the -atmosphere. - -When the piston has reached the top, the regulating valve R is -closed, and the condensing valve H opened, and another descent -produced, as before, and so the process is continued. [Pg071] - -The manipulation necessary in working this engine was, therefore, -the alternate opening and closing of two valves; the regulating -and condensing valves. When the piston reached the top of the -cylinder, the former was to be closed, and the latter opened; and, -on reaching the bottom, the former was to be opened, and the -latter closed. - - -(42.) The duty of working the engine requiring no great amount of -labour, or skill, was usually entrusted to boys, called, _cock -boys_. It happened that one of the most important improvements -which has ever been made in the working of steam engines was due -to the ingenuity of one of these boys. It is said that a lad, -named _Humphrey Potter_, was employed to work the cocks of an -atmospheric engine, and being tempted to escape from the -monotonous drudgery to which his duty confined him, his ingenuity -was sharpened so as to prompt him to devise some means by which he -might indulge his disposition to play without exposing himself to -the consequences of suspending the performance of the engine. On -observing the alternate ascending and descending motion of the -beam above him, and considering it in reference to the labour of -his own hands, in alternately raising and lowering the levers -which governed the cocks, he perceived a relation which served as -a clue to a simple contrivance, by which the steam engine, for the -first time, became an automaton. When the beam arrived at the top -of its play, it was necessary to open the steam valve by raising a -lever, and to close the injection valve by raising another. This -he saw could be accomplished by attaching strings of proper length -to these levers, and tying them to some part of the beam. These -levers required to be moved in the opposite direction when the -beam attained the lowest point of its play. This he saw could be -accomplished by strings, either connected with the outer arm of -the beam, or conducted over rods or pulleys. In short, he -contrived means of so connecting the levers which governed the two -cocks by strings with the beam, that the beam opened and closed -these cocks with the most perfect regularity and certainty as it -moved upwards and downwards. - -Besides rendering the machine independent of manual [Pg072] -superintendence, this process conferred upon it much greater -regularity of performance than any manual superintendence could -ensure. - -This contrivance of Potter was very soon improved by the -substitution of a bar, called a _plug frame_, which was suspended -from the arm of the beam, and which carried upon it pins, by which -the arms of the levers governing the cocks were struck as the -plug-frame ascended and descended, so as to be opened and closed -at the proper times. - -The engine thus improved required no other attendance except to -feed the boiler occasionally by the cock T, and to attend the -furnace. - - -(43.) However the merit of the discovery of the physical -principles on which the mechanical application of steam depends -may be awarded, it must be admitted that the engine contrived by -Newcomen and his associates, considered as a practical machine, -was immeasurably superior to that which preceded it; superior, -indeed, to such a degree, that while the one was incapable of any -permanently useful application, the other soon became a machine of -extensive utility in the drainage of mines; and, even at the -present time, the atmospheric engine is not unfrequently used in -preference to the modern steam engine, in districts where fuel is -abundant and cheap; the expense of constructing and maintaining it -being considerably less than that of an improved steam engine. The -low pressure of the steam used in working it, rendered it -perfectly safe. While Savery's engine, to work with effect, -required that the steam confined in the vessels should have a -bursting pressure amounting to about thirty pounds per square -inch, the pressure of steam in the boiler and cylinder of the -atmospheric engine required only a pressure about one pound per -square inch. The high pressure also of the steam used in Savery's -engine, was necessarily accompanied, as we shall presently -explain, by a greatly increased temperature. The effect of this -was, to weaken and gradually destroy the vessels, especially those -which, like the steam vessels V and V' (_fig._ 12.), were -alternately heated and cooled. - -Besides these defects, the power of Savery's engines was [Pg073] -also very restricted, both as to the quantity of water raised and as -to the height to which it was elevated. On the other hand, the -atmospheric engine was limited in its power only by the dimensions -of its piston. Another considerable advantage which the atmospheric -engine possessed over that of Savery, was the facility with which it -was capable of driving machinery by means of the working-beam. The -merit, however, of Newcomen's engine, regarded as an invention, and -apart from merely practical considerations, must be ascribed -principally to its mechanism and combinations. We find in it no new -principle, and scarcely even a novel application of a principle. The -agency of the atmospheric pressure acting against a vacuum, or -partial vacuum, had been long known: the method of producing a -vacuum by the condensation of steam had been suggested by Papin, and -carried into practical effect by Savery. The mechanical power -obtained from the direct pressure of the elastic force of steam, -used in the atmospheric engine to balance the atmosphere during the -ascent of the piston, was suggested by De Caus and Lord Worcester. -The boiler, gauge pipes, and the regulator, were all borrowed from -the engine of Savery. The idea of using the atmospheric pressure -against a vacuum or partial vacuum, to work a piston in a cylinder, -had been suggested by Otto Guericke, an ingenious German -philosopher, who invented the air-pump; and this, combined with the -production of a vacuum by the condensation of steam, was -subsequently suggested by Papin. The use of a working-beam could not -have been unknown. Nevertheless, the judicious combination of these -scattered principles must be acknowledged to deserve considerable -credit. In fact, the mechanism contrived by Newcomen rendered a -machine which was before altogether inefficient, highly efficient: -and, as observed by Tredgold, such a result, considered in a -practical sense, should be more highly valued than the fortuitous -discovery of a physical principle. The method of condensing the -steam by the sudden injection of water, and of expelling the air and -water from the cylinder by the injection of steam, are two -contrivances not before in use, which are quite essential to the -[Pg074] effective operation of the engine. These processes, which -are still necessary to the operation of the improved steam engine, -appear to be wholly due to the inventors of the atmospheric engine. - -[Illustration: ATMOSPHERIC ENGINE.] - - FOOTNOTES: - - [9] This pipe is represented as proceeding from the force-pipe - above the cistern C, in the perspective view of Savery's - engine at the head of this chapter. - - [10] Hot water being lighter than cold, floats on the surface. - - [11] "Captain" is a title given in Cornwall to the - superintendent of the works connected with a mine. - - [12] As the calculation of the power of an engine depends on - the number of square inches in the section of the piston, it - may be useful to give a rule for computing the number of - square inches in a circle. The following rule will always give - the dimensions with sufficient accuracy:--_Multiply the number_ - _of inches in the diameter by itself; divide the product by 14,_ - _and multiply the quotient thus obtained by 11, and the result_ - _will be the number of square inches in the circle_. Thus, if - there be 12 inches in the diameter, this multiplied by itself - gives 144, which divided by 14 gives 10-4/44, which multiplied - by 11 gives 115, neglecting fractions. There are, therefore, - 115 square inches in a circle whose diameter is 12 inches. - - [13] The external cylinder is not represented in the diagram. - - [14] Also called the _snifting_ valve, from the peculiar noise - made by the air and steam escaping from it. - - [15] Also called the _injection valve_. - -[Pg075] - - - - -[Illustration: GREENOCK, IN 1824.] - -CHAP. III. - - PROGRESS OF THE ATMOSPHERIC ENGINE. -- SMEATON'S IMPROVEMENTS. - -- BRINDLEY, ENGINEER OF THE BRIDGEWATER CANAL. -- INVENTS THE - SELF-REGULATING FEEDER. -- JAMES WATT. -- HIS DESCENT AND - PARENTAGE. -- ANECDOTES OF HIS BOYHOOD. -- HIS EARLY AQUIREMENTS. - -- GOES TO LONDON. -- RETURNS TO GLASGOW. -- IS APPOINTED - INSTRUMENT-MAKER TO THE UNIVERSITY. -- OPENS A SHOP IN GLASGOW. - -- HIS FRIENDS AND PATRONS. -- ADAM SMITH. -- DR. BLACK. -- ROBERT - SIMSON. -- PROFESSOR ROBISON. -- WATT'S PERSONAL CHARACTER. -- - INDUSTRIOUS AND STUDIOUS HABITS. -- HIS ATTENTION FIRST - DIRECTED TO STEAM. -- EXPERIMENTS ON HIGH-PRESSURE STEAM. -- - REPAIRS AN ATMOSPHERIC MODEL. -- EXPERIMENTAL INQUIRY - CONSEQUENT ON THIS. -- ITS RESULTS. -- DISCOVERS THE GREAT - DEFECTS OF THE ATMOSPHERIC ENGINE. -- DISCOVERY BY EXPERIMENT - OF THE EXPANSION WHICH WATER UNDERGOES IN EVAPORATION. -- - DISCOVERS THE LATENT HEAT OF STEAM. -- IS INFORMED BY DR. BLACK - OF THE THEORY OF LATENT HEAT. - - -(44.) The atmospheric engine was brought to a state of -considerable efficiency and improvement by Mr. Beighton, in 1718. -From that time it continued in use without any change in its -[Pg076] principle, and with little improvement in its structure, -for half a century. Although engines of this kind continued to be -extensively constructed, they were usually executed by ordinary -mechanics, incapable of applying to them the just principles of -practical science; and, consequently, little attention was paid to -their proportions. It was not until about the year 1772, that Mr. -John Smeaton, the celebrated engineer, applied the powers of his -mind to the investigation of this machine, as he had previously -done with such success to wind and water mills. Although he did -not introduce any new principle into the atmospheric engine, yet -it derived greatly augmented power from the proportions which he -established for engines of different magnitudes. - -In 1759, Mr. James Brindley, whose name is so celebrated as the -engineer of the Duke of Bridgewater's canal, obtained a patent for -some improvements in the atmospheric engine. He proposed that the -boiler should be made of wood and stone, with a stove or -fire-place of cast iron within it, so that the fire should be -surrounded on every side by water. The chimney was to be an iron -pipe or tube, conducted through the boiler; so that the heated -air, in passing from the fire, should impart a portion of its heat -to the water. He also proposed a method of feeding the boiler, -which, by self-acting machinery, would keep the water in the -boiler at a fixed level, independently of any attention on the -part of the engine-man. This was to be accomplished by a buoy or -float upon the surface of the water in the boiler, which should -communicate with a valve in the feed-pipe, so that when the level -of the water in the boiler fell, the float or buoy, falling with -it, would open the valve and supply the feed. It is stated, in the -_Biographia Britannica_, that Mr. Brindley, in 1756, undertook to -erect an engine at Newcastle-under-Lyne; but he is said to have -been discouraged by the obstacles which were thrown in his way, -and to have abandoned the steam engine. - -The interval between the invention of the atmospheric engine, and -the amelioration it received at the hands of Smeaton, has been -rendered memorable by the advent of one who was destined to work a -mighty change in the condition [Pg077] of the human race by the -application of his vast genius to the adaptation of steam power to -the uses of life. - - -(45.) JAMES WATT was born at Greenock, in Scotland, on the -nineteenth day of January, in the year 1736.[16] - -The great-grandfather of Watt, a farmer in Aberdeenshire, was killed -in one of the battles of Montrose. The victorious party, not -thinking death a sufficient expiation for the political opinions in -support of which he had fought and bled, punished him in the person -of his son, by confiscating his little property. Thomas Watt, the -son, thus deprived of support, was received by distant relations, -and, for a time, applied himself to study, by which he was enabled, -after the restoration of tranquillity, to establish himself at -Greenock as a teacher of practical mathematics and navigation. He -resided in the burgh or barony of Crawford's Dyke, and attained a -position of sufficient respectability to be elected to the office of -baron-baillie, or chief magistrate, and died in 1734, at the -advanced age of ninety-two years. - -Thomas Watt had two sons. The elder, John, adopted the profession -of his father, and was a teacher of mathematics and navigation at -Glasgow: he died in 1737, at the age of fifty years. The second -son, James, the father of the celebrated engineer, was, during a -quarter of a century, treasurer of the town council of Greenock, -and a local magistrate. He was remarked for the ardent zeal and -enlightened spirit with which he discharged his public duties. His -business was that of a ship-chandler, builder, and general -merchant; but, unhappily, notwithstanding his active industry, he -lost, in the decline of his life, by unsuccessful commercial -speculations, a part of the property which he had so honourably -acquired. He died in 1782, at the age of eighty-four years. - -JAMES WATT, to whom the world is so largely indebted for the -extension and improvement of steam power, had from his birth an -extremely delicate constitution. From his mother, [Pg078] whose -family name was Muirhead, he received his first lessons in -reading, and he learned from his father writing and arithmetic. -Although he was entered as a pupil in the grammar school of -Greenock, yet such was his delicate state of health, that his -attendance there was so interrupted by constant indisposition that -he could derive but little benefit from the opportunities of -instruction which it afforded. For a great period of the year he -was confined to his room, where he devoted himself to study -without the aid of instruction. It was in the retirement of the -sick chamber that the high intellectual faculties of Watt, which -were destined to produce such precious fruits, began to unfold -themselves. He was too sickly to be subjected to the restraints -which the business of education usually imposes on children. His -parents, therefore, found it necessary to leave him at liberty to -choose his occupations and amusements. The following anecdotes -will show the use he made of this freedom. - -A friend of his father found the boy one day stretched upon the -hearth tracing with chalk various lines and angles. "Why do you -permit this child," said he, "to waste his time so; why not send -him to school?" Mr. Watt replied, "You judge him hastily; before -you condemn us, ascertain how he is employed." On examining the -boy, then six years of age, it was found that he was engaged in -the solution of a problem of Euclid! - -Having observed the tendency of his son's mind, Mr. Watt placed at -his disposal a collection of tools. These he soon learned to use -with the greatest skill. He took to pieces and put together, again -and again, all the children's toys which he could procure; and he -was constantly employed in making new ones. Subsequently he used -his tools in constructing a little electrical machine, the sparks -proceeding from which became a great subject of amusement to all -the playfellows of the poor invalid. - -Though endowed with great retentive powers, Watt would probably -never have figured among the prodigies of a common school: he would -have been slow to commit his lessons to memory, from the repugnance -which he would feel to repeat like a parrot anything which he did -not perfectly [Pg079] understand. The natural tendency of his mind -to meditate on whatever came before it, would give him, to -superficial observers, the appearance of dullness. Happily, however, -he had a parent who was sufficiently clear-sighted, and who -entertained high hopes of the growing faculties of his son. More -distant and less sagacious relations were not so sanguine. One day -Mrs. Muirhead, the aunt of the boy, reproaching him for what she -conceived to be listless idleness, desired him to take a book and -occupy himself usefully. "More than an hour has now passed away," -said she, "and you have not uttered a single word. Do you know what -you have been doing all this time? You have taken off, and put on, -repeatedly, the lid of the tea-pot; you have been holding the -saucers and the spoons over the steam, and you have been -endeavouring to catch the drops of water formed on them by the -vapour. Is it not a shame for you to waste your time so?" - -Mrs. Muirhead was little aware that this was the first experiment -in the splendid career of discovery which was subsequently to -immortalise her little nephew. She did not see, as we now can, in -the little boy playing with the tea-pot, the great engineer -preluding to those discoveries which were destined to confer on -mankind benefits so inestimable. - -One of the social qualities of mind which was remarkable -throughout his life, was the singular felicity and grace with -which he related anecdotes. This power was manifested even in his -earliest childhood. The following is an extract from a letter -written by Mrs. Marion Campbell, his cousin, and the playfellow of -his childhood:-- - -"He was not fourteen when his mother brought him to Glasgow to -visit a friend of hers; his brother John accompanied him. On Mrs. -Watt's return to Glasgow, some weeks after, her friend said, 'You -must take your son James home; I cannot stand the degree of -excitement he keeps me in; I am worn out for want of sleep. Every -evening before ten o'clock, our usual hour of retiring to rest, he -contrives to engage me in conversation, then begins some striking -tale, and, whether humorous or pathetic, the interest is so -overpowering that the family all listen to him with breathless -attention, and hour after hour strikes unheeded.'" [Pg080] - -Watt had a younger brother, John, who was subsequently lost by -shipwreck, in a voyage from Scotland to the United States. This lad, -having determined on following the business of his father, left James -more completely at liberty to choose his own occupation. But such a -choice was difficult for a student who commanded equal success in -every thing to which he directed his attention. - -The excursions which he was in the habit of making on the Scottish -mountains surrounding Loch Lomond, naturally directed his -attention to botany and mineralogy, in each of which he attained -considerable knowledge. His love of anecdote and romance was -likewise gratified by the scenery which he enjoyed in these walks; -and the traditions and popular songs with which they made him -acquainted. When from ill-health, as constantly happened, he was -confined to the house, he devoted himself to chemistry, natural -philosophy, and even to medicine and surgery. In chemistry he -acquired some experimental skill, and studied with eager zeal the -elements of natural philosophy by S'. Gravesande. His own unhappy -maladies prompted him to read works on surgery and medicine; and -to such an extent did the activity of his mind impel him on these -subjects, that he was found one day dissecting, in his room, the -head of a child, who had died of some unknown disease, with a view -to ascertain the cause of its death. - -In 1775, at the age of nineteen, at the recommendation of Dr. -Dick, professor of natural philosophy in the university of -Glasgow, he went to London, where he employed himself in the house -of Mr. John Morgan, a mathematical instrument maker, in Finch -Lane, Cornhill, to whom he apprenticed himself for three years. He -remained, however, only a year, at the expiration of which -(probably owing to his delicate state of health) he was released -from his apprenticeship, and returned to Glasgow, with the -intention of establishing himself in business as an optician and -mathematical instrument maker. In the fulfilment of this -intention, however, he was obstructed by the interposition of the -Corporation of Trades in that town, who regarded him as an -intruder, not qualified by the necessary apprenticeship to carry -on business. All means of conciliation being [Pg081] exhausted, -the Professors of the University interfered, and gave him the use -of three apartments within the college, for carrying on his -business, and likewise appointed him mathematical instrument maker -to the University. Soon afterwards the opposition of the local -trades seems to have given way, and he opened a shop in Glasgow -for the sale of mathematical instruments. - -After the celebrity at which he has arrived, it will be easily -believed that every trace of his earlier connection with Glasgow -college is carefully cherished. There are accordingly preserved at -that place little instruments and pieces of apparatus of exquisite -workmanship, which were executed entirely by the hand of Watt, at -a time when he was not in a condition to command the aid of -workmen under him. - -At the time of obtaining this appointment in the University, Watt -was in his twenty-first year. His natural talents and winning -manners were speedily the means of gaining for him the esteem and -friendship of all those eminent persons connected at the time with -that university whose regard was most valued. Among these the -earliest of his friends and patrons were--ADAM SMITH, the author of -"The Wealth of Nations;" BLACK, afterwards celebrated for his -chemical discoveries, and more especially for his theory of latent -heat; and ROBERT SIMSON, rendered illustrious by his works on -ancient geometry. In releasing Watt from the persecution of the -Glasgow corporation, these distinguished persons first imagined that -they were conferring a benefit merely on an industrious and clever -artisan, whose engaging manners won their regard; but a short -acquaintance with him was sufficient to convince them how superior -his mind was to his position, and they conceived towards him the -most lively friendship. His shop became the common rendezvous, the -afternoon lounge, of all who were most distinguished for literary -and scientific attainments among the professors and students. There -they met to discuss the topics of the day in art, science, and -literature. Among these students, the name which afterwards attained -the highest distinctions, and among these distinctions, not the -least, the lasting personal friendship and esteem of Watt himself, -was ROBISON, [Pg082] the author of a well known work on Mechanics, -and one of the contributors to the _Encyclopoedia Britannica_. - -The following extract from an unpublished manuscript by Robison -himself will show at once the estimation in which Watt was held, -and will illustrate one of the most interesting traits of his -personal character:-- - -"I had always, from my earliest youth, a great relish for the -natural sciences, and particularly for mathematical and mechanical -philosophy, when I was introduced by Drs. Simson, Dick, and Moor, -gentlemen eminent for their mathematical abilities, to Mr. Watt. I -saw a workman, and expected no more; but was surprised to find a -philosopher as young as myself, and always ready to instruct me. I -had the vanity to think myself a pretty good proficient in my -favourite study, and was rather mortified at finding Mr. Watt so -much my superior. . . . Whenever any puzzle came in the way of any -of the young students, we went to Mr. Watt. He needed only to be -prompted, for every thing became to him the beginning of a new and -serious study, and we knew that he would not quit it till he had -either discovered its insignificancy, or had made something of it. -He learnt the German language in order to peruse Leupold's -'Theatrum Machinarum;' so did I, to know what he was about. -Similar reasons made us both learn the Italian language. * * * -When to his superiority of knowledge is added the _naïve_ -simplicity and candour of Mr. Watt's character, it is no wonder -that the attachment of his acquaintances was strong. I have seen -something of the world, and am obliged to say I never saw such -another instance of general and cordial attachment to a person -whom all acknowledged to be their superior. But that superiority -was concealed under the most amiable candour, and a liberal -allowance of merit to every man. Mr. Watt was the first to ascribe -to the ingenuity of a friend things which were nothing but his own -surmises, followed out and embodied by another. I am the more -entitled to say this, as I have often experienced it in my own -case." - -Watt never permitted the inquiries which arose out of these -reunions to interfere with the discharge of the duties of his -workshop. There he passed the day, devoting the [Pg083] night to -study. Every inquiry appeared to him to be attractive in -proportion to its difficulty, and to have charms in proportion as -it was removed from the common routine of his business. As an -example of this may be mentioned the fact, that, being himself so -insensible to the charms of music that he could not distinguish -one note from another, he was actually induced to undertake the -construction of an organ, in which he was nevertheless completely -successful. The instrument he constructed, as might have been -expected, contained many improvements in its mechanism; but what -is much more remarkable, its tone and its musical qualities -commanded the admiration of all the professional musicians who -heard it. In the construction of this instrument Watt showed that -vigorous spirit of investigation which characterised all the -subsequent labours of his life. He made out the scale of -temperament by the aid of the phenomena of beats, of which he -could only obtain a knowledge by a profound but obscure work -published by Dr. Robert Smith of Cambridge. - -The earliest occasion on which the attention of Watt is said to -have been called to the agency of steam, was in the year 1759, -when his friend Robison entertained some speculations for applying -that agent as a means of propelling wheel carriages; and he -consulted Watt on the subject. No record, however, has been -preserved of any experiments which were tried on this occasion; -nor does it appear that the inquiry was carried farther than a -verbal discussion, such as habitually took place on other subjects -of science between Watt and his friends. - - -(46.) In 1762, Watt tried some experiments on the force of steam -at a high pressure, confined in a close digester; and he then -constructed a small model to show how motion could be obtained -from that power. The practicability of what has since been called -the _High Pressure Engine_, was demonstrated by him on this -occasion; but he did not pursue the inquiry, on account of the -supposed danger of working with such compressed steam as was -required. - -It is usual to provide, in the cabinets of experimental apparatus -for the instruction of the students of universities, [Pg084] -small working models of the most useful machines. In the -collection for the illustration of the lectures delivered to the -Natural Philosophy class in the University of Glasgow was a -working model of Newcomen's atmospheric engine, applied to a pump -for raising water; which, however, had never been found to work -satisfactorily. The Professor of Experimental Philosophy of that -day, Dr. John Anderson (the founder of the celebrated Andersonian -Institution), sent this model in 1763 to Watt's workshop, to be -repaired. Its defects soon disappeared, and it was made to work to -the satisfaction of the professor and students. - -This simple discharge of his duty, however, did not satisfy the -artisan; and his wonted activity of mind rendered this model a -subject of profound meditation, and led him into a course of -practical inquiry respecting it, which formed the commencement of a -most brilliant career of mechanical discovery. The improvement--we -might almost say the creation--of the steam engine, by this great -man, must not therefore be regarded, as so often happens with -mechanical discoveries, as the result of fortuitous observation, or -even of a felicitous momentary inspiration. Watt, on the other hand, -conducted his investigation by a course of deep thought, and of -experiments marked by the last refinement of delicacy and address. -If he had received a more extended and liberal education, one would -have thought that he had adopted for his guide the celebrated maxim -of Bacon:-- - -"To write, speak, meditate, or act, when we are not provided with -_facts_ to direct our thoughts, is to navigate a coast full of -dangers without a pilot, and to launch into the immensity of the -ocean without either rudder or compass." - -The model which he had repaired, had a cylinder of only two inches -diameter, and six inches stroke. After he had put it in complete -order, he found, that although the boiler was much larger in -proportion to the cylinder than those of real engines, yet, that -it was incapable of supplying the cylinder with steam in -sufficient quantity to keep it at work. To enable it to continue -to move, he found it necessary to lessen the quantity of water -raised by its pump, so as to [Pg085] reduce the load on its -piston very much below the proper standard according to the common -rules for large engines. - -He ascribed the great inferiority in the performance of the model, -compared with the performance of the large engines, to the small -size of the cylinder, and to its material. The cylinder of the -model was brass, while those of large engines were of cast iron; -and brass being a better conductor of heat than iron, he concluded -that more heat in proportion was lost from this cause in the -model, than in the larger engines. He observed that the small -cylinder was so heated when the steam was admitted into it, that -it could not be touched by the hand; but, nevertheless, that this -heat contributed nothing to its performance, inasmuch as before -the piston descended, the cylinder required to be cooled. - - -(47.) His first attempt to improve the engine, was by using a -wooden cylinder instead of an iron one. He accordingly made a -model with a cylinder of wood, soaked in linseed oil, and baked to -dryness. With this he made numerous experiments, and found that it -required a less quantity of water to be thrown into the cylinder -to condense the steam, and that it was worked with a less supply -of steam from the boiler than was necessary with the metallic -cylinder. - -Still he found that the force with which the piston descended was -considerably less than that which the atmospheric pressure ought -to supply, supposing a tolerably perfect vacuum to be produced -under the piston. This led him to suspect that the water injected -into the cylinder was not perfectly effectual in condensing the -steam. The experiments which he had previously made on the -increased temperature at which water boils under pressures greater -than that of the atmosphere, led him by analogy to the conclusion -that it would boil at lower temperatures if it were submitted to a -pressure less than the atmosphere, and he was aware that Dr. -Cullen and others had then recently discovered that in vacuo, -water would boil at so low a temperature as 100°. These notions -suggested the probability that the water injected into the -cylinder being heated by the condensed steam, might produce vapour -of a low temperature [Pg086] and reduced pressure under the -piston, which would account for the deficiency he observed in the -power of the engine. - -No means occurred to him by which he could ascertain, by direct -experiment, the temperatures at which water would boil under -pressures less than that of the atmosphere. He sought, however, to -determine it by the following method. Having ascertained, by -repeating and multiplying the experiments which he had tried in -1762, on high-pressure steam, he obtained a table of the -temperatures at which water boils at various pressures greater -than that of the atmosphere. These results he laid down in a -series forming a curve, of which the abscissa represented the -temperatures, and the ordinates the pressures. He then continued -this curve, backwards as it were, and obtained, by analogy, an -approximation to the boiling temperatures, corresponding to -pressures less than that of the atmosphere. In other words, having -obtained by his experiments a notion, however imperfect, of the -law or rule observed by the temperatures at which water boils at -different pressures _greater_ than that of the atmosphere, he -calculated by the same law or rule what the pressures would be at -different pressures _less_ than that of the atmosphere. - -Applying these results to the interior of the cylinder of the -atmospheric engine, he obtained an approximation to the pressure -of the vapour which would be produced from the warm water formed -by the cold water injected into the cylinder, and the steam -condensed by it; and he accordingly found that vapour, having a -pressure seriously injurious to the power of the engine would be -produced in the cylinder, unless considerably more water of -injection was thrown in than was customary. - -It was apparent that the actual quantity of steam usefully -employed in the cylinder at each stroke, was only the quantity -which filled the cylinder; and therefore, in order to ascertain -the quantity of steam lost by the imperfections of the machine, it -was necessary to compare the actual quantity of steam transmitted -by the boiler to the cylinder at each stroke, with the quantity -which would just fill the cylinder. The difference would of course -be wasted. But to determine [Pg087] the actual quantity of steam -supplied by the boiler to the cylinder, there was no other means -than by observing the quantity of water evaporated in the boiler. -That being observed, it was necessary to know the quantity of -steam which that water formed; and it was therefore necessary to -determine the quantity or volume of steam which a given volume of -water produced. - - -(48.) On considering more attentively the operation of the -machine, the following circumstances gradually unfolded themselves -to him. - -Let us suppose the piston at the top of the cylinder, and the -space in the cylinder below it, filled with steam so as to balance -the pressure of the atmosphere above the piston. Under such -circumstances the steam, as will presently be explained, must have -the temperature of boiling water. But that the steam should have, -and should maintain, this temperature, it was evidently necessary -that the inner surface of the cylinder in contact with it should -have the same temperature: for if it had a lower temperature, it -would take heat from the steam, and reduce the temperature of the -latter. Now the cylinder being a mass of metal, has a quality in -virtue of which heat passes freely through its dimensions, so that -its inner surface could not be maintained at a temperature more -elevated than that of its dimensions extending from the inner -surface to the outer surface. Therefore, to maintain the steam -contained in the cylinder at the proper temperature, it was -essential that the whole of the solid metal composing the cylinder -should be itself at that temperature. - -Things being in this state, it was required that a vacuum should -be produced under the piston to give effect to the atmospheric -pressure above it, by relieving it from the pressure below. This, -indeed, would appear to have been attained by introducing as much -cold water within the cylinder as would be sufficient to reconvert -the steam contained in it into water; but Watt found, in his -experiments on the atmospheric model, that the piston would not -descend with the proper force, unless a vastly greater quantity of -water were introduced into the cylinder than the quantity which he -had ascertained to be [Pg088] necessary for the reconversion of -the steam into water. The cause of this he perceived and fully -explained. - -If we suppose as much, and no more, cold water introduced into the -cylinder as would reconvert the steam contained in it into water, -then we should have in the bottom of the cylinder a quantity of -warm water with a vacuum above it: but the entire mass of metal -composing the cylinder itself, which was previously at the -temperature of boiling water, would still be at the same -temperature. The warm water, resting in contact with this metal in -the bottom of the cylinder, would be immediately heated by it, and -would rise in its temperature, while the metal of the cylinder -itself would be somewhat lowered in temperature by the heat which -it would thus impart to the warm water contained in it. Under -these circumstances, as we shall presently explain, steam would be -produced from the water, which would fill the cylinder; and -although such steam would not have a mechanical pressure equal in -amount to the atmosphere, and therefore would not altogether -prevent the piston from descending if it had no load to move, yet -it would deprive the engine of so great a portion of its -legitimate power as to render it altogether inefficient. But this -defect would be removed by throwing into the cylinder a sufficient -quantity of cold water, not only to destroy the steam contained in -it, but also to cool the entire mass of metal composing the -cylinder itself, until it would be reduced to such a temperature -that the vapour proceeding from the water contained in it would -have so small a pressure that it would not seriously or -injuriously obstruct the descent of the piston. - -The piston being made to descend with such force as to render the -machine practically efficient, it would then be necessary again to -make it ascend; and to accomplish this, Watt found that the boiler -should supply a quantity of steam many times greater than was -necessary to fill the cylinder. Mature reflection on the -circumstances which have been just explained, enabled him to -discover how this undue quantity of steam was rendered necessary. - -Let it be recollected, that when the piston has reached the bottom -of the cylinder, the whole mass of the cylinder, and [Pg089] the -piston itself, are reduced to so low a temperature that the vapour -of water, having the same temperature, has no pressure sufficiently -great to obstruct the action of the machine. When, in order to make -the piston ascend, steam is introduced from the boiler into the -cylinder under the piston, this steam encounters, in the first -instance, the cold surfaces of the metal forming the bottom of the -cylinder and the bottom of the piston. The first effect of this is -to convert the steam which comes from the boiler into water, an -effect which is produced by that steam imparting its heat to the -metal with which it comes into contact. This destruction of steam -continues until the metal exposed to contact with it has been heated -up to the temperature of boiling water. Then, and not till then, the -steam below the piston will have a pressure equal to that of the -atmosphere above it, and the piston will begin to ascend. As it -ascends, however, the sides of the cylinder which it exposes to the -contact of the steam are cold, and partially destroy the steam. -Steam, therefore, must be supplied from the boiler to replace the -steam thus destroyed; nor can the piston reach the top of the -cylinder until such a quantity of steam shall have flowed from the -boiler into the cylinder, as shall be sufficient not only to fill -the cylinder under the piston, but likewise, by its condensation, to -raise the whole mass of the cylinder and piston to the temperature -of boiling water. - -Such were the circumstances which forced themselves upon the -attention of Watt, in the course of repairing, and subsequently -trying, the model of the atmospheric engine, at Glasgow. Being -informed generally of the uses of the engine in the drainage of -mines, and of the vast expense attending its operation, by reason -of the quantity of fuel which it consumed, he saw how important -any improvement would be by which the extensive sources of waste -which had thus presented themselves could be removed. He saw also, -that all that portion of steam which was expended, not in filling -the cylinder under the piston, but in heating the great mass of -metal composing the cylinder and piston, from a low temperature to -that of boiling water, upon each stroke of the piston, was so much -heat lost, and that the proportion of the fuel expended in -evaporating the steam thus wasted would be saved, if by any -[Pg090] expedient _he could make the piston descend without -cooling the cylinder_. But in order to estimate the full amount of -this waste, and to discover the most effectual means of preventing -it, it was necessary to investigate the quantity of heat necessary -for the evaporation of a given quantity of water; also, the -quantity of steam which a given quantity of water would produce, -as well as other circumstances connected with the temperature and -pressure of steam. He, therefore, applied himself to make -experiments with a view to elucidate these questions; and -succeeded in obtaining results which led to the discovery of some -of the most important of those physical phenomena, on the due -application of which, the efficacy of the steam engine, which he -afterwards invented, depended, and which also form striking facts -in the general physics of heat. - - -(49.) The first question to which he directed his experiments, was -the determination of the extent to which water enlarged its -volume, or magnitude, when it passed into steam. To ascertain -this, he filled a thin Florence flask with steam, of a pressure -equal to the atmosphere, and weighed it accurately. The same flask -was then filled with water, and weighed again. Finally, the weight -of the flask itself was ascertained. It is evident, that by such -means, the exact weight of the steam which filled the flask, and -of the same bulk of water, would be obtained. He found that the -water weighed about eighteen hundred times more than the steam; -from whence he inferred that the steam which filled the flask -contained about eighteen hundred times less water than the flask -would contain.[17] - -[Pg091] Having once ascertained this point, he was able, by -observing the quantity of water evaporated in the boiler of the -atmospheric model, to compute the volume of steam which was -supplied to the cylinder. It was evident, that for every cubic -inch of water evaporated in the boiler, eighteen hundred cubic -inches of steam were supplied to the cylinder. Having accurately -observed the evaporation of the boiler for a short time, and the -number of strokes made by the piston in the same time, he found -that the quantity of water evaporated in the boiler would supply -about four times as much steam as the cylinder would require. He -consequently inferred, that about three-fourths of the steam -produced was wasted. - -The next question to which he directed his experiments, was to -ascertain the quantity of cold water necessary to be injected into -the cylinder, in order to condense the steam contained in it. To -ascertain this, he attached a pipe to a boiler, by which he was -enabled to conduct the steam from the boiler into a glass jar -containing cold water at fifty-two degrees of temperature. The -steam, as it passed from the boiler through the pipe, was -condensed by the cold water, and continued to be so condensed, -until, by the heat which it imparted to the water, the latter -began to boil, and would then condense no more steam. On comparing -the water in the glass jar, when boiling, with the water -originally contained in it at fifty-two [Pg092] degrees, the -quantity was found to be increased in the proportion of six to -seven, very nearly; from which he inferred, that to reduce one -ounce of steam to water, it was necessary to mix about six ounces -of cold water with it. - -He was further led to the conclusion, that steam contains a vast -quantity of heat, by the following experiment. He heated, in a -close digester, a quantity of water several degrees above the -common boiling point. When thus heated, by opening a stop-cock, he -allowed the compressed steam to escape into a cold vessel; in -three or four seconds, he found that the heat of the water in the -digester was reduced from a very high temperature to the common -boiling point; yet, that all the steam which escaped from it, and -which carried off with it the superabundant heat, formed only a -few drops of water when condensed; from which he inferred, that -this small quantity of water, in the form of steam, contained as -much heat as was sufficient to raise all the water in the digester -from the boiling point to the temperature at which it was before -the steam was allowed to escape. - -Having thus ascertained the exact quantity of cold water which -ought to be injected into the cylinder in order to condense the -steam which filled the cylinder, he found, on comparing the -quantity necessary to be injected in order to enable the piston to -descend, that this quantity was about four times as great as that -which was necessary to condense the steam. This led him to the -conclusion, that about four times as much heat was destroyed in -the cylinder as needed to be destroyed, if the object were the -mere condensation of the steam. This result fully corroborated the -other conclusion, deduced, from the proportion which he found -between the quantity of steam supplied by the boiler and the -actual contents of the cylinder. - - -(50.) Watt was forcibly struck with these circumstances, not only -on account of their importance in an economical point of view, -when their relation to steam power was considered, but still more -so, as indicating phenomena in the physics of heat altogether -novel to him. - -He, therefore, eagerly sought his friend Dr. Black, to whom he -communicated these results. Then, for the first time, he [Pg093] -was informed, by Black, of the theory of LATENT HEAT, which had -recently been discovered by him, and of which these very phenomena -formed the basis. - -Some passages in the works of Dr. Robison produced an erroneous -impression, that a large share of the merit of the discoveries of -Watt which have been just explained was due to Dr. Black, to whose -instructions on the subject of latent heat Watt was represented to -have owed the knowledge of those facts which led to his principal -inventions and improvements. We shall here give, in the words of -Watt himself, his explanation of the circumstances which led to -this error. This explanation is given in a letter addressed by -Watt to Dr. Brewster, in May 1814, and prefixed to the third -volume of Brewster's edition of Robison's Mechanical Philosophy:-- - - "The representations of friends whose opinions I highly value - induce me to avail myself of this opportunity of noticing an error - into which not only Dr. Robison, but apparently also Dr. Black, - has fallen, in relation to the _origin_ of my improvements upon - the steam engine, and which not having been publicly controverted - by me, has, I am informed, been adopted by almost every subsequent - writer upon the subject of latent heat. - - "Dr. Robison, in the article Steam Engine, after passing an - encomium upon me, dictated by the partiality of friendship, - qualifies me as the '_pupil_ and intimate friend of Dr. Black,'--a - description which not being there accompanied with any inference, - did not particularly strike me at the time of its first perusal. - He afterwards, in the dedication to me of his edition of Dr. - Black's lectures upon chemistry, goes the length of supposing me - to have professed to owe my improvements upon the steam engine to - the instructions and information I had received from that - gentleman, which certainly was a misapprehension; as, though I - have always felt and acknowledged my obligations to him for the - information I had received from his conversation, and particularly - for the knowledge of the doctrine of latent heat, I never did nor - _could_ consider my improvements as originating in those - communications. He is also mistaken in his assertion (p. 8. of the - preface to the above work), that 'I had attended two courses - [Pg094] of the doctor's lectures;' for, unfortunately for me, the - necessary avocations of my business prevented me from attending - his or any other lectures at college; and as Dr. Robison was - himself absent from Scotland for four years at the period referred - to, he must have been misled by erroneous information. In p. 184. - of the lectures, Dr. Black says, 'I have the pleasure of thinking - that the knowledge we have acquired concerning the nature of - elastic vapours, in consequence of my fortunate observation of - what happens in its formation and condensation, has contributed in - no inconsiderable degree to the public good by _suggesting_ to my - friend Mr. Watt of Birmingham, then of Glasgow, his improvement on - this useful engine' (meaning the steam engine of which he is then - speaking). There can be no doubt from what follows in his - description of the engine, and from the very honourable mention - which he has made of me in various parts of his lectures, that he - did not mean to lessen any merit that might attach to me as an - inventor; but, on the contrary, he was always disposed to give me - fully as much praise as I deserved. - - "And were that otherwise doubtful, it would, I think, be evident - from the following quotation from a letter of his to me, dated - 13th February 1783, where, speaking of an intended publication by - a friend of mine, on subjects connected with the history of steam, - he says, 'I think it is very proper for you to give him a short - account of your discoveries and speculations; _and particularly_ - _to assert clearly and fully your sole right to the honour of_ - _the improvements of the steam engine_.' And in a written - testimonial which he very kindly gave me, on the occasion of a - trial at law against a piracy of my invention in 1796-7, after - giving a short account of the invention, he adds, '_Mr. Watt was_ - _the sole inventor of the capital improvement and contrivance_ - _above mentioned._' - - "Under this conviction of his candour and friendship, it is very - painful to me to controvert any assertion or opinion of my revered - friend; yet, in the present case I find it necessary to say, that - he appears to me to have fallen into an error; and I hope, in - addition to my assertion, to make that appear by the short history - I have given of my invention, in my [Pg095] notes upon Dr. - Robison's essay, as well as by the following account of the state - of my knowledge previous to my receiving any explanation of the - doctrine of latent heat; and also from that of the facts which - principally guided me in the invention. - - "It was known very long before my time, that steam was condensed - by coming into contact with cold bodies, and that it communicated - heat to them; witness the common still, &c. &c. - - "It was known, by some experiments of Dr. Cullen and others, that - water and other liquids boiled in vacuo at very low heats; water - below 100°. - - "It was known to some philosophers that the capacity or - equilibrium of heat, as we then called it, was much smaller in - mercury and tin than in water. - - "It was also known that evaporation caused the cooling of the - evaporating liquid, and bodies in contact with it. - - "I had myself made experiments to determine the following facts:-- - - "First, the capacities of heat for iron, copper, and some sorts of - wood, comparatively with water. - - "Second, the bulk of steam compared with that of water. - - "Third, the quantity of water evaporated in a certain boiler by a - pound of coals. - - "Fourth, the elasticities of steam at various temperatures greater - than that of boiling water, and an approximation to the law which - it followed at other temperatures. - - "Fifth, how much water in the form of steam was required every - stroke by a small Newcomen's engine, with a wooden cylinder six - inches diameter, and twelve inches stroke. - - "Sixth, the quantity of cold water required in every stroke to - condense the steam in that cylinder, so as to give it a working - power of about 7 lb. on the inch. - - "Here I was at a loss to understand how so much cold water could - be heated so much by so small a quantity of water in the form of - steam; and I accordingly applied to Dr. Black, and then first - understood what was called latent heat. - - "But this theory, though useful in determining the quantity of - injection necessary where the quantity of water [Pg096] - evaporated by the boiler, and used by the cylinder, was known, and - in determining, by the quantity and heat of the hot water emitted - by Newcomen's engines, the quantity of steam required to work them - did not lead to the improvements I afterwards made in the engine. - These improvements proceeded upon the old established fact, that - steam was condensed by the contact of cold bodies; and the later - known one, that water boiled in vacuo at heats below 100°, and - consequently that a vacuum could not be obtained unless the - cylinder and its contents were cooled every stroke to below that - heat." - -[Illustration: LOCH LOMOND.] - - FOOTNOTES: - - [16] We are indebted for many of the anecdotes of the life of - Watt to the _Eloge Historique_, recently published by M. Arago, - who was furnished with all the documents and circumstances - relating to this celebrated person which were considered proper - for publication, by his son, the present James Watt, Esq., of - Aston Hall, near Birmingham, and to the notes added to this - memoir by Mr. Muirhead, a relative of Mr. Watt. - - [17] The following is the account of these experiments given - in Watt's own words:-- - - "It being evident that there was a great error in Dr. - Desagulier's calculations of Mr. Beighton's experiments on the - bulk of steam, a Florence flask, capable of containing about a - pound of water, had about one ounce of distilled water put - into it; a glass tube was fitted into its mouth, and the - joining made tight by lapping that part of the tube with - packthread covered with glazier's putty. When the flask was - set upright, the tube reached down near to the surface of the - water, and in that position the whole was placed in a tin - reflecting oven before a fire until the water was wholly - evaporated, which happened in about an hour, and might have - been done sooner, had I not wished the heat not much to exceed - that of boiling water. As the air in the flask was heavier - than the steam, the latter ascended to the top, and expelled - the air through the tube. When the water was all evaporated, - the oven and flask were removed from the fire, and a blast of - cold air was directed against one side of the flask, to - collect the condensed steam in one place. When all was cold, - the tube was removed, the flask and its contents were weighed - with care; and the flask being made hot, it was dried by - blowing into it by bellows, and when weighed again was found - to have lost rather more than four grains, estimated at 4-1/3 - grains. When the flask was filled with water, it was found to - contain about 17-1/8 ounces avoirdupois of that fluid which - gave about 1800 for the expansion of water converted into - steam of the heat of boiling water. - - "This experiment was repeated with nearly the same result, and - in order to ascertain whether the flask had been wholly filled - with steam, a similar quantity of water was for the third time - evaporated; and, while the flask was still cold, it was placed - inverted with its mouth (contracted by the tube) immersed in a - vessel of water, which it sucked in as it cooled, until in the - temperature of the atmosphere it was filled to within half an - ounce measure of water. - - "In repetitions of this experiment at a later date, I - simplified the apparatus by omitting the tube, and laying the - flask upon its side in the oven, partly closing its mouth by a - cork, having a notch on one side, and otherwise proceeding as - has been mentioned." - -[Pg097] - - - - -[Illustration: GLASGOW.] - -CHAP. IV. - - EXPOSITION OF PHYSICAL PRINCIPLES. -- THERMOMETER. -- METHOD OF - GRADUATING IT. -- FREEZING AND BOILING POINTS. -- LATENT HEAT OF - WATER. -- QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER. - -- QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO - ICE. -- PROCESS OF BOILING. -- OF RECONVERSION OF STEAM INTO - WATER. -- QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO - STEAM. -- BOILING POINT OF WATER. -- DIFFERENT IN DIFFERENT - PLACES. -- DEPENDS ON THE BAROMETER. -- VARIES WITH THE - PRESSURE. -- EXPERIMENTAL PROOF OF THIS. -- BOILS AT LOWER - TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE - ATMOSPHERE. -- SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS - THE SAME. -- THE FUEL NECESSARY TO EVAPORATE WATER THE SAME, - WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS - EVAPORATED. -- MECHANICAL FORCE OBTAINED BY EVAPORATION. -- THIS - FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES. - - -(51.) We shall pause here to put the reader in possession of the -physical and mechanical principles connected with the evaporation -of water and other liquids, which are necessary to enable him to -understand the full extent of the value and the merit of the -discoveries of Watt, and to comprehend the [Pg098] structure and -operation of the steam engine in its improved form, as it has -passed to us from his hands. - -As we shall frequently have occasion to refer to the indications -of a thermometer, we shall first explain the principle of that -instrument as it is commonly used in this country. - -The thermometer is an instrument used for the purpose of measuring -and indicating the temperature or sensible heat of material -substances. - -Heat, like all other physical agents, can only be measured by its -effects. One of these effects best suited for this purpose, is the -change of dimension which all bodies undergo in consequence of -their change of temperature. In general, when heat is applied to a -material substance, that substance undergoes an enlargement of -bulk; and if heat be abstracted from it, it suffers a diminution -of bulk. This variation of magnitude is not always in the same -proportion as the increase or diminution of temperature; but it is -so when applied to certain substances and between certain limits. -One of the substances whose expansion and contraction through an -extensive range of temperature has been found to be nearly -uniform, and which is attended with other convenient qualities for -a thermometer, is the liquid called _mercury_ or _quicksilver_. A -mercurial thermometer is constructed in the following way:-- - -A glass tube is made with a small and uniform bore: upon the end -of this tube, a bulb is blown, having a magnitude very great -compared with the bore of the tube. Let us suppose this bulb and a -part of the tube to be filled with mercury. If the mercury -contained in the bulb be heated, it will expand, and being more -susceptible of expansion than the glass which contains it, the -bulb will be too small for its augmented volume: the mercury in -the bulb can only, therefore, obtain room for its increased bulk -by pressing the mercury in the tube upwards, which it will -accordingly do. The increase of volume which the mercury in the -bulb therefore undergoes, will be exhibited by the increased -length of the column in the tube. Since the bore of the tube is -made so exceedingly minute compared with the magnitude of the -bulb, a very small quantity of mercury forced [Pg099] from the -bulb into the tube, will cause a considerable increase of the -length of the column. Small degrees of expansion will therefore be -rendered very apparent, and may be accurately measured. The -following is the method by which the thermometer called -_Fahrenheit's thermometer_ is graduated. - -The tube and bulb being prepared and supplied with mercury, as -already explained, let the instrument be plunged in a vessel of -melting ice. It will be found that the mercury will stand in the -tube at a certain point, from which it will not vary so long as -any ice remains not completely melted in the vessel. Let a mark be -made on the tube, or on a scale attached to the tube, at the point -corresponding to the top of the column: the point thus marked is -called the _freezing point_. - -Now let the instrument be immersed in a vessel of boiling water, -the barometer at the time having the height of thirty inches. It -will be found that so long as the water is kept boiling, the -column of mercury in the tube will remain stationary. Let the -point corresponding with the top of the column be marked on the -tube, or on the scale attached to it. This is called the _boiling -point_. Let the space on the scale between the freezing and -boiling points be now divided into 180 equal parts: each of these -parts is called a _degree_. Let the same divisions be continued -upon the scale below the freezing point, until thirty-two -divisions be taken; let the lowest division be then marked 0, and -let the successive divisions upwards from that be numbered 1, 2, -3, &c. In like manner, let the same divisions be continued above -the boiling point, as far as the tube will admit. - -It is evident that, under these circumstances, the freezing point -will be marked by 32, and the boiling point by 212. It is usual to -express the degrees of a thermometer in the same manner as the -degrees of a circle, by placing a small ° above the number. Thus -the freezing point is expressed by 32°, and the boiling point by -212°. - -The reason the degrees were commenced at 32° below the freezing -point was, because, when the thermometer was invented, that -temperature was supposed to be the lowest degree of cold possible, -being that of a certain mixture of [Pg100] snow and salt. This, -however, has since been found to be an error, very much lower -temperatures being obtained by various physical expedients. - -The temperature of a body is, then, that elevation to which the -thermometer would rise when immersed in that body. Thus, if in -plunging the thermometer in water we found the mercury to rise or -fall to the division marked 100, we should then say, the -temperature of the water was 100°. - -Let us suppose a spirit lamp, or other regular source of heat, -applied to a bath of mercury, so as to maintain the mercury at a -fixed temperature of 200°, and let another vessel, containing a -quantity of ice at a temperature of 20° be immersed in the -mercury. Let a thermometer be placed in the mercury, and another -in the ice. The following effects will then ensue. The thermometer -immersed in the ice will be observed gradually to rise from 20° -upwards, until it indicates the temperature of 32°. It will then -become stationary, and the ice which had hitherto remained in a -solid state will begin to melt and be converted into water. This -process of liquefaction will continue for a considerable time, -during which the thermometer immersed in the ice will constantly -be maintained at 32°. At the moment, however, when the last -portion of ice is liquefied, the thermometer will begin again to -rise. The coincidence of this ascent of the thermometer with the -completion of the liquefaction of the ice, may be very easily -observed, because the ice being lighter, bulk for bulk, than -water, will float on the surface, and so long as a particle of it -remains unmelted it will be distinctly seen. - -Now it cannot be doubted that, during the whole of this process, -the mercury, supposed to be maintained at 200°, constantly imparts -heat to the ice; yet, from the moment the liquefaction begins, -until it is completed, no increased temperature is exhibited by -the thermometer immersed in the melting ice. If during this part -of the process no heat were received by the ice from the mercury, -the consequence would be, that the application of the lamp would -cause the temperature of the mercury to rise above 200°, which may -be easily demonstrated by withdrawing the vessel of ice from the -mercurial bath during the process of liquefaction. The moment -[Pg101] it is withdrawn, the thermometer immersed in the mercury, -instead of remaining fixed at 200°, will begin to rise, although -the action of the lamp remains the same as before; from which it -is evident that the heat which now causes the mercury to rise -above 200° was before received by the melting ice. - -The heat which thus enters ice in the process of liquefaction, and -which is not indicated by the thermometer, is for this reason -called _latent heat_. It will be perceived that this phrase is the -name of a fact, and not of an hypothesis. That heat really enters -the water, and is contained in it, has been established by the -experiments; and to declare that it is present there, is to -declare an established fact. To call it by the name _latent_ heat, -is to declare another established fact, viz., that it is not -sensible to the thermometer. - -These facts show us that heat is capable of existing in bodies in -two distinct states, in one of which it is sensible to the -thermometer, and in the other not. Heat which is sensible to the -thermometer is called, for distinction, _sensible_ or _free heat_. -It may be here observed, that heat which is sensible to the -thermometer is also perceptible by the senses, and heat not -sensible to the thermometer is not perceptible by the senses. -Thus, ice at 32° and water at 32° _feel_ equally cold, and yet we -have seen that the latter contains considerably more heat than the -former. - -Dr. Black, who first noticed the remarkable fact to which we have -now alluded, inferred that ice is converted into water by -communicating to it a certain quantity or dose of heat, which -enters into combination with it in a manner analogous to that -which takes place when bodies combine chemically. The heat, thus -combined with the solid ice, loses its property of affecting the -senses or the thermometer, and the effects therefore bear a -resemblance to those cases of chemical combination in which the -constituent elements change their sensible properties when they -form the compound. - -The fact that the thermometer immersed in the ice remains stationary -only as long as the process of liquefaction is going on, shows that -this absorption of heat is necessarily connected with that process, -and that, were it not for the conversion of [Pg102] the solid ice -into liquid water, the heat which is so received would be sensible, -and would cause the thermometer immersed in the ice to rise. Before -the time of Black it was supposed that the slightest addition of -heat would cause solid ice to be converted into water, and that the -thermometer would immediately pass from the freezing temperature to -higher degrees. The experiments above described, however, show the -falsehood of such a supposition. If, while the mercurial bath, in -which the ice is immersed, is maintained at the temperature of 200°, -the length of time necessary to complete the liquefaction of the ice -be observed, it would be found that that time is about twenty-eight -times the length of time which it would take to raise the liquid -water from 32° to 37°; and if it be assumed that the same quantity -of heat is imparted to the ice, during the process of liquefaction, -during each minute, as is imparted to the water, during each minute, -in rising from 32° to 37°, it will follow, that to liquefy the ice -requires twenty-eight times as much heat as is necessary to raise -the water from 32° to 37°. It appears, therefore, that, instead of a -small quantity of heat being necessary to melt the ice, a very -considerable portion is absorbed in that process. - -Having ascertained the remarkable fact, that heat is absorbed in a -large quantity in the conversion of ice into water, without -rendering the body so absorbing it warmer, let us now inquire what -the exact quantity of heat so absorbed is. We have already stated -that, if the quantity communicated in equal times be the same, the -heat necessary to liquefy a given weight of ice would be -twenty-eight times as much as would be necessary to raise the same -weight of water from 32° to 37°; or, if the heat necessary to -raise water through every 5° be the same, that quantity of heat -would be sufficient to raise water from 32° to 172°: and hence we -infer, that as much heat is absorbed in the liquefaction of a -given quantity of ice as would raise the same quantity of water -through 140 degrees of the thermometric scale. - - -(52.) Let us now examine the analogous effects produced by the -continued application of heat to water in the liquid state. - -Let a small quantity of water be placed in a glass flask of -considerable size, and then closed so as to prevent the escape -[Pg103] of any vapour. Let this vessel be now placed over the -flame of a spirit lamp, so as to cause the water it contains to -boil. For a considerable time the water will be observed to boil, -and apparently to diminish in quantity, until at length all the -water disappears, and the vessel is apparently empty. If the -vessel be now removed from the lamp, and suspended in a cool -atmosphere, the whole of the interior of its surface will -presently appear to be covered with a dewy moisture; and at length -a quantity of water will collect in the bottom of it, equal to -that which had been in it at the commencement of the process. That -no water has at any period of the experiment escaped from it, may -be easily determined, by performing the experiment with the glass -flask suspended from the arm of a balance, counterpoised by a -sufficient weight suspended from the other arm. The equilibrium -will be preserved throughout, and the vessel will be found to have -the same weight, when to all appearance it is empty, as when it -contains the liquid water. It is evident, therefore, that the -water exists in the vessel in every stage of the process, but that -it becomes invisible when the process of boiling has continued for -a certain length of time; and it may be shown that it will -continue to be invisible, provided the flask be exposed to a -temperature considerably elevated. Thus, for example, if it be -suspended in a vessel of boiling water, the water which it -contains will continue to be invisible; but the moment it is -withdrawn from the boiling water, and exposed to the cold air, the -water will again become visible, as above mentioned, forming a dew -on the inner surface, and finally collecting in the bottom, as in -the commencement of the experiment. - -In fact, the liquid has, by the process of boiling, been converted -into _vapour_, or _steam_, which is a body similar in its leading -properties to common air, and, like it, is invisible. It will -hereafter appear that it likewise possesses the property of -elasticity, and other mechanical qualities enjoyed by gases in -general. - - -(53.) Again, let an open vessel be filled with water at 60°, and -placed in a mercurial bath, which is maintained, by a fire or lamp -applied to it, at the temperature of 230°. Place a thermometer in -the water, and it will be observed gradually to [Pg104] rise as -the temperature of the water is increased by the heat which it -receives from the mercury in which it is immersed. The water will -steadily rise in this manner until it attains the temperature of -212°; but here the thermometer immersed in it will become -stationary. At the same time the water contained in the vessel -will become agitated, and its surface will present the same -appearance as if bubbles of air were rising from the bottom, and -issuing at the top. A cloudy vapour will be given off in large -quantities from its surface. This process is called _ebullition_ -or _boiling_. If it be continued for any considerable time, the -quantity of water in the vessel will be sensibly diminished; and -at length every particle of it will disappear, and the vessel will -remain empty. During the whole of this process, the thermometer -immersed in the water will remain stationary at 212°. - -Now, it will be asked, what has become of the water? It cannot be -imagined that it has been annihilated. We shall be able to answer -this by adopting means to prevent the escape of any particle of -matter from the vessel containing the water, into the atmosphere -or elsewhere. Let us suppose that the top of the vessel containing -the water is closed, with the exception of a neck communicating -with a tube, and let that tube be carried into another close -vessel removed from the cistern of heated mercury, and plunged in -another cistern of cold water. Such an apparatus is represented in -_fig._ 15. - -[Illustration: _Fig._ 15.] - -A is a cistern of heated mercury, in which the glass vessel B, -containing water, is immersed. From the top of the vessel B -proceeds a glass tube C, inclining downwards, and entering a glass -vessel D, which is immersed in a cistern E of cold water. If the -process already described be continued until the water by constant -ebullition has disappeared, as already mentioned, [Pg105] from -the vessel B, it will be found that a quantity of water will be -collected in the vessel D; and if this water be weighed, it will -be found to have exactly the same weight as the water had which -was originally placed in the vessel B. It is, therefore, quite -apparent that the water has passed by the process of boiling from -the one vessel to the other; but, in its passage, it was not -perceptible by the sight. The tube C and the upper part of the -vessel B, had the same appearance, exactly, as if they had been -filled with atmospheric air. That they are not merely filled with -atmospheric air may, however, be easily proved. When the process -of boiling first commences, it will be found that the tube C is -cold, and the inner surface dry. When the process of ebullition -has continued a short time, the tube C will become gradually -heated, and the inner surface of it covered with moisture. After a -time, however, this moisture disappears, and the tube attains the -temperature 212°. In this state it continues until the whole of -the water is discharged from the vessel B to the vessel D. - - -(54.) These effects are easily explained. The water in the vessel -B is incapable of receiving any higher temperature than 212°, -consistently with its retaining the liquid form. Small portions, -therefore, are constantly converted into steam by the heat -received from the surrounding mercury, and bubbles of steam are -formed on the bottom and sides of the vessel B. These bubbles, -being very much lighter, bulk for bulk, than water, rise rapidly -through the water, just in the same manner as bubbles of air -would, and produce that peculiar agitation at its surface which -has been taken as the external indication of boiling. They escape -from the surface, and collect in the upper part of the vessel. The -steam thus collected, when it first enters the tube C, is cooled -below the temperature of 212° by the surface of the tube; and -consequently, being incapable of remaining in the state of vapour -at any lower temperature than 212°, it is reconverted into water, -and forms the dewy moisture which is observed in the commencement -of the process on the interior of the tube C. At length, however, -the whole of the tube C is heated to the temperature of 212°, and -the moisture which was previously collected upon its inner -[Pg106] surface is again converted into steam. As the quantity of -steam evolved from the water in B increases, it drives before it -the steam previously collected in the tube C, and forces it into -the vessel B. Here it encounters the inner surface of this vessel, -which is kept constantly cold by being surrounded with the cold -water in which it is immersed; and the vapour, being thus -immediately reduced below the temperature of 212°, is reconverted -into water. At first it collects in a dew on the surface of the -vessel D; but as this accumulates, it drops into the bottom of the -vessel, and forms a more considerable quantity. As the quantity of -water is observed to be gradually diminished in the vessel B, the -quantity will be found to be gradually increased in the vessel D; -and if the operation be suspended at any stage of the process, and -the water in the two vessels weighed, it will be found that the -weight of the water in D is exactly equal to the weight which the -water in B has lost. - - -(55.) The demonstration is, therefore, perfect, that the gradual -diminution of the boiling water in the vessel B is produced by the -conversion of that water into steam by the heat. In the process -first described, when the top of the vessel B was supposed to be -open, this steam made its escape into the air, where it was first -dispersed, and subsequently cooled in separate particles, and was -deposited in minute globules of moisture on the ground and on -surrounding objects. - - -(56.) In reviewing this process, we are struck by the fact, that -the continued application of heat to the vessel B is incapable of -raising the temperature of the water contained in it above 212°. -This presents an obvious analogy to the process of liquefaction, -and leads to inquiries of a similar nature, which are attended -with a like result. We must either infer, that the water, having -arrived at 212°, received no more heat from the mercury; or that -such heat, if received, is incapable of affecting the thermometer; -or, finally, that the steam which passes off carries this heat -with it. That the water receive heat from the mercury, will be -proved by the fact, that, if the vessel B be removed from the -mercury, other things remaining as before, the temperature of the -mercury will rapidly rise, and if the fire be continued, it will -even boil; but so long as the [Pg107] vessel B remains immersed, -it prevents the mercury from increasing in temperature. It -therefore receives that heat which would otherwise raise the -temperature of the quicksilver. - -[Illustration: _Fig._ 16.] - -If a thermometer be immersed in the steam which collects in the -upper part of the vessel B, it will show the same temperature (of -212°) as the water from which it is raised. The heat, therefore, -received from the mercury, is clearly not imparted in a sensible -form to the steam, which has the same temperature in the form of -steam as it had in the form of water. What has been already -explained respecting liquefaction would lead us, by analogy, to -suspect that the heat imparted by the mercury to the water has -become latent in the steam, and is instrumental to the conversion -of water into steam, in the same manner as heat has been shown to -be instrumental to the conversion of ice into water. As the fact -was in that case detected by mixing ice with water, so we shall, -in the present instance, try it by a like test, viz. by mixing -water with steam. Let about five ounces and a half of water, at -the temperature of 32°, be placed in a vessel A (_fig._ 16.), and -let another vessel B, in which water is kept constantly boiling at -the temperature of 212°, communicate with A by a pipe C proceeding -from the top, so that the steam may be conducted from B, and -escape from the mouth of the pipe at some depth below the surface -of the water in A. As the steam issues from the pipe, it will be -immediately reconverted into water by the cold water which it -encounters; and, by continuing this process, the water in A will -be gradually heated by the steam combined with it and received -through the pipe C. If this process be continued until the water -in A is raised to the temperature of 212°, it will boil. Let it -then be weighed, and it will be found to weigh six ounces and a -half: from whence we infer, that one ounce of water has been -received from the vessel B in the form of steam, and has been -reconverted into water by the inferior temperature of the water in -A. Now, this ounce of water received in the form of steam into the -vessel A had, when in that form, the temperature of 212°. It is -now [Pg108] converted into the liquid form, and still retains the -same temperature of 212°; but it has caused the five ounces and a -half of water with which it has been mixed, to rise from the -temperature of 32° to the temperature of 212°,--and this, _without -losing any temperature itself_. It follows, therefore, that, in -returning to the liquid state, it has parted with as much heat as -is capable of raising five times and a half its own weight of -water from 32° to 212°. This heat was combined with the steam, -though not sensible to the thermometer; and was, therefore, -_latent_. Had it been sensible in the water in B, it would have -caused the water to have risen through a number of thermometric -degrees, amounting to five times and a half the excess of 212° -above 32°; that is, through five times and a half 180°; for it has -caused five times and a half its own weight of water to receive an -equal increase of temperature. But five times and a half 180° is -990°, or, to use round numbers (for minute accuracy is not here -our object), 1000°. It follows, therefore, that an ounce of water, -in passing from the liquid state at 212° to the state of steam at -212°, receives as much heat as would be sufficient to raise it -through 1000 thermometric degrees, if that heat, instead of -becoming latent, had been sensible. - - -(57.) In order to derive all the knowledge from these experiments -which they are capable of imparting, it will be necessary to -examine very carefully how water comports itself under a variety -of different circumstances. - -If water be boiled in an open vessel, with a thermometer immersed, -on different days, it will be observed that the fixed temperature -which it assumes in boiling will be subject to a variation within -certain small limits. Thus, at one time, it will be found to boil -at the temperature of 210°; while, at others, the thermometer -immersed in it will rise to 213°; and, on different occasions, it -will fix itself at different points within these limits. It will -also be found, if the same experiment be performed at the same -time in distant places, that the boiling points will be subject to -a like variation. Now, it is natural to inquire what cause -produces this variation; and we shall be led to the discovery of -the cause, by examining what other physical effects undergo a -simultaneous change. [Pg109] - -If we observe the height of the barometer at the time of making -each experiment, we shall find a very remarkable correspondence -between it and the boiling temperature. Invariably, whenever the -barometer stands at the same height, the boiling temperature will -be the same. Thus, if the barometer stands at 30 inches, the -boiling temperature will be 212°. If the barometer fall to 29-1/2 -inches, the thermometer stands at a small fraction above 211°. If -the barometer rise to 30-1/2 inches, the boiling temperature rises -to nearly 213°. The variation in the boiling temperature is, then, -accompanied by a variation in the pressure of the atmosphere -indicated by the barometer; and it is constantly found that the -boiling point will remain unchanged, so long as the atmospheric -pressure remains unchanged, and that every increase in the one -causes a corresponding increase in the other. - - -(58.) From these facts it must be inferred, that the pressure -excited on the surface of the water has a tendency to resist its -ebullition, and to make it necessary, before it can boil, that it -should receive a higher temperature; and, on the contrary, that -every diminution of pressure on the surface of the water will give -an increased facility to the process of ebullition, or will cause -that process to take place at a lower temperature. As these facts -are of the utmost importance in the theory of heat, it may be -useful to verify them by direct experiment. - -If the variable pressure excited on the surface of the water by -the atmosphere be the cause of the change in the boiling -temperature, it must happen, that any change of pressure produced -by artificial means on the surface of the water must likewise -change the boiling point, according to the same law. Thus, if a -pressure considerably greater than the atmospheric pressure be -excited on a liquid, the boiling point may be expected to rise -considerably above 212°; and, on the other hand, if the surface of -the water be relieved from the pressure of the atmosphere, and be -submitted to a considerably diminished pressure, the water would -boil below 212°. - -[Illustration: _Fig._ 17.] - -Let B (_fig._ 17.) be a strong spherical vessel of brass, supported -on a stand S, under which is placed a large spirit lamp L, or other -means of heating it. In the top of this vessel are three apertures, -in two of which are screwed a [Pg110] thermometer T, the bulb of -which enters the hollow brass sphere, and a stop-cock C, which may -be closed or opened at pleasure, to confine the steam, or allow it -to escape. In the third aperture at the top, is screwed a long -barometer tube, open at both ends. The lower end of this tube -extends nearly to the bottom of the spherical vessel B. In the -bottom of this vessel is placed a quantity of mercury, the surface -of which rises to some height above the lower end of the tube A. -Over the mercury is poured a quantity of water, so as to half fill -the vessel B. Matters being thus arranged, the screws are made -tight, so as to confine the water, and the lamp is allowed to act on -the vessel; the temperature of the water is raised, and steam is -produced, which, being confined within the vessel, exerts its -pressure on the surface of the water, and resists its ebullition. -The pressure of the steam acting on the surface of the water is -communicated to the surface of the mercury, and it forces a portion -of the mercury into the tube A, which presently rises above the -point where the tube is screwed into the top of the vessel B. As the -action of the lamp continues, the thermometer T exhibits a gradually -increasing temperature; while the column of mercury in A shows the -force with which the steam presses on the surface of the water in -B,--this column being balanced by the pressure of the steam. Thus, -the temperature and pressure of the steam at the same moment may -always be observed by inspecting the thermometer T and the tube A. -When the column in the tube A has risen to the height of 30 inches -above the level of the mercury in the vessel B, then the pressure of -the steam will be equivalent to double the pressure of the -atmosphere, because, the tube A being open at the top, the -atmosphere presses on the [Pg111] surface of the mercury in it. The -thermometer T will be observed gradually to rise until it attains -the temperature of 212°; but it will not stop there, as it would do -if immersed in water boiled in an open vessel. It will, on the other -hand, continue to rise; and when the column of mercury in A has -attained the height of 30 inches, the thermometer T will have risen -to 251°,--being 39° above the ordinary boiling point. - -During the whole of this process, the surface of the water being -submitted to a constantly increasing pressure, its ebullition is -prevented, and it continues to receive heat without boiling. That -it is the increased pressure which resists its ebullition, and -causes it to receive a temperature above 212°, may be easily -shown. Let the stop-cock C be opened; immediately the steam in B, -having a pressure considerably greater than that of the -atmosphere, will rush out, and will continue to issue from C, -until its pressure is balanced by the atmosphere. At the same time -the column of mercury in A will be observed rapidly to fall, and -to sink below the orifice by which it is inserted in the vessel B. -The thermometer T will also fall until it attains the temperature -of 212°. At that point, however, it will remain stationary; and -the water will now be distinctly heard to be in a state of rapid -ebullition. If the stop-cock C be once more closed, the -thermometer will begin to rise, and the column of mercury -ascending in A will be again visible. - -If, instead of a stop-cock being at C, the aperture were made to -communicate with a valve, like the safety-valve of a steam engine, -loaded with a certain weight,--say at the rate of 15 lbs. on the -square inch,--then the thermometer T, and the mercury in the tube -A, would not rise indefinitely as before. The thermometer would -continue to rise till it attained the temperature of 251°; and the -mercury in the tube A would rise to the height of 30 inches. At -this limit the resistance of the valve would be balanced by the -pressure of the steam; and as fast as the water would have a -tendency to produce steam of a higher pressure, the valve would be -raised and the steam suffered to escape; the thermometer T and the -column of mercury in A remaining stationary during this process. -If the valve were loaded more heavily, the phenomena would be -[Pg112] the same, only that the mercury in T and A would become -stationary at certain heights. But, on the other hand, if the -valve were loaded at a less pressure than 15 lbs. on the square -inch, then the mercury in the two tubes would become stationary at -lower points. - - -(59.) These experiments show that every increase of pressure above -the ordinary pressure of the atmosphere causes an increase in the -temperature at which water boils. We shall now inquire whether a -diminution of pressure will produce a corresponding effect on the -boiling point. - -This may be easily accomplished by the aid of an air pump. Let -water at the temperature of 200° be placed in a glass vessel under -the receiver of an air pump, and let the air be gradually -withdrawn. After a few strokes of the pump, the water will boil; -and if the mercurial gauge of the pump be observed, it will be -found that its altitude will be about 23-1/2 inches. Thus the -pressure to which the water is submitted has been reduced from the -ordinary pressure of the atmosphere expressed by the column of 30 -inches of mercury, to a diminished pressure expressed by 23-1/2 -inches; and we find that the temperature at which the water boils -has been lowered from 212° to 200°. Let the same experiment be -repeated with water at the temperature of 180°, and it will be -found that a further rarefaction of the air is necessary, but the -water will at length boil. If the gauge of the pump be now -observed, it will be found to stand at about fifteen inches, -showing, that at the temperature of 180° water will boil under -half the ordinary pressure of the atmosphere. These experiments -may be varied and repeated; and it will be always found, that, as -the pressure is diminished or increased, the temperature at which -the water will boil will be also diminished or increased. - - -(60.) The same effects may be exhibited in a striking manner -without an air pump, by producing a vacuum by the condensation of -steam. Let a small quantity of water be placed in a thin glass -flask, and let it be boiled by holding it over a spirit lamp. When -the steam is observed to issue abundantly from the mouth of the -flask, let it be quickly corked and removed from the lamp. The -process of boiling will then cease, and the water will become -quiescent; but if the flask be plunged [Pg113] in a vessel of -cold water, the water it contains will again pass into a state of -violent ebullition, thus exhibiting the singular fact of water -being boiled by cooling it. This effect is produced by the cold -medium in which the flask is immersed, causing the steam above the -surface of the water in it to be condensed, and therefore -relieving the water from its pressure. The water, under these -circumstances, boils at a lower temperature than when submitted to -the pressure of the uncondensed vapour. - - -(61.) There is no limit to the temperature to which water may be -raised, if it be submitted to a sufficient pressure to resist its -tendency to take the vaporous form. If a strong metallic vessel be -nearly filled with water, so as to prevent the liquid from -escaping by any force which it can exert, the water thus inclosed -may be heated to any temperature whatever without boiling; in -fact, it may be made red-hot; and the temperature to which it may -be raised will have no limit, except the strength of the vessel -containing it, or the point at which the metal of which it is -formed may begin to soften or to be fused. - - -(62.) The following table will show the temperature at which water -will boil under different pressures of the atmosphere corresponding -to the altitudes of the barometer between 26 and 31 inches. - - Barometer. Boiling Point. - 26 inches 204°·91 - 26·5 205°·79 - 27 206°·67 - 27·5 207°·55 - 28 208°·43 - 28·5 209°·31 - 29 210°·19 - 29·5 211°·07 - 30 212° - 30·5 212°·88 - 31 213°·76 - -From this table it appears, that, for every tenth of an inch which -the barometric column varies between these limits, the boiling -temperature changes by the fraction of a degree expressed by the -decimal ·176, or nearly by the vulgar fraction 1/6. - - -(63.) In the experiment already described, by which the latent -[Pg114] heat of steam was determined, the water was supposed to be -boiled under the ordinary pressure of the atmosphere. Having seen, -however, that water may boil at different temperatures, under -different pressures, the inquiry presents itself, whether the heat -absorbed in vaporisation at different temperatures, and under -different pressures, is subject to any variation? Experiments of -the same nature as those already described, instituted upon water -in a state of ebullition at different temperatures, as well below -as above 212°, have led to the discovery of a very remarkable fact -in the theory of vapour. It has been found that the heat absorbed -by vaporisation is always less, the higher the temperature at -which the ebullition takes place; and less, by the same amount as -the temperature of ebullition is increased. Thus, if water boil at -312°, the heat absorbed in ebullition will be less by 100° than if -it boiled at 212°; and again, if water be boiled under a -diminished pressure, at 112°, the heat absorbed in vaporisation -will be 100° more than the heat absorbed by water boiled at 212°. -It follows, therefore, that the actual consumption of heat in the -process of vaporisation must be the same, whatever be the -temperature at which the vaporisation takes place; for whatever -heat is saved in the sensible form, is consumed in the latent -form, and _vice versâ_. - -Let us suppose a given weight of water at the temperature of 32° -to be exposed to any regular source by which heat may be supplied -to it. If it be under the ordinary atmospheric pressure, the first -180° of heat which it receives will raise it to the boiling point, -and the next 1000° will convert it into steam. Thus, in addition -to the heat which it contains at 32°, the steam at 212° contains -1180° of heat. But if the same water be submitted to a pressure -equal to half the atmospheric pressure, then the first 148° of -heat which it receives will cause it to boil, and the next 1032° -will convert it into vapour. Thus, steam at the temperature of -180° contains a quantity of heat more than the same quantity of -water at 32°, by 1032° added to 148°, which gives a sum of 1180°. -Steam, therefore, raised under the ordinary pressure of the -atmosphere at 212°, and steam raised under half that pressure at -180°, contain the same quantity of heat,--with this difference -[Pg115] only--that the one has more latent heat, and less sensible -heat, than the other. - -From this fact, that the sum of the latent and sensible heats of -the vapour of water is constant, it follows that the same quantity -of heat is necessary to convert a given weight of water into -steam, at whatever temperature, or under whatever pressure, the -water may be boiled. It follows, also, that, in the steam engine, -equal weights of high-pressure and low-pressure steam are produced -by the same consumption of fuel; and that, in general, the -consumption of fuel is proportional to the quantity of water -vaporised, whatever the pressure of the steam may be.[18] - - -(64.) Having explained the conditions under which, by supplying -heat to water, it is converted into steam, and, by abstracting -heat from steam, it may be reconverted into water, let us now -consider the mechanical force which is developed in these -phenomena. - -[Illustration: _Fig._ 18.] - -Let A B (_fig._ 18.) be a tube, or cylinder, the base of which is -equal to a square inch, and let a piston P move in it so as to be -steam-tight. Let it be supposed, that under this piston there is, -in the bottom of the cylinder, a cubic inch of water between the -bottom of the piston and the bottom of the tube; let the piston be -counterbalanced by a weight W acting over a pulley, which will be -just sufficient to counterpoise the weight of the piston, so as -leave no force tending to keep the piston down, except the force -of the atmosphere acting above it. Under the circumstances here -supposed, the piston being in contact with the water, and all air -being excluded, it will be pressed down by the weight of the -atmosphere, which we will suppose to be fifteen pounds, the -magnitude of the piston being a square inch. [Pg116] - -Now let the flame of a lamp be applied at the bottom of the tube; -the water under the piston having its temperature thereby -gradually raised, and being submitted to no pressure save that of -the atmosphere above the piston, it will begin to be converted -into steam when it has attained the temperature of 212°. According -as it is converted into steam, it will cause the piston to ascend -in the tube until all the water has been evaporated. If the tube -were constructed of sufficient length, the piston then would be -found to have risen to the height of about seventeen hundred -inches, or one hundred and forty-two feet; since, as has been -already explained, water passing into steam under the ordinary -pressure of the atmosphere undergoes an increase of bulk in the -proportion of about seventeen hundred to one. - -Now in this process, the air above the piston, which presses on it -with a force equal to fifteen pounds, has been raised one hundred -and forty-two feet. It appears, therefore, that, by the -evaporation of a cubic inch of water under a pressure equal to -fifteen pounds per square inch, a mechanical force of this amount -is developed. - -It is evident that fifteen pounds raised one hundred and forty-two -feet successively, is equivalent to one hundred and forty-two -times fifteen pounds raised one foot. Now, one hundred and -forty-two times fifteen is two thousand one hundred and thirty, -and therefore the force thus obtained is equal to two thousand one -hundred and thirty pounds raised one foot high. This being within -about 110 pounds of a ton, it may be stated, in round numbers, -that, by the evaporation of a cubic inch of water under these -circumstances, a force is obtained equal to that which would raise -a ton weight a foot high. - -The augmentation of volume which water undergoes in passing into -steam under the pressure here supposed, may be easily retained in -the memory, from the accidental circumstance that a cubic inch of -water is converted into a cubic foot of steam, very nearly. A -cubic foot contains one thousand seven hundred and twenty-eight -cubic inches,--which is little different from the proportion which -steam bears to water, when raised under the atmospheric pressure. -[Pg117] - - -(65.) It will, therefore, be an advantage to retain in memory the -following general facts:-- - -1. _A cubic inch of water evaporated under the ordinary -atmospheric pressure, is converted into a cubic foot of steam._ - -2. _A cubic inch of water evaporated under the atmospheric -pressure, gives a mechanical force equal to what would raise about -a ton weight a foot high._ - - -(66.) Let us, again, suppose the piston P (_fig._ 23.) to be -restored to its original position, with the liquid water beneath -it; and, in addition to the weight of the atmosphere which before -pressed it down, let us suppose another weight of fifteen pounds -laid upon it, so that the water below shall be pressed by double -the weight of the atmosphere. If the lamp were now applied, and at -the same time a thermometer were immersed in the water, it would -be found that the water would not begin to be converted into steam -until it attained the temperature of about 250°. The piston would -then begin, as before, to ascend, and the water to be gradually -converted into vapour. The water being completely evaporated, it -would be found that the piston would be raised to a height little -more than half its former height, or 72 feet. The mechanical -effect, therefore, thus obtained, will be equivalent to double the -former weight raised half the former height. - -In like manner, if the piston were loaded with thirty pounds in -addition to the atmosphere, the whole pressure on the water being -then three times the pressure first supposed, the piston would be -raised to somewhat more than one third of its first height by the -evaporation of the water. This would give a mechanical force -equivalent to three times the original weight raised a little more -than one third of the original height. - -In general, as the pressure on the piston is increased, the height -to which the piston would be raised by the evaporation of the -water will be diminished in a proportion somewhat less than the -proportion in which the pressure on the piston is increased. If -the temperature at which the water is converted into steam under -these different pressures were the same, then the height to which -the piston would be raised by the evaporation of the water would -be diminished in precisely [Pg118] the same proportion as the -pressure on the piston is increased; and, in that case, the whole -mechanical force developed by the evaporation of the water would -remain exactly the same under whatever pressure the water might be -boiled. We shall explain hereafter the extent to which the -variation of temperature in the water and steam corresponding to -the variation of pressure modifies this law; but, as the effect of -the difference of temperatures is not considerable, it will be -convenient to register in the memory the following important -practical conclusion:-- - - -(67.) _A cubic inch of water converted into steam will supply a -mechanical force very nearly equal to a ton weight raised a foot -high; and this force will not be subject to considerable -variation, whatever be the temperature or pressure at which the -water may be evaporated._ - -[Illustration: GLASGOW.] - - FOOTNOTES: - - [18] The preceding paragraphs, and some other parts of the - present volume on the general properties of Heat, are taken - from my Treatise on Heat, in the _Cabinet Cyclopoedia_, to - which those who desire more detailed explanation and more - copious illustration should refer. - -[Pg119] - - - - -[Illustration: GLASGOW COLLEGE.] - -CHAP. V. - - WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE - WITH A DUE ECONOMY OF FUEL. -- CONCEIVES THE NOTION OF - CONDENSING OUT OF THE CYLINDER. -- DISCOVERS SEPARATE - CONDENSATION. -- INVENTS THE AIR-PUMP. -- SUBSTITUTES STEAM - PRESSURE FOR ATMOSPHERIC PRESSURE. -- INVENTS THE STEAM CASE, - OR JACKET. -- HIS FIRST EXPERIMENTS TO REALISE THESE - INVENTIONS. -- HIS EXPERIMENTAL APPARATUS. -- DIFFICULTIES OF - BRINGING THE IMPROVED ENGINES INTO USE. -- WATT PRACTISES AS A - CIVIL ENGINEER. -- HIS PARTNERSHIP WITH ROEBUCK. -- HIS FIRST - PATENT. -- DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE. - - -(68.) At the period to which we have now brought the history of -the invention of the steam engine, Watt had obtained, chiefly by -his own experiments, a sufficient knowledge of the phenomena which -have been just explained, to enable him to arrive at the -conclusion that a very small proportion of the whole mechanical -effect attending the evaporation was really rendered available by -the atmospheric engine; and that, [Pg120] therefore, extensive -and injurious sources of waste existed in its machinery. - -He perceived that the principal source of this wasteful -expenditure of power consisted in the quantity of steam which was -condensed at each stroke of the piston, in heating the cylinder -previous to the ascent of the piston. Yet, as it was evident that -that ascent could not be accomplished in a cold cylinder, it was -apparent that this waste of power must be inevitable, unless some -expedient could be devised, by which _a vacuum could be maintained -in the cylinder, without cooling it_. But, to produce such a -vacuum, the steam must be condensed; and, to condense the steam, -its temperature must be lowered to such a point that the vapour -proceeding from it shall have no injurious pressure; yet, if -condensed steam be contained in a cylinder at a high temperature, -it will return to the temperature of the cylinder, recover its -elasticity, and resist the descent of the piston. - -Having reflected on these circumstances, it became apparent to -Watt, that a vice was inherent in the structure of the atmospheric -engine, which rendered a large waste of power inevitable; this -vice arising from the fact, that the condensation of the steam was -incompatible with the condition of maintaining the elevated -temperature of the cylinder in which that condensation took place. -It followed, therefore, either that the steam must be imperfectly -condensed, or that the condensation could not take place in the -cylinder. It was in 1765, that, pondering on these circumstances, -the happy idea occurred to him, that the production of a vacuum -could be equally effected, though _the place_ where the -condensation of the steam took place were not the cylinder itself. -He saw, that if a vessel in which a vacuum was produced were put -into communication with another containing an elastic fluid, the -elastic fluid would rush into the vacuum, and diffuse itself -through the two vessels; but if, on rushing into such vacuum, this -elastic fluid, being vapour, were there condensed, or restored to -the liquid form, that then the space within the two vessels would -be equally rendered a vacuum;--that, under such circumstances, one -of the vessels might be maintained at any temperature, however -high, while [Pg121] the other might be kept at any temperature, -however low. This felicitous conception formed the first step in -that splendid career of invention and discovery which has -conferred immortality on the name of Watt. He used to say, that -the moment the idea of separate condensation occurred to -him,--that is, of condensing, in one vessel kept cold, the steam -coming from another vessel kept hot,--all the details of his -improved engine rushed into his mind in such rapid succession, -that, in the course of a day, his invention was so complete that -he proceeded to submit it to experiment. - -[Illustration: _Fig._ 19.] - -To explain the first conception of this memorable invention; let a -tube or pipe, S (_fig._ 19.), be imagined to proceed from the -bottom of the cylinder A B to a vessel, C, having a stop-cock, D, -by which the communication between the cylinder and the vessel C -may be opened or closed at pleasure. If we suppose the piston P at -the top of the cylinder, and the space below it filled with steam, -the cylinder and steam being at the usual temperature, while the -vessel C is a vacuum, and maintained at a low temperature. Then, -on opening the cock D, the steam will rush from the cylinder A B -through the tube S, and, passing into the cold vessel C, will be -condensed by contact with its cold sides. This process of -condensation will be rendered instantaneous if a jet of cold water -is allowed to play in the vessel C. When the steam thus rushing -into C, has been destroyed, and the space in the cylinder A B -becomes a vacuum, then the pressure of the atmosphere being -unobstructed, the piston will descend with the force due to the -excess of the pressure of the atmosphere above the friction. When -it has descended, suppose the stop-cock D closed, and steam -admitted from [Pg122] the boiler through a proper cock or valve -below the piston, the cylinder and piston being still at the same -temperature as before. The steam on entering the cylinder, not -being exposed to contact with any surface below its own -temperature, will not be condensed, and therefore will immediately -cause the piston to rise, and the piston will have attained the -top of the cylinder when as much steam shall have been supplied by -the boiler as will fill the cylinder. When this has taken place, -suppose the communication with the boiler cut off, and the cock D -once more opened: the steam will again rush through the pipe S -into the vessel C, where encountering the cold surface and the jet -of cold water, it will be condensed, and the vacuum, as before, -will be produced in the cylinder A B; that cylinder still -maintaining its temperature, the piston will again descend, and so -the process may be continued. - - -(69.) Having carried the invention to this point, Watt saw that -the vessel C would gradually become heated by the steam which -would be continually condensed in it. To prevent this, as well as -to supply a constant jet of cold water, he proposed to keep the -vessel C submerged in a cistern of cold water, from which a pipe -should conduct a jet to play within the vessel, so as to condense -the steam as it would pass from the cylinder. - -But here a difficulty presented itself, against which it was -necessary to provide. The cold water admitted through the jet to -condense the steam, mixed with the condensed steam itself, would -gradually collect in the vessel C, and at length choke it. To -prevent this, Watt proposed to put the vessel C in communication -with a pump F, which might be wrought by the engine itself, and by -which the water, which would collect in the bottom of the vessel -C, would be constantly drawn off. This pump would be evidently -rendered the more necessary, since more or less atmospheric air, -always combined with water in its common state, would enter the -vessel C by the condensing jet. This air would be disengaged in -the vessel C by the heat of the steam condensed therein; and it -would rise through the tube S, and vitiate the vacuum in the -cylinder;--an effect which would be rendered the more injurious, -[Pg123] inasmuch as, unlike steam, this elastic fluid would be -incapable of being condensed by cold. The pump F, therefore, by -which Watt proposed to draw off the water from the vessel C, might -also be made to draw off the air, or the principal part of it. - -The vessel C was subsequently called a _condenser_; and, from the -circumstances just adverted to, the pump F has been called the -_air-pump_. - -These--namely, the cylinder, the condenser, and the air-pump--were -the three principal parts in the invention, as it first presented -itself to the mind of Watt--and even before it was reduced to a -model, or submitted to experiment. But, in addition to these, -other two improvements offered themselves in the very first stage -of its progress. - -In the atmospheric engine, the piston was maintained steam-tight -in the cylinder by supplying a stream of cold water above it, by -which the small interstices between the piston and cylinder would -be stopped. It is evident that the effect of this water as the -piston descended would be to cool the cylinder, besides which any -portion of it which might pass between the piston and cylinder and -which would pass below the piston, would boil the moment it would -fall into the cylinder, which itself would be maintained at the -boiling temperature. This water, therefore, would produce steam, -the pressure of which would resist the descent of the piston. - -Watt perceived, that even though this inconvenience were removed -by the use of oil or tallow upon the piston, still, that as the -piston would descend in the cylinder, the cold atmosphere would -follow it; and would, to a certain extent, lower the temperature -of the cylinder. On the next ascent of the piston, this -temperature would have to be again raised to 212° by the steam -coming from the boiler, and would entail upon the machine a -proportionate waste of power. - -If the atmosphere of the engine-house could be kept heated to the -temperature of boiling water, this inconvenience would be removed. -The piston would then be pressed down by air as hot as the steam -to be subsequently introduced into it. On further consideration, -however, it occurred to Watt that it would be still more -advantageous if the cylinder itself could be [Pg124] worked in an -atmosphere of steam, having only the same pressure as the -atmosphere. Such steam would press the piston down as effectually -as the air would; and it would have the further advantage over -air, that if any portion of it leaked through between the piston -and cylinder, it would be condensed, which could not be the case -with atmospheric air. He therefore determined on surrounding the -cylinder by an external casing, the space between which and the -cylinder he proposed to be filled with steam supplied from the -boiler. The cylinder would thus be enclosed in an atmosphere of -its own, independent of the external air, and the vessel so -enclosing it would only require to be a little larger than the -cylinder, and to have a close cover at the top, the centre of -which might be perforated with a hole to admit the rod of the -piston to pass through, the rod being made smooth, and so fitted -to the perforation that no steam should escape between them. This -method would be attended also with the advantage of keeping the -cylinder and piston always heated, not only inside but outside; -and Watt saw that it would be further advantageous to employ the -pressure of steam to drive the piston in its descent instead of -the atmosphere, as its intensity or force would be much more -manageable; for, by increasing or diminishing the heat of the -steam in which the cylinder was enclosed, its pressure might be -regulated at pleasure, and it might be made to urge the piston -with any force that might be required. The power of the engine -would therefore be completely under control, and independent of -all variations in the pressure of the atmosphere. - - -(70.) This was a step which totally changed the character of the -machine, and which rendered it a STEAM ENGINE instead of an -ATMOSPHERIC ENGINE. Not only was the vacuum below the piston now -produced by the property of steam, in virtue of which it is -reconverted into water by cold; but the pressure which urged the -piston into this vacuum was due to the elasticity of steam. - -The external cylinder, within which the working cylinder was -enclosed, was called THE JACKET, and is still very generally used. - -[Illustration: _Fig._ 20.] - - -(71.) The first experiment in which Watt attempted to [Pg125] -realise, on a small scale, his conceptions, was made in the -following manner. The cylinder of the engine was represented by a -brass syringe A B (_fig._ 20.) an inch and a third in diameter, and -ten inches in length, to which a top and a bottom of tin plate was -fitted. Steam was conveyed by a pipe, S, from a small boiler into -the lower end of this syringe, a communication being made with the -upper end of the syringe by a branch pipe D. For the greater -convenience of the experiment, it was found desirable to invert the -position of the cylinder, so that the steam should press the piston -P upwards instead of downwards. The piston-rod R therefore was -presented downwards. An eduction pipe E was also inserted in the top -of the cylinder, which was carried to the condenser. The piston-rod -was made hollow, or rather a hole was drilled longitudinally through -it, and a valve was fitted at its lower end, to carry off the water -produced by the steam, which [Pg126] would be condensed in the -cylinder in the commencement of the process. The condenser used in -this experiment operated without injection, the steam being -condensed by the contact of cold surfaces. It consisted of two thin -pipes F, G of tin, ten or twelve inches in length, and the sixth of -an inch in diameter, standing beside each other perpendicularly, and -communicating at the top with the eduction pipe, which was provided -with a valve opening upwards. At the bottom these two pipes -communicated with another tube I of about an inch in diameter, by a -horizontal pipe, having in it a valve, M, opening towards I, fitted -with a piston K, which served the office of the air-pump, being -worked by the hand. This piston, K, had valves in it opening -upwards. These condensing pipes and air-pump were immersed in a -small cistern, filled with cold water. The steam was conveyed by the -steam-pipe S to the bottom of the cylinder, a communication between -the top and bottom of the cylinder being occasionally opened by a -cock, C, placed in the branch pipe. The eduction pipe leading to the -condenser also had a cock, L, by which the communication between the -top of the cylinder and the condenser might be opened and closed at -pleasure. In the commencement of the operation, the cock N admitting -steam from the boiler, and the cock L opening a communication -between the cylinder and the condenser, and the cock C opening a -communication between the top and bottom of the cylinder, being all -open, steam rushed from the boiler, passing through all the pipes, -and filling the cylinder. A current of mixed air and steam was thus -produced through the eduction pipe E, through the condensing pipes F -and G, and through the air-pump I, which issued from the valve H in -the eduction pipe, and from the valve in the air-pump piston, all of -which opened upwards. The steam also in the cylinder passed through -the hole drilled in the piston-rod, and escaped, mixed with air, -through the valve in the lower end of that rod. This process was -continued until all the air in the cylinder, pipes, and condenser, -was blown out, and all these spaces filled with pure steam. The -cocks L, C, and N, were then closed, and the atmospheric pressure -closed the valve H and the valves in the air-pump piston. The cold -surfaces condensing the steam in [Pg127] the pipes F and G, and in -the lower part of the air-pump, a vacuum was produced in these -spaces. The cock C being now closed, and the cocks L and N being -open, the steam in the upper part of the cylinder rushed through the -pipe E into the condenser, where it was reduced to water, so that a -vacuum was left in the upper part of the cylinder. The steam from -the boiler passing below the piston, pressed it upwards with such -force, that it lifted a weight of eighteen pounds hung from the end -of the piston-rod. When the piston reached the top of the cylinder, -the cocks L and N were closed, and the cock C opened. All -communication between the cylinder and the boiler, as well as -between the cylinder and the condenser, were now cut off, and the -steam in the cylinder circulated freely above and below the piston, -by means of the open tube D. The piston, being subject to equal -forces upwards and downwards, would therefore descend by its own -weight, and would reach the bottom of the cylinder. The air-pump -piston meanwhile being drawn up, the air and the condensed steam in -the tubes F and G were drawn into the air-pump I, through the open -horizontal tube at the bottom. Its return was stopped by the valve -M. By another stroke of the air-pump, this water and air were drawn -out through valves in the piston, which opened upwards. The cock C -was now closed, and the cocks L and N opened, preparatory to another -stroke of the piston. The steam in the upper part of the cylinder -rushed, as before, into the tubes F and G, and was condensed by -their cold surfaces, while steam from the boiler coming through the -pipe S, pressed the piston upwards. The piston again ascended with -the same force as before, and in the same manner the process was -continually repeated. - - -(72.) The quantity of steam expended in this experimental model in -the production of a given number of strokes of the piston was -inferred from the quantity of water evaporated in the boiler; and -on comparing this with the magnitude of the cylinder and the -weight raised by the pressure of the steam, the contrivance was -proved to affect the economy of steam, as far as the imperfect -conditions of such a model could have permitted. A larger model -was next constructed, having an outer cylinder, or steam case, -surrounding the working cylinder, and [Pg128] the experiments -made with it fully realised Watt's expectations, and left no doubt -of the great advantages which would attend his invention. The -weights raised by the piston proved that the vacuum in the -cylinder produced by the condensation was almost perfect; and he -found that when he used water in the boiler which by long boiling -had been well cleared of air, the weight raised was not much less -than the whole amount of the pressure of the steam upon the -piston. In this larger model, the cylinder was placed in the usual -position, with a working lever and other apparatus similar to that -employed in the Atmospheric Engine. - - -(73.) It was in the beginning of the year 1765, Watt being then in -the twenty-ninth year of his age, that he arrived at these great -discoveries. The experimental models just described, by which his -invention was first reduced to a rude practical test, were fitted -up at a place called Delft House, in Glasgow. It will doubtless, -at the first view, be a matter of surprise that improvements of -such obvious importance in the economy of steam power, and capable -of being verified by tests so simple, were not immediately adopted -wherever atmospheric engines were used. At the time, however, -referred to, Watt was an obscure artisan, in a provincial town, -not then arrived at the celebrity to which it has since attained, -and the facilities by which inventions and improvements became -public were much less than they have since become. It should also -be considered that all great and sudden advances in the useful -arts are necessarily opposed by the existing interests with which -their effects are in conflict. From these causes of opposition, -accompanied with the usual influence of prejudice and envy, Watt -was not exempt, and was not therefore likely suddenly to -revolutionise the arts and manufactures of the country by -displacing the moving powers employed in them, and substituting an -engine, the efficacy and power of which depended mainly on -physical principles, then altogether new and but imperfectly -understood. - -Not having the command of capital, and finding it impracticable to -inspire those who had, with the same confidence in the advantages -of his invention which he himself felt, he was [Pg129] unable to -take any step towards the construction of engines on a large -scale. Soon after this, he gave up his shop in Glasgow, and -devoted himself to the business of a Civil Engineer. In this -capacity he was engaged to make a survey of the river Clyde, and -furnished an elaborate and valuable Report upon its projected -improvements. He was also engaged in making a plan of the canal, -by which the produce of the Monkland Colliery was intended to be -carried to Glasgow, and in superintending the execution of that -work. Besides these, several other engineering enterprises -occupied his attention, among which may be mentioned, the -navigable canal across the isthmus of Crinan, afterwards completed -by Rennie; improvements proposed in the ports of Ayr, Glasgow, and -Greenock; the construction of the bridges at Hamilton, and at -Rutherglen; and the survey of the country through which the -celebrated Caledonian canal was intended to be carried. - -"If, forgetful of my duties as the organ of this academy," says M. -Arago (whose eloquent observations on the delays of this great -invention, addressed to the assembled members of the National -Institute of France, we cannot forbear to quote), "I could think -of making you smile, rather than expressing useful truths, I would -find here matter for a ludicrous contrast. I would call to your -recollection the authors, who at our weekly sittings demand with -all their might and main (_à cor et à cris_) an opportunity to -communicate some little remark--some small reflection--some -trifling note, conceived and written the night before; I would -represent them to you cursing their fate, when according to your -rules, the reading of their communication is postponed to the next -meeting, although during this cruel week, they are assured that -their important communication is deposited in our archives in a -sealed packet. On the other hand, I would point out to you the -creator of a machine, destined to form an epoch in the annals of -the world, undergoing patiently and without murmur, the stupid -contempt of capitalists,--conscious of his exalted genius, yet -stooping for eight years to the common labour of laying down -plans, taking levels, and all the tedious calculations connected -with the routine of common engineering. While in this conduct you -cannot fail to recognise the serenity, [Pg130] the moderation, -and the true modesty of his character, yet such indifference, -however noble may have been its causes, has something in it not -altogether blameless. It is not without reason that society visits -with severe reprobation those who withdraw gold from circulation -and hoard it in their coffers. Is he less culpable who deprives -his country, his fellow citizens, his age, of treasures a thousand -times more precious than the produce of the mine; who keeps to -himself his immortal inventions, sources of the most noble and -purest enjoyment of the mind, who abstains from conferring upon -labour those powers, by which would be multiplied in an infinite -proportion the products of industry, and by which, with advantage -to civilisation and human nature, he would smooth away the -inequalities of the conditions of man."[19] - - -(74.) Although Watt was thus attracted by pursuits foreign to his -recent investigations respecting the improvement of steam power, -he never lost sight of that object. It was not until the year -1768, three years after his great discoveries, that any step was -taken to enable him to carry them into effect on a large scale. At -that time his friends brought him into communication with Dr. -Roebuck, the proprietor of the Carron Iron Works, who rented -extensive coal works at Kinneal from the Duchess of Hamilton. Watt -was first employed by Roebuck as a civil engineer; but when he -made known to him the improvements he had projected in the steam -engine, Roebuck proposed to take out a patent for an engine on the -principle of the model which had been fitted up at Delft House, -and to join Watt in a partnership, for the construction of such -engines. Sensible of the advantages to be derived from the -influence of Roebuck, and from his command of capital, Watt agreed -to cede to him two thirds of the advantages to be derived from the -invention. A patent was accordingly taken out on the fifth of -January, 1769, nearly four years after the invention had been -completed; and an experimental engine on a large scale was -constructed by him, and fitted up at Kinneal House. In the first -trial this machine more than fulfilled Watt's anticipations. Its -[Pg131] success was complete. In the practical details of its -construction, however, some difficulties were still encountered, -the greatest of which consisted in packing the piston, so as to be -steam-tight. The principle of the new engine did not admit of -water being kept upon the piston, to prevent leakage, as in the -old engines; he was therefore obliged to have his cylinders much -more accurately bored, and more truly cylindrical, and to try a -great variety of soft substances for packing the piston, which -would make it steam-tight without great friction, and maintain it -so in a situation perfectly dry, and at the temperature of boiling -water. - -While Watt was endeavouring to overcome these and other -difficulties, in the construction of the machine, his partner, Dr. -Roebuck, became embarrassed, by the failure of his undertaking in -the Borrowstowness coal and salt works; and he was unable to -supply the means of prosecuting with the necessary vigour the -projected manufacture of the new engines. - -The important results of Watt's labours having happily at this -time become more publicly known, Mr. Matthew Boulton, whose -establishment at Soho, near Birmingham, was at that time the most -complete manufactory for metal-work in England, and conducted with -unexampled enterprise and spirit, proposed to purchase Dr. -Roebuck's interest in the patent. This arrangement was effected in -the year 1773, and in the following year Mr. Watt removed to Soho, -where a portion of the establishment was allotted to him, for the -erection of a foundery, and other works necessary to realise his -inventions on a grand scale. - -The patent which had been granted in 1769 was limited to a period -of fourteen years, and would consequently expire about the year -1783. From the small progress which had hitherto been made in the -construction of engines upon the new principle, and from the many -difficulties still to be encountered, and the large expenditure of -capital which must obviously be incurred before any return could -be obtained, it was apparent that unless an extension of the -patent right could be obtained, Boulton and Watt could never -expect any advantage adequate to the risk of their great [Pg132] -enterprise. In the year 1774 an application was accordingly made -to parliament for an extension of the patent, which was supported -by the testimony of Dr. Roebuck, Mr. Boulton, and others, as to -the merits and probable utility of the invention. An Act was -accordingly passed, in 1775, extending the term of the patent -until the year 1800. - - -(75.) The following abstract of this Act may not be uninteresting -at this time, when the anticipations expressed in it have been so -successfully and extensively realised:-- - -"An Act for vesting in James Watt, engineer, his executors, -administrators, and assigns, the sole use and property of certain -steam engines, commonly called fire engines, of his invention, -throughout his majesty's dominions, for a limited time: - -"And whereas the said James Watt hath employed many years, and a -considerable part of his fortune, in making experiments upon steam -engines, commonly called fire engines, with a view to improve -those very useful machines, by which several very considerable -advantages over the common steam engines are acquired; but upon -account of the many difficulties which always arise in the -execution of such large and complex machines, and of the long time -requisite to make the necessary trials, he could not complete his -intention before the end of the year 1774, when he finished some -large engines as specimens of his construction, which have -succeeded, so as to demonstrate the utility of the said invention: - -"And whereas, in order to manufacture these engines with the -necessary accuracy, and so that they may be sold at moderate -prices, a considerable sum of money must be previously expended in -erecting mills and other apparatus; and as several years and -repeated proofs will be required before any considerable part of -the public can be fully convinced of the utility of the invention, -and of their interest to adopt the same, the whole term granted by -the said letters patent may probably elapse before the said James -Watt can receive an advantage adequate to his labour and -invention: - -"And whereas, by furnishing mechanical power at much less expense, -and in more convenient forms, than has hitherto been done, his -engines may be of great utility, in facilitating [Pg133] the -operations in many great works and manufactures of this kingdom; -yet it will not be in the power of the said James Watt to carry -his invention into that complete execution which he wishes, and so -as to render the same of the highest utility to the public of -which it is capable, unless the term granted by the said letters -patent be prolonged, and his property in the said invention -secured for such time as may enable him to obtain an adequate -recompense for his labour, time, and expense: - -"To the end, therefore, that the said James Watt may be enabled -and encouraged to prosecute and complete his said invention, so -that the public may reap all the advantages to be derived -therefrom in their fullest extent: it is enacted, - -"That from and after the passing of this Act, the sole privilege -and advantage of making, constructing, and selling the said -engines hereinbefore particularly described, within the kingdom of -Great Britain, and his majesty's colonies and plantations abroad, -shall be, and are hereby declared to be, vested in the said James -Watt, his executors, administrators, and assigns, for and during -the term of twenty-five years," &c. &c. - - -(76.) Thus protected and supported, Watt now directed the whole -vigour of his mind to perfect the practical details of his -invention, and the result was, the construction on a large scale -of the engine which has since been called his SINGLE ACTING STEAM -ENGINE. - -It is necessary to recollect, that notwithstanding the extensive and -various application of steam power in the arts and manufactures, at -the time to which our narrative has now reached, the steam engine -had never been employed for any other purpose save that of raising -water by working pumps. The motion, therefore, which was required -was merely an upward force, such as was necessary to elevate the -piston of a pump, loaded with the column of water which it raised. -The following then is a description of the improved engine of Watt, -by which such work was proposed to be performed:-- - -[Illustration: _Fig._ 21.] - -In the cylinder represented at C (_fig._ 21.), the piston P moves -steam-tight. It is closed at the top, and the piston [Pg134] rod, -being accurately turned, runs in a steam-tight collar, B, -furnished with a stuffing-box, and is constantly lubricated with -melted tallow. A funnel is screwed into the top of the cylinder, -through which, by opening a stop-cock, melted [Pg135] tallow is -permitted from time to time to fall upon the piston within the -cylinder, so as to lubricate it, and keep it steam-tight. Two -boxes, A A, called the upper and lower steam boxes, contain valves -by which steam from the boiler may be admitted and withdrawn. -These steam boxes are connected by a tube of communication T, and -they communicate with the cylinder at the top and bottom by short -tubes represented in the figure. The upper steam box A contains -one valve, by which a communication with the boiler may be opened -or closed at pleasure. The lower valve box contains two valves. -The lower valve I communicates with the tube T', leading to the -condenser D, which being opened or closed, a communication is made -or cut off at pleasure, between the cylinder C and the condenser -D. A second valve, or upper valve H, which is represented closed -in the figure, may be opened so as to make a free communication -between the cylinder C and the tube T, and by that means between -the cylinder C, below the piston and the space above the piston. -The condenser D is submerged in a cistern of cold water. At the -side there enters it a tube, E, governed by a cock, which being -opened or closed to any required extent, a jet of cold water may -be allowed to play in the condenser, and may be regulated or -stopped, at pleasure. This jet, when playing, throws the water -upwards in the condenser towards the mouth of the tube T', as -water issues from the rose of a watering pot. The tube S proceeds -from the boiler, and terminates in the steam box A, so that the -steam supplied from the boiler constantly fills that box. The -valve G is governed by levers, whose pivots are attached to the -framing of the engine, and is opened or closed at pleasure, by -raising or lowering the lever G'. The valve G, when open, will -therefore allow steam to pass from the boiler through the short -tube to the top of the piston, and this steam will also fill the -tube T. If the lower valve H be closed, its circulation beyond -that point will be stopped; but if the valve H be open, the valve -I being closed, then the steam will circulate equally in the -cylinder, above and below the piston. If the valve I be open, then -steam will rush through the tube T' into the condenser; but this -escape of the steam will be [Pg136] stopped, if the valve I be -closed. The valve H is worked by the lever H', and the valve I by -the lever I'. - -The valve G is called the upper steam valve, H the lower steam -valve, I the exhausting valve, and E the condensing valve. - -From the bottom of the condenser D proceeds a tube leading to the -air-pump, which is also submerged in the cistern of cold water. In -this tube is a valve M, which opens outwards from the condenser -towards the air-pump. In the piston of the air-pump N is a valve -which opens upwards. The piston-rod Q of the air-pump is attached -to a beam of wood called a plug frame, which is connected with the -working beam by a flexible chain playing on the small arch-head -immediately over the air-pump. From the top of the air-pump barrel -above the piston proceeds a pipe or passage leading to a small -cistern, B, called the hot well. The pipe which leads to this -well, is supplied with a valve, K, which opens outwards from the -air pump barrel towards the well. From the nature of its -construction, the valve M admits the flow of water from the -condenser towards the air-pump, but prevents its return; and, in -like manner, the valve K admits the flow of water from the upper -part of the air-pump barrel into the hot well B, but obstructs its -return. - -Let us now consider how these valves should be worked in order to -move the piston upwards and downwards with the necessary force. It -is in the first place necessary that all the air which fills the -cylinder, the tubes and the condenser shall be expelled. To -accomplish this it is only necessary to open at once the three -valves G, H, and I. The steam then rushing from the boiler through -the steam-pipe S, and the open valve G will pass into the cylinder -above the piston, will fill the tube T, pass through the lower -steam valve H, will fill the cylinder C below the piston, and will -pass through the open valve I into the condenser. If the valve E -be closed so that no jet shall play in the condenser, the steam -rushing into it will be partially condensed by the cold surfaces -to which it will be exposed; but if the boiler supply it through -the pipe S in sufficient abundance, it will rush with violence -through the cylinder and all the passages, and its pressure in -the [Pg137] condenser D, combined with that of the heated air -with which it is mixed, will open the valve M, and it will rush -through mixed with the air into the air-pump barrel N. It will -press the valves in the air-pump piston upwards, and, opening -them, will rush through, and will collect in the air-pump barrel -above the piston. It will then, by its pressure, open the valve K, -and will escape into the cistern B. - -Throughout this process the steam, which mixed with the air fills -the cylinder, condenser and air-pumps will be only partially -condensed in the last two, and it will escape mixed with air -through the valve K, and this process will continue until all the -atmospheric air which at first filled the cylinder, tubes, -condenser and air-pump barrel shall be expelled through the valve -K, and these various spaces shall be filled with pure steam. When -that has happened let us suppose all the valves closed. In closing -the valve I the flow of steam to the condenser will be stopped, -and the steam contained in it will speedily be condensed by the -cold surface of the condenser, so that a vacuum will be produced -in the condenser, the condensed steam falling in the form of water -to the bottom. In like manner, and for like reasons, a vacuum will -be produced in the air-pump. The valve M, and the valves in the -air-pump piston will be closed by their own weight. - -By this process, which is called _blowing through_, the -atmospheric air, and other permanent gases, which filled the -cylinder, tubes, condenser and air-pump are expelled, and these -spaces will be a vacuum. The engine is then prepared to be -started, which is effected in the following manner:--The upper -steam valve G is opened, and steam allowed to flow from the boiler -through the passage leading to the top of the cylinder. This steam -cannot pass to the bottom of the cylinder, since the lower steam -valve H is closed. The space in the cylinder below the piston -being therefore a vacuum, and the steam pressing above it the -piston will be pressed downwards with a corresponding force. When -it has arrived at the bottom of the cylinder the steam valve G -must be closed, and at the same time the valve H opened. The valve -I leading to the condenser being also closed, the steam [Pg138] -which fills the cylinder above the piston is now admitted to -circulate through the open valve H below the piston, so that the -piston is pressed equally upwards and downwards by steam, and -there is no force to resist its movement save its friction with -the cylinder. The weight of the pump rods on the opposite end of -the beam being more than equivalent to overcome this the piston is -drawn to the top of the cylinder, and pushes before it the steam -which is drawn through the tube T, and the open valve H, and -passes into the cylinder C below the piston. - -[Illustration: _Fig._ 22.] - -When the piston has thus arrived once more at the top of the -cylinder, let the valve H be closed, and at the same time the -valves G and I opened, and the condensing cock E also opened, so -as to admit the jet to play in the condenser. The steam which -fills the cylinder C below the piston, will now rush through the -open valve I into the condenser which has been hitherto a vacuum, -and there encountering the jet, will be instantly converted into -water, and a mixture of condensed steam and injected water will -collect in the bottom of the condenser. At the same time, the -steam proceeding from the boiler by the steam pipe S to the upper -steam box A, will pass through the open steam valve G to the top -of the piston, but cannot pass below it because of the lower steam -valve H being closed. The piston, thus acted upon above by the -pressure of the steam, and the space in the cylinder below it -being a vacuum, its downward motion is resisted by no force but -the friction, and it is therefore driven to the bottom of the -cylinder. During its descent the valves G, I, and E remained open. -At the moment it arrives at the bottom of the cylinder, all these -three valves are closed, and the valve H opened. The steam which -fills the cylinder above the piston is now permitted to circulate -below it, by the open valve H, and the piston being consequently -pressed equally upwards and downwards will be drawn upwards as -before by the preponderance of the pump rods at the opposite end -of the beam. The weight of these rods must also be sufficiently -great to draw the air-pump piston N upwards. As this piston rises -in the air-pump, it leaves a vacuum below it into which the water -and air collected in the condenser will be drawn through the valve -M, which opens outwards. When the [Pg139] air-pump piston has -arrived at the top of the barrel, which it will do at the same -time that the steam piston arrives at the top of the cylinder, the -water and the chief part of the air or other fluids which may have -been in the condenser will be drawn into the barrel of the -air-pump, and the valve M being closed by its own weight, assisted -by the pressure of these fluids they cannot return into the -condenser. At the moment the steam piston arrives at the top of -the cylinder, the valve H is closed, and the three valves G, I, -and E are opened. The effect of this change is the same as was -already described in the former case, and the piston will in the -same manner and from the same causes be driven downwards. The -air-pump piston will at the same time descend by the force of its -own weight, aided by the weight of the plug-frame attached to its -rod. As it descends, the air below it will be gradually compressed -above the surface of the water in the bottom of the barrel, until -its pressure becomes sufficiently great to open the valves in the -air-pump piston. When this happens, the valves in the air-pump -piston, as represented on a large scale in _fig._ 22., will be -opened, and the air will pass through them above the piston. When -the piston comes in contact with the water in the bottom of the -barrel, this water will likewise pass through the open valves. -When the piston has arrived at the bottom of the air-pump barrel, -the valves in it will be closed by the pressure of the fluids -above them. The next ascent of the steam piston will draw up the -air-pump piston, and with it the fluids in the pump barrel above -it. As the air-pump [Pg140] piston approaches the top of its -barrel, the air and water above it will be drawn through the valve -K into the hot cistern B. The air will escape in bubbles through -the water in that cistern, and the warm water will be deposited in -it. - -The magnitude of the opening in the condensing valve E, must be -regulated by the quantity of steam admitted to the cylinder. As -much water ought to be supplied through the injection valve as -will be sufficient to condense the steam contained in the -cylinder, and also to reduce the temperature of the water itself, -when mixed with the steam, to a sufficiently low degree to prevent -it from producing vapour of a pressure which would injuriously -affect the working of the piston. It has been shown, that five and -a half cubic inches of ice-cold water mixed with one cubic inch of -water in the state of steam would produce six and a half cubic -inches of water at the boiling temperature. If then the cylinder -contained one cubic inch of water in the state of steam, and only -five and a half cubic inches of water were admitted through the -condensing jet, supposing this water, when admitted, to be at the -temperature of 32°, then the consequence would be that six and a -half cubic inches of water at the boiling temperature would be -produced in the condenser. Steam would immediately arise from -this, and at the same time the temperature of the remaining water -would be lowered by the amount of the latent heat taken up by the -steam so produced. This vapour would rise through the open -exhausting valve I, would fill the cylinder below the piston, and -would impair the efficiency of the steam above pressing it down. -The result of the inquiries of Watt respecting the pressure of -steam at different temperatures, showed, that to give efficiency -to the steam acting upon the piston it would always be necessary -to reduce the temperature of the water in the condenser to 100°. - -Let us then see what quantity of water at the common temperature -would be necessary to produce these effects. - -If the latent heat of steam be taken at 1000°, a cubic inch of -water in the state of steam may be considered for the purposes of -this computation, as equivalent to one cubic inch of water at -1212°. Now the question is, how many cubic inches of water at 60° -must be mixed with this, in order that the [Pg141] mixture may -have the temperature of 100°? This will be easily computed. As the -cubic inch of water at 1212° is to be reduced to 100°, it must be -deprived of 1112° of its temperature. On the other hand, as many -inches of water at 60° as are to be added, must be raised in the -same mixture to the temperature of 100°, and therefore each of -these must receive 40° of temperature. The number of cubic inches -of water necessary to be added will therefore be determined by -finding how often 40° are contained in 1112°. If 1112 be divided -by 40, the quotient will be 27·8. Hence it appears, that to reduce -the water in the condenser to the temperature of 100°, supposing -the temperature of the water injected to be 60°, it will be -necessary to supply by the injection cock very nearly twenty-eight -times as much water as passes through the cylinder in the state of -steam; and therefore if it be supposed that all the water -evaporated in the boiler passes through the cylinder, it follows -that about twenty-eight times as much water must be thrown into -the condenser as is evaporated in the boiler. - -From these circumstances it will be evident that the cold cistern -in which the condenser and air-pump are submerged, must be -supplied with a considerable quantity of water. Independently of -the quantity drawn from it by the injection valve, as just -explained, the water in the cistern itself must be kept down to a -temperature of about 60°. The interior of the condenser and -air-pump being maintained by the steam condensed in them at a -temperature not less than 100°; the outer surfaces of these -vessels consequently impart heat to the water in the cold cistern, -and have therefore a tendency to raise the temperature of that -water. To prevent this, a pump called the _cold pump_, represented -at L in _fig._ 21., is provided. By this pump water is raised from -any convenient reservoir, and driven through proper tubes into the -cold cistern. This cold pump is wrought by the engine, the rod -being attached to the beam. Water being, bulk for bulk, heavier -the lower its temperature, it follows that the water supplied by -the cold pump to the cistern will have a tendency to sink to the -bottom, pressing upwards the warmer water contained in it. A -waste-pipe is provided, by which this [Pg142] water is drained -off, and the cistern therefore maintained at the necessary -temperature. - -From what has been stated, it is also evident that the hot well B, -into which the warm water is thrown by the air-pump, will receive -considerably more water than is necessary to feed the boiler. A -waste-pipe, to carry off this, is also provided; and the quantity -necessary to feed the boiler is pumped up by a small pump, O, the -rod of which is attached to the beam, as represented in _fig._ -21., and which is worked by the engine. The water raised by this -pump is conducted to a reservoir from which the boiler is fed, by -means which will be hereafter explained. - -We shall now explain the manner in which the machine is made to -open and close the valves at the proper times. By referring to the -explanation already given, it will be perceived that at the moment -the piston reaches the top of the cylinder, the upper steam valve -G must be open, to admit the steam to press it down; while the -exhausting valve I must be opened, to allow the steam to pass to -the condenser; and the condensing valve E must be opened, to let -in the water necessary for the condensation of the steam; and at -the same time the lower steam valve H must be closed, to prevent -the passage of the steam which has been admitted through G. The -valves G, I, and E must be kept open, and the valve H kept closed, -until the piston arrives at the bottom of the cylinder, when it -will be necessary to close all the three valves, G, I, and E, and -to open the valve H, and the same effects must be produced each -time the piston arrives at the top and bottom of the cylinder. All -this is accomplished by a system of levers, which are exhibited in -_fig._ 21. The pivots on which these levers play are represented -on the framing of the engine, and the arms of the levers G', H', -and I', communicating with the corresponding valves G, H, and I, -are represented opposite a bar attached to the rod of the -air-pump, called the _plug frame_. This bar carries certain pegs -and detents, which act upon the arms of the several levers in such -a manner that, on the arrival of the beam at the extremities of -its play upwards and downwards, the levers are so struck that the -valves are opened and closed at the proper [Pg143] times. It is -needless to explain all the details of this arrangement. Let it be -sufficient, as an example of all, to explain the method of working -the upper steam valve G. When the piston reaches the top of the -cylinder, a pin strikes the arm of the lever G', and throws it -upwards: this, by means of the system of levers, pulls the arm of -the valve G downwards, by which the upper steam valve is raised -out of its seat, and a passage is opened from the steam pipe to -the cylinder. The valve is maintained in this state until the -piston reaches the bottom of the cylinder, when the arm G' is -pressed downwards, by which the arm G is pressed upwards, and the -valve restored to its seat. By similar methods the levers -governing the other three valves, H, I, and E, are worked. - -[Illustration: _Fig._ 23.] - -[Illustration: _Fig._ 24.] - -The valves used in these engines were of the kind called _spindle -valves_. They consisted of a flat circular plate of bell metal, A -B, _fig._ 23., with a round spindle passing perpendicularly -through its centre, and projecting above and below it. This valve, -having a conical form, was fitted very exactly, by grinding into a -corresponding circular conical seat, A B C D, _fig._ 24., which -forms the passage which it is the office of the valve to open and -close. When the valve falls into its seat, it fits the aperture -like a plug, so as entirely to stop it. The spindle plays in -sockets or holes, one above and the other below the aperture which -the valve stops; these holes keep the valve in its proper -position, so as to cause it to drop exactly into its place. - -In the experimental engine made by Mr. Watt at Kinneal, he used -cocks, and sometimes sliding covers, like the regulator described -in the old engines; but these he found very soon to become leaky. -He was, therefore, obliged to change them for the spindle valves -just described, which, being truly [Pg144] ground, and accurately -fitted in the first instance, were not so liable to go out of -order. These valves are also called _puppet clacks_, or _button -valves_. - -In the earlier engines constructed by Watt, the condensation was -produced by the contact of cold surfaces, without injection. The -reason of rejecting the method of condensing by injection was, -doubtless, to avoid the injurious effects of the air, which would -always enter the condenser, in combination with the water of -condensation, and vitiate the vacuum. It was soon found, however, -that a condenser acting by cold surfaces without injection, being -necessarily composed of narrow pipes or passages, was liable to -incrustation from bad water, by which the conducting power of the -material of the condenser was diminished; so that, while its outer -surface was kept cold by the water of the cold cistern, the inner -surface might, nevertheless, be so warm that a very imperfect -condensation would be produced. - -[Illustration: SOHO, BIRMINGHAM.] - - FOOTNOTES: - - [19] Eloge, p. 308. - -[Pg145] - - - - -[Illustration: BIRMINGHAM.] - -CHAP. VI. - - CORRESPONDENCE OF WATT WITH SMEATON. -- FAILURE OF CONDENSATION - BY SURFACE. -- IMPROVEMENTS IN CONSTRUCTION OF PISTON. -- METHOD - OF PACKING. -- IMPROVEMENTS IN BORING THE CYLINDERS. -- - DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES. -- - GREATLY INCREASED ECONOMY OF FUEL. -- EXPEDIENTS TO FORCE THE - NEW ENGINES INTO USE. -- CORRESPONDENCE WITH SMEATON. -- - EFFICIENCY OF FUEL IN THE NEW ENGINES. -- DISCOVERY OF THE - EXPANSIVE ACTION OF STEAM. -- WATT STATES IT IN A LETTER TO DR. - SMALL. -- ITS PRINCIPLE EXPLAINED. -- MECHANICAL EFFECT - RESULTING FROM IT. -- COMPUTED EFFECT OF CUTTING OFF STEAM AT - DIFFERENT PORTIONS OF THE STROKE. -- PRODUCES A VARIABLE POWER. - -- EXPEDIENTS FOR EQUALISING THE POWER. -- LIMITATION OF THE - EXPANSIVE PRINCIPLE IN WATT'S ENGINES. -- ITS MORE EXTENSIVE - APPLICATION IN THE CORNISH ENGINES. - - -(77.) In a letter addressed by Watt to Smeaton, dated April, 1766, -Watt refers to some of these practical difficulties which he had -to encounter. "I have been," says he, "tormented with exceedingly -bad health, resulting from the operation of an anxious mind, the -natural consequence of staking everything [Pg146] upon the cast -of a die; for in that light I look upon every project which has -not received the sanction of repeated success. - -"I have made considerable alterations in our engine lately, -particularly in the condenser. That which I used at first was -liable to be impaired, from incrustations from bad water; -therefore we have substituted one which works by an injection. In -pursuing this idea I have tried several kinds, and have at last -come to one, which I am not inclined to alter. It consists of a -jack-head pump, shut at bottom, with a common clack bucket, and a -valve in the cover of the pump, to discharge the air and water. -The eduction steam pipe, which comes from the cylinder, -communicates with this pump both above and below the bucket, and -has valves to prevent anything from going back from the pump to -the eduction pipe. The bucket descends by its own weight, and is -raised by the engine when the great piston descends, being hung to -the outer end of the great lever: the injection is made both into -the upper part of this pump and into the eduction pipe, and -operates beyond my ideas in point of quickness and perfection." - -Besides the difficulty arising from incrustation, Watt found the -tubulated condensers, and indeed all other expedients for -condensing by cold surfaces, subject to a fatal objection. They -did not condense instantaneously, and although they were capable -of ultimately effecting the condensation, yet that process was not -completed until a great part of the stroke of the piston was made. -Thus during more or less of the stroke the uncondensed steam -resisted the piston, and robbed the moving power of a part of its -effect. This objection has ever attended condensation by surface. - -[Illustration: _Fig._ 25.] - -[Illustration: _Fig._ 26.] - - -(78.) Another source of difficulty arose from the necessity of -constructing the piston and cylinder with greater precision than -had been usual in the old engines. To fit the cover to the -cylinder so as to be steam-tight; to construct the piston rod so -as to move through it without allowing the escape of steam, and -yet at the same time without injurious friction; to connect the -piston rod with the piston, so as to drive the [Pg147] latter -through the cylinder with a perfectly straight and parallel -motion; to make such connection perfectly centrical and firm, and -yet to allow the piston in its ascent to come nearly into contact -with the cover of the cylinder--were all difficulties peculiar to -the new engine. In the atmospheric engine the shank of the piston -rod was rough and square, and the rod was secured to the piston by -two or four branches or stays, as represented in _fig._ 25. It is -evident that such a construction would be inadmissible in an -engine in which the piston in its ascent must be brought nearly -into contact with the close cover of the cylinder. Besides this -the piston rod of an atmospheric engine might throughout its whole -length have any form which was most convenient, and required no -other property than the strength necessary to work the beam. In -the new engine, on the contrary, it was necessary that it should -be accurately turned and finely polished, so as to pass through -the hole in the top of the cylinder, and be maintained in it -steam-tight. This was effected by a contrivance called a -_stuffing-box_ B, represented in _fig._ 26. A hole is made in the -cover of the cylinder very little greater in magnitude than the -diameter of the piston rod. Above this hole is a cup in which, -around the piston, is placed a stuffing of hemp or tow, which is -saturated with oil or melted tallow. This collar of hemp is -pressed down by another piece, also perforated with a hole through -which the piston rod plays, and which is screwed down on the said -collar of hemp. - - -(79.) Although the imperfect manner in which the interior of the -cylinders was then formed impaired the efficiency of the [Pg148] -new engines, yet such imperfections were not so injurious as in -the old atmospheric engines. Any imperfection of form of the inner -surface of the cylinder would necessarily cause more or less steam -or air to escape between the piston and cylinder. In the improved -engine this steam passing into the vacuum below the piston would -rush into the condenser, and be there condensed, so that its -effect in resisting the motion of the piston would necessarily be -trifling. But on the other hand, any escape of air between the -piston and cylinder of an atmospheric engine would introduce an -elastic fluid under the piston, which would injuriously affect the -action of the machine. - -[Illustration: _Fig._ 27.] - -To make the pistons move sufficiently steam-tight in these early -imperfect cylinders, Watt contrived a packing formed of a collar -of hemp, or tow, as represented in _fig._ 27. The bottom of the -piston was formed of a circular plate of a diameter nearly, but -not altogether equal to the interior diameter of the cylinder. The -part of the piston above this was considerably less in diameter, -so that the piston was surrounded by a circular groove or channel -two inches wide, into which hemp or soft rope, called _gasket_, -was run, so as to form the packing. The top of the piston was -placed over this, having a rim or projecting part, which entered -the circular groove and pressed upon the packing, the cover being -pressed downwards by screws passing through the piston. The lower -part of the groove round the piston was rounded with a curve, so -that the pressure on the packing might force the latter against -the inner surface of the cylinder. This packing was kept supplied -with melted tallow, as already described, from the funnel, screwed -into the top of the cylinder. The metallic edges of the piston -were by this means prevented from coming into contact with the -surface of the cylinder, which was only pressed upon by the -stuffing or packing projecting beyond these. - - -(80.) Improved methods of boring soon, however, relieved [Pg149] -the engine from a part of these imperfections, and Watt writes to -Mr. Smeaton in the letter above quoted as follows:-- - -"Mr. Wilkinson has improved the art of boring cylinders; so that I -promise, upon a 72 inch cylinder, being not further distant from -absolute truth than the thickness of a thin sixpence in the worst -part. I am labouring to improve the regulators; my scheme is to -make them acute conical valves, shut by a weight, and opened by -the force of the steam. They bid fair for success, and will be -tried in a few days." - -The person here alluded to was Mr. John Wilkinson, of Bersham near -Chester, who, about the year 1775, contrived a new machine for -accurately boring the insides of cylinders. The cylinder being -first obtained from the foundery with a surface as accurate as the -process of casting would admit, had its inner surface reduced to -still greater accuracy by this machine, which consists of a -straight central bar extended along the axis of the cylinder, -which was made to revolve slowly round it. During the operation of -boring, the borer or cutter was fitted to slide along this bar, -which being perfectly straight, served as a sort of ruler to guide -the borer or cutter in its progress through the cylinder. In this -manner the interior surface of the cylinder was rendered not only -true and straight in its longitudinal direction, but also -perfectly circular in its cross section. - -The grease found to be most eligible for lubrication was the -tallow of beef or mutton; but in the earlier cylinders this was -soon consumed by reason of the imperfection of the boring, and the -piston being left dry ceased to be steam-tight. To prevent this, -Watt sought for some substance, which while it would thicken the -tallow, and detain it around the piston, would not be subject to -decomposition by heat. Black lead dust was used for this purpose, -but was soon found to wear the cylinder. In the mean while, -however, the improved method of boring supplied cylinders which -rendered this expedient unnecessary. - -When the inner surface of the cylinder is perfectly true and -smooth, the packing of the piston is soon rendered solid and hard, -being moulded to the cylinder by working, so as to fit it -perfectly. When by wear it became loose, it was [Pg150] only -necessary to tighten the screws by which the top and bottom of the -piston were held together. The packing being compressed by those -means, was forced outwards towards the surface of the cylinder, so -as to be rendered steam-tight. - - -(81.) It was not until about the year 1778, nine years after the -date of the patent, and thirteen after the invention of separate -condensation, that any impression was produced on the mining -interests by the advantages which were presented to them by these -vast improvements. This long interval, however, had not elapsed -without considerable advantage; for although all the great leading -principles of the contrivance were invented so early as the year -1765, yet the details of construction had been in a state of -progressive and continued improvement from the time Watt joined -Dr. Roebuck, in 1769, to the period now adverted to. - -The advantages which the engine offered in the form in which it -has been just described, were numerous and important, as compared -even with the most improved form of the atmospheric engine; and it -should be remembered, that that machine had also gone on -progressively improving, and was probably indebted for some of its -ameliorations to hints derived from the labours of Watt, and to -the adoption of such of his expedients as were applicable to this -imperfect machine, and could be adopted without an infraction of -his patent. - -In the most improved forms to which the atmospheric engine had -then attained, the quantity of steam wasted at each stroke of the -piston was equal to the contents of the cylinder. Such engines, -therefore, consumed twice the fuel which would be requisite, if -all sources of waste could have been removed. In Watt's engines, -the steam consumed at each stroke of the piston amounted only to -1-1/4 times the contents of the cylinder. The waste steam, -therefore, per stroke, was only a quarter of what was usefully -employed. The absolute waste, therefore, of the best atmospheric -engines was four times that of the improved engine, and -consequently the saving of fuel in the improved engines amounted -to about three eighths of all the fuel consumed in atmospheric -engines of the same power. [Pg151] - - -(82.) But independently of this saving of steam, which would -otherwise be wasted, the power of Watt's engine, as compared with -the atmospheric engine, was so much augmented that the former -would work against a resistance of ten pounds on the square inch -under the same circumstances in which the latter would not move -against more than seven pounds. The cause of this augmentation of -power is easily explained. In the atmospheric engine the -temperature of the condensed steam could not be reduced below 152° -without incurring a greater loss than would be compensated by the -advantage to be obtained from any higher degree of condensation. -Now steam raised from water at 152° has a pressure of nearly four -pounds per square inch. This pressure, therefore, acted below the -piston resisting the atmospheric pressure above. In Watt's engine, -however, the condenser was kept at a temperature of about 100°, at -which temperature steam has a pressure of less than one pound per -square inch. A resisting force upon the piston of three pounds per -square inch was therefore saved in Watt's engine as compared with -the atmospheric engine. - - -(83.) Besides these direct sources of economy, there were other -advantages incidental to Watt's engine. An atmospheric engine -possessed very limited power of adaptation to a varying load. The -moving power being the atmospheric pressure, was not under -control, and, on the other hand, was subject to variations from -day to day and from hour to hour, according to the changes of the -barometer. In the first construction of such an engine, therefore, -its power being necessarily adapted to the greatest load which it -would have to move, whenever the load upon its pumps was -diminished, the motion of the piston in descending would be -rapidly accelerated in consequence of the moving power exceeding -the resistance. By this the machinery would be subject to sudden -shocks, which were productive of rapid wear, and exposed the -machinery to the danger of fracture. To remedy this inconvenience, -the following expedient was provided in the atmospheric engine: -whenever the load on the engine was materially diminished, the -quantity of water admitted through the injection valve to condense -the steam was proportionally [Pg152] diminished. An imperfect -condensation being therefore produced, vapour remained in the -cylinder under the piston, the pressure of which resisted the -atmosphere, and mitigated the force of the machine. Besides this, -a cock was provided in the bottom of the cylinder, called an _air -cock_, by which atmospheric air could be admitted to resist the -piston whenever the motion was too rapid. - -These expedients, however, were all attended with a waste of fuel -in relation to the work done by the engine; for it is evident that -the consumption of steam was necessarily the same, whether the -engine was working against its full load or against a reduced -resistance. - -On the other hand, in the improved engine of Watt, when the load, to -work against which the engine exerted its full power, was -diminished, a cock or valve was provided in the steam pipe leading -from the boiler, which was called a _throttle valve_, by adjusting -which the passage in that pipe could be more or less contracted. By -regulating this cock the supply of steam from the boiler was -checked, and the quantity transmitted to the cylinder diminished, so -that its effect upon the piston might be rendered equal to the -amount of the diminished resistance. By this means the quantity of -steam transmitted to the cylinder was rendered exactly proportional -to the work which the engine had to perform. If, under such -circumstances, the boiler was worked to its full power, so as to -produce steam as fast as it would when the engine was working at -full power, then no saving of fuel would be effected, since the -surplus steam produced in the boiler would necessarily escape at the -safety valve. But in such case the fireman was directed to limit the -fuel of the furnace until the discharge at the safety valve ceased. - -By these expedients, the actual consumption of fuel in one of -these improved engines was always in the exact proportion of the -work which it performed, whether it worked at full power or at any -degree under its regular power. - - -(84.) Notwithstanding these and other advantages attending the new -engines, Boulton and Watt experienced difficulties all but -insurmountable in getting them into use. No manufactory existed in -the country possessing machinery capable of [Pg153] executing -with the necessary precision the valves and other parts which -required exact execution, and the patentees were compelled to -construct machinery at Soho for this purpose; and even after they -succeeded in getting the cylinders properly bored, the piston rods -exactly turned and polished, the spindle valves constructed so as -to be steam-tight, and every other arrangement completed which was -necessary for the efficiency of the machine, the novelty of the -engine, and the difficulty which was supposed to attend its -maintenance in good working order, formed strong objections to its -adoption. - -To remove such objections, great sacrifices were necessary on the -part of Boulton and Watt; and they accordingly resolved to -undertake the construction of the new engines without any profit, -giving them to the parties requiring their use at first cost, on -the condition of being remunerated by a small share of what they -would save in fuel. - -"We have no objection," writes Mr. Boulton, "to contract with the -Carron Company to direct the making of an engine to return the -water for their mills. * * * * We do not aim at profits in engine -building, but shall take our profits out of the saving of fuel; so -that if we save nothing, we shall take nothing. Our terms are as -follows: we will make all the necessary plans, sections, and -elevations for the building, and for the engine with its -appurtenances, specifying all cast and forged iron work, and every -other particular relative to the engine. We will give all -necessary directions to your workmen, which they must implicitly -obey. We will execute, for a stipulated price, the valves, and all -other parts which may·require exact execution, at Soho; we will -see that all the parts are put together, and set to work, -properly; we will keep our own work in repair for one year, and we -have no other objection to seven years than the inconvenience of -the distance. We will guarantee that the engine so constructed -shall raise at least 20,000 cubic feet of water twenty-four feet -high with each hundred weight of coals burnt. - -"When all this is done, a fair and candid comparison shall be made -between it, and your own engine, or any other engine in Scotland, -from which comparison the amount of savings in fuel shall be -estimated, and that amount being [Pg154] divided into three -parts, we shall be entitled to one of those parts, in recompense -for our patent licence, our drawings, &c. &c. Our own share of -savings shall be estimated in money, according to the value of -your coals delivered under the boiler, and you shall annually pay -us that sum, during twenty-five years from the day you begin to -work; provided you continue the use of the engine so long. And in -case you sell the engine, or remove it to any other place, you -must previously give us notice, for we shall then be entitled to -our third of the savings of fuel, according to the value of coals -at such new place. This is a necessary condition, otherwise the -engine which we make for you at an expense of two thousand pounds -may be sold in Cornwall for ten thousand pounds. - -"Such parts of the engine as we execute at Soho we will be paid -for at a fair price; I conclude, from all the observations I have -had an opportunity of making, that our engines are four times -better than the common engines. In boilers, which are a very -expensive article, the savings will be in proportion to the -savings of coal. If you compare our engine with the common engine -(not in size, but in power), you will find the original expense of -erecting one to be nearly the same. - -"Mr. Wilkinson has bored us several cylinders, almost without -error; that of fifty inches diameter, which we put up at Tipton, -does not err the thickness of an old shilling in any part; so that -you must either improve your method of boring, or we must furnish -the cylinder to you." - -The reluctance of mining companies to relinquish the old engines, -even on these terms, led them to propose to Mr. Watt to grant them a -licence for the use of his condenser, to be applied to the -atmospheric engine, without the introduction of other improvements. -Such a proposition was made to him by Mr. Smeaton, in the year 1778, -to which he returned the following answer:-- - -"I have several times considered the propriety of the application -of my condensers to common engines, and have made experiments with -that view upon our engine at Soho, but have never found such -results as would induce me to try [Pg155] it any where else; and, -in consequence, we refused to make that application to Wheal -Virgin engines in Cornwall, and to some others; our reasons were, -that though it might have enabled them to have gone deeper with -their present engines, yet, the savings of fuel would not have -been great, in comparison to the complete machine. By adding -condensers to engines that were not in good order, our engine -would have been introduced into that country (which we look upon -as our richest mine) in an unfavourable point of view, and without -such profits as would have been satisfactory either to us or to -the adventurers; and if we had granted the use of condensers to -one, we must have done so to all, and thereby have curtailed our -profits, and perhaps injured our reputation. Besides, where a new -engine is to be erected, and to be equally well executed in point -of workmanship and materials, an engine of the same power cannot -be constructed materially cheaper on the old plan than on ours; -for our boiler and cylinder are much smaller, and the building, -the lever, the chains, together with all the pump and pit work, -are only the same. * * * * - -"We charge our profits in proportion to the saving made in fuel by -our engine, when compared with a common one which burns the same -kind of coals; we ask one third of these savings to be paid us -annually, or half yearly; the payment being redeemable in the -option of our employer, at ten years' purchase; and when the coals -are low priced, we should also make some charge as engineers. In -all these comparisons our own interest has made us except your -(Mr. Smeaton) improved engines, unless we were allowed a greater -proportion of the savings." - -Their exertions to improve the manufacture of engines at Soho is -shown by the following letter from Mr. Boulton, in the same -correspondence to Mr. Smeaton:-- - -"We are systematising the business of engine making, as we have -done before in the button manufactory; we are training up workmen, -and making tools and machines to form the different parts of Mr. -Watt's engines with more accuracy, and at a cheaper rate than can -possibly be done by the ordinary methods of working. Our workshop -and apparatus will be of [Pg156] sufficient extent to execute all -the engines which are likely to be soon wanted in this country; -and it will not be worth the expense for any other engineers to -erect similar works, for that would be like building a mill to -grind a bushel of corn. - -"I can assure you from experience, that our small engine at Soho -is capable of raising 500,000 cubic feet of water 1 foot high with -every 112 lbs. of coals, and we are in hopes of doing much more. -Mr. Watt's engine has a very great advantage in mines, which are -continually working deeper: suppose, for instance, that a mine is -50 fathoms deep, you may have an engine which will be equal to -draining the water when the mine is worked, to 100 fathoms deep, -and yet you can constantly adapt the engine to its load, whether -it be 50 or 100 fathoms, or any intermediate depth; and the -consumption of coals will be less in proportion when working at -the lesser than at the greater depths; supposing it works, as our -engines generally do, at 11 lbs. per square inch, when the mine -becomes 100 fathoms deep." - - -(85.) The great improvement which has been introduced within the -last half century, in the details of Watt's steam engine, will be -rendered manifest by comparing the effects of a given weight of -fuel here supplied by Mr. Boulton with the effects which the same -weight of fuel is now known to produce in the best pumping engines -worked in Cornwall. One of these engines, in good working order, -has been known to raise 125,000,000 lbs. 1 foot high, by the -combustion of a bushel of coals. But the average performance of -even the best engines is below this amount. If we take it at -90,000,000, this will be equivalent to the weight of about 1-1/2 -million cubic feet of water, a bushel of coals being 3/4 cwt. It -will therefore follow that, with the present engines, one hundred -weight of coals is capable of raising about two million cubic feet -of water one foot high, being a duty four times that assigned to -the early engines by Mr. Boulton. - - -(86.) At the time that Watt, in conjunction with Dr. Roebuck, -obtained the patent for his improved engine, the idea occurred to -him, that the steam which had impelled the piston in its descent -rushed from the cylinder with a mechanical force much more than -sufficient to overcome any resistance [Pg157] which it had to -encounter in its passage to the condenser; and that such force -might be rendered available as a moving power, in addition to that -already obtained from the steam during the stroke of the piston. -This notion involved the whole principle of the expansive action -of steam, which subsequently proved to be of such importance in -the performance of steam engines. Watt was, however, so much -engrossed at that time, and subsequently, by the difficulties he -had to encounter in the construction of his engines, that he did -not attempt to bring this principle into operation. It was not -until after he had organised that part of the establishment at -Soho which was appropriated to the manufacture of steam engines, -that he proceeded to apply the expansive principle. Since the date -of the patent which he took out for this (1782), was subsequent to -the application of the same principle by another engineer, named -Hornblower, it is right to state, that the claim of Mr. Watt to -this important step in the improvement of the steam engine, is -established by a letter addressed by him to Dr. Small, of -Birmingham, dated Glasgow, May, 1769:-- - -"I mentioned to you a method of still doubling the effect of the -steam, and that tolerably easy, by using the power of steam -rushing into a vacuum, at present lost. This would do little more -than double the effect, but it would too much enlarge the vessels -to use it all: it is peculiarly applicable to wheel engines, and -may supply the want of a condenser, where the force of steam only -is used; for open one of the steam valves, and admit steam until -one fourth of the distance between it and the next valve is filled -with steam, then shut the valve, and the steam will continue to -expand, and to press round the wheel, with a diminishing power, -ending in one fourth of its first exertion. The sum of the series -you will find greater than one half, though only one fourth of -steam was used. The power will indeed be unequal, but this can be -remedied by a fly, or by several other means." - -In 1776 the engine, which had been then recently erected at Soho, -was adapted to act upon the principle of expansion. When the -piston had been pressed down in the cylinder for a certain portion -of the stroke, the further supply of steam [Pg158] from the -boiler was cut off, by closing the upper steam valve, and the -remainder of the stroke was accomplished by the expansive power of -the steam which had already been introduced into the cylinder. - - -(87.) To make this method of applying the force of steam -intelligible, some previous explanation of mechanical principles -will be necessary. - -If a body which offers a certain resistance be urged by a certain -moving force, the motion which it will receive will depend on the -relation between the energy of the moving force and the amount of -the resistance opposed to it. If the moving force be precisely -equal to the resistance, the motion which the body will receive -will be perfectly uniform. - -If the energy of the moving force be greater than the resistance, -then its surplus or excess above the amount of resistance will be -expended in imparting momentum to the mass of the body moved, and -the latter will, consequently, continually acquire augmented -speed. The motion of the body will, therefore, be in this case -accelerated. - -If the energy of the moving force be less in amount than the -resistance, then all that portion of the resistance which exceeds -the amount of the moving force will be expended in depriving the -mass of the body of momentum, and the body will therefore be moved -with continually diminished speed until it be brought to rest. - - -(88.) Whenever, therefore, a uniform motion is produced in a body, -it may be taken as an indication of the equality of the moving -force to the resistance; and, on the other hand, according as the -speed of the body is augmented or diminished, it may be inferred -that the energy of the moving force has been greater or less than -the resistance. - -It is an error to suppose that rest is the only condition possible -for a body to assume when under the operation of two or more -mechanical forces which are in equilibrium. By the laws of motion -the state of a body which is not under the operation of any -external force must be either in a state of rest or of uniform -motion. Whichever be its state, it will suffer no change if the -body be brought under the operation of two or more forces which -are in equilibrium; for to suppose [Pg159] such forces to produce -any change in the state of the body, whether from rest to motion, -or _vice versâ_, or in the velocity of the motion which the body -may have previously had, would be equivalent to a supposition that -the forces applied to the body being in equilibrium were capable -of producing a dynamical effect, which would be a contradiction in -terms. This, though not always clearly understood by mere -practical men, or by persons superficially informed, is, in fact, -among the fundamental principles of mechanical science. - - -(89.) When the piston is at the top of the cylinder, and about to -commence its motion downwards, the steam acting upon it will have -not only to overcome the resistance arising from the friction of -the various parts of the engine, but will also have to put in -motion the whole mass of matter of the piston pump rods, pump -pistons, and the column of water in the pump barrels. Besides -imparting to this mass the momentum corresponding to the velocity -with which it will be moved, it will also have to encounter the -resistance due to the preponderance of the weight of the water and -pump rods over that of the steam piston. The pressure of steam, -therefore, upon the piston at the commencement of the stroke must, -in accordance with the mechanical principles just explained, have -a greater force than is equal to all the resistances which it -would have to overcome, supposing the mass to be moving at a -uniform velocity. The moving force, therefore, being greater than -the resistance, the mass, when put in motion, will necessarily -move with a gradually augmented speed, and the piston of the -engine which has been described in the last chapter would -necessarily move from the top to the bottom of the cylinder with -an accelerated motion, having at the moment of its arrival at the -bottom a greater velocity than at any other part of the stroke. As -the piston and all the matter which it has put in motion must at -this point come to rest, the momentum of the moving mass must -necessarily expend itself on some part of the machinery, and would -be so much mechanical force lost. It is evident, therefore, -independently of any consideration of the expansive principle, to -which we shall presently refer, that the action of the [Pg160] -moving power in the descent of the piston ought to be suspended -before the arrival of the piston at the bottom of the cylinder, in -order to allow the momentum of the mass which is in motion to -expend itself, and to allow the piston to come gradually to rest -at the termination of the stroke. - -Thus, if we were to suppose that after the piston had descended -through three fourths of the whole length of the cylinder, and had -acquired a certain velocity, the steam above it were suddenly -condensed, so as to leave a vacuum both above and below it, the -piston, being then subject to no impelling force, would still move -downwards, in virtue of the momentum it had acquired, until the -resistance would deprive it of that momentum, and bring it to -rest; and if the remaining fourth part of the cylinder were -necessary for the accomplishment of this, then it is evident that -that part of the stroke would be accomplished without further -expenditure of the moving power. - -In fact, this part of the stroke would be made by the expenditure -of that excess of moving power, which, at the commencement of the -stroke, had been employed in putting the machinery and its load in -motion, and in subsequently accelerating that motion. - -Although under such circumstances the resistance, during the -operation of the moving power, shall not have been at any time -equal to the moving power, since while the motion was accelerated -it was less, and while retarded greater than that power, yet as -the whole moving power has been expended upon the resistance, the -mechanical effect which the moving power has produced under such -circumstances will be equal to the actual amount of that power. If -in an engine of this kind the steam was not cut off till the -conclusion of the stroke, a part of the moving power would be lost -upon those fixed points in the machinery which would sustain the -shock produced by the instantaneous cessation of motion at the end -of the stroke. - -Independently, therefore, of any consideration of the expansive -principle, it appears that, in an engine of this kind, the steam -ought to be cut off before the completion of the stroke. [Pg161] - -[Illustration: _Fig._ 28.] - - -(90.) To render the expansive action of steam intelligible, let A -B (_fig._ 28.) represent a cylinder whose area we will suppose, -for the sake of illustration, to be a square foot, and whose -length, A B, shall also be a foot. If steam of a pressure equal to -the atmosphere be supplied to this cylinder, it will exert a -pressure of about one ton on the piston; and if such steam be -uniformly supplied from the boiler, the piston will be moved from -A to B with the force of one ton, and that motion will be uniform -if the piston be opposed throughout the same space by a resistance -equal to a ton. When the piston has arrived at B, let us suppose -that the further supply of steam from the boiler is stopped by -closing the upper steam valve, and let us also suppose the -cylinder to be continued downwards so that B C shall be equal to A -B, and suppose that B C has been previously in communication with -the condenser, and is therefore a vacuum. The piston at B will -then be urged with a force of one ton downwards, and as it -descends the steam above it will be diffused through an increased -volume, and will consequently acquire a diminished pressure. We -shall, for the present, assume that this diminution of pressure -follows the law of elastic fluids in general; that it will be -decreased in the same proportion as the volume of the steam is -augmented. While the piston, therefore, moves from B downwards it -will be urged by a continually decreasing force. Let us suppose, -that by some expedient, it is also subject to a continually -decreasing resistance, and that this resistance decreases in the -same proportion as the force which urges the piston. In that case -the motion of the piston would continue uniform. When the piston -would arrive at P', the middle of the second cylinder, then the -space occupied by the steam being increased in the proportion of 2 -to 3, the pressure on the piston would be diminished in the -proportion of 3 to 2, and the pressure at B being one ton, it -would be two-thirds of a ton at P'. In like manner when the piston -would arrive at C, the space occupied by the steam being double -that which [Pg162] it occupied when the piston was at B, the -pressure of the steam would be half its pressure at B, and -therefore at the termination of the stroke, the pressure on the -piston would be half a ton. - -If the space from B to C, through which the steam is here supposed -to act expansively, be divided into ten equal parts, the pressure -on the piston at the moment of passing each of those divisions -would be calculated upon the same principle as in the cases now -mentioned. After moving through the first division, the volume of -the steam would be increased in the proportion of 10 to 11, and -therefore its pressure would be diminished in the proportion of 11 -to 10. The pressure, therefore, driving the piston at the end of -the first of these ten divisions would be 10/11ths of a ton. In -like manner, its pressure at the second of the divisions would be -10/12ths of a ton, and the third 10/13ths of a ton; and so on, as -indicated in the figure. - -Now if the pressure of the steam through each of these divisions -were to continue uniform, and, instead of gradually diminishing, -to suffer a sudden change in passing from one division to another, -then the mechanical effect produced from B to C would be obtained -by taking a mean or average of the several pressures throughout -each of the ten divisions. In the present case it has been -supposed that the force on the piston at B was 2240 pounds. To -obtain the pressure in pounds corresponding to each of the -successive divisions, it will therefore only be necessary to -multiply 2240 by 10, and to divide it successively by 11, 12, 13, -&c. The pressures, therefore, in pounds, at each of the ten -divisions, will be as follows:-- - - 1st 2036·3 - 2d 1866·6 - 3d 1723·1 - 4th 1600·0 - 5th 1493·3 - 6th 1400·0 - 7th 1317·6 - 8th 1244·4 - 9th 1179·0 - 10th 1120·0 - -If the mean of these be taken by adding them together [Pg163] and -dividing by 10, it will be found to be 1498 pounds. It appears, -therefore, that the pressures through each of the ten divisions -being supposed to be uniform (which however, strictly, they are -not,) the mechanical effect of the steam from B to C would be the -same as if it acted uniformly throughout that space upon the -piston with a force of about 1500 pounds, being rather less than -three-fourths of its whole effect from A to B. - -But it is evident that this principle will be equally applicable -if the second cylinder had any other proportion to the first. Thus -it might be twice the length of the first; and in that case, a -further mechanical effect would be obtained from the expansion of -the steam. - -The more accurate method of calculating the effect of the -expansion from B to C, would involve more advanced mathematical -principles than could properly be introduced here; but the result -of such a computation would be that the actual average effect of -the steam from B to C would be equal to a uniform pressure through -that space, amounting to one thousand five hundred and forty-five -pounds, being greater than the result of the above computation, -the difference being due to the expansive action through each of -the ten divisions, which was omitted in the above computation. - - -(91.) It is evident that the expansive principle, as here explained, -involves the condition of a variation in the intensity of the moving -power. Thus, if the steam act with a uniform energy on the piston so -long as its supply from the boiler continues, the moment that supply -is stopped, by closing the steam valve, the steam contained in the -cylinder will fill a gradually increasing volume by the motion of -the piston, and therefore will act above the piston with a gradually -decreasing energy. If the resistance to the moving power produced by -the load, friction, &c. be not subject to a variation corresponding -precisely to such variation in the moving power, then the -consequence must be that the motion imparted to the load will cease -to be uniform. If the energy of the moving power at any part of the -stroke be greater than the resistance, the motion produced will be -accelerated; if it be less, the motion will be retarded; and if it -be at one time greater, and another [Pg164] time less, as will -probably happen, then the motion will be alternately accelerated and -retarded. This variation in the speed of the body moved will not, -however, affect the mechanical effect produced by the power, -provided that the momentum imparted to the moving mass be allowed to -expend itself at the end of the stroke, so that the piston may be -brought to rest as nearly as possible by the resistance of the load, -and not by any shock on any fixed points in the machine. This is an -object which, consequently, should be aimed at with a view to the -economy of power, independently of other considerations connected -with the wear and tear of the machinery. So long as the engine is -only applied to the operation of pumping water, great regularity of -motion is not essential, and, therefore, the variation of speed -which appears to be an almost inevitable consequence of any -extensive application of the expansive principle, is of little -importance. In the patent which Watt took out for the application of -the expansive principle, he specified several methods of producing a -uniform effect upon a uniform resistance, notwithstanding the -variation of the energy of the power which necessarily attended the -expansion of the steam. This he proposed to accomplish by various -mechanical means, some of which had been previously applied to the -equalisation of a varying power. One consisted in causing the piston -to act on a lever, which should have an arm of variable length, the -length increasing in the same proportion as the energy of the moving -power diminished. This was an expedient which had been already -applied in mechanics for the purpose of equalising a varying power. -A well-known example of it is presented in the main-spring and fuzee -of a watch. According as the watch goes down, the main-spring -becomes relaxed, and its force is diminished; but, at the same time, -the chain by which it drives the fuzee acts upon a wheel or circle, -having a diameter increased in the same proportion as the energy of -the spring is diminished. - -Another expedient consisted in causing the moving power, when -acting with greatest energy, to lift a weight which should be -allowed to descend again, assisting the piston when the energy of -the moving force was diminished. [Pg165] - -Another method consisted in causing the moving force, when acting -with greatest energy, to impart momentum to a mass of inert -matter, which should be made to restore the same force when the -moving power was more enfeebled. We shall not more than allude -here to these contrivances proposed by Watt, since their -application has never been found advantageous in cases where the -expansive principle is used. - - -(92.) The application of the expansive principle in the engines -constructed by Boulton and Watt, was always very limited, by -reason of their confining themselves to the use of steam having a -pressure not much exceeding that of the atmosphere. If the -principle of expansion, as above explained, be attentively -considered, it will be evident that the extent of its application -will mainly depend on the density and pressure of the steam -admitted from the boiler. If the density and pressure be not -considerable when the steam is cut off, the extent of its -subsequent expansion will be proportionally limited. It was in -consequence of this, that this principle from which considerable -economy of power has been derived, was applied with much less -advantage by Mr. Watt than it has since been by others, who have -adopted the use of steam of much higher pressure. In the engines -of Boulton and Watt, where the expansive principle was applied, -the steam was cut off after the piston had performed from one half -to two thirds of the stroke, according to the circumstances under -which the engine was worked. The decreasing pressure produced by -expansion was, in this case, especially with the larger class of -engines, little more than would be necessary to allow the momentum -of the mass moved to spend itself, before the arrival of the -piston at the end of the stroke. - -Subsequently, however, boilers producing steam of much higher -pressure were applied, and the steam was cut off when the piston -had performed a much smaller part of the whole stroke. The great -theatre of these experiments and improvements has been the mining -districts in Cornwall, where, instead of working with steam of a -pressure not much exceeding that of the atmosphere, it has been -found advantageous to use steam whose pressure is at least four -times as great as [Pg166] that of the atmosphere; and instead of -limiting its expansion to the last half or fourth of the stroke, -it is cut off after the piston has performed one fourth part of -the stroke or less, all the remainder of the stroke being -accomplished by the expansive power of the steam, and by momentum. - -[Illustration: BRIDGE OVER THE CLYDE AT HAMILTON, DESIGNED BY -WATT.] - -[Pg167] - - - - -[Illustration: DOUBLE-ACTING ENGINE, ZINC WORKS, CITY ROAD, -LONDON.] - -CHAP. VII. - - PROPERTIES OF STEAM. -- COMMON STEAM. -- SUPERHEATED STEAM. -- - LAW OF DALTON AND GAY LUSSAC. -- LAW OF MARIOTTE. -- RELATION - BETWEEN TEMPERATURE AND PRESSURE OF COMMON STEAM. -- EFFECTS OF - THE EXPANSION OF COMMON STEAM. -- MECHANICAL EFFECTS OF STEAM. - -- METHOD OF EQUALISING THE EXPANSIVE FORCE. -- HORNBLOWER'S - ENGINE. -- WOOLF'S ENGINE. -- WATT'S ATTEMPTS TO EXTEND THE - STEAM ENGINE TO MANUFACTURES. -- PAPIN'S PROJECTED APPLICATIONS - OF THE STEAM ENGINE. -- SAVERY'S APPLICATIONS OF THE ENGINE TO - MOVE MACHINERY. -- JONATHAN HULL'S APPLICATION TO WATER WHEELS. - -- STEWART'S APPLICATION OF THE ENGINE TO MILL WORK. -- - WASHBOROUGH'S APPLICATION OF THE FLY WHEEL AND CRANK. -- WATT'S - SECOND PATENT. -- DOUBLE-ACTION VALVES. - - -(93.) Since the application of the expansive action of steam -involves the consideration of its properties when it ceases to be -in contact with the water from which it was produced, and likewise -the variation of its pressure in different states of [Pg168] -density and at different temperatures, it is necessary here to -explain some of the most important of these properties of vapour. - -Steam may exist in two states, distinguished from each other by -the following circumstances:-- - -1st. It may be such that the abstraction from it of any portion of -heat, however small, will cause its partial condensation. - -2d. It may be such as to admit of the abstraction of heat from it -without undergoing any other change than that which air would -undergo under like circumstances, viz. a diminution of temperature -and pressure. - - -(94.) We shall call, for distinction, the former _Common Steam_, -and the latter _Superheated Steam_. - -[Illustration: _Fig._ 29.] - -To explain the circumstances out of which these properties arise, -let B (_fig._ 29.) be imagined to be a vessel filled with water, -communicating by a pipe and stopcock with another vessel A, which -in the commencement of the process may be conceived to be filled -with air. Let D be a pipe and stopcock at the top of this vessel. -If the vessel B be heated, and the two cocks be opened, the steam -proceeding from the water in B will blow the air out of the vessel -A through the open stopcock D, in the same manner as air is blown -from a steam engine. When the vessel A by these means has been -filled with pure steam, let both stopcocks be closed. If the steam -in A, under these circumstances, have a pressure of 15 lbs. per -square inch, its temperature will be found to be 213°. Now, if any -heat be abstracted from this steam, its temperature will fall, and -a portion of it will be reconverted into water. - -Again, suppose the vessel A to be filled with pure steam which has -been produced from the heated water in B, the stopcock C being -open. Let the stopcock C be then closed, and the water in B be -heated to a higher temperature, the temperature and pressure of -the steam in A being observed. If the stopcock C be now opened, -the steam in A will be immediately observed to rise to the more -elevated temperature which has been imparted to the water in B, -and at the same time it will acquire an increased pressure. -[Pg169] - -The increase of temperature which it has received would of itself -produce an increased pressure; but that this is not the sole cause -of the augmented pressure in the present case might be proved by -weighing the vessel A. It would be found to have increased weight, -which could only arise from its having received from the water in -B an additional quantity of vapour. The increased pressure -therefore, which the steam in A has acquired, is due conjointly to -its increased density and its increased temperature. In general, -if the water in the vessel B be raised or lowered in temperature, -the steam in the vessel A will rise and fall in temperature in a -corresponding manner, always having the same temperature as the -water in B. If the weight of the vessel A were observed, it would -be found to increase with every increase of temperature, and to -diminish with every diminution of temperature, proving that the -augmented temperature of the water in B produces an augmented -density of the steam in A. The same pressure would be found always -to correspond to the same temperature and density, so that if the -numerical amount of any one of the three quantities, the -temperature, the pressure, or the density, were known, the other -two must necessarily be determined, the same temperature always -corresponding to the same pressure, and _vice versâ_. And in like -manner, steam produced under these circumstances of the same -density cannot have different pressures. It must be observed that -the steam here produced receives all the heat which it possesses -from the water from which it is raised. Now it is easily -demonstrable, that this is the least quantity of heat which is -compatible with the steam maintaining the vaporous form; for if -the stopcock C be closed so as to separate the steam in A from the -water in B, and that any portion of heat, however small, be then -abstracted from the steam in A, some portion of the steam will be -reconverted into water. - -This then, according to the definition already given, is _Common -Steam_. - - -(95.) Let us now suppose that the vessel A, being in communication -with the vessel B by the open stopcock, has been filled with pure -steam of any given temperature. The steam which it thus contains -will be common steam, and, as has been [Pg170] shown (94.), it -cannot lose any portion of heat, however small, without being -partially condensed; but let the stopcock C be closed, and let the -steam in A be then exposed to any source of heat by which its -temperature may be raised any required number of degrees. From the -steam thus obtained heat may be abstracted without producing any -condensation; and such abstraction of heat may be continued -without producing condensation, until the steam is cooled down to -that temperature at which it was raised from the water in B, when -the stopcock C was opened. Any further reduction of temperature -would be attended with condensation. - -If after increasing the temperature of the steam in A, the -stopcock C being shut so as to render it superheated steam, its -pressure be observed, the pressure will be found to be increased, -but not to that amount which it would have been increased had the -steam in A been raised to the same temperature by heating the -water in B to that temperature, and keeping the stopcock open. In -fact, its present augmented pressure will be due only to its -increased temperature, since its density remains unchanged. But if -in these circumstances the stopcock C be suddenly opened, the -pressure of the steam in A will as suddenly rise to that pressure -which in common steam corresponds to its temperature; and if the -vessel A were weighed, it would be found to have increased in -weight, proving that the steam contained in it has received -increased density by an increased quantity of vapour proceeding -from the water in A. In fact, by opening the stopcock the steam -which was before superheated steam, has become common steam. It -has the greatest density which steam of that temperature can have; -and consequently, if any heat be abstracted from it, a partial -condensation will ensue. - -To render these general principles more intelligible, let us -suppose that the water in B is raised to the temperature of 213°, -the stopcock C being open; the vessel A will then be filled with -steam of the same temperature, and having a pressure of 15 lbs. -per square inch. This will be common steam. If the stopcock be now -closed, and the whole apparatus be exposed to the temperature of -243°; the steam in A will preserve the same density, but its -pressure will be [Pg171] increased from 15 lbs. to a little more -than 16 lbs. per square inch. Let the stopcock C be then opened -and while the temperature of the steam in A shall continue to be -243°, the pressure will suddenly rise from 16 lbs. to about 26 -lbs. per square inch. The weight of the steam in A will be at the -same time increased in the same proportion of 16 to 26 as its -pressure. The steam thus produced in A will then be common steam, -and any abstraction of heat from it would be attended with partial -condensation. - - -(96.) The law, according to which the pressure of elastic fluids -in general, whether gases or vapours, increases with their -temperature, was simultaneously discovered by Dalton and Gay -Lussac. If the pressure which the gas or vapour would have at the -temperature of melting ice, were expressed by 10,000, then the -increase of pressure which it would receive for every degree of -temperature by which it would be raised, its volume being supposed -to be preserved, would be expressed by 208-1/3. Thus, if the -pressure of gas, or vapour, on a surface of a certain magnitude at -the temperature of 32° were 10,000 ounces, then the same gas or -vapour would acquire an additional pressure of 208-1/3 ounces for -every degree of temperature which would be imparted to it above -32°. This law is common to all gases and vapours. - -It may be objected that water cannot exist in the state of vapour -under the usual pressures at so low a temperature as melting ice. -This, however, does not hinder the application of the above law, -for that law will equally hold good by computing the pressure -which the vapour would have if it were a permanent gas, and if it -could therefore exist in the elastic form at that low temperature. - - -(97.) Another law, common to all elastic fluids, and of equal -importance with the former, was discovered by Mariotte. By this -law it appears that every gas or vapour, so long as its -temperature is unchanged, will have a pressure directly -proportional to its density. If therefore, while we compress steam -into half its volume, we could preserve its temperature unaltered, -we should increase its pressure in a two-fold proportion; but if -the process of compression should cause its temperature to -increase, [Pg172] then its increase of pressure will be greater -than its increase of density, since it will be due conjointly to -the increase of density and to the increase of temperature. In -this case the increased pressure may be deduced from the combined -application of the two laws just explained; that of Mariotte will -determine that increase of pressure which is due to the increase -of density, and that of Dalton and Gay Lussac will determine the -further increase of pressure which will be due to the increase of -temperature. The full investigation of these effects, and the -formulæ expressing them, will be found in the Appendix to this -volume. - - -(98.) The fixed relations which exist between the temperatures of -common steam and its pressure and density, have never been -discovered from any general physical principles. The pressures and -the densities however, which correspond to a great variety of -temperatures throughout the thermometric scale, have been -ascertained by extensive series of experiments instituted by -philosophers of this and other countries. From a comparison of the -temperatures and pressures thus found by experiment, empirical -formulæ have been constructed, which exhibit, with an approximation -sufficiently close for practice, this relation; and these formulæ -may accordingly be used for the computation of tables exhibiting the -pressures, temperatures, and densities of common steam; and such -tables will have sufficient numerical accuracy for all practical -purposes. These formulæ, and the tables resulting from them, will be -found in the Appendix to this volume. - - -(99.) It has been explained, that to effect the conversion of -water into steam, it is only necessary to impart to it as much -heat as, added to the temperature which it has, would, if it -continued in the liquid form, raise it to the temperature of -1212°. This condition is necessary, and sufficient to effect the -transition of water into vapour. If, for example, as much heat -were imparted to the water evaporated, as would maintain it in the -liquid state to 1300°, then the steam so produced would be -superheated steam, having 80° of heat more than is necessary to -maintain it in the vaporous form. From such steam, therefore, 80° -of heat may be abstracted without producing any condensation. -[Pg173] - - -(100.) Common steam being raised from water at any pressure and -temperature, and being afterwards separated from the water, if the -same steam be compressed into a small volume, or allowed to expand -into a greater volume, it will still maintain its quality of -common steam, and will have the same pressure and temperature, -whatever volume it may assume, as it would have if immediately -raised from water at that pressure. Thus if steam be raised from -water under a pressure of 30 lbs. per square inch, and, being -separated from the water, be allowed to dilate, until its pressure -is reduced to 15 lbs. per square inch, its temperature will then -be reduced to 213°, which is that temperature which it would have -if immediately raised from water under a pressure of 15 lbs. per -square inch; and if any heat be abstracted from such steam, -whether under its original pressure, or under the diminished -pressure of 15 lbs. per square inch, a condensation will be -produced, the amount of which will be the same, if the same -quantity of heat be abstracted from the steam. These are -consequences which immediately flow from the fact, that the sum of -the latent and sensible heats of steam is always the same.[20] - -It appears, therefore, that supposing the steam used in an engine -to receive no additional heat after it leaves the boiler, however -it may be changed in its density by subsequent expansion, it will -still retain its character of common steam, and cannot lose any -portion of heat, however small, without suffering partial -condensation. The mechanical force also exerted by such steam, -after expansion, must be computed in the same manner as if it were -raised immediately. - - -(101.) If the law of Mariotte were strictly applicable to steam, -its mechanical effect would be the same as has been already -explained in all states of density; but since its temperature will -rise and fall as its density is increased or diminished, a -corresponding change will be produced in its [Pg174] mechanical -efficacy. It is therefore necessary in the calculation of the -mechanical effect of steam, whether it be used at a uniform -pressure without the principle of expansion, or with the -application of that principle to any given extent, to take into -account the combined operation of the laws of Mariotte and Dalton. -Formulæ exhibiting the relation between the temperatures, -pressures, volumes, and densities of steam, and the mechanical -effect produced by the evaporation of water, whether acting with -or without expansion, together with the tables necessary for the -practical application of these, will be found in the Appendix. - - -(102.) One of the methods of equalising the varying force of -expanding steam, would be to work it at the same time in two -cylinders connected with the same beam; so that while its force in -one would be augmented, its force in the other would be -diminished, the combination of the two producing a uniform effect. -Soon after the expansive principle was promulged by Mr. Watt, this -expedient was accordingly resorted to by an engineer named -Hornblower. - -[Illustration: _Fig._ 30.] - -In the year 1781, Hornblower conceived the notion of working an -engine with two cylinders of different sizes, by allowing the -steam to flow freely from the boiler until it fills the smaller -cylinder, and then permitting it to expand into the greater one, -employing it thus to press down two pistons in the following -manner. - -Let C, _fig._ 30., be the centre of the great working-beam, -carrying two arch heads, on which the chains of the piston rods -play. The distances of these arch heads from the centre C must be -in the same proportion as the length of the cylinders, in order -that the same play of the beam may correspond to [Pg175] the -plays of both pistons. Let F be the steam-pipe from the boiler, -and G a valve to admit the steam above the lesser piston. H is a -tube by which a communication may be opened by the valve I, -between the top and bottom of the lesser cylinder B. K is a tube -communicating by the valve L, between the bottom of the lesser -cylinder B and the top of the greater cylinder A. M is a tube -communicating, by the valve N, between the top and bottom of the -greater cylinder A; and P a tube leading to the condenser by the -exhausting valve O. - -At the commencement of the operation, suppose all the valves -opened, and steam allowed to flow through the engine until the air -be completely expelled, and then let all the valves be closed. To -start the engine, let the exhausting valve O and the steam valves -G and L be opened, as in _fig._ 30. The steam will flow freely -from the boiler, and press upon the lesser piston, and at the same -time the steam below the greater piston will flow into the -condenser, leaving a vacuum in the greater cylinder. The valve L -being opened, the steam which is under the piston in the lesser -cylinder will flow through K, and press on the greater piston, -which, having a vacuum beneath it, will consequently descend. At -the commencement of the motion, the lesser piston is as much -resisted by the steam below it, as it is urged by the steam above -it; but after a part of the descent has been effected, the steam -below the piston, in the lesser cylinder, passing into the -greater, expands into an increased space, and therefore loses part -of its elastic force. The steam above the lesser piston retaining -its full force by having a free communication with the boiler by -the valve G, the lesser piston will be urged by a force equal to -the excess of the pressure of this steam above the diminished -pressure of the expanded steam below it. As the pistons descend, -the steam which is between them is continually increasing in its -bulk, and therefore decreasing in its pressure, from whence it -follows, that the force which resists the lesser piston is -continually decreasing, while that which presses it down remains -the same, and therefore the effective force which impels it must -be continually increasing. [Pg176] - -On the other hand, the force which urges the greater piston is -continually decreasing, since there is a vacuum below it, and the -steam which presses it is continually expanding into an increased -bulk. - -[Illustration: _Fig._ 31.] - -Impelled in this way, let us suppose the pistons to have arrived -at the bottoms of the cylinders, and let the valves G, L, and O, -be closed, and the valves I and N opened. No steam is allowed to -flow from the boiler, G being closed, nor any allowed to pass into -the condenser, since O is closed, and all communication between -the cylinders is stopped by closing L. By opening the valve I, a -free communication is made between the top and bottom of the -lesser piston through the tube H, so that the steam which presses -above the lesser piston will exert the same pressure below it, and -the piston is in a state of indifference. In the same manner the -valve N being open, a free communication is made between the top -and bottom of the greater piston, and the steam circulates above -and below the piston, and leaves it free to rise. A counterpoise -attached to the pump-rods, in this case, draws up the piston, as -in Watt's single engine; and when they arrive at the top, the -valves I and N are closed, and G, L, and O, opened, and the next -descent of the pistons is produced in the manner already -described, and so the process is continued. - -The valves are worked by the engine itself, by means similar to -some of those already described. By computation, we find the power -of this engine to be nearly the same as a similar engine on Watt's -expansive principle. It does not, however, appear, that any -adequate advantage was gained by this modification of the -principle, since no engines of this construction are now made. - - -(103.) The use of two cylinders was revived by Arthur Woolf in 1804, -who, in this and the succeeding year, obtained patents for the -application of steam raised under a high pressure to double-cylinder -engines. The specification of his patent states, that he has proved -by experiment that steam raised [Pg177] under a safety-valve loaded -with any given number of pounds upon the square inch will, if -allowed to expand into as many times its bulk as there are pounds of -pressure on the square inch, have a pressure equal to that of the -atmosphere. Thus, if the safety-valve be loaded with four pounds on -the square inch, the steam, after expanding into four times its -bulk, will have the atmospheric pressure; if it be loaded with 5, 6, -or 10 lbs. on the square inch, it will have the atmospheric pressure -when it has expanded into 5, 6, or 10 times its bulk, and so on. It -was, however, understood in this case, that the vessel into which it -was allowed to expand should have the same temperature as the steam -before it expands. - -It is very unaccountable how a person of Mr. Woolf's experience in -the practical application of steam could be led into errors so -gross as those involved in the averments of this patent; and it is -still more unaccountable how the experiments could have been -conducted which led him to conclusions not only incompatible with -all the established properties of elastic fluids, but even -involving in themselves palpable contradiction and absurdity. If -it were admitted that every additional pound avoirdupois which -should be placed upon the safety-valve would enable steam, by its -expansion into a proportionally enlarged space, to attain a -pressure equal to the atmosphere, the obvious consequence would -be, that a physical relation would subsist between the atmospheric -pressure and the pound avoirdupois! It is wonderful that it did -not occur to Mr. Woolf, that, granting his principle to be true at -any given place, it would necessarily be false at another place, -where the barometer would stand at a different height! Thus, if -the principle were true at the foot of a mountain, it would be -false at the top of it; and if it were true in fair weather, it -would be false in foul weather, since these circumstances would be -attended by a change in the atmospheric pressure, without making -any change in the pound avoirdupois.[21] - -[Pg178] - - -(104.) For several years after the extension of Watt's first -patent had been obtained from parliament, he was altogether -engrossed by the labour of bringing to perfection the application -of the steam-engine to the drainage of mines, and in surmounting -the numerous difficulties which presented themselves to its -general adoption, even after its manifold advantages were -established and admitted. When, however, these obstacles had been -overcome, and the works for the manufacture of engines for pumping -water, at Soho, had been organised and brought into active -operation, he was relieved from the pressure of these anxieties, -and was enabled to turn his attention to the far more extensive -and important uses of which he had long been impressed with the -conviction that the engine was capable. His sagacious mind enabled -him to perceive that the machine he had created was an infant -force, which by the fostering influence of his own genius would -one day extend its vast power over the arts and manufactures, the -commerce and the civilisation of the world. Filled with such -aspirations, he addressed his attention about the year 1779, to -the adaptation of the steam-engine to move machinery, and thereby -to supersede animal power, and the natural agents, wind and water. - -The idea that steam was capable of being applied extensively as a -prime mover, had prevailed from a very early period; and now that -we have seen its powers so extensively brought to bear, it will -not be uninteresting to revert to the faint traces by which its -agency was sketched in the crude speculations of the early -mechanical inventors. - - -(105.) Papin, to whom the credit of discovering the method of -producing a vacuum by the condensation of steam is due, was the -earliest and most remarkable of those projectors. With very limited -powers of practical application, he was, nevertheless, peculiarly -happy in his mechanical conceptions; and had his experience and -opportunities been proportionate to the clearsighted character of -his mind, he would doubtless have anticipated some of the most -memorable of his successors in the progressive improvement of the -steam engine. - -In his work already cited, after describing his method of -imparting an alternate motion to a piston by the atmospheric -[Pg179] pressure acting against a vacuum produced by the -condensation of steam, he stated that his invention, besides being -applicable to pumping water, could be available for rowing vessels -against wind and tide, which he proposed to accomplish in the -following manner. - -Paddle-wheels, such as have since been brought into general use, -were to be placed at the sides, and attached to a shaft extending -across the vessel. Within the vessel, and under this shaft, he -proposed to place several cylinders supplied with pistons, to be -worked by the atmospheric pressure. On the piston-rods were to be -constructed racks furnished with teeth: these teeth were to work in -the teeth of wheels or pinions, placed on the shaft of the -paddle-wheels. These pinions were not to be fixed on the shaft, but -to be connected with it by a ratchet; so that when they turned in -one direction, they would revolve without causing the shaft to -revolve; but when driven in the other direction, the catch of the -ratchet-wheel would act upon the shaft so as to compel the shaft and -paddle-wheels to revolve with the motion of the pinion or wheel upon -it. By this arrangement, whenever the piston of any cylinder was -forced down by the atmospheric pressure, the rack descending would -cause the corresponding pinion of the paddle-shaft to revolve; and -the catch of the ratchet wheel, being thus in operation, would cause -the paddle-shaft and paddle-wheels also to revolve; but whenever the -piston would rise, the rack driving the pinion in the opposite -direction, the catch of the ratchet wheel would merely fall from -tooth to tooth, without driving the paddle-shaft. - -It is evident that by such an arrangement a single cylinder and -piston would give an intermitting motion to the paddle-shaft, the -motion of the wheel being continued only during the descent of the -piston; but if several cylinders were provided, then their motion -might be so managed, that when one would be performing its -ascending stroke, and therefore giving no motion to the -paddle-shaft, another should be performing its descending stroke, -and therefore driving the paddle-shaft. As the interval between -the arrival of the piston at the bottom of the cylinder and the -commencement [Pg180] of its next descent would have been, in the -imperfect machine conceived by Papin, much longer than the time of -the descent, it was evident that more than two cylinders would be -necessary to insure a constantly acting force on the paddle-shaft, -and, accordingly, Papin proposed to use several cylinders. - -In addition to this, Papin proposed to construct a boiler having a -fireplace surrounded on every side by water, so that the heat -might be imparted to the water with such increased rapidity as to -enable the piston to make four strokes per minute. These projects -were promulged in 1690, but it does not appear that they were ever -reduced to experiment. - - -(106.) Savery proposed, in his original patent, in 1698, to apply -his steam engine as a general prime mover for all sorts of -machinery, by causing it to raise water to make an artificial -fall, by which overshot water-wheels might be driven. This -proposal was not acted on during the lifetime of Savery, but it -was at a subsequent period partially carried into effect. Mr. -Joshua Rigley erected several steam engines on this principle at -Manchester, and other parts of Lancashire, to impel the machinery -of some of the earliest manufactories and cotton mills in that -district. The engines usually raised the water from sixteen to -twenty feet high, from whence it was conveyed to an overshot -wheel, to which it gave motion. The same water was repeatedly -elevated by the engine, so that no other supply was necessary, -save what was sufficient to make good the waste. These engines -continued in use for some years, until superseded by improved -machines.[22] - - -(107.) In 1736, Jonathan Hulls obtained a patent for a method of -towing ships into or out of harbour against wind and tide. This -method was little more than a revival of that proposed by Papin in -1690. The motion, however, was to be communicated to the -paddle-shaft by a rope passing over a pulley fixed on an axis, and -was to be maintained during the returning stroke of the piston by -the descent of a weight which was elevated during the descending -stroke. There is no record, however, of this plan, any more than -that of Papin, ever having been reduced to experiment. - - -(108.) During the early part of the last century the [Pg181] -manufactures of this country had not attained to such an extent as -to render the moving power supplied by water insufficient or -uncertain to any inconvenient degree; and accordingly mills, and -other works in which machinery required to be driven by a moving -power, were usually built along the streams of rivers. About the -year 1750 the general extension of manufactures, and their -establishment in localities where water power was not accessible, -called the steam engine into more extensive operation. In the year -1752, Mr. Champion, of Bristol, applied the atmospheric engine to -raise water, by which a number of overshot wheels were driven. -These were applied to move extensive brass-works in that -neighbourhood, and this application was continued for about twenty -years, but ultimately given up on account of the expense of fuel -and the improved applications of the steam engine. - -About this time Smeaton applied himself with great activity and -success to the improvement of wind and water mills, and succeeded -in augmenting their useful effect in a twofold proportion with the -same supply of water. From the year 1750 until the year 1780 he -was engaged in the construction of his improved water mills, which -he erected in various parts of the country, and which were -imitated so extensively that the improvement of such mills became -general. In cases where a summer drought suspended the supply of -water, horse machinery was provided, either to work the mill or to -throw back the water. These improvements necessarily obstructed -for a time the extension of steam power to mill work; but the -increase of manufactures soon created a demand for power greatly -exceeding what could be supplied by such limited means. - -In the manufacture of iron, it is of great importance to keep the -furnaces continually blown, so that the heat may never be abated -by day or night. In the extensive ironworks at Colebrook Dale, -several water-wheels were used in the different operations of the -manufacture of iron, especially in driving the blowers of the iron -furnaces. These wheels were usually driven by the water of a -river, but in the summer months the supply became so short that it -was insufficient to work them all. Steam engines were accordingly -erected to [Pg182] return the water for driving these wheels. -This application of the engine as an occasional power for the -supply of water-wheels having been found so effectual, returning -engines were soon adopted as the permanent and regular means of -supplying water-wheels. The first attempt of this kind is recorded -to have been made by Mr. Oxley, in 1762, who constructed a machine -to draw coals out of a pit at Hartley colliery, in Northumberland. -It was originally intended to turn the machine by a continuous -circular motion received from the beam of the engine; but that -method not being successful, the engine was applied to raise water -for a wheel by which the machine was worked. This engine was -continued in use for several years, and though it was at length -abandoned, on account of its defective construction, it -nevertheless established the practicability of using steam power -as a means of driving water wheels.[23] - - -(109.) In the year 1777, Mr. John Stewart read a paper before the -Royal Society, describing a method for obtaining a continued -circular motion for turning all kinds of mills from the -reciprocating motion of a steam engine. He proposed to accomplish -this by means of two endless chains passing over pulleys, which -should be moved upwards and downwards by the motion of the engine, -in the manner of a window sash. The joint pins of the links of the -two chains worked in teeth at the opposite sides of a cog wheel, -to which they imparted a circular motion, first by one chain, and -then by the other, acting alternately on opposite sides of the -wheel. One chain impelled it during the descent of the piston, and -the other during the ascent; but one of these chains always passed -over its pulleys so as to produce no effect on one side of the cog -wheel, whilst the other chain worked on the opposite side to turn -it round. For this purpose each chain was provided with a catch, -to prevent its circulating over its pulleys in one direction, but -to allow it free motion in the other. The cog wheel thus kept in -revolution might be applied to the axis of any mill which the -engine was required to work. Thus, if it were applied to a -flour-mill, the millstone itself would perform the office of a -fly-wheel to regulate the intermission of [Pg183] the power, and -in other mills a fly-wheel might be added for this purpose. - -The hints obtained by Mr. Stewart from Papin's contrivance, before -mentioned, will not fail to be perceived. In Mr. Stewart's paper -he notices indirectly the method of obtaining a continued circular -motion from a reciprocating motion by means of a crank or winch, -which, he says, occurs naturally in theory, but in practice would -be impossible, from the nature of the motion of the engine, which -depends on the force of the steam, and cannot be ascertained in -its length. Therefore, on the first variation, the machine would -be either broken in pieces or turned back. Such an opinion, -pronounced by a man of considerable mechanical knowledge and -ingenuity, against a contrivance which, as will presently appear, -proved in practice, not less than in theory, to be the most -effectual means of accomplishing the end here pronounced to be -impossible, is sufficiently remarkable. It might cast some doubt -on the extent of Mr. Stewart's practical knowledge, if it did not -happen to be in accordance with a judgment so generally -unimpeachable as that of Mr. Smeaton. This paper of Mr. Stewart's -was referred by the council of the Royal Society to Mr. Smeaton, -who remarked upon the difficulty arising from the absolute -stopping of the whole mass of moving power, whenever the direction -of the motion is changed; and observed, that although a fly-wheel -might be applied to regulate the motion, it must be such a large -one as would not be readily controlled by the engine itself; and -he considered that the use of such a fly-wheel would be a greater -incumbrance to a mill than a water-wheel to be supplied by water -pumped up by the engine. This engineer, illustrious as he was, not -only fell into the error of Mr. Stewart in respect of the crank, -but committed the further blunder of condemning the very expedient -which has since rendered the crank effectual. It will presently -appear that the combination of the crank and fly-wheel have been -the chief means of establishing the dominion of the steam engine -over manufactures. - - -(110.) In 1779, Mr. Matthew Wasbrough, an engineer at Bristol, -took out a patent for the application of a steam engine [Pg184] -to produce a continuous circular motion by means of ratchet -wheels, similar to those previously used by Mr. Oxley, at Hartley -colliery; to which, however, Mr. Wasbrough added a fly-wheel to -maintain and regulate the motion. Several machines were -constructed under this patent; and among others, one was erected -at Mr. Taylor's saw-mills and block manufactory at Southampton. In -1780, one was erected at Birmingham, where the ratchet work was -found to be subject to such objections, that one of the persons -about the works substituted for it the simple crank, which has -since been invariably used. A patent was taken out for this -application of the crank in the same year, by Mr. James Pickard, -of Birmingham. It will presently appear, however, that the -suggestion of this application of the crank was derived from the -proceedings of Watt, who was at the same time engaged in similar -experiments. - - -(111.) The single-acting steam engine, as constructed by Watt, was -not adapted to produce continuous uniform motion of rotation, for -the following reasons:-- - -_First._ The effect required was that of an uniformly acting -force. The steam engine, on the other hand, supplied an -intermitting force. Its operation was continued during the -descending motion of the piston, but it was suspended during the -ascent of the piston. To produce the continued effect now -required, either its principle of operation should be altered, or -some expedient should be devised for maintaining the motion of the -revolving shaft during the ascent of the piston, and the -consequent suspension of the moving power. - -_Secondly._ The action of the steam engine was rectilinear. It was -a power which acted in a straight line, viz., in the direction of -the cylinder. The motion, however, required to be produced, was a -circular motion--a motion of rotation around the axis or shaft of -the mill. - -The steps by which Watt proceeded to accomplish these objects have -been recorded by himself as follows, in his notes upon Dr. -Robison's article on the steam engine:-- - -"I had very early turned my mind to the producing of continued -motion round an axis; and it will be seen, by reference to my -first specification in 1769, that I there described [Pg185] a -steam wheel, moved by the force of steam, acting in a circular -channel against a valve on one side, and against a column of -mercury, or some other fluid metal, on the other side. This was -executed upon a scale of about six feet diameter at Soho, and -worked repeatedly, but was given up, as several practical -objections were found to operate against it; similar objections -lay against other rotative engines, which had been contrived by -myself and others, as well as to the engines producing rotatory -motions by means of ratchet wheels. - -"Having made my single reciprocating engines very regular in their -movements, I considered how to produce rotative motions from them -in the best manner; and amongst various schemes which were -subjected to trial, or which passed through my mind, none appeared -so likely to answer the purpose as the application of the crank, -in the manner of the common turning lathe; but as the rotative -motion is produced in that machine by impulse given to the crank -in the descent of the foot only, it requires to be continued in -its ascent by the energy of the wheel, which acts as a fly; being -unwilling to load my engine with a fly-wheel heavy enough to -continue the motion during the ascent of the piston (or with a -fly-wheel heavy enough to equalise the motion, even if a -counterweight were employed to act during that ascent), I proposed -to employ two engines, acting upon two cranks fixed on the same -axis, at an angle of 120° to one another, and a weight placed upon -the circumference of the fly-wheel at the same angle to each of -the cranks, by which means the motion might be rendered nearly -equal, and only a very light fly-wheel would be requisite. - -"This had occurred to me very early; but my attention being fully -employed in making and erecting engines for raising water, it -remained _in petto_ until about the year 1778 or 1789, when Mr. -Wasbrough erected one of his ratchet-wheel engines at Birmingham, -the frequent breakages and irregularities of which recalled the -subject to my mind, and I proceeded to make a model of my method, -which answered my expectations; but having neglected to take out a -patent, the invention was communicated by a workman employed -to [Pg186] make the model, to some of the people about Mr. -Wasbrough's engine, and a patent was taken out by them for the -application of the crank to steam engines. This fact the said -workman confessed, and the engineer who directed the works -acknowledged it; but said, nevertheless, that the same idea had -occurred to him prior to his hearing of mine, and that he had even -made a model of it before that time; which might be a fact, as the -application to a single crank was sufficiently obvious. - -"In these circumstances, I thought it better to endeavour to -accomplish the same end by other means, than to enter into -litigation; and if successful, by demolishing the patent, to lay -the matter open to every body. Accordingly, in 1781, I invented -and took out a patent for several methods of producing rotative -motions from reciprocating ones; amongst which was the method of -the sun-and-planet wheels. This contrivance was applied to many -engines, and possesses the great advantage of giving a double -velocity to the fly-wheel; but is perhaps more subject to wear, -and to be broken under great strains, than a simple crank, which -is now more commonly used, although it requires a fly-wheel of -four times the weight, if fixed upon the first axis; my -application of the double engine to these rotative machines -rendered the counterweight unnecessary, and produced a more -regular motion." - - -(112.) Watt's second patent here referred to, was dated 25th -October, 1781, and was entitled "A patent for certain new methods -of applying the vibrating or reciprocating motions of steam or -fire engines to produce a continued rotative or circular motion -round an axis or centre, and thereby to give motion to the wheels -of mills and other machines." - -All the methods specified in this patent were intended to be -worked by the single-acting engine, already described, a -counterweight being applied to impel the machinery during the -returning stroke of the engine, which weight would be elevated -during the descent of the piston. There were five different -expedients proposed in the specification for producing a rotatory -motion; but, of these five, two only were ever applied in -practice. [Pg187] - - -(113.) Suppose a rod or bar attached by a pin or joint at the -upper extremity to the working end of the beam of the engine, and -by a similar pin or joint at the lower extremity to an iron wheel -fixed on the extremity of the axis of the fly-wheel. One half of -this wheel is formed of a solid semicircle of cast iron, while the -other half is constructed of open spokes, so as to be as light as -is consistent with strength. The position of the wheel on the axis -is such that during the returning stroke of the piston, when the -operation of the steam is suspended, the heavy semicircle of the -wheel will be descending, and by its weight will draw down the -connecting bar, and thereby draw down the working end of the beam, -and draw up the piston in the cylinder. When the piston descends -and is driven by the power of the steam, the heavy semicircle of -the above-mentioned wheel will be drawn upwards, and in the same -way the motion will be continued. - -[Illustration: _Fig._ 32.] - - -(114.) The second method of producing a rotatory motion, which was -subsequently continued for many years in practical operation, was -that which was called the _Sun-and-planet Wheels_. A toothed wheel -A (_fig._ 32.), called the sun wheel, was fixed on the axle of the -fly-wheel, to which rotation was to be imparted. The wheel B, -called the planet wheel, having an equal diameter, was fastened on -the end I of the connecting rod H I, so as to be incapable of -revolving. During the descent of the piston, the working end of -the beam was drawn upwards, and the end I of the connecting rod -travelled from C to D, through the dotted semicircle C I D. The -wheel B not being capable of revolving on the centre I, would, -during this motion, drive the sun wheel A. During the ascent of -the steam piston, the working end of the beam would descend, and -the centre I of [Pg188] the planet wheel B would be driven -downwards from D to C, through the other dotted semicircle, and -would consequently continue to drive the sun wheel round in the -same direction. - -This contrivance, although in the main inferior to the more simple -one of the crank, is not without some advantages; among others, it -gives to the sun wheel double the velocity which would be -communicated by the crank; for in the crank one revolution only on -the axle is produced by one revolution of the crank, but in the -sun-and-planet wheel, two revolutions of the sun wheel are -produced by one of the planet wheel; thus a double velocity is -obtained from the same motion of the beam. This will be evident -from considering that when the planet wheel is in its highest -position, its lowest tooth is engaged with the highest tooth of -the sun wheel; as the planet wheel passes from the highest -position, its teeth drive those of the sun wheel before them, and -when it comes into the lowest position, the highest tooth of the -planet wheel is engaged with the lowest of the sun wheel: but then -half of the sun wheel has _rolled off_ the planet wheel, and, -therefore, the tooth which was engaged with it in its highest -position, must now be distant from it by half the circumference of -the wheel, and must, therefore, be again in the highest position; -so that while the planet wheel has been carried from the top to -the bottom, the sun wheel has made a complete revolution. - -This advantage of giving an increased velocity may be obtained -also by the crank, by placing toothed wheels on its axle. -Independently of the greater expense attending the construction of -the sun-and-planet wheel, its liability to go out of order, and -the rapid wear of the teeth, and other objections, rendered it -inferior to the crank, which has entirely superseded it. - - -(115.) Although by these contrivances Watt succeeded in obtaining -a continuous circular motion from the reciprocating motion of -the steam engine, the machine was still one of intermitting, -instead of continuous action. The expedient of a counterweight, -elevated during the descending stroke, and giving back the power -expended on it in the interval of the returning stroke, did not -satisfy the [Pg189] fastidious mechanical taste of Watt. He -soon perceived that all which he proposed to accomplish by the -application of two cylinders and pistons working alternately, -could be attained with greater simplicity and effect by a single -cylinder, if he could devise means by which the piston might be -impelled by steam upwards as well as downwards. To accomplish -this, it was only necessary to throw the lower end of the cylinder -into alternate communication with the boiler, while the upper end -would be put into communication with the condenser. If, for -example, during the descent of the piston, the upper end of the -cylinder communicated with the boiler, and the lower end with the -condenser; and, on the other hand, during the ascent of the -piston, the lower end communicated with the boiler, and the upper -end with the condenser; then the piston would be driven -continually, whether upwards or downwards, by the power of steam -acting against a vacuum. Watt obtained his third patent for this -contrivance, on the 12th of March, 1782. - -This change in the principle of the machine involved several other -changes in the details of its mechanism. - -[Illustration: _Fig._ 33.] - - -(116.) It was necessary, in the first place, to provide means for -admitting and withdrawing the steam at either end of the cylinder. -For this purpose let B and B' (_fig._ 33.) be two steam-boxes, B -the upper, and B' the lower, communicating respectively with the -top and bottom of the cylinder by proper passages D D'. Let two -valves be placed in B, one, S, above the passage D, and the other, -C, below it; and in like manner two other valves in the lower -valve-box, B', one, S', above the passage D', and the other, C', -below it. Above the valve S in the upper steam-box is an opening -at which the steam-pipe from the boiler enters, and below the -valve C is another opening, at which enters the exhausting-pipe -leading to the condenser. In like manner, above the valve S' in -the lower steam-box enters a steam-pipe leading from the boiler, -and below the valve C' enters an exhausting-pipe leading to -[Pg190] the condenser. It is evident, therefore, that steam can -always be admitted above the piston by opening the valve S, and -below it by opening the valve S'; and, in like manner, steam can -be withdrawn from the cylinder above the piston, and allowed to -pass to the condenser, by opening the valve C, and from below it -by opening the valve C'. - -[Illustration: _Fig._ 34.] - -Supposing the piston P to be at the top of the cylinder, and the -cylinder below the piston to be filled with pure steam, let the -valves S and C' be opened, the valves C and S' being closed as -represented in _fig._ 34. Steam from the boiler will, therefore, -flow in through the open valve S, and will press the piston -downwards, while the steam that has filled the cylinder below the -piston will pass through the open valve C' into the exhausting-pipe -leading to the condenser, and being condensed will leave the -cylinder below the piston a vacuum. The piston will, therefore, be -pressed downwards by the action of the steam above it, as in the -single-acting engine. Having arrived at the bottom of the cylinder, -let the valves S and C' be both closed, and the valves S' and C be -opened, as represented in _fig._ 34. Steam will now be admitted -through the open valve S' and through the passage D' below the -piston, while the steam which has just driven the piston downwards, -filling the cylinder above the piston, will be drawn off through the -open valve C, and the exhausting-pipe, into the condenser, leaving -the cylinder above the piston a vacuum. The piston will, therefore, -be pressed upwards by the action of the steam below it, against the -vacuum above it, and will ascend with the same force as that with -which it had descended. - -This alternate action of the piston upwards and downwards may -evidently be continued by opening and closing the valves alternately -in pairs. Whenever the piston is at the top of the cylinder, as -represented in _fig._ 33., the valves S and C', that is, the upper -steam-valve and the lower exhausting-valve, are opened, and the -valves C and S', that is, the upper exhausting-valve and the lower -steam-valve, are closed; and [Pg191] when the piston has arrived at -the bottom of the cylinder, as represented in _fig._ 34., the valves -C and S', that is, the upper exhausting-valve and the lower -steam-valve, are opened, and the valves S and C', that is, the upper -steam-valve and the lower exhausting-valve, are closed. - -If these valves, as has been here supposed, be opened and closed -at the moments at which the piston reaches the top and bottom of -the cylinder, it is evident that they may be all worked by a -single lever connected with them by proper mechanism. When the -piston arrives at the top of the cylinder, this lever would be -made to open the valves S and C', and at the same time to close -the valves S' and C; and when it arrives at the bottom of the -cylinder, it would be made to close the valves S and C', and to -open the valves S' and C. - -If, however, it be desired to cut off the steam before the arrival -of the piston at the termination of its stroke, whether upwards or -downwards, then the steam-valves must be closed before the arrival -of the piston at the end of its stroke; and as the exhausting-valve -ought to be left open until the stroke is completed, these valves -ought to be moved at different times. In that case separate levers -should be provided for the different valves. We shall, however, -return again to the subject of the valves which regulate the -admission of steam to the cylinder and its escape to the condenser. - - -(117.) It will be remembered that in the single-acting engine the -process of condensation was suspended while the piston ascended in -the cylinder, and therefore the play of the jet of cold water in the -condenser was stopped during this interval. In the double-acting -engine, however, the flow of steam from the cylinder to the -condenser is continued, whether the piston ascends or descends, and -therefore a constant condensation of steam must be produced. The -condensing jet, therefore, does not in this case, as in the former, -play with intervals of intermission. A constant jet of cold water -must be maintained in the condenser. - -It will presently appear that in the double-acting engine applied -to manufactures, the motion of the piston was subject to more or -less variation of speed, and the quantity of steam [Pg192] -admitted to the cylinder was subject to a corresponding change. -The quantity of steam, therefore, drawn into the condenser was -subject to variation, and required a considerable change in the -quantity of cold water admitted through the jet to condense it. To -regulate this, the valve or cock by which the water was admitted -into the condenser was worked in the double-acting engine by a -lever furnished with an index, by which the quantity of condensing -water admitted into the condenser could be regulated. This index -played upon a graduated arch, by which the engine-man was enabled -to regulate the supply. - -[Illustration: HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF -WATT.] - - FOOTNOTES: - - [20] These effects are explained in my Treatise on Heat; and - they have lately been verified by experiments made with - locomotive engines by M. de Pambour, who found that the steam - raised from the boiler of a locomotive engine, under a - pressure of above 50 lbs. per square inch, was in the state of - common steam as it issued from the chimney at a very diminished - pressure and at a lower pressure. - - [21] It is strange that this absurdity has been repeatedly - given as unquestionable fact in various encyclopædias, as well - as in by far the greater number of treatises expressly on the - subject. - - [22] Farey, Treatise on the Steam Engine, p. 122. - - [23] Farey on the Steam Engine, p. 297. - -[Pg193] - - - - -[Illustration: DOUBLE-ACTING ENGINE.--CITY SAW-MILLS.] - -CHAP. VIII. - - METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE - DOUBLE-ACTING ENGINE. -- RACK AND SECTOR. -- PARALLEL MOTION. -- - CONNECTING ROD AND CRANK. -- FLY-WHEEL. -- THROTTLE-VALVE. -- - GOVERNOR. -- CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING - ENGINE. -- ECCENTRIC. -- COCKS AND VALVES. -- SINGLE-CLACK VALVE. - -- DOUBLE-CLACK VALVE. -- CONICAL VALVES. -- SLIDE VALVES. -- - MURRAY'S SLIDES. -- THE D VALVE. -- SEAWARD'S SLIDES. -- SINGLE - COCK. -- FOUR-WAY COCK. -- PISTONS. -- COMMON HEMP-PACKED PISTON. - -- WOOLFE'S PISTON. -- METALLIC PISTONS. -- CARTWRIGHT'S ENGINE. - -- CARTWRIGHT'S PISTON. -- BARTON'S PISTON. - - -(118.) In the single-acting engine, the force of the piston acted -on the beam only during its descent; and this force was -transmitted from the piston to the beam, as we have seen, by a -flexible chain, extended from the end of the piston-rod, [Pg194] -and playing upon the arch head of the beam. In the double-acting -engine, however, the force of the steam pressing the piston -upwards must likewise be transmitted to the beam, so as to drive -the latter upwards while the piston ascends. This action could not -be accomplished by a chain connecting the piston with the arch -head of the beam. - -Where the mechanical action to be transmitted is a _pull_, and not -a _push_, a flexible chain, cord, or strap, is sufficient; but if -a _push_ or _thrust_ is required to be transmitted, then the -flexibility of the medium of mechanical communication afforded by -a chain renders it inapplicable. In the double-acting engine, -during the descent, the piston-rod still pulls the beam down; and -so far a chain connecting the piston-rod with the beam would be -sufficient to transmit the action of the one to the other; but in -the ascent, the beam no longer pulls up the piston-rod, but is -pushed up by it. A chain from the piston-rod to the arch head, as -described in the single-acting engine, would fail to transmit this -force. If such a chain were used with the double engine, where -there is no counterweight on the opposite end of the beam, the -consequence would be, that in the ascent of the piston the chain -would slacken, and the beam would still remain depressed. It is -therefore necessary that some other mechanical connection be -contrived between the piston-rod and the beam, of such a nature -that in the _descent_ the piston-rod may _pull_ the beam down, and -may _push_ it up in the _ascent_. - -[Illustration: _Fig._ 35.] - -Watt first proposed to effect this by attaching to the end of the -piston-rod a straight rack, faced with teeth, which should work in -corresponding teeth raised on the arch head of the beam, as -represented in _fig._ 35. If his improved steam engines required no -further precision of operation and construction than the atmospheric -engines, this might have been sufficient; but in these engines it -was indispensably necessary that the piston-rod should be guided -with a smooth and even motion through the stuffing-box in the top of -the cylinder, otherwise any shake or irregularity would cause it to -work loose in the stuffing-box, and either to admit the air, or to -let the steam escape. Under these circumstances, the motion of -[Pg195] the rack and toothed arch head were inadmissible, since it -was impossible by such means to impart to the piston-rod that smooth -and equable motion which was requisite. Another contrivance which -occurred to Watt was, to attach to the top of the piston-rod a bar, -which should extend above the beam, and to use two chains or straps, -one extending from the top of the bar to the lower end of the arch -head, and the other from the bottom of the bar to the upper end of -the arch head. By such means the latter strap would pull the beam -down when the piston would descend, and the former would pull the -beam up when the piston would ascend. These contrivances, however, -were superseded by the celebrated mechanism since called the -_Parallel Motion_, one of the most ingenious mechanical combinations -connected with the history of the steam engine. - - -(119.) It will be observed that the object was to connect by some -inflexible means the end of the piston-rod with the extremity of -the beam, and so to contrive the mechanism, that while the end of -the beam would move alternately up and down in part of a circle, -the end of the piston-rod connected with the beam should move up -and down in a straight line. If the end of the piston-rod were -fastened upon the end of the beam by a pivot without any other -connection, it is evident that, being moved up and down in the -arch of a circle, it would be drawn to the left and the right -alternately, and would consequently either be broken or bent, or -would work loose in the stuffing-box. Instead of connecting the -end of the rod immediately with the end of the beam by a pivot, -Watt proposed to connect them by certain moveable rods, so -arranged that, as the end of the beam would move up and down in -the circular arch, the rods would so accommodate themselves to -that motion, that the end connected with the piston-rod should not -be disturbed from its rectilinear course. - -To explain the principle of the mechanism called the parallel -motion, let us suppose that O P (_fig._ 36.) is a rod or lever -moveable on a centre O, and that the end P of this rod shall move -through a circular arch P P' P'' P''' a vertical plane, and let its -play be limited by two stops S, which shall prevent its ascent -above the point P, and its descent below [Pg196] the point P'''. -Let the position of the rod and the limitation of its play be such -that the straight line A B drawn through P and P''', the extreme -positions of the lever O P, shall be a vertical line. - -[Illustration: _Fig._ 36.] - -Let _o_ be a point on the other side of the vertical line A B, and -let the distance of O to the right of A B be the same as the -distance of _o_ to the left of A B. Let _o p_ be a rod equal in -length to O P, moving like O P on the centre _o_, so that its -[Pg197] extremity _p_ shall play upwards and downwards through the -arch _p p' p'' p'''_, its play being limited in like manner by stops -_s_. - -Now, let us suppose that the ends P _p_ of these two rods are joined -by a link P _p_, the connection being made by a pivot, so that the -angles formed by the link and the rods shall be capable of changing -their magnitude. This link will make the motion of one rod depend on -that of the other, since it will preserve their extremities P _p_ -always at the same distance from each other. If, therefore, we -suppose the rod O P to be moved to the position O P''', its extremity -P tracing the arch P P' P'' P''', the link connecting the rods will at -the same time drive the extremity _p_ of the rod _o p_ through the -arch _p p' p'' p'''_ so that when the extremity of the one rod arrives -at P''', the extremity of the other rod will arrive at _p'''_. By this -arrangement, in the simultaneous motion of the rods, whether upwards -or downwards, through the circular arches to which their play is -limited, the extremities of the link joining them will deviate from -the vertical line A B in opposite directions. At the limits of their -play, the extremities of the link will always be in the line A B; -but in all intermediate positions, the lower extremity of the link -will be to the right of A B, and its upper extremity to the left of -A B. So far as the derangement of the lower extremity of the link is -concerned, the matter composing the link would be transferred to the -right of A B, and so far as the upper extremity of the link is -concerned, the matter composing it would be transferred to the left -of A B. - -By the combined effects of these contrary derangements of the -extremities of the link from the vertical line, it might be -expected that a point would exist, in the middle of the link, -where the two contrary derangements would neutralise each other, -and which point would therefore be expected to be disturbed -neither to the right nor to the left, but to be moved upwards and -downwards in the vertical line A B. Such is the principle of the -parallel motion; and in fact the middle point of the link will -move for all practical purposes accurately in the vertical line A -B, provided that the angular play of the levers O P and _o p_ does -not exceed a certain [Pg198] limit, within which, in practice, -their motion may always be restrained. - -To trace the motion of the middle point of the link more minutely, -let P P' P'' P''' be four positions of the lever O P, and let _p_ _p'_ -_p''_ _p'''_ be the four corresponding positions of the lever _o p_. In -the positions O P _o p_, the link will take the position P _p_, in -which the entire link will be vertical, and its middle point _x_ -will therefore be in the vertical line A B. - -When the one rod takes the position O P', the other rod will have -the position _o p'_; and the link will have the position P' _p'_. -The middle point of the link will be at _x'_, which will be found -to be on the vertical line A B. Thus one half of the link P' _x'_ -will be to the left of the vertical line A B; while the other -half, _p' x'_, will be to the right of the vertical line; the -derangement from the vertical line affecting each half of the link -in contrary directions. - -Again, taking the one rod in the position O P'', the corresponding -position of the other rod will be _o p''_, and the position of the -link will be P'' _p''_. If the middle point of the link in this -position be taken, it will be found to be at _x''_, on the vertical -line A B; and, as before, one half of the link P'' _x''_ will be -thrown to the left of the vertical line, while the other half _p'' -x''_, will be thrown to the right of the vertical line. - -Finally, let the one rod be in its lowest position, O P''', while -the other rod shall take the corresponding position, _o p'''_. The -direction of the link P''' _p'''_ will now coincide with the vertical -line; and its middle point _x'''_ will therefore be upon that line. -The previous derangement of the extremities of the rod, to the -right and to the left, are now redressed, and all the parts of the -rod have assumed the vertical position. - -It is plain, therefore, that by such means the alternate motion of -a point such as P or _p_, upwards and downwards in a circular -arch, may be made to produce the alternate motions of another -point _x_, upwards and downwards in a straight line. - - -(120.) Although the guidance of the air-pump rod in a true -vertical line is not so necessary as that of the steam piston, -[Pg199] and as the air-pump piston is always brought down by its -own weight and that of its rod, the connection of the air-pump -piston-rod with the beam, by any contrivance of the kind now -described, was not so necessary. Nevertheless, by a slight -addition to the mechanical contrivance which has been just -described, Watt obtained the means of at once preserving the true -rectilinear motion of both piston-rods. - -[Illustration: _Fig._ 37.] - -Let the lever represented by O P in _fig._ 36. be conceived to be -prolonged to twice its length, as represented in _fig._ 37., so -that O P' shall be twice O P. Let the points P _p_ be connected by -a link as before. Let a link P' _x'_, equal in length to the link -P _p_ be attached to the point P', and let the extremity _x'_ of -this link be connected with the point _p_ by another link, equal -in length to P P', by pivots at _x'_ and _p_, so that the figure P -P' _x' p_ shall be a jointed parallelogram, the angles of which -will be capable of altering their magnitude with every change of -position of the rods _o p_ and O P. Thus, when the rod O P -descends, the angles of the parallelogram at P and _x'_ will be -diminished in magnitude, while the angles at P' and _p_ will be -increased in magnitude. Now, let a line be conceived to be drawn -from O to _x'_. It is evident that that line will pass through the -middle point of the link _p_ P, for the triangle O P _x_ is in all -respects similar to the greater triangle O P' _x'_ only on half -the scale, so that every side of the one is [Pg200] half the -corresponding side of the other. Therefore P _x_ is half the -length of P' _x'_; but P' _x'_ was made equal to P _p_, and -therefore _p x_ is half of P _p_, that is to say, _x_ is the -middle point of P _p_. - -It has been already shown, that in the alternate motion of the -rods _o p_, O P in ascending and descending, the point _x_ is -moved upwards and downwards in a true vertical line. Now since the -triangle O P _x_ is in all respects similar to O P' _x'_, and -subject to a similar motion during the ascent and descent of the -rods, it is apparent that the point _x'_ must be subject to a -motion in all respects similar to that which affects the points -_x_, except that the point _x'_ will move through double the -space. In fact, the principle of the mechanism is precisely -similar to that of the common pantograph, where two rods are so -connected as that the motion of the one governs the motion of the -other, so that whatever line or figure may be described by one, a -similar line or figure must be described by the other. Since, -then, the point _x_ is moved upwards and downwards in a vertical -straight line, the point _x'_ will also be moved in a vertical -straight line of double the length. - -If such an arrangement of mechanism as has been here described can -be connected with the beam of the steam engine, so that while the -point _x'_ is attached to the top of the steam piston, and the -space through which it ascends and descends shall be equal to the -length of the stroke of that piston, the point _x_ shall be -attached to the rod of the air-pump piston, the stroke of the -latter being half that of the steam piston, then the points _x'_ -and _x_ will guide the motion of the two pistons so as to preserve -them in true vertical straight lines. - -The manner in which these ideas are reduced to practice admits of -easy explanation: let the point O be the centre of the great -working beam, and let O P' be the arm of the beam on the side of -the steam cylinder. Let P be a pivot upon the beam, at the middle -point between its centre O and its extremity P'; and let the links -P p, P' _x'_, and P _p_ be jointed together, as already described. -Let the point or pivot _o_ be attached to some part of the fixed -framing of the engine or engine house, and let the rod _o p_, -equal to half the arm of the beam, be attached by a pivot to the -corner of the parallelogram at [Pg201] _p_. Let the end of the -steam piston-rod be attached to the corner of the parallelogram -_x'_, and let the end of the air-pump be attached to the middle -point _x_ of the link P _p_; by which arrangement it is evident -that the rectilinear motion of the two piston-rods will be -rendered compatible with the alternate circular motions of the -points P' and P on the beam. - -Among the many mechanical inventions produced by the fertile -genius of Watt, there is none which has excited such universal, -such unqualified, and such merited admiration as that of the -parallel motion. It is indeed impossible, even for an eye -unaccustomed to view mechanical combinations, to behold the beam -of a steam engine moving the pistons, through the instrumentality -of the parallel motion, without an instinctive feeling of pleasure -at the unexpected fulfilment of an end by means having so little -apparent connection with it. When this feeling was expressed to -Watt himself, by those who first beheld the performance of this -exquisite mechanism, he exclaimed with his usual vivacity, that he -himself, when he first beheld his own contrivance in action, was -affected by the same sense of pleasure and surprise at its -regularity and precision. He said, that he received from it the -same species of enjoyment that usually accompanies the first view -of the successful invention of another person. - -"Among the parts composing the steam engine, you have doubtless," -says M. Arago, "observed a certain articulated parallelogram. At -each ascent and descent of the piston, its angles open and close -with the sweetness--I had almost said with the grace--which charms -you in the gestures of a consummate actor. Follow with your eye -alternately the progress of its successive changes, and you will -find them subject to the most curious geometrical conditions. You -will see, that of the four angles of the jointed parallelogram, -three describe circular arches, but the fourth which holds the -piston-rod is moved nearly in a straight line. The immense utility -of this result strikes mechanicians with even less force than the -simplicity of the means by which Watt has attained it." - -The parallel motion, of which there are several other varieties, -depending, however, generally upon the same [Pg202] principle, -formed part of a patent which Mr. Watt obtained in the year 1784, -another part of which patent was for a locomotive engine, by which -a carriage was to be propelled on a road. In a letter to Mr. -Smeaton dated 22d October, in the same year, Watt says,-- - -"I have lately contrived several methods of getting entirely rid -of all the chains and circular arches about the great levers of -steam engines, and nevertheless making the piston-rods ascend and -descend perpendicularly, without any sliding motions or -right-lined guides, merely by combinations of motions about -centres; and with this further advantage, that they answer equally -well to push upwards as to pull downwards, so that this method is -applicable to our double engines which act both in the ascent and -descent of their pistons. - -"A rotative engine of this species with the new motion which is -now at work in our manufactory (but must be sent away very soon) -answers admirably. It has cost much brain work to contrive proper -working gear for these double engines, but I have at last done it -tolerably well, by means of the circular valves, placed in an -inverted position, so as to be opened by the force of the steam; -and they are kept shut by the working gear. We have erected an -engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield, -London." - -[Illustration: _Fig._ 38.] - - -(121.) By the contrivance which has been explained above, the -force of the piston in ascending and descending would be conveyed -to the working end of the beam; and the next problem which Watt -had to solve was, to produce by the force exerted by the working -end of the beam in ascending and descending a continuous motion of -rotation. In the first instance he proposed to accomplish this by -a crank placed upon the axle to which rotation was to be imparted, -and driven by a rod connecting it with the working end of the -beam. Let K (_fig._ 38.) be the centre, to which motion is to be -imparted by the working end H of the beam. On the axle K suppose a -short lever K I to be fixed so that when K I is turned round the -centre K, the axle must turn with it. Let an iron rod, the weight -of which shall balance the piston and piston-rod at the other end -of the beam, be connected by joints with the working end H of the -beam, and the extremity I of the [Pg203] lever K I. As the end H -of the beam is moved upwards and downwards, the lever K I will be -turned round the centre K, taking successively the positions -represented by faint lines in the figure; and thus a motion of -continued rotation will be imparted to the axle K. - -This simple and effectual expedient of producing a continued -rotatory motion by a crank was abandoned by Watt, as already -explained, by reason of a patent having been obtained upon -information of his experiments surreptitiously procured. To avoid -litigation, he therefore substituted for the crank the sun and -planet wheel already described; but at the expiration of the -patent, which restricted the use of the crank, the sun and planet -wheel was discontinued in Watt's engine, and the crank restored. - - -(122.) Whether the crank or the sun and planet wheel be used, -there is still a difficulty in the maintenance of a regular motion -of rotation. In the various positions which the crank and -connecting rod assume throughout a complete revolution, there are -two in which the moving power loses all influence in impelling the -crank. These positions are those which the crank assumes when the -piston is at the top and bottom of the [Pg204] cylinder, and is -just about to change the direction of its motion. When the piston -is at the bottom of the cylinder, the pivot I (_fig._ 38.), by -which the connecting rod H I is attached to the end of the crank, -is immediately over the axle K of the crank, and under the pivot -H, which joins the upper end of the connecting rod with the beam. -In fact, in this position the connecting rod and crank are in the -same straight line, extending from the end of the beam to the axle -of the crank. The steam, on entering the cylinder below the -piston, and pressing it upwards, would produce a corresponding -downward force on the connecting rod at H, which would be -continued along the connecting rod and crank to the axle K. It is -evident that such a force could have no tendency to turn the crank -round, but would expend its whole energy in pressing the axle K -downwards. - -The other position in which the power loses its effect upon the -crank is when the piston is at the top of the cylinder. In this -case, the working end of the beam will be at the lowest point of -its play, and the crank-pin I will be immediately below the axle -K; so that K will be placed immediately between H and I. When the -steam presses on the top of the piston, it will expend its force -in drawing the end H of the connecting rod upwards, by which the -crank-pin I will likewise be drawn upwards. It is evident that -this force can have no effect in turning the crank round, but will -expend its whole energy in producing an upward strain on the axle -K. - -If the crank were absolutely at rest in either of the positions -above described, it is apparent that the engine could not be put -in motion by the steam; but if the engine has been previously in -motion, then the mass of matter forming the crank, and the axle on -which the crank is formed, having already had a motion of -rotation, will have a tendency to preserve the momentum it has -received, and this tendency will be sufficient to throw the crank -K I out of either of those critical positions which have been -described. Having once escaped these dead points, then the -connecting rod forming an angle, however obtuse or acute, with the -crank, the pressure or pull upon the former will have a tendency -to produce rotation in the latter. As the crank revolves, however, -the influence [Pg205] of the connecting rod upon it will vary -according to the angle formed by the connecting rod and crank. -When that angle is a right angle, then the effect of the -connecting rod on the crank is greatest, since the force upon it -has the advantage of the whole leverage of the crank; but -according as the angle formed by the crank and connecting rod -becomes more or less acute or obtuse in the successive attitudes -which they assume in the revolution of the crank, the influence of -the connecting rod over the crank varies, changing from nothing at -the two dead points already described, to the full effect produced -in the two positions where they are at right angles. In -consequence of this varying leverage, by which the force with -which the connecting rod is driven by the steam is transmitted to -the axle on which the crank revolves, a corresponding variation of -speed would necessarily be produced in the motion imparted to the -crank. The speed at the dead points would be least, being due -altogether to the momentum already imparted to the revolving mass -of the crank and axle; and it would gradually increase and be -greatest at the points where the effect of the crank on the -connecting rod is greatest. Although this change of speed would -not affect the actual mechanical efficacy of the machine, and -although the same quantity of steam would perform the same work at -the varying velocity as it would do if the velocity were -regulated, yet this variation of speed would be incompatible with -the purposes to which it was now proposed that the steam engine -should be applied in manufactures. In these a regular uniform -motion should be imparted to the main axle. - - -(123.) One of the expedients which Watt proposed for the -attainment of this end was, by placing two cranks on the same -axle, in different positions, to be worked by different cylinders, -so that while one crank should be at its dead points, the other -should be in the attitude most favourable for its action. This -expedient has since, as we shall see, been carried into effect in -steam vessels; but one more simple and efficient presented itself -in the use of a _fly-wheel_. - -On the main axle driven by the crank Watt placed a large wheel of -metal, as represented in _fig._ 43., called a _fly-wheel_. This -wheel being well constructed, and nicely balanced on its [Pg206] -axle, was subject to very little resistance from friction; any -moving force which it would receive it would therefore retain, and -would be ready to impart such moving force to the main axle -whenever that axle ceased to be driven by the power. When the -crank, therefore, is in those positions in which the action of the -power upon it is most efficient, a portion of the energy of the -power is expended in increasing the velocity of the mass of matter -composing the fly-wheel. As the crank approaches the dead points, -the effect of the moving power upon the axle and upon the crank is -gradually enfeebled, and at these points vanishes altogether. The -momentum which has been imparted to the fly-wheel then comes into -play, and carries forward the axle and crank out of the dead -points with a velocity very little less than that which it had -when the crank was in the most favourable position for receiving -the action of the moving power. - -By this expedient, the motion of revolution received by the axle -from the steam piston is subject to no other variation than just -the amount of change of momentum in the great mass of the -fly-wheel, which is sufficient to extricate the crank twice in -every revolution from the mechanical dilemma to which its peculiar -form exposes it; and this change of velocity may be reduced to as -small an amount as can be requisite by giving the necessary weight -and magnitude to the fly-wheel. - - -(124.) By such arrangements the motion imparted to the main axle K -would be uniform, provided that the moving power of the engine be -always proportionate to the load which it drives. But in the -general application of the steam engine to manufactures it was -evident that the amount of the resistance to which any given -machine would be subject must be liable to variation. If, for -example, the engine drive a cotton-mill, it will have to impart -motion to all the spinning frames in that mill. The operation of -one or more of these may from time to time be suspended, and the -moving power would be relieved from a corresponding amount of -resistance. If, under such circumstances, the energy of the moving -power remained the same, the velocity with which the machines -would be driven would be subject to variation, being increased -whenever the operation of any portion of the machines usually -[Pg207] driven by it is suspended; and, on the other hand, -diminished when any increased number of machines are brought into -operation. In fine, the speed would vary nearly in the inverse -proportion of the load driven, increasing as the load is -diminished, and _vice versâ_. - -On the other hand, supposing that no change took place in the -amount of the load driven by the engine, and that the same number -of machines of whatever kind would have to be continually driven, -the motion imparted to the main axle would still be subject to -variation by the changes inevitable to the moving power. The -piston of the engine being subject to an unvaried resistance, a -uniform motion could only be imparted to it, by maintaining a -corresponding uniformity in the impelling power. This would -require a uniform supply of steam from the boiler, which would -further imply a uniform rate of evaporation in the boiler, unless -means were provided in the admission of steam from the boiler to -the cylinder to prevent any excess of steam which might be -produced in the boiler from reaching the cylinder. - -[Illustration: _Fig._ 39.] - -[Illustration: _Fig._ 40.] - -This end was attained by a contrivance afterwards called the -_throttle-valve_. An axis A B (_figs._ 39, 40.) was placed across -the steam pipe in a ring of cast-iron D E, of proper thickness. On -this axis was fastened a thin circular plate T, of nearly the same -diameter as the steam pipe. On the outer end B of this axle was -placed a short lever or handle B C, by which it could be turned. -When the circular plate T was turned into such a position as to be -at right angles to the length of the tube, it stopped the passage -within the tube altogether, so that no steam could pass from the -boiler to the engine. On the other hand, when the handle was -turned through a fourth of a revolution from this position, then -the circular plate T had its plane in the direction of the length -of the tube, so that its edge would be presented towards the -current of steam flowing from the boiler to the cylinder. In that -position the passage within the tube [Pg208] would be necessarily -unobstructed by the throttle-valve. In intermediate positions of -the valve, as that represented in _figs._ 39, 40., the passage -might be left more or less opened, so that steam from the boiler -might be admitted to the cylinder in any regulated quantity -according to the position given to the lever B C. - -A view of the throttle-valve taken by a section across the steam -pipe is exhibited in _fig._ 40., and a section of it through the -axis of the steam pipe is represented in _fig._ 39. The form of -the valve is such, that, if accurately constructed, the steam in -passing from the boiler would have no effect by its pressure to -alter any position which might be given to the valve; and any -slight inaccuracy of form which might give a tendency to the steam -to alter the position would be easily counteracted by the friction -of the valve upon its axle. The latter might be regulated at -pleasure. - -By this expedient, however the evaporation of water in the boiler -might vary within practical limits, the supply of steam to the -cylinder would be rendered regular and uniform. If the boiler became -too active, and produced more steam than was necessary to move the -engine with its load at the requisite speed, then the throttle-valve -was shifted so as to contract the passage and limit the supply of -steam. If, on the other hand, the process of evaporation in the -boiler was relaxed, then the throttle-valve was placed with its edge -more directed towards the steam. Independently of the boiler, if the -load on the engine was lightened, then the same supply of steam to -the cylinder would unduly accelerate the motion. In this case, -likewise, the partial closing of the throttle-valve would limit the -supply of steam and regulate the motion; and if, on the other hand, -the increase of load upon the engine rendered necessary an increased -supply of steam, then the opening of the throttle-valve would -accomplish the purpose. By these means, therefore, a uniform motion -might be maintained, provided the vigilance of the engine-man -was sufficient for the due management of the lever B C, and -provided that the furnace under the boiler was kept in sufficient -activity to supply the greatest amount of steam which would be -necessary [Pg209] for the maintenance of a uniform motion with the -throttle-valve fully opened. - - -(125.) Watt, however, soon perceived that the proper manipulation -of the lever B C would be impracticable with any degree of -vigilance and skill which could be obtained from the persons -employed to attend the engine. He, therefore, adapted to this -purpose a beautiful application of a piece of mechanism, which had -been previously used in the regulation of mill-work, and which has -since been well known by the name of the _Governor_, and has -always been deservedly a subject of much admiration. - -The governor is an apparatus by which the axle of the fly-wheel is -made to regulate the throttle-valve, so that the moment that the -axle begins to increase its velocity, it shifts the position of -the throttle-valve, so as to limit the supply of steam from the -boiler, and thereby to check the increase of speed. And on the -other hand, whenever the velocity of the axle is diminished, the -lever B C is moved in the contrary direction, so as to open more -fully the passage for the steam, and accelerate the motion of the -engine. - -A small grooved wheel A B (_fig._ 41.) is attached to a vertical -spindle supported in pivots or sockets C and D, in which it is -capable of revolving. An endless cord works in the groove A B, and -is carried over proper pulleys to the axle of the fly-wheel, where -it likewise works in a groove. When this cord is properly -tightened the motion of the fly-wheel will give motion to the -wheel A B, so that the velocity of the one will be subject to all -the changes incidental to the velocity of the other. By this means -the speed of the grooved wheel A B may be considered as -representing the speed of the fly-wheel, and of the machinery -which the axle of the fly-wheel drives. - -[Illustration: _Fig._ 41.] - -It is evident that the same end might be attained by substituting -for the grooved wheel A B a toothed wheel, which might be -connected by other toothed wheels, and proper shafts, and axles -with the axle of the fly-wheel. - -A ring or collar E is placed on the upright spindle, so as to be -capable of moving freely upwards and downwards. To this ring are -attached by pivots two short levers, E F, the [Pg210] pivots or -joints at E allowing these levers to play upon them. At F these -levers are joined by pivots to other levers F G, which cross each -other at H, where an axle or pin passes through them, and attaches -them to the upright spindle C D. These intersecting levers are -capable, however, of playing on this axle or pin H. To the ends G -of these levers are attached two heavy balls of metal I. The -levers F G pass through slits in a metallic arch attached to the -upright spindle, so as to be capable of revolving upon it. If the -balls I are drawn outwards from the vertical axis, it is evident -that the ends F of the levers will be drawn down, and therefore -the pivots E likewise drawn down. In fact, the angles E F H will -become more acute, and the angle F E F more obtuse. By these means -the sliding ring E will be drawn down. To this sliding ring E, and -immediately above it, is attached a grooved collar, which slides -on the vertical spindle upwards and downwards with the ring E. In -the grooved collar are inserted the prongs of a fork K, formed at -the end of the lever K L, the fulcrum or pivot of the lever being -at L. By this arrangement, when the divergence of the balls I -causes the collar E to be drawn down, the fork K, whose prongs are -inserted in the groove of that collar, is likewise drawn down; -and, on the other hand, when, by reason of the balls I falling -towards the [Pg211] vertical spindle, the collar E is raised, the -fork K is likewise raised. - -The ascent and descent of the fork K necessarily produce a -contrary motion in the other end N of the lever. This end is -connected by a rod, or system of rods, with the end M of the short -lever which works the throttle-valve T. By such means the motion -of the balls I, towards or from the vertical spindle, produces in -the throttle-valve a corresponding motion; and they are so -connected that the divergence of the balls I will cause the -throttle-valve to close, while their descent towards the vertical -spindle will cause it to open. - -These arrangements being comprehended, let us suppose that, either -by reason of a diminished load upon the engine or an increased -activity of the boiler, the speed has a tendency to increase. This -would impart increased velocity to the grooved wheel A B, which -would cause the balls I to revolve with an accelerated speed. The -centrifugal force which attends their motion would therefore give -them a tendency to move from the axle, or to diverge. This would -cause, by the means already explained, the throttle-valve T to be -partially closed, by which the supply of steam from the boiler to -the cylinder would be diminished, and the energy of the moving -power, therefore, mitigated. The undue increase of speed would -thereby be prevented. - -If, on the other hand, either by an increase of the load, or a -diminished activity in the boiler, the speed of the machine was -lessened, a corresponding diminution of velocity would take place -in the grooved wheel A B. This would cause the balls I to revolve -with less speed, and the centrifugal force produced by their -circular motion would be diminished. This force being thus no -longer able fully to counteract their gravity, they would fall -towards the spindle, which would cause, as already explained, the -throttle-valve to be more fully opened. This would produce a more -ample supply of steam to the cylinder, by which the velocity of -the machine would be restored to its proper amount. - -[Illustration: _Fig._ 42.] - - -(126.) The principle which renders the governor so perfect a -regulator of the velocity of the machine is difficult to be -[Pg212] explained without having recourse to the aid of the -technical language of mathematical physics. As, however, this -instrument is of such great practical importance, and has -attracted such general admiration, it may be worth while here to -attempt to render intelligible the mechanical principles which -govern its operation. Let S (_fig._ 42.) be the point of -suspension of a common pendulum S P, and let P O P' be the arch of -its vibration, so that the ball P shall swing or vibrate -alternately to the east and to the west of the lowest point O, -through the arches O P' and O P. It is a property of such an -instrument that, provided the arch in which it vibrates be not -considerable in magnitude, the time of its vibration will be the -same whether the arch be long or short. Thus, for example, if the -pendulum, instead of vibrating in the arch P P', vibrated in the -arch _p p'_, the time which it would take to perform its -vibrations would be the same. If, however, the magnitude of the -arch of vibration be increased, then a variation will take place -in the time of vibration; but unless the arch of vibration be -considerably increased, this variation will not be great. - -Now let it be supposed that while the pendulum P P' continues to -vibrate east and west through the arch P P', it shall receive such -an impulse from north and south as would, if it were not in a -state of previous vibration, cause it to vibrate between north and -south, in an arch similar to the arch P P'. This second vibration -between north and south [Pg213] would not prevent the continuance -of the other vibration between east and west; but the ball P would -be at the same time affected by both vibrations. While, in virtue -of the vibration from east to west, the ball would swing from P to -P', it would, in virtue of the other vibration, extend its motion -towards the north to a distance from the line W E equal to half a -vibration, and will return from that distance again to the -position P'. While returning from P' to P, its second vibration -will carry it towards the south to an equal distance on the -southern side of W E, and it will return again to the position P. -If the combination of these two motions or vibrations be -attentively considered, it will be perceived that the effect on -the ball will be a circular motion, precisely similar to the -circular motion of the balls of the governor already described. - -Now the time of vibration of the pendulum S P between east and -west will not in any way be affected by the second vibration, -which it is supposed to receive between north and south, and -therefore the time the pendulum takes in moving from P to P' and -back again from P' to P will be the same whether it shall have -simultaneously or not the other vibration between north and south. -Hence it follows that the time of revolution of the circular -pendulum will be equal to the time of similar vibrations of the -same pendulum, if, instead of having a circular motion, it were -allowed to vibrate in the manner of a common pendulum. - -If this point be understood, and if it also be remembered that the -time of vibration of a common pendulum is necessarily the same -whether the arch of vibration be small or great, it will be easily -perceived that the revolving pendulum or governor will have nearly -the same time of revolution whether it revolve in a large circle -or a small one: in other words, whether the balls revolve at a -greater or a less distance from the central spindle or axis. This, -however, is to be understood only approximately. When the angle of -divergence of the balls is as considerable as it usually is in -governors, the time of revolution at different distances from the -axis will therefore be subject to some variation, but to a very -small one. [Pg214] - -The centrifugal force (which is the name given in mechanics to -that influence which makes a body revolving in a circle fly from -the centre) depends conjointly on the velocity of revolution, and -on the distance of the revolving body from the centre of the -circle. If the velocity of revolution be the same, then the -centrifugal force will increase in the same proportion as the -distance of the revolving body from the centre. If, on the other -hand, the distance of the revolving body from the centre remain -the same, the centrifugal force will increase in the same -proportion as the square of the time of vibration diminishes, or, -in other words, it will increase in the same proportion as the -square of the number of revolutions per minute. It follows from -this, therefore, that the greater is the divergence of the balls -of the governor, and the more rapidly they revolve, the greater -will be their centrifugal force. Now this centrifugal force, if it -were not counterbalanced, would give the balls a constant tendency -to recede from the centre; but from the construction of the -apparatus, the further they are removed from the centre the -greater will be the effect of their gravitation in resisting the -centrifugal force. - -It is evident that the ball at P will have a greater tendency to -fall by gravitation towards O than it would have at _p_, because -the acclivity of the arch descending towards O at P is greater -than its acclivity at _p_. The gravitation, therefore, or tendency -of the ball to fall towards the central axis being greater at P -than at _p_ it will be able to resist a greater centrifugal force. -This increased centrifugal force, which the ball would have -revolving at the distance P above what it would have at the -distance _p_, is produced partly by the greater distance of the -ball from the central axis, and partly by the greater velocity of -its motion. But it will be evident that the time of its revolution -may nevertheless be the same, or nearly the same, at both -distances. If it should appear that the actual velocity of its -motion of revolution at P be greater than its velocity at _p_, in -the same proportion as the circles in which they revolve, then it -is evident that the time of revolution would be as much increased -by the greater space which P will have to travel over, as it will -have to be [Pg215] diminished by the greater speed with which -that space is traversed. The time of revolution, therefore, may be -the same, or nearly the same, in both cases. - -If this explanation be comprehended, it will not be difficult to -apply it to the actual case of the governor. If a sudden increase -of the energy of the moving power, or a diminution of the load, -should give the machine an increased velocity, then the increased -speed of the balls of the governor will give them an increased -centrifugal force, which for the moment will be greater than the -tendency of their gravitation to make them fall towards the -vertical axis. This centrifugal force, therefore, prevailing, the -balls will recede from the axis; but as they recede, their -gravitation towards the vertical axis will, as has been already -explained, be increased, and will become equal to the centrifugal -force produced by the increased velocity, provided that velocity -do not exceed a certain limit. When the balls, by diverging, get -such increased gravitation as to balance the centrifugal force, -then they will continue to revolve at a fixed distance from the -vertical axis. When this happens, the time of the revolution must -be nearly the same as it was before their increased divergence; in -other words, the proportion of the moving power to the load will -be so restored by the action of the levers of the governor on the -throttle-valve that the machine will move at its former velocity, -or nearly so. - -The principle on which the governor acts, as just explained, -necessarily supposes temporary disarrangements of the speed. In -fact, the governor, strictly speaking, does not maintain a uniform -velocity, but restores it after it has been disturbed. When a -sudden change of motion of the engine takes place, the governor -being immediately affected will cause a corresponding alteration -in the throttle-valve; and this will not merely correct the change -of motion, but it will, as it were, overdo it, and will cause a -derangement of speed of the opposite kind. Thus if the speed be -suddenly increased to an undue amount, then the governor being -affected will first close the throttle-valve too much, so as to -reduce the speed below the proper limit. This second error will -again affect the governor in the contrary way, and the speed -[Pg216] will again be increased rather too much. In this way a -succession of alterations of effect will ensue until the governor -settles down into that position in which it will maintain the -engine at the proper speed. - -To prevent the inconvenience which would attend any excess of such -variations, the governor is made to act with great delicacy on the -throttle-valve, so that even a considerable change in the -divergence of the balls shall not produce too much alteration in -the opening of that valve: the steam in the boiler should have at -least 2 lbs. per square inch pressure more than is generally -required in the cylinder. This excess is necessary to afford scope -for that extent of variation of the power which it is the duty of -the throttle-valve to regulate. - -The governor is usually so adjusted as to make thirty-six -revolutions per minute, when in uniform motion; but if the motion -is increased to the rate of thirty-nine revolutions, the balls -will fly to the utmost extent allowed them, being the limitation -of the grooves in which their rods move; and if, on the other -hand, the speed be diminished to thirty-four revolutions per -minute, they will collapse to the lowest extent of their play. The -duty of the governor, therefore, is to correct smaller casual -derangements of the velocity; but if any permanent change to a -considerable extent be made either in the load driven by the -machine or in the moving power supplied to it from the boiler, -then a permanent change is necessary to be made in the connection -between the governor and the throttle-valve, so as to render the -governor capable of regulating those smaller changes to which the -speed of the machine is liable. - - -(127.) Having thus explained the principal mechanical contrivances -provided by Watt for the maintenance and regulation of the -rotatory motion to be produced by his double-acting steam engine, -let us now consider the machine as a whole, and investigate the -process of its operation. A section of this engine is represented -in _fig._ 43. - -[Illustration: _Fig._ 43.] - -Steam is supplied from the boiler to the cylinder by the steam -pipe S. The throttle-valve T in that pipe, near the cylinder, is -regulated by a system of levers connected with [Pg217] the -governor. The piston P is accurately fitted in the steam cylinder -C by packing, as already described in the single-acting engine. -This piston, as it moves, divides the cylinder into two -compartments, between which there is no communication by which -steam or any other elastic fluid can pass. The upper steam box B -is divided into three compartments by the two valves. Above the -upper steam valve V is a compartment communicating with the steam -pipe; below the upper exhausting valve E is another compartment -communicating with the eduction pipe which leads to the condenser. -By the valves V and E a communication may be opened or closed -between the boiler on the one hand, or the condenser on the other, -and the top of the cylinder. The continuation S' of the steam pipe -leads to the lower box B', which, like the upper, is divided into -three compartments by two valves V' and E'. The upper compartment -communicates with the steam pipe, and thereby with the boiler; and -the lower compartment communicates with the eduction pipe, and -thereby with the condenser. By means of the two valves V' and E', -a communication may be opened or closed between the steam pipe on -the one hand, or the exhausting pipe on the other, and the lower -part of the cylinder. The four valves V, E, V', and E' are -connected by a system of levers with a handle or spanner _m_, -which, being driven downwards or upwards, is capable of opening or -closing the valves in pairs, in the manner already described -(116.). The condensers, the air-pump, and the hot-water pump, are -in all respects similar to those already described in the -single-acting engine, except that the condensing jet is governed -by a lever I, by which it is allowed to play continually in the -condenser, and by which the quantity of water admitted through it -is regulated. The cold-water pump N is worked by the engine as -already described in the single-acting engine, and supplies the -cistern in which the air-pump and condenser are submerged, so as -to keep down its temperature to the proper limit. On the air-pump -rod R are two pins properly placed, so as to strike the spanner -_m_, upwards and downwards, at the proper times, when the piston -approaches the termination of the stroke at the top or bottom of -the cylinder. The pump L [Pg218] conducts the warm water drawn by -the air-pump from the condenser to a proper reservoir for feeding -the boiler. The vertical motion of the piston-rod in a straight -line is rendered compatible with the circular motion of the end of -the beam by the parallel motion already described. The point _b_, -on the beam, moves upwards and downwards in a circular arch, of -which the axis of the beam is the centre. In like manner the point -_d_ of the rod _d c_ moves upwards and downwards, in a similar -arch of which the fixed pivot _c_ is the centre. The joint or bar -_d b_, which joins these two pivots, will be moved so that its -middle point _e_ will ascend and descend nearly in a straight -line, as has been already explained (120.); [Pg219] opposite this -point _e_ is attached the piston-rod of the air-pump, which is -accordingly guided upwards and downwards by this means. The -jointed parallelogram _b d g f_ is attached to the beam by pivots; -and, as has been explained (120.), the point _g_ will be moved -upwards and downwards in a straight line, through twice the space -through which the point _e_ is moved. To the point _g_ the rod of -the steam piston is attached. Thus, the rods of the steam piston -and air-pump are moved by the same system of jointed bars, and -moved through spaces which are in the proportion of two to one. - -Although this system of jointed rods forming the parallel motion, -appears in the figure to consist only of one parallelogram _b d g -f_, and one rod _c d_, called the _radius rod_, it is, in fact, -double, a similar parallelogram and radius rod being attached to -corresponding points, and in the same manner on the other side of -the beam; but from the view given in the cut, the one set of rods -hides the other. The two systems of rods thus attached to opposite -sides of the beam at several inches asunder, are connected by -cross rods, the ends of which form the pivots or joints, and -extend between the parallelograms. The ends of these rods are only -visible in the figure. It is to the middle of one of these rods, -the end of which is represented at _e_, that the air-pump -piston-rod is attached; and it is to the middle of another, the -end of which is represented at _g_, that the steam piston-rod is -attached. These two piston-rods, therefore, are driven, not -immediately by either of the parallelograms forming the parallel -motion, but by the bars extending between them. - -To the working end of the beam H is attached a rod of cast-iron O, -called the _connecting rod_, the lower end of which is attached to -the crank by a pivot. The weight of the connecting rod is so made, -that it shall balance the weight of the piston-rods of the air-pump -and cylinder on the other side of the beam; and the weight of the -piston-rod of the cold-water pump N nearly balances the weight of -the piston-rod of the hot-water pump L. Thus, so far as the weights -of the machinery are concerned, the engine is in equilibrium, and -the piston would rest in any position indifferently in the cylinder. - -The axis of the fly-wheel on which the crank is formed is [Pg220] -square in the middle part, where the fly-wheel is attached to it, -but has cylindrical necks at each end, which rest in sockets or -bearings supported by the framing of the machine, in which sockets -the axis revolves freely. On the axle of the crank is placed the -fly-wheel, and connected with its axle is the governor Q, which -regulates the throttle-valve T in the manner already described. - -Let us now suppose the engine to be in full operation. The piston -being at the top of the cylinder, the spanner _m_ will be raised -by the lower pin on the air-pump rod, and the upper steam valve V, -and the lower exhausting valve E', will be opened, while the upper -exhausting valve E and the lower steam valve V' are closed. Steam -will, therefore, be admitted above the piston, and the steam which -filled the cylinder below it will be drawn off to the condenser, -where it will be converted into water. The piston will, therefore, -be urged by the pressure of the steam above it to the bottom of -the cylinder. As it approaches that limit, the spanner _m_ will be -struck downwards by the upper pin on the air-pump rod, and the -valves V and E' will be closed, and at the same time the lower -steam valve V' and the upper exhausting valve E will be opened. -Steam will, therefore, be admitted below the piston, while the -steam above it will be drawn off into the condenser, and converted -into water. The pressure of the steam, therefore, below the piston -will urge it upwards, and in the same manner the motion will be -continued. - -While this process is going on in the cylinder and the condenser, -the water formed in the condenser will be gradually drawn off by -the operation of the air-pump piston, in the same manner as -explained in the single-acting engine; and at the same time the -hot water thrown into the hot well by the air-pump piston will be -carried off by the hot-water pump L. - -Such are the chief circumstances attending the continuance of the -operation of the double-acting engine. It is only necessary here to -recall what has been already explained respecting the operation of -the fly-wheel. The commencement of the motion of the piston from the -top and bottom of the cylinder is produced, not by the pressure of -the steam upon it upwards or downwards, which must, for the reasons -[Pg221] already explained, be entirely inefficient; but by the -momentum of the fly-wheel, which extricates the crank from those -positions in which the moving power cannot affect it. - -The manner in which the motion of the crank affects the connecting -rod at the dead points produces an effect of great importance in -the operation of the engine. When the crank-pin is approaching the -lowest point of its play, and therefore the piston approaching the -top of the cylinder, the motion of the crank-pin becomes nearly -horizontal, and consequently its effect in drawing the connecting -rod and the working end of the beam downwards and the piston -upwards, is extremely small. The consequence of this is, that as -the piston approaches the top of the cylinder, its motion becomes -very rapidly retarded; and as the motion of the crank-pin at its -lowest point is actually horizontal, the piston is brought to a -state of rest by this gradually retarded motion at the top of the -cylinder. In like manner, when the crank-pin moves from its dead -point upwards, its motion at first is very nearly horizontal, and -consequently its effect in driving the working end of the beam -upwards, and the piston downwards, is at first very small, but -gradually accelerated. The effect of this upon the piston is, that -it arrives at and departs from the top of the stroke with a very -slow motion, being absolutely brought to rest at that point. - -The same effect is produced when the piston arrives at the bottom -of the cylinder. This retardation and suspension of the motion of -the piston at the termination of the stroke affords time for the -process of condensation to be effected, so that when the moving -power of the steam upon the piston can come into action, the -condensation shall be sufficiently complete. As the piston -approaches the top of the cylinder, and its motion becomes slow, -the working gear is made to open the lower exhausting valve; the -steam enclosed in the cylinder below the piston, and which has -just driven the piston upwards, presses with an elastic force of -17 lbs. per square inch on every part of the interior of the -cylinder, while the uncondensed vapour in the condenser presses -with a force of about 2 lbs. per square inch. The steam, -therefore, will have a tendency to rush from the cylinder to the -[Pg222] condenser through the open exhausting valve, with an -excess of pressure amounting to 15 lbs. per square inch, while the -piston pauses at the top of the cylinder. This process goes on, -and when the piston has descended by the motion of the fly-wheel, -a sufficient distance from the top of the cylinder to call the -moving force of the steam into action, the exhaustion will be -complete, and the pressure of the uncondensed vapour in the -cylinder will become the same as in the condenser. - -The pressure of steam in the cylinder, and of uncondensed vapour -in the condenser, varies, within certain limits, in different -engines, and therefore the amount here assigned to them must be -taken merely as an example. - -The size of the valves by which the steam is allowed to pass from -the cylinder to the condenser should be such as to cause the -condensation to take place in a sufficiently short time, to be -completed when the steam impelling the piston is called into -action. - -Watt, in the construction of his engines, made the -exhaustion-valves with a diameter which was one fifth of the -diameter of the cylinder, and therefore the actual magnitude of -the aperture for the escape of the steam was one twenty-fifth of -the magnitude of the cylinder; but the spindle of the valve -diminished this so that the available space for the escape of -steam did not exceed one twenty-seventh of the magnitude of the -cylinder. This was found to produce a sufficiently rapid -condensation. - -It was usual to make the steam valves of the same magnitude as the -exhausting valves, but the flow of steam through the former was -resisted by the throttle-valve, while no obstruction was opposed -to its passage through the latter. - -The rapidity with which the cylinder must be exhausted by the -condenser will, however, depend upon the velocity with which the -piston is moved in it. The magnitude, therefore, of the exhausting -valves which would be sufficient for an engine which acts with a -slow motion would be too small where a rapid motion is required. - -In the single-acting steam engine, where the moving force always -acted downwards on the piston, the pressure upon [Pg223] all the -joints of the machinery by which the force of the piston was -conveyed to the working parts, always took place in the same -direction, and consequently whatever might be the mechanical -connection by which the several joints were formed, the pins by -which they were connected, must always come to a bearing in their -respective sockets, however loosely they may have been fitted. For -the same reason, however, that the arch head and chain were -abandoned as a means of connecting the steam piston with the beam, -and the parallel motion substituted, it was also necessary in the -double-acting engine, where all joints whatever were driven -alternately in opposite directions, to fit the connecting pins -with the greatest accuracy in their sockets, and to abandon all -connection of the parts by chains. If any sensible looseness was -left in the joints, a violent jerk would be produced every time -the motion of the piston was reversed. Any looseness either in the -pivots or joints of the parallel motion of the working beam, the -connecting rod, or crank, would, at every change of stroke, be so -accumulated as to produce upon the machinery the effects of -percussion, and would consequently be attended with the danger of -straining and breaking the moveable parts of the mechanism. - -To secure, therefore, the necessary accuracy of the joints, Watt -contrived that every joint in the engine should admit of the size of -the socket being exactly adapted to the size of the pin, so as -always to make a good fitting by closing the socket upon the pin, -when any looseness would be produced by wear. With this view, all -the joints were fitted with sockets made of brass or gun-metal, -capable of adjustment. Each socket was composed of two pieces, -accurately fitted into a cell or groove, in which one of the brasses -can be moved towards the other by means of a wedge or screw. Each -brass has in it a semi-cylindrical cavity, and the two cavities -being opposed to each other, form a socket for the joint-pin. One of -the two brasses can always be tightened round that pin, so as to -enclose it tight between the two semi-cylindrical cavities, and to -prevent any looseness taking place. The brasses, and other parts of -such a joint, are represented [Pg224] in _fig._ 44. These joints -still continue to be used in the engines as now constructed. - -[Illustration: _Fig._ 44.] - -The motion of the working beam, and the pump-rods which it drives, -and of the connecting rod, ought, if the whole were constructed -with perfect precision, to take place in the same or parallel -vertical planes; but this supposes a perfection of execution which -could hardly have been expected in the early manufacture of such -engines, whatever may have been attained by improvements which -have been since made. In the details of construction, Watt saw -that there would be a liability to lateral strain, owing to the -planes of the different motions not being truly vertical and truly -parallel, and that if a provision were not made for such lateral -motion, the machinery would be subject to constant strain in its -joints and rapid wear. He provided against this by constructing -the main joints by which the great working lever was connected -with the pistons and connecting rod, so as to form universal -joints, giving freedom of motion laterally as well as vertically. - -The great lever, or working beam, was so called from being -originally made from a beam of oak. It is now, however, -universally constructed of cast-iron. The connecting rod is also -made of cast-iron, and attached to the beam and to the crank by -axles or pivots. - -The mechanism by which the four valves are opened and closed, is -subject to considerable variation in different engines. They have -been described above as being opened and closed simultaneously by -a single lever. Sometimes, however, they are opened alternately in -pairs by two distinct levers driven by two pins attached to the -air-pump rod. One pin strikes the lever, which opens and closes -the upper steam valve, and lower exhausting valve; the other -strikes that which opens and closes the lower steam valve and -upper exhausting valve. - -Since the date of the earlier double-acting engines, constructed -by Boulton and Watt, a great variety of mechanical expedients have -been practised for working the valves, by which the steam is -admitted to and withdrawn from the [Pg225] cylinder. We shall -here describe a few of these methods:-- - - -(128.) The method of working the valves by pins on the air-pump -rod driving levers connected with the valves has been, in almost -all modern double-acting machines, superseded by an apparatus -called an _eccentric_, by which the motion of the axle of the -fly-wheel is made to open and close the valves at the proper -times. - -[Illustration: _Fig._ 45.] - -An eccentric is a metallic circle attached to a revolving axle, so -that the centre of the circle shall not coincide with the centre -round which the axle revolves. Let us suppose that G (_fig._ 45.), -is a square revolving shaft. Let a circular plate of metal B D, -having its centre at C, have a square hole cut in it, corresponding -to the shaft G, and let the shaft G pass through this square -aperture, so that the circular plate B D shall be fastened upon the -shaft, and capable of revolving with it as the shaft revolves. The -centre C of the circular plate B D will be carried round the centre -G of the revolving shaft, and will describe round it a circle, the -radius of which will be the distance of the centre C of the circular -plate from the centre of the shaft. Such circular plate so placed -upon a shaft, and revolving with it, is _an eccentric_. - -Let E F be a metallic ring, formed of two semicircles of metal -screwed together at H, so as to be capable, by the adjustment of -the screws, of having the circular aperture formed by the ring -enlarged and diminished within certain [Pg226] small limits. Let -this circular aperture be supposed to be equal to the magnitude of -the eccentric B D. To the circular ring E F let an arm L M be -attached. If the ring E F be placed around the eccentric B D, and -that the screws H be so adjusted as to allow the eccentric B D to -revolve within the ring E F, then while the eccentric revolves, -the ring not partaking of its revolution, the arm L M will be -alternately driven to the right and to the left, by the motion of -the centre C of the eccentric as it revolves round the centre G of -the axle. When the centre C of the eccentric is in the same -horizontal line with the centre G, and to the left of it, then the -position of L M will be that which is represented in _fig._ 45.; -but when, after half a revolution of the main axle, the centre C -of the eccentric is thrown on the other side of the centre G, then -the point M will be transferred to the right, to a distance equal -to twice the distance C G. Thus as the eccentric B D revolves -within the ring E F, that ring, together with the arm L M, will be -alternately driven, right and left, through a space equal to twice -the distance between the centre of the eccentric and the centre of -the revolving shaft. - -If we suppose a notch formed at the extremity of the arm L M, which -is capable of embracing a lever N M, moveable on a pivot at N, the -motion of the eccentric would give to such a lever an alternate -motion from right to left, and _vice versâ_. If we suppose another -lever N O connected with N M, and at right angles to it, forming -what is called a bell-crank, then the alternate motion received by -M, from right to left, would give a corresponding motion to the -extremity O of the lever N O, upwards and downwards. If this last -point O were attached to a vertical arm or shaft, it would impart to -such arm or shaft an alternate motion upwards and downwards, the -extent of which would be regulated by the length of the levers -respectively. - -By such a contrivance the revolution of the fly-wheel shaft is -made to give an alternate vertical motion of any required extent -to a vertical shaft placed near the cylinder, which may be so -connected with the valves as to open and close them. Since the -upward and downward motion of this vertical shaft is governed by -the alternate motion of the centre [Pg227] C to the right and to -the left of the centre G, it is evident that by the adjustment of -the eccentric upon the fly-wheel shaft, the valves may be opened -and closed at any required position of the fly-wheel and crank, -and therefore at any required position of the piston in the -cylinder. - -Such is the contrivance by which the valves, whatever form may be -given to them, are now almost universally worked in double-acting -steam engines. - - * * * * * - -Having described the general structure and operation of the steam -engine as improved by Watt, we shall now explain, in a more -detailed manner, some parts of its machinery which have been -variously constructed, and in which more or less improvements have -been made. - - -OF THE COCKS AND VALVES. - - -(129.) In the steam engine, as well as in every other machine in -which fluids act, it is necessary to open or close, occasionally, -the tubes or passages through which these fluids move. The -instruments by which this is accomplished are called cocks or -valves. - -Cocks or valves may be classified by the manner in which they are -opened: 1st, they may be opened by a motion similar to the lid of -a box upon its hinges; 2d, they may be opened by being raised -directly upwards, in the same manner as the lid of a pot or -kettle; 3d, they may be opened by a sliding motion, like that of -the sash of a window or the lid of a box which slides in grooves; -4th, they may be opened by a motion of revolution, in the same -manner as the cock of a beer-barrel is opened or closed. The term -_valve_ is more properly applied to the first and second of these -classes; the third class are usually called _slides_, and the -fourth _cocks_. - - -(130.) The single clack valve is the most simple example of the -first class. It is usually constructed by attaching to a plate of -metal larger than the aperture which the valve is intended to -stop, a piece of leather, and to the under side of this leather -another piece of metal smaller than the aperture. The leather -[Pg228] extending on one side beyond the larger metallic plate, -and being flexible, forms the hinge on which the valve plays. Such -a valve is usually closed by its own weight, and opened by the -pressure of the fluid which passes through it. It is also held -closed more firmly by the pressure of the fluid whose return it is -intended to obstruct. An example of this valve occurs in the steam -engine, in the passage between the condenser and the air-pump. The -aperture which it stops is there a seat inclined at an angle whose -inclination is such as to render the weight of the valve -sufficient to close it. In cases where the valve is exposed to -heat, as in the example just mentioned, where it is continually in -contact with the hot water flowing from the condenser to the -air-pump, the use of leather is inadmissible, and in that case the -metallic surface of the valve is ground smooth to fit its seat. - -The extent to which such a valve should be capable of opening, -ought to be such that the aperture produced by it shall be equal -to the aperture which it stops. This will be effected if the angle -through which it rises be about 30°. - -[Illustration: _Fig._ 46.] - -The valve by which the air and water collected in the bottom of -the air-pump are admitted to pass through the air-pump piston is a -double clack, consisting of two semicircular plates, having the -hinges on the diameters of these semicircles, as represented in -_fig._ 46. - - -(131.) Of the valves which are opened by a motion perpendicular to -their seat, the most simple is a flat metallic plate, made larger -than the orifice which it is intended to stop, and ground so as to -rest in steam-tight contact with the surface surrounding the -aperture. Such a valve is usually guided in its perpendicular -motion by a spindle passing through its centre, and sliding in -holes made in cross bars extending above and below the seat of the -valve. - -The conical steam-valves, which have been already described -(116.), usually called spindle-valves, are the most common of this -class. The best angle to be given to the conical seat is found in -practice to be 45°. With a less inclination the valve has a -tendency to be fastened in its seat, and a greater inclination -would cause the top of the valve to occupy [Pg229] unnecessary -space in the valve-box. The area, or transverse section of the -valve-box, should be rather more than double the magnitude of the -upper surface of the valve, in order to allow a sufficiently free -passage for the steam, and the play of the valve should be such as -to allow it to rise from its seat to a height not less than one -fourth of the diameter of its upper surface. - -The valves coming under this class are sometimes formed as spheres -or hemispheres resting in a conical seat, and in such cases they -are generally closed by their own weight, and opened by the -pressure of the fluid which passes through them. - - -(132.) One of the advantages attending the use of slides, compared -with the other form of valves, is the simplicity with which the -same slide may be made to govern several passages, so that a -single motion with a slide may perform the office of two or more -motions imparted to independent valves. - -In most modern engines the passage of the steam to and from the -cylinder is governed by slides of various forms, some of which we -shall now explain. - -[Illustration: _Fig._ 47.] - - -(133.) In _figs._ 47. and 48. is represented a slide-valve -contrived by Mr. Murray of Leeds. A B is a steam-tight case -attached to the side of the cylinder; E F is a rod, which receives -an alternate motion, upwards and downwards, from the eccentric, or -from whatever other part of the engine is intended to move the -slide. This rod, passing through a stuffing-box, moves the slide G -upwards and downwards. S is the mouth of the steam pipe coming -from the boiler; T is the mouth of a tube or pipe leading to the -condenser; H is a passage leading to the top, and I to the bottom, -of the cylinder. In the position of the slide represented in -_fig._ 47., the steam coming from the boiler through S passes -through the space H to the top of the cylinder, while the steam -from the bottom of the cylinder passes through the space I into -the tube T, and goes to the condenser. When the rod [Pg230] E F -is raised to the position represented in _fig._ 48., then the -passage H is thrown into communication with the tube T, while the -passage I is made to communicate with the tube S. Steam, -therefore, passes from the boiler through I below the piston, -while the steam which was above the piston, passing through H into -T, goes to the condenser. Thus the single slide G performs the -office of the four valves described in (116.). - -[Illustration: _Fig._ 48.] - - -(134.) The slide G has always steam of a full pressure behind it, -while the steam in front of it escaping to the condenser, exerts -but little pressure upon it. It is therefore always forcibly -pressed against the surfaces in contact with which it moves, and -is thereby maintained steam-tight. Indeed this pressure would -rapidly wear the rubbing surfaces, unless they were made -sufficiently extensive, and hardened so as to resist the effects -of the friction. Where fresh water is used, as in land boilers, -the slide may be made of hardened steel; and in the case of marine -boilers, it may be constructed of gun-metal. In this and all other -contrivances in which the apertures by which the steam is admitted -to and withdrawn from the piston are removed to any considerable -distance from the top and bottom of the cylinder, there is a waste -of steam, for the steam consumed at each stroke of the piston is -not only that which would fill the capacity of the cylinder, but -also the steam which fills the passage between the slide G and the -top or bottom of the cylinder. Any arrangement which would throw -the passages H and I on the other side of the slide G, that is, -between S and G, instead of being, as they are, between G and the -top and bottom of the cylinder, would remove this defect. This is -accomplished by a slide, which is usually called the D valve, -because, being semi-cylindrical in its form, and hollow, its cross -section resembles the letter D. This slide, which is that which at -present is in most general use, is represented in _figs._ 49, 50.; -E is the rod by which the slide is moved, passing [Pg231] through -a stuffing-box F; G G is the slide represented by a vertical -section, _a a_ being a passage in it extending from the top to the -bottom; S is the mouth of the great steam pipe coming from the -boiler; P is the pipe leading to the condenser; T H is a hollow -space formed in the slide always in communication with the steam -pipe S, and consequently always filled with steam from the boiler. -A transverse section of the slide and cylinder is represented in -_fig._ 51., where _a_ represents the top of the passage marked _a_ -in _fig._ 49. In the position of the slide represented in _fig._ -49., the steam filling the space T H has access to the top of the -cylinder, but is excluded from the bottom. The steam which was -below the piston, passing up the passage _a_, escapes through the -tube P to the condenser. When the piston has descended, the rod E -moves the slide downwards, so as to give it the position -represented in FIG. 50. The steam in T H has now access to the -bottom of the cylinder, while the steam above the piston passing -through P escapes to the condenser. In this way the operation of -the piston is continued and the steam consumed at each stroke only -exceeds the capacity of the cylinder by what is necessary to fill -the passages between the slide and the cylinder. - -[Illustration: _Fig._ 49.] - -[Illustration: _Fig._ 50.] - -[Illustration: _Fig._ 51.] - -In a slide constructed in this manner, the steam filling the space -T H has a tendency to press the slide back, so as to break the -contact of the rubbing surfaces, and thereby to cause the steam to -leak from the space T H to the back of the slide. This is -counteracted by the packing _x_, at the back of the slide. - -In engines of very long stroke, the extent of the rubbing surfaces -of slides of this kind renders it difficult to keep [Pg232] them -in steam-tight contact and to insure their uniform wear. In such -cases, therefore, separate slides, upon the same principle, are -provided at the top and bottom of the cylinder, moved, however, by -a single rod of communication. - - -(135.) In slides, as we have here described them, the same motion -which admits steam to either end of the cylinder, withdraws it -from the other end. Such an arrangement is only compatible with -the operation of a cylinder which works without expansion; for in -such a cylinder the full flow of steam to the piston is only -interrupted for a moment during the change of position of the -slide. But if the steam act expansively, it would be necessary to -move the slide, so as to stop its flow to one end of the cylinder, -without at the same time obstructing the escape of steam from the -other end to the condenser. It would therefore be necessary that -the slide should close the passage leading to the cylinder at one -end, without at the same time obstructing the communication -between the passage from the cylinder to the condenser at the -other end. On the arrival of the piston, however, at the bottom of -the cylinder, it would be necessary immediately to put the lower -passage to the cylinder in communication with the steam pipe, and -the upper passage in communication with the condenser. This would -necessarily suppose two motions of the slide as well as some -modifications in its length. Let the length of the slide be such -that when the passage to the top of the cylinder is stopped, the -lower part of the slide shall not reach the passage to the lower -part of the cylinder; and let such a provision be made in the -mechanism by which the rod E governing the slide is driven that it -shall receive two motions during the descent of the piston, the -first to be imparted to it at the moment the steam is to be cut -off, and the second just before the termination of the stroke. Let -the position of the slide, at the commencement of the stroke, be -represented in _fig._ 52., and let it be required that the steam -shall be cut off at one half of the stroke. When the piston has -made half the stroke, the rod governing the slide is moved -downwards, so as to throw the slide into the position represented -in _fig._ 53. The passage between the steam pipe and the cylinder -is [Pg233] now stopped at both ends; but the passage from the -bottom of the cylinder to the condenser remains open. During the -remainder of the stroke, therefore, the steam in the cylinder -works expansively. As the piston approaches the bottom of the -cylinder, another motion is imparted to the rod governing the -slide, by which the latter is thrown into the position represented -in _fig._ 54. Steam now flows below the piston while the steam -above it passes to the condenser. In a similar manner, by two -motions successively imparted to the slide during the ascent of -the piston, the steam may be cut off at half stroke; and it is -evident that by regulating the time at which these motions are -given to the slide, the steam may be worked expansively, to any -required extent. - -[Illustration: _Fig._ 52.] - -[Illustration: _Fig._ 53.] - -[Illustration: _Fig._ 54.] - -It is easy to conceive various mechanical means by which, in the -same engine, the point at which the steam is cut off may be -regulated at pleasure. - -In cases where the motion of the piston is very rapid, as in -locomotive engines, it is desirable that the passages to and from -the cylinder should be opened very suddenly. This is difficult to -be accomplished with any form of slide consisting of a single -aperture; but if, instead of admitting the steam to the cylinder -by a single aperture, the same magnitude of opening were divided -among several apertures, then a proportionally less extent of -motion in the slide would clear the passage for the steam, and -consequently greater suddenness of opening would be effected. -[Pg234] - -The great advantages in the economy of fuel resulting from the -application of the expansive principle have, of late years forced -themselves on the attention of engineers, and considerable -improvements have been made in its application, especially in the -case of marine engines used for long voyages, in which the economy -of fuel has become an object of the last importance. The mechanism -by which expansive slides are moved, is made capable of adjustment, -so that the part of the stroke at which the steam is cut off, can be -altered at pleasure. The working power of the engine, therefore, -instead of being controlled by the throttle-valve, is regulated by -the greater or less extent to which the expansive principle is -applied. Steam of the same pressure is admitted to the cylinder in -all cases; but it is cut off at a greater or less portion of the -stroke, according to the power which the engine is required to -exert. - -The last degree of perfection has been conferred on this principle -by connecting the governor with the mechanism by which the slide -is moved, so that the governor instead of acting on the -throttle-valve, is made to act upon the slide. By this means when, -by reason of any diminution of the resistance, the motion of the -engine is accelerated, the balls of the governor diverging shift -the cam or lever which governs the slide, so that the steam is cut -off after a shorter portion of the stroke, the expansive principle -is brought into greater play, and the quantity of steam admitted -to the cylinder at each stroke is diminished. If, on the other -hand, the resistance to the machine be increased, so as to -diminish the velocity of the engine, then the balls collapsing the -levers of the governor shift the cam which moves the slides, so as -to increase the portion of the stroke made by the piston before -the steam is cut off, and thereby to increase the amount of -mechanical power developed in the cylinder at each stroke. The -extent to which the expansive principle is capable of being -applied, more especially in marine engines, has been hitherto -limited by the necessity of using steam of very high pressure, -whenever the steam is cut off after the piston has performed only -a small part of the stroke. A method, however, is now (March, -1840) under experimental trial, by [Pg235] Messrs. Maudsley and -Field, by which the expansive principle may be applied to any -required extent without raising the steam in the boiler above the -usual pressure of from three to five pounds per square inch. This -method consists in the use of a piston of great magnitude. The -force urging the piston is thus obtained not by an excessive -pressure on a limited surface, but by a moderate pressure diffused -over a large surface. The entire moving force acting on the piston -before the steam is cut off, is considerably greater than the -resistance; but during the remainder of the stroke this force is -gradually enfeebled until the piston is brought to the extremity -of its play. - -[Illustration: _Fig._ 55.] - - -(136.) Mr. Samuel Seaward, of the firm of Messrs. Seawards, -engineers, has contrived an improved system of slides, for which -he has obtained a patent. A section of Seaward's slides is -represented in _fig._ 55. The steam pipe proceeding from the -boiler to the cylinder is represented at A A, and it communicates -with passages S and S' leading to the top and bottom of the -cylinder. These passages are formed in nozzles of iron or other -hard metal cast upon the side of the cylinder. These nozzles -present a smooth face outwards, upon which the slides B B', also -formed with smooth faces, play. The slides B B' are attached by -knuckle-joints to rods E E', which move through stuffing-boxes, -and the [Pg236] connection of these rods with the slides is such -that the slides have play so as to detach their surfaces easily -from the smooth surfaces of the nozzles when not pressed against -these surfaces. The steam in the steam pipe A A will press against -the backs of the slides B B', and keep their faces in steam-tight -contact with the smooth surfaces of the nozzles. These slides may -be opened or closed by proper mechanism at any point of the -stroke. When steam is to be admitted to the top of the cylinder, -the upper slide is raised and the passage S opened; and when it is -to be admitted to the bottom of the cylinder, the lower slide is -raised and the passage S' opened; and its communication to the top -or bottom of the cylinder is stopped by the lowering of these -slides respectively. On the other side of the cylinder are -provided two passages C C' leading to a pipe G, which is continued -to the condenser. On this pipe are cast nozzles of iron or other -metal presenting smooth faces towards the cylinder, and having -passages D D' communicating between the top and bottom of the -cylinder respectively and the pipe G G leading to the condenser. -Two slides _b b'_, having smooth faces turned from the cylinder, -and pressing upon the faces of the nozzles D D', are governed by -rods playing through stuffing-boxes, in the same manner as already -described. The faces of these slides being turned from the -cylinder, the steam in the cylinder having free communication with -them, has a tendency to keep them by its pressure in steam-tight -contact with the surfaces in which the apertures leading to the -condenser are formed. These two slides may be opened or closed -whenever it is necessary. - -When the piston commences its descent, the upper steam slide is -raised, so as to open the passage S, and admit steam above the -piston; and the lower exhausting slide _b'_ is also raised, so as -to allow the steam below the piston to escape through G to the -condenser, the other two passages S' and C being closed by their -respective slides. The slide which governs S is lowered at that -part of the stroke at which the steam is intended to be cut off, -the other slides remaining unchanged; and when the piston has -reached the bottom of the cylinder, the lower steam slide opens -the passage S', and [Pg237] the upper exhausting slide opens the -passage C; and at the same time the lower exhausting slide closes -the passage C'. Steam being admitted below the piston through S', -and at the same time the steam above it being drawn away to the -condenser through the open passage C and the tube G, the piston -ascends. When it has reached that point at which the steam is -intended to be cut off, the slide which governs S' is lowered, the -other slides remaining unaltered, and the upward stroke is -completed in the same manner as the downward. - -These four slides may be governed by a single lever, or they may -be moved by separate means. From the small spaces between the -several slides and the body of the cylinder, it will be evident -that the waste of steam by this contrivance will be very small. - -In the slide valves commonly used, the packing of hemp at the back -of the slide, by which the pressure necessary to keep the slide in -steam-tight contact is obtained, requires constant attention from -the engine-man while the engine is at work. Any neglect of this -will produce a corresponding loss in the power of the engine; and -accordingly it is found that in many cases where engines work -inefficiently, the defect is owing either to ignorance or want of -attention on the part of the engine-man in the packing of the -slides. In Seaward's slides no hemp packing is used, nor is any -attention on the part of the engine-man required after the slides -are first adjusted. The slides receive the pressure necessary to -keep them in steam-tight contact with the surfaces of the nozzles -from the steam itself, which acts behind them. - -The eduction and steam slides being independent of each other, -they may be adjusted so that the engine shall work expansively in -any required degree; and this may be accomplished either by -working the slides by separate mechanism, or by a single -eccentric. - -One of the advantages claimed by the patentees for these slides -is, that the engines are secured from the accidents which arise -from the accumulation of water within the steam cylinder. If such -a circumstance should occur, the action of the piston will press -the water against the faces of the steam [Pg238] slides, and the -play allowed to them by their connection with the rods which move -them permits their faces to be raised from the surfaces of the -nozzles, so that the water collected in the cylinder shall be -driven into the steam pipe, and sent back from thence to the -boiler. - -[Illustration: _Fig._ 56.] - -[Illustration: _Fig._ 57.] - -[Illustration: _Fig._ 58.] - - -(137.) Of the cocks or valves which are opened and closed by the -motion of an axis passing through their centre, the throttle-valve, -whether worked by hand or by the governor, is an example. But the -most common form for cocks is that of a cylindrical or slightly -conical plug (_fig._ 56.), inserted in an aperture of corresponding -magnitude passing across the pipe or passage which the cock is -intended to open or close. One or more holes are pierced -transversely in the cock, and when the cock is turned so that these -holes run in the direction of the tube, the passage through the tube -is opened; but when the passage through the cock is placed at right -angles to the tube, then the sides of the tube stop the ends of the -passage in the cock, and the passage through the tube is obstructed. -The simple cock is designed to open or close the passage through a -single tube. When the cock is turned, as in _fig._ 57., so that the -passage through the cock shall be at right angles to the length of -the tube, then the passage through the tube is stopped; but when the -cock is turned from that position through a quarter of a revolution, -as in _fig._ 58., then the passage through the cock takes the -direction of the passage through the tube, and the cock is opened, -and the passage through the tube unobstructed. In such a cock the -passage may be more or less _throttled_ by [Pg239] adjusting the -position of the cock, so that a part of the opening in it shall be -covered by the side of the tube. - - -(138.) It is sometimes required to put one tube or passage -alternately in communication with two others. This is accomplished -by a _two-way cock_. In this cock the passage is curved, opening -usually at points on the surface of the cock, at right angles to -each other. Such a cock has already been described, and its use -illustrated in the description of the Marquis of Worcester's -engine (17.); the two-way cock, as represented at K and R (_fig._ -4.), being the means by which steam and water are alternately -supplied to the two forcing vessels. - -[Illustration: _Fig._ 59.] - -[Illustration: _Fig._ 60.] - - -(139.) When it is required to put four passages alternately in -communication by pairs, a _four-way cock_ is used. Such a cock has -two curved passages (_fig. 59._), each similar to the curved -passage in the two-way cock. Let S C B T be the four tubes which -it is required to throw alternately into communication by pairs. -When the cock is in the position _fig._ 59., the tube S -communicates with T, and the tube C with B. By turning the cock -through a quarter of a revolution, as in _fig._ 60., the tube S is -made to communicate with B, and the tube C with T; and if the cock -continue to be turned at intervals through a quarter of a -revolution, these changes of communication will continue to be -alternately made. It is evident that this may be accomplished by -turning the cock continually in the same direction. - -The four-way cock is sometimes used as a substitute for the valves -or slides in a double-acting steam engine to conduct the steam to -and from the cylinder. If S represent a pipe conducting steam from -the boiler, C that which leads to the condenser, T the tube which -leads to the top of the cylinder, and B that which leads to the -bottom, then when the cock is in the position (_fig._ 59.), steam -would flow from the boiler to [Pg240] the top of the piston, -while the steam below it would be drawn off to the condenser; and -in the position (_fig._ 60.), steam would flow from the boiler to -the bottom of the piston, while the steam above it would be drawn -off to the condenser. Thus by turning the cock through a quarter -of a revolution towards the termination of each stroke, the -operation of the machine would be continued. - -One of the disadvantages which is inseparable from the use of a -four-way cock for this purpose is the loss of the steam at each -stroke, which fills the tubes between the cock and the ends of the -cylinder. This disadvantage could only be avoided by the -substitution of two two-way cocks (138.) instead of a four-way -cock. A two-way cock at the top of the cylinder would open an -alternate communication between the cylinder and steam pipe, and -the cylinder and condenser, while a similar office would be -performed by another two-way cock at the other end. - -The friction on cocks of this description is more than on other -valves; but this is in some degree compensated by the great -simplicity of the instrument. When the cock is truly ground into -its seat, being slightly conical in its form, the pressure of the -steam has a tendency to keep the surfaces in contact; but this -pressure also increases the friction, and has a tendency to wear -the seat of the cock into an elliptical shape. Consequently, such -cocks require to be occasionally ground and refitted. - - -(140.) The four-way cock, as above described, admits the steam to -one end of the piston at the same moment that it stops it at the -other end. It would therefore be inapplicable where steam is -worked expansively. A slight modification, however, analogous to -that already described in the slides, will adapt it to expansive -action. This will be accomplished by giving to one of the passages -through the cock one aperture larger than the other, and working -the cock so that this passage shall always be used to conduct -steam to the cylinder; also by enlarging both apertures of the -other passage, and using it always to conduct steam from the -cylinder. The effect of such an arrangement will be readily -understood. - -[Illustration: _Fig._ 61.] - -[Illustration: _Fig._ 62.] - -[Illustration: _Fig._ 63.] - -[Illustration: _Fig._ 64.] - -Let the position of the cock at the commencement of the [Pg241] -descending stroke be represented in _fig._ 61. Steam flows from S -through T to the top of the cylinder, while it escapes from B -through C from the bottom of the cylinder. When the piston has -arrived at that point at which the steam is to be cut off, let the -cock be shifted to the position represented in _fig._ 62. The -passage of steam from the boiler is now stopped, but the escape of -steam from the bottom of the cylinder through C continues, and the -cock is maintained in this position until the piston approaches -the bottom of the cylinder, when it is further shifted to the -position represented in _fig._ 63. Steam now flows from S through -B to the bottom of the cylinder, while the steam from the top of -the cylinder escapes through C to the condenser. When the piston -has arrived at that point where the steam is to be cut off, the -cock is shifted to the position represented in _fig._ 64. The -communication between the steam and the bottom of the piston is -now stopped, while the communication between the top of the -cylinder and the condenser is still open. During the next double -stroke of the piston the position of the cock is similarly -changed, but in the contrary direction, and in the same way the -motion is continued. Under these circumstances the cock, instead -[Pg242] of being moved constantly in the same direction, as in the -case of the common four-way cock, will require to be moved -alternately in opposite directions. - - -PISTONS. - - -(141.) The office of a piston being to divide a cylinder into two -compartments by a movable partition which shall obstruct the passage -of any fluid from one compartment to the other, it is evident that -the two conditions which such an instrument ought to fulfil are, -_first_, that the contact of its sides with the surface of the -cylinder shall be so close and tight throughout its entire play that -no steam or other fluid can pass between them; _secondly_, that it -shall be so free from friction, notwithstanding this necessary -tightness, that it shall not absorb any injurious quantity of the -moving power. - -Since, however accurately the surfaces of the piston and cylinder -may be constructed, there will always be in practice more or less -imperfection of form, it is evident that the contact of the -surface of the piston with the cylinder throughout the stroke can -only be maintained by giving to the circumference of the piston -sufficient elasticity to accommodate itself to such inequalities -of form. The substance, whatever it may be, used for this purpose, -and by which the piston is surrounded, is called _packing_. - -In steam pistons the material used for packing must be such as is -capable of resisting the united effects of heat and moisture. -Hence leather and other animal substances are inapplicable. - -The packing used for steam pistons is therefore of two kinds, -_vegetable packing_, usually hemp, or _metallic packing_. - -The common hemp-packed piston has been already in part described -(79.). The bottom of the piston is a circular plate just so much -less in diameter than the cylinder as is sufficient to allow its -free motion in ascending and descending. A little above its lowest -point this plate begins gradually to diminish in thickness, until -its diameter is reduced to from one to two inches less than that -of the cylinder, leaving therefore around [Pg243] it a hollow -space, as represented in _fig._ 65. The cover of the piston is a -plate similarly formed, being in like manner gradually reduced in -thickness downwards, so as to correspond with the lower plate. In -the hollow space which thus surrounds the piston a packing of -unspun hemp or soft rope, called _gasket_, is introduced by -winding it round the piston so as to render it an even and compact -mass. When the space is thus filled up, the top of the piston is -attached to the bottom by screws. The curved form of the space -within which the hempen packing is confined is such that when the -screws are tightened, that part of the packing which is nearest to -the top and bottom of the piston is forced against the cylinder, -so as to produce upon the two parallel rings as much pressure as -is necessary to render it steam-tight. When by use the packing is -worn down so as to produce leakage, the cover of the cylinder must -be removed, and the screws connecting the top and bottom of the -piston tightened: this will force out the packing and render the -piston steam-tight. This packing is lubricated by melted tallow -let down upon the piston from the funnel inserted in the top of -the cylinder, furnished with a stop-cock to prevent the escape of -steam. The lower end of the piston-rod is formed slightly conical, -the thickest part of the cone being downward. It is passed up -through the piston, and a nut or wedge between the top and bottom -is inserted so as to secure the piston in its position upon the -rod. - -[Illustration: _Fig._ 65.] - -The process of removing the top of the cylinder for the purpose of -tightening the screws in the piston is one of so laborious a -nature, that the men entrusted with the superintendence of these -machines are tempted to allow the engine to work notwithstanding -injurious leakage at the piston, rather than incur the labour of -tightening the screws as often as it is necessary to do so. - -To avoid this inconvenience, the following method of [Pg244] -tightening the packing of the piston without removing the lid of -the cylinder, was contrived by Woolf. The head of each of the -screws was formed into a toothed pinion, and as these screws were -placed at equal distances from the centre of the piston, these -several pinions were driven by a large toothed wheel, revolving on -the piston-rod as an axis. By such an arrangement it is evident -that if any one of the screws be turned, a like motion will be -imparted to all the others through the medium of the large central -wheel. Woolf accordingly formed, on the head of one of the screws, -a square end. When the piston was brought to the top of the -cylinder, this square end entered an aperture made in the under -side of the cover of the cylinder. This aperture was covered by a -small circular piece screwed into the top of the cylinder, which -was capable of being removed so as to render the square head of -the screw accessible. When this was done, a proper key being -applied to the square head of the screw, it was turned; and by -being turned, all the other screws were in like manner moved. In -this way, instead of having to remove the cover of the cylinder, -which in large cylinders was attended with great labour and loss -of time, the packing was tightened by merely unscrewing a piece in -the top of the cylinder not much greater in magnitude than the -head of one of the screws. - -This method was further simplified by causing the great circular -wheel already described to move upon the piston-rod, not as an -axis, but as a screw, the thread being cut upon a part of the -piston-rod which worked in a corresponding female screw cut upon -the central plate. By such means, the screw whose head was let -into the cover of the cylinder which turned, would cause this -circular plate to be pressed downwards by the force of the screw -constructed on the piston-rod. This circular plate thus pressed -downwards, acted upon pins or plugs which pressed together the top -and bottom of the cylinder in the same manner as they were pressed -together by the screws connecting them as already described. - - -METALLIC PISTONS. - - -(142.) The notion of constructing a piston so as to move -steam-tight in the cylinder without the use of packing of -vegetable [Pg245] matter was first suggested by the Rev. Mr. -Cartwright, a gentleman well known for other mechanical -inventions. A patent was granted in 1797 for a new form of steam -engine, in which he proposed to use the vapour of alcohol to work -the piston instead of the steam of water: and since the principle -of the engine excluded the use of lubrication by oil or tallow, he -substituted a piston formed of metallic rings pressed against the -surface of the cylinder by springs, so as to be maintained in -steam-tight contact with it, independently either of packing or -lubrication. Although the engine for which this form of piston was -intended never came into practical use, yet it is so simple and -elegant in its structure, and forms a link so interesting in the -history of the steam engine, that some explanation of it ought not -to be omitted in this work. - -The steam-pipe from the boiler is represented cut off at B (_fig._ -66.); T is a spindle-valve, for admitting steam above the piston, -and R is a spindle-valve in the piston; D is a curved pipe forming -a communication between the cylinder and the condenser, which is -of very peculiar construction. Cartwright proposed effecting a -condensation without a jet, by exposing the steam to contact with -a very large quantity of cold surface. For this purpose, he formed -his condenser by placing two cylinders nearly equal in size, one -within the other, allowing the water of the cold cistern in which -they were placed to flow through the inner cylinder, and to -surround the outer one. Thus, the thin space between the two -cylinders formed the condenser. - -[Illustration: _Fig._ 66.] - -The air-pump is placed immediately under the cylinder, and the -continuation of the piston-rod works its piston, which is solid -and without a valve. F is the pipe from the condenser to the -air-pump, through which the condensed steam is drawn off through -the valve G on the ascent of the piston, and on the descent this -is forced through a tube into a hot well H, for the purpose of -feeding the boiler through the feed-pipe I. In the top of the hot -well H is a valve which opens inwards, and is kept closed by a -ball floating on the surface of the liquid. The pressure of the -condensed air above the surface of the liquid in H forces it -through I into the boiler. When the air accumulates in too great a -degree [Pg246] in H, the surface of the liquid is pressed so low -that the ball falls and opens the valve, and allows it to escape. -The air in H is that which is pumped from the condenser with the -liquid, and from which it was disengaged. - -Let us suppose the piston at the top of the cylinder: it strikes -the tail of the valve T, and raises it, while the stem of the -piston-valve R strikes the top of the cylinder, and is pressed -into its seat. A free communication is at the same time open -between the cylinder, below the piston and the condenser, through -the tube D. The pressure of the steam [Pg247] thus admitted above -the piston acting against the vacuum below it, will cause its -descent. On arriving at the bottom of the cylinder, the tail of -the piston-valve R will strike the bottom, and it will be lifted -from its seat, so that a communication will be opened through it -with the condenser. At the same moment, a projecting spring K, -attached to the piston-rod, strikes the stem of the steam-valve T, -and presses it into its seat. Thus while the further admission of -steam is cut off, the steam above the piston flows into the -condenser, and the piston being relieved from all pressure, is -drawn up by the momentum of the fly-wheel, which continues the -motion it received from the descending force. On the arrival of -the piston again at the top of the cylinder, the valve T is opened -and R closed, and the piston descends as before, and so the -process is continued. - -The mechanism by which motion is communicated from the piston to -the fly-wheel is peculiarly elegant. On the axis of the fly-wheel -is a small wheel with teeth, which work in the teeth of another -larger wheel L. This wheel is turned by a crank, which is worked -by a cross-piece attached to the end of the piston-rod. Another -equal-toothed wheel M is turned by a crank, which is worked by the -other end of the cross-arm attached to the piston-rod. - -One of the peculiarities of this engine is, that the liquid which -is used for the production of steam in the boiler circulates -through the machine without either diminution or admixture with -any other fluid, so that the boiler never wants more feeding than -what can be supplied from the hot well H. This circumstance forms -an important feature in the machine, as it allows of ardent -spirits being used in the boiler instead of water, which, since -they boil at low heats, promised a saving of fuel. The inventor -proposed that the engine should be used as a still, as well as a -mechanical power, in which case the whole of the fuel would be -saved. - -[Illustration: _Fig._ 67.] - -[Illustration: _Fig._ 68.] - - -(143.) That part of Cartwright's piston which in the common piston -is occupied by the packing of gasket, already explained (141.), -was filled by a number of rings, one placed within and above -another, and divided into three or four [Pg248] segments. Two -rings of brass were made of the full size of the cylinder, and so -ground as to fit the cylinder nearly steam-tight. These were cut -into several segments A A A (_fig._ 67.), and were placed one -above the other, so as to fill the space between the top and -bottom plates of the piston. The divisions of the segments of the -one ring were made to fit between the divisions of the other. -Within these another series of rings, B B B, were placed, -similarly constructed, so as to fit within the first series in the -same manner as the first series were made to fit within the -cylinder. The joints of the upper series of each set of rings are -exhibited in the plan (_fig._ 67.); the places of the joints of -the lower series are shown by dotted lines; the position of the -rings of each series one above the other is shown in the section -(_fig._ 68.). The joints of the inner series of rings are so -placed as to lie between those of the outer series, to prevent the -escape of steam which would take place by one continued joint from -top to bottom of the packing. The segments into which the rings -are divided are pressed outwards by steel springs in the form of -the letter V, the springs which act upon the outer series of -segments abutting upon the inner series, and those which act on -the inner series abutting upon the solid centre of the piston: -these springs are represented in _fig._ 67. - -[Illustration: _Fig._ 69.] - -[Illustration: _Fig._ 70.] - - -(144.) An improved form was given to the metallic piston by -Barton. Barton's piston consists of a solid cylinder of cast iron, -represented at A in section in _fig._ 69., and in plan in [Pg249] -_fig._ 70. In the centre of this is a conical hole, increasing in -magnitude downwards, to receive the piston-rod, in which the -latter is secured by a cross-pin B. A deep groove, square in its -section, is formed around the piston, so that while the top and -bottom of the piston form circles equal in magnitude to the -section of the cylinder, the intermediate part of the body of the -piston forms a circle less than the former by the depth of the -groove. Let a ring of brass, cast iron, or cast steel, be made to -correspond in magnitude and form with this groove, and let it be -divided as represented in _fig._ 70., into four segments C C C C, -and four corresponding angular pieces D D D D. Let the groove -which surrounds the piston be filled by the four segments with the -four wedge-like angular pieces within them, and let the latter be -urged against the former by eight spiral springs, as represented -in _fig._ 69. and _fig._ 70. These springs will abut against the -solid centre by the piston, and will urge the segments C against -the cylinder. The spiral springs which urge the wedges are -confined in their action by steel pins which pass through their -centre, and by being [Pg250] confined in cylindrical cavities -worked into the wedges and into corresponding parts of the solid -centre of the piston, as the segments C wear, the springs urge the -wedges outwards, and the points of the latter protruding, are -gradually worn down so as to fill up the spaces left between the -segments, and thus to complete the outer surface of the piston. - -Various other forms of metallic pistons have been proposed, but as -they do not differ materially in principle from those we have just -described, it will not be necessary here to describe them. - -[Illustration: ENGINE AT THE CITY SAW MILLS.] - -[Pg251] - - - - -[Illustration: FURNACE AT THE CITY SAW MILLS.] - -CHAP. IX. - - CONSTITUENTS OF COAL. -- PROCESS OF COMBUSTION. -- HEAT EVOLVED - IN IT. -- FORM AND STRUCTURE OF BOILER. -- WAGGON BOILER. -- - FURNACE. -- METHOD OF FEEDING IT. -- COMBUSTION OF GAS IN FLUES. - -- CONSTRUCTION OF GRATE AND ASH-PIT. -- MAGNITUDE OF HEATING - SURFACE OF BOILER. -- STEAM-SPACE AND WATER-SPACE IN BOILER. -- - POSITION OF FLUES. -- METHOD OF FEEDING BOILER. -- LEVEL GAUGES. - -- SELF-REGULATING FEEDERS. -- STEAM-GAUGE. -- BAROMETER-GAUGE. -- - INDICATOR. -- COUNTER. -- SAFETY-VALVE. -- FUSIBLE PLUGS. -- - SELF-REGULATING DAMPER. -- SELF-REGULATING FURNACE. -- POWER AND - DUTY OF ENGINES. -- HORSE-POWER OF STEAM ENGINES. -- EVAPORATION - PROPORTIONAL TO HORSE-POWER. -- SOURCES OF LOSS OF POWER. -- - ABSENCE OF GOOD PRACTICAL RULES FOR POWER. -- COMMON RULES - FOLLOWED BY ENGINE MAKERS. -- DUTY DISTINGUISHED FROM POWER. -- - DUTY OF BOILERS. -- PROPORTION OF STROKE TO DIAMETER OF - CYLINDER. -- DUTY OF ENGINES. -- CORNISH SYSTEM OF INSPECTION. -- - ITS GOOD EFFECTS. -- HISTORICAL DETAIL OF THE DUTY OF CORNISH - ENGINES. - - -(145.) The machinery which has been explained in the preceding -chapters, consisting of the cylinder with its passages and valves, -the piston-rod, parallel motion, beam, connecting-rod and crank, -together with the condenser, air-pump, and other appendages, -having no source of moving power in themselves, must be regarded -as mere instruments by which the mechanical effect developed by -the furnace and the boiler is transmitted to the working point and -so [Pg252] modified as to be adapted to the uses to which the -machine is applied. The boiler is at once a magazine in which the -moving power is stored in sufficient quantity to supply the -demands of the engine and an apparatus in which that power is -fabricated. The mechanical effect evolved in the conversion of -water into steam by heat, is the process by which the power of the -steam-engine is produced, and space is provided in the boiler, -capacious enough to contain as much steam as is necessary for the -engine, besides a sufficient quantity of water to continue that -supply undiminished, notwithstanding the constant drafts made upon -it by the cylinder: even the water itself, from the evaporation of -which the mechanical power is produced, ought to be regarded as an -instrument by which the effect of the heat of the combustible is -rendered mechanically efficient, inasmuch as the same heat, -applied not only to other liquids but even to solids, would -likewise be productive of mechanical effects. The boiler and its -furnace are therefore parts of the steam-engine, the construction -and operation of which are entitled to especial attention. - - -(146.) COAL, the combustible almost universally used in -steam-engines, is a substance, the principal constituents of which -are _carbon_ and _hydrogen_, occasionally mixed with sulphur in a -small proportion, and earthy incombustible matter. In different -sorts of coal the proportions of these constituents vary, but in -coal of good quality about three quarters of the whole weight of -the combustible is carbon. - -When carbon is heated to a temperature of about 700° in an -atmosphere of pure oxygen, it will combine chemically with that -gas, and the product will be the gas called _carbonic acid_. The -volume of carbonic acid produced by this combination, will be -exactly equal to that of the oxygen combined with the carbon, and -therefore the weight of a given volume of the gas will be -increased by the weight of carbon which enters the combination. It -is found that two parts by weight of oxygen combined with three of -carbon, form carbonic acid. The weight of the carbonic acid, -therefore, produced in the combustion, will be greater than the -weight of the oxygen, bulk for bulk, in the proportion of five to -two, the volume being the same and the gases being [Pg253] -compared at the same temperatures and under equal pressures. In -this combination heat is evolved in very large quantities. This -effect arises from the heat previously latent in the carbon and -oxygen being rendered sensible in the process of combustion. The -carbonic acid proceeding from the combustion is by such means -raised to a very high temperature, and the carbon during the -process acquires a heat so intense as to become luminous; no -flame, however, is produced. - -Hydrogen, heated to a temperature of about 1000°, in contact with -oxygen will combine with the latter, and a great evolution of heat -will attend the process; the gases will be rendered luminous, and -flame will be produced. The product of this process will be water, -which being exposed to the intense heat of combustion, will be -immediately converted into steam. Hydrogen combines with eight -times its own weight of oxygen, producing nine times its own -weight of water. - -Hydrogen gas is, however, not usually disengaged from coal in a -simple form, but combined chemically with a certain portion of -carbon, the combination being called carburetted hydrogen. Pure -hydrogen burns with a very faintly luminous blue flame, but -carburetted hydrogen gives that bright flame occasionally having -an orange or reddish tinge, which is seen to issue from burning -coals: this is the gas used for illumination, being expelled from -the coal by the process of coking, and conducted to the various -burners through proper pipes. - -The sulphur, which in a very small proportion is contained in -coals, is also combustible, and combines in the process of -combustion with oxygen, forming sulphurous acid: it is also -sometimes evolved in combination with hydrogen, forming -sulphuretted hydrogen. - -Atmospheric air consists of two gases, azote and oxygen, mixed -together in the proportion of four to one; five cubic feet of -atmospheric air consisting of four cubic feet of azote and one of -oxygen. Any combustible will combine with the oxygen contained in -atmospheric air, if raised to a temperature somewhat higher than -that which is necessary to cause its combustion in an atmosphere -of pure oxygen. - -If coals, therefore, or other fuel exposed to atmospheric [Pg254] -air, be raised to a sufficiently high temperature, their -combustible constituents will combine with the oxygen of the -atmospheric air, and all the phenomena of combustion will ensue. -In order, however, that the combustion should be continued, and -should be carried on with quickness and activity, it is necessary -that the carbonic acid, and other products, should be removed from -the combustible as they are produced, and fresh portions of -atmospheric air brought into contact with it; otherwise the -combustible would soon be surrounded by an atmosphere composed -chiefly of carbonic acid to the exclusion of atmospheric air, and -therefore of uncombined oxygen, and consequently the combustion -would cease, and the fuel be extinguished. To maintain the -combustion, therefore, a current of atmospheric air must be -constantly carried through the fuel: the quantity and force of -this current must depend on the quantity and quality of the fuel -to be consumed. It must be such that it shall supply sufficient -oxygen to the fuel to maintain the combustion, and not more than -sufficient, since any excess would be attended with the effect of -absorbing the heat of combustion, without contributing to the -maintenance of that effect. - -Heat is communicated from body to body in two ways, by radiation -and by contact. - -Rays of heat issue from a heated body, and are dispersed through -the surrounding space in a manner, and according to laws, similar -to those which govern the radiation of light. The heat thus -radiated meeting other bodies is imparted to them, and penetrates -them with more or less facility according to their physical -qualities. - -A heated body also brought into contact with another body of lower -temperature, communicates heat to that other body, and will -continue to do so until the temperature of the two bodies in -contact shall be equalised. Heat proceeds from fuel in a state of -combustion in both these ways: the heated fuel radiates heat in -all directions around it, and the heat thus radiated will be -imparted to all parts of the furnace which are exposed to the -fuel. - -The gases, which are the products of the combustion, escape from -the fuel at a very high temperature, and consequently, in -acquiring that temperature they absorb a considerable [Pg255] -quantity of the heat of combustion. But besides the gases actually -formed in the process of combustion, the azote forming four fifths -of the air carried through the fuel to support the combustion, -absorbs heat from the combustible, and rises into the upper part -of the furnace at a high temperature. These various gases, if -conducted directly to the chimney, would carry off with them a -considerable quantity of the heat. Provision should therefore be -made to keep them in contact with the boiler such a length of time -as will enable them to impart such a portion of the heat which -they have absorbed from the fuel, as will still leave them at a -temperature sufficient, and not more than sufficient, to produce -the necessary draft in the chimney. - - -(147.) The forms of boiler which have been proposed as the most -convenient for the attainment of all these requisite purposes have -been very various. If strength alone were considered, the -spherical form would be the best; and the early boilers were very -nearly hemispheres, placed on a slightly concave base. The form -adopted by Watt, called the waggon boiler, consists of a -semi-cylindrical top, flat perpendicular sides, flat ends, and a -slightly concave bottom. The steam intended to be used in boilers -of this description did not exceed the pressure of the external -atmosphere by more than from 3 to 5 lbs. per square inch; and the -flat sides and ends, though unfavourable to strength, could be -constructed sufficiently strong for this purpose. In a boiler of -this sort, the air and smoke passing through the flues that are -carried round it, are in contact at one side only with the boiler. -The brickwork, or other materials forming the flue, must therefore -be non-conductors of heat, that they may not absorb any -considerable portion of heat from the air passing in contact with -them. A boiler of this form is represented in _fig._ 71. - -The grate and a part of the flues are rendered visible by the -removal of a portion of the surrounding masonry in which the -boiler is set. The interior of the boiler is also shown by cutting -off one half of the semi-cylindrical roof. A longitudinal vertical -section is shown in _fig._ 72., and a cross section in _fig._ 73. -A horizontal section taken above the level of the grate, and below -the level of the water in the boiler, showing [Pg256] the course -of the flues, is given in _fig._ 74. The corresponding parts in -all the figures are marked by the same letters. - -[Illustration: _Fig._ 71.] - -The door by which fuel is introduced upon the grate is represented -at A, and the door leading to the ash-pit at B. The fire bars at C -slope downwards from the front at an angle of about 25°, giving a -tendency to the fuel to move from the front towards the back of -the grate. The ash-pit D is constructed of such a magnitude, form, -and depth, as to admit a current of atmospheric air to the -grate-bars, sufficient to sustain the combustion. The form of the -ash-pit is usually wide below, contracting towards the top. - -[Illustration: _Fig._ 72.] - -The fuel when introduced at the fire-door A, should be laid on that -part of the grate nearest to the fire-door, called the dead plates: -there it is submitted to the process of coking, by which the gases -and volatile matter which it contains are expelled, and being -carried by a current of air, admitted [Pg257] through small -apertures in the fire-door over the burning fuel in the hinder part -of the grate, they are burnt. When the fuel in front of the grate -has been thus _coked_, it is pushed back, and a fresh feed -introduced in front. The coal thus pushed back soon becomes vividly -ignited, and by continuing this process, the fuel spread over the -grate is maintained in the most active state of combustion at the -hinder part of the grate. By such an arrangement, the smoke produced -by the combustion of the fuel may be burnt before it enters the -flues. The flame and heated air proceeding from the burning fuel -arising from the grate, and rushing towards the back of the furnace, -passes over the _fire-bridge_ E, and is carried through the flue F -which passes under the boiler. This flue (the cross section of which -is shown in _fig._ 73., by the dark shade put under the boiler) is -very nearly equal in width to the bottom of the boiler, the space at -the bottom of the boiler, near the corners, being only what is -sufficient to give the weight of the boiler support on the masonry -forming the [Pg258] sides of the flue. The bottom of the boiler -being concave, the flame and heated air as they pass along the flue -rise to the upper part by the effects of their high temperature, and -_lick_ the bottom of the boiler from the fire-bridge at E to the -further end G. - -[Illustration: _Fig._ 73.] - -[Illustration: _Fig._ 74.] - -At G the flue rises to H, and turning to the side of the boiler at -I I, conducts the flame in contact with the side from the back to -the front; it then passes through the flue K across the front, and -returns to the back by the other side [Pg259] flue L. The side -flue is represented, stripped of the masonry, in _fig._ 71., and -also appears in the plan in _fig._ 74., and in the cross section -in _fig._ 73. The course of the air is represented in _fig._ 74. -by the arrows. From the flue L the air is conducted into the -chimney at M. - -By such an arrangement, the flame and heated air proceeding from -the grate are made to circulate round the boiler, and the length -and magnitude of the flues through which it is conducted should be -such, that when it shall arrive at the chimney its temperature -shall be reduced, as nearly as is consistent with the maintenance -of draught in the chimney, to the temperature of the water with -which it is in contact. - -The method of feeding the furnace, which has been described above, -is one which, if conducted with skill and care, would produce a -much more perfect combustion of the fuel than would attend the -common method of filling the grate from the back to the front with -fresh fuel, whenever the furnace is fed. This method, however, is -rarely observed in the management of the furnace. It requires the -constant attention of the stokers (such is the name given to those -who feed the furnaces). The fuel must be supplied, not in large -quantities, and at distant intervals, but in small quantities and -more frequently. On the other hand, the more common practice is to -allow the fuel on the grate to be in a great degree burned away, -and then to heap on a large quantity of fresh fuel, covering over -with it the burning fuel from the back to the front of the grate. -When this is done, the heat of the ignited coal acting upon the -fresh fuel introduced, expels the gases combined with it and, -mixed with these, a quantity of carbon, in a state of minute -division, forming an opaque black smoke. This is carried through -the flues and drawn up the chimney. The consequence is, that not -only a quantity of solid fuel is sent out of the chimney -unconsumed, but the hydrogen and other gases also escape unburned, -and a proportional waste of the combustible is produced; besides -which, the nuisance of an atmosphere filled with smoke ensues. -Such effects are visible to all who observe the chimneys of -steam-vessels, while the engine is in operation. When the furnaces -are thus filled with fresh fuel, a large volume of [Pg260] dense -black smoke is observed to issue from the chimney. This gradually -subsides as the fuel on the grate is ignited, and does not -reappear until a fresh feed is introduced. - -This method of feeding, by which the furnace would be made to -consume its own smoke, and the combustion of the fuel be rendered -complete, is not however free from counteracting effects. In -ordinary furnaces the feed can only be introduced by opening the -fire-doors, and during the time the fire-doors are opened a volume -of cold air rushes in, which passing through the furnace is -carried through the flues to the chimney. Such is the effect of -this in lowering the temperature of the flues, that in many cases -the loss of heat occasioned is greater than any economy of fuel -obtained by the complete consumption of smoke. Various methods, -however, may be adopted by which fuel may be supplied to the grate -without opening the fire-doors, and without disturbing the supply -of air to the fire. A hopper built into the front of the furnace, -with a moveable bottom or valve, by which coals may be allowed to -drop in from time to time upon the front of the grate, would -accomplish this. - - -(148.) In order to secure the combustion of the gases evolved from -the coals placed in the front of the grate, it is necessary that a -supply of atmospheric air should be admitted with them over the -burning fuel. This is effected by small apertures or regulators, -provided in the fire-doors, governed by sliding-plates, by which -they may be opened or closed to any required extent. - -A patent has recently been granted to Mr. Williams, one of the -directors of the City of Dublin Steam Navigation Company, for a -method of consuming the unburned gases which escape from the -grate, and are carried through the flues. This method consists in -introducing into the flue tubes placed in a vertical position, the -lower ends of which being inserted in the bottom of the flue are -made to communicate with the ash-pit, and the upper ends of which -are closed. The sides and tops of these tubes are pierced with -small holes, through which atmospheric air drawn from the ash-pit -issues in jets. The oxygen supplied by this air immediately -combines with the carburetted hydrogen, which [Pg261] having -escaped from the furnace unburned is carried through the flues at -a sufficient temperature to enter into combination with the oxygen -admitted through holes in the tubes. A number of jets of flame -thus proceed from these holes, having an appearance similar to the -flame of a gas-lamp. - -It is evident that such tubes must be inefficient unless they are -placed in the flues so near the furnace, that the temperature of -the unburned gases shall be sufficiently high to produce their -combustion. - - -(149.) The magnitude of the grate and ash-pit must be determined by -the rate at which the evaporation is required to be conducted in the -boiler and the quality of the fuel. It must be a matter of regret, -that the proportions of the various parts of steam-engines, with -their boilers and furnaces, have not been determined by any exact or -satisfactory experiments; and those who project and manufacture the -engines themselves, are not less in ignorance on those points than -others. With coals of the common quality a certain average -proportion must exist between the necessary magnitude of the -grate-surface and the quantity of water to be evaporated in a given -time in the boiler. But what that proportion is for any given -quality of fuel, is at present unascertained. Each engine-maker -follows his own rule, and the rule thus followed is in most cases a -matter of bare conjecture, unsupported by any experimental evidence. -Some engine-makers will allow a square foot of grate-surface for -every cubic foot of water per hour, which is expected to be -evaporated in the boiler; others allow only half a square foot: and -practice varies between these limits. Bituminous coals which melt -and cake, and which burn with much flame and smoke, must be spread -more thinly on the grate than other descriptions of fuel, otherwise -a considerable quantity of combustible gases would be dismissed into -the flues unburnt. Such coals therefore, other circumstances being -the same, require a larger portion of grate-surface; and the same -may be said of coals which produce clinkers in their combustion, and -form lumps of vitrified matter on the grate, by which the spaces -between the grate-bars are speedily closed up. When such fuel is -used, the grate-bars require to be frequently raked [Pg262] out, -otherwise the spaces between them being obstructed, the draught -would become insufficient for the due combustion of the fuel. - -To facilitate the raking out of the grate, the bars are placed -with their ends towards the fire-door: they are usually made of -cast-iron, from two to two inches and a half wide on the upper -surface, with intervals of nearly half an inch between them. The -bars taper downwards, their under surfaces being much narrower -than their upper, the spaces between them thus widening, to -facilitate the fall of the ashes between them. The grate-bars -slope downwards from the front to the back. The height of the -centre of the bottom of the boiler, above the front of the grate, -is usually about two feet, and about three feet above the back of -it. The concave bottom of the boiler, however, brings its surfaces -at the slide closer to the grate. - - -(150.) Between the evaporating power of the boiler, and the -magnitude of surface it exposes to the action of the furnace, -there is a relation which, like that of the grate surface, has -never been ascertained by any certain or satisfactory experimental -investigation; much less have the different degrees of efficiency -attending different parts of the boiler-surface been determined. -That part of the surface of the boiler immediately over and around -the grate, is exposed to the immediate radiation of the burning -fuel, and is therefore probably the most efficient in the -production of steam. The tendency of flame and heated air to rise, -would naturally bring them in the flues into closer contact with -those parts of the boiler-surface which are horizontal in their -position, and which form the tops of the flues, than with those -which are lateral or vertical in their position, and which form -the sides of the flues. In a boiler constructed like that already -described, the flue-surface therefore, which would be most -efficient, would be the concave bottom of the boiler extending -from the fire-bridge to its remote end. In some boilers, -especially those in which steam of high pressure is produced, the -form is cylindrical, the middle flue being formed into an -elliptical tube the greater axis of which is horizontal from end -to end of the boiler. It seems doubtful, however, [Pg263] whether -in such a boiler the heat produces any useful effect on the water -below the flue, the water above being always at a higher -temperature, and therefore lighter than that below, and -consequently no currents being established between the upper and -lower strata of the water. - -It was considered by Mr. Watt, but we are not aware on what -experimental grounds, that from eight to ten square feet of -heating surface were sufficient to produce the evaporation of one -cubic foot of water per hour. The practice of engine-makers since -that time has been to increase the allowance of heating surface -for the same rate of evaporation. Engine-builders have varied very -much in this respect, some allowing twelve, fifteen, and even -eighteen square feet of surface for the same rate of evaporation. -It must, however, still be borne in mind, that whether this -increased allowance did or did not produce the actual evaporation -imputed to it, has not been, as far as we are informed, ever -accurately ascertained. The production of a given rate of -evaporation by a moderate heat diffused over a larger surface, -rather than by a fiercer temperature confined to a smaller -surface, is attended with many practical advantages. The plates of -the boiler acted upon by the fire are less exposed to oxydisation, -and the boiler will be proportionally more durable. - - -(151.) Besides presenting to the action of the fire a sufficient -surface to produce steam at the required rate, the capacity of the -boiler must be proportioned to the quantity of water to be -evaporated. The space within the boiler is appropriated to a -twofold purpose: 1st, To contain the water to be evaporated; 2dly, -To contain a quantity of ready made steam for the supply of the -cylinder. If the space appropriated to the steam did not bear a -considerable proportion to the magnitude of the cylinder, the -momentary expansion of the steam passing to the cylinder from the -boiler at each stroke would reduce the pressure of the steam in a -great proportion, and unless the pressure in the boiler were -considerably greater than that which the steam is intended to have -in the cylinder, the pressure in the latter would be reduced below -the proper amount. The proportion of the [Pg264] steam space in -the boiler to the magnitude of the cylinder has been very -variously estimated, nor can it be said that any practical rule of -a general kind has been adopted. It is held by some that the -steam-space will be sufficient if it contain five times the -quantity of steam consumed at each stroke, while others maintain -that it should contain at least ten times that quantity, and -opinions vary between these limits. - - -(152.) The proportion of water-space in the boiler to its -evaporating power should also be regulated, so that the -introduction of the feed at a comparatively low temperature may -not unduly chill the water in the boiler. Supposing the feed to be -introduced in a low pressure boiler at the temperature of 100°, -and that the necessary temperature within the boiler be 225°, the -quantity of water it contains should be about five times the -quantity evaporated, and therefore also five times the quantity -introduced through the feed per hour. For every cubic foot of -water per hour therefore, intended to be evaporated by the boiler, -water-space for five cubic feet should be provided. It is, -however, right to repeat that this (like almost every other so -called rule) is the result not of any exact general calculation, -but one deduced from the custom which has obtained among the -manufacturers of steam-engines. - - -(153.) The surface of the water in the boiler should always be -above the range of the flues. When the heated air in the flues -acts upon a part of the boiler within which water is contained, -the water within receiving an increased temperature becomes, bulk -for bulk, lighter than the strata of water above it, and ascends. -It is replaced by the descending strata, which, in their turn -receiving increased temperature, rise to the surface; or if the -action of the heat convert the water into steam, the bubbles of -steam rise to the surface, fresh portions of water continually -coming into contact with the boiler-plates on which the heated air -or flame acts. By this process the boiler-plates are continually -cooled, either by being successively washed by water at a lower -temperature, or by the heat taken from them becoming latent in the -steam bubbles formed in contact with them. But if the heat act -[Pg265] upon a part of the boiler containing steam within it, -which steam being a slow recipient of heat, and no currents being -established, nor any phenomenon produced in which heat is rendered -latent, the heat of the fire communicated to the boiler-plates -accumulates in them, and raises their temperature to an injurious -degree. The plates may by this means be softened, so as to cause -the boiler to burst, or the difference between the expansion of -the highly heated plates thus exposed to fire in contact with -steam and that of the plates which are cooled by contact with -water, may cause the joinings of the boiler-plates to open, and -the boiler to leak. By whatever means, therefore, the boiler be -fed, care should be taken that the evaporation should not be -allowed to reduce the level of the water in it below the highest -flue. - - -(154.) As the water by which the boiler is fed must always have a -much lower temperature than that at which the boiler is -maintained, the supply of the feed will have a constant tendency -to lower the temperature of the water, and this tendency will be -determined by the proportion between the magnitude of the feed and -the quantity of water in the boiler. - -Since it is requisite that the level of the water in the boiler -shall not suffer any considerable change, it is evident that the -magnitude of the feed must be equal to the quantity of water -evaporated. If it were less, the level of the water would -continually fall by reason of the excess of the evaporation over -the feed; and if it were greater, the level would rise by the -accumulation of water in the boiler. If therefore the quantity of -water-space allowed in the boiler be five times the volume of -water evaporated per hour, the quantity introduced by the feed per -hour, whether continuously or at intervals, must be of the same -amount. Since the process of evaporation is continuous, the -variation of level of water in the boiler will be entirely -dependent on the intervals between the successive feeds. If the -feed be continuous, and always equal to the evaporation, then the -level of the water in the boiler will undergo no change; but if -while the evaporation is continuous the feed be made at intervals, -then the change of level of water in the boiler as [Pg266] well -as its change of temperature, will be subject to a variation -proportional to the intervals between the successive feeds. It is -manifest, therefore, that the feed should either be uninterrupted -or be supplied at short intervals, so that the change of level and -temperature of the water in the boiler should not be considerable. - - -(155.) Different methods have been, from time to time, suggested -for indicating the level of the water in the boiler. We have -already mentioned the two gauge-pipes used in the earlier -steam-engines (31.), and which are still generally continued. -There are, however, some other methods which merit our attention. - -[Illustration: _Fig._ 75.] - -A weight F (_fig._ 75.), half immersed in the water in the boiler, -is supported by a wire, which, passing steam-tight through a small -hole in the top, is connected by a flexible string, or chain, -passing over a wheel W, with a counterpoise A, which is just -sufficient to balance F when half immersed. If F be raised above the -water, _A_ being lighter will no longer balance it, and F will -descend pulling up A, and turning the wheel W. If, on the other -hand, F be plunged deeper in the water, A will more than balance it, -and will pull it up, so that the only position in which F and A will -balance each other is, when F is half immersed. The wheel W is so -adjusted, that when two pins placed on its rim are in the horizontal -position, the water is at its proper level. Consequently it follows, -that if the water rise above this level, the weight F is lifted and -A falls, so that the pins come into another position. If, on the -other hand, the level of the water fall, F falls and A rises, so -that the pins assume a different position. Thus, in general, the -position of the pins becomes an indication of the quantity of water -in the boiler. - - -(156.) Another method is to place a glass tube (_fig._ 76.), with -one end T entering the boiler above the proper level, and the -other end T' entering it below the proper level. It must [Pg267] -be evident that the water in the tube will always stand at the -same level as the water in the boiler, since the lower part has a -free communication with that water, while the surface is submitted -to the pressure of the same steam as the water in the boiler. This -and the last-mentioned gauge have the advantage of addressing the -eye of the engineer at once, without any adjustment; whereas the -gauge-cocks must be both opened, whenever the depth is to be -ascertained. - -[Illustration: _Fig._ 76.] - -These gauges, however, require the frequent attention of the -engine-man; and it becomes desirable either to find some more -effectual means of awakening that attention, or to render the -supply of the boiler independent of any attention. In order to -enforce the attention of the engine-man to replenish the boiler -when partially exhausted by evaporation, a tube was sometimes -inserted at the lowest level to which it was intended that the -water should be permitted to fall. This tube was conducted from -the boiler into the engine-house, where it terminated in a -mouth-piece or whistle, so that whenever the water fell below the -level at which this tube was inserted in the boiler, the steam -would rush through it, and issuing with great velocity at the -mouth-piece, would summon the engineer to his duty with a call -that would rouse him even from sleep. - -[Illustration: _Fig._ 77.] - - -(157.) In the most effectual of these methods, the task of -replenishing the boiler should still be executed by the engineer; -and the utmost that the boiler itself was made to do, was to give -due notice of the necessity for the supply of water. The -consequence was, among other inconveniences, that the level of the -water was subject to constant variation. - -To remedy this a method has been invented, by which [Pg268] the -engine is made to feed its own boiler. The pipe G (_fig._ 77.), -which leads from the hot water pump, terminates in a small cistern -C in which the water is received. In the bottom of this cistern, a -valve V is placed, which opens upwards, and communicates with a -feed-pipe, which descends into the boiler below the level of the -water in it. The stem of the valve V is connected with a lever -turning on the centre D, and loaded with a weight F dipped in the -water in the boiler in a manner similar to that described in -_fig._ 75., and balanced by a counterpoise A in exactly the same -way. When the level of the water in the boiler falls, the float F -falls with it, and pulling down the arm of the lever raises the -valve V, and lets the water descend into the boiler from the -cistern C. When the boiler has thus been replenished, and the -level raised to its former place, F will again be raised, and the -valve V closed by the weight A. In practice, however, the valve V -adjusts itself by means of the effect of the water on the weight -F, so as to permit the water from the feeding-cistern C to flow in -a continued stream, just sufficient in quantity to supply the -consumption from evaporation, and to maintain the level of the -water in the boiler constantly the same. - -By this arrangement the boiler is made to replenish itself, or, -more properly speaking, it is made to receive such a supply, as -that it never wants replenishing, an effect which no effort of -attention on the part of an engine-man could produce. But this is -not the only good effect produced by this contrivance. A part of -the steam which originally left the boiler, and having discharged -its duty in moving the piston, was condensed and reconverted into -water, and lodged by the air-pump in the hot well (_fig._ 77.), is -here again restored to the source from which it came, bringing -back all the unconsumed portion of its heat preparatory to being -once more put in circulation through the machine. - -The entire quantity of hot water pumped into the cistern C, is not -always necessary for the boiler. A waste-pipe may be provided for -carrying off the surplus, which may be turned to any purpose for -which it may be required; or it may be discharged into a cistern -to cool, preparatory to [Pg269] being restored to the cold -cistern, in case water for the supply of that cistern be not -sufficiently abundant. - -[Illustration: _Fig._ 78.] - - -(158.) Another method of arranging a self-regulating feeder is -shown in _fig._ 78. A is a hollow ball of metal attached to the -end of a lever, whose fulcrum is at B. The other arm of the lever -C is connected with the stem of a spindle-valve, communicating -with a tube which receives water from the feeding-cistern. Thus, -when the level of the water in the boiler subsides, the ball A -preponderating over the weight of the opposite arm, the lever -falls, the arm C rises and opens the valve, and admits the feeding -water. This apparatus will evidently act in the same manner and on -the same principles as that already described. [Pg270] - -The mouth of the tube by which the feed is introduced should be -placed at that part of the boiler which is nearest the end of the -flues which issue into the chimney. By such means the temperature -of the water in contact with those flues will be lowest at the -place where the temperature of the heated air intended to act upon -it is also lowest. The difference of the temperatures will -therefore be greater than it would be if the point of the boiler -containing water of a higher temperature was left in contact with -this part of the flue. - -[Illustration: _Fig._ 79.] - - -(159.) It is necessary to have a ready method of ascertaining at -all times the pressure of the steam which is used in working the -engine. For this purpose a bent tube containing mercury is -inserted into some part of the apparatus, which has free -communication with the steam. Let A B C (_fig._ 79.) be such a -tube. The pressure of the steam forces the mercury down in the leg -A B, and up in the leg B C. If the mercury in both legs be at -exactly the same level, the pressure of the steam must be exactly -equal to that of the atmosphere; because the steam pressure on the -mercury in A B balances the atmospheric pressure on the mercury in -B C. If, however, the level of the mercury in B C be above the -level of the mercury in B A, the pressure of the steam will exceed -that of the atmosphere. The excess of its pressure above that of -the atmosphere may be found by observing the difference of the -level of the mercury in the tubes B C and B A, allowing a pressure -of one pound on each square inch for every two inches in the -difference of the levels. - -If, on the contrary, the level of the mercury in B C should fall -below its level in A B, the atmospheric pressure will [Pg271] -exceed that of the steam, and the quantity of the excess may be -ascertained exactly in the same way. - -If the tube be glass, the difference of levels of the mercury -would be visible; but it is most commonly made of iron; and in -order to ascertain the level, a thin wooden rod with a float is -inserted in the open end of B C, so that the portion of the stick -within the tube indicates the distance of the level of the mercury -from its mouth. A bulb or cistern of mercury might be substituted -for the leg A B, as in the common barometer. This instrument is -called the _steam-gauge_. - -If the steam-gauge be used as a measure of the strength of the -steam which presses on the piston, it ought to be on the same side -of the throttle-valve (which is regulated by the governor) as the -cylinder; for if it were on the same side of the throttle-valve -with the boiler, it would not be affected by the changes which the -steam may undergo in passing through the throttle-valve, when -partially closed by the agency of the governor. - -For boilers in which steam of very high pressure is used, as in -those of locomotive engines, a steam-gauge, constructed on the above -principle, would have inconvenient or impracticable length. In such -boilers the pressure of the steam is equal to four or five times -that of the atmosphere, to indicate which the column of mercury in -the steam-gauge would be four or five feet in height. In such cases -a thermometer-gauge may be used with advantage. The principle of -this gauge is founded on the fact, that between the pressure and -temperature of steam produced in contact with water there is a fixed -relation, the same temperature always corresponding to the same -pressure. If, therefore, a thermometer be immersed in the boiler -which shall show the temperature of the steam, a scale may be -attached to it, on which shall be engraved the corresponding -pressures. Such gauges are now very generally used on locomotive -engines. - -[Illustration: _Fig._ 80.] - - -(160.) The force with which the piston is pressed depends on two -things, 1st, the actual strength of the steam which presses on it; -and, 2dly, on the actual strength of the vapour which resists it. -For although the vacuum produced by the method of separate -condensation be much more perfect than [Pg272] what had been -produced in the atmospheric engines, yet still some vapour of a -small degree of elasticity is found to be raised from the hot -water in the bottom of the condenser before it can be extracted by -the air-pump. One of these pressures is indicated by the -steam-gauge already described; but still, before we can estimate -the force with which the piston descends, it is necessary to -ascertain the force of the vapour which remains uncondensed, and -resists the motion of the piston. Another gauge, called the -barometer-gauge, is provided for this purpose. A glass tube A B -(_fig._ 80.), more than thirty inches long and open at both ends, -is placed in an upright or vertical position, having the lower end -B immersed in a cistern of mercury C. To the upper end is attached -a metal tube, which communicates with the condenser, in which a -constant vacuum, or rather high degree of rarefaction, is -sustained. The same vacuum must therefore exist in the tube A B, -above the level of the mercury, and the atmospheric pressure on -the surface of the mercury in the cistern C will force the mercury -up in the tube A B, until the column which is suspended in it is -equal to the difference between the atmospheric pressure and the -pressure of the uncondensed steam. The difference between the -column of mercury sustained in this instrument and in the common -barometer, will determine the strength of the uncondensed steam, -allowing a force proportional to one pound per square inch for -every two inches of mercury in the difference of the two columns. -In a well-constructed engine which is in good order, there is very -little difference between the altitude in the barometer-gauge and -the common barometer. - -To compute the force with which the piston descends, thus becomes a -very simple arithmetical process. First, ascertain the difference of -the levels of the mercury in the steam-gauge; this gives the excess -of the steam pressure above the atmospheric pressure. Then find the -height of the mercury in the barometer-gauge; this gives the excess -of the atmospheric pressure above the uncondensed steam. Hence, if -these two heights be added together, we shall obtain the [Pg273] -excess of the impelling force of the steam from the boiler, on the -one side of the piston, above the resistance of the uncondensed -steam on the other side: this will give the effective impelling -force. Now, if one pound be allowed for every two inches of mercury -in the two columns just mentioned, we shall have the number of -pounds of impelling pressure on every square inch of the piston. -Then, if the number of square inches in the section of the piston be -found, and multiplied by the number of pounds on each square inch, -the force with which it moves will be obtained. - -From what we have stated it appears that, in order to estimate the -force with which the piston is urged, it is necessary to refer to -both the barometer and the steam-gauge. This double computation -may be obviated by making one gauge serve both purposes. If the -end C of the steam-gauge (_fig._ 79.), instead of communicating -with the atmosphere were continued to the condenser, we should -have the pressure of the steam acting upon the mercury in the tube -B A, and the pressure of the uncondensed vapour which resists the -piston acting on the mercury in the tube B C. Hence the difference -of the levels of the mercury in the tubes would at once indicate -the difference between the force of the steam and that of the -uncondensed vapour, which is the effective force with which the -piston is urged. - - -(161.) But these methods of determining the effective force by -which the piston is urged, can only be regarded as approximations, -and not very perfect ones. If the condensation of steam on one -side of the piston were instantaneously effected, or the -uncondensed vapour were of the same tension during the whole -stroke; and if, besides this, the pressure of steam on the piston -were of uniform intensity from the beginning to the end of the -stroke, then the steam and barometer gauges taken together would -become an accurate index of the effective force of steam on the -piston: but such is not the case. When the steam is first admitted -through the steam-valve it acts on the piston with a pressure -which is first slightly diminished, and afterwards a little -increased, until it arrives at that part of the stroke at which -the steam-valve is closed, after which the pressure is diminished. -The [Pg274] pressure, therefore, urging the piston is subject to -variation; but the pressure of the uncondensed vapour on the other -side of the piston is subject to still greater change. At the -moment the exhausting-valve is opened, the piston is relieved from -the pressure upon it by the commencement of the condensation; but -this process during the descent of the piston is gradual, and the -vacuum is rendered more and more perfect, until the piston has -nearly attained the limit of its play. These variations, both as -well of the force urging the piston as of the force resisting it, -are such as not to be capable of being accurately measured by a -mercurial column, since they would produce oscillations in such a -column, which would render any observations of its mean height -impracticable. - -To measure the mean efficient force of the piston, taking into -account these circumstances, Mr. Watt invented an instrument, -which, like all his mechanical inventions, has answered its -purpose perfectly, and is still in general use. This instrument, -called an _indicator_, consists of a cylinder of about 1-3/4 inch -in diameter, and 8 inches in length. It is bored with great -accuracy, and fitted with a solid piston moving steam-tight in it -with very little friction. The rod of this piston is guided in the -direction of the axis of the cylinder through a collar in the top, -so as not to be subject to friction in any part of its play. At -the bottom of the cylinder is a pipe governed by a stop-cock and -turned in a screw, by which the instrument may be screwed on the -top of the steam-cylinder of the engine. In this position, if the -stop-cock of the indicator be opened, a free communication will be -made between the cylinder of the indicator and that of the engine. -The piston-rod of the indicator is attached to a spiral spring, -which is capable of extension and compression, and which by its -elasticity is capable of measuring the force which extends or -compresses it in the same manner as a spring steel-yard or -balance. If a scale be attached to the instrument at any point on -the piston-rod to which an index might be attached, then the -position of that index upon the scale would be governed by the -position of the indicator-piston in its cylinder. If any force -pressed the indicator-piston upwards, so as to compress the -spring, [Pg275] the index would rise upon the scale; and if, on -the other hand, a force pressed the indicator-piston downwards, -then the spiral spring would be extended, and the index on the -piston-rod descend upon the scale. In each case the force of the -spring, whether compressed or extended, would be equal to the -force urging the indicator-piston, and the scale might be so -divided as to show the amount of this force. - -Now, let the instrument be supposed to be screwed upon the top of -the cylinder of a steam-engine, and the stop-cock opened so as to -leave a free communication between the cylinder of the indicator -below its piston and the cylinder of the steam-engine above the -steam-piston. At the moment the upper steam-valve is opened, the -steam rushing in upon the steam-piston will also pass into the -indicator, and press the indicator-piston upwards: the index upon -its piston-rod will point upon the scale to the amount of pressure -thus exerted. As the steam-piston descends, the indicator-piston -will vary its position with the varying pressure of the steam in -the cylinder, and the index on the piston-rod will play upon the -scale, so as to show the pressure of the steam at each point -during the descent of the piston. - -If it were possible to observe and record the varying position of -the index on the piston-rod of the indicator, and to refer each of -these varying positions to the corresponding point of the descending -stroke, we should then be able to declare the actual pressure of the -steam at every point of the stroke. But it is evident that such an -observation would not be practicable. A method, however, was -contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt, -by which this is perfectly effected. A square piece of paper, or -card, is stretched upon a board, which slides in grooves formed in a -frame. This frame is placed in a vertical position near the -indicator, so that the paper may be moved in a horizontal direction -backwards and forwards, through a space of fourteen or fifteen -inches. Instead of an index a pencil is attached to the indicator of -the piston-rod: this pencil is lightly pressed by a spring against -the paper above mentioned, and as the paper is moved in a horizontal -direction [Pg276] under the pencil, would trace upon the paper a -line. If the pencil were stationary this line would be straight and -horizontal, but if the pencil were subject to a vertical motion, the -line traced on the paper moved under the pencil horizontally would -be a curve, the form of which would depend on the vertical motion of -the pencil. The board thus supporting the paper is put into -connexion by a light cord carried over pulleys with some part of the -parallel motion, by which it is alternately moved to the right and -to the left. As the piston ascends or descends, the whole play of -the board in the horizontal direction will therefore represent the -length of the stroke, and every fractional part of that play will -correspond to a proportional part of the stroke of the steam-piston. - -[Illustration: _Fig._ 81.] - -The apparatus being thus arranged, let us suppose the steam-piston -at the top of the cylinder commencing its descent. As it descends, -the pencil attached to the indicator piston-rod varies its height -according to the varying pressure of the steam in the cylinder. At -the same time the paper is moved uniformly under the pencil, and a -curved line is traced upon it from right to left. When the piston -has reached the bottom of the cylinder, the upper exhausting-valve -is opened, and the steam drawn off to the condenser. The -indicator-piston being immediately relieved from a part of the -pressure acting upon it descends, and with it the pencil also -descends; but at the same time the steam-piston has begun to ascend, -and the paper to return from left to right under the pencil. While -the steam-piston continues to ascend, the condensation becomes more -and more perfect, and the vacuum in the cylinder, and therefore also -in the indicator, being gradually increased in power, the -atmospheric pressure above the indicator-piston presses it downwards -and stretches the spring. The pencil meanwhile, with the paper -moving under it from right to left, traces a second curve. As the -former curve showed the actual pressure of the steam impelling the -piston in its descent, this latter will show the pressure of the -uncondensed steam raising the piston in its ascent, and a comparison -of the two will exhibit the effective force on the piston. _Fig._ -81. represents such a diagram as would be [Pg277] produced by this -instrument. A B C is the curve traced by the pencil during the -descent of the piston, and C D E that during its ascent. A is the -position of the pencil at the moment the piston commences its -descent, B is its position at the middle of the stroke, and C at the -termination of the stroke. On closing the upper steam-valve and -closing the exhausting-valve, the indicator-piston being gradually -relieved from the pressure of the steam the pencil descends, and at -the same time the paper moving from left to right, the pencil traces -the curve C D E, the gradual descent of this curve showing the -progressive increase of the vacuum. As the atmospheric pressure -constantly acts above the piston of the indicator, its position will -be determined by the difference between the atmospheric pressure and -the pressure of the steam below it; and therefore the difference -between the heights of the pencil at corresponding points in the -ascending and descending stroke, will express the difference between -the pressure of the steam impelling the piston in the ascent and -resisting it in the descent at these points. Thus at the middle of -the stroke, the line B D will express the extent to which the spring -governing the indicator-piston would be stretched by the difference -between the force of steam impelling the piston at the middle of the -descending stroke, and the force of steam resisting it at the middle -of the ascending stroke. The force therefore measured by the line B -D will be the effective force on the piston at that point; and the -same may be said of every part of the diagram produced by the -indicator. - -The whole mechanical effect produced by the stroke of the piston -being composed of the aggregate of all its varying effects -throughout the stroke, the determination of its amount [Pg278] is -a matter of easy calculation by the measurement of the diagram -supplied by the indicator. Let the horizontal play of the pencil -from A to C be divided into any proposed number of equal parts, -say ten: at the middle of the stroke, B D expresses the effective -force on the piston, and if this be considered to be uniform -through the tenth part of the stroke, as from _f_ to _g_, then the -number of pounds expressed by B D multiplied by the tenth part of -the stroke expressed in parts of a foot, will be the mechanical -effect through that part of the stroke expressed in pounds' weight -raised one foot. In like manner _m n_ will express the effective -force on the piston after three fourths of the stroke have been -performed, and if this be multiplied by a tenth part of the stroke -as before, the mechanical effect similarly expressed will be -obtained; and the same process being applied to any successive -tenth part of the stroke, and the numerical results thus obtained -being added together, the whole effect of the stroke will be -obtained, expressed in pounds' weight raised one foot. - - -(162.) By means of the indicator, the actual mechanical effect -produced by each stroke of the engine can be obtained, and if the -actual number of strokes made in any given time be known, the -whole effect of the moving power would be determined. An -instrument called a _counter_ was also contrived by Watt, to be -attached either to the working beam or to any other reciprocating -part of the engine. This instrument consisted of a train of -wheel-work with governing hands or indices moved upon divided -dials, like the hand of a clock. A record of the strokes was -preserved by means precisely similar to those by which the hands -of a clock or time-piece indicated and recorded the number of -vibrations of the pendulum or balance-wheel. - - -(163.) To secure the boiler from accidents arising from the steam -contained in it acquiring an undue pressure, a safety-valve is -used, similar in principle to those adopted in the early engines. -This valve is represented in _fig._ 71. at N. It is a conical -valve, kept down by a weight sliding on a rod upon it. When the -pressure of the steam overcomes the force of this weight, it -raises the valve and escapes, being carried off through the tube. -[Pg279] - -With a view to the economy of heat, this waste steam tube is -sometimes conducted into the feeding cistern, where the steam -carried off by it is condensed, and heats the feeding water. - -The magnitude of the safety-valve should be such that, when open, -steam should be capable of passing through it as rapidly as it is -generated in the boiler. The superficial magnitude, therefore, of -such valves must be proportional to the evaporating power of the -boiler. In low pressure boilers the steam is generally limited to -five or six pounds' pressure per square inch, and consequently the -load over the safety-valve in pounds would be found by multiplying -the superficial magnitude of its smallest part by these numbers. -In boilers in which the steam is maintained at a higher pressure, -it would be inconvenient to place upon the safety-valve the -necessary weight. In such cases a lever is used, the shorter arm -of which presses down the valve, and the longer arm is held down -by a weight capable of adjustment, so that the pressure on the -valve may be regulated at discretion. Two safety-valves should be -provided on all boilers, one of which should be locked up, so that -the persons in care of the engine should have no power to increase -the load upon it. In such case, however, it is necessary that a -handle connected with the valve should project outside the box -containing it, so that it may always be possible for the engineer -to ascertain that the valve is not locked in its seat, a -circumstance which is liable to happen. - -Sometimes also two safety-valves are provided, one loaded a little -heavier than the other. The escape of steam from the lighter valve -in this case gives notice to the engine-man of the growing -increase of pressure, and warns him to check the production of -steam. The lever by which the safety-valve is held down is -sometimes acted on by a spiral spring, capable of being so -adjusted as to produce any required pressure on the valve. This -arrangement is adopted in locomotive engines, where steam of very -high pressure is used; and in such cases also there are always -provided two such valves, one of which cannot be increased in its -pressure. - -The pipe by which the boiler is fed with water will [Pg280] -necessarily act as a safety-valve, for when the pressure of the -steam increases in an undue degree, it will press the water in the -boiler up through the feed-pipe, so as to discharge it into the -feed-cistern, a circumstance which would immediately give notice -of the internal state of the boiler. The steam-gauge, already -described (_fig._ 79.), would also act as a safety-valve; for if -the pressure of steam in the boiler should be so augmented as to -blow the mercury out of the steam-gauge, the steam would then -issue through the gauge, and the pressure of the boiler be -reduced, provided that the magnitude of the tube forming the -steam-gauge were sufficient for this purpose. - - -(164.) In high pressure boilers which are exposed to extreme -temperatures and pressures, and which are therefore subject to -danger of explosion, a plug of metal is sometimes inserted, which is -capable of being fused at a temperature above which the boiler -should not be permitted to be raised. If the pressure of steam -increase beyond the proper limit, the temperature of the water and -steam will undergo a corresponding increase; and if the metal of the -plug be capable of being fused at such a temperature, the plug will -fall out of the boiler, and the steam and water will issue from it. -Various alloys of metal are fusible at temperatures sufficiently low -for this purpose. An alloy composed of one part of lead, three of -tin, and five of bismuth, will fuse at the common temperature of -boiling water; and alloys of the same metals, in various -proportions, will fuse at different temperatures from 200° to 400°. - -Although fusible plugs may be used, in addition to other means of -insuring safety, they ought not to be exclusively relied on at the -ordinary working pressure of the boiler. The fusible plug ought to -be capable of more than resisting the pressure; but if it be so, -its point of fusion would be one at which the steam would have a -pressure of at least two atmospheres above its working pressure. -The plug would therefore be capable of being fused only as soon as -the steam would acquire a pressure of 30 lbs. per inch above its -regular working pressure. - -When a boiler ceases to be worked, and the furnace has been -extinguished, the space within it appropriated to steam [Pg281] -will be left a vacuum by the condensation of the steam with which -it was previously filled. The external pressure of the atmosphere -acting on the boiler would, under such circumstances, have a -tendency to crush it inwards. To prevent this, a safety-valve is -provided, opening inwards, and balanced by a weight sufficient to -keep it closed until it be relieved from the pressure of the steam -below. - -A large aperture closed by a flange secured with screws, -represented at O in _fig._ 71., called the _man-hole_, is provided -to admit persons into the boiler for the purpose of cleaning or -repairing its interior. - - -(165.) The manner in which the governor regulates the supply of -steam from the boiler to the cylinder, proportioning the quantity -to the work to be done, and thereby sustaining a uniform motion, -has been already explained (p. 125.). Since then the _consumption_ -of steam in the engine is subject to variation, owing to the -various quantities of work it may have to perform, it is evident -that the _production_ of steam in the boiler should be subject to -a proportional variation. For otherwise, one of two effects would -ensue: the boiler would either fail to supply the engine with -steam, or steam would accumulate in the boiler from being produced -in too great abundance, and would escape at the safety-valve, and -thus be wasted. - -In order to vary the production of steam in proportion to the -demands of the engine, it is necessary to stimulate or mitigate -the furnace, as the evaporation is to be augmented or diminished. - -The activity of the furnace must depend on the current of air -which is drawn through the grate-bars, and this will depend on the -magnitude of the space afforded for the passage of that current -through the flues. A plate called a _damper_ is accordingly placed -with its plane at right angles to the flue, so that by raising and -lowering it in the same manner as the sash of a window is raised -or lowered, the space allowed for the passage of air through the -flue may be regulated. This plate might be regulated by the hand, -so that by raising or lowering it the draught might be increased -or diminished, and a corresponding effect produced on the [Pg282] -evaporation in the boiler: but the force of the fire is rendered -uniformly proportional to the rate of evaporation by the following -arrangement, without the intervention of the engineer. The column -of water sustained in the feed-pipe (_figs._ 71, 72.) represents -by its weight the difference between the pressure of steam within -the boiler and that of the atmosphere. If the engine consumes -steam faster than the boiler produces it, the steam contained in -the boiler acquires a diminished pressure, and consequently the -column of water in the feed-pipe will fall. If, on the other hand, -the boiler produce steam faster than the engine consumes it, the -accumulation of steam in the boiler will cause an increased -pressure on the water it contains, and thereby increase the height -of the column of water sustained in the feed-pipe. This column -therefore necessarily rises and falls with every variation in the -rate of evaporation in the boiler. A hollow float P is placed upon -the surface of the water of this column; a chain connected with -this float is carried upwards, and passed over two pulleys, after -which it is carried downwards through an aperture leading to the -flue which passes beside the boiler: to this chain is attached the -damper. By such an arrangement it is evident that the damper will -rise when the float P falls, and will fall when the float P rises, -since the weight of the damper is so adjusted, that it will only -balance the float P when the latter rests on the surface of the -water. - -Whenever the evaporation of the boiler is insufficient, it is -evident from what has been stated, that the float P will fall and -the damper will rise, and will afford a greater passage for air -through the flue. This will stimulate the furnace, will augment -its heating power, and will therefore increase the rate of -evaporation in the boiler. If, on the other hand, the production -of steam in the boiler be more than is requisite for the supply of -the engine, the float will be raised and the damper let down, so -as to contract the flue, to diminish the draught, to mitigate the -fire, and therefore to check the evaporation. In this way the -excess, or defect, of evaporation in the boiler is made to act -upon the fire, so as to render the heat proceeding from the -combustion as nearly as possible proportional to the wants of the -engine. [Pg283] - - -(166.) The method of feeding the furnace by hand through the -fire-door being subject to the double objection of admitting more -cold air over the fuel than is necessary for its combustion, and -the impracticability of insuring that regular attendance on the -part of the stokers, directed the attention of engineers to the -construction of self-regulating furnaces. The most effectual of -these, and that which has come into most general use, was invented -by Mr. William Brunton of Birmingham. - -The advantages proposed to be attained by him were those expressed -in his patent:-- - -"First, I put the coal upon the grate by small quantities, and at -very short intervals, say every two or three seconds. 2dly, I so -dispose of the coals upon the grate, that the smoke evolved must -pass over that part of the grate upon which the coal is in full -combustion, and is thereby consumed. 3dly, As the introduction of -coal is uniform in short spaces of time, the introduction of air -is also uniform, and requires no attention from the fireman. - -"As it respects economy: 1st, The coal is put upon the fire by an -apparatus driven by the engine, and so contrived that the quantity -of coal is proportioned to the quantity of work which the engine -is performing; and the quantity of air admitted to consume the -smoke is regulated in the same manner. 2dly, The fire-door is -never opened, excepting to clean the fire; the boiler, of course, -is not exposed to that continual irregularity of temperature which -is unavoidable in the common furnace, and which is found -exceedingly injurious to boilers. 3dly, The only attention -required is to fill the coal-receiver every two or three hours, -and clean the fire when necessary. 4thly, The coal is more -completely consumed than by the common furnace, as all the effect -of what is termed stirring up the fire (by which no inconsiderable -quantity of coal is passed into the ash-pit), is attained without -moving the coal upon the grate." - -A circular grate is placed on a vertical revolving shaft; on the -lower part of this shaft, under the ash-pit, is placed a toothed -wheel driven by a pinion. This pinion is placed on another vertical -shaft, which ascends above the boiler; and [Pg284] on the other end -of this is placed a bevelled wheel driven by a pinion. This pinion -is attached to a shaft, which takes its motion from the axis of the -fly-wheel, or any other revolving shaft connected with the engine. A -constant motion of revolution is therefore imparted to the circular -grate, and its velocity being proportional to that of the engine, -will necessarily be also proportional to the quantity of fuel which -ought to be consumed. Through that part of the boiler which is over -the fire-grate a vertical tube or opening is made directly over that -part of the furnace which is most distant from the flues. Over this -opening a hopper is placed, which contains the fuel by which the -boiler is to be fed; and in the bottom of this hopper is a sliding -valve, capable of being opened or closed, so as to regulate the -quantity of fuel supplied to the fire-grate. The fuel dropping in in -small quantities through this open valve falls on the grate, and is -carried round by it, so as to leave a fresh portion of the grate to -receive succeeding feeds. The coals admitted through the hopper are -previously broken to a proper size; and in some forms of this -apparatus there are two rollers, at a regulated distance asunder, -the surfaces of which are formed into blunt angular points, and -which are kept in slow revolution by the engine. Between these -rollers the coals must pass before they reach the valve through -which the furnace is fed, and they are thus broken and reduced to a -regulated size. The valve which regulates the opening through which -the feed is admitted, is connected by chains and pulleys with the -self-regulating damper already described, so that in proportion as -the damper is raised, the valve governing the feed may be opened. -Thus, while the quantity of air admitted by the damper is increased -according to the demands of the engine, the quantity of fuel -admitted for the feed is increased by opening the valve in the -bottom of the hopper in the same proportion. Apertures are also -provided in the front of the grate, governed by regulators, by which -the quantity of air necessary and sufficient to produce the -combustion of the gas evolved from the fuel is admitted, these -openings being also connected with the self-regulating damper. - -A considerable portion of the heat imparted to the water [Pg285] -in the boiler escapes by radiation from the surface of the boiler, -steam-pipes, and other parts of the machinery in contact with the -steam and hot water. The effects of this are rendered very -apparent in marine engines, where a large quantity of water is -found to be condensed in the great steam-pipes leading from the -boiler to the cylinder. In stationary land boilers this loss of -heat is usually diminished, and in some cases in a great degree -removed, by surrounding the boiler with non-conducting substances. -In some cases the boiler is built round in brick work. In -Cornwall, where the economy is regarded perhaps to a greater -extent than elsewhere, the boiler and steam-pipes are surrounded -with a packing of sawdust, which being almost a non-conductor of -heat, is impervious to the heat proceeding from the surfaces with -which it is in contact, and consequently confines all the heat -within the boiler. In marine boilers it has been the practice -recently to clothe the boiler and steam-pipes with a coating of -felt, which is attended with a similar effect. When these remedies -are properly applied, the loss of heat proceeding from the -radiation of the boiler is reduced to an extremely small amount. -The engine-houses of some of the Cornish engines, where the boiler -generates steam at a very high temperature, are nevertheless -frequently maintained at a lower temperature than the external -air, and on entering them they have in a great degree the effect -of a cave. - - -(167.) All mechanical action is measured by the amount of force -exercised, or resistance overcome, and the space through which -that force has acted, or through which the resistance has been -moved. - -The gross amount of mechanical action developed by the moving -power of an engine, is expended partly on moving the engine -itself, and partly on overcoming the resistance on which the -engine is intended to act. That part of the mechanical energy of -the moving power which is expended on the resistance or load which -the engine moves exclusively, and of the power expended on moving -the engine itself, is called _the useful effect_ of the machine. - -The _gross effect_, therefore, exceeds the _useful effect_ by the -[Pg286] amount of power spent in moving the engine, or which may -be wasted or destroyed in any way by the engine. - -It is usual to express and estimate all mechanical effect whatever -by nature of the resistance overcome, by an equivalent weight -raised a certain height. Thus, if an engine exerts a certain power -in driving a mill, in drawing a carriage on a road, or in -propelling a vessel on water, the resistance against which it has -to act must be equal to a definite amount of weight. If a carriage -be drawn, the traces are stretched by the tractive power, by the -same tension that would be given to them if a certain weight were -appended to them. If the paddle-wheels of a boat are made to -revolve, the water opposes to them a resistance equal to that -which would be produced, if instead of moving the water the wheel -had to raise some certain weight. In any case, therefore, weight -becomes the exponent of the energy of the resistance against which -the moving power acts. - -But the amount of mechanical effect depends conjointly on the -amount of resistance, and the space through which that resistance -is moved. The quantity of this effect, therefore, will be -increased in the same proportion, whether the quantity of -resistance or the space through which that resistance is moved be -augmented. Thus, a resistance of one hundred pounds, moved through -two feet, is mechanically equivalent to a resistance of two -hundred pounds moved through one foot, or of four hundred pounds -moved through six inches. To simplify, therefore, the expression -of mechanical effect, it is usual to reduce it invariably to a -certain weight raised one foot. If the resistance under -consideration be equivalent to a certain weight raised through ten -feet, it is always expressed by ten times the amount of that -weight raised through one foot. - -It has also been usual in the expression of mechanical effect, to -take the pound weight as the unit of weight, and the foot as the -unit of length, so that all mechanical effect whatsoever is -expressed by a certain number of pounds raised one foot. - - -(168.) The gross effect of the moving power in a steam-engine, is -the whole mechanical force developed by the evaporation [Pg287] -of water in the boiler. A part of this effect is lost by the -partial condensation of the steam before it acts upon the piston, -and by the imperfect condensation of it subsequently: another -portion is expended on overcoming the friction of the different -moving parts, and in acting against the resistance which the air -opposes to the machine. If the motion be subject to sudden shocks, -a portion of the power is then lost by the destruction of momentum -which such shocks produce. But if those parts of the machine which -have a reciprocating motion be, as they ought to be, brought -gradually to rest at each change of direction, then no power is -absorbed in this way. - - -(169.) The useful effect of an engine is variously denominated -according to the relation under which it is considered. If it be -referred to the time during which it is produced, it is called -POWER. - - -(170.) If it be referred to the fuel, by the combustion of which -the evaporation has been effected, it is called DUTY. - - -(171.) When steam-engines were first brought into use, they were -commonly applied to work pumps for mills which had been previously -worked or driven by horses. In forming their contracts, the first -steam-engine builders found themselves called upon to supply -engines capable of executing the same work as was previously -executed by some certain number of horses. It was therefore -convenient, and indeed necessary, to be able to express the -performance of these machines by comparison with the animal power -to which manufacturers, miners, and others, had been so long -accustomed. When an engine, therefore, was capable of performing -the same work in a given time as any given number of horses of -average strength usually performed, it was said to be an engine of -so many horses' power. Steam-engines had been in use for a -considerable time before this term had acquired any settled or -uniform meaning, and the nominal power of engines was accordingly -very arbitrary. At length, however, the use of steam-engines -became more extended, and the confusion and inconvenience arising -out of all questions respecting the performance of engines, -rendered it necessary that some fixed [Pg288] and definite -meaning should be assigned to the terms by which the powers of -this machine were expressed. To have abandoned the term -_horse-power_, which had been so long in use, would have been -obviously inconvenient; nor could there be any objection to its -continuance, provided all engine-makers, and all those who used -engines, could be brought to agree upon some standard by which the -unit of horse-power might be defined. The performance of a horse -of average strength working for eight hours a day was therefore -selected as a standard, or unit, of steam-engine power. Smeaton -estimated that such an animal, so working, was capable of -performing a quantity of work equal in its mechanical effect to -22,916 lbs. raised one foot per minute, while Desaguliers -estimated the same power at 27,500 lbs. raised through the same -height in the same time. The discrepancy between these estimates -probably arose from their being made from the performances of -different classes of horses. Messrs. Boulton and Watt caused -experiments to be made with the strong horses used in the -breweries in London, and from the result of these trials they -assigned 33,000 lbs. raised one foot per minute, as the value of a -horse's power. This is the unit of engine-power now universally -adopted; and when an engine is said to be of so many horses' -power, what is meant is, that that engine, in good working order -and properly managed, is capable of moving a resistance equal to -33,000 lbs. through one foot per minute. Thus an engine of ten -horse-power is one that would raise 330,000 lbs. weight one foot -per minute. - -Whether this estimate of an average horse's power be correct or -not, in reference to the actual work which the animal is capable -of executing, is a matter of no present importance in its -application to steam-power. The steam-engine is no longer used to -replace the power of horses, and therefore no contracts are based -upon such a comparison. The term horse-power, therefore, as -applied to steam-engines, must be understood to have no reference -whatever to the actual animal power, but must be taken as a term -having no other meaning than the expression of the ability of the -[Pg289] machine to move the amount of resistance above mentioned -through one foot per minute. - - -(172.) It has been already explained (67.) that the conversion of -a given volume of water into steam is productive of a certain -definite amount of mechanical force, this amount depending on the -pressure under which the water is evaporated, and the extent to -which the expansive principle is used in working the steam. It is -evident that this amount of mechanical effect is a major limit, -which cannot be exceeded by the power of the engine. - -If the steam be not worked expansively, then the whole power of -the water, transmitted in the form of steam from the boiler to the -working machinery, will be a matter of easy calculation, when the -pressure at which the steam is worked is known. A table, -exhibiting the mechanical power of a cubic foot of water converted -into steam at various pressures, expressed in an equivalent number -of pounds' weight raised one foot high, is given in the Appendix -to this volume. Where much accuracy is sought for, the pressure at -which the steam is used must be taken into account; but by -reference to the table it will be seen, that when steam is worked -without expansion, its mechanical effect varies very little with -the pressure. It may therefore be assumed, as has been already -stated, that for every cubic inch of water transmitted in the form -of steam to the cylinders, a force is produced, represented by a -ton weight raised a foot high. Now, as 33,000 lbs. is very nearly -15 tons, it follows that 15 cubic inches of water converted into -steam per minute, or 900 cubic inches per hour, will produce a -mechanical force equal to one horse. If, therefore, to 900 cubic -inches be added the quantity of water per hour necessary to move -the engine itself, independently of its load, we shall obtain the -quantity of water per hour which must be supplied by the boiler to -the engine for each horse-power, and this will be the same -whatever may be the magnitude or proportions of the cylinder. - - -(173.) The quantity of power expended in working the engine -itself, independently of that required to move its load, will be -less in proportion to the degree of perfection which [Pg290] may -be attained in the construction of the engine, and to the order in -which it is kept while working. Engines vary one from another so -much in these respects, that it is scarcely possible to lay down -any general rules for the quantity of power to be allowed over and -above what is necessary to move the load. The means whereby -mechanical power is expended in working the engine may be -enumerated as follows:-- - -_First._ Steam in passing from the boiler to the cylinder is -liable to lose its temperature by the radiation of the steam-pipes -and other passages through which it is conducted. Since the steam -produced in the boiler is in contact with water, it will be common -steam (94.), and consequently the least loss of heat will cause a -partial condensation. To whatever extent this condensation may be -carried, a proportional loss of power, in reference to the heat -obtained from the fuel, will be entailed upon the engine. - -It has been said that the force necessary to move the steam from -the boiler to the cylinder through passages more or less -contracted, subject to the friction of the pipes and tubes through -which it moves, should be taken into account in estimating the -power, and a corresponding deduction made. This, however, is not -the case: the steam having passed into the cylinder remains common -steam, its pressure being diminished by reason of the force -expended in thus moving it from the boiler to the cylinder. But -its mechanical efficacy at the reduced pressure is not sensibly -different from the efficacy which it had in the boiler. If at the -reduced pressure its volume were the same, then a loss of effect -would be sustained equivalent to the difference of the pressures; -but its volume being augmented in very nearly the same proportion -as its pressure is diminished, the mechanical efficacy of a given -weight of steam in the cylinder will be sensibly the same as in -the boiler. - -_Second._ The radiation of heat from the cylinder and its -appendages, will cause a partial condensation of steam, and -thereby produce a diminished mechanical effect. - -_Third._ The steam, which at each stroke of the piston fills the -passages between the steam-valves and the piston, at the [Pg291] -moment the latter commences the stroke will be inefficient. If it -were possible for the piston to come into steam-tight contact with -each end of the cylinder, and that the steam-valve should be in -immediate contact with the side or top of the piston, then the -whole of the steam which would pass through the steam-valve would -be efficient; but as some space, however small, must remain -between the piston and the ends of the cylinder, and between the -side of the cylinder and the steam-valve, there will always be a -volume of steam bearing a sensible proportion to the magnitude of -the cylinder, which at each stroke of the piston will be -inefficient. This volume of steam is called the _clearance_. - -_Fourth._ Since the piston must move in steam-tight contact with -the cylinder, it must have a definite amount of friction with the -sides of the cylinder by whatever means it may be packed. This -friction will produce a corresponding resistance to the moving -power. - -_Fifth._ The various joints of the machinery where steam is -contained are subject to leakage, and whatever amount of steam -shall thus escape must be placed to the account of power lost. - -_Sixth._ When the eduction-valve is opened to admit the steam to -the condenser, a certain force is required to expel the steam from -the cylinder. This force reacts upon the piston, and counteracts -to a proportional extent the moving power of the steam on the -other side. Besides this the water in the condenser cannot be -conveniently reduced below the temperature of about 100°, and at -this temperature steam has a pressure of about 1 lb. per square -inch. This vapour will continue to fill the cylinder, and will -resist the moving power which impels the piston. - -_Seventh._ Power must be provided for opening and closing the -valves or slides, for working the air-pump, hot-water pump, and -cold-water pump, and finally to overcome the friction on the -journals and centres of the parts of the parallel motion, the main -axle of the beam, the connecting rod, crank, and fly-wheel axle. - -It will be apparent how very much these sources of resistances -must vary in different engines, and how rough [Pg292] an -approximation any general estimate must be of their gross amount. - - -(174.) There are many circumstances which obstruct the practical -application of any standard of engine-power: the magnitude of -furnace, and the extent of heating surface necessary to produce -any required rate of evaporation in the boiler, are unascertained; -each engine-maker has his own rule in these matters, and all the -rules are equally unsupported by any experimental test entitled to -respect. Thus the circumstances that govern the rate of -evaporation in the boiler may be regarded as almost wholly -unknown. But supposing the rate of evaporation to be ascertained, -the amount of power absorbed by the condensation of steam on its -passage to the cylinder, the imperfect condensation of the same -steam after it has worked the piston, the friction of the various -moving parts of the machinery, and, above all, the difference of -effect of these losses of power in engines constructed on -different scales of magnitude, are absolutely unknown. We are, -therefore, not placed in a condition to assign any thing more than -a general account of what has been the practice of engine-makers -in constructing engines which are nominally of a certain power. - -In common low-pressure engines of the larger kind, to which class -alone we at present refer, it has been usual, with the same fuel -and under like circumstances, to allow from 10 to 18 square feet -of heating surface in the boiler for every nominal horse-power of -the engine. Within these wide limits the practice of engine-makers -has varied. It is not, however, to be supposed, that the boiler -with 18 square feet of surface per horse-power has the same -evaporating power as that which has but 10. This difference, -therefore, amounts to nothing more than different manufacturers of -steam-engines putting into circulation boilers having powers -_really_ different while they are _nominally_ the same. The -magnitude of the cylinder is regulated by the nominal power of the -engine, and it is usual so to regulate the evaporating power of -the boiler, that the piston shall move at the average rate of 200 -feet per minute. This being assumed, it is customary to allow -about 22 square inches of piston [Pg293] surface for every -nominal horse-power of the engine. If this power were in -conformity to the standard already defined, this amount of surface -moved at 200 feet per minute would be impelled by a pressure -amounting to 7-1/2 lbs. per square inch. The safety-valve of the -boiler of such engines is usually loaded at from 4 to 5 lbs. per -square inch, and consequently the steam in the boiler will have a -pressure of from 19 to 20 lbs. per square inch. If, therefore, the -effective pressure on the piston be really only 7-1/2 lbs. per -square inch, the pressure expended in overcoming the friction of -the engine, and the loss consequent on the partial condensation of -steam on one side and its imperfect condensation on the other, -would amount to from 12 to 13 lbs. per square inch, or nearly -double the assumed useful effect of the engine. - -Messrs. Maudslay and Field are accustomed to allow an evaporation -of ten gallons, or 1·6 cubic feet of water per hour, for each -nominal horse-power of the engine. They also allow about 22 square -inches of piston surface per nominal horse-power, the piston being -supposed to move at the rate of 200 feet per second.[24] - -The quantity of grate surface necessary in proportion to the power -of the engine, has been equally unascertained, and engine-makers -vary in their practice from half a square foot to one square foot -per nominal horse-power. - -The proportion which the magnitude of the heating surface of the -boiler, and the fire surface of the grate bears to the evaporating -power of the boiler, has not been determined by experiment, nor, -so far as we are informed, by any well-ascertained practical -results. - -The estimates or rather conjectures of engine-makers, of the -evaporation necessary to produce one horse-power, vary from one to -two cubic feet of water per hour. It has been [Pg294] already -shown that the evaporation of 900 cubic inches, or little more -than half a cubic foot per hour, evolves a gross mechanical effect -representing one horse-power; from which it appears, that if the -evaporation of the boilers of steam engines were what engineers -suppose them to be, the gross mechanical power produced in them -for every nominal horse-power of the engine varies in actual -amount from the power of two to that of four horses. - -The above estimates must be understood as referring to -double-acting steam engines above thirty-horse power. The -circumstances attending the performance of single-acting engines -applied to the drainage of mines, have been ascertained with much -greater precision. This has been mainly owing to a spirited system -of general inspection, which has been established in Cornwall, to -which we shall hereafter more particularly advert. - - -(175.) In expressing the duty of engines, it would have been -desirable that the duty of the boiler should have been separated -from that of the engine. - -The duty of a boiler is estimated by the volume of water -evaporated by a given quantity of fuel, independently of the time -which such evaporation may take. The duty, therefore, will be -expressed by the number of cubic feet of water evaporated, divided -by the number of bushels of coal necessary for that evaporation, -supposing the bushel of coal to be the unit of fuel. It will be -observed that the _duty_ of an engine or boiler is entirely -distinct from, and independent of, its _power_. One boiler may be -greater than another in power to any extent, while it may be equal -to or less than it in duty. A bushel of coals may evaporate the -same number of cubic feet of water under two boilers, but may take -twice as great a time to produce such evaporation under one than -under the other. In such a case the power of one boiler will be -double that of the other, while their duty will be the same. - -In like manner, a bushel of coals consumed in working two engines -may produce the same useful effect, but it may produce that useful -effect in the one in half the time it takes to produce it in the -other. In that case the _duty_ of the engines will be the same, -but the _power_ of the one will be double that of the other. -[Pg295] - -In fine, _power_ has reference to _time_,--_duty_, to _fuel_. The -more rapidly the engine produces its mechanical effect, the -greater its power will be, whatever may be the fuel consumed in -working it. And, on the other hand, the greater the useful effect -produced by a given weight of fuel, the greater will be the duty, -however long the time may be which the fuel may take to produce -the useful effect. - - -(176.) The proportion of the stroke to the diameter of the -cylinder must be determined by the velocity intended to be given -to the piston. With the same capacity of cylinder, and the same -evaporation in the boiler, the velocity of the piston will augment -as the magnitude of its diameter is diminished. - -The proportion of the diameter to the stroke of the cylinder is -very various. In engines used for steam-vessels the length of the -cylinder very little exceeds its diameter. In land engines, -however, the proportion of the length to the diameter is greater. -It is maintained by some that the proportion of the diameter and -length of the cylinder should be such as to render its surface -exposed to the cooling of the external air, the smallest possible. -Tredgold has maintained that since, during the stroke, the steam -is gradually exposed to contact with the surface of the cylinder -from the top to the bottom, the mean surface exposed in contact -with steam being half that of the entire cylinder, the proportion -of the diameter to the stroke should be such that the surface of -half the length of the cylinder, added to the magnitude of the top -and bottom, shall be a minimum. If this principle be admitted, -then the best proportion of the diameter to the stroke would be -that of one to two, the length of the stroke being twice the -diameter of the cylinder; but since the whole surface of the -cylinder is constantly exposed to the cooling effects of the air, -and since in the intervals of the stroke there is no sensible -change of the temperature of the surface, the loss of heat by -cooling will in effect be the same, especially in double-acting -engines, as if the cylinder were constantly filled with steam. If -this be admitted, then the object should be to give the cylinder -such a proportion, that its entire surface, including the top and -bottom, shall be a minimum. [Pg296] The proportion given by this -condition would be very nearly that which is observed in the -cylinders of marine engines, viz. that the length of the cylinder -should be equal to its diameter. - -If in a low-pressure engine the pressure of steam in the cylinder -be taken at 17 lbs. per square inch, then the volume of steam will -be about fifteen hundred times that of the water which produces -it. For every cubic foot of water, therefore, in the effective -evaporation of the boiler, 1500 cubic feet of steam will be passed -through the cylinder. If it be intended that the motion of the -piston shall be at the rate of 25 strokes per minute, or 1500 -strokes per hour, then the capacity of that portion of the -cylinder between the steam-valve and the piston at the end of the -stroke, must consist of half as many cubic feet as there are cubic -feet per hour evaporated in the boiler. If the steam, therefore, -be cut off at half stroke, the number of cubic feet of space in -the cylinder will be equal to the number of cubic feet of water -effectively evaporated by the boiler; and if a cubic foot of water -effectively evaporated be taken as the measure of a horse-power, -then there would be as many cubic feet in the capacity of the -cylinder as is equal to the nominal power of the engine. - - -(177.) The duty of engines varies according to their form and -magnitude, the circumstances under which they are worked, and the -purposes to which they are applied. In double-acting engines -working without expansion, the coal consumed per nominal -horse-power per hour varies from 7 to 12 lbs. An examination of -the steam-logs of several government steamers made by me a few -years since, gave, as the average of consumption of fuel at that -time of the best class of marine engines, about 8 lbs. per nominal -horse-power per hour. Since, however, no account could be obtained -of the actual evaporation of water in the boiler, nor, with the -necessary degree of precision, of the quantity and pressure of the -steam which passed through the cylinders, this estimate must be -regarded as an approximation subject to several causes of error. -The question of the duty of boilers and engines applied to the -[Pg297] general purposes of manufactures and navigation, is one -which has not yet been satisfactorily investigated; and it were -much to be desired that the proprietors of such engines should -combine to establish a strict analysis of their performance in -reference to their consumption of fuel, their evaporation of -water, and their useful effects. The results of such an -investigation, if properly conducted, would perhaps tend more to -the improvement of the steam engine than any discoveries in -science, or inventions in mechanical detail likely to be made in -the present stage of the progress of that machine. - - -(178.) A strict investigation of this kind has been for many years -carried on respecting the performance of the steam engines used -for the drainage of the mines in Cornwall; and it has been -attended with effects the most beneficial to the interests of -those concerned in them. The engines to which this important -inquiry has been applied being used for the purpose of pumping, -are generally single-acting engines, in which steam is used -expansively to a great extent. The steam is produced under a very -high pressure in the boiler, and being admitted to the cylinder is -cut off after a small portion of the entire stroke has been made, -the remainder of the stroke being produced by the expansion of the -steam. - -About the year 1811, a number of the proprietors of the principal -Cornish mines agreed to establish this system of inspection, under -the management and direction of Captain Joel Lean, and to publish -monthly reports. In these reports were stated the following -particulars:--1. The load per square inch on the piston; 2. The -consumption of coal in bushels; 3. The number of strokes made by -the engine; 4. The length of the strokes in the pumps; 5. The load -in pounds; 6. The duty of the engine, expressed by the number of -pounds raised one foot high by the consumption of a bushel of -coals; 7. The number of strokes per minute; 8. The diameter and -stroke of the cylinder, and a general description of the engine. -When these reports were commenced, the number of engines brought -under inspection was twenty-one. In the year 1813 it increased -to twenty-nine; in 1814 to thirty-two; in 1820 the number -reported upon increased [Pg298] to forty; in 1828 the number was -fifty-seven; and in 1836 it was sixty-one. This gradual increase -in the number of engines brought under this system of inspection, -was produced by the good effects which attended it. These -beneficial consequences were manifested, not only in the improved -performance of the same engines, but in the gradually improved -efficiency of those which were afterwards constructed. - -The following table taken from the statement of the duty of -Cornish engines by Thomas Lean and brother, lately published by -the British Association, will show in a striking manner the -improvement of the Cornish engines, from the commencement of this -system of inspection to the present time. The duty is expressed by -the number of pounds raised one foot high by the consumption of a -bushel of coals. - - --------------------------------------------------------------- - | No. of | Average Duty of the | Average Duty of the best - Years.|Engines.| Whole. | Engine. - --------------------------------------------------------------- - 1812 | 21 | 19,300,000 | - 1813 | 29 | 19,500,000 | 26,400,000 - 1814 | 32 | 20,600,000 | 32,000,000 - 1815 | 35 | 20,500,000 | 28,700,000 - 1816 | 35 | 23,000,000 | 32,400,000 - 1817 | 35 | 26,500,000 | 41,600,000 - 1818 | 36 | 25,400,000 | 39,300,000 - 1819 | 40 | 26,300,000 | 40,000,000 - 1820 | 46 | 28,700,000 | 41,300,000 - 1821 | 45 | 28,200,000 | 42,800,000 - 1822 | 52 | 28,900,000 | 42,500,000 - 1823 | 52 | 28,200,000 | 42,100,000 - 1824 | 49 | 28,300,000 | 43,500,000 - 1825 | 56 | 32,000,000 | 45,400,000 - 1826 | 51 | 30,500,000 | 45,200,000 - 1827 | 51 | 32,100,000 | 59,700,000 - 1828 | 57 | 37,100,000 | 76,800,000 - 1829 | 53 | 41,700,000 | 77,000,000 - 1830 | 56 | 43,300,000 | 78,000,000 - 1831 | 58 | 43,400,000 | 71,100,000 - 1832 | 59 | 45,000,000 | 85,000,000 - 1833 | 56 | 46,600,000 | 84,300,000 - 1834 | 52 | 47,800,000 | 90,900,000 - 1835 | 51 | 47,800,000 | 91,700,000 - 1836 | 61 | 46,600,000 | 85,400,000 - 1837 | 58 | 47,000,000 | 87,200,000 - 1838 | 61 | 48,700,000 | 84,200,000 - --------------------------------------------------------------- - -[Pg299] As an example of the beneficial effects produced upon the -efficiency of an individual engine by the first application of -this system of inspection, the case of the Stray Park engine may -be mentioned. This engine, constructed by Boulton and Watt, had a -sixty inch cylinder, and when first reported in 1811, its duty -amounted to 16,000,000 pounds. After having been reported on for -three years, its duty was found to have increased to 32,000,000; -this estimate being taken from the average result of twelve -months' performance. Its duty was doubled in less than three -years. - -It will appear, by inspection of the duties registered in the -preceding table, that the augmentation of the efficiency of the -engines has not been the effect of any great or sudden improvement, -but has rather resulted from the combination of a great number of -small improvements in the details of the operation of these -machines. In these improvements more is due to the successful -application of practical experience than to any new principles -developed by scientific research. Mr. John Taylor, in his "Records -of Mining," has traced the successive improvements on which the -increased duty of engines depends, and has connected these -improvements with their causes in the order of their dates. The -following results, abridged from his estimates, may not be -uninteresting:-- - -In 1769, soon after the date of the earliest discoveries of Mr. -Watt, but before they had come into practical application, Smeaton -computed that the average duty of fifteen atmospheric engines, -working at Newcastle-on-Tyne, was 5,590,000. The duty of the best -of these engines was 7,440,000, and that of the worst 3,220,000. - -In 1772, Smeaton commenced his improvements on the atmospheric -engine, and raised the duty to 9,450,000. - -In 1776, Watt obtained a duty of 21,600,000. - -At this time Smeaton acknowledged that Watt's engines gave a duty -amounting to double that of his own. - -In 1778-79, Watt reported a duty of 23,400,000. - -From 1779 to 1788, Watt introduced the application of expansion, -and raised the duty to 26,600,000. [Pg300] - -In 1798, an engine by Boulton and Watt, erected at Herland, was -reported as giving a duty of 27,000,000. - -This engine, which was probably the best which at that time had -ever been erected, attracted the particular attention of Mr. Watt, -who, on visiting Cornwall, went to see it, and had many -experiments tried with it. It was under the care of Mr. Murdock, -the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt -inspected it he pronounced it perfect, and that further -improvement could not be expected. How singular an instance this -of the impossibility, even of the most sagacious, to foresee the -results of mechanical improvement! In twenty years afterwards the -average duty of the best engine was nearly 40,000,000, and in -forty years it was above 84,000,000. - -[Illustration: BOILER MANUFACTORY.] - - FOOTNOTES: - - [24] If 22 square inches of piston surface be allowed to - represent a horse-power, the power of an engine may always be - computed by dividing the square of the diameter of the piston - expressed in inches by 28. And, on the other hand, to find the - diameter of piston which would correspond to any given power, - multiply the number of horses' power by 28, and take the - square root of the product. These rules, however, cannot be - applied if the piston be supposed to move with any other - velocity; since, in that case, the same amount of piston - surface would cease to represent a horse-power, unless the - effective pressure on the piston were at the same time - changed. - -[Pg301] - - - - -[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.] - -CHAP. X. - - NOTICE OF THE LIFE OF MR. WATT. -- HIS FRIENDS AND ASSOCIATES - AT BIRMINGHAM. -- INVENTION OF THE COPYING PRESS. -- HEATING BY - STEAM. -- DRYING LINEN BY STEAM. -- THEORY OF THE COMPOSITION OF - WATER. -- FIRST MARRIAGE OF WATT. -- DEATH OF HIS FIRST WIFE. -- - HIS SECOND MARRIAGE. -- DEATH OF HIS YOUNGER SON. -- EXTRACTS - FROM HIS LETTERS. -- CHARACTER OF WATT BY LORD BROUGHAM. -- BY - SIR WALTER SCOTT. -- BY LORD JEFFREY. -- OCCUPATION OF HIS OLD - AGE. -- INVENTION OF MACHINE FOR COPYING SCULPTURE. -- HIS LAST - DAYS. -- MONUMENTS. - - -(179.) Having brought this historical analysis of the invention -and application of the steam engine to the date of the decease of -the illustrious man, to the powers of whose mind the world stands -indebted for the benefits conferred upon [Pg302] mankind by that -machine, it will perhaps not be deemed an improper digression in -this work, to devote some pages to a notice of the principal -labours of the same mind in other departments of art and science, -and to circumstances connected with his personal history and the -close of his life, which cannot fail to possess general interest. - -At the period when Watt, having connected himself in partnership -with Boulton, went to reside at Soho, near Birmingham, a number of -persons, some of whom have since attained great celebrity by their -discoveries and their works, and all of whom were devoted to -inquiries connected with the arts and sciences, resided in that -neighbourhood. Among these may be mentioned PRIESTLEY, whose -discoveries in physical science have rendered his name immortal; -DARWIN, the philosopher and poet; WITHERING, a distinguished -physician and botanist; KEIR, a chemist, who published a translation -of Macquer, with annotations; GALTON, the ornithologist; and -EDGEWORTH, whose investigations respecting wheeled carriages and -other subjects, have rendered him well known. A society was formed -by these and other individuals, of which Boulton and Watt were -leading members, the meetings of which were held monthly on the -evening of full moon, and which was thence called the _Lunar -Society_. At the meetings of this society, subjects connected with -the arts and sciences were discussed, and out of those discussions -occasionally arose suggestions not unattended with important and -advantageous consequences. At one of these meetings, Darwin stated -that he had discovered a pen formed with two quills, by means of -which, at a single operation, an original and a copy of a letter -might be produced. Watt almost instantly observed that he thought he -could find a better expedient, and that he would turn it in his mind -that night. By the next morning the COPYING PRESS was invented, for -which he afterwards obtained a patent. - -This machine, which is now so generally used in counting-houses, -consists of a rolling-press, by which a leaf of thin paper, -previously damped, is pressed upon the letter to be copied. The -writing, of which the ink is not yet quite dry, leaves its -impression upon the thin paper thus pressed upon [Pg303] it, and -the copy taken in this manner is read through the semi-transparent -paper. If a letter be written with ink suitable for this purpose, -a copy may be taken at any time within several hours after the -letter is written. - -The method of heating apartments and buildings by steam, which has -since been improved and brought into extensive use, was likewise -brought forward by Watt. Although this contrivance had been -previously pointed out by Sir Hugh Platt about the middle of the -seventeenth century, and by Colonel Cooke in 1745, yet these -suggestions remained barren. Mr. Watt gave detailed methods of -heating buildings by steam[25]; and also invented a machine for -drying linen by steam, a description of which he communicated to -Dr. Brewster, which was read in December, 1824, before the Society -for promoting Useful Arts in Scotland.[26] - -But the circumstance, exclusive of those connected with the -invention of the steam engine, which is by far the most memorable -in the career of Watt, is the share which he had in the discovery -of the composition of water. As this circumstance has recently -excited much interest, and led to some controversy, we shall here -state, as distinctly as possible, the leading facts connected with -it. - -Water, which was so long held to be a simple element, has, in -modern times, been proved to be a substance consisting of two -aeriform bodies or gases chemically combined. These two gases are -those called in chemistry _oxygen_ and _hydrogen_. If eight grains -weight of oxygen be mixed with one grain weight of hydrogen, and -the mixture be submitted to such effects as would cause the -chemical combination of these two airs, it would be converted into -nine grains weight of pure water. - -If, on the other hand, nine grains weight of pure water be -submitted to any conditions which would separate its constituent -parts, the result would be eight grains weight of oxygen gas, and -one grain weight of hydrogen gas. There are a variety of methods -in physics by which these effects would be [Pg304] produced. It -will be sufficient here to state one method of producing each of -the above changes. - -If eight grains weight of oxygen be inclosed in a strong vessel -with one grain weight of hydrogen, all other substances being -excluded, and the mixture be inflamed, an explosion will take -place, the gases will disappear, and a small quantity of water -will be the only substance remaining in the vessel. If this water -be weighed, it will be found to weigh exactly nine grains. - -It is known that the metals have a strong attraction for oxygen -gas, and this attraction is promoted by elevating their -temperature. If a glass tube be filled with iron wire heated to -redness, and to one end of this tube a small vessel of boiling -water be attached, the steam evolved from the water will force its -way through the spaces between the red-hot wires in the tube, and -would be expected to issue from the remote end; but if the -substance issuing from the remote end of the tube be examined, it -will be found to be not steam, but hydrogen gas. If the quantity -of this gas be ascertained by weight, and also the quantity of -weight lost by the vessel of water at the other end of the tube, -it will be found that the loss of weight of the water by -evaporation will be nine times the weight of the hydrogen which -has issued from the remote end of the tube. If the weight of the -tube with the wire contained in it be next ascertained, it will be -found to be increased by eight times the weight of the hydrogen -which has issued from its remote end. From this it follows that -the weight of the hydrogen which has escaped from the tube, added -to the increase of weight which has been given to the wire in the -tube, makes up the whole weight of the water evaporated. If the -wire in the tube be next examined, it will be found that it has -suffered oxydation, or, in other words, that a new substance has -been formed in it called the oxyde of iron,--such substance being -a chemical compound formed of oxygen gas and iron. - -It follows, therefore, that in this process the vapour of the -water, in passing through the tube, has been decomposed, and that, -having given up to the iron its oxygen, the hydrogen [Pg305] -alone escaped from the other end; and for every nine grains weight -of steam which passed through the tube, eight grains of oxygen -have been combined with the iron, and one grain of hydrogen has -escaped from the end of the tube. - -Such are the class of effects on which the modern discovery of the -composition of water has been based. The merit of that discovery -has been shared between the celebrated English chemist, CAVENDISH, -and the not less celebrated French chemist, LAVOISIER, the chief -merit, however, being ascribed to the former. - -We shall now briefly state the facts which led to this discovery, -with their dates, which will necessarily show the share which Watt -had in it. - -When pure hydrogen gas is burned in an atmosphere of common air, -the process which takes place is now known to be nothing more than -the chemical combination of the hydrogen with eight times its own -weight of oxygen taken from the atmosphere, and the product of the -combustion is a quantity of water nine times the weight of the -hydrogen consumed. In the year 1776, Macquer, a well-known chemist -of that day, having held a saucer of white porcelain over a flame -of hydrogen which was burning at the mouth of a bottle, observed -that no smoke was produced and no soot deposited on the saucer. On -the other hand, he found that after the lapse of some time drops -of a clear pellucid liquid were perceptible on the saucer: this -liquid he submitted to analysis, and found it to be pure water. -Macquer mentioned this fact without comment or inference. It did -not occur to him that the water thus produced upon the saucer was -a substance which contained the hydrogen, which disappeared upon -combustion from the bottle. - -On the 18th of April, 1781, Mr. Warltire addressed a letter to Dr. -Priestley, dated Birmingham, which letter is published in Dr. -Priestley's _Experiments on Air_, printed at Birmingham in 1781, -in which Warltire informs Priestley that he had fired a mixture of -hydrogen and common air in close glass vessels, and that, although -previously to firing the mixture the vessels were clean and dry, a -dewy deposit was [Pg306] observed afterwards on their sides. In -fact, water was present which was not present before. - -The mixture was in this case fired by passing an electric spark -through the vessel; and it is now known that the effect produced -was the combination of the hydrogen, which formed part of the -mixture of airs in the vessel with the oxygen, which also formed -part of the same mixture. - -It appears, from expressions in Warltire's letter, that the same -experiment had been previously made by Priestley, and the same -result observed by him. - -The inference deduced from this by Warltire, and apparently -acquiesced in by Priestley, was, that whenever hydrogen was fired -in atmospheric air, the moisture, which is always more or less -sustained in the latter, was deposited; but neither of these -chemists perceived the real cause of the production of the water. - -In the beginning of 1783, and not later than the 21st of April, -this experiment of Warltire and Priestley was repeated by -Cavendish, with this difference, that, instead of exploding the -mixture of hydrogen and common air, Cavendish exploded a mixture -of hydrogen and oxygen. He observed that water was present after -the explosion, but _inferred nothing_. - -In a published paper dated April, 1783, Priestley announced a -further and most important result of his experiments. This was, -that in examining the weight of water produced by the explosion of -a mixture of oxygen and hydrogen, _that weight was found to be -precisely equal to the sum of the weights of the two gases_, which -disappeared in the process. - -Immediately on observing this, Priestley, being then, as has been -already stated, Watt's near neighbour, communicated to the latter -what he had observed; upon which Watt immediately, viz. by a letter -dated the 26th of the same month, declared that the inevitable -consequence which followed from Priestley's observations was, that -water was a substance compounded of oxygen and hydrogen deprived of -[Pg307] a quantity of heat which was previously latent in them.[27] -The letter containing this inference was communicated immediately by -Priestley to Sir Joseph Banks, then President of the Royal Society, -to be laid before that body; and it is accordingly printed with its -proper date in the 74th volume of the _Philosophical Transactions_. - -About two months after the date of Mr. Watt's letter just quoted, -Lavoisier made experiments on the combustion of oxygen and -hydrogen, and read a memoir before the Academy of Sciences in -Paris, in which his views of the formation of water by the -combination of these gases were developed. This paper, by -Lavoisier, was afterwards printed in the Memoirs of the Academy in -the year 1784. The experiments are there stated to have been made -in the month of June, 1783; and it is stated that Sir Charles -Blagden, who was present at the experiments, told Lavoisier that -Mr. Cavendish had already burned the same gases in close vessels, -and obtained a very sensible quantity of water. - -On the 15th of January, 1784, the celebrated paper by Cavendish, -entitled "Experiments on Air," was read before the Royal Society, -and in this paper the composition of water by the union of oxygen -and hydrogen is explained. - -In a controversy which afterwards ensued on the respective -[Pg308] claims of Cavendish and Lavoisier to credit for the -discovery of the composition of water, Sir Charles Blagden stated -that he had told Lavoisier, in June, 1783, more than Lavoisier -acknowledged, that he had not only told him that water was -produced by the combustion of the gases, but that his information -embraced the whole theory of the composition of water. This -declaration of Blagden was subsequent in date to January, 1784, -and there is no evidence of any explanation of this theory, verbal -or otherwise, having been given by Cavendish, or any other person, -antecedent to April, 1783. - -From this brief statement of the facts and dates it will appear -that the merit of the discovery of the FACT, that the weight of -water resulting from the combustion of oxygen and hydrogen, is -equal to the sum of the weights of the oxygen and hydrogen which -disappear in the combustion, is due to Priestley; and that the -merit of the INFERENCE from that fact, that water is a compound -body, whose constituents are oxygen and hydrogen, is due to -Watt.[28] Whether those who subsequently deduced the same -inference, and promulgated the same theory, were or were not -informed of Mr. Watt's solution of the phenomenon, or what credit -may be due to any person, however eminent, who at any time -posterior to Mr. Watt's letter to Priestley, asserted that they -had, at a time antecedently to that, made the same inference -without having published it, or communicated it in such a manner -as to establish their claim upon rational and credible evidence, -are questions which we shall not here discuss, being contented -with establishing the right of Mr. Watt to the merit of the -discovery of the THEORY which explained the FACT discovered by -Priestley. - -Even in his declining years, after he had withdrawn from the -active pursuits of his business, the least excitement was -sufficient to call into play the slumbering powers of his -inventive genius. No object could present itself to his notice -[Pg309] without receiving from that genius adaptation in form and -construction to useful purposes. As an example of this restless -activity of mind the following anecdote may be mentioned:-- - -A company at Glasgow had erected on the right bank of the Clyde -extensive buildings and powerful engines for supplying water to -the town. After this expense it was found that a source of water, -of very superior quality, existed on the left bank of the river. -To change the site of the establishment, after the expense which -had been incurred in its erection could not be contemplated, and -they therefore proposed to carry across the bottom of the river a -flexible suction pipe, the mouth of which should terminate in the -source from which the pure water was to be derived. This pipe was -to be supported by a flooring constructed upon the bed of the -river; but it was soon apparent that the construction of such a -flooring on a shifting and muddy bottom, full of inequalities, and -under several feet depth of water would require a greater -expenditure of capital than could with propriety be afforded. In -this difficulty the aged mechanician, for whom Glasgow itself had -been the earliest stage of professional labour, was applied to, -and instantly solved the problem. His attention is said to have -been attracted by a lobster which had been served at table: he set -himself about to contrive how, by mechanism, he could make an -apparatus of iron with joints which should have all the -flexibility of the tail of the lobster. He therefore proposed that -an articulated suction-pipe, capable of accommodating itself to -all the inequalities and to the possible changes of the bed of the -river, should be carried across it; that this flexible pipe should -be two feet in diameter, and one thousand feet in length. This -project the company accordingly caused to be executed after the -plans and drawings of Watt with the most complete success.[29] - -[Pg310] Among the less prominent, though not less useful services -rendered by Watt to his country, may be mentioned the introduction -of the use of chlorine in bleaching. That invention of Berthollet -was introduced into England by Watt after his visit to Paris at -the close of the year 1786. He constructed all the necessary -apparatus for it, directed its erection, and superintended its -first performances. He then left it to his wife's father, Mr. -Macgregor, to carry on the processes. - -When the properties of the gases began to occupy the attention of -chemists, attempts were made to apply them as a means of curing -diseases of the lungs. Dr. Beddoes pursued this inquiry with great -activity, and established, through the means of private -subscription, at Clifton, an institution in which this method of -cure was carefully investigated. The Pneumatic Institution (for so -it was called) has been rendered celebrated for having at its head -Humphry Davy, just then commencing his scientific career. Among -its founders was also numbered James Watt. Not content, however, -with affording the institution the sanction of his name, he -designed and caused to be constructed, at Soho, the apparatus used -for making the gases and administering them to the patients. - -As the exalted powers of the mind of Watt, unfolded in his -numerous mechanical and philosophical inventions and discoveries, -have commanded the admiration and respect of his species, the -affection and love of his fellow men would not have been less -conciliated, had the qualities of his heart, as developed in his -private and personal relations, been as well known as the products -of his genius. - -In the year 1764, Watt being then in the twenty-ninth year of his -age, married his cousin, Miss Miller. At this time he had fallen -into a state of despondency from his disappointments, which -produced a serious attack of nervous illness. The accomplishments -and superior understanding, the mildness of temper and goodness of -disposition of his wife, soon restored him to health. Of this -marriage four children, two sons and two daughters, were the -issue. Two of these children died in infancy; another, a daughter, -was married to Mr. Miller of Glasgow; and the fourth is the -[Pg311] present Mr. James Watt. In September, 1773, while her -husband was engaged in the design of the Caledonian canal in the -North of Scotland, Mrs. Watt died in child-bed of a fifth child, -who was still-born: "Would that I might here transcribe," says M. -Arago, "in all their simple beauty, some lines of the journal in -which he daily recorded his inmost thoughts, his fears, his hopes! -Would that you could see him, after this heavy affliction, pausing -on the threshold of that home, where 'HIS KIND WELCOMER' awaited -him no more; unable to summon courage to enter those rooms where -he was never more to meet 'THE COMFORT OF HIS LIFE!' Possibly, so -faithful a picture of a very deep sorrow might at last put to -silence those obstinate theorists, who, without being struck by -the thousands of instances to the contrary, do yet refuse -qualities of the heart to every man whose intellect has been -fostered by the fertile, sublime, and imperishable truths of the -exact sciences!" - -After the lapse of some years Watt married Miss Macgregor, a -person who is represented to have possessed qualities of mind -which rendered her a companion every way suitable to her husband. -This lady survived Watt, and died in 1832 at an advanced age. Two -children were the issue of this second marriage. - -In the year 1800 the extended patent right, which had been granted -to Boulton and Watt for their improved engine, expired, and at -this time Mr. Watt retired altogether from business. He was -succeeded by his two sons, the present Mr. James Watt, and -Gregory, one of the children of his second marriage. The works at -Soho continued to be conducted by the present Mr. Boulton, the son -of the partner of Mr. Watt, and the two Messrs. Watt. In 1804 -Gregory Watt died at the age of twenty-seven, of a disease of the -chest. This afflicting event was deeply felt by Mr. Watt; but he -did not sink under it into that state of despondency in which he -has been represented to have fallen by M. Arago. On the contrary, -he continued to show the same activity of mind which had -characterised his whole [Pg312] life; nor did he lose that -interest which he always took in the pursuit of literature and in -society. The state of his feelings under this affliction is shown -by the following extracts from letters written by him at that -time, which have been published by Mr. Muirhead. - - - "Heathfield, January 26th, 1805. - - * * "I, perhaps, have said too much to you and Mrs. Campbell - on the state of my mind. I, therefore, think it necessary to - say that _I am not low spirited_; and were you here, you would - find me as cheerful in the company of my friends as usual; my - feelings for the loss of poor Gregory are not passion, but a - deep regret that such was his and my lot. - - "I know that all men must die, and I submit to the decrees - of nature, I hope with due reverence to the Disposer of - Events. Yet one stimulus to exertion is taken away, and, - somehow or other, I have lost my relish for my usual avocations. - Perhaps time may remedy that in some measure; meanwhile, - I do not neglect the means of amusement which are in my power." - - "Heathfield, April 8th, 1805. - - * * "It is rather mortifying to see how easily the want of - even the best of us is dispensed with in the world; but it is - very well it should be so. We here, however, cannot help - feeling a terrible blank in our family. When I look at my - son's books, his writings and drawings, I always say to - myself, where are the mind that conceived these things, and - the hands that executed them? In the course of nature, he - should have said so of mine; but it was otherwise ordered, and - our sorrow is unavailing. As Catullus says:-- - - -- 'Nunc it, per iter tenebricosum, - Illuc, unde negant redire quemquam. - At vobis male sit, malæ tenebræ - Orci, quæ omnia bella devoratis!' - - "But Catullus was a heathen; let us hope that he (G.) is now - rejoicing in another and a better world, free from our cares, - griefs, and infirmities. Some one has said, I shall not wholly - die; and Gregory's name, his merits and virtues, will live at - least as long as those do who knew him. You are not, from - this, to conceive that we give way to grief; on the contrary, - you will find us as cheerful as we ought to be, and as much - disposed to enjoy the friends we have left as ever; but we - should approach to brutes if we had no regrets." - -Mr. Watt, at the date of these letters, had entered on his -seventieth year, a period after which great mental exertions are -rarely made. [Pg313] - -In the summer of 1819, symptoms of indisposition manifested -themselves which soon rendered Watt aware of his approaching -dissolution. "I am very sensible," said he to his afflicted -friends, "of the attachment you show me, and I hasten to thank you -for it, as I am now come to my last illness." He died on the 25th -of August, 1819. His remains were deposited in the church of -Handsworth, near his estate of Heathfield. His son has raised over -his grave a Gothic chapel, in the centre of which is placed a -statue by Chantrey. - -The personal character of Watt could not fail to excite the -admiration and the love of those distinguished persons, whose -pride and happiness it was to be admitted to a share in the -friendship of the great engineer. Among these were reckoned some -of the men who will leave upon the present age the deepest and -most lasting impressions of their genius, and such persons have -bequeathed to posterity the sentiments with which he inspired -them. We cannot here do more justice to the personal character of -the subject of this notice than by repeating the portraiture of it -which has been given by three of the most distinguished of his -friends, and of the most illustrious men of the present age. - -At a meeting convened in 1824, for erecting a monument to Watt, -Lord Brougham pronounced a speech, from which we extract the -following observations:-- - - "I had the happiness of knowing Mr. Watt, for many years, in the - intercourse of private life; and I will take upon me to bear a - testimony in which all who had that gratification I am sure will - join, that they who only knew his public merit, prodigious as - that was, knew but half his worth. Those who were admitted to - his society will readily allow that anything more pure, more - candid, more simple, more scrupulously loving of justice, than - the whole habits of his life and conversation, proved him to be, - was never known in society. One of the most astonishing - circumstances in this truly great man, was the versatility of - his talents. His accomplishments were so various, the powers of - his mind were so vast, and yet of such universal application, - that it was hard to say whether we should most admire the - extraordinary grasp of his understanding, or the accuracy of - nice research with which he could bring it to bear upon the most - minute objects of investigation. I forget of whom it was said, - that his mind resembled the trunk of an elephant, which can pick - up [Pg314] straws, and tear up trees by the roots. Mr. Watt, in - some sort, resembled the greatest and most celebrated of his own - inventions, of which we are at a loss whether most to wonder at - the power of grappling with the mightiest objects, or of - handling the most minute; so that, while nothing seems too large - for its grasp, nothing seems too small for the delicacy of its - touch, which can cleave rocks, and pour forth rivers from the - bowels of the earth, and, with perfect exactness, though not - with greater ease, fashion the head of a pin, or strike the - impress of some curious die. Now, those who knew Mr. Watt, had - to contemplate a man whose genius could create such an engine, - and indulge in the most abstruse speculations of philosophy, and - could at once pass from the most sublime researches of geology - and physical astronomy, the formation of our globe, and the - structure of the universe, to the manufacture of a needle or a - nail; who could discuss, in the same conversation, and with - equal accuracy, if not with the same consummate skill, the most - forbidding details of art and the elegances of classical - literature, the most abstruse branches of science and the - niceties of verbal criticism. - - "There was one quality in Mr. Watt which most honourably - distinguished him from too many inventors, and was worthy of - all imitation--he was not only entirely free from jealousy, - but he exercised a careful and scrupulous self-denial, and was - anxious not to appear, even by accident, as appropriating to - himself that which he thought belonged to others. I have heard - him refuse the honour universally ascribed to him, of being - the inventor of the steam engine, and call himself simply its - improver; though, in my mind, to doubt his right to that - honour, would be as inaccurate as to question Sir Isaac - Newton's claim to his greatest discoveries, because Descartes - in mathematics, and Galileo in astronomy and mechanics, had - preceded him; or to deny the merits of his illustrious - successor, because galvanism was not his discovery, though, - before his time, it had remained as useless to science as the - instrument called a steam engine was to the arts before Mr. - Watt. The only jealousy I have known him to betray, was with - respect to others, in the nice adjustment he was fond of - giving to the claims of inventors. Justly prizing scientific - discovery above all other possessions, he deemed the title to - it so sacred, that you might hear him arguing by the hour to - settle disputed rights; and if you ever perceived his temper - ruffled, it was when one man's invention was claimed by, or - given to another; or when a clumsy adulation pressed upon - himself that which he knew to be not his own." - -In the preface to the _Monastery_ Sir Walter Scott speaks of Watt -in the following terms:-- - - "There were assembled about half a score of our northern - lights. * * Amidst this company stood Mr. Watt, the man whose - genius discovered the means of multiplying our national - resources to a degree, perhaps, even beyond his own stupendous - powers of calculation and combination; bringing the treasures - of the abyss to the summit of the earth--giving the feeble arm - of man the momentum of an Afrite--commanding manufactures to - arise as the rod of the prophet produced water in the - desert--affording the means of dispensing with that time and - tide which wait for no man--and of sailing without that wind - which defied the command and threats of Xerxes himself. This - potent commander of the elements--this abridger of time and - space--this magician, whose cloudy [Pg315] machinery has - produced a change on the world, the effects of which, - extraordinary as they are, are, perhaps, only now beginning to - be felt--was not only the most profound man of science--the - most successful combiner of powers, and calculator of numbers, - as adapted to practical purposes--was not only one of the most - generally well informed, but one of the best and kindest of - human beings. - - "There he stood, surrounded by the little band I have - mentioned of northern literati, men not less tenacious, - generally speaking, of their own fame and their own opinions, - than the national regiments are supposed to be jealous of the - high character which they have won upon service. Methinks I - yet see and hear what I shall never see or hear again. In his - eighty-second year, the alert, kind, benevolent old man, had - his attention alive to every one's question, his information - at every one's command. - - "His talents and fancy overflowed on every subject. One gentleman - was a deep philologist--he talked with him on the origin of the - alphabet, as if he had been coeval with Cadmus; another a - celebrated critic--you would have said the old man had studied - political economy and belles lettres all his life. Of science it is - unnecessary to speak--it was his own distinguished walk. And yet, - Captain Clutterbuck, when he spoke with your countryman, Jedediah - Cleishbotham, you would have sworn he had been coeval with Claverse - and Burley, with the persecutors and persecuted, and could number - every shot the dragoons had fired at the fugitive Covenanters. In - fact, we discovered that no novel of the least celebrity escaped - his perusal, and that the gifted man of science was as much - addicted to the productions of your native country, in other words, - as shameless and obstinate a peruser of novels, as if he had been a - very milliner's apprentice of eighteen." - -In the Edinburgh newspaper, called the _Scotsman_, of the 4th -September, 1819, immediately after the decease of Watt, the -following sketch was published from the pen of Lord Jeffrey:-- - - "This name fortunately needs no commemoration of ours; for he - that bore it survived to see it crowned with undisputed and - unenvied honours; and many generations will probably pass away - before it shall have gathered 'all its fame.' We have said - that Mr. Watt was the great _improver_ of the steam engine; - but, in truth, as to all that is admirable in its structure, - or vast in its utility, he should rather be described as its - _inventor_. It was by his inventions, that its action was so - regulated as to make it capable of being applied to the finest - and most delicate manufactures, and its power so increased, as - to set weight and solidity at defiance. By his admirable - contrivance, it has become a thing stupendous alike for its - force and its flexibility--for the prodigious power which it - can exert, and the ease, and precision, and ductility with - which it can be varied, distributed, and applied. The trunk of - an elephant, that can pick up a pin or rend an oak, is as - nothing to it. It can engrave a seal, and crush masses of - obdurate metal before it--draw out, without breaking, a thread - as fine as gossamer, and lift a ship of war like a bauble in - the air. It can embroider muslin, and forge anchors--cut steel - into ribands, and impel loaded vessels against the fury of the - winds and waves. [Pg316] - - "It would be difficult to estimate the value of the benefits - which these inventions have conferred upon this country. There - is no branch of industry that has not been indebted to them; - and, in all the most material, they have not only widened most - magnificently the field of its exertions, but multiplied a - thousand fold the amount of its productions. It is our - improved steam engine that has fought the battles of Europe, - and exalted and sustained, through the late tremendous - contest, the political greatness of our land. It is the same - great power which now enables us to pay the interest of our - debt, and to maintain the arduous struggle in which we are - still engaged (1819), with the skill and capital of countries - less oppressed with taxation. But these are poor and narrow - views of its importance. It has increased indefinitely the - mass of human comforts and enjoyments, and rendered cheap and - accessible all over the world the materials of wealth and - prosperity. It has armed the feeble hand of man, in short, - with a power to which no limits can be assigned; completed the - dominion of mind over the most refractory qualities of matter; - and laid a sure foundation for all those future miracles of - mechanic power which are to aid and reward the labours of - after generations. It is to the genius of one man, too, that - all this is mainly owing; and certainly no man ever bestowed - such a gift on his kind. The blessing is not only universal, - but unbounded; and the fabled inventors of the plough and the - loom, who were deified by the erring gratitude of their rude - contemporaries, conferred less important benefits on mankind - than the inventor of our present steam engine. - - "This will be the fame of Watt with future generations; and it is - sufficient for his race and his country. But to those to whom he - more immediately belonged, who lived in his society and enjoyed his - conversation, it is not, perhaps, the character in which he will be - most frequently recalled--most deeply lamented--or even most highly - admired. Independently of his great attainments in mechanics, Mr. - Watt was an extraordinary, and in many respects a wonderful man. - Perhaps no individual in his age possessed so much and such varied - and exact information--had read so much, or remembered what he had - read so accurately and well. He had infinite quickness of - apprehension, a prodigious memory, and a certain rectifying and - methodising power of understanding, which extracted something - precious out of all that was presented to it. His stores of - miscellaneous knowledge were immense; and yet less astonishing than - the command he had at all times over them. It seemed as if every - subject that was casually started in conversation with him, had - been that which he had been last occupied in studying and - exhausting;--such was the copiousness, the precision, and the - admirable clearness of the information which he poured out upon it - without effort or hesitation. Nor was this promptitude and compass - of knowledge confined in any degree to the studies connected with - his ordinary pursuits. That he should have been minutely and - extensively skilled in chemistry and the arts, and in most of the - branches of physical science, might perhaps have been conjectured; - but it could not have been inferred from his usual occupations, and - probably is not generally known, that he was curiously learned in - many branches of antiquity, metaphysics, medicine, and etymology; - and perfectly at home in all the details of architecture, music, - and law. He was well acquainted, too, with most of the modern - languages, and familiar with their most recent literature. Nor was - it at all extraordinary to hear the great mechanician and engineer - detailing and expounding, for hours together, [Pg317] the - metaphysical theories of the German logicians, or criticising the - measures or the matter of the German poetry. - - "His astonishing memory was aided, no doubt, in a great measure, - by a still higher and rarer faculty--by his power of digesting, - and arranging in its proper place, all the information he - received; and of casting aside and rejecting, as it were - instinctively, whatever was worthless or immaterial. Every - conception that was suggested to his mind seemed instantly to - take its place among its other rich furniture, and to be - condensed into the smallest and most convenient form. He never - appeared, therefore, to be at all incumbered or perplexed with - the _verbiage_ of the dull books he perused, or to the idle talk - to which he listened; but to have at once extracted, by a kind - of intellectual alchemy, all that was worthy of attention, and - to have reduced it, for his own use, to its true value and to - its simplest form. And thus it often happened, that a great deal - more was learned from his brief and vigorous account of the - theories and arguments of tedious writers, than an ordinary - student could ever have derived from the most painful study of - the originals; and that errors and absurdities became manifest - from the mere clearness and plainness of his statement of them, - which might have deluded and perplexed most of his hearers - without that invaluable assistance. - - "It is needless to say that, with those vast resources, his - conversation was at all times rich and instructive in no ordinary - degree: but it was, if possible, still more pleasing than wise; and - had all the charms of familiarity with all the substantial - treasures of knowledge. No man could be more social in his spirit, - less assuming or fastidious in his manners, or more kind and - indulgent toward all who approached him. He rather liked to - talk,--at least in his latter years; but though he took a - considerable share of the conversation, he rarely suggested the - topics on which it was to turn, but readily and quietly took up - whatever was presented by those around him, and astonished the idle - and barren propounders of an ordinary theme by the treasures which - he drew from the mine they had unconsciously opened. He generally - seemed, indeed, to have no choice or predilection for one subject - of discourse rather than another; but allowed his mind, like a - great cyclopædia, to be opened at any letter his associates might - choose to turn up, and only endeavoured to select from his - inexhaustible stores, what might be best adapted to the taste of - his present hearers. As to their capacity he gave himself no - trouble; and indeed such was his singular talent for making all - things plain, clear, and intelligible, that scarcely any one could - be aware of such a deficiency in his presence. His talk, too, - though overflowing with information, had no resemblance to - lecturing or solemn discoursing, but, on the contrary, was full of - colloquial spirit and pleasantry. He had a certain quiet and grave - humour which ran through most of his conversation; and a vein of - temperate jocularity, which gave infinite zest and effect to the - condensed and inexhaustible information which formed its main - staple and characteristic. There was a little air of affected - testiness, and a tone of pretended rebuke and contradiction, with - which he used to address his younger friends, that was always felt - by them as an endearing mark of his kindness and familiarity; and - prized, accordingly, far beyond all the solemn compliments that - ever proceeded from the lips of authority. His voice was deep and - powerful, though he commonly spoke in a low and somewhat monotonous - tone, which harmonised admirably with the weight and brevity of his - observations, and set off to the greatest advantage the pleasant - [Pg318] anecdotes, which he delivered with the same grave brow, and - the same calm smile playing soberly on his lips. There was nothing - of effort, indeed, or impatience, any more than of pride or levity, - in his demeanour; and there was a finer expression of reposing - strength, and mild self-possession in his manner, than we ever - recollect to have met with in any other person. He had in his - character the utmost abhorrence for all sorts of forwardness, - parade, and pretensions; and, indeed, never failed to put all such - impostures out of countenance, by the manly plainness and honest - intrepidity of his language and deportment. - - "In his temper and dispositions, he was not only kind and - affectionate, but generous, and considerate of the feelings of - all around him; and gave the most liberal assistance and - encouragement to all young persons who showed any indications of - talent, or applied to him for patronage or advice. His health, - which was delicate from his youth upwards, seemed to become - firmer as he advanced in years; and he preserved, up almost to - the last moment of his existence, not only the full command of - his extraordinary intellect, but all the alacrity of spirit and - the social gaiety which had illumined his happiest days. His - friends in this part of the country never saw him more full of - intellectual vigour and colloquial animation--never more - delightful or more instructive--than in his last visit to - Scotland in autumn 1817. Indeed, it was after that time that he - applied himself, with all the ardour of early life, to the - invention of a machine for mechanically copying all sorts of - sculpture and statuary; and distributed among his friends some - of its earliest performances, as the productions of a young - artist just entering on his eighty-third year. - - "This happy and useful life came, at last, to a gentle close. - He had suffered some inconvenience through the summer; but was - not seriously indisposed till within a few weeks of his death. - He then became perfectly aware of the event which was - approaching; and with his usual tranquillity and benevolence - of nature, seemed only anxious to point out to the friends - around him, the many sources of consolation which were - afforded by the circumstances under which it was about to take - place. He expressed his sincere gratitude to Providence for - the length of days with which he had been blessed, and his - exemption from most of the infirmities of age; as well as for - the calm and cheerful evening of life that he had been - permitted to enjoy, after the honourable labours of the day - had been concluded. And thus, full of years and honours, in - all calmness and tranquillity, he yielded up his soul without - pang or struggle; and passed from the bosom of his family to - that of his God." - -The English nation has ever shown itself insensible to the claims -of genius and high intellectual endowments, except where the -results have been brought directly to bear in statesmanship or -war. Of this inability to appreciate the highest order of -intellectual excellence Watt affords a striking example. When it -was suggested to the British government by those better capable -than that government was of appreciating the genius of this great -man, that the nation would do itself honour by erecting a splendid -monument at his own [Pg319] cost to him to whom it was so deeply -indebted for the extension of its resources and the augmentation -of its power, the reply was that such a measure could not be -adopted as it might be drawn into a precedent in like cases -thereafter! A precedent in like cases!! When will the time arrive -when the world will produce a like case? The monument which has -been erected in Westminster Abbey was in fact raised by private -subscription, the nation having thus stigmatised itself through -the act of its government with the everlasting disgrace of -refusing the honour proposed to it. The other statues and -monuments which have been erected to this great man, have been for -the most part raised by the filial piety and the never-dying -affection and veneration of the present Mr. James Watt. A statue -has been presented by him to the University of Glasgow, and placed -in one of the halls of that college. The inhabitants of Greenock -have also erected a marble statue of Watt, for which, and for a -library, a building has been erected at the expense of about -3,500_l._ which has been defrayed by Mr. James Watt. A colossal -bronze statue has been erected on a handsome granite pedestal, -standing at one of the corners of George Square, Glasgow. The -monument in Westminster Abbey, erected by the subscription raised -at the public meeting already alluded to, is a colossal statue of -Carrara marble, by Chantrey. - -Watt was elected a fellow of the Royal Society of Edinburgh in -1784; of the Royal Society of London in 1785; a member of the -Batavian Society in 1787; and a corresponding member of the -Institut of France in 1808. The degree of Doctor of Laws was -conferred upon him by the University of Glasgow, in 1806; and in -1814, the highest scientific honour which can be attained by a -philosopher, was conferred on him by the Academy of Sciences of -the Institut of France, who nominated him one of its eight foreign -associates. - -On the pedestal of the monument in Westminster Abbey is engraved -the following inscription from the pen of Lord Brougham:-- -[Pg320] - - NOT TO PERPETUATE A NAME WHICH MUST ENDURE WHILE THE PEACEFUL - ARTS FLOURISH, BUT TO SHOW THAT MANKIND HAVE LEARNED TO HONOUR - THOSE WHO BEST DESERVE THEIR GRATITUDE, THE KING HIS MINISTERS, - AND MANY OF THE NOBLES AND COMMONERS OF THE REALM RAISED THIS - MONUMENT TO - - JAMES WATT, - - WHO DIRECTING THE FORCE OF AN ORIGINAL GENIUS, EARLY EXERCISED - IN PHILOSOPHIC RESEARCH TO THE IMPROVEMENT OF THE STEAM - ENGINE, ENLARGED THE RESOURCES OF HIS COUNTRY, INCREASED THE - POWER OF MAN, AND ROSE TO AN EMINENT PLACE AMONG THE MOST - ILLUSTRIOUS FOLLOWERS OF SCIENCE AND THE REAL BENEFACTORS OF - THE WORLD. BORN AT GREENOCK MDCCXXXVI. DIED AT HEATHFIELD IN - STAFFORDSHIRE MDCCCXIX. - -[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.] - - FOOTNOTES: - - [25] See Buchanan on the Economy of Fuel and Management of - Heat, especially as it relates to heating and drying by means - of Steam. - - [26] See Brewster's Edinburgh Encyclopædia, article - STEAM-DRYING MACHINE. - - [27] The following are the words in which Watt makes this - remarkable announcement to Priestley:-- - - "Let us now consider what obviously happens in the deflagration - of the inflammable (hydrogen) and dephlogisticated air (oxygen). - These two kinds of air unite with violence; they become red hot, - and upon cooling, totally disappear. When the vessel is cooled, - a quantity of water is found in it equal to the weight of the - air employed. This water is then the only remaining product of - the process; and water, light, and heat are all the products. - - "Are we not then authorised to conclude, that water is composed - of dephlogisticated air (oxygen) and phlogiston (hydrogen), - deprived of part of their latent or elementary heat; that - dephlogisticated or pure air (oxygen) is composed of water - deprived of its phlogiston (hydrogen), and united to elementary - heat and light; and that the latter are contained in it in a - latent state, so as not to be sensible to the thermometer or to - the eye; and if light be only a modification of heat, or a - circumstance attending it, or a component part of the - inflammable air (hydrogen), then pure or dephlogisticated air - (oxygen) is composed of water deprived of its phlogiston - (hydrogen), and united to elementary heat." - - [28] Those who desire to investigate this controversy more in - detail will find very full information on the subject in the - Translation of Arago's Eloge, with notes and appendix by J. P. - Muirhead, Esq. Murray, London, 1839. - - [29] An account of this remarkable apparatus, accompanied by an - engraving made from a drawing supplied by Watt, was communicated - by Sir John Robison to the _Edinburgh Philosophical Journal_ in - 1820. _See_ vol. iii, p. 60. - -[Pg321] - - - - -[Illustration] - -CHAP. XI. - -LOCOMOTIVE ENGINES ON RAILWAYS. - - NON-CONDENSING ENGINES. -- LEUPOLD'S ENGINE.-TREVETHICK AND - VIVIAN. -- EFFECTS OF RAILWAY TRANSPORT. -- HISTORY OF THE - LOCOMOTIVE ENGINE. -- BLENKINSOP. -- MESSRS. CHAPMAN. -- WALKING - ENGINE. -- MR. STEPHENSON'S ENGINES AT KILLINGWORTH. -- - LIVERPOOL AND MANCHESTER RAILWAY. -- EXPERIMENTAL TRIAL. -- THE - ROCKET. -- THE SANSPAREIL. -- THE NOVELTY. -- SUBSEQUENT - IMPROVEMENTS IN THE LOCOMOTIVE ENGINE. -- LARDNER'S EXPERIMENTS - IN 1832. -- ADOPTION OF BRASS TUBES. -- MR. BOOTH'S REPORT. -- - DETAILED DESCRIPTION OF THE MOST IMPROVED LOCOMOTIVE ENGINES. - -- POWER OF LOCOMOTIVE ENGINES. -- EVAPORATION OF BOILERS. -- - LARDNER'S EXPERIMENTS IN 1838. -- RESISTANCE TO RAILWAY TRAINS. - -- RESTRICTIONS ON GRADIENTS. -- COMPENSATING EFFECT OF - GRADIENTS. -- EXPERIMENT WITH THE HECLA. -- METHODS OF - SURMOUNTING STEEP INCLINATIONS. - - -(180.) In the various modifications of the steam engine which we -have hitherto considered, the pressure introduced on one side of the -piston derives its efficacy either wholly or partially from the -vacuum produced by condensation on the other side. This always -requires a condensing apparatus, and a constant and abundant supply -of cold water. An engine of this kind must therefore necessarily -have considerable dimensions and weight, and is inapplicable to uses -in which a small and light machine only is admissible. If the -condensing apparatus be dispensed with, the piston will always be -resisted by a force equal to the atmospheric [Pg322] pressure, and -the only part of the steam pressure which will be available as a -moving power, is that part by which it exceeds the pressure of the -atmosphere. Hence, in engines which do not work by condensation, -steam of a much higher pressure than that of the atmosphere is -indispensably necessary, and such engines are therefore called -_high-pressure engines_. - -We are not, however, to understand that every engine, in which -steam is used of a pressure exceeding that of the atmosphere, is -what is meant by an _high-pressure engine_; for in the ordinary -engines in common use, constructed on Watt's principle, the -safety-valve is loaded with from 3 to 5 lbs. on the square inch; -and in Woolf's engines, the steam is produced under a pressure of -40 lbs. on the square inch. These would therefore be more properly -called _condensing engines_ than _low-pressure engines_; a term -quite inapplicable to those of Woolf. In fact, by _high-pressure -engines_ is meant engines in which no vacuum is produced, and, -therefore, in which the piston works against a pressure equal to -that of the atmosphere. - -In these engines the whole of the condensing apparatus, viz. the -cold-water cistern, condenser, air-pump, cold-water pump, &c., are -dispensed with, and nothing is retained except the boiler, -cylinder, piston, and valves. Consequently, such an engine is -small, light, and cheap. It is portable also, and may be moved, if -necessary, along with its load, and is therefore well adapted to -locomotive purposes. - - -(181.) High-pressure engines were one of the earliest forms of the -steam engine. The contrivance, which is obscurely described in the -article already quoted (7.), from the Century of Inventions, is a -high-pressure engine; for the power there alluded to is the -elastic force of steam working against the atmospheric pressure. -Newcomen, in 1705, applied the working-beam, cylinder, and piston -to the atmospheric engine; and Leupold, about 1720, combined the -working-beam and cylinder with the high-pressure principle, and -produced the earliest high-pressure engine worked by a cylinder -and piston. The following is a description of Leupold's engine:-- -[Pg323] - -[Illustration: _Fig._ 82.] - -A (_fig._ 82.) is the boiler, with the furnace beneath it; C C are -two cylinders with solid pistons P P', connected with the -working-beams B B', to which are attached the pump-rods R R', of -two forcing pumps F F', which communicate with a great force-pipe -S; G is a _four-way cock_ (66.) already described. In the position -in which it stands in the figure, the steam issues from below the -piston P into the atmosphere, and the piston is descending by its -own weight; steam from the boiler is at the same time pressing up -the piston P', with a force equal to the difference between the -pressure of the steam and that of the atmosphere. Thus the piston -R of the forcing-pump is being drawn up, and the piston P' is -forcing the piston R' down, and thereby driving water into the -force-pipe [Pg324] S. On the arrival of the piston P at the -bottom of the cylinder C, and P' at the top of the cylinder C', -the position of the cock is changed as represented in _fig._ 83. -The steam, which has just pressed up the piston P', is allowed to -escape into the atmosphere, while the steam, passing from the -boiler below the piston P, presses it up, and thus P ascends by -the steam pressure, and P' descends by its own weight. By these -means the piston R is forced down, driving before it the water in -the pump-cylinder into the force-pipe S, and the piston R' is -drawn up to allow the other pump-cylinder to be re-filled; and so -the process is continued. - -[Illustration: _Fig._ 83.] - -A valve is placed in the bottom of the force-pipes, to prevent the -water which has been driven into it from returning. This valve -opens upwards; and, consequently, the weight of the water pressing -upon it only keeps it more effectually closed. On each descent of -the piston, the pressure transmitted to the valve acting upwards -being greater than the weight of the water resting upon it, forces -it open, and an increased quantity of water is introduced. - - -(182.) From the date of the improvement of Watt until the -commencement of the present century, non-condensing engines were -altogether neglected in these countries. In the year 1802, Messrs. -Trevethick and Vivian constructed the first non-condensing engine -of this kind which was ever brought into extensive practical use -in this kingdom. A section of this machine, made by a vertical -plane, is represented in _fig._ 84. - -The boiler A B is a cylinder with flat circular ends. The -fire-place is constructed in the following manner:--A tube enters -the cylindrical boiler at one end; and, proceeding onwards near -the other extremity, is turned and recurved, so as to be carried -back parallel to the direction in which it entered. It is thus -conducted out of the boiler, at another part of the same end at -which it entered. One of the ends of this tube communicates with -the chimney E, which is carried upwards as represented in the -figure. The other mouth is furnished [Pg325] with a door; and in -it is placed the grate, which is formed of horizontal bars, -dividing the tube into two parts; the upper part forming the -fire-place, and the lower the ash-pit. The fuel is maintained in a -state of combustion, on the bars, in that part of the tube -represented at C D; and the flame is carried by the draught of the -chimney round the curved flue, and issues at E into the chimney. -The flame is thus conducted through the water, so as to expose the -latter to as much heat as possible. - -[Illustration: _Fig._ 84.] - -A section of the cylinder is represented at F, immersed in the -boiler, except a few inches of the upper end, where the four-way -cock G is placed for regulating the admission of the steam. A tube -is represented at H, which leads from this four-way cock into the -chimney; so that the waste steam, after working the piston, is -carried off through this tube, and passes into the chimney. The -upper end of the piston-rod is furnished with a cross-bar, which -is placed in a direction at right angles to the length of the -boiler, and also to the [Pg326] piston-rod. This bar is guided in -its motion by sliding on two iron perpendicular rods fixed to the -sides of the boiler, and parallel to each other. To the ends of -this cross-bar are joined two connecting rods, the lower ends of -which work two cranks fixed on an axis extending across and -beneath the boiler, and immediately under the centre of the -cylinder. This axis is sustained in bearings formed in the legs -which support the boiler, and upon its extremity is fixed the -fly-wheel as represented at B. A large-toothed wheel is placed on -this axis; which, being turned with the cranked axle, communicates -motion to other wheels; and through them, to any machinery which -the engine may be applied to move. - -[Illustration: _Fig._ 85.] - -As the four-way cock is represented in the figure, the steam -passes from the boiler through the curved passage G above the -piston, while the steam below the piston is carried off through a -tube which does not appear in the figure, by which it is conducted -to the tube H, and thence to the chimney. The steam, therefore, -which passes above the piston presses it downwards; while the -pressure upwards does not exceed that of the atmosphere. The -piston will therefore descend with a force depending on the excess -of the pressure of the steam produced in the boiler above the -atmospheric pressure. When the piston has arrived at the bottom of -the cylinder, the cock is made to assume the position represented -in _fig._ 85. This effect is produced by the motion of the -piston-rod. The steam now passes from above the piston, through -the tube H, into the chimney, while the steam from the boiler is -conducted through another tube below the piston. The pressure -above the piston, in this case, does not exceed that of the -atmosphere; while the pressure below it will be that of the steam -in the boiler. The piston will therefore ascend with the -difference of these pressures. On the arrival of the piston at the -top of the cylinder, the four-way cock is again turned to the -position represented in _fig._ 85., and the piston again descends; -and in the same manner the process is continued. A safety-valve is -placed on the boiler at V, loaded with a weight W, proportionate -[Pg327] to the strength of the steam with which it is proposed to -work. - -In the engines now described, this valve was frequently loaded at -the rate of from 60 to 80 lbs. on the square inch. As the boilers of -high-pressure engines were considered more liable to accidents from -bursting than those in which steam of a lower pressure was used, -greater precautions were taken against such effects. A second -safety-valve was provided, which was not left in the power of the -engine-man. By this means he had a power to diminish the pressure of -the steam, but could not increase it beyond the limit determined by -the valve which was removed from his interference. The greatest -cause of danger, however, arose from the water in the boiler being -consumed by evaporation faster than it was supplied; and therefore -falling below the level of the tube containing the furnace. To guard -against accidents arising from this circumstance, a hole was bored -in the boiler, at a certain depth, below which the water should not -be allowed to fall; and in this hole a plug of metal was soldered -with lead, or with some other metal, which would fuse at that -temperature which would expose the boiler to danger. Thus, in the -event of the water being exhausted, so that its level would fall -below the plug, the heat of the furnace would immediately melt the -solder, and the plug would fall out, affording a vent for the steam, -without allowing the boiler to burst. The mercurial steam-gauge, -already described, was also used as an additional security. When the -force of the steam exceeded the length of the column of mercury -which the tube would contain, the mercury would be blown out, and -the tube would give vent to the steam. The water by which the boiler -was replenished was forced into it by a pump worked by the engine. -In order to economise the heat, this water was contained in a tube -T, which surrounded the pipe H. As the waste steam, after working -the piston, passed off through H, it imparted a portion of its heat -to the water contained in the tube T, which was thus warmed to a -certain temperature before it was forced into the boiler by the -pump. Thus a part of the heat, which was originally [Pg328] carried -from the boiler in the form of steam, was returned again to the -boiler with the water with which it was fed. - -It is evident that engines constructed in this manner may be -applied to all the purposes to which the condensing engines are -applicable. - - -(183.) Two years after the date of the patent of this engine, its -inventor constructed a machine of the same kind for the purpose of -moving carriages on railroads; and applied it successfully, in the -year 1804, on the railroad at Merthyr Tydvil, in South Wales. It -was in principle the same as that already described. The cylinder -however was in a horizontal position, the piston-rod working in -the direction of the line of road: the extremity of the -piston-rod, by means of a connecting rod, worked cranks placed on -the axletree, on which were fixed two cogged wheels: these worked -in others, by which their motion was communicated finally to -cogged wheels fixed on the axle of the hind wheels of the -carriage, by which this axle was kept in a state of revolution. -The hind wheels being fixed on the axletree, and turning with it, -were caused likewise to revolve; and so long as the weight of the -carriage did not exceed that which the friction of the road was -capable of propelling, the carriage would thus be moved forwards. -On this axle was placed a fly-wheel to continue the rotatory -motion at the termination of each stroke. The fore wheels are -described as being capable of turning like the fore wheels of a -carriage, so as to guide the vehicle. The projectors appear to -have contemplated, in the first instance, the use of this carriage -on common roads; but that notion seems to have been abandoned, and -its use was only adopted on the railroad before mentioned. On the -occasion of its first trial, it drew after it as many carriages as -contained ten tons of iron a distance of nine miles; which stage -it performed without any fresh supply of water, and travelled at -the rate of five miles an hour. - - -(184.) Capital and skill have of late years been directed with -extraordinary energy to the improvement of inland transport; and -this important instrument of national wealth and civilisation has -received a proportionate impulse. Effects are now witnessed, -which, had they been narrated a few years [Pg329] since, could -only have been admitted into the pages of fiction or volumes of -romance. Who could have credited the possibility of a ponderous -engine of iron, loaded with some hundred passengers, in a train of -carriages of corresponding magnitude, and a large quantity of -water and coal, taking flight from Manchester and arriving at -Liverpool, a distance of above thirty miles, in little more than -an hour? And yet this is a matter of daily and almost hourly -occurrence. The rapidity of transport thus attained is not less -wonderful than the weights transported. Its capabilities in this -respect far transcend the exigencies even of the two greatest -commercial marts in Great Britain. Loads, varying from fifty to -one hundred and fifty tons, are transported at the average rate of -fifteen miles an hour; and in one instance we have seen a load--we -should rather say a _cargo_--of waggons, conveying merchandise to -the amount of two hundred and thirty tons gross, transported from -Liverpool to Manchester at the average rate of twelve miles an -hour. - -The astonishment with which such performances must be viewed, -might be qualified, if the art of transport by steam on railways -had been matured, and had attained that full state of perfection -which such an art is always capable of receiving from long -experience, aided by great scientific knowledge, and the unbounded -application of capital. But such is not the present case. The art -of constructing locomotive engines, so far from having attained a -state of maturity, has not even emerged from its infancy. So -complete was the ignorance of its powers which prevailed, even -among engineers, previous to the opening of the Liverpool railway, -that the transport of heavy goods was regarded as the chief object -of the undertaking, and its principal source of revenue. The -incredible speed of transport, effected even in the very first -experiments in 1830, burst upon the public, and on the scientific -world, with all the effect of a new and unlooked-for phenomenon. -On the unfortunate occasion which deprived this country of Mr. -Huskisson, the wounded body of that statesman was transported a -distance of about fifteen miles in twenty-five minutes, being at -the rate of thirty-six miles an hour. The revenue of the road -arising from passengers since its opening, [Pg330] has, contrary -to all that was foreseen, been nearly double that which has been -derived from merchandise. So great was the want of experience in -the construction of engines, that the company was at first -ignorant whether they should adopt large steam engines fixed at -different stations on the line, to pull the carriages from station -to station, or travelling engines to drag the loads the entire -distance. Having decided on the latter, they have, even to the -present moment, laboured under the disadvantage of the want of -that knowledge which experience alone can give. The engines have -been constantly varied in their weight and proportions, in their -magnitude and form, as the experience of each successive month has -indicated. As defects became manifest they were remedied; -improvements suggested were adopted; and each year produced -engines of such increased power and efficiency, that their -predecessors were abandoned, not because they were worn out, but -because they had been outstripped in the rapid march of -improvement. Add to this, that only one species of travelling -engine has been effectively tried; the capabilities of others -remain still to be developed; and even that form of engine which -has received the advantage of a course of experiments on so grand -a scale to carry it towards perfection, is far short of this -point, and still has defects, many of which, it is obvious, time -and experience will remove. - -If, then, the locomotive engine, subject thus to all the -imperfections inseparable from a novel contrivance--with the -restrictions on the free application of skill and capital, arising -from the nature of the monopolies granted to railway companies--with -the disadvantage of very limited experience, the great parent of -practical improvement, having been submitted to experiments hitherto -only on a limited scale, and confined almost to one form of -machine;--if, under such disadvantages, such effects have been -produced as are now daily witnessed by the public, what may not be -looked for from this extraordinary power when the enterprise of the -country shall be more unfettered--when greater fields of experience -are opened--when time, ingenuity, and capital have removed or -diminished existing imperfections, and have brought to light new and -more powerful principles? This is not mere speculation [Pg331] on -abstract possibilities, but refers to what is in actual progress. -The points of greatest wealth and population--the centres of largest -capital and most active industry throughout the country--will soon -be connected by lines of railway; and various experiments are -proposed, with more or less prospect of success, for the application -of steam engines on stone roads where the intercourse is not -sufficient to render railways profitable. - -The important commercial and political effects attending such -increased facility and speed in the transport of persons and goods, -are too obvious to require any very extended notice here. A part of -the price (and in many cases a considerable part) of every article -of necessity or luxury, consists of the cost of transporting it from -the producer to the consumer; and consequently every abatement or -saving in this cost must produce a corresponding reduction in the -price of every article transported; that is to say, of every thing -which is necessary for the subsistence of the poor, or for the -enjoyment of the rich--of every comfort, and of every luxury of -life. The benefit of this will extend, not to the consumer only, but -to the producer: by lowering the expense of transport of the -produce, whether of the soil or of the loom, a less quantity of that -produce will be spent in bringing the remainder to market, and -consequently a greater surplus will reward the labour of the -producer. The benefit of this will be felt even more by the -agriculturist than by the manufacturer; because the proportional -cost of transport of the produce of the soil is greater than that of -manufactures. If two hundred quarters of corn be necessary to raise -four hundred, and one hundred more be required to bring the four -hundred to market, then the net surplus will be one hundred. But if -by the use of steam carriages the same quantity can be brought to -market with an expenditure of fifty quarters, then the net surplus -will be increased from one hundred to one hundred and fifty -quarters; and either the profit of the farmer, or the rent of the -landlord, must be increased by the same amount. - -But the agriculturist would not merely be benefited by an -increased return from the soil already under cultivation. Any -[Pg332] reduction in the cost of transporting the produce to -market would call into cultivation tracts of inferior fertility, -the returns from which would not at present repay the cost of -cultivation and transport. Thus land would become productive which -is now waste, and an effect would be produced equivalent to adding -so much fertile soil to the present extent of the country. It is -well known, that land of a given degree of fertility will yield -increased produce by the increased application of capital and -labour. By a reduction in the cost of transport, a saving will be -made which may enable the agriculturist to apply to tracts already -under cultivation the capital thus saved, and thereby increase -their actual production. Not only, therefore, would such an effect -be attended with an increased extent of cultivated land, but also -with an increased degree of cultivation in that which is already -productive. - -It has been said, that in Great Britain there are above a million -of horses engaged in various ways in the transport of passengers -and goods, and that to transport each horse requires as much land -as would, upon an average, support eight men. If this quantity of -animal power were displaced by steam engines, and the means of -transport drawn from the bowels of the earth, instead of being -raised upon its surface, then, supposing the above calculation -correct, as much land would become available for the support of -human beings as would suffice for an additional population of -eight millions; or, what amounts to the same, would increase the -means of support of the present population by about one third of -the present available means. The land which now supports horses -for transport would then support men, or produce corn for food. - -The objection that a quantity of land exists in the country -capable of supporting horses alone, and that such land would be -thrown out of cultivation, scarcely deserves notice here. The -existence of any considerable quantity of such land is extremely -doubtful. What is the soil which will feed a horse and not feed -oxen or sheep, or produce food for man? But even if it be admitted -that there exists in the country a small portion of such land, -that portion cannot exceed, nor indeed equal, what would be -sufficient for the number of horses [Pg333] which must after all -continue to be employed for the purposes of pleasure, and in a -variety of cases where steam must necessarily be inapplicable. It -is to be remembered, also, that the displacing of horses in one -extensive occupation, by diminishing their price must necessarily -increase the demand for them in others. - -The reduction in the cost of transport of manufactured articles, -by lowering their price in the market, will stimulate their -consumption. This observation applies of course not only to home -but to foreign markets. In the latter we already in many branches -of manufactures command a monopoly. The reduced price which we -shall attain by cheapness and facility of transport will still -further extend and increase our advantages. The necessary -consequence will be, an increased demand for manufacturing -population; and this increased population again reacting on the -agricultural interests, will form an increased market for that -species of produce. So interwoven and complicated are the fibres -which form the texture of the highly civilised and artificial -community in which we live, that an effect produced on any one -point is instantly transmitted to the most remote and apparently -unconnected parts of the system. - -The two advantages of increased cheapness and speed, besides -extending the amount of existing traffic, call into existence new -objects of commercial intercourse. For the same reason that the -reduced cost of transport, as we have shown, calls new soils into -cultivation, it also calls into existence new markets for -manufactured and agricultural produce. The great speed of transit -which has been proved to be practicable, must open a commerce -between distant points in various articles, the nature of which -does not permit them to be preserved so as to be fit for use -beyond a certain time. Such are, for example, many species of -vegetable and animal food, which at present are confined to -markets at a very limited distance from the grower or feeder. The -truth of this observation is manifested by the effects which have -followed the intercourse by steam on the Irish Channel. The -western towns of England have become markets for a prodigious -quantity of Irish produce, which it had been previously [Pg334] -impossible to export. If animal food be transported alive from the -grower to the consumer, the distance of the market is limited by -the power of the animal to travel, and the cost of its support on -the road. It is only particular species of cattle which bear to be -carried to market on common roads and by horse carriages. But the -peculiar nature of a railway, the magnitude and weight of the -loads which may be transported on it, and the prodigious speed -which may be attained, render the transport of cattle, of every -species, to almost any distance, both easy and cheap. In process -of time, when the railway system becomes extended, the metropolis -and populous towns will therefore become markets, not as at -present to districts within limited distances of them, but to the -whole country. - -The moral and political consequences of so great a change in the -powers of transition of persons and intelligence from place to place -are not easily calculated. The concentration of mind and exertion -which a great metropolis always exhibits, will be extended in a -considerable degree to the whole realm. The same effect will be -produced as if all distances were lessened in the proportion in -which the speed and cheapness of transit are increased. Towns at -present removed some stages from the metropolis, will become its -suburbs; others, now at a day's journey, will be removed to its -immediate vicinity; business will be carried on with as much ease -between them and the metropolis, as it is now between distant points -of the metropolis itself. Let those who discard speculations like -these as wild and improbable, recur to the state of public opinion, -at no very remote period, on the subject of steam navigation. Within -the memory of persons who have not yet passed the meridian of life, -the possibility of traversing by the steam engine the channels and -seas that surround and intersect these islands, was regarded as the -dream of enthusiasts. Nautical men and men of science rejected such -speculations with equal incredulity, and with little less than scorn -for the understanding of those who could for a moment entertain -them. Yet we have witnessed steam engines traversing not these -channels and seas alone, but sweeping the face of the waters round -every coast in Europe. The [Pg335] seas which interpose between our -Asiatic dominions and Egypt, and those which separate our own shores -from our West Indian possessions, have offered an equally -ineffectual barrier to its powers, and the establishment of a -regular steam communication between the capitals of the Old and New -World has ceased to be a question of practicability, having become -merely one of commercial profit. If steam be not used as the only -means of connecting the most distant points of our planet, it is not -because it is inadequate to the accomplishment of that end, but -because the supply of the material, from which at the present moment -it derives its powers, is restricted by local and accidental -circumstances.[30] - -We propose in the present chapter to lay before our readers some -account of the means whereby the effects above referred to have -been produced; of the manner and degree in which the public have -availed themselves of these means; and of the improvements of -which they seem to us to be susceptible. - - -(185.) It is a singular fact, that in the history of this -invention considerable time and great ingenuity were vainly -expended in attempting to overcome a difficulty, which in the end -turned out to be purely imaginary. To comprehend distinctly the -manner in which a wheel carriage is propelled by steam, suppose -that a pin or handle is attached to the spoke of the wheel at some -distance from its centre, and that a force is applied to this pin -in such a manner as to make the wheel revolve. If the tire of the -wheel and the surface of the road were absolutely smooth and free -from friction, so that the face of the tire would slide without -resistance upon the road, then the effect of the force thus -applied would be merely to cause the wheel to turn round, the -carriage being stationary, the surface of the tire slipping or -sliding upon the road as the wheel is made to revolve. But if, on -the other hand, the pressure of the face of the tire upon the road -is such as to produce between them such a degree of adhesion as -will render it impossible for the wheel to slide or slip upon the -road by [Pg336] the force which is applied to it, the consequence -will be, that the wheel can only turn round in obedience to the -force which moves it by causing the carriage to advance, so that -the wheel will roll upon the road, and the carriage will be moved -forward, through a distance equal to the circumference of the -wheel, each time it performs a complete revolution. - -It is obvious that both of these effects may be partially produced; -the adhesion of the wheel to the road may be insufficient to prevent -slipping altogether, and yet it may be sufficient to prevent the -wheel from slipping as fast as it revolves. Under such circumstances -the carriage would advance and the wheel would slip. The progressive -motion of the carriage during one complete revolution of the wheel -would be equal to the difference between the complete circumference -of the wheel and the portion through which in one revolution it has -slipped. - -When the construction of travelling steam engines first engaged -the attention of engineers, and for a considerable period -afterwards, a notion was impressed upon their minds that the -adhesion between the face of the wheel and the surface of the road -must necessarily be of very small amount, and that in every -practical case the wheels thus driven would either slip -altogether, and produce no advance of the carriage, or that a -considerable portion of the impelling power would be lost by the -partial slipping or sliding of the wheels. It is singular that it -should never have occurred to the many ingenious persons who for -several years were engaged in such experiments and speculations, -to ascertain by experiment the actual amount of adhesion in any -particular case between the wheels and the road. Had they done so, -we should probably now have found locomotive engines in a more -advanced state than that to which they have attained. - -To remedy this imaginary difficulty, Messrs. Trevethick and Vivian -proposed to make the external rims of the wheels rough and uneven, -by surrounding them with projecting heads of nails or bolts, or by -cutting transverse grooves on them. They proposed, in cases where -considerable elevations were to be ascended, to cause claws or -nails to project from the surface during the ascent, so as to take -hold of the road. [Pg337] - -In seven years after the construction of the first locomotive -engine by these engineers, another locomotive engine was -constructed by Mr. Blinkensop, of Middleton Colliery, near Leeds. -He obtained a patent, in 1811, for the application of a rack-rail. -The railroad thus, instead of being composed of smooth bars of -iron, presented a line of projecting teeth, like those of a -cog-wheel, which stretched along the entire distance to be -travelled. The wheels on which the engine rolled were furnished -with corresponding teeth, which worked in the teeth of the -railroad, and, in this way, produced a progressive motion in the -carriage. - -The next contrivance for overcoming this fictitious difficulty, -was that of Messrs. Chapman, who, in the year 1812, obtained a -patent for working a locomotive engine by a chain extending along -the middle of the line of railroad, from the one end to the other. -This chain was passed once round a grooved wheel under the centre -of the carriage; so that, when this grooved wheel was turned by -the engine, the chain being incapable of slipping upon it, the -carriage was consequently advanced on the road. In order to -prevent the strain from acting on the whole length of the chain, -its links were made to fall upon upright forks placed at certain -intervals, which between those intervals sustained the tension of -the chain produced by the engine. Friction-rollers were used to -press the chain into the groove of the wheel, so as to prevent it -from slipping. This contrivance was soon abandoned, for the very -obvious reason that a prodigious loss of force was incurred by the -friction of the chain. - -The following year, 1813, produced a contrivance of singular -ingenuity, for overcoming the supposed difficulty arising from the -want of adhesion between the wheels and the road. This was no -other than a pair of mechanical legs and feet, which were made to -walk and propel in a manner somewhat resembling the feet of an -animal. - -[Illustration: _Fig._ 86.] - -A sketch of these propellers is given in _fig._ 86. A is the -carriage moving on the railroad, L and L' are the legs, F and F' -the feet. The foot F has a joint at O, which corresponds to the -ankle; another joint is placed at K, which corresponds to the -knee; and a third is placed at L, which corresponds to [Pg338] -the hip. Similar joints are placed at the corresponding letters in -the other leg. The knee-joint K is attached to the end of the -piston of the cylinder. When the piston, which is horizontal, is -pressed outwards, the leg L presses the foot F against the ground, -and the resistance forces the carriage A onwards. As the carriage -proceeds, the angle K at the knee becomes larger, so that the leg -and thigh take a straighter position; and this continues until the -piston has reached the end of its stroke. At the hip L there is a -short lever L M, the extremity of which is connected by a cord or -chain with a point S, placed near the shin of the leg. When the -piston is pressed into the cylinder, the knee K is drawn towards -the engine, and the cord M S is made to lift the foot F from the -ground; to which it does not return until the piston has arrived -at the extremity of the cylinder. On the piston being again driven -out of the cylinder, the foot F, being placed on the road, is -pressed backwards by the force of the piston-rod at K; but the -friction of the ground preventing its backward motion, the -re-action causes the engine to advance: and in the same manner -this process is continued. - -Attached to the thigh at N, above the knee, by a joint, is a -horizontal rod N R, which works a rack R. This rack has beneath it -a cog-wheel. This cog-wheel acts in another rack below it. By -these means, when the knee K is driven _from_ the engine, the rack -R is moved _backwards_; but the cog-wheel acting on the other rack -beneath it, will move the latter _in the contrary direction_. The -rack R being then moved _in the_ [Pg339] _same direction with the -knee_ K, it follows that the other rack will always be moved _in a -contrary direction_. The lower rack is connected by another -horizontal rod with the thigh of the leg L F', immediately above -the knee at N'. When the piston is forced _inwards_, the knee K' -will thus be forced _backwards_; and when the piston is forced -_outwards_, the knee K' will be drawn _forwards_. It therefore -follows, that the two knees K and K' are pressed _alternately -backwards_ and _forwards_. The foot F', when the knee K' is drawn -forward, is lifted by the means already described for the foot F. - -It will be apparent, from this description, that the piece of -mechanism here exhibited is a contrivance derived from the motion -of the legs of an animal, and resembling in all respects the fore -legs of a horse. It is however to be regarded rather as a specimen -of great ingenuity than as a contrivance of practical utility. - - -(186.) It was about this period that the important fact was first -ascertained that the adhesion or friction of the wheels with the -rails on which they moved was amply sufficient to propel the -engine, even when dragging after it a load of great weight; and -that in such case, the progressive motion would be effected -without any slipping of the wheels. The consequence of this fact -rendered totally useless all the contrivances for giving wheels a -purchase on the road, such as racks, chains, feet, &c. The -experiment by which this was determined appears to have been first -tried on the Wylam railroad; where it was proved, that when the -road was level, and the rails clean, the adhesion of the wheels -was sufficient, in all kinds of weather, to propel considerable -loads. By manual labour it was first ascertained how much weight -the wheels of a common carriage would overcome without slipping -round on the rail, and having found the proportion which that bore -to the weight, they then ascertained that the weight of the engine -would produce sufficient adhesion to drag after it on the railroad -the requisite number of waggons.[31] - -In 1814, an engine was constructed at Killingworth, by Mr. -Stephenson, having two cylinders with a cylindrical [Pg340] -boiler, and working two pair of wheels, by cranks placed at right -angles; so that when the one was in full operation, the other was -at its dead points. By these means the propelling power was always -in action. The cranks were maintained in this position by an -endless chain, which passed round two cogged wheels placed under -the engine, and which were fixed on the same axles on which the -wheels were placed. The wheels in this case were fixed on the -axles, and turned with them. - -[Illustration: _Fig._ 87.] - -This engine is represented in _fig._ 87., the sides being open, to -render the interior mechanism visible. A B is the cylindrical -boiler; C C are the working cylinders; D E are the cogged wheels -fixed on the axle of the wheels of the engine, and surrounded by -the endless chain. These wheels being equal in magnitude, perform -their revolutions in the same time; so that, when the crank F -descends to the lowest point, the crank G rises from the lowest -point to the horizontal position D; and, again, when the crank F -rises from the lowest point to the horizontal position E, the -other crank rises to the highest point; and so on. A very -beautiful contrivance was adopted in this engine, by which it was -suspended on springs of steam. Small cylinders, represented at H, -are screwed by flanges to one side of the boiler, and project -within it a few inches; they have free communication at the top -with the water or steam of the boiler. Solid pistons are -represented at I, which move steam-tight in these [Pg341] -cylinders; the cylinders are open at the bottom, and the -piston-rods are screwed on the carriage of the engine, over the -axle of each pair of wheels, the pistons being presented upwards. -As the engine is represented in the figure, it is supported on -four pistons, two at each side. The pistons are pressed upon by -the water or steam which occupies the upper chamber of the -cylinder; and the latter being elastic in a high degree, the -engine has all the advantage of spring suspension. The defect of -this method of supporting the engine is, that when the steam loses -that amount of elasticity necessary for the support of the -machine, the pistons are forced into the cylinders, and the -bottoms of the cylinders bear upon them. All spring suspension is -then lost. This mode of suspension has consequently since been -laid aside. - -In an engine subsequently constructed by Mr. Stephenson, for the -Killingworth railroad, the mode adopted of connecting the wheels -by an endless chain and cog-wheels was abandoned; and the same -effect was produced by connecting the two cranks by a straight -rod. All such contrivances, however, have this great defect, that, -if the fore and hind wheels be not constructed with dimensions -accurately equal, there must necessarily be a slipping or dragging -on the road. The nature of the machinery requires that each wheel -should perform its revolution exactly in the same time; and -consequently, in doing so, must pass over exactly equal lengths of -the road. If, therefore, the circumference of the wheels be not -accurately equal, that wheel which has the lesser circumference -must be dragged along so much of the road as that by which it -falls short of the circumference of the greater wheel; or, on the -other hand, the greater wheel must be dragged in the opposite -direction, to compensate for the same difference. As no mechanism -can accomplish a perfect equality in four, much less in six, -wheels, it may be assumed that a great portion of that dragging -effect is a necessary consequence of the principle of this -machine; and even were the wheels, in the first instance, -accurately constructed, it is not possible that their wear could -be so exactly uniform as to continue equal. - - -(187.) The next stimulus which the progress of this [Pg342] -invention received, proceeded from the great national work -undertaken at Liverpool, by which that town and the extensive -commercial mart of Manchester were connected by a double line of -railway. When this project was undertaken, it was not decided what -moving power it might be most expedient to adopt as a means of -transport on the proposed road: the choice lay between horse -power, fixed steam engines, and locomotive engines; but the first, -for many obvious reasons, was at once rejected in favour of one or -other of the last two. - -The steam engine may be applied, by two distinct methods, to move -waggons either on a turnpike road or on a railway. By the one -method the steam engine is fixed, and draws the carriage or train -of carriages towards it by a chain extending the whole length of -road on which the engine works. By this method the line of road -over which the transport is conducted is divided into a number of -short intervals, at the extremity of each of which an engine is -placed. The waggons or carriages, when drawn by any engine to its -own station, are detached, and connected with the extremity of the -chain worked by the next stationary engine; and thus the journey -is performed, from station to station, by separate engines. By the -other method the same engine draws the load the whole journey, -travelling with it. - -The Directors of the Liverpool and Manchester railroad, when that -work was advanced towards its completion, employed, in the spring -of the year 1829, Messrs. Stephenson and Lock, and Messrs. Walker -and Rastrick, experienced engineers, to visit the different -railways, where practical information respecting the comparative -effects of stationary and locomotive engines was likely to be -obtained; and from these gentlemen they received reports on the -relative merits, according to their judgment of the two methods. -The particulars of their calculations are given at large in the -valuable work of Mr. Nicholas Wood on railways; to which we refer -the reader, not only on this, but on many other subjects connected -with the locomotive steam engine, into which it would be foreign -to our object to enter. The result of the comparison of the two -systems was, that the capital [Pg343] necessary to be advanced to -establish a line of stationary engines was considerably greater -than that which was necessary to establish an equivalent power in -locomotive engines; that the annual expense by the stationary -engines was likewise greater; and that, consequently, the expense -of transport by the latter was greater, in a like proportion. The -subjoined table exhibits the results numerically:-- - - ----------------------------------------------------------------- - | | | Expense of - | | | taking a - | | Annual | Ton of Goods - | Capital. | Expense. | a Mile. - ----------------------------------------------------------------- - | £ s. d.| £ s. d.| - Locomotive engines | 58,000 0 0 | 25,517 8 2 | 0·164 penny - Stationary engines | 121,496 7 0 | 42,031 16 5 | 0·269 - |--------------------------------------------- - Locomotive system | | | - less | 63,496 7 0 | 16,514 8 3 | 0·105 - ----------------------------------------------------------------- - -On the score of economy, therefore, the system of locomotive -engines was entitled to a preference; but there were other -considerations which conspired with this to decide the choice of -the Directors in its favour. An accident occurring in any part of -a road worked by stationary engines must necessarily produce a -total suspension of work along the entire line. The most vigilant -and active attention on the part of every workman, however -employed, in every part of the line, would therefore be necessary; -but, independently of this, accidents arising from the fracture or -derangement of any of the chains, or from the suspension of the -working of any of the fixed engines, would be equally injurious, -and would effectually stop the intercourse along the line. On the -other hand, in locomotive engines an accident could only affect -the particular train of carriages drawn by the engine to which the -accident might occur; and even then the difficulty could be -remedied by having a supply of spare engines at convenient -stations along the line. It is true that the _probability_ of -accident is, perhaps, less in the stationary than in the -locomotive system; but the _injurious consequences_, when accident -_does_ happen, are prodigiously greater in the former. "The one -system," says Mr. Walker, "is like a chain extending from -Liverpool to Manchester, the failure [Pg344] of a single link of -which would destroy the whole; while the other is like a number of -short and unconnected chains," the destruction of any one of which -does not interfere with the effect of the others, and the loss of -which may be supplied with facility. - -The decision of the Directors was, therefore, in favour of -locomotive engines; and their next measure was to devise some -means by which the inventive genius of the country might be -stimulated to supply them with the best possible form of engines -for this purpose. With this view, it was proposed and carried into -effect to offer a prize for the best locomotive engine which might -be produced under certain proposed conditions, and to appoint a -time for a public trial of the claims of the candidates. A premium -of five hundred pounds was accordingly offered for the best -locomotive engine to run on the Liverpool and Manchester railway; -under the condition that it should produce no smoke; that the -pressure of the steam should be limited to fifty pounds on the -inch; and that it should draw at least three times its own weight, -at the rate of not less than ten miles an hour; that the engine -should be supported on springs, and should not exceed fifteen feet -in height. Precautions were also proposed against the consequences -of the boiler bursting; and other matters not necessary to mention -more particularly here. This proposal was announced in the spring -of 1829, and the time of trial was appointed in the following -October. The engines which underwent the trial were, the Rocket, -constructed by Mr. Stephenson; the Sanspareil, by Hackworth; and -the Novelty, by Messrs. Braithwaite and Ericson. Of these, the -Rocket obtained the premium. A line of railway was selected for -the trial, on a level piece of road about two miles in length, -near a place called Rainhill, between Liverpool and Manchester; -the distance between the two stations was a mile and a half, and -the engine had to travel this distance backwards and forwards ten -times, which made altogether a journey of thirty miles. The Rocket -performed this journey twice: the first time in 2 hours 14 minutes -and 8 seconds; and the second time in 2 hours 6 minutes and 49 -seconds. Its speed at different parts of the journey varied: its -greatest rate of motion was [Pg345] rather above 29 miles an -hour; and its least, about 11-1/2 miles an hour. The average rate -of the one journey was 13-4/10 miles an hour; and of the other, -14-2/20 miles. This was the only engine which performed the -complete journey proposed, the others having been stopped from -accidents which occurred to them in the experiment. The Sanspareil -performed the distance between the stations eight times, -travelling 22-1/2 miles in 1 hour 37 minutes and 16 seconds. The -greatest velocity to which this engine attained was something less -than 23 miles per hour. The Novelty had only passed twice between -the stations when the joints of the boiler gave way, and put an -end to the experiment. - - -(188.) The great object to be attained in the construction of -these engines was, to combine with sufficient lightness the -greatest possible heating power. The fire necessarily acts on the -water in two ways: first, by its radiant heat; and second, by the -current of heated air which is carried by the draught through the -flues, and finally passes into the chimney. To accomplish this -object, therefore, it is necessary to expose to both these sources -of heat the greatest possible quantity of surface in contact with -the water. These ends were attained by the following admirable -arrangement in the Rocket:-- - -[Illustration: _Fig._ 88.] - -[Illustration: _Fig._ 89.] - -This engine is represented in _fig._ 88. It is supported on four -wheels; the principal part of the weight being thrown on one pair, -which are worked by the engine. The boiler consists of a cylinder -six feet in length, with flat ends; the chimney issues from one -end, and to the other end is attached a square box B, the bottom -of which is furnished with the grate on which the fuel is placed. -This box is composed of two casings of iron, one contained within -the other, having between them a space about three inches in -breadth; the magnitude of the box being three feet in length, two -feet in width, and three feet in depth. The casing which surrounds -the box communicates with the lower part of the boiler by a pipe -marked C; and the same casing at the top of the box communicates -with the upper part of the boiler by another pipe marked D. When -water is admitted into the boiler, therefore, it flows freely, -through the pipe C, into the casing which [Pg346] surrounds the -furnace or fire-box, and fills this casing to the same level as -that which it has in the boiler. When the engine is at work, the -boiler is kept about half filled with water; and, consequently, -the casing surrounding the furnace is completely filled. The steam -which is generated in the water contained in the casing finds its -exit through the pipe D, and escapes into the upper part of the -boiler. A section of the engine, taken at right angles to its -length, is represented at _fig._ 89. Through the lower part of the -boiler pass a number of copper tubes of small size, which -communicate at one end with the fire-box, and at the other with -the chimney, and form a passage for the heated air from the -furnace to the chimney. The ignited fuel spread on the grate at -the bottom of the fire-box disperses its heat by radiation, and -acts in this manner on the whole surface of the casing surrounding -the fire-box; and thus raises the temperature of the thin shell of -water contained in that casing. The chief [Pg347] part of the -water in the casing, being lower in its position than the water in -the boiler, acquires a tendency to ascend when heated, and passes -into the boiler; so that a constant circulation of the heated -water is maintained, and the water in the boiler must necessarily -be kept at nearly the same temperature as the water in the casing. -The air which passes through the burning fuel, and which fills the -fire-box, is carried by the draught through the tubes which extend -through the lower part of the boiler; and as these tubes are -surrounded on every side with the water contained in the boiler, -this air transmits its heat through these tubes to the water. It -finally issues into the chimney, and rises by the draught. The -power of this furnace must necessarily depend on the power of -draught in the chimney; and to increase this, and at the same time -to dispose of the waste steam after it has worked the piston, this -steam is carried off by a pipe L, which passes from the cylinder -to the chimney, and escapes there in a jet which is turned -upwards. By the velocity with which it issues from this jet, and -by its great comparative levity, it produces a strong current -upwards in the chimney, and thus gives force to the draught of the -furnace. In _fig._ 89. the grate-bars are represented at the -bottom of the fire-box at F. There are two cylinders, one of which -works each wheel; one only appearing in the drawing _fig._ 88., -the other being concealed by the engine. The spokes which these -cylinders work are placed at right angles on the wheels; the -wheels being fixed on a common axle, with which they turn. - -In this engine, the surface of water surrounding the fire-box, -exposed to the action of radiant heat, amounted to twenty square -feet, which received heat from the surface of six square feet of -burning fuel on the bars. The surface exposed to the action of the -heated air amounted to 118 square feet. The engine drew after it -another carriage, containing fuel and water; the fuel used was -coke, for the purpose of avoiding the production of smoke. - - -(189.) The Sanspareil of Mr. Hackworth is represented in _fig._ -90.; the horizontal section being exhibited in _fig._ 91. - -[Illustration: _Fig._ 90.] - -[Illustration: _Fig._ 91.] - -The draught of the furnace is produced in the same manner as in -the Rocket, by ejecting the waste steam coming from [Pg348] the -cylinder into the chimney; the boiler, however, differs -considerably from that of the Rocket. A recurved tube passes -through the boiler, somewhat similar to that already described in -the early engine of Messrs. Trevethick and Vivian. In the -horizontal section (_fig._ 91.), D expresses the opening of the -furnace at the end of the boiler, beside the chimney. The -grate-bars appear at A, supporting the burning fuel; and a curved -tube passing through the boiler, and terminating in the chimney, -is expressed at B, the direction [Pg349] of the draught being -indicated by the arrow; C is a section of the chimney. The -cylinders are placed, as in the Rocket, on each side of the -boiler; each working a separate wheel, but acting on spokes placed -at right angles to each other. The tube in which the grate and -flue are placed diminishes in diameter as it approaches the -chimney. At the mouth where the grate was placed, its diameter was -two feet; and it was gradually reduced, so that, at the chimney, -its diameter was only fifteen inches. The grate-bars extended five -feet into the tube. The surface of water exposed to the radiant -heat of the fire was sixteen square feet; and that exposed to the -action of the heated air and flame was about seventy-five square -feet. The magnitude of the grate, or sheet of burning fuel which -radiated heat, was ten square feet. - - -(190.) The Novelty, of Messrs. Braithwaite and Ericson, is -represented in _fig._ 92.; and a section of the generator and -boiler is exhibited in _fig._ 93.; the corresponding parts in the -two figures are marked by the same letters. - -[Illustration: _Fig._ 92.] - -A is the generator or receiver containing the steam which works -the engine; this communicates with a lower generator B, which -extends in a horizontal direction the entire length of the -carriage. Within the generator A is contained the furnace F, which -communicates in a tube C, carried up through the generator, and -terminated at the top by sliding shutters, which exclude the air, -and which are only opened to supply fuel to the grate F. Below the -grate the furnace is not open, as usual, to the atmosphere, but -communicates, [Pg350] by a tube E, with a bellows D; which is -worked by the engine, and which forces a constant stream of air, -by the tube E, through the fuel on F, so as to keep that fuel in -vivid combustion. The heated air contained in the furnace F is -driven on, by the same force, through a small curved tube marked -_e_, which circulates like a worm (as represented in _fig._ 93.) -through the horizontal generator or receiver; and, tapering -gradually, until reduced to very small dimensions, it finally -issues into the chimney G. The air in passing along this tube, -imparts its heat to the water by which the tube is surrounded, and -is brought to a considerably reduced temperature when discharged -into the chimney. The cylinder, which is represented at K, works -one pair of wheels, by means of a bell-crank, the other pair, when -necessary, being connected with them. - -[Illustration: _Fig._ 93.] - -In this engine, the magnitude of the surface of burning fuel on -the grate-bars is less than two square feet; the surface exposed -to radiant heat is nine and a half square feet; and the surface of -water exposed to heated air is about thirty-three square feet. - -The superiority of the Rocket may be attributed chiefly to the -greater quantity of surface of the water which is exposed to the -action of the fire. With a less extent of grate-bars than the -Sanspareil, in the proportion of three to five, it exposes a -greater surface of water to radiant heat, in the proportion of -four to three; and a greater surface of water to heated air, in -the proportion of more than three to two. It was found that -the Rocket, compared with the Sanspareil, consumed fuel, in -the evaporation of a given quantity of water, [Pg351] in the -proportion of eleven to twenty-eight. The suggestion of using the -tubes to conduct through the water the heated air to the chimney -is due to Mr. Booth, treasurer of the Liverpool and Manchester -Railway Company. - - -(191.) The object to be effected in the boilers of these engines -is, to keep a small quantity of water at an excessive temperature, -by means of a small quantity of fuel kept in the most active state -of combustion. To accomplish this, it is necessary, first, so to -shape the boiler, furnace, and flues, that the water shall be in -contact with as extensive a surface as possible, every part of -which is acted on, either immediately, by the heat radiating from -the fire, or mediately, by the air which has passed through the -fire, and which finally rushes into the chimney: and, secondly, -that such a forcible draught should be maintained in the furnace, -that a quantity of heat shall be extricated from the fuel, by -combustion, sufficient to maintain the water at the necessary -temperature, and to produce the steam with sufficient rapidity. To -accomplish these objects, therefore, the chamber containing the -grate should be completely surrounded by water, and should be -below the level of the water in the boiler. The magnitude of the -surface exposed to radiation should be as great as is consistent -with the whole magnitude of the machine. The comparative advantage -which the Rocket possessed in these respects over the other -engines will be evident on inspection. In the next place, it is -necessary that the heat, which is absorbed by the air passing -through the fuel, and keeping it in a state of combustion, should -be transferred to the water before the air escapes into the -chimney. Air being a bad conductor of heat, to accomplish this it -is necessary that the air in the flues should be exposed to as -great an extent of surface in contact with the water as possible. -No contrivance can be less adapted for the attainment of this end -than one or two large tubes traversing the boiler, as in the -earliest locomotive engines: the body of air which passed through -the centre of these tubes had no contact with their surface, and, -consequently, passed into the chimney at nearly the same -temperature as that which it had when it quitted the fire. The -only portion of air which imparted its heat to the water [Pg352] -was that portion which passed next to the surface of the tube. - -Several methods suggest themselves to increase the surface of -water in contact with a given quantity of air passing through it. -This would be accomplished by causing the air to pass between -plates placed near each other, so as to divide the current into -thin strata, having between them strata of water, or it might be -made to pass between tubes differing slightly in diameter, the -water passing through an inner tube, and being also in contact -with the external surface of the outer tube. Such a method would -be similar in principle to the steam-jacket used in Watt's steam -engines, or to the condenser of Cartwright's engine already -described. But, considering the facility of constructing small -tubes, and of placing them in the boiler, that method, perhaps, -is, on the whole, the best in practice; although the shape of a -tube, geometrically considered, is most unfavourable for the -exposure of a fluid contained in it to its surface. The air which -passes from the fire-chamber, being subdivided as it passes -through the boiler by a great number of very small tubes, may be -made to impart all its excess of heat to the water before it -issues into the chimney. This is all which the most refined -contrivance can effect. The Rocket engine was traversed by -twenty-five tubes, each three inches in diameter; and the -principle has since been carried to a much greater extent. - -The abstraction of a great quantity of heat from the air before it -reaches the chimney is attended with one consequence, which, at -first view, would present a difficulty apparently insurmountable; -the chimney would, in fact, lose its power of draught. This -difficulty, however, was removed by using the waste steam, which -had passed from the cylinder after working the engine, for the -purpose of producing a draught. This steam was urged through a jet -presented upwards in the chimney, and driven out with such force -in that direction as to create a sufficient draught to work the -furnace. - -It will be observed that the principle of draught in the Novelty -is totally distinct from this: in that engine the draught is -produced by a bellows worked by the engine. The question, as far -as relates to these two methods, is, whether more power [Pg353] -is lost in supplying the steam through the jet, as in the Rocket, -or in working the bellows, as in the Novelty. The force requisite -to impel the steam through the jet must be exerted by the -returning stroke of the piston, and, consequently, must rob the -working effect to an equivalent amount. On the other hand, the -power requisite to work the bellows in the Novelty must be -subducted from the available power of the engine. The former -method has been hitherto found to be the more effectual and -economical. - -The importance of these details will be understood, when it is -considered that the only limit to the attainment of speed by -locomotive engines is the power to produce, in a given time, a -certain quantity of steam. Each stroke of the piston causes one -revolution of the wheels, and consumes four cylinders full of -steam: consequently, a cylinder of steam corresponds to a certain -number of feet of road travelled over: hence it is that the -production of a rapid and abundant supply of heat, and the -imparting of that heat quickly and effectually to the water, is -the key to the solution of the problem to construct an engine -capable of rapid motion. - -The method of subdividing the flue into tubes was carried much -further by Mr. Stephenson after the construction of the Rocket; -and, indeed, the principle was so obvious, it is only surprising -that, in the first instance, tubes of smaller diameter than three -inches were not used. In engines since constructed, the number of -tubes vary from ninety to one hundred and twenty, the diameter -being reduced to two inches or less; and in some instances tubes -have been introduced, even to the number of one hundred and fifty, -of one and a half inch diameter. In the Meteor, twenty square feet -are exposed to radiation, and one hundred and thirty-nine to the -contact of heated air; in the Arrow, twenty square feet to -radiation, and one hundred and forty-five to the contact of heated -air. The superior economy of fuel gained by this means will be -apparent by inspecting the following table, which exhibits the -consumption of fuel which was requisite to convey a ton weight a -mile in each of four engines, expressing also the rate of the -motion:-- [Pg354] - - ------------------------------------------------------------ - | Average Rate of | Consumption of Coke - Engines. | Speed in Miles per | in Pounds per Ton - | Hour. | per Mile. - ------------------------------------------------------------ - No. 1. Rocket | 14 | 2·41 - 2. Sanspareil | 15 | 2·47 - 3. Phoenix | 12 | 1·42 - 4. Arrow | 12 | 1·25 - ------------------------------------------------------------ - -(192.) Since the period at which this railway was opened for the -actual purposes of transport, the locomotive engines have been in -a state of progressive improvement. Scarcely a month has passed -without suggesting some change in the details, by which fuel might -be economised, the production of steam rendered more rapid, the -wear of the engine rendered slower, the proportionate strength of -the different parts improved, or some other desirable end -obtained. - -Engines constructed in the form of the Rocket, were subject to two -principal defects. The cylinders, being placed outside the engine, -were exposed to the cold of the atmosphere, which produced a waste -of heat more or less considerable by condensation. The points at -which the power of the steam to turn the wheels was applied, being -at the extremities of the axle and on the exterior of the wheel, a -considerable strain was produced, owing to the distance of the -point of application of the power from the centre of resistance. -If it were possible that the impelling power could act in drawing -the train at all times with equal energy on both sides of the -engine, then no injurious strain would be produced; but from the -relative position of the points on the opposite wheels to which it -was necessary to attach the connecting rods, it was inevitable -that, at the moment when one of the pistons exerts its full power -in driving the wheel, the other piston must be altogether -inactive. The impelling power, therefore, at alternate moments of -time, acted on opposite wheels, and on each of them at the -greatest possible distance from the centre of the axle. - -[Illustration: _Fig._ 94.] - - -(193.) The next step in the improvement of the machine was made -with a view to remove these two defects. The cylinders were -transferred from the exterior of the engine to the [Pg355] -interior of the casing called the smoke-box, B, _fig._ 94., which -supports the chimney, and which receives the heated air issuing -from the tubes which traverse the boiler. Thus placed, the -cylinders are always maintained as hot as the air which issues -from the flues, and all condensation of steam by their exposure is -prevented. The piston-rods are likewise brought closer together, -and nearer the centre of the engine: the connecting rods, no -longer attached to the wheels, are made to act upon two cranks -constructed upon the axle of the wheels, and placed at right -angles to each other. From the position of these cranks, one would -always be at its dead point when the other is in full action. The -action of the steam upon them would, therefore, be generally -unequal; but this would not produce the same strain as when the -connecting rods are attached to points upon the exterior of the -wheels, owing to the cranks being constructed on the axle at -points so much nearer its centre. By this means it was found that -the working of the machine was more even, and productive of much -less strain, than in the arrangement adopted in the Rocket, and -the earlier engines. On the other hand, a serious disadvantage was -incurred by a double-cranked axle. The weakness necessarily -arising from such a form of axle could only be removed by great -thickness [Pg356] and weight of metal; and even this precaution, -at first, did not prevent their occasional fracture. The forging -of them was, however, subsequently much improved: the cranks, -instead of being formed by bending the metal when softened by -heat, were made by cutting the square of the crank out of the -solid metal; and now it rarely happens that one of these axles -fails. - -The adoption of smaller tubes, and a greater number of them, with -a view more perfectly to extract the heat from the air in passing -to the chimney, rendered a more forcible draft necessary. This was -accomplished by the adoption of a more contracted blast-pipe -leading from the eduction-pipes of the cylinders and presented up -the chimney. A representation of such a blast-pipe, with the two -tubes leading from the cylinders and uniting together near the -point, which is presented up the chimney, is given at _p p_ in -_fig._ 104. The engine thus improved is represented in _fig._ 94. - -A represents the cylindrical boiler, the lower half of which is -traversed by tubes, as described in the Rocket. They are usually -from eighty to one hundred in number, and about an inch and a half -in diameter; the boiler is about seven feet in length; the -fire-chamber is attached to one end of it, at F, as in the Rocket, -and similar in construction: the cylinders are inserted in a -chamber at the other end, immediately under the chimney. The -piston-rods are supported in the horizontal position by guides; -and connecting rods extend from them, under the engine, to the two -cranks placed on the axle of the large wheels. The effects of an -inequality in the road are counteracted by springs, on which the -engine rests; the springs being below the axle of the great -wheels, and above that of the less. The steam is supplied to the -cylinders, and withdrawn, by means of the common sliding valves, -which are worked by an eccentric wheel placed on the axle of the -large wheels of the carriage. The motion is communicated from this -eccentric wheel to the valve by sliding rods. The stand is placed -for the attendant at the end of the engine, next the fire-place F; -and two levers L project from the end which communicate with the -valves by means of rods, by which the engine is governed so as to -reverse the motion. [Pg357] - -The wheels of these engines have been commonly constructed of wood -with strong iron ties, furnished with flanges adapted to the -rails. But Mr. Stephenson afterward substituted, in some -instances, wheels of iron with hollow spokes. The engine draws -after it a tender carriage containing the fuel and water; and, -when carrying a light load, is capable of performing the whole -journey from Liverpool to Manchester without a fresh supply of -water. When a heavy load of merchandise is drawn, it is usual to -take in water at the middle of the trip. - - -(194.) In reviewing all that has been stated, it will be perceived -that the efficiency of the locomotive engines used on this railway -is mainly owing to three circumstances: 1st, The unlimited power -of draft in the furnace, by projecting the waste steam into the -chimney; 2d, The almost unlimited abstraction of heat from the air -passing from the furnace, by arrangement of tubes traversing the -boiler; and, 3d, Keeping the cylinders warm, by immersing them in -the chamber under the chimney. There are many minor details which -might be noticed with approbation, but these constitute the main -features of the improvements. - -The successive introduction of improvements in the engines, some -of which we have mentioned, was accompanied by corresponding -accessions to their practical power, and to the economy of fuel. -In the spring of the year 1832, I made several experiments on the -Manchester Railway, with a view to determine, in the actual state -of the locomotive engines at that time, their powers with respect -to the amount of load and the economy of fuel, from which I select -the following as examples:-- - - - I. - - On Saturday, the 5th of May, the engine called the "Victory" - took 20 waggons of merchandise, weighing gross 92 tons 19 cwt. - 1 qr., together with the tender containing fuel and water, of - the weight of which I have no account, from Liverpool to - Manchester (30 miles), in 1 h. 34 min. 45 sec. The train - stopped to take in water half-way, for 10 minutes, [Pg358] - not included in the above-mentioned time. On the inclined - plane rising 1 in 96, and extending 1-1/2 mile, the engine was - assisted by another engine called the "Samson," and the ascent - was performed in 9 minutes. At starting, the fire-place was - well filled with coke, and the coke supplied to the tender - accurately weighed. On arriving at Manchester, the fire-place - was again filled, and the coke remaining in the tender - weighed. The consumption was found to amount to 929 pounds net - weight, being at the rate of one third of a pound per ton per - mile. - - Speed on the level was 18 miles an hour; on a fall of 4 feet - in a mile, 21-1/2 miles an hour; fall of 6 feet in a mile, - 25-1/2 miles an hour; on the rise over Chatmoss, 8 feet in a - mile, 17-5/8 miles an hour; on level ground sheltered from the - wind, 20 miles an hour. The wind was moderate, but direct - ahead. The working wheels slipped three times on Chatmoss, and - the train was retarded from 2 to 3 minutes. - - The engine, on this occasion, was not examined before or after - the journey, but was presumed to be in good working order. - - - II. - - On Tuesday, the 8th of May, the same engine performed the same - journey, with 20 waggons, weighing gross 90 tons 7 cwt. 2 - qrs., exclusive of the unascertained weight of the tender. The - time of the journey was 1 h. 41 min. The consumption of coke - 1040 lbs. net weight, estimated as before. Rate of speed:-- - - Level 17-5/8 miles per hour. - Fall of 4 feet in a mile 22 - -- 6 22-1/2 - Rise of 8 15 - - On this occasion there was a high wind ahead on the quarter, - and the connecting rod worked hot, owing to having been keyed - too tight. On arriving at Manchester, I caused the cylinders - to be opened, and found that the pistons were [Pg359] so - loose, that the steam blew through the cylinders with great - violence. By this cause, therefore, the machine was robbed of - a part of its power during the journey; and this circumstance - may explain the slight decrease in speed, and increase in the - consumption of fuel, with a lighter load, in this journey, - compared with that performed on the 5th of May. - - The Victory weighs 8 tons 2 cwt., of which 5 tons 4 cwt. rest - on the drawing wheels. The cylinders are 11 inches diameter, - and 16 inches stroke, and the diameter of the drawing wheels - is 5 feet. - - - III. - - On the 29th of May, the engine called the "Samson" (weighing - 10 tons 2 cwt., with 14-inch cylinders, and 16-inch stroke; - wheels 4 feet 6 inches diameter, both pairs being worked by - the engine; steam 50 lbs. pressure, 130 tubes) was attached to - 50 waggons, laden with merchandise; net weight about 150 tons; - gross weight, including waggons, 223 tons 6 cwt. The tender - weighed 7 tons, making a gross load (including the engine) of - 240 tons 8 cwt. The engine with this load travelled from - Liverpool to Manchester (30 miles) in 2 hours and 40 min., - exclusive of delays upon the road for watering, &c.; being at - the rate of nearly 12 miles an hour. The speed varied - according to the inclinations of the road. Upon a level, it - was 12 miles an hour; upon a descent of 6 feet in a mile, it - was 16 miles an hour; upon a rise of 8 feet in a mile, it was - about 9 miles an hour. The weather was calm, the rails very - wet; but the wheels did not slip, even in the slowest speed, - except at starting, the rails being at that place soiled and - greasy with the slime and dirt to which they are always - exposed at the stations. The coke consumed in this journey, - exclusive of what was raised in getting up the steam, was 1762 - lbs., being at the rate of a quarter of a pound per ton per - mile. - - -(195.) The great original cost, and the heavy expense of keeping -the engines used on the railway in repair, have pressed severely -on the resources of the undertaking. One of the best [Pg360] -constructed of the later engines costs originally 1500_l._ and -sometimes more. The original cost, however, is far from being the -principal source of expense: the wear and tear of these machines, -and the occasional fracture of those parts on which the greatest -strain has been laid, have greatly exceeded what the directors had -anticipated. Although this source of expense must be in part -attributed to the engines not having yet attained that state of -perfection, in the proportion and adjustment of their parts, of -which they are susceptible, and to which experience alone can -lead, yet there are some obvious defects which demand attention. - -The heads of the boilers are flat, and formed of iron, similar to -the material of the boilers themselves. The tubes which traverse -the boiler were, until recently, copper, and so inserted into the -flat head or end as to be water-tight. When the boiler was heated, -the tubes were found to expand in a greater degree than the other -parts of the boiler; which frequently caused them either to be -loosened at the extremities, so as to cause leakage, or to bend -from want of room for expansion. The necessity of removing and -refastening the tubes caused, therefore, a constant expense. - -It will be recollected that the fire-place is situated at one end -of the boiler, immediately below the mouths of the tubes: a -powerful draft of air, passing through the fire, carries with it -ashes and cinders, which are driven violently through the tubes, -and especially the lower ones, situated near the fuel. These tubes -are, by this means, subject to rapid wear, the cinders continually -acting upon their interior surface. After a short time it becomes -necessary to replace single tubes, according as they are found to -be worn, by new ones; and it not unfrequently happens, when this -is neglected, that tubes burst. After a certain length of time the -engines require new tubing. This wear of the tubes might possibly -be avoided by constructing the fire-place in a lower position, so -as to be more removed from their mouths; or, still more -effectually, by interposing a casing of metal, which might be -filled with water, between the fire-place and those tubes which -are the most exposed to the cinders and ashes. The unequal -expansion of the tubes [Pg361] and boilers appears to be an -incurable defect, if the present form of the engine be retained. -If the fire-place and chimney could be placed at the same end of -the boiler, so that the tubes might be recurved, the unequal -expansion would then produce no injurious effect; but it would be -difficult to clean the tubes, if they were exposed, as they are at -present, to the cinders. The next source of expense arises from -the wear of the boiler-heads, which are exposed to the action of -the fire. - -A considerable improvement was subsequently introduced into the -method of tubing, by substituting brass for copper tubes. I am not -aware that the cause of this improvement has been discovered; but -it is certain, whatever be the cause, that brass tubes are subject -to considerably slower wear than copper ones. - - -(196.) The expense of locomotive power having so far exceeded what -was anticipated at the commencement of the undertaking, it was -thought advisable, about the beginning of the year 1834, to -institute an inquiry into the causes which produced the -discrepancy between the estimated and actual expenses, with a view -to the discovery of some practical means by which they could be -reduced. The directors of the company, for this purpose, appointed -a sub-committee of their own body, assisted by Mr. Booth, their -treasurer, to inquire and report respecting the causes of the -amount of this item of their expenditure, and to ascertain whether -any and what measures could be devised for the attainment of -greater economy. A very able and satisfactory report was made by -this committee, or, to speak more correctly, by Mr. Booth. - -It appears that, previous to the establishment of the railway, -Messrs. Walker and Rastrick, engineers, were employed by the -company to visit various places where steam power was applied on -railways, for the purpose of forming an estimate of the probable -comparative expense of working the railway by locomotive and by -fixed power. These engineers recommended the adoption of -locomotive power; and their estimate was, that the transport might -be effected at the rate of ·278 of a penny, or very little more -than a farthing per ton per mile. In the year [Pg362] 1833, five -years after this investigation took place, it was found that the -actual cost was ·625 of a penny, or something more than a -halfpenny, per ton per mile, being considerably above double the -estimated rate. Mr. Booth very properly directed his inquiries to -ascertain the cause of this discrepancy, by comparing the various -circumstances assumed by Messrs. Walker and Rastrick, in making -their estimate, with those under which the transport was actually -effected. The first point of difference which he observed was the -_speed_ of transport: the estimate was founded on an assumed speed -of ten miles an hour, and it was stated that a four-fold speed -would require an addition of 50 per cent. to the power, without -taking into account wear and tear. Now, the actual speed of -transport being double the speed assumed in the statement, Mr. -Booth holds it to be necessary to add 25 per cent. on that score. - -The next point of difference is in the amount of the loads: the -estimate is founded upon the assumption, that every engine shall -start with its full complement of load, and that with this it -shall go the whole distance. "The facts, however, are," says Mr. -Booth, "that, instead of a _full load_ of profitable carriage -_from_ Manchester, about half the waggons _come back empty_; and, -instead of the tonnage being conveyed the whole way, many thousand -tons are conveyed only half the way; also, instead of the daily -work being uniform, it is extremely fluctuating." It is further -remarked, that in order to accomplish the transport of goods from -the branches and from intermediate places, engines are despatched -several times a-day, from both ends of the line, _to clear the -road_; the object of this arrangement being rather to lay the -foundation of a beneficial intercourse in future, than with a view -to any immediate profit. Mr. Booth makes a rough estimate of the -disadvantages arising from these circumstances, by stating them at -33 per cent. in addition to the original estimate. - -The next point of difference is the fuel. In the original -estimate, _coal_ is assumed as the fuel, and it is taken at the -price of five shillings and ten-pence per ton: now the act of -parliament forbids the use of coal which would produce smoke; the -company have, therefore, been obliged to use _coke_, at [Pg363] -seventeen shillings and sixpence a ton.[32] Taking coke, then, to -be equivalent to coal, ton for ton, this would add ·162 to the -original estimate. - -These several discrepancies being allowed for, and a proportional -amount being added to the original estimate, the amount would be -raised to ·601 of a penny per ton per mile, which is within one -fortieth of a penny of the actual cost. This difference is -considered to be sufficiently accounted for by the wear and tear -produced by the very rapid motion, more especially when it is -considered that many of the engines were constructed before the -engineer was aware of the great speed that would be required. - -"What, then," says Mr. Booth, in the Report already alluded to, "is -the result of these opposite and mutually counteracting -circumstances? and what is the present position of the company in -respect of their moving power? Simply, that they are still in a -course of experiment, to ascertain practically the best -construction, and the most durable materials, for engines required -to transport greater weights, and at greater velocities, than had, -till very recently, been considered possible; and which, a few years -ago, it had not entered into the imagination of the most daring and -sanguine inventor to conceive: and farther, that these experiments -have necessarily been made, not with the calm deliberation and quiet -pace which a salutary caution recommends,--making good each step in -the progress of discovery before advancing another stage,--but -amidst the bustle and responsibilities of a large and increasing -traffic; the directors being altogether ignorant of the time each -engine would last before it would be laid up as inefficient, but -compelled to have engines, whether good or bad; being aware of -various defects and imperfections, which it was impossible at the -time to remedy, yet obliged to keep the machines in motion, under -all the disadvantages of heavy repairs, constantly going on during -the night, in order that the requisite number of engines might be -ready for the morning's work. Neither is this great experiment yet -complete; it is still going forward. But the most prominent -difficulties have been in a great measure surmounted, [Pg364] and -your committee conceive that they are warranted in expecting, that -the expenditure in this department will, ere long, be materially -reduced,--more especially when they consider the relative -performances of the engines at the _present time_, compared with -what it was two years ago." - -In the half year ending 31st December, 1831, the six best engines -performed as follows:-- - - Miles. - Planet 9,986 - Mercury 11,040 - Jupiter 11,618 - Saturn 11,786 - Venus 12,850 - Etna 8,764 - ------ - Making in all 66,044 - ------ - -In the half year ending 31st December, 1833, the six best engines -performed as follows:-- - - Miles. - Jupiter 16,572 - Saturn 18,678 - Sun 14,552 - Etna 17,763 - Ajax 11,678 - Firefly 15,608 - ------ - Making in all 95,851 - ------ - - -(197.) Since the date to which the preceding observations refer, -the locomotive engine has undergone several improvements in detail -of considerable importance; among which, the addition of a third -pair of wheels deserves to be particularly mentioned. An engine -supported on three pair of wheels has great security in the event -of the fracture of any one of the axles,--the remaining axles and -wheels being sufficient for the support of the machine. Connected -with this change is another, recommended by Mr. Robert Stephenson, -by which the flanges are removed from the driving wheels, those -upon the remaining pairs of wheels being sufficient to keep the -engine in its position upon the rails. We shall now describe a -locomotive engine similar in construction to those almost [Pg365] -universally used at present on railroads, as well in this kingdom -as in other countries.[33] - -The external appearance of the engine and tender is shown in the -engraving at the head of this chapter. In _fig._ 97. is exhibited -a vertical section of the engine made by a plane carried through -its length; and in _fig._ 98. is exhibited a corresponding section -of its tender,--the tender being supposed to be joined on to the -engine at the part where the connecting points appear to be broken -in the drawing. In _fig._ 99. is exhibited the plan of the working -machinery, including the cylinders, pistons, eccentrics, &c. which -are under the boiler, by the operation of which the engine is -driven. _Fig._ 100. represents the tender, also taken in plan. - -In _fig._ 101. is represented an elevation of the hinder end of -the engine next the fire-box; and in _fig._ 102. is represented a -cross vertical section through the fire-box, and at right angles -to the length of the engine, showing the interior of the boiler -above and beside the fire-box, the rivets and bolts connecting the -internal and external fire-boxes, the regulator, steam funnel, and -steam dome. - -In _fig._ 103. is represented an elevation of the front of the -engine next the smoke-box, showing the cylinder covers W, buffers -T, &c.; and in _fig._ 104. is represented a section of the -interior of the smoke-box, made by a vertical plane at right -angles to the engine, showing the tube plate forming the foremost -end of the boiler, the branches S of the steam-pipe leading to the -cylinders, the blast-pipe _p_, the cylinders H, and the chimney G. - -The same letters of reference are placed at corresponding parts in -the different figures. - -The boiler, as has been explained in the engines already -described, is a cylinder placed upon its side, the section of -which is exhibited at A, _fig._ 97. The fire-box consists of two -casings of metal, one within the other. The fire-grate is -represented at D. The tubes by which the products of combustion -are [Pg366] drawn from the fire-box to the smoke-box F are -represented at E. Upon the smoke-box is erected the chimney G. In -the engine from which this drawing has been taken, and which was -used on the London and Birmingham Railway, the boiler is a -cylinder 7-1/2 feet long, and 3-1/2 feet in diameter. It is formed -of wrought-iron plates 5/16 of an inch in thickness, overlapping -each other, and bound together by iron rivets 7/8 of an inch in -diameter and 1-3/4 inch apart. One of these rivets, as it joins -two plates, is represented in _fig._ 95. The boiler is clothed -with a boarding of wood _a_, an inch in thickness, and bound round -by iron hoops screwed together at the bottom. Wood being a slow -conductor of heat, this covering has the effect of keeping the -boiler warm, and checking the condensation of steam which would -otherwise be produced by the rapid motion of the engine through -the cold air. - -[Illustration: _Fig._ 95.] - -[Illustration: _Fig._ 96.] - -The external fire-box, B B, is a casing nearly square in its plan, -being four feet wide outside, and three feet seven and a half -inches long, measured in the direction of the boiler. It is -constructed of wrought-iron plates, similar to those of the -boiler. This box descends about two feet below the boiler, the top -being semi-cylindrical, as seen in _fig._ 102., of a somewhat -greater diameter than the boiler, and concentrical with it. The -front of the fire-box next the end of the boiler has a circular -opening equal in size to the end of the boiler. To the edge of -this opening the boiler is fastened by angle irons, and rivets in -the manner represented in _fig._ 96. These rivets are seen in -section in _fig._ 97. - -The internal fire-box C, _fig._ 97., is similar in shape to the -external, only it is flat at the top, and close every where except -at the bottom. Between it and the external fire-box an open space -of three inches and a half is left all round, and on the side next -the boiler this space is increased to four inches. This internal -fire-box is made of copper plates, 7/16 [Pg367] of an inch in -thickness, every where except next the boiler, where the thickness -is 7/8. - -As the sides and front of the external fire-box, and all the -surfaces bounding the internal fire-box, are flat, their form is -unfavourable for the resistance of pressure. Adequate means are, -therefore, provided for strengthening them. The plates forming the -internal fire-box are bent outwards near the bottom, until they -are brought into contact with those of the external fire-box, to -which they are attached by copper rivets, as represented at _f_ in -_fig._ 97. The plates forming the bounding surfaces of the two -fire-boxes are fastened together by stays represented at _k_ in -_figs._ 97. and 102. These stays, which are of copper, have a -screw cut upon them through their whole length, and holes are made -through the plates of both fire-boxes tapped with corresponding -threads. The copper screws are then passed through them, and -rivets formed on their heads within and without, as seen in _fig._ -102. These screw rivets connect all parts of the plating of the -two fire-boxes which are opposed to each other: they are placed at -about four inches apart over the sides and back of the internal -fire-place and that part of the front which is below the boiler. - -[Illustration: _Fig._ 97. - -LONGITUDINAL VERTICAL SECTION OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 98. - -LONGITUDINAL VERTICAL SECTION OF THE TENDER.] - -[Illustration: _Fig._ 99. - -PLAN OF THE WORKING MACHINERY OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 100. - -PLAN OF THE TENDER.] - -[Illustration: _Fig._ 101. - -ELEVATION OF THE HINDER END OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 102. - -CROSS VERTICAL SECTION OF THE ENGINE THROUGH THE FIRE-BOX.] - -[Illustration: _Fig._ 103. - -ELEVATION OF THE FOREMOST END OF THE ENGINE.] - -[Illustration: _Fig._ 104. - -CROSS VERTICAL SECTION OF ENGINE THROUGH THE SMOKE-BOX.] - -As the top of the internal fire-box cannot be strengthened by -stays of this kind, ribs of wrought-iron, which are seen in their -length at _l_, in _fig._ 97., and of which an end view is seen in -_fig._ 102., are attached by bolts to it. These ribs are hollowed -out, as seen in _fig._ 97., between bolt and bolt, in order to -break their contact with the roof of the fire-box, and allow a -more free passage to the heat through it. If they were in -continuous contact with the fire-box, the metal composing them -would become more highly heated, and would soon wear out, besides -intercepting heat from the water. This part of the fire-box is -subject to rapid wear, unless care be taken that the level of the -water be preserved at its proper height in the boiler. Even when -the boiler is properly filled, the depth of water above the roof -of the fire-box is not considerable, and on the least neglect the -roof may be exposed to the contact of steam, in which case it will -soon be destroyed. - -To prevent accidents arising from this cause, a leaden plug, -[Pg368] represented at _m_, _figs._ 97. and 102., is inserted in -the roof of the internal fire-box. If the water be allowed to -subside, this plug will melt out before the copper is very -injuriously heated, and the steam rushing out at the aperture will -cause the fire to be extinguished. - -Copper fire-boxes are almost universally used; but sometimes, from -the consideration of cheapness, the internal fire-box is -constructed of iron. - -In the plating which forms the back of the external fire-box, an -oval aperture is formed, as represented in the back view of the -engine, _fig._ 101., for the fire-door _g_. The plating of the -internal fire-box around this aperture is bent at right angles to -meet that of the external fire-box, to which it is fastened by a -row of copper rivets. The fire-door is formed of two plates of -wrought-iron, riveted together with a space of nine inches and a -half between them. The air between these plates being an imperfect -conductor of heat, keeps the outer plate of the fire-door at a -moderate temperature. - -In that part of the surface of the internal fire-box which forms -the end of the boiler, holes are made to receive the extremities -of the tubes, by which the air proceeding from the fire is drawn -to the smoke-box at the remote end of the boiler. These tubes are -represented in longitudinal section at E, _fig._ 97., and their -ends are seen in the surface of the internal fire-box in _fig._ -102., and in the remote end of the boiler where they terminate in -the smoke-box in _fig._ 104. These tubes are formed of the best -rolled brass, and their thickness in the engine, to which we now -refer, is 1/13 of an inch. After the brass plating is bent into -the form of a tube, and being overlapped, is properly soldered -together, and the edges smoothed off, the tubes are made perfectly -cylindrical by being drawn through a circular steel die. - -[Illustration: _Fig._ 105.] - -The tube-plates (as those parts of the boiler ends in which the -tubes are inserted are called) are bored with holes in corresponding -positions, truly cylindrical, and corresponding in magnitude to the -tubes, so that the tubes, when passed into them, will be just in -contact with them. The length of the tubes is so regulated, that -when extending from end to end of the boiler, and passing through -the holes, they shall [Pg369] project at each end a little beyond -the holes. The manner of fastening them so as to be water-tight is -as follows:--A steel hoop or ferrule, made slightly conical, a -section of which is exhibited at C. _fig._ 105., the smaller end of -which is a little less than the internal diameter of the tube, but -which increases towards the outer end, is driven in as represented -in the figure. It acts as a wedge, and forces the tube into close -contact with the edges of the hole in the tube-plate. - -When particular tubes in a boiler are worn out, and require to be -replaced, their removal is easily effected. It is only necessary -to cut the steel ferrule on the inside, and to bend it off from -contact with the tube, by which means it can be loosened and -withdrawn, and the tube removed. - -In the engine to which this description refers there were one -hundred and twenty-four tubes, the external diameter of which was -1-5/8 inch. The distance between tube and tube was 3/4 of an inch. -The number of tubes vary in different engines, some having so many -as one hundred and fifty, while the number in some is less than -ninety. The evaporating power of an engine greatly depends on the -proper number and magnitude of its tubes; and the experience which -engineers have had on railways have led them gradually to increase -the number of tubes, and diminish their magnitude. In the Rocket, -already mentioned as having gained the prize on the opening of the -Liverpool and Manchester Railway, the number of tubes was -twenty-four, and their diameter three inches; but in all the -engines subsequently made their number was augmented, and their -diameter diminished. The practical inconvenience which limits the -size of the tubes is their liability to become choked by cinders -and ashes, which get wedged in them when they are too small, and -thereby obstruct the draft, and diminish the evaporating power of -the boiler. The tubes now in use, of about an inch and a [Pg370] -half internal diameter, not only require to be cleared of the -ashes and cinders, which get fastened in them after each journey, -but it is necessary throughout a journey of any length that the -tubes should be picked and cleaned by opening the fire door at -convenient intervals. - -The substitution of brass for copper tubes, which has been already -mentioned as so great an improvement in the construction of -locomotive engines, is ascribed to Mr. Dixon, who suggested them -in 1833, being then the resident engineer of the Liverpool and -Manchester Railway. They are said to last six or eight times as -long as copper tubes of the same dimensions. - -When tubes fail, they are usually destroyed by the pressure of the -water crushing them inwards: the water enters through the rent made -in the tube, and flowing upon the fire extinguishes it. When a -single tube thus fails upon a journey, the engine, notwithstanding -the accident, may generally be made to work to the end of its -journey by plugging the ends of the broken tube with hard wood; the -water in contact with which will prevent the fire from burning it -away. - -Tubes of the dimensions here referred to weigh about sixteen -pounds, and lose from six to seven pounds before they are worn -out. Their cost is about one pound each. - -The tubes act as stays, connecting the ends of the boiler to -strengthen them. Besides these, there are rods of wrought iron -extended from end to end of the boiler above the roof of the -internal fire-place. These rods are represented at _o_ in their -length in _fig._ 97., and an end view of them is seen in _fig._ -102. The smoke-box F, _fig._ 97. 104., containing the cylinders, -steam-pipe, and blast-pipe, is four feet wide, and two feet long. -It is formed of wrought iron plates, half an inch thick on the -side next the boiler, and a quarter of an inch elsewhere. The -plates are riveted in the same manner as those of the fire-box -already described. From the top of the smoke-box, which, like the -fire-box, is semi-cylindrical, as seen in elevation in _fig._ -103., and in section _fig._ 104., rises the chimney G, fifteen -inches diameter, and formed of 1/8 inch iron plates, riveted and -bound round by hoops. It is flanged to the top of the [Pg371] -smoke-box, as represented in _fig._ 104. Near the bottom of the -smoke-box the working cylinders are placed, side by side, in a -horizontal position, with the slide valves upwards. In the top of -the external fire-box a circular aperture is formed fifteen inches -in diameter, and upon this aperture is placed the steam-dome T -(_figs._ 97. 101, 102.) two feet high, and attached around the -circular aperture by a flange and screw secured by nuts. This -steam dome is made of brass 3/8 inch thick. In stationary boilers, -where magnitude is not limited, it has been already explained, -that the space allowed for steam is sufficiently large to secure -the complete separation of the vapour from the spray which is -mixed with it when it issues immediately from the water. In -locomotive boilers sufficient space cannot be allowed for this, -and the separation of the water from the steam is effected by the -arrangement here represented. A funnel-shaped tube _d'_ (_figs._ -97. 102.), with its wide end upwards, rises into the steam-dome, -and reaches nearly to the top of it. This funnel bends towards the -back of the fire-box, and is attached by a flange and screws to -the great steam-pipe S, which traverses the whole length of the -boiler. The steam rising from the boiler fills the steam-dome T, -and descends in the funnel-shaped tube _d'_. The space it has thus -to traverse enables the steam to disengage itself almost -completely from the priming. The wider part of the great -steam-pipe _a_ is flanged and screwed at the hinder end to a -corresponding aperture in the back plate of the fire-box. This -opening is covered by a circular plate, secured by screws, having -a stuffing-box in its centre, of the same kind as is used for the -piston-rods of steam-cylinders. Through this stuffing-box the -spindle _a''_ of the regulator passes, and to its end is attached a -winch _h'_, by which the spindle _a''_ is capable of being turned. -This winch is limited in its play to a quarter of a revolution. -The other end of the spindle _a''_ is attached to a plate _e'_ seen -edgeways in _fig._ 97., and the face of which is seen in _fig._ -102.: this circular plate _e_ is perforated with two apertures -somewhat less than quadrants. That part of the plate, therefore, -which remains not pierced forms two solid pieces somewhat greater -than quadrants. This plate is ground so as to move in steam-tight -[Pg372] contact with a fixed plate under it, which terminates at -the wide end of the conical mouth of the steam-pipe S. This fixed -circular plate is likewise pierced with two nearly quadrantal -apertures, corresponding with those in the movable plate _e'_. -When the movable plate _e'_ is turned round by the winch _h'_, the -apertures in it may be made to correspond with those of the fixed -circular plate on which it moves, in which position the steam-pipe -S communicates with the funnel _d'_ by the two quadrantal -apertures thus open. If, on the other hand, the winch _h'_ be -moved from this position through a quarter revolution, then the -quadrantal openings in the movable plate will be brought over the -solid parts of the fixed plate on which it moves, and these solid -parts being a little more than quadrants, while the openings are a -little less, all communication between the steam-pipe S and the -funnel _d'_ will be stopped, for in this case the quadrantal -openings in the fixed and movable plates respectively will be -stopped by the solid parts of these plates. It will be evident -that as the winch _h'_ of the regulator is moved from the former -position to the latter, in every intermediate position the -aperture communicating between the funnel _d'_ and the steam-pipe -S will be less in magnitude than the complete quadrant. It will in -fact be composed of two openings having the form of _sectors_ of a -circle less than a quadrant, and these sectors may be made of any -magnitude, however small, until the opening is altogether closed. - -By such means the admission of steam from the boiler to the -steam-pipe S may be regulated by the winch _h'_. - -The steam being admitted to the steam-pipe passes through it to -the front end of the boiler, and the pipe being enclosed within -the boiler the temperature of the steam is maintained. The -steam-pipe passing through the tube-plate at the front end of the -boiler is carried to a small distance from the tube-plate in the -same direction, where it is flanged on to a cross horizontal pipe -proceeding to the right and to the left as represented in _fig._ -104. This cross pipe is itself flanged to two curved steam-pipes S -(_fig._ 104.), by which the steam is conducted to the valve-boxes -V V. The lower ends of these curved arms are flanged on to the -valve-boxes of the two cylinders [Pg373] at the ends nearest to -the boiler. The opening of one of these is exhibited in the right -hand cylinder in _fig._ 99. By these pipes the steam is conducted -into the valve-boxes or steam-chests, from which it is admitted by -slide-valves to the cylinders to work the pistons in the same -manner as has been already described in the large stationary -engines. - -On the upper sides of the cylinders are formed the steam-chests or -valve-boxes, which are exhibited at U (_figs._ 97. 99. 104.). -These are made of cast-iron half an inch thick, and are bolted to -the upper side of each cylinder. At the front end they are also -secured by bolts to the smoke-box, and at the hinder end are -attached to the tube-plate. These valve-boxes communicate with the -passages _m_ and _n_ _fig._ 99. leading to the top and bottom of -the cylinder: these are called the steam-ports. They also -communicate with a passage _o_ leading to the mouth of a curved -horizontal pipe _p'_ connecting the front ends of the two -cylinders, as seen in _figs._ 99. 104. These curved pipes unite in -a single vertical pipe _p_, called the _blast-pipe_, seen in -_figs._ 97. 104.: this vertical pipe becomes gradually small -towards the top, and terminates a little above the base of the -funnel or chimney G. In the valve-box is placed the slide-valve -_v_ to which is attached the spindle _l'_. This spindle moves -through a stuffing-box _k'_, and is worked by gearing, which will -be described hereafter. According to the position given to the -slide, a communication may be opened between the steam-chest, or -the waste-port, and either end of the cylinders. Thus when the -slide is in the position represented in _fig._ 97. the steam-chest -communicates with the front end of the cylinder, while the -waste-port communicates with the hinder end. If, on the other -hand, the spindle _l'_ being pressed forward, move the slide to -its extreme opposite position, the steam-port _n_ would -communicate with the waste-port _o_, while the steam-chest would -communicate with the steam-port _m_, steam would, therefore, be -admitted to the hinder end of the cylinder, while the foremost end -would communicate with the waste-port. It will be perceived that -this arrangement is precisely similar to that of the slide-valves -already described (133.). The slide-valve is represented on a -larger scale in _fig._ 106., where A is the hinder steam-port, -[Pg374] B the foremost steam-port, and C the waste-port. The -surfaces D, separating the steam-ports from the waste-ports, are -called the bars: they are planed perfectly smooth, so that the -surfaces F and G of the slide-valve, also planed perfectly smooth, -may move in steam-tight contact with them. These surfaces are kept -in contact by the pressure of the steam in the steam-chest, by -which the slide-valve is always pressed down. In its middle -position, as represented by the dotted lines in the figure, both -the steam-ports are stopped by the slide-valve, so that at that -moment no steam is admitted to either end of the cylinder. On -either side of this intermediate position the slide has an inch -and a half play, which is sufficient to open successively the two -steam-ports. - -[Illustration: _Fig._ 106.] - -The cylinders are inserted at one end in the plate of the -smoke-box, and at the other in the tube-plate of the boiler. They -are closed at either end by cast iron covers, nearly an inch -thick, flanged on by bolts and screws. In the cover of the -cylinder attached to the tube-plate is a stuffing-box, in which -the piston rod plays. The metallic pistons used in locomotive -engines do not differ materially from those already described, and -therefore need not be here particularly noticed. From their -horizontal position they have a tendency to wear unequally in the -cylinders, their weight pressing them on one side only; but from -their small magnitude this effect is found to be imperceptible in -practice. In the engine here described the stroke of the piston is -eighteen inches, and this is the most usual length of stroke in -locomotive engines. The piston, in its play, comes at either end -within about half an inch of the inner surface of the covers of -the cylinders, this space being allowed to prevent collision. In -the foremost cover of the cylinder is inserted a cock _q'_ (_fig._ -97. 99.), by which any water which may collect in the cylinder by -condensation or priming may be discharged. A cock _r'_ (_fig._ -97.), communicating with a small tube proceeding from the branches -of the waste pipe _p'_ (_fig._ 104.), is likewise provided to -discharge from that pipe any water which may be [Pg375] collected -in it. After the steam has been admitted to work the piston -through the slide-valve, and has been discharged through the -waste-port by shifting that valve, it passes through the pipe _p'_ -into the blast-pipe _p_, from the mouth of which it issues, with -great force, up the funnel G. When the motion of the engine is -rapid, the steam from the two cylinders proceeds in an almost -uninterrupted current from the blast-pipe, and causes a strong -draft up the chimney. The heated air which passes from the mouths -of the tubes into the smoke-box is drawn up by this current, and a -corresponding draft is produced in the fire-box. - -[Illustration: _Fig._ 107.] - -The piston-rods Y terminate in a fork, by which they are attached -to cross heads Z, the ends of which are confined by guide-bars A', -in which they are allowed to play backwards and forwards through a -space equal to the stroke of the piston. To these cross heads Z, -between the prongs of the fork in which the piston terminates, are -attached the foremost ends of the connecting rods B'. These rods -are, therefore, driven backwards and forwards by the motion -imparted to the cross head Z by the piston-rods Y. The connecting -rods B' are attached at the hinder ends to two cranks formed upon -the axles C' of the driving wheels D'. These two cranks are formed -upon the axles precisely at right angles to each other. The -left-hand crank is represented in its horizontal position, in -_fig._ 99., and the right-hand crank is seen in its vertical -position. A cranked axle is represented on a larger scale in -_fig._ 107., and the two cranks are seen in a position oblique to -the plane of the figure. As this axle is the instrument by which -the impelling force is conveyed to the load, and as it has to -support a great portion of the weight of the engine, it is -constructed with great strength and precision. It is made all in -one [Pg376] piece, and of the best wrought iron called Back -Barrow, or scrap iron. In the engine here described its extreme -length is six feet and a half, and its diameter is five inches. At -the centre part A it is cylindrical, and is increased to five -inches and a quarter at C, where the cranks are formed. The sides -D of the cranks are four inches thick, and the crank pins B, which -are truly cylindrical, are five inches diameter, and three inches -in length, the brasses at the extremities of the connecting rods -which play upon them having a corresponding magnitude. The -distance from the centre of the crank-pins B to the centre of the -axle A must be exactly equal to half the stroke of the piston, and -is, therefore, in this case precisely nine inches. Upon the parts -F, which are seven inches and a half long, the great driving -wheels are firmly fastened, so as to be prevented from turning or -shaking upon the axle. The axle projects beyond the wheels at G, -where it is reduced to three inches and an eighth diameter. These -projecting parts G are five inches long, having collars at the -outer ends. Brasses are fixed at the outside frame of the engine -which rest upon these projections G of the axle, and upon these -brasses the weight of the engine is supported. The entire axle is -accurately turned in a lathe, and each of the crank-pins B is -likewise turned by suspending the axle on centres corresponding -with the centres of the crank-pins, and made on strong cast iron -arms, which are firmly fixed on the ends of the axle, and project -beyond the cranks so as to balance the axle, and enable it to turn -round on the centre of the crank-pin. The axle is by such means -made perfectly true, and the cranks are made of exactly the proper -length, and precisely at right angles to each other. The corners -of the cranks are champered off, as shown in the figure, and the -ends of the cylindrical parts well rounded out. - -The strength and accuracy of construction indispensable in these -cranked axles, in order to make them execute their work, render -them very expensive. Those which are here described cost about -50_l._ each. When properly constructed, however, they are seldom -broken, but are sometimes bent when the engine escapes from the -rails. - -The proper motion to admit and withdraw the steam from [Pg377] -either end of the cylinder is imparted to the slide-valves by -eccentrics, in a manner and on a principle so similar to that -already described in large stationary engines, that it will not be -necessary here to enter into any detailed explanation of the -apparatus for communicating this motion, which is exhibited in -plan and section in _figs._ 97. 99. The eccentrics are attached to -the cranked axles at E' E''. The eccentric E' imparts motion by a -rod _e''_ to a lever _h''_, formed on an axle extending across the -frame of the engine. This conveys motion to another lever _l''_, -projecting from the same axle. This lever _l''_ is jointed to -horizontal links _m''_, which at the foremost ends are attached to -the spindle _l'_, by which the slide is driven. By these means the -motion received by the eccentric from the great working axle -conveys to the spindle _l'_ an alternate movement backwards and -forwards, and the points at which it is reversed will be regulated -by the position given to the eccentric upon the great axle. The -eccentric is formed in two separate semicircles, and is keyed on -to the great axle, and consequently any position may be given to -it which may be required. The position to be given to the -eccentrics should be such that they shall be at right angles to -their respective cranks, and they should be fixed a quarter of a -revolution behind the cranks so as to move the slides to that -extent in advance of the pistons, since by the position of the -levers _h''_ and _l''_, the motion of the eccentric becomes reversed -before it reaches the valve spindle. - -The performance of the engine is materially affected by the -position of the eccentrics on the working axle. The slide should -begin to uncover the steam-port a little before the commencement -of the stroke of the piston, in order that the steam impelling the -piston should be shut off, and the steam about to impel it in the -contrary direction admitted before the termination of the stroke. -Through this small space the steam, therefore, must act in -opposition to the motion of the piston. This is called the _lead_ -of the slide, and the extent generally given to it is about a -quarter of an inch. This is accomplished by fixing the eccentrics -not precisely at right angles to the respective cranks, but a -little in advance of that position. The introduction of the steam -to [Pg378] the piston before the termination of the stroke has -the effect of bringing it gradually to rest at the end of the -stroke, and thereby diminishing the jerk or shock produced by the -rapid change of motion. In stationary engines, where the -reciprocations of the engine are slow, the necessity for this -provision does not arise; but in locomotive engines in which the -motion of the piston is changed from four to six times in a -second, it becomes necessary. The steam admitted to the piston -before the termination of the stroke acts as a spring-cushion to -assist in changing its motion, and if it were not applied, the -piston could not be kept tight upon the piston-rod. Another -advantage which is produced by allowing some lead to the slide is -that the waste steam which has just impelled the piston begins to -make its escape through the waste-port before the commencement of -the next stroke, so that when the impelling steam begins to -produce the returning stroke, there is less waste steam on the -other side of the piston to resist it. - -When the motion of the engine is very rapid, the resistance of the -waste steam, as it escapes from the blast-pipe to the piston, has -been generally supposed to be very considerable, though we are not -aware of any direct experiments by which its amount has been -ascertained. In the account of the locomotive engine which has -been here described, supplied by Mr. Stephenson for the last -edition of Tredgold on the Steam Engine, he states, that the -average resisting pressure of the waste steam throughout the -stroke is 6 lbs. per square inch, when running at the usual rate -of from 25 to 28 miles an hour, and that at greater velocities -this negative pressure has been found to increase to more than -double that amount. No experiments are, however, cited from which -this inference has been drawn. - -It has been also thought that the pressure of steam upon the -piston in the cylinder, at high velocities, is considerably below -the pressure of steam in the boiler; but this has not been, so far -as we are informed, ascertained by any satisfactory experimental -test. Mr. Stephenson likewise states, that this loss of pressure, -causes the negative pressure or resistance of the waste steam to -amount to [Pg379] from 30 to 40 per cent. of the positive -pressure upon the piston when the engine is running very fast, and -that therefore the power of the engine is diminished nearly one -half. - -But it will be perceived that besides the uncertainty which -attends the estimate of the actual amount of pressure on the -piston compared with the pressure of steam in the boiler, the -inference here drawn does not appear to be compatible with what -has been already proved respecting the mechanical effect of steam. -No change of pressure which may take place between the boiler and -the cylinder can affect the practical efficacy of the steam. As -the steam passes through the engine, whatever change of pressure -it may be subject to, it still remains common steam; and though -its pressure may be diminished, its volume being increased in a -nearly equal proportion, its mechanical effect will remain the -same. The power of the engine, therefore, estimated as it ought to -be, by the whole mechanical effect produced, will not be altered -otherwise than by the effect of the increased resistance produced -by the blast-pipe. What that resistance is, we repeat, has not, so -far as we know, been ascertained by direct experiment, and there -are circumstances attending it which render it probable that, even -at high velocities, it is less in amount than Mr. Stephenson's -estimate. - -The position of the eccentrics which is necessary to make the -pistons drive the engine forward must be directly the reverse of -that which would cause them to drive the engine backwards. To be -able, therefore, to reverse the motion of the engine, it would -only be necessary to be able to reverse the position of the -eccentrics, which may be accomplished by either of two expedients. - -_First_, The eccentrics may be capable of revolving on the great -working axle, and also of sliding upon it through a small space. -Their revolution on the axle may be checked by letting a pin -attached to a collar fastened on the axle fall into a hole on the -side of the eccentric. Such a pin will drive the eccentric round -with the axle, and the position of this pin and the hole will -determine the position of the eccentric with reference to the -crank. At a short distance [Pg380] on the other side of the -eccentric may be a corresponding collar with a pin in the opposite -position. By moving the eccentric longitudinally on the axle, the -former pin may be withdrawn from the hole, and the latter allowed -to fall into the hole on the other side. Proper mechanism may be -provided by which the position of the eccentric may thus be -reversed in reference to the crank, and by such means the motion -of the engine may be reversed. - -_Secondly_, Supposing the eccentrics which drive the engine -forward to be immovably fixed upon the axle, two other eccentrics -may be provided attached to other parts of the same axle, and -having a position exactly the reverse with reference to the -cranks. Proper mechanism may be provided, by which either or both -pairs of eccentrics may be thrown in or out of gear. Such are the -means adopted in the engine which has been already described. The -eccentrics for driving the engine backwards are placed outside the -cranks at F' F''. A hand lever _w''_ _fig._ 101. is provided, by -which the engine man may throw either pair of eccentrics into or -out of gear, so as to make the engine work either backwards or -forwards. - -[Illustration: 108.] - -[Illustration: 109.] - -[Illustration: 110.] - -As all the moving parts of the engine require to be constantly -lubricated with oil to diminish the friction, and keep them cool, -oil-cups for this purpose are fixed upon them. In some engines -these oil-cups are attached separately to all the moving parts: in -others they are placed near each other in a row on the boiler, and -communicate by small tubes with the several parts required to be -lubricated. One of these is requisite for each end of the -connecting rods, for each of the guides of the piston-rods, for -the piston-rod itself, the spindle of the slide-valve, and other -parts. An elevation of one of these oil-cups is shown in _fig._ -108., a vertical section in _fig._ 109., and horizontal plan in -_fig._ 110. The cup A is made of brass with a cover B. This cover -has a piece projecting from it turning upon a pin in a socket C at -the side of the cup A, and square at the end, resting upon a small -spring at the bottom of the socket to hold it either open or shut. -In the bottom of the [Pg381] cup is inserted an iron tube D -extending nearly to the top. This tube projects from the bottom of -the cup, where it is tapped for the purpose of fixing the cup on -the part of the engine which it is intended to lubricate. The hole -into which the cup is screwed communicates with the rubbing -surface, and some cotton thread is passed through the tube dipping -into the oil in the cup at the one end and touching the moving -part at the other. This thread acts as a siphon, and constantly -drops oil on the rubbing surface. - -[Illustration: _Fig._ 111.] - -The tender is a carriage attached behind the engine and close to it, -carrying coke for the supply of the furnace, and water for the -boiler. The coke is contained in the space R'', (_fig._ 98. 100.) -surrounded by a tank I'' containing water to feed the boiler. The -feed for the boiler is conducted from the tank through a pipe -descending downwards and in a curved direction, P'' Q'', _fig._ 98., -and connected with a horizontal pipe K, _fig._ 97. A cock is -provided at P'', by which the supply of water to this pipe may be cut -off at pleasure. Another cock is provided at _t'_, _fig._ 97., where -the curved pipe joins the horizontal pipe by which the quantity of -water supplied to K may be regulated by opening the cock more or -less fully. The handle of this cock rises through the floor of the -engine, so that the engineer may regulate it at discretion. The pipe -K being conducted under the engine, as represented in _fig._ 97., -terminates in a vertical pipe, of greater diameter, containing two -valves, both of which open upwards, and between these valves to this -vertical pipe is attached a force-pump, by which the water is drawn -from the horizontal pipe K into the vertical pipe K', and from the -latter is driven into a delivery-pipe by which it is forced into the -boiler. The details of the interior of this feed-pump are -represented on a larger scale in _fig._ 111. The extremity of the -horizontal pipe K' is represented in section at H, where it is -joined on by a screw to the bottom of the vertical pipe which is -represented in _fig._ 97. at K, and which is here represented in -section. The vertical pipe, represented in _fig._ 97. consists of -several parts screwed together by nuts and bolts passing through -flanges. The lowest piece I is attached by a flange to the piece L: -within these is contained the valve Q resting in a seat made -conical, so that the ball [Pg382] which forms the valve shall rest -in water-tight contact with it. The ball is turned and ground to an -accurate sphere, and whatever position it assumes upon its seat its -contact will be perfect. It is guided in its upward and downward -motion by several vertical bars which confine it, and which are -united at the top, so as to limit the upward motion of the ball. A -screw V' is inserted in the bottom of the piece I, by removing which -access can be obtained to the valve. The piece L is secured to the -short pipe G by nuts and bolts passed through a flange. The pipe G -is cast upon the end of the feed-pump A. On the foremost end of this -feed-pump is constructed a stuffing-box C of the usual form, having -a gland D forced against packing by nuts and screws E. The plunger B -is turned so as to be truly cylindrical, and moves in water-tight -contact through the gland D. The plunger not being in contact with -the inner surface of the pump-barrel A, the latter need not be -ground. The horizontal rod by which the plunger B is driven is -attached at its foremost extremity to an arm which projects from the -rod of the steam-piston, and consequently this plunger is moved -through a space equal to the stroke of the steam-piston. In this -case that space is eighteen inches. The [Pg383] upper end of the -vertical tube G is attached by screws and a flange to a piece P -containing a valve R similar in all respects to the lower valve Q, -and like it opening upwards. A screw V is introduced at the top by -which access may be obtained to this valve. This screw also presses -on the crown of the guides of the valve, so as to hold it down by -regulated pressure. At the side of this upper piece P is inserted a -horizontal tube M connected with the end of the delivery-pipe N. -This latter is continued to the boiler with which it communicates at -the fire-box. When the plunger B is drawn out of the pump-barrel A, -the spherical valve Q being relieved from its downward pressure is -raised, and water passes from the pipe H through the valve Q into -the vertical pipe G; the lower valve Q then closes and stops the -return of the water. The plunger B returning into the pump-barrel A -then forces the water against the upper valve R and drives it -through the delivery-tube N, from which its return is prevented by -the valve R. When the delivery-tube N is filled with water -throughout its whole length, every stroke of the plunger will -evidently drive into the boiler a volume of water equal to the -magnitude of a part of the plunger eighteen inches in length. - -Until within the last few years, locomotive engines were supported -on only four wheels; they are, however, now almost universally -supported on six, the driving wheels being in the middle. To give -greater security to the position of the engine between the rails -it is usual to construct flanges on the tires of all the six -wheels. Mr. Stephenson, however, has been in the practice of -constructing the driving wheels without flanges, and with tires -truly cylindrical, depending on the flanges of the two pairs of -smaller wheels to maintain the engine between the rails. The -wheels of the engine here described are constructed in this -manner. The driving wheels D' are fixed on the cranked axle C', -and are five feet in diameter. The other wheels L' M', the one -being placed immediately behind the smoke-box, and the other -immediately behind the fire-box, are each three feet six inches in -diameter, and have a flange upon their tires, which running on -the [Pg384] inside of each rail keeps the engine between the -rails. Each pair of these small wheels, like the driving-wheels, -is fixed upon their axle. The axles are 3-5/8 inches diameter, and -project beyond the wheels, the projecting part supporting the -frame of the engine and turning in brasses. Upon these brasses -rest springs, which bear the whole weight of the engine. These -springs having nothing between them and the road but the wheels -and axles intercept and equalise the sudden shocks produced by the -rapid motion upon the road. - -When an engine is required for the transport of very heavy loads, -such as those of merchandise, the adhesion of one pair of working -wheels is found to be insufficient, and, in such cases, one of the -two pairs of wheels L' M' is made of the same diameter as the -wheels which are placed upon the working axle, and a bar is -attached to points on the outside of the wheels at equal distances -from their centre, connecting them in such a manner that any force -applied to make one pair of wheels revolve must necessarily impart -the same motion to the other pair. By such means the force of the -steam is made to drive both pairs of wheels, and consequently a -proportionally increased adhesion is obtained. - -The velocity which an engine is capable of imparting to the load -which it draws depends upon the rate at which the pistons are -capable of being moved in the cylinders. By every motion of each -piston backwards and forwards one revolution of the driving wheels -is produced, and by each revolution of the driving wheels, -supposing them not to slip upon the rails, the load is driven -through a distance upon the road equal to their circumference. As -the two cylinders work together, it follows, that a quantity of -steam sufficient to fill four cylinders supplied by the boiler to -the engine will move the train through a distance equal to the -circumference of the driving wheels; and in accomplishing this, -each piston must move twice from end to end of the cylinder; each -cylinder must be twice filled with steam from the boiler; and that -steam must be twice discharged from the cylinder through the -blast-pipe into the chimney. - -[Pg401] If the driving wheels be five feet in diameter their -circumference will be fifteen feet seven inches. To drive a train -with a velocity of thirty miles an hour, it will be necessary that -the engine should be propelled through a space of forty-five feet -per second. To accomplish this with five-feet wheels they must be -therefore made to revolve at the rate of very nearly three -revolutions per second; and as each revolution requires two -motions of the piston in the cylinder, it follows that each piston -must move three times forwards and three times backwards in the -cylinder in a second; that steam must be admitted six times per -second from the steam-chest to each cylinder, and discharged six -times per second from each cylinder into the blast-pipe. The -motion, therefore, of each piston, supposing it to be uniform, -must divide a second into six equal parts, and the puffs of the -blast-pipe in the chimney must divide a second into twelve equal -parts. The motion of the slides and other reciprocating parts of -the machinery must consequently correspond. - -This motion of the reciprocating parts of the machinery being -found to be injurious to it, and to produce very rapid wear, -attempts have been made to remedy the defect, and to obtain -greater speed with an equal or diminished rate of motion of the -piston, by the adoption of driving wheels of greater diameter, and -on several of the great lines of railway the magnitude of the -wheels for the passenger-engines have been increased to five feet -and a half and six feet diameter; but such engines have not been -sufficiently long in use to afford grounds for forming a practical -estimate of their effects. Experiments of a much bolder -description have, however, been tried on one of the great lines of -railway by the adoption of driving wheels of much greater -diameter. In some cases their magnitude has been increased even to -ten feet; but from various experiments to which these engines have -been submitted by myself and others, as well as from the -experience which appears to be obtained from the results of their -ordinary work, it does not appear that any advantages have -attended them, and they have been accordingly for the most part -abandoned. - -The pressure of steam in the boiler is limited by two -safety-valves, [Pg402] represented in _fig._ 97. at N and O. The -valve at N is under the control of the engineer, but the valve at -O is inaccessible to him. The structure of the safety-vale -represented at N is exhibited on a larger scale in _fig._ 112., -which represents its section, and _fig._ 113., which shows a plan -of the valve-seat with the valve removed. The valve A, which is -made of brass, is mitred round the edge at an angle of 45°, and -has a spindle, or stalk B, cast upon it, projecting downwards from -the middle of it. The valve-seat C is also made of brass, and cast -with a flange at the bottom to attach it to the boiler. The mitred -surface of the valve is ground into the valve-seat, so as to rest -in steam-tight contact with it. Across the valve-seat, which is -two and a half inches in diameter, is cast a thin piece D, seen in -plan in _fig._ 113. and in section in _fig._ 112. which extends -from the top to the bottom, and has a longitudinal hole through -it, in which the spindle B of the valve works: by this hole it is -guided when it rises from its seat. A projection E is cast upon -the seat of the valve, in which a standard F is inserted. This -standard is forked at the top, and receives the end of a lever G, -which turns in it upon a centre. A rod H is jointed to this lever -by another pin at three inches from the former, and the lower end -of this rod, ground to a point, presses upon the centre of the -valve A. At the other end of the lever, which is broken off in -_fig._ 112., at a distance of three feet from the centre pin, -inserted in the fork of the pillar F, the rod of a common -spring-balance _w_, _fig._ 101., is attached by a finger-nut _n_. -The bottom of this spring-balance is secured on to the fire-box. -This balance is screwed up by the finger-nut on the valve-lever -until the required pressure on the lever is produced through the -medium of the rod H, this pressure being generally fifty pounds -per square inch above the atmosphere. When the pressure of the -steam in the boiler exceeds this, the valve A is raised from its -seat, and the steam escapes. - -[Illustration: _Fig._ 112.] - -[Illustration: _Fig._ 113.] - -It is evident that the sliding weight by which the pressure -[Pg403] of the safety-valve is sometimes regulated in stationary -engines would not be admissible in a locomotive engine, since the -motion of the engine would constantly jolt it up and down, and -cause the steam to escape. One of the disadvantages attending the -use of the spring-valve is that it cannot be opened to let the -steam escape without increasing its force, so that the steam, when -escaping, must really have a greater pressure than that to which -the valve has been previously adjusted. The longer the lever is, -the greater will be this difference of pressure, inasmuch as a -given elevation of the pin governing the rod H would cause a -proportionally greater motion in that end of the lever attached to -the spring. - -The second safety-valve O is enclosed in a case, so that it is -inaccessible, and its purpose is to limit the power of the -engineer to increase the pressure of steam in the boiler. This -valve is similar in construction to the former, but instead of -being held down by a lever, is pressed upon by several small -elliptical springs placed one above another over the valve, and -held down by a screw which turns in a frame Y, fixed into the -valve-seat. By this screw the pressure on the valve can be -adjusted to any required degree; and if the open safety-valve be -screwed down to a greater pressure, the steam will begin to escape -from this second valve. - -Also in the case where the boiler produces surplus steam faster -than its escape can be effected at the valve N, the pressure will -sometimes be increased until the valve O is opened, and its escape -will take place from both valves. - -The whole weight of the engine bears upon those parts of the six -axles R', _fig._ 99., which project beyond the wheels. Boxes are -formed in which these parts of the axles turn, and through the -medium of which the weight of the engine rests upon them. Over -these boxes are constructed oil or grease cups, by means of which -the axles are constantly lubricated. It is usual to lubricate the -axles of the engine itself with oil: the axles of the tender, and -other coaches and waggons, are lubricated with a mixture of oil -and tallow. In the middle of the box in which the axle turns, and -between the two oil-cups, is cast a socket, in which the end of -the spindle on [Pg404] which the spring presses rests. The -springs are composed of a number of steel-plates, laid, in the -usual manner, one above the other, increasing in length upwards. -In the engine here described, the plates forming the springs of -the driving wheels are thirteen in number, each of which is four -inches in width, and 5/16ths of an inch in thickness. The springs -upon the other wheels are three inches in width. The springs of -the driving wheels are below the axle, while those of the smaller -wheels are above it. - -Buffers D'' are placed behind the tender, which act upon a spring C -(_fig._ 100.), to break the collision, when the waggons or -carriages strike upon the tender, and similar buffers are attached -to all passenger-coaches. Some of these buffers are constructed -with a system of springs similar to C, but more elastic, and -combined in greater number under the framing of the carriage, so -that a considerable play is allowed to them. In some cases the -rods of the buffers are made to act upon strong spiral springs -inserted in the sides of the framing of the carriage. This -arrangement gives greater play to the buffers; and as every coach -in a train has several buffers, the combined effect of these is -such, that a considerable shock given to either end of the train -may be rendered harmless by being spent upon the elasticity of -these several systems of springs. - -In order to give notice of the approach of a train, a -steam-whistle Z', _fig._ 97. 101., is placed immediately above the -fire-box at the back of the engine. This is an apparatus composed -of two small hemispheres of brass, separated one from the other by -a small space. Steam is made to pass through a hollow space -constructed in the lower hemisphere, and escapes from a very -narrow circular opening round the edge of that hemisphere, rushing -up with a force proportionate to its pressure. The edge of the -upper hemisphere presented downwards encounters this steam, and an -effect is produced similar to the action of air in organ pipes. A -shrill whistle is produced, which can be heard at a very -considerable distance, and, differing from all ordinary sounds, it -never fails to give timely notice of the approach of a train. - -The water tank I'', _fig._ 98. 100., which is constructed on the -tender, is formed of wrought-iron plates 1/8 of an inch thick, -[Pg405] riveted at the corners by angle iron already described. -This tank is 9 feet long, 6-3/4 feet wide, and 2-1/4 feet deep. -The top is covered with a board K'', and a raised platform N'' is -constructed behind, divided into three parts, covered with leads, -which open on hinges. The middle lid covers an opening to the tank -by which water is let in: the lids at either side cover boxes in -which are contained the tools necessary to be carried with the -engine. The curved pipe P'', _fig._ 98., leading from the bottom of -the tank to the pipe Q'', is of copper. The pipe Q'', connecting the -latter with the feed-pipe K', _fig._ 99., is sometimes formed of -leather or India-rubber cloth, having a spiral spring on the -inside to prevent it from collapsing. It is necessary that this -pipe Q'' should have a power of yielding to a sufficient degree to -accommodate itself to the inequalities of motion between the -engine and tender. A metal pipe is sometimes used, supplied with a -double ball and socket, and a telescopic joint, having sufficient -play to allow for the lateral and longitudinal inequalities of -motion of the engine and tender. The weight of an engine, such as -that here described, supplied with its proper quantity of water -and fuel, is about 12 tons: the tender, when empty, weighs about -3-1/4 tons; and when filled with water and fuel its weight is 7 -tons. The tank contains 700 gallons of water, and the tender is -capable of carrying about 800 weight of coke. This supply is -sufficient for a trip of from thirty to forty miles with an -ordinary load. - - -(198.) It is not usual to express the power of locomotive engines -in the same manner as that of other engines by the term -horse-power. Indeed, until the actual amount of resistance opposed -to these machines, under the various circumstances in which they -are worked, shall be ascertained with some degree of precision, it -is impossible that their power or efficiency can be estimated with -any tolerable degree of approximation. The quantity of water -evaporated, and passed in steam through the cylinders, supplies a -major limit to the power exerted; but even this necessary element -for the calculation of the efficacy of these machines has not been -ascertained by a sufficiently extensive course of observation and -experiment. Mr. Stephenson states, that the engine which [Pg406] -has been here described is capable of evaporating 77 cubic feet of -water per hour, while the early locomotives could only evaporate -16 cubic feet per hour. This evaporation, however, is inferior to -that which I have ascertained myself to be produced by engines in -regular operation on some of the northern railways. In an -experiment made in July, 1839, with the Hecla engine, I found that -the evaporation in a trip of ninety-five miles, from Liverpool to -Birmingham, was at the rate of 93·2 cubic feet per hour, and in -returning the same distance it was at the rate of 85·7 cubic feet -per hour, giving a mean of 89 cubic feet per hour nearly. The -Hecla weighed 12 tons; and its dimensions and proportions -corresponded very nearly with those of the engine above described. - -In a course of experiments which I made upon the engines then in -use on the Grand Junction Railway in the autumn of 1838 I found -that the ordinary evaporating power of these engines varied from -eighty to eighty-five cubic feet per hour. - -Engines of much greater dimensions, and consequently of greater -evaporating power, are used on the Great Western Railway. In the -autumn of 1838 experiments were made upon these engines by Mr. -Nicholas Wood and myself, when we found that the most powerful -engine on that line, the North Star, drawing a load of 110-1/2 -tons gross, engine and tender inclusive, at 30-1/2 miles an hour, -evaporated 200 cubic feet of water per hour. The same engine -drawing a load of 194-1/2 tons at 18-1/2 miles an hour evaporated -141 cubic feet per hour, and when drawing 45 tons at 38-1/2 miles -an hour evaporated 198 cubic feet of water per hour. - -It has been already shown that a cubic foot of water evaporated -per hour produces a gross amount of mechanical force very little -less than two-horse power, and consequently the gross amount of -mechanical power evolved in these cases by the evaporation of the -locomotive boilers will be very nearly twice as many horse-power -as there are cubic feet of water evaporated per hour. Thus the -evaporation of the Hecla, in the experiments made in July, 1839, -gave a gross power of about one hundred and eighty horses, while -the evaporation of the North Star gave a power of about four -hundred horses. In stationary engines about half the gross -[Pg407] power evolved in the evaporation is allowed for waste, -friction, and other sources of resistance not connected with the -load. What quantity should be allowed for this in locomotive -engines is not yet ascertained, and therefore it is impossible to -state what proportion of the whole evaporation is to be taken as -representing the useful horse-power. - - -(199.) The great uniformity of resistance produced by the traction -of carriages upon a railway is such as to render the application -of steam power to that purpose extremely advantageous. So far as -this resistance depends on mechanical defects, it is probably -rendered as uniform as is practicable, and in proportion to the -quantity of load carried is reduced to as small an amount as it is -likely to attain under any practicable circumstances. Until a -recent period this resistance was ascribed altogether, or nearly -so, to mechanical causes. The inequalities of the road-surface, -the friction of the axles of the wheels in their bearings, and the -various sources of resistance due to the machinery of the engine, -being the principal of these resistances, were for the most part -independent of the speed with which the train was moved; and it -was accordingly assumed in all calculations respecting the power -of locomotive engines that the resistance would be practically the -same whatever might be the speed of the train. It had been well -understood that so far as the atmosphere might offer resistance to -the moving power this would be dependent on the speed, and would -increase in a very high ratio with the speed; but it was -considered that the part of the resistance due to this cause -formed a fraction of the whole amount so insignificant that it -might be fairly disregarded in practice, or considered as a part -of the actual computed resistance taken at an average speed. - -It has been, until a late period, accordingly assumed that the -total amount of resistance to railway trains which the locomotive -engines have had to overcome was about the two hundred and -fiftieth part of the gross weight of the load drawn: some -engineers estimated it at a two hundred and twentieth; others at a -two hundred and fiftieth; others at a three hundred and thirtieth -part of the load; and the two hundred and fiftieth part of the -gross load drawn may perhaps be [Pg408] considered as a mean -between these much varying estimates. What the experiments were, -if any, on which these rough estimates were based, has never -appeared. Each engineer formed his own valuation of this effect, -but none produced the experimental grounds of their opinion. It -has been said that the trains run down the engine, or that the -drawing chains connecting the engine slacken in descending an -inclination of sixteen feet in a mile, or 1/330. Numerous -experiments, however, made by myself, as well as the constant -experience now daily obtained on railways, show that this is a -fallacious opinion, except at velocities so low as are never -practised on railways. - - -(200.) In the autumn of 1838 a course of experiments was commenced -at the suggestion of some of the proprietors of the Great Western -Railway Company, with a view to determine various points connected -with the structure and the working of railways. A part of these -experiments were intended to determine the mean amount of the -resisting force opposed to the moving power, and this part was -conducted by me. After having tried various expedients for -determining the mean amount of resistance to the moving power, I -found that no method gave satisfactory results except one founded on -observing the motion of trains by gravity down steep inclined -planes. When a train of waggons or coaches is placed upon an -inclined plane so steep that it shall descend by its gravity without -any moving power, its motion, when it proceeds from a state of rest, -will be gradually accelerated, and if the resistance to that motion -was, as it has been commonly supposed to be, uniform and independent -of the speed, the descent would be uniformly accelerated: in other -words, the increase of speed would be proportional to the time of -the motion. Whatever velocity the train would gain in the first -minute, it would acquire twice that velocity at the end of the -second minute, three times that velocity at the end of the third -minute, and so on; and this increase of velocity would continue to -follow the same law, however extended the plane might be. That such -would be the law which the descending motion of a train would follow -had always been supposed, up to the time of the experiments now -referred to; and it was even maintained by some that [Pg409] such a -law was in strict conformity with experiments made upon railways and -duly reported. The first experiments instituted by me at the time -just referred to afforded a complete refutation of this doctrine. It -was found that the acceleration was not uniform, but that with every -increase of speed the acceleration was lessened. Thus if a certain -speed were gained by a train in one second when moving at five miles -an hour, a much less speed was gained in one second when moving ten -miles an hour, and a comparatively small speed was gained in the -same time when moving at fifteen miles an hour, and so on. In fact, -the augmentation of the rate of acceleration appeared to diminish in -a very rapid proportion as the speed increased: this suggested to me -the probability that a sufficiently great increase of speed would -destroy all acceleration, and that the train would at length move at -a uniform velocity. In effect, since the moving power which impels a -train down an inclined plane of uniform inclination is that fraction -of the gross weight of the train which acts in the direction of the -plane, this moving power must be necessarily invariable; and as any -acceleration which is produced must arise from the excess of this -moving power over the resistance opposed to the motion of the train, -from whatever causes that resistance may arise, whenever -acceleration ceases, the moving force must necessarily be equal to -the resistance; and therefore, when a train descends an inclined -plane with a uniform velocity, the gross resistance to the motion of -the train must be equal to the gross weight of the train resolved in -the direction of the plane; or, in other words, it must be equal to -that fraction of the whole weight of the train which is expressed by -the inclination of the plane. Thus if it be supposed that the plane -falls at the rate of one foot in one hundred, then the force -impelling the train downwards will be equal to the hundredth part of -the weight of the train. So long as the resistance to the motion of -the train continues to be less than the hundredth part of its -weight, so long will the motion of the train be accelerated; and the -more the hundredth part of the weight exceeds the resistance, the -more rapid will the acceleration be; and the less the hundredth part -of the weight [Pg410] exceeds the resistance, the less rapid will -the acceleration be. If it be true that the amount of resistance -increases with the increase of speed, then a speed may at length be -attained so great that the amount of resistance to the motion of the -train will be equal to the hundredth part of the weight. When that -happens, the moving power of a hundredth part of the weight of the -train being exactly equal to the resistance to the motion, there is -no excess of power to produce acceleration, and therefore the motion -of the train will be uniform. - -Founded on these principles, a vast number of experiments were -made on planes of different inclinations, and with loads of -various magnitudes; and it was found, in general, that when a -train descended an inclined plane, the rate of acceleration -gradually diminished, and at length became uniform; that the -uniform speed thus attained depended on the weight, form, and -magnitude of the train and the inclination of the plane; that the -same train on different inclined planes attained different uniform -speeds--on the steeper planes a greater speed being attained. From -such experiments it followed, contrary to all that had been -previously supposed, that the amount of resistance to railway -trains had a dependence on the speed; that this dependence was of -great practical importance, the resistance being subject to very -considerable variation at different speeds, and that this source -of resistance arises from the atmosphere which the train -encounters. This was rendered obvious by the different amount of -resistance to the motion of a train of coaches and to that of a -train of low waggons of equal weight. - -The former editions of this work having been published before the -discovery which has resulted from these experiments, the average -amount of resistance to railway trains, there stated, and the -conclusions deduced therefrom, were in conformity with what was -then known. It was stated that the resistance to the moving power -was practically independent of the speed, and on level rails was -at the average rate of about seven pounds and a half per ton. This -amount would be equivalent to the gravitation of a load down an -inclined plane falling 1/300, and consequently in ascending such a -plane the moving power would have to encounter twice [Pg411] the -resistance opposed to it on a level. As it was generally assumed -that a locomotive-engine could not advantageously vary its -tractive power beyond this limit, it was therefore inferred that -gradients (as inclinations are called) ought not to be constructed -of greater steepness than 1/300. It was supposed that in -descending gradients more steep than this the train would be -accelerated and would require the use of the brake to check its -motion, while in ascending such planes the engine would be -required to exert more than twice the ordinary tractive power -required on level rails. As the resistance produced by the air was -not taken into consideration, no distinction was made between -heavy trains of goods presenting a frontage and magnitude bearing -a small proportion to their gross weight and lighter trains of -passenger-coaches presenting great frontage and great magnitude in -proportion to their weight. The result of the experiments above -explained leads to inferences altogether at variance with those -which have been given in former editions of the present work, and -which were then universally admitted by railway engineers. The -tendency of the results of these experiments show that low -gradients on railways are not attended with the advantageous -effects which have been hitherto ascribed to them; that, on the -contrary, the resistance produced by steeper gradients can be -compensated by slackening the speed, so that the power shall be -relieved from as much atmospheric resistance by the diminution of -velocity as is equal to the increased resistance produced by the -gravity of the plane which is ascended. And, on the other hand, in -descending the plane the speed may be increased until the -resistance produced by the atmosphere is increased to the same -amount as that by which the train is relieved of resistance by the -declivity down which it moves. Thus, on gradients, the inclination -of which is confined within practical limits, the resistance to -the moving-power may be preserved uniform, or nearly so, by -varying the velocity. - - -(201.) The series of experiments which have established these -general conclusions have not yet been sufficiently extended -and varied to supply a correct practical estimate of the limit -which it would be most advantageous to impose upon the [Pg412] -gradients of railways; but it is certain that railways may be laid -down, without practical disadvantage, with gradients considerably -steeper than those to which it has been hitherto the practice to -recommend as a limit. - -The principle of compensation by varied speed being admitted, it -will follow that the time of transit between terminus and terminus -of a line of railway laid down with gradients, varying from twenty -to thirty feet a mile, will be practically the same as it would be -on a line of the same length constructed upon a dead level; and -not only will the time of transport be equal, but the quantity of -moving power expended will not be materially different. The -difference between the circumstances of the transport in the two -cases will be merely that, on the undulating line, a varying -velocity will be imparted to the train and a varying resistance -opposed to the moving power; while on the level line the train -would be moved at a uniform speed, and the engine worked against a -uniform resistance. These conclusions have been abundantly -confirmed by the experiments made in last July with the Hecla -engine above referred to. The line of railway between Liverpool -and Birmingham on which the experiment was made extended over a -distance of ninety-five miles, and the gradients on which the -effects were observed varied from a level to thirty feet per mile, -a great portion of the line being a dead level. The following -table shows the uniform speed with which the train ascended and -descended the several gradients, and also the mean of the ascent -and descent in each case, as well as the speed upon the level -parts of the line:-- - - -------------------------------------------------- - | Speed. | - |---------------------------------------- - Gradient.| Ascending. | Descending. | Mean. - -------------------------------------------------- - One in |Miles per hour.|Miles per hour.| - 177 | 22·25 | 41·32 | 31·78 - 265 | 24·87 | 39·13 | 32·00 - 330 | 25·26 | 37·07 | 31·16 - 400 | 26·87 | 36·75 | 31·81 - 532 | 27·35 | 34·30 | 30·82 - 590 | 27·37 | 33·16 | 30·21 - 650 | 29·03 | 32·58 | 30·80 - | | |-------- - Level | | | 30·93 - -------------------------------------------------- - -[Pg413] From this table it is apparent that the gradients do -possess the compensating power with respect to speed already -mentioned. The discrepancies existing among the mean values of the -speed are only what may be fairly ascribed to casual variations in -the moving power. The experiment was made under favourable -circumstances: little disturbance was produced from the -atmosphere; the day was quite calm. In the same experiment it was -found that the water evaporated varied very nearly in proportion -to the varying resistance, and the amount of that evaporation may -be taken as affording an approximation to the mean amount of -resistance. Taking the trip to and from Birmingham over the -distance of 190 miles, the mean evaporation per mile was 3·36 -cubic feet of water. The volume of steam produced by this quantity -of water will be determined approximately by calculating the -number of revolutions of the driving wheels necessary to move the -engine one mile. The driving wheels being 5 feet in diameter, -their circumference was 15·7 feet, and consequently in passing -over a mile they would have revolved 336·3 times. Since each -revolution consumes four cylinders full of steam, the quantity of -steam supplied by the boiler to the cylinders per mile will be -found by multiplying the contents of the cylinder by four times -336·3, or 1345·2. - -The cylinders of the Hecla were 12-1/2 inches diameter, and 18 -inches in length, and consequently their contents were 1·28 cubic -feet for each cylinder: this being multiplied by 1345·2 gives -1721·86 or 1722 cubic feet of steam per mile. It appears, -therefore, that supposing the priming either nothing or -insignificant, which was considered to be the case in these -experiments, 3·36 cubic feet of water produced 1722 cubic feet of -steam, of the density worked in the cylinders. The ratio, -therefore, of the volume of this steam to that of the water -producing it, was 1722 to 3·36, or 512·5 to 1. The pressure of -steam of this density would be 54·5 pounds per square inch.[34] -Such, therefore, was the limit of the average total pressure of -the steam in the cylinders. In this experiment the safety-valve of -the boiler was screwed down to 60 pounds per square [Pg414] inch -above the atmospheric pressure, which was therefore the major -limit of the pressure of steam in the boiler; but as the actual -pressure in the boiler must have been less than this amount, the -difference between the pressure in the cylinder and boiler could -not be ascertained. This difference, however, would produce no -effect on the moving power of the steam, since the pressure of -steam in the cylinders obtained by the above calculation is quite -independent of the pressure in the boiler, or of any source of -error except what might arise from priming. The pressure of 54·5 -pounds per square inch, calculated above, being the total pressure -of the steam on the pistons, let 14·5 pounds be deducted from it, -to represent the atmospheric pressure against which the piston -must act, and the remaining 40 pounds per square inch will -represent the whole available force drawing the train and -overcoming all the resistances arising from the machinery of the -engine, including that of the blast-pipe. The magnitude of a -12-1/2 inch piston being 122·7 square inches, the total area of -the two pistons would be 245·2 square inches, and the pressure -upon each of 40 pounds per inch would give a total force of 9816 -on the two pistons. Since this force must act through a space of -three feet, while the train is impelled through a space of 15·7 -feet, it must be reduced in the proportion of 3 to 15·7, to obtain -its effect at the point of contact of the wheels upon the rails: -this will give 1875 pounds as the total force exerted in the -direction of the motion of the train. The gross weight of the -train being 80 tons, including the engine and tender, this would -give a gross moving force along the road of about 23·4 pounds per -ton of the gross load, this force being understood to include all -the resistances due to the engine. This resistance corresponds to -the gravitation of a plane rising at the rate of 1/95, and -therefore it appears that such would be the inclination of the -plane by the gravitation of which the gross resistance would be -doubled, instead of such inclination being about 1/300, as has -been hitherto supposed. - -Since the remarkable and unexpected results of this series of -experiments became known various circumstances were brought to -light, which were before unnoticed, and which [Pg415] abundantly -confirm them. Among these may be mentioned the fact, that in -descending the Madeley plane, on the Grand Junction Railway, which -falls for above three miles at the rate of twenty-nine feet a -mile, the steam can never be entirely cut off. But, on the other -hand, to maintain the necessary speed in descending, the power of -the engine is always necessary. As this plane greatly exceeds that -which would be sufficient to cause the free motion of the train -down it, the power of the engine expended in descending it, -besides all that part of the gravitating power of the plane which -exceeds the resistance due to friction and other mechanical causes -must be worked against the atmosphere. - -This estimate of the resistance is also in conformity with the -results of a variety of experiments made by me with trains of -different magnitudes down inclined planes of various inclinations. - - -(202.) In laying out a line of railway the disposition of the -gradients should be such as to preserve among them as uniform a -character as is practicable, for the weight and power of the -engine must necessarily be regulated by the general steepness of -the gradients. Thus if upon a railway which is generally level, -like that between Liverpool and Manchester, one or two inclined -planes of a very steep character occur, as happens upon that line, -then the engine which is constructed to work upon the general -gradients of the road is unfit to draw the same load up those -inclinations which form an exception to the general character of -the gradients. In such cases some extraordinary means must -generally be provided for surmounting those exceptionable -inclinations. Several expedients have been proposed for this -purpose, among which the following may be mentioned:-- - -1. Upon arriving at the foot of the plane the load is divided, and -the engine carries it up in several successive trips, descending -the plane unloaded after each trip. The objection to this method -is the delay which it occasions--a circumstance which is -incompatible with a large transport of passengers. From what has -been stated, it would be necessary, when the engine is fully -loaded on a level, to divide its load into two or more parts, to -be successively [Pg416] carried up when the incline rises 52 feet -per mile. This method has been practised in the transport of -merchandise occasionally, when heavy loads were carried on the -Liverpool and Manchester line, upon the Rainhill incline. - -2. A subsidiary or assistant locomotive engine may be kept in -constant readiness at the foot of each incline, for the purpose of -aiding the different trains, as they arrive, in ascending. The -objection to this method is the cost of keeping such an engine -with its boiler continually prepared, and its steam up. It is -necessary to keep its fire continually lighted, whether employed -or not; otherwise, when the train would arrive at the foot of the -incline, it should wait until the subsidiary engine was prepared -for work. In cases where trains would start and arrive at stated -times, this objection, however, would have less force. This method -is at present generally adopted on the Liverpool and Manchester -line. - -3. A fixed steam-engine may be erected on the crest of the -incline, so as to communicate by ropes with the train at the foot. -Such an engine would be capable of drawing up one or two trains -together, with their locomotives, according as they would arrive, -and no delay need be occasioned. This method requires that the -fixed engine should be kept constantly prepared for work, and the -steam continually up in the boiler. - -4. In working on the level, the communication between the boiler -and the cylinder in the locomotives may be so restrained by -partially closing the throttle-valve, as to cause the pressure -upon the piston to be less in a considerable degree than the -pressure of steam in the boiler. If under such circumstances a -sufficient pressure upon the piston can be obtained to draw the -load on the level, the throttle-valve may be opened on approaching -the inclined plane, so as to throw on the piston a pressure -increased in the same proportion as the previous pressure in the -boiler was greater than that upon the piston. If the fire be -sufficiently active to keep up the supply of steam in this manner -during the ascent, and if the rise be not greater in proportion -than the power thus obtained, the locomotive will draw the load up -the incline without further assistance. It is, however, to be -observed, that in this case [Pg417] the load upon the engine must -be less than the amount which the adhesion of its working wheels -with the railroad is capable of drawing; for this adhesion must be -adequate to the traction of the same load up the incline, -otherwise, whatever increase of power might be obtained by opening -the throttle-valve, the drawing wheels would revolve without -causing the load to advance. This method has been generally -practised upon the Liverpool and Manchester line in the transport -of passengers; and, indeed, it is the only method yet discovered -which is consistent with the expedition necessary for that species -of traffic. - -In the practice of this method considerable aid may be derived -also by suspending the supply of feeding water to the boiler -during the ascent. It will be recollected that a reservoir of cold -water is placed in the tender which follows the engine, and that -the water is driven from this reservoir into the boiler by a -forcing pump, which is worked by the engine itself. This pump is -so constructed that it will supply as much cold water as is equal -to the evaporation, so as to maintain constantly the same quantity -of water in the boiler. But it is evident, on the other hand, that -the supply of this water has a tendency to check the rate of -evaporation, since in being raised to the temperature of the water -with which it mixes it must absorb a considerable portion of the -heat supplied by the fire. With a view to accelerate the -production of steam, therefore, in ascending the inclines, the -engine man may suspend the action of the forcing pump, and thereby -stop the supply of cold water to the boiler; the evaporation will -go on with increased rapidity, and the exhaustion of water -produced by it will be repaid by the forcing pump on the next -level, or still more effectually on the next descending incline. -Indeed the feeding pump may be made to act in descending an -incline, if necessary, when the action of the engine itself is -suspended, and when the train descends by its own gravity, in -which case it will perform the part of a brake upon the descending -train. - -5. The mechanical connexion between the piston of the cylinder and -the points of contact of the working wheels with the road may be -so altered, upon arriving at the incline, as to [Pg418] give the -piston a greater power over the working wheels. This may be done -in an infinite variety of ways, but hitherto no method has been -suggested sufficiently simple to be applicable in practice; and -even were any means suggested which would accomplish this, unless -the intensity of the impelling power were at the same time -increased, it would necessarily follow that the speed of the -motion would be diminished in exactly the same proportion as the -power of the piston over the working wheels would be increased. -Thus, on the inclined plane, which rises fifty-five feet per mile, -upon the Liverpool line, the speed would be diminished to nearly -one fourth of its amount upon the level. - -[Illustration] - - FOOTNOTES: - - [30] Some of the preceding observations on inland transport, - as well as other parts of the present chapter, appeared in - articles written by me in the _Edinburgh Review_ for October, - 1832, and October, 1834. - - [31] Wood on Railroads, 2d edit. - - [32] The cost of coke has risen considerably since the date of - this report. - - [33] I am indebted to the enlarged edition of Tredgold on the - Steam Engine, published by Mr. Weale, for the drawings of this - engine. The details of the machine are very fully given in - that work, the description of them being supplied by Mr. - Stephenson himself. - - [34] See Table of Pressures, Temperatures, and Volumes, in - appendix. - -[Pg419] - - - - -[Illustration] - -CHAP. XII. - -LOCOMOTIVE ENGINES ON TURNPIKE ROADS. - - RAILWAYS AND STONE ROADS COMPARED. -- MR. GURNEY'S STEAM - ENGINE. -- CONVENIENCE AND SAFETY OF STEAM CARRIAGES. -- - HANCOCK'S STEAM ENGINE. -- OGLE'S STEAM ENGINE. -- TREVETHICK'S - INVENTION. -- DR. CHURCH'S STEAM ENGINE. - - -(203.) We have hitherto confined our observations on steam-power, -as a means of transport by land, to its application on railways. -But modern speculation has not stopped there; various attempts -have been made, and attended with more or less success, to work -steam-carriages on common roads. The mere practicability of this -project had long been regarded as very questionable; but enough -has been done to show that the only doubt which can attend it, is -as to whether it can be profitably resorted to, as a means of -transport, and this question [Pg420] has been materially affected -by the recent extension of railways. In comparing the effect of a -stone road with an iron railway, there are two circumstances which -give great superiority and advantage to the latter: first, the -resistance opposed by a railway to the moving power, no matter -what that moving power may be, is considerably less in proportion -to the load than on a stone road. The average resistance on a good -level stone road, to the motion of carriages drawn at the speed -usually attained by the application of horse-power, may be taken -at about a thirty-sixth part of the load, while the resistance to -a load drawn upon a railway _at the same speed_ probably does not -amount to a tenth part of this resistance. Thus the moving power, -whatever it may be, would produce on a railway ten times the -useful effect which it would produce on a stone road; secondly, -the resistance which is opposed to the moving power on a level -railway is much more uniform than on a stone road, and, -consequently, the moving power is less subjected to jerks and -inequalities. This renders the application of inanimate power more -easy on the railway. Those inequalities of surface which increase -the amount of resistance on stone roads as compared with railways -also produce a jolting motion in the carriage, to counteract -which, the use of springs become necessary. These springs render -the motion of that part of the carriage which rests upon them -different from that part of the carriage which supports them; and -in the application of steam-machinery it becomes necessary so to -connect the moving power with the wheels that the machinery may -have one motion, and the wheels which are put in mechanical -connexion with that machinery, and driven by it, shall have -another motion. This, it is true, is the case with locomotive -engines on railways; but owing to the greater smoothness and -equality of the railway surface the difference between the motion -of the carriage body suspended on springs and that of the wheels -is much less than it would be on a stone road. - -But besides the greater smoothness of railways compared with stone -roads, the latter have another disadvantage, the effects of which -have probably been exaggerated by those who are opposed to this -application of steam-power. One of the [Pg421] laws of adhesion -long since developed by experiment, and established as a principle -of practical science, is that the adhesion is greater between -surfaces of the same than between surfaces of a different kind. Thus -between two metals of the same kind, the adhesion corresponding to -any given pressure is greater than between two metals of different -kinds; between two metals of any sort the adhesion is greater than -between metal and stone, or between metal and wood. Hence, the -wheels of steam-carriages running on a railroad have a greater -adhesion with the road, and therefore offer a greater resistance to -slip round without the advance of the carriage, than wheels would -offer on a turnpike road; for on a railroad the iron tire of the -wheel rests in contact with the iron rail, while on a common road -the iron tire rests in contact with the surface of stone, or -whatever material the road may be composed of. Besides this, the -dust and loose matter which necessarily collect on a common road, -when pressed between the wheels and the solid base of the road, act -somewhat in the manner of rollers, and give the wheels a greater -facility to slip than if the road were swept clean, and the wheels -rested in immediate contact with its hard surface. The truth of this -observation is illustrated on the railroads themselves, where the -adhesion is found to be diminished whenever the rails are covered -with any extraneous matter, such as dust or moist clay. Although the -adhesion of the wheels of a carriage with a common road, however, be -less than those of the wheels of a steam-carriage with a railroad, -yet still the actual adhesion on turnpike roads is greater in amount -than has been generally supposed, and is quite sufficient to propel -carriages drawing after them loads of large amount. - -The relative facility with which carriages are propelled on -railroads and turnpike-roads equally affects any moving power, -whether that of horses or steam engines; and whether loads be -propelled by the one power or the other, the railroad, as compared -with the turnpike-road, will always possess the same proportionate -advantage; and a given amount of power, whether of the one kind or -the other, will always perform a quantity of work less in the same -proportion on a [Pg422] turnpike-road than on a rail-road. But, on -the other hand, the expense of original construction, and of -maintaining the repairs of a rail-road, is to be placed against the -certain facility which it offers to draught. - -In the attempts which have been made to adapt locomotive engines to -turnpike-roads, the projectors have aimed at the accomplishment of -two objects: first, the construction of lighter and smaller engines; -and, secondly, increased power. These ends, it is plain, can only be -attained, with our present knowledge, by the production of steam of -very high temperature and pressure, so that the smallest volume of -steam shall produce the greatest possible mechanical effect. The -methods of propelling the carriage have been in general similar to -that used in the railroad engines, viz. either by cranks placed on -the axles, the wheels being fixed upon the same axles, or by -connecting the piston rods with the spokes of the wheels. In some -carriages, the boiler and moving power, and the body of the carriage -which bears the passengers, are placed on the same wheels. In -others, the engine is placed on a separate carriage, and draws after -it the carriage which transports the passengers, as is always the -case on railways. - -The chief difference between the steam engines used on railways, and -those adapted to propel carriages on turnpike roads, is in the -structure of the boiler. In the latter it is essential that, while -the power remains undiminished, the boiler should be lighter and -smaller. The accomplishment of this has been attempted by various -contrivances for so distributing the water as to expose a -considerable quantity of surface in contact with it to the action of -the fire: spreading it in thin layers on flat plates; inserting it -between plates of iron placed at a small distance asunder, the fire -being admitted between the intermediate plates; dividing it into -small tubes, round which the fire has play; introducing it between -the surfaces of cylinders placed one within another, the fire being -admitted between the alternate cylinders,--have all been resorted to -by different projectors. - - -(204.) First and most prominent in the history of the application of -steam to the propelling of carriages on turnpike roads stands the -name of Mr. Goldsworthy Gurney, a medical [Pg423] gentleman, and -scientific chemist, of Cornwall. In 1822, Mr. Gurney succeeded Dr. -Thompson as lecturer on chemistry at the Surrey Institution; and, in -consequence of the results of some experiments on heat, his -attention was directed to the project of working steam-carriages on -common roads; and he subsequently devoted his exertions in -perfecting a steam-engine capable of attaining the end he had in -view. - -The mistake which so long prevailed in the application of -locomotives on railroads, and which, as we have shown, materially -retarded the progress of that invention, was shared by Mr. Gurney. -Without reducing the question to the test of experiment, he took for -granted, in his first attempts, that the adhesion of the wheels with -the road was too slight to propel the carriage. He was assured, he -says, by eminent engineers, that this was a point settled by actual -experiment. It is strange, however, that a person of his quickness -and sagacity did not inquire after the particulars of these "actual -experiments." So, however, it was; and, taking for granted the -inability of the wheels to propel, he wasted much labour and skill -in the contrivance of levers and propellers, which acted on the -ground in a manner somewhat resembling the feet of horses, to drive -the carriage forward. After various fruitless attempts of this kind, -the experience acquired in the trials to which they gave rise at -last forced the truth upon his notice, and he found that the -adhesion of the wheels was not only sufficient to propel the -carriage heavily laden on level roads, but was capable of causing it -to ascend all the hills which occur on ordinary turnpike-roads. In -this manner it ascended all the hills between London and Barnet, -London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old -Highgate Hill, the last at one point rising one foot in nine. - -[Illustration: _Fig._ 114.] - -[Illustration: _Fig._ 115.] - -The boiler of Mr. Gurney's engine is so constructed, that there is -no part of it in which metal exposed to the action of the fire is -out of contact with water. If it be considered how rapidly the -action of an intense furnace destroys metal when water is not -present to prevent the heat from accumulating, the advantage of -this circumstance will be appreciated. In the boiler of Mr. -Gurney, the grate-bars [Pg424] themselves are tubes filled with -water, and form, in fact, a part of the boiler itself. This boiler -consists of three strong metal cylinders placed in a horizontal -position one above the other. A section, made by a perpendicular -or vertical plane, is represented in _fig._ 114. The ends of the -three cylinders just mentioned are represented at D, H, and I. In -the side of the lowest cylinder D are inserted a row of tubes, a -ground plan of which is represented in _fig._ 115. These tubes, -proceeding from the side of the lowest cylinder D, are inclined -[Pg425] slightly upwards, for a reason which I shall presently -explain. From the nature of the section, only one of these tubes -is visible in _fig._ 114. at C. The other extremities of these -tubes at A are connected with the same number of upright tubes, -one of which is shown at E. The upper extremities G of these -upright tubes are connected with another set of tubes K, equal in -number, proceeding from G, inclining slightly upwards, and -terminating in the second cylinder H. - -[Illustration: _Fig._ 116.] - -An end view of the boiler is exhibited in _fig._ 116., where the -three cylinders are expressed by the same letters. Between the -cylinders D and H there are two tubes of communication B, and two -similar tubes between the cylinders H and I. From the nature of -the section these appear only as a single tube in _fig._ 114. From -the top of the cylinder I proceeds a tube N, by which steam is -conducted to the engine. - -It will be perceived that the space F is enclosed on every side by -a grating of tubes, which have free communication with the -cylinders D and H, which cylinders have also a free communication -with each other by the tubes B. It follows, [Pg426] therefore, -that if water be supplied to the cylinder I, it will descend -through the tubes, and first filling the cylinder D and the tubes -C, will gradually rise in the tubes B and E, will next fill the -tubes K and the cylinder H. The grating of water-pipes C E K forms -the furnace, the pipes C being the fire-bars, and the pipes E and -K being the back and roof of the stove. The fire-door, for the -supply of fuel, appears at M, fig. 116. The flue issuing between -the tubes F is conducted over the tubes K, and the flame and hot -air are carried off through a chimney. That portion of the heat of -the burning fuel, which in other furnaces destroys the bars of the -grate, is here expended in heating the water contained in the -tubes C. The radiant heat of the fire acts upon the tubes K, -forming the roof of the furnace, on the tube E at the back of it, -and partially on the cylinders D and H, and the tubes B. The draft -of hot air and flame passing into the flue at A acts upon the -posterior surfaces of the tubes E, and the upper sides of the -tubes K, and finally passes into the chimney. - -As the water in the tubes C E K is heated, it becomes specifically -lighter than water of a less temperature, and consequently -acquires a tendency to ascend. It passes, therefore, rapidly into -H. Meanwhile the colder portions descend, and the inclined -positions of the tubes C and K give play to this tendency of the -heated water, so that a prodigiously rapid circulation is -produced, when the fire begins to act upon the tubes. When the -water acquires such a temperature that steam is rapidly produced, -steam-bubbles are constantly formed in the tubes surrounding the -fire; and if these remained stationary in the tubes, the action of -the fire would not only decompose the steam, but render the tubes -red hot, the water not passing through them to carry off the heat. -But the inclined position of the tubes, already noticed, -effectually prevents this injurious consequence. A steam-bubble, -which is formed either in the tubes C or K, having a tendency to -ascend proportional to its lightness as compared with water, -necessarily rushes upwards; if in C towards A, and if in K towards -H. But this motion of the steam is also aided by the rapid -circulation of the water which is continually maintained [Pg427] -in the tubes, otherwise it might be possible, notwithstanding the -levity of steam compared with water, that a bubble might remain in -a narrow tube without rising. To bring the matter to the test of -experiment, I have connected two cylinders, such as D and H, by a -system of glass tubes, such as represented at C E K. The rapid and -constant circulation of the water was then made evident: bubbles -of steam were formed in the tubes, it is true; but they passed -with great rapidity into the upper cylinder, and rose to the -surface, so that the glass tubes never acquired a higher -temperature than that of the water which passed through them. - -Every part of the boiler being cylindrical, it has the form which, -mechanically considered, is most favourable to strength, and -which, within given dimensions, contains the greatest quantity of -water. It is also free from the defects arising from unequal -expansion, which are found to be most injurious in tubular -boilers. The tubes C and K can freely expand in the direction of -their length, without being loosened at their joints, and without -straining any part of the apparatus; the tubes E, being short, are -subject to a very slight degree of expansion; and it is obvious -that the long tubes, with which they are connected, will yield to -this without suffering a strain, and without causing any part of -the apparatus to be loosened. - -When water is converted into steam, any foreign matter which may -be combined with it is disengaged, and is deposited on the bottom -of the vessel in which the water is evaporated. All boilers, -therefore, require occasional cleansing, to prevent the crust thus -formed from accumulating; and this operation, for obvious reasons, -is attended with peculiar difficulty in tubular boilers. In the -case before us, the crust of deposited matter would gather and -thicken in the tubes C and K, and if not removed, would at length -choke them. But besides this, it would be attended with a still -worse effect; for, being a bad conductor, it would intercept the -heat in its transit from the fire to the water, and would cause -the metal of the tube to become unduly heated. Mr. Gurney of -course foresaw this inconvenience, and contrived an ingenious -chemical method of removing it, by occasionally injecting [Pg428] -through the tubes such an acid as would combine with the deposit, -and carry it away. This method was effectual; and although its -practical application was found to be attended with difficulty in -the hands of common workmen, Mr. Gurney was persuaded to adhere to -it by the late Dr. Wollaston, until experience proved the -impossibility of getting it effectually performed, under the -circumstances in which boilers are commonly used. Mr. Gurney then -adopted a method of removing the deposit by mechanical means. -Opposite the mouths of the tubes, and on the other side of the -cylinders D and H, are placed a number of holes, which, when the -boiler is in use, are stopped by pieces of metal screwed into -them. When the tubes require to be cleaned, these stoppers are -removed, and an iron scraper is introduced through the holes into -the tubes, which, being passed backwards and forwards, removes the -deposit. - -In these engines the draught through the furnace was produced by -projecting the waste steam up the chimneys as is practised in -railway engines; a method so perfectly effectual, that it is -unlikely to be superseded by any other. The objection which has -been urged against it in locomotive engines, working on -turnpike-roads, is, that the noise which it produces has a -tendency to frighten horses. - -In the engines on the Liverpool road, the steam is allowed to pass -directly from the eduction pipe of the cylinder to the chimney, -and it there escapes in puffs corresponding with the alternate -motion of the pistons, and produces a noise, which, although -attended with no inconvenience on the railroad, would perhaps be -objectionable on turnpike-roads. In the engine used in Mr. -Gurney's steam-carriage, the steam which passes from the cylinders -is conducted to a receptacle, which he calls a blowing box. This -box serves the same purpose as the upper chamber of a smith's -bellows. It receives the steam from the cylinders in alternate -puffs, but lets it escape into the chimney in a continued stream -by a number of small jets. Regular draught is by this means -produced, and no noise is perceived. Another exit for the steam is -also provided, by which the conductor is enabled to increase or -diminish, or to suspend altogether, the draught [Pg429] in the -chimney, so as to adapt the intensity of the fire to the -exigencies of the road. This is a great convenience in practice; -because on some roads a draught is scarcely required, while on -others a powerful blast is indispensable. - -Connected with this blowing box is another apparatus of -considerable practical importance. The pipe through which the -feeding water is conducted from the tank is carried through this -blowing box, within which it is coiled in a spiral form, so that -an extensive thread of the water is exposed to the heat of the -waste steam which has escaped from the cylinders, and which is -enclosed in this blowing box. In passing through this pipe the -feeding water is raised from the ordinary temperature of about 60° -to the temperature of 212°. Fuel is thus economised and weight -diminished; but there is another still greater advantage attending -this process. The feeding water in the worm just mentioned, while -it takes up the heat from the surrounding steam in the blowing -box, condenses a part of the waste steam, which is thence -conducted to the tank, from which the feeding water is pumped. - -When steam is generated so rapidly as is necessarily the case in -locomotive boilers, it rises with great violence in numerous -bubbles from the bottom of the boiler to the surface of the water, -and puts the liquid into a state of foaming turbulence not unlike -the sea in a storm. As the steam rushes from the surface into the -upper part of the boiler, under these circumstances, it carries -with it a spray by which water is scattered in minute subdivision -among the steam, and floats there like the spray which rises from -the base of a cascade. If the steam be conducted immediately to -the cylinder from the boiler in this state, it will carry with it -the water which is thus suspended in it, which will pass through -the cylinder, and finally be driven into the atmosphere upon the -returning stroke of the piston. The hot water thus carried off -possesses none of the mechanical properties of steam, and is -wholly inefficient as a moving power, and is therefore an -extensive source of the waste of heat. In every boiler, some means -should be provided for the separation of the water thus suspended -in the steam, before the steam is conducted to the cylinder. In -ordinary boilers, the large space which [Pg430] remains above the -surface of the water serves this purpose. The steam being there -subject to no agitation or disturbance, the water mechanically -suspended in it descends by its own gravity, and leaves pure steam -in the upper part. In the small tubular boilers, this has been a -matter, however, of greater difficulty. The contracted space in -which the ebullition takes place causes the water to be mixed with -the steam in a greater quantity than could happen in common -boilers; and the want of the same steam-room renders the -separation of the water from the steam a matter of some -difficulty. These inconveniences have been attempted to be -overcome by various contrivances. I have already described the -rapid and regular circulation effected by the arrangement of the -tubes. By this a regularity in the currents is established, which -has a tendency to diminish the mixture of water with the steam. In -addition to this, a method of separation is provided in the vessel -I, which is a strong iron cylinder of some magnitude, placed out -of the immediate influence of the fire. A partial separation of -the steam from the water takes place in the cylinder H; and the -steam with the water mechanically suspended in it, technically -called moist steam, rises into the _separator_ I. Here, being free -from all agitation and currents, and being, in fact, quiescent, -the particles of water fall to the bottom, while the pure steam -remains at the top. This separator, therefore, serves all the -purposes of the steam-room above the surface of the water in the -large plate boilers. The dry steam is thus collected and ready for -the supply of the engine through the tube N, while the water, -which is disengaged from it, is collected at the bottom of the -separator, and is conducted through the tube T to the lowest -vessel D, to be again circulated through the boiler. - -The pistons of the engine work on the axles of the hind wheels of -the carriage which bears the engine, by cranks, as in the -locomotives on the Manchester railway, so that the axle is kept in -a constant state of rotation while the engine is at work. The -wheels placed on this axle are not permanently fixed or keyed upon -it, as in the Manchester locomotives; but they are capable of -turning upon it in the same manner as ordinary carriage wheels. -Immediately within [Pg431] these wheels there are fixed upon the -axles two projecting spokes or levers, which revolve with the -axle, and which take the position of two opposite spokes of the -wheel. These may be occasionally attached to the wheel or detached -from it; so that they are capable of compelling the wheels to turn -with the axle, or leaving the axle free to turn independently of -the wheel, or the wheel independent of the axle, at the pleasure -of the conductor. It is by these levers that the engine is made to -propel either or both of the wheels. If both pairs of spokes are -thrown into connexion with the wheels, the crank shaft or axle -will cause both wheels to turn with it, and in that case the -operation of the carriage is precisely the same as those of the -locomotives already described upon the Liverpool and Manchester -line; but this is rarely found to be necessary, since the adhesion -of one wheel with the road is generally sufficient to propel the -carriage, and consequently only one pair of these fixed levers are -used, and the carriage propelled by only one of the two hind -wheels. The fore wheels of the carriage turn upon a pivot similar -to those of a four-wheeled coach. The position of these wheels is -changed at pleasure by a pinion and circular rack, which is moved -by the conductor, and in this manner the carriage is guided with -precision and facility. - -The force of traction necessary to propel a carriage upon common -roads must vary with the variable quality of the road, and -consequently the propelling power, or the pressure upon the pistons -of the engine, must be susceptible of a corresponding variation; but -a still greater variation becomes necessary from the undulations and -hills which are upon all ordinary roads. This necessary change in -the intensity of the impelling power is obtained by restraining the -steam in the boiler by the throttle-valve, as already described in -the locomotive engines on the railroad. This principle, however, is -carried much further in the present case. The steam in the boiler -maybe at a pressure of from 100 to 200 lbs. on the square inch; -while the steam on the working piston may not exceed 30 or 40 lbs. -on the inch. Thus an immense increase of power is always at the -command of the conductor; so that when a hill is encountered, or a -rough piece of road, [Pg432] he is enabled to lay on power -sufficient to meet the exigency of the occasion. - -The two difficulties which have been always apprehended in the -practical working of steam-carriages upon common roads are, first, -the command of sufficient power for hills and rough pieces of -road; and, secondly, the apprehended insufficiency of the adhesion -of the wheels with the road to propel the carriage. The former of -these difficulties has been met by allowing steam of very great -pressure to be constantly maintained in the boiler with perfect -safety. As to the second, all experiments tend to show that there -is no ground for the supposition that the adhesion of the wheels -is in any case insufficient for the purposes of propulsion. Mr. -Gurney states, that he has succeeded in driving carriages thus -propelled, up considerable hills on the turnpike roads about -London. He made a journey to Barnet with only one wheel attached -to the axle, which was found sufficient to propel the carriage up -all hills upon that road. The same carriage, with only one -propelling wheel, also went to Bath, and surmounted all the hills -between Cranford Bridge and Bath, going and returning. - -A double stroke of the piston produces one revolution of the -propelling wheels, and causes the carriage to move through a space -equal to the circumference of those wheels. It will therefore be -obvious, that the greater the diameter of the wheels, the better -adapted the carriage is for speed; and, on the other hand, wheels -of smaller diameter are better adapted for power. In fact, the -propelling power of an engine on the wheels will be in the inverse -proportion of their diameter. In carriages designed to carry great -weights at a moderate speed, smaller wheels will be used; while in -those intended for the transport of passengers at considerable -velocities, wheels of at least 5 feet diameter are most -advantageous. - - -(205.) Among the numerous popular prejudices to which this new -invention has given rise, one of the most mischievous in its -effects and most glaring in its falsehood, is the notion that -carriages thus propelled are more injurious to roads than -carriages drawn by horses. This error has been successfully -exposed in the evidence taken before the committee of the [Pg433] -House of Commons upon steam carriages. It is there demonstrated, -not only that carriages thus propelled do not wear a turnpike road -more rapidly than those drawn by horses, but that, on the other -hand, the wear by the feet of horses is far more rapid and -destructive than any which could be produced by the wheels of -carriages. Steam carriages admit of having the tires of the wheels -broad, so as to act upon the road more in the manner of rollers, -and thereby to give consistency and firmness to the material of -which the road is composed. The driving wheels being proved not to -slip upon the road, do not produce any effects more injurious than -the ordinary rolling wheels; consequently the wear occasioned by a -steam carriage upon a road, is not more than that produced by a -carriage drawn by horses, of an equivalent weight and the same or -equal tires; but the wear produced by the pounding and digging of -horses' feet in draught is many times greater than that produced -by the wear of any carriage. Those who still have doubts upon this -subject, if there be any such persons, will be fully satisfied by -referring to the evidence which accompanies the report of the -committee of the House of Commons, printed in October, 1831. - -The weight of machinery necessary for steam carriages is sometimes -urged as an objection to their practical utility. Mr. Gurney -states, that, by successive improvements in the details of the -machinery, the weight of his carriages, without losing any of the -propelling power, may be reduced to 35 cwt., exclusive of the -load, and fuel and water: but thinks that it is possible to reduce -the weight still further. - -A steam carriage constructed by Mr. Gurney, weighing 35 cwt., -working for 8 hours, is found, according to his statement, to do -the work of about 30 horses. He calculates that the weight of his -propelling carriage, which would be capable of drawing 18 persons, -would be equal to the weight of 4 horses; and the carriage in -which these persons would be drawn would have the same weight as a -common stage coach capable of carrying the same number of persons. -Thus the weight of the whole--the propelling carriage and the -carriage for passengers taken together--would be the same [Pg434] -with the weight of a common stage coach, with 4 horses inclusive. - -There are two methods of applying locomotives upon common roads to -the transport of passengers or goods; the one is by causing the -locomotive to carry, and the other to draw the load; and different -projectors have adopted the one and the other method. Each is -attended with its advantages and disadvantages. If the same -carriage transport the engine and the load, the weight of the -whole will be less in proportion to the load carried; also a -greater pressure may be produced on the wheels by which the load -is propelled. It is also thought that a greater facility in -turning and guiding the vehicle, greater safety in descending the -hills, and a saving in the original cost, will be obtained. On the -other hand, when the passengers are placed in the same carriage -with the engine, they are necessarily more exposed to the noise of -the machinery and to the heat of the boiler and furnace. The -danger of explosion is so slight, that, perhaps, it scarcely -deserves to be mentioned; but still _the apprehension_ of danger -on the part of the passengers, even though groundless, should not -be disregarded. This apprehension will be obviously removed or -diminished by transferring the passengers into a carriage separate -from the engine; but the greatest advantage of keeping the engine -separate from the passengers is the facility which it affords of -changing one engine for another in case of accident or derangement -on the road, in the same manner as horses are changed at the -different stages: or, if such an accident occur in a place where a -new engine cannot be procured, the load of passengers may be -carried forward by horses, until it is brought to some station -where a locomotive may be obtained. There is also an advantage -arising from the circumstance, that when the engines are under -repair, or in process of cleaning, the carriages for passengers -are not necessarily idle. Thus the same number of carriages for -passengers will not be required when the engine is used to draw as -when it is used to carry. - -In case of a very powerful engine being used to carry great loads, -it would be quite impracticable to place the engine [Pg435] and -loads on four wheels, the pressure being such as no turnpike road -could bear. In this case it would be indispensably necessary to -place a part of the load at least upon separate carriages to be -drawn by the engine. - -In the comparison of carriages propelled by steam with carriages -drawn by horses, there is no respect in which the advantage of the -former is so apparent as the safety afforded to the passenger. -Steam power is under the most perfect control, and a carriage thus -propelled is capable of being guided with the most admirable -precision. It is also capable of being stopped almost suddenly, -whatever be its speed: it is capable of being turned within a -space considerably less than that which would be necessary for -four-horse coaches. In turning sharp corners, there is no danger, -with the most ordinary care on the part of the conductor. On the -other hand, horse power, as is well known, is under very imperfect -control, especially when horses are used adapted to that speed -which at present is generally considered necessary for the -purposes of travelling. "The danger of being run away with and -overturned," says Mr. Farey, in his evidence before the House of -Commons, "is greatly diminished in a steam coach. It is very -difficult to control four such horses as can draw a heavy stage -coach ten miles an hour, in case they are frightened or choose to -run away; and, for such quick travelling, they must be kept in -that state of courage that they are always inclined to run away, -particularly down hill, and at sharp turns in the road. Steam -power has very little corresponding danger, being perfectly -controllable, and capable of having its power reversed, to retard -in going down hill. It must be carelessness that would occasion -the overturning of a steam carriage. The chance of breaking down -has been hitherto considerable, but it will not be more than in -stage coaches when the work is truly proportioned and properly -executed. The risk from explosion of the boiler is the only new -cause of danger, and that I consider not equivalent to the danger -from horses." - -That the risk of accident from explosion is extremely slight, may -be proved by the fact that the railway between Liverpool and -Manchester has now been in operation for about ten [Pg436] years, -and that other railways more extensive in length have been worked -for a considerable time, and that no instance has ever yet -occurred of an accident to passengers from the explosion of a -boiler. Generally these machines, when they fail, are attended -with no other effect than the extinction of the fire, by the water -of the boiler flowing in upon it. I am not aware of more than one -instance, in which a serious accident has been produced by -explosion; and in that instance, the sufferers were only the -engineer and stoker. In the steam-engine of Mr. Gurney, the -carriage is drawn after the engine, as represented in _fig._ 117. - -[Illustration: _Fig._ 117.] - -[Illustration: _Fig._ 118.] - - -(206.) In the boiler to be used in the steam carriage projected by -Mr. Walter Hancock, the subdivision of the water is accomplished -by dividing a case or box by a number of [Pg437] thin plates of -metal, like a galvanic battery, the water being allowed to flow -between every alternate pair of plates, at E, _fig._ 118., and the -intermediate spaces H forming the flue through which the flame and -hot air are propelled. - -In fact, a number of thin plates of water are exposed on both -sides to the most intense action of flame and heated air; so that -steam of a high pressure is produced in great abundance and with -considerable rapidity. The plates forming the boiler are bolted -together by strong iron ties, extending across the boiler, at -right angles to the plates, as represented in the figure. The -distance between the plates is two inches. - -There are ten flat chambers of this kind for water, and -intermediately between them ten flues. Under the flues is the -fire-place, or grate, containing six square feet of fuel in vivid -combustion. The chambers are all filled to about two thirds of -their depth with water, and the other third is left for steam. The -water chambers, throughout the whole series, communicate with each -other both at top and bottom, and are held together by two large -bolts. By releasing these bolts, at any time, the chambers fall -asunder; and by screwing them up they may be all made tight again. -The water is supplied to the boiler by a forcing-pump, and the -steam issues from the centre of one of the flues at the top. - -These boilers are constructed to bear a pressure of 400 or 500 -lbs. on the square inch; but the average pressure of the steam on -the safety valve is from 60 to 100. There are 100 square feet of -surface in contact with the water exposed to the fire. The stages -which such an engine performs are eight miles, at the end of which -a fresh supply of fuel and water are taken in. It requires about -two bushels of coke for each stage. - -The steam carriage of Mr. Hancock differs from that of Mr. Gurney -in this--that in the former the passengers and engine are all -placed on the same carriage. The boiler is placed behind the -carriage; and there is an engine-house between the boiler and the -passengers, the latter being placed in the fore part of the -vehicle; so that all the machinery is behind them. The carriages -are adapted to carry 14 [Pg438] passengers, and weigh, exclusive -of their load, about 3-1/2 tons, the tires of the wheels being -about 3-1/2 inches in breadth. Mr. Hancock states, that the -construction of his boiler is of such a nature, that, even in the -case of bursting, no danger is to be apprehended, nor any other -inconvenience than the stoppage of the carriage. He states that, -while travelling about nine miles an hour, and working with a -pressure of about 100 lbs. on the square inch, loaded with -thirteen passengers, the carriage was suddenly stopped. At first -the cause of the accident was not apparent; but, on opening one of -the cocks of the boiler, it was found that it contained neither -steam nor water. Further examination proved that the boiler had -burst. On unscrewing the bolts, it was found that there were -several large holes in the plates of the water-chamber, through -which the water had flowed on the fire, but neither noise nor -explosion, nor any dangerous consequences, ensued. - - -(207.) Mr. Nathaniel Ogle of Southampton obtained a patent for a -locomotive carriage, and worked it for some time experimentally; -but as his operations do not appear to have been continued, I -suppose he was unsuccessful in fulfilling those conditions, -without which the machine could not be worked with economy and -profit. In his evidence before a committee of the House of -Commons, he has thus described his contrivance:-- - -"The base of the boiler and the summit are composed of cross -pieces, cylindrical within and square without; there are holes -bored through these cross pieces, and inserted through the whole -is an air tube. The inner hole of the lower surface, and the under -hole of the upper surface, are rather larger than the other ones. -Round the air tube is placed a small cylinder, the collar of which -fits round the larger aperture on the inner surface of the lower -frame, and the under surface of the upper frame-work. These are -both drawn together by screws from the top; these cross pieces are -united by connecting pieces, the whole strongly bolted together; -so that we obtain, in one tenth of the space, and with one tenth -of the weight, the same heating surface and power as is now -obtained in other and low-pressure boilers, with incalculably -[Pg439] greater safety. Our present experimental boiler contains -250 superficial feet of heating surface in the space of 3 feet 8 -inches high, 3 feet long, and 2 feet 4 inches broad, and weighs -about 8 cwt. We supply the two cylinders with steam, communicating -by their pistons with a crank axle, to the ends of which either -one or both wheels are affixed as may be required. One wheel is -found to be sufficient, except under very difficult circumstances, -and when the elevation is about one foot in six to impel the -vehicle forward. - -"The cylinders of which the boiler is composed are so small as to -bear a greater pressure than could be produced by the quantity of -fire beneath the boiler; and if any one of these cylinders should -be injured by violence, or any other way, it would become merely a -safety valve to the rest. We never, with the greatest pressure, -burst, rent, or injured our boiler; and it has not once required -cleaning, after having been in use twelve months." - -Dr. Church of Birmingham has obtained a succession of patents for -contrivances connected with a locomotive engine for stone roads; -and a company, consisting of a considerable number of individuals, -possessing sufficient capital, has been formed in Birmingham, for -carrying into effect his designs, and working carriages on his -principle. The present boiler of Dr. Church is formed of copper. -The water is contained between two sheets of copper, united -together by copper nails, in a manner resembling the way in which -the cloth forming the top of a mattress or cushion is united with -the cloth which forms the bottom of it, except that the nails or -pins, which bind the sheets of copper, are much closer together. -The water, in fact, seems to be "quilted" or "padded" in between -two sheets of thin copper. This double sheet of copper is formed -into an oblong rectangular box, the interior of which is the -fire-place and ash-pit, and over the end of which is the -steam-chest. The great extent of surface exposed to the immediate -action of the fire causes steam to be produced with great -rapidity. - -Various other projects for the application of steam engines on -common roads were in a state of progressive improvement, [Pg440] -when the greater advantages attending railways were considered so -manifest, that considerable doubts were raised, whether, supposing -the problem of the application of the steam engine on common roads -to be successfully solved, it could ever be attended with the same -economy and effect, as by the adoption of a railway. Among the -projects which promised a successful issue, may be mentioned the -locomotive engines contrived by Messrs. Maudslay and Field, by -Colonel Maceroni, and by Mr. Scott Russell. These and others have, -however, been abandoned, mainly, we believe, from the impression, -that wherever traffic can exist, sufficiently extensive to render -the application of steam power profitable, a railway must always -supersede a common road; and that, even in the limited traffic to -be expected on branches to the great railways, horse power applied -to railways would be attended with more economy than steam power -applied on stone roads. - -[Illustration] - -[Pg441] - - - - -[Illustration] - -CHAP. XIII. - -STEAM NAVIGATION. - - FORM AND ARRANGEMENT OF MARINE ENGINES. -- EFFECTS OF SEA WATER - IN BOILERS. -- REMEDIES FOR THEM. -- BLOWING OUT. -- INDICATORS - OF SALTNESS. -- SEAWARD'S INDICATOR. -- HIS METHOD OF BLOWING - OUT. -- FIELD'S BRINE PUMPS. -- TUBULAR CONDENSERS APPLIED BY - MR. WATT. -- HALL'S CONDENSERS. -- COPPER BOILERS. -- PROCESS OF - STOKING. -- MARINE BOILERS. -- MEANS OF ECONOMISING FUEL. -- - COATING MARINE BOILERS WITH FELT. -- NUMBER AND ARRANGEMENT OF - FURNACES AND FLUES. -- HOWARD'S ENGINE. -- APPLICATION OF THE - EXPANSIVE PRINCIPLE IN MARINE ENGINES. -- RECENT IMPROVEMENTS - OF MESSRS. MAUDSLAY AND FIELD. -- HUMPHRYS' ENGINE. -- COMMON - PADDLE-WHEEL. -- FEATHERING PADDLES. -- MORGAN'S WHEELS. -- THE - SPLIT PADDLE. -- PROPORTION OF POWER TO TONNAGE. -- IMPROVED - EFFICIENCY OF MARINE ENGINES. -- IRON STEAM-VESSELS. -- - STEAM-NAVIGATION TO INDIA. - - -(208.) Among the many ways in which the steam-engine has -ministered to the advancement of civilisation and the social -progress of the human race, there is none more [Pg442] important -or more interesting than its application to navigation. Before it -lent its giant powers to the propulsion of ships, locomotion over -the waters of the deep was attended with so much danger and -uncertainty that, as a common proverb, it became the type and the -representative of every thing which was precarious and perilous. -The application, however, of steam to navigation has rescued the -mariner and the voyager from many of the dangers of wind and -water; and even in its present state, putting out of view its -probable improvement, it has rendered all voyages of moderate -length as safe, and very nearly as regular, as journeys over-land. -As a means of transport by sea, the application of this power may -be considered as established; and it is now receiving improvements -by which its extension to the longest class of ocean voyages is a -question not of practicability, but merely of profit. - -The manner in which the steam-engine is rendered an instrument for -the propulsion of vessels must in its general features be so -familiar to every one as to require but short explanation. A shaft -is carried across the vessel, being continued on either side -beyond the timbers: to the extremities of this shaft, on the -outside of the vessel, are fixed a pair of wheels constructed like -undershot water-wheels, having attached to their rims a number of -flat boards called _paddle-boards_. As the wheels revolve, these -paddle-boards strike the water, driving it in a direction contrary -to that in which it is intended the vessel should be propelled. -The moving force imparted to the water thus driven backwards is -necessarily accompanied by a re-action upon the vessel through the -medium of the paddle-shaft, by which the vessel is propelled -forwards. On the paddle-shaft two cranks are constructed, similar -to the cranks already described on the axle of the driving wheels -of a locomotive engine. These cranks are placed at right angles to -each other, so that when either is in its highest or lowest -position the other shall be horizontal. They are driven by two -steam-engines, which are placed in the hull of the vessel below -the paddle-shaft. In the earlier steam-boats a single steam-engine -was used, and in that case the unequal action of the engine on the -crank was equalised by a fly-wheel. This, however, has been long -[Pg443] since abandoned in European vessels, and the use of two -engines is now almost universal. By the relative position of the -cranks it will be seen, that when either crank is at its dead -points, the other will be in the positions most favourable to its -action, and in all intermediate positions the relative efficiency -of the cranks will be such as to render their combined action very -nearly uniform. - -The steam-engines used to impel vessels may be either condensing -engines, similar to those of Watt, and such as are used in -manufactures generally, or they may be non-condensing and -high-pressure engines, similar in principle to those used on -railways. Low-pressure condensing engines are, however, universally -used for marine purposes in Europe and to some extent in the United -States. In the latter country, however, high-pressure engines are -also in pretty general use, on rivers where lightness is a matter of -importance. - -The arrangement of the parts of a marine engine differs in some -respects from that of a land engine. The limitation of space, -which is unavoidable in a vessel, renders greater compactness -necessary. The paddle-shaft on which the cranks to be driven by -the engine are constructed being very little below the deck of the -vessel, the beam and connecting rod could not be placed in the -position in which they usually are in land engines, without -carrying the machinery to a considerable elevation above the deck. -This is done in the steam-boat engines used on the American -rivers; but it would be inadmissible in steam-boats in general, -and more especially in sea-going steamers. The connecting rods, -therefore, instead of being presented downwards towards the cranks -which they drive, must, in steam-vessels, be presented upwards, -and the impelling force received from below. If, under these -circumstances, the beam were in the usual position above the -cylinder and piston-rod, it must necessarily be placed between the -engine and the paddle-shaft. This would require a depth for the -machinery which would be incompatible with the magnitude of the -vessel. The beam, therefore, of marine engines, instead of being -above the cylinder and piston, is placed below them. To the top of -the [Pg445] piston-rods cross pieces are attached of greater -length than the diameter of the cylinders, so that their -extremities shall project beyond the cylinders. To the ends of -these cross pieces are attached by joints the rods of a parallel -motion: these rods are carried downwards, and are connected with -the ends of two beams below the cylinder, and placed on either -side of it. The opposite ends of these beams are connected by -another cross piece, to which is attached a connecting rod, which -is continued upwards to the crank-pin, to which it is attached, -and which it drives. Thus the beam, parallel motion, and -connecting rod of a marine engine, is similar to that of a land -engine, only that it is turned upside down; and in consequence of -the impossibility of placing the beam directly over the -piston-rod, two beams and two systems of parallel motion are -provided, one on each side of the engine, acted upon by, and -acting on the piston-rod and crank by cross pieces. - -The proportion of the cylinders differs from that usually observed -in land engines, for like reasons. The length of the cylinder of -land engines is generally greater than its diameter, in the -proportion of about two to one. The cylinders of marine engines -are, however, commonly constructed with a diameter very little -less than their length. In proportion, therefore, to their power -their stroke is shorter, which infers a corresponding shortness of -crank and a greater limitation of play of all the moving parts in -the vertical direction. The valves and the gearing by which they -are worked, the air-pump, the condenser, and other parts of the -marine engines, do not materially differ from those already -described in land engines. - -[Illustration: _Fig._ 119.] - -These arrangements of a marine engine will be more clearly -understood by reference to _fig._ 119.[35], in which is -represented a longitudinal section of a marine engine with its -boiler as placed in a steam-vessel. The sleepers of oak, -supporting the engine, are represented at X, the base of the -engine being secured to these by bolts passing through them -[Pg446] and the bottom timbers of the vessel; S is the steam-pipe -leading from the steam-chest in the boiler to the slides _c_, by -which it is admitted to the top and bottom of the cylinder. The -condenser is represented at B, and the air-pump at E. The hot well -is seen at F, from which the feed is taken for the boiler; L is -the piston-rod connected by the parallel motion _a_ with the beam -H, working on a centre K, near the base of the engine. The other -end of the beam I drives the connecting rod M, which extends -upwards to the crank which it works upon the paddle-shaft O. Q R -is the framing by which the engine is supported. The beam here -exhibited is shown on dotted lines as being on the further side of -the engine. A similar beam similarly placed, and moving on the -same axis, must be understood to be at this side connected with -the cross head of the piston in like manner by a parallel motion, -and with a cross piece attached to the lower end of the connecting -rod and to the opposite beam. The eccentric which works the slides -is placed upon the paddle shaft O, and the connecting arm which -drives the slides may be easily detached when the engine requires -to be stopped. The section of the boiler, grate, and flues, is -represented at W U. The safety-valve _y_ is enclosed beneath a -pipe carried up beside the chimney, and is inaccessible to the -engine-man; _h_ are the cocks for blowing the salted water from -the boiler; and I I the feed-pipe. - -The general arrangement of the engine-room of a steam-vessel is -represented in _fig._ 120. - -The nature of the effect required to be produced by marine engines -does not render either necessary or possible that great regularity -of action which is indispensable in a steam-engine applied to the -purposes of manufacture. The agitation of the surface of the sea -will cause the immersion of the paddle-wheels to be subject to -great variation, and the resistance produced by the water to the -engine will undergo a corresponding change. The governor, -therefore, and other parts of the apparatus, contrived for giving -to the engine that great regularity required in manufactures, are -omitted in nautical engines, and nothing is introduced save what -is [Pg447] necessary to maintain the machine in its full working -efficiency. - -[Illustration: _Fig._ 120.] - -[Illustration: _Fig._ 121.] - -To save space, marine boilers are constructed so as to produce the -necessary quantity of steam within the smallest possible -dimensions. With this view a more extensive surface in proportion -to the capacity of the boiler is exposed to the action of the -fire. The flues, by which the flame and heated air are conducted -to the chimney, are so constructed that the heat may act upon the -water on every side in thin oblong shells or plates. This is -accomplished by constructing the flues so as to traverse the -boiler backwards and forwards several times before they terminate -[Pg448] in the chimney. Such an arrangement renders the expense of -the boilers greater, but their steam-producing power is -proportionally augmented, and experiments made by Mr. Watt, at -Birmingham, have proved that such boilers with the same -consumption of fuel will produce, as compared with common land -boilers, an increased evaporation in the proportion of about three -to two. - -[Illustration: _Fig._ 122.] - -[Illustration: _Fig._ 123.] - -The form and arrangement of the water-spaces and flues in marine -boilers may be collected from the sections of the boilers used in -some of the government steamers, exhibited in _figs._ 121, 122, -123. A section made by a horizontal plane passing through the -flues is exhibited in _fig._ 121. The furnaces F communicate in -pairs with the flues E, the air following the course through the -flues represented by the arrows. The flue E passes to the back of -the boiler, then returns to the front, then to the back again, and -is finally carried back to the front, where it communicates at C -with the curved flue B, represented in the transverse vertical -section, _fig._ 122. This curved flue B finally terminates in the -chimney A. There are in this case three independent boilers, each -worked by two furnaces communicating with the same system of -flues; and in the curved flues B, _fig._ 122., by which the air is -finally conducted through the chimney, are placed three -independent [Pg449] dampers, by means of which the furnace of -each boiler can be regulated independently of the other, and by -which each boiler may be separately detached from communication -with the chimney. The letters of reference in the horizontal -section, _fig._ 121., correspond with those in the transverse -vertical section, _fig._ 122., E representing the commencement of -the flues, and C their termination. - -[Illustration: _Fig._ 124.] - -A longitudinal section of the boiler made by a vertical plane -extending from the front to the back is given in _fig._ 123., -where F, as before, is the furnace, G the grate-bars sloping -downwards from the front to the back, H the fire-bridge, C the -commencement of the flues, and A the chimney. An elevation of the -front of the boiler is represented in _fig._ 124., showing two of -the fire-doors closed, and the other two removed, displaying the -position of the grate-bars in front. Small openings are also -provided, closed by proper doors, by which access can be had to -the under side of the flues between the foundation timbers of the -engine for the purpose of cleaning them. - -Each of these boilers can be worked independently of the others. -By this means, when at sea, the engine may be worked by any two of -the three boilers, while the third is being cleaned and put in -order. In all sea-going steamers multiple boilers are at present -provided for this purpose. - -In the boilers here represented the flues are all upon the same -level, winding backwards and forwards without passing one above -the other. In other boilers, however, the flues, [Pg450] after -passing backwards and forwards near the bottom of the boiler, turn -upwards and pass backwards and forwards through a level of the -water nearer its surface, finally terminating in the chimney. More -heating surface is thus obtained with the same capacity of boiler. - -The most formidable difficulty which has been encountered in the -application of the steam-engine to sea-voyages has arisen from the -necessity of supplying the boiler with sea-water instead of pure -fresh water. The sea-water is injected into the condenser for the -purpose of condensing the steam, and it is thence, mixed with the -condensed steam, conducted as feeding water into the boiler. - - -(209.) Sea-water holds, as is well known, certain alkaline -substances in solution, the principal of which is muriate of soda, -or common salt. Ten thousand grains of pure sea-water contain two -hundred and twenty grains of common salt, the remaining -ingredients being thirty-three grains of sulphate of soda, -forty-two grains of muriate of magnesia, and eight grains of -muriate of lime. The heat which converts pure water into steam -does not at the same time evaporate those salts which the water -holds in solution. As a consequence it follows, that as the -evaporation in the boiler is continued, the salt, which was held -in solution by the water which has been evaporated, remains in the -boiler, and enters into solution with the water remaining in it. -The quantity of salt contained in sea-water being considerably -less than that which water is capable of holding in solution, the -process of evaporation for some time is attended with no other -effect than to render the water in the boiler a stronger solution -of salt. If, however, this process be continued, the quantity of -salt retained in the boiler having constantly an increasing -proportion to the quantity of water, it must at length render the -water in the boiler a saturated solution--that is, a solution -containing as much salt as at the actual temperature it is capable -of holding in solution. If, therefore, the evaporation be -continued beyond this point, the salt disengaged from the water -evaporated instead of entering into solution with the water -remaining in the boiler will be precipitated in the form of -sediment; and if the process be continued in the [Pg451] same -manner, the boiler would at length become a mere salt-pan. - -But besides the deposition of salt sediment in a loose form, some -of the constituents of sea-water having an attraction for the iron -of the boiler, collect upon it in a scale or crust in the same -manner as earthy matters held in solution by spring-water are -observed to form and become incrusted on the inner surface of -land-boilers and of common culinary vessels. - -The coating of the inner surface of a boiler by incrustation and -the collection of salt sediment in its lower parts, are attended -with effects highly injurious to the materials of the boiler. The -crust and sediment thus formed within the boiler are almost -non-conductors of heat, and placed, as they are, between the water -contained in the boiler and the metallic plates which form it, -they obstruct the passage of heat from the outer surface of the -plates in contact with the fire to the water. The heat, therefore, -accumulating in the boiler-plates so as to give them a much higher -temperature than the water within the boiler, has the effect of -softening them, and by the unequal temperature which will thus be -imparted to the lower plates which are incrusted, compared with -the higher parts which may not be so, an unequal expansion is -produced, by which the joints and seams of the boiler are loosened -and opened, and leaks produced. - -These injurious effects can only be prevented by either of two -methods; first, by so regulating the feed of the boiler that the -water it contains shall not be suffered to reach the point of -saturation, but shall be so limited in its degree of saltness that -no injurious incrustation or deposit shall be formed; secondly, by -the adoption of some method by which the boiler may be worked with -fresh water. This end can only be attained by condensing the steam -by a jet of fresh water, and working the boiler continually by the -same water, since a supply of fresh water sufficient for a boiler -worked in the ordinary way could never be commanded at sea. - - -(210.) The method by which the saltness of the water in the boiler -is most commonly prevented from exceeding a certain [Pg452] limit -has been to discharge from the boiler into the sea a certain -quantity of over-salted water, and to supply its place by -sea-water introduced into the condenser through the injection-cock -for the purpose of condensing the steam, this water being mixed -with the steam so condensed, and being, therefore, a weaker -solution of salt than common sea-water. To effect this, cocks -called _blow-off cocks_, are usually placed in the lower parts of -the boiler, where the over-salted, and therefore heavier, parts of -the water collect. The pressure of the steam and incumbent weight -of the water in the boiler force the lower strata of water out -through these cocks; and this process, called _blowing out_, is, -or ought to be, practised at such intervals as will prevent the -water from becoming over salted. When the salted water has been -blown out in this manner, the level of the water in the boiler is -restored by a feed of corresponding quantity. - -This process of blowing out, on the due and regular observance of -which the preservation and efficiency of the boiler mainly depend, -is too often left at the discretion of the engineer, who is, in most -cases, not even supplied with the proper means of ascertaining the -extent to which the process should be carried. It is commonly -required that the engineer should blow out a certain portion of the -water in the boiler every two hours, restoring the level by a feed -of equivalent amount; but it is evident that the sufficiency of the -process founded on such a rule must mainly depend on the supposition -that the evaporation proceeds always at the same rate, which is far -from being the case with marine boilers. An indicator, by which the -saltness of the water in the boiler would always be exhibited, ought -to be provided, and the process of blowing out should be regulated -by the indications of that instrument. To blow out more frequently -than is necessary is attended with a waste of fuel; for hot water is -thus discharged into the sea while cold water is introduced in its -place, and consequently all the heat necessary to produce the -difference of the temperatures of the water blown out and the feed -introduced is lost. If, on the other hand, the process of blowing -out be observed less frequently than is necessary, then more or less -incrustation and deposit [Pg453] may be produced, and the injurious -effects already described ensue. - -As the specific gravity of water holding salt in solution is -increased with every increase of the strength of the solution, any -form of hydrometer capable of exhibiting a visible indication of -the specific gravity of the water contained in the boiler would -serve the purpose of an indicator, to show when the process of -blowing out is necessary, and when it has been carried to a -sufficient extent. The application of such instruments, however, -would be attended with some practical difficulties in the case of -sea-boilers. - -The temperature at which a solution of salt boils under a given -pressure varies considerably with the strength of the solution; -the more concentrated the solution is, the higher will be its -boiling temperature under the same pressure. A comparison, -therefore, of a steam-gauge attached to the boiler, and a -thermometer immersed in it, showing the pressure and the -temperature, would always indicate the saltness of the water; and -it would not be difficult so to graduate these instruments as to -make them at once show the degree of saltness. - -If the application of the thermometer be considered to be attended -with practical difficulty, the difference of pressures under which -the salt water of the boiler and fresh water of the same -temperature boil, might be taken as an indication of the saltness -of the water in the boiler, and it would not be difficult to -construct upon this principle a self-registering instrument, which -would not only indicate but record from hour to hour the degree of -saltness of the water. A small vessel of distilled water being -immersed in the water of the boiler would always have the -temperature of that water, and the steam produced from it -communicating with a steam-gauge, the pressure of such steam would -be indicated by that gauge, while the pressure of the steam in the -boiler under which pressure the salted water boils might be -indicated by another gauge. The difference of the pressures -indicated by the two gauges would thus become a test by which the -saltness of the water in the boiler would be measured. The two -pressures might be made to act on opposite ends of the same column -of [Pg454] mercury contained in a siphon tube, and the difference -of the levels of the two surfaces of the mercury would thus become -a measure of the saltness of the water in the boiler. A -self-registering instrument founded on this principle formed part -of the self-registering steam-log which I proposed to introduce -into steam-vessels some time since. - - -(211.) The Messrs. Seaward of Limehouse have adopted, in some of -their recently constructed engines, a method of indicating the -saltness of the water, and of measuring the quantity of salted -water or brine discharged, by blowing out. A glass-gauge, similar -in form to that already described in land engines (156.), is -provided to indicate the position of the surface of the water in -the boiler. In this gauge two hydrometer balls are provided, the -weight of which in proportion to their magnitude is such that they -would both sink to the bottom in a solution of salt of the same -strength as common sea-water. When the quantity of salt exceeds -5/32 parts of the whole weight of the water, the lighter of the -two balls will float to the top; and when the strength is further -increased until the proportion of salt exceeds 6/32 parts of the -whole, then the heavier ball will float to the top. The actual -quantity of salt held in solution by sea-water in its ordinary -state is 1/32 part of its whole weight; and when by evaporation -the proportion of salt in solution has become 9/32 parts of the -whole, then a deposition of salt commences. With an indicator such -as that above described, the ascent of the lighter hydrometer ball -gives notice of the necessity for blowing out, and the ascent of -the heavier may be considered as indicating the approach of an -injurious state of saltness in the boiler. - -[Illustration: _Fig._ 125.] - -The ordinary method of blowing out the salted water from a boiler -is by a pipe having a cock in it leading from the boiler through -the bottom of the ship, or at a point low down at its side. -Whenever the engineer considers that the water in the boiler has -become so salted that the process of blowing out should commence, -he opens the cock communicating by this pipe with the sea, and -suffers an indefinite and uncertain quantity of water to escape. -In this way he discharges, according to the magnitude of the -boiler, from two to six tons [Pg455] of water, and repeats this -at intervals of from two to four hours, as he may consider to be -sufficient. If, by observing this process, he prevents the boiler -from getting incrusted during the voyage, he considers his duty to -be effectually discharged, forgetting that he may have blown out -many times more water than is necessary for the preservation of -the boiler, and thereby produced a corresponding and unnecessary -waste of fuel. In order to limit the quantity of water discharged, -Messrs. Seaward have adopted the following method. In _fig._ 125. -is represented a transverse section of a part of a steam-vessel; W -is the water-line of the boiler, B is the mouth of a blow-off -pipe, placed near the bottom of the boiler. This pipe rises to A, -and turning in the horizontal direction, A C is conducted to a -tank T, which contains exactly a ton of water. This pipe -communicates with the tank by a cock D, governed by a lever H. -When this lever is moved to D', the cock D is open, and when it is -moved to K, the cock D is closed. From the same tank there -proceeds another pipe E, which issues from the side of the -[Pg456] vessel into the sea governed by a cock F, which is -likewise put in connection with the lever H, so that it shall be -opened when the lever H is drawn to the position F', the cock D' -being closed in all positions of the lever between K and F'. Thus, -whenever the cock F communicating with the sea is open, the cock D -communicating with the boiler is closed, and _vice versâ_, both -cocks being closed when the lever is in the intermediate position -K. By this arrangement the boiler cannot, by any neglect in -blowing off, be left in communication with the sea, nor can more -than a ton of water be discharged except by the immediate act of -the engineer. The injurious consequences are thus prevented which -sometimes ensue when the blow-off cocks are left open by any -neglect on the part of the engineer. When it is necessary to blow -off, the engineer moves the lever H, to the position D'. The -pressure of the steam in the boiler on the surface of the water W -forces the salted water or brine up the pipe B A, and through the -open cock C into the tank, and this continues until the tank is -filled: when that takes place, the lever is moved from the -position D' to the position F', by which the cock D is closed, and -the cock F opened. The water in the tank flows through the pipe E -into the sea, air being admitted through the valve V, placed at -the top of the tank, opening inwards. A second ton of brine is -discharged by moving the lever back to the position D', and -subsequently returning it to the position F'; and in this way the -brine is discharged ton by ton, until the supply of water from the -feed which replaces it has caused both the balls in the indicator -to sink to the bottom. - - -(212.) A different method of preserving the requisite freshness of -the water in the boiler has been adopted by Messrs. Maudslay and -Field, and introduced with success into the Great Western and -other steam-vessels. Pumps called _brine-pumps_ are put into -communication with the lower part of the boiler, and so -constructed as to draw the brine therefrom, and drive it into the -sea. These brine-pumps are worked by the engine, and their -operation is constant. The feed-pumps are likewise worked by the -engine, and they bear such a proportion to the brine-pumps that -the quantity of salt discharged in a given time in the brine is -equal to the quantity of salt [Pg457] introduced in solution by -the water of the feed-pumps. By this means the same actual -quantity of salt is constantly maintained in the boiler, and -consequently the strength of the solution remains invariable. If -the brine discharged by the brine-pumps contains 5/32 parts of -salt while the water introduced by the feed-pumps contains only -1/32 part, then it is evident that five cubic feet of the feeding -water will contain no more salt than is contained in one cubic -foot of brine. Under such circumstances the brine-pumps would be -so constructed as to discharge 1/5 of the water introduced by the -feed-pumps, so that 4/5 of all the water introduced into the -boiler would be evaporated, and rendered available for working the -engine. - -To save the heat of the brine, a method has been adopted in the -marine engines constructed by Messrs. Maudslay and Field similar -to one which has been long practised in steam-boilers, and in -various apparatus for the warming of buildings. The current of -heated brine is conducted from the boiler through a tube which is -contained in another, through which the feed is introduced. The -warm current of brine, therefore, as it passes out, imparts a -considerable portion of its heat to the cold feed which comes in; -and it is found that by this expedient the brine discharged into -the sea may be reduced to a temperature of about 100°. - -This expedient is so effectual that when the apparatus is properly -constructed, and kept in a state of efficiency, it may be regarded -as nearly a perfect preventive against the incrustation, and the -deposition of salt in the boilers, and is not attended with any -considerable waste of fuel. - - -(213.) About the year 1776, Mr. Watt invented a tubular condenser, -with a view to condense the steam drawn off from the cylinder -without the process of injection. This apparatus consisted of a -number of small tubes connecting the top and bottom of the -condenser, arranged in a manner not very different from that of -the tubes which traverse the boiler of a locomotive engine. These -tubes were continually surrounded by cold water, and the steam, as -it escaped from the cylinder passing through them, was condensed -by their cold surfaces, and collected in the form of water in a -reservoir below, from [Pg458] whence it was drawn off by a pump -in the same manner as in engines which condensed by injection. One -of the advantages proposed by this expedient was, that no -atmospheric air would be introduced into the condenser, as is -always the case when condensation by injection is practised. Cold -water, which is injected, has always combined with it more or less -common air. When this water is mixed with the condensed steam, the -elevation of its temperature disengages the air combined with it, -and this air circulating to the cylinder, vitiates the vacuum. One -of the purposes for which the air-pump in condensing steam-engines -was provided, and from which it took its name, was to draw off -this air. If, however, a tubular condenser could be made to act -with the necessary efficiency, no injection water would be -introduced for condensation, and the pump would have no other duty -except to remove the small quantity of water produced by the -condensed steam. That water being subsequently carried back to the -boiler by the feed-pumps, a constant system of circulation would -be maintained, and the boiler would never require any fresh supply -of water, except what might be necessary to make good the waste by -leakage and other causes. - -This contrivance has been of late years revived by Mr. Samuel Hall -of Basford, near Nottingham, with a view to supersede in marine -engines the necessity of using sea-water in the boilers. Mr. Hall -proposes to make marine boilers with fresh water to condense the -steam without injection, by a tubulated condenser, and to provide -by the distillation of sea-water the small quantity of fresh water -which would be necessary to make good the waste. These condensers -have been introduced into several steam-vessels: in some they have -been continued, and in others abandoned, and various opinions are -entertained of their efficacy. I have not been able to obtain the -results of any satisfactory experiments on them, and cannot -therefore form a judgment of their usefulness. Mr. Watt abandoned -these condensers from finding that the condensation of the steam -was not sufficiently sudden, and that consequently at the -commencement of the stroke the piston was subject to a resistance -which [Pg459] injuriously diminished the amount of the moving -power, whereas condensation by jet was almost instantaneous, and -the efficiency of the piston throughout the entire stroke was more -uniform. - -Mr. Watt also found that a fur collected around the tubes of the -condenser, so as to obstruct the free passage of heat from the -steam to the water of the cold cistern; and that, consequently, -the efficiency of the condenser was gradually impaired, and could -only be restored by frequent cleansing. - -It is stated by Mr. Hall that a vacuum is preserved in his -condensers as perfect as that which is maintained in the ordinary -condensers by injection. It is objected, on the other hand, that -without the injection water and the air which accompanies it being -introduced into his condensers, Mr. Hall uses as large and -powerful an air-pump as those which are used in engines of equal -power condensing by injection; that, consequently, the vacuum -which is maintained is produced, not as it ought to be altogether -by the condensation of steam, but by the air-pump drawing off the -uncondensed steam. To whatever extent this may be true, the -efficacy of the machine, as indicated by the barometer-gauge, is -only apparent; since as much power is necessary to pump away any -portion of uncondensed vapour as is obtained by the vacuum -produced by the absence of that vapour. - -A tubular condenser of the form proposed by Mr. Hall is represented -in _fig._ 126.; _a_ is the upper part of the condenser to which -steam is admitted from the slide after having worked the piston; _k_ -is the section of a thin plate, forming the top of the condenser, -perforated with small holes, in which the tubes are inserted so as -to be steam-tight and water-tight. Water is admitted to flow around -these tubes between the top _k_ and the bottom _d_ of the condenser, -so as to keep them constantly at a low temperature. The steam passes -from _a_ through the tubes to the lower chamber _f_ of the -condenser, where it is reduced to water by the cold to which it has -been exposed. A supply of cold water is constantly pumped through -the condenser, so as to keep the tubes at a low temperature. The -air-pump _g_ is of the usual construction, having valves in the -piston opening upwards, and [Pg460] similar valves in the cover of -the pump also opening upwards. The water formed by the condensed -steam in _f_ is drawn through the foot-valve, and after passing -through the piston-valves, is discharged by the up-stroke of the -piston into the hot well. Any air, or other permanent gas, which may -be admitted by leakage through the tubes of the condenser, or by any -other means, is likewise drawn out by this pump, and when drawn into -the hot well is carried from thence to the feeding apparatus of the -boiler, to which it is transferred by the feed-pump. - -[Illustration: _Fig._ 126.] - -A provision is likewise made by which the steam escaping at the -safety-valve is condensed and carried away to the feeding cistern. - - -(214.) One of the remedies proposed for the evil consequences -arising from incrustation is the substitution of copper for iron -boilers. The attraction which produces the adhesion of the -calcareous matter held in solution by salt water to the surface of -iron has no existence in copper, and all the saline and other -alkaline matter precipitated in the boiling water in [Pg461] -copper boilers is suspended in a loose form, and carried off by -the process of blowing out. - -Besides the injury arising from the deposition of salt and the -incrustation on the inner surface of boilers, an evil of a -formidable kind attends the accumulation of soot mixed with salt -in the flues, which proceeds from the leaks. In the seams of the -boiler there are numerous apertures, of dimensions so small as to -be incapable of being rendered stanch by any practicable means, -through which the water within the boiler filters, and the salt -which it carries with it mixes with the soot, forming a compound -which rapidly corrodes the boilers. This process of corrosion in -the flues takes place not less in copper than in iron boilers. In -cleansing the flues of a copper boiler, the salt and soot which -was thrown out upon the iron-plates which formed the flooring of -the engine-room, having remained there for some time, left behind -it a permanent appearance of copper on the iron flooring, arising -from the precipitation of the copper which had combined with the -soot and salt in the flues.[36] In this case the leaks from whence -the salt proceeded were found, on careful examination, so -unimportant, that the usual means to stanch them could not be -resorted to without the risk of increasing the evil. - - -(215.) In the application of the steam-engine to the propulsion of -vessels in voyages of great extent, the economy of fuel acquires -an importance greater than that which appertains to it in -land-engines, even in localities the most removed from coal-mines, -and where its expense is greatest. The practical limit to -steam-voyages being determined by the greatest quantity of coals -which a steam-vessel can carry, every expedient by which the -efficiency of the fuel can be increased becomes a means, not -merely of a saving of expense, but of an increased extension of -steam-power to navigation. Much attention has been bestowed on the -augmentation of the duty of engines in the mining districts of -Cornwall, where the question of their efficiency is merely a -question of economy, but far greater care should be given to this -subject when the practicability of maintaining intercourse by -steam between distant points of the globe will perhaps depend on -the effect produced by a given quantity [Pg462] of fuel. So long -as steam-navigation was confined to river and channel transport, -and to coasting voyages, the speed of the vessel was a paramount -consideration, at whatever expenditure of fuel it might be -obtained; but since steam-navigation has been extended to -ocean-voyages, where coals must be transported sufficient to keep -the engine in operation for a long period of time without a fresh -relay, greater attention has been bestowed upon the means of -economising it. - -Much of the efficiency of fuel must depend on the management of -the fires, and therefore on the skill and care of the stokers. -Formerly the efficiency of firemen was determined by the abundant -production of steam, and so long as the steam was evolved in -superabundance, however it might have blown off to waste, the duty -of the stoker was considered as well performed. The regulation of -the fires according to the demands of the engine were not thought -of, and whether much or little steam was wanted, the duty of the -stoker was to urge the fires to their extreme limit. - -Since the resistance opposed by the action of the paddle-wheels of -a steam-vessel varies with the state of the weather, the -consumption of steam in the cylinders must undergo a corresponding -variation; and if the production of steam in the boilers be not -proportioned to this, the engines will either work with less -efficiency than they might do under the actual circumstances of -the weather, or more steam will be produced in the boilers than -the cylinders can consume, and the surplus will be discharged to -waste through the safety-valves. The stokers of a marine engine, -therefore, to perform their duty with efficiency, and obtain from -the fuel the greatest possible effect, must discharge the -functions of a self-regulating furnace, such as has been already -described: they must regulate the force of the fires by the amount -of steam which the cylinders are capable of consuming, and they -must take care that no unconsumed fuel is allowed to be carried -away from the ash-pit. - - -(216.) Until within a few years of the present time the heat -radiated from every part of the surface of the boiler was allowed -to go to waste, and to produce injurious effects on those parts of -the vessel to which it was transmitted. This evil, [Pg463] -however, has been lately removed by coating the boilers, -steam-pipes, &c. of steam-vessels with felt, by which the escape -of heat from the surface of the boiler is very nearly, if not -altogether, prevented. This felt is attached to the boiler-surface -by a thick covering of white and red lead. This expedient was -first applied in the year 1818 to a private steam-vessel of Mr. -Watt's called the _Caledonia_, and it was subsequently adopted in -another vessel, the machinery of which was constructed at Soho, -called the _James Watt_. - -The economy of fuel depends in a considerable degree on the -arrangement of the furnaces, and the method of feeding them. In -general each boiler is worked by two or more furnaces communicating -with the same system of flues. While the furnace is fed, the door -being open, a stream of cold air rushes in, passing over the burning -fuel and lowering the temperature of the flues: this is an evil to -be avoided. But, on the other hand, if the furnaces be fed at -distant intervals, then each furnace will be unduly heaped with -fuel, a great quantity of smoke will be evolved, and the combustion -of the fuel will be proportionally imperfect. The process of coking -in front of the grate, which would insure a complete combustion of -the fuel, has been already described (147.). A frequent supply of -coals, however, laid carefully on the front part of the grate, and -gradually pushed backwards as each fresh feed is introduced, would -require the fire-door to be frequently opened, and cold air to be -admitted. It would also require greater vigilance on the part of the -stokers than can generally be obtained in the circumstances in which -they work. In steam-vessels the furnaces are therefore fed less -frequently, fuel introduced in greater quantities, and a less -perfect combustion produced. - -When several furnaces are constructed under the same boiler, -communicating with the same system of flues, the process of -feeding, and consequently opening one of them, obstructs the due -operation of the others, for the current of cold air which is thus -admitted into the flues checks the draft and diminishes the -efficiency of the furnaces in operation. It was formerly the -practice in vessels exceeding one hundred horse-power, to place -four furnaces under each boiler, communicating with the same -system of flues. Such an arrangement [Pg464] was found to be -attended with a bad draft in the furnaces, and therefore to -require a greater quantity of heating surface to produce the -necessary evaporation. This entailed upon the machinery the -occupation of more space in the vessel in proportion to its power; -it has therefore been more recently the practice to give a -separate system of flues to each pair of furnaces, or, at most, to -every three furnaces. When three furnaces communicate with a -common flue, two will always be in operation, while the third is -being cleared out; but if the same quantity of fire were divided -among two furnaces, then the clearing out of one would throw out -of operation half the entire quantity of fire, and during the -process the evaporation would be injuriously diminished. It is -found by experience, that the side plates of furnaces are liable -to more rapid destruction than their roofs, owing, probably, to a -greater liability to deposit. Furnaces, therefore, should not be -made narrower than a certain limit. Great depth from front to back -is also attended with practical inconvenience, as it renders -firing tools of considerable length, and a corresponding extent of -stoking room necessary. It is recommended, by those who have had -much practical experience in steam-vessels, that furnaces six feet -in depth from front to back should not be less than three feet in -width, to afford means of firing with as little injury to the side -plates as possible, and of keeping the fires in the condition -necessary for the production of the greatest effect. The tops of -the furnaces almost never decay, and seldom are subject to an -alteration of figure, unless the level of the water be allowed to -fall below them.[37] - - -(217.) A form of marine engine was some years since proposed and -patented by Mr. Thomas Howard, possessing much novelty and -ingenuity, and having pretensions to a very extraordinary economy -of fuel, in addition to the advantages claimed by Mr. Hall. In Mr. -Howard's engines, the steam, as in Mr. Hall's, is constantly -reproduced from the same water, so that pure or distilled water -may be used; but Mr. Howard dispenses altogether with the use of a -boiler. - -A quantity of mercury is placed in a shallow wrought-iron -vessel over a coke fire, by which it is maintained at a [Pg465] -temperature varying from 400° to 500°. The surface exposed to the -fire was computed at three fourths of a square foot for each -horse-power. The upper surface of the mercury was covered by a -very thin plate of iron in contact with it, and so contrived as to -present about four times as much surface as that exposed beneath -the fire. Adjacent to this a vessel of water was placed, -maintained nearly at the boiling point, and communicating by a -nozzle and valve with the chamber immediately above the mercury. -At intervals corresponding to the motion of the piston a small -quantity of water was injected from this vessel, and thrown upon -the plate of iron resting upon the hot mercury. From this it -received not only the heat necessary to convert it into common -steam, but to give it the qualities of highly superheated steam. -In fact, the steam thus produced had a temperature considerably -above that which corresponded to its pressure, and was, therefore, -capable of being deprived of more or less of its heat without -being condensed. (94.) The quantity of water injected into the -steam-chamber was regulated by the power at which the engine was -intended to be worked. The fire was supplied with air by a blower -subject to exact regulation. The steam thus produced was conducted -to a chamber surrounding the working cylinder, and this chamber -itself was enclosed by another space through which the air from -the furnace passed before it reached the flue. By this contrivance -the air imparted its redundant heat to the steam, as the latter -passed to the cylinder, and raised its temperature to about 400°, -the pressure, however, not exceeding 25 lbs. per square inch. The -valves, governing the admission of steam to the piston, were -adapted for expansive action. - -The vacuum on the opposite side was maintained by condensation in -the following manner:--The condenser was a copper vessel placed in -a cistern of cold water, and the steam was admitted to it from the -cylinder by an eduction pipe in the usual way. A jet was -introduced from an adjacent vessel filled with distilled water, -and the condensing water and condensed steam were pumped from the -condenser as in common engines. The warm water thus pumped out of -the [Pg466] condenser was drawn through a copper worm, carried -with many coils through a cistern of cold water, so that when it -arrived at the end of this pipe it was reduced nearly to the -temperature of the atmosphere. The pipe was thus brought to the -vessel of distilled water already mentioned, and the water -supplied by it replaced. The water admitted to the condenser -through the condensing jet being purged of air, a small air-pump -was sufficient, since it had only to exhaust the condenser and -tubes at starting, and to remove the air which might be admitted -by leakage. Mr. Howard stated that the condensation took place as -rapidly and perfectly as in the best engines of the common kind. - -An engine of this construction was in the spring of 1835 placed in -the government steamer called the _Comet_. It was stated, that -though the machinery was not advantageously constructed, a part of -the engine being old, and not made expressly for a boiler of this -kind, the vessel performed a voyage from Falmouth to Lisbon, in -which the consumption of fuel did not exceed a third of her former -consumption when worked by Boulton and Watt's engines, the former -consumption of coals being about eight hundred pounds per hour, -and the consumption of Mr. Howard's engine being less than two -hundred and fifty pounds of coke per hour. - -The advantages claimed for this contrivance were the following: -_first_, the small space and weight occupied by the machinery, -arising from the absence of a boiler; _second_, the diminished -consumption of fuel; _third_, the reduced size of the flues; -_fourth_, the removal of the injurious effects arising from -deposit and incrustation; _fifth_, the absence of smoke. - - -(218.) The method by which the greatest quantity of practical effect -can be obtained from a given quantity of fuel must, however, mainly -depend on the extended application of the expansive principle. This -has been the means by which an extraordinary amount of duty has been -obtained from the Cornish engines. The difficulty of the application -of this principle in marine engines has arisen from the objections -entertained in Europe to the use of steam of high pressure under the -circumstances in which the engine must be worked at sea. To apply -the expansive principle, it is necessary that the moving power at -the commencement of the stroke shall considerably exceed the -[Pg467] resistance, its force being gradually attenuated till the -completion of the stroke, when it will at length become less than -the resistance. This condition may, however, be attained with steam -of limited pressure, if the engine be constructed with a sufficient -quantity of piston-surface. This method of rendering the expansive -principle available at sea, and compatible with low-pressure steam, -has recently been brought into operation by Messrs. Maudslay and -Field. Their improvement consists in adapting two steam-cylinders in -one engine, in such a manner that the steam shall act simultaneously -on both pistons, causing them to ascend and descend together. The -piston-rods are both attached to the same horizontal cross-head, -whereby their combined action is applied to one crank by means of a -connecting rod placed between the pistons. - -[Illustration: _Fig._ 127.] - -A section of such an engine, made by a plane passing through the -two piston-rods P P' and cylinders, is represented in _fig._ 127. -The piston-rods are attached to a cross-head C, [Pg468] which -ascends and descends with them. This cross-head drives upwards and -downwards an axle D, to which the lower end of the connecting rod -E is attached. The other end of the connecting rod drives the -crank-pin F, and imparts revolution to the paddle-shaft G. A rod H -conveys motion by means of a beam I to the rod K of the air-pump -E. - - -(219.) Connected with this, and in the same patent, another -improvement is included, consisting of the application of a hollow -wrought-iron framing carried across the vessel above the machinery, -to support the whole of the bearings of the crank-shaft. A plan of -this, including the cylinders and paddle-wheel, is represented in -_fig._ 128. The advantages proposed by these improvements are -simplicity of construction, more direct action on the crank, economy -of space and weight of material, combined with increased area of the -piston, whereby a given evaporating power of the boiler is rendered -productive, by extended application of the expansive principle, of a -greater moving power than in former arrangements. Consequently, -under like circumstances, greater power and economy of fuel is -obtained, with the further advantage at sea, that when the engine is -reduced in its speed, either by the vessel being deeply laden with -coal, as is the case at the commencement of a long sea voyage, or by -head winds, more steam may be given to the cylinders, and -consequently more speed imparted to the vessel, all the steam -produced in the boiler being usefully employed. - - -(220.) Another improvement, having the same objects, and analogous -to the preceding, has been likewise patented by Messrs. Maudslay -and Field. This consists in the adoption of a cylinder of greater -diameter, having two piston-rods P P', as represented in _fig._ -129., of considerable length, connected at the top by a cross-head -C. From this cross-head is carried downwards the connecting rod D, -which drives the crank-pin E, and thereby works the paddle-shaft -S. In this case the paddle-shaft is extended immediately above the -piston, and the double piston-rod has sufficient length to be -above the paddle-shaft when the piston is at the bottom of its -stroke. This improvement is intended to be applied more -particularly for engines for river navigation, the advantages -resulting from [Pg469] it being that a paddle-shaft placed at a -given height from the bottom of the vessel will be enabled to -receive a longer stroke of piston than by any other arrangement -now in use. A more [Pg470] compact and firm connection of the -cylinder with the crank-shaft bearings is effected by it, and a -cylinder of much greater diameter may be applied by which the -expansive action of steam may be more fully brought into play; and -a more direct action of the steam-power on the crank with a less -weight of materials and a greater economy of space may be obtained -than by any of the arrangements of marine engines hitherto used. - -[Illustration: _Fig._ 128.] - -[Illustration: _Fig._ 129.] - - -(221.) Mr. Francis Humphrys has obtained a patent for a form of -marine engine, by which some simplification of the machinery is -attained, and the same power comprised within more limited -dimensions. In this engine there is attached to the piston of the -cylinder, instead of a piston-rod, a hollow casing D D (_fig._ -130.), which moves through a stuffing-box G, constructed in a -manner similar to the stuffing-box of a piston-rod. In the figure, -this casing is presented in section, but [Pg471] its form is that -of a long narrow slit, or opening, rounded at either end as -exhibited in the plan (_fig._ 131) of the cylinder-cover. The -crank C is driven by the other end of the connecting rod H, the -crank-shaft being immediately above the centre of the piston and -the connecting rod passing through the oblong opening D, and -descending into the hollow piston-rod it is attached to an axis I -at the bottom of the piston. A box or cover K K encloses the -cross-piece or axis I with its bearings, and is [Pg472] attached -so as to be steam-tight to the bottom of the piston. A hollow -space L L is cast in the bottom of the cylinder for the reception -of the box K K, when the piston is at the bottom of the cylinder. - -[Illustration: _Fig._ 130.] - -[Illustration: _Fig._ 131.] - -By this arrangement the force by which the piston is driven in its -ascent and descent is communicated to the connecting rod, not, as -usual, through the intervention of a piston-rod, but directly from -the piston itself by the cross-pin I, and from thence to the crank -C, which it drives without the intervention of beams, cross-heads, -or any similar appendage. - -The slide-valves regulating the admission and eduction of steam -are represented at _a_; the rod of the air-pump is shown at _d_, -being worked by a crank placed on the centre of the great crank -shaft.[38] - - -(222.) To obtain from the moving power its full amount of -mechanical effect in propelling the vessel, it would be necessary -that its force should propel, by constantly acting against the -water in a horizontal direction, and with a motion contrary to the -course of the vessel. No system of mechanical propellers has, -however, yet been contrived capable of perfectly accomplishing -this. Patents have been granted for many ingenious mechanical -combinations to impart to the propelling surfaces such angles as -appeared to the respective contrivers most advantageous. In most -of these the mechanical complexity has formed a fatal objection. -No part of the machinery of a steam-vessel is so liable to become -deranged at sea as the paddle-wheels; and, therefore, that -simplicity of construction which is compatible with those repairs -which are possible on such emergencies is quite essential for safe -practical use. - -[Illustration: _Fig._ 132.] - -The ordinary paddle-wheel, as has been already stated, is a wheel -revolving upon a shaft driven by the engine, and carrying upon its -circumference a number of flat boards, called paddle-boards, which -are secured by nuts and braces in a fixed position; and that -position is such that the planes [Pg473] of the paddle-boards -diverge nearly from the centre of the shaft on which the wheel -turns. The consequence of this arrangement is that each -paddle-board can only act in that direction which is most -advantageous for the propulsion of the vessel when it arrives near -the lowest point of the wheel. In _fig._ 132. let O be the shaft -on which the common paddle-wheel revolves; the position of the -paddle-boards are represented at A, B, C, &c.; X, Y represents the -water line, the course of the vessel being supposed to be from X -to Y; the arrows represent the direction in which the paddle-wheel -revolves. The wheel is immersed to the depth of the lowest -paddle-board, since a less degree of immersion would render a -portion of the surface of each paddle-board mechanically useless. -In the position A the whole force of the paddle-board is efficient -for propelling the vessel; but as the paddle enters the water in -the position H, its action upon the water, not being horizontal, -is only partially effective for propulsion: a part of the force -which drives the paddle is expended in depressing the water, and -the remainder in driving it contrary to the course of the vessel, -and, therefore, by its re-action producing a certain propelling -effect. The tendency, however, of the paddle entering the water at -H, is to form a hollow or trough, which the water, by its ordinary -property, has a continual tendency to fill up. After passing the -lowest point A, as the paddle approaches the position B, where it -[Pg474] emerges from the water, its action again becomes oblique, -a part only having a propelling effect, and the remainder having a -tendency to raise the water, and throw up a wave and spray behind -the paddle-wheel. It is evident that the more deeply the -paddle-wheel becomes immersed, the greater will be the proportion -of the propelling power thus wasted in elevating and depressing -the water; and if the wheel were immersed to its axis, the whole -force of the paddle-boards, on entering and leaving the water, -would be lost, no part of it having a tendency to propel. If a -still deeper immersion take place, the paddle-boards above the -axis would have a tendency to retard the course of the vessel. -When the vessel is, therefore, in proper trim, the immersion -should not exceed nor fall short of the depth of the lowest -paddle; but for various reasons it is impossible in practice to -maintain this fixed immersion: the agitation of the surface of the -sea, causing the vessel to roll, will necessarily produce a great -variation in the immersion of the paddle-wheels, one becoming -frequently immersed to its axle, while the other is raised -altogether out of the water. Also the draught of water of the -vessel is liable to change, by the variation in her cargo; this -will necessarily happen in steamers which take long voyages. At -starting they are heavily laden with fuel, which as they proceed -is gradually consumed, whereby the vessel is lightened. - - -(223.) To remove this defect, and economise as much as possible -the propelling effect of the paddle-boards, it would be necessary -so to construct them that they may enter and leave the water -edgeways, or as nearly so as possible; such an arrangement would -be, in effect, equivalent to the process called feathering, as -applied to oars. Any mechanism which would perfectly accomplish -this would cause the paddles to work in almost perfect silence, -and would very nearly remove the inconvenient and injurious -vibration which is produced by the action of the common paddles. -But the construction of feathering paddles is attended with great -difficulty, under the peculiar circumstances in which such wheels -work. Any mechanism so complex that it could not be easily -repaired when deranged, with such engineering implements and -skill [Pg475] as can be obtained at sea, would be attended with -great objections; and the efficiency of its propelling action -would not compensate for the dangers which must attend upon the -helpless state of a steamer, deprived of her propelling agents. - -Feathering paddle-boards must necessarily have a motion -independently of the motion of the wheel, since any fixed position -which could be given to them, though it might be most favourable to -their action in one position would not be so in their whole course -through the water. Thus the paddle-board when at the lowest point -should be in a vertical position, or so placed that its plane, if -continued upwards, would pass through the axis of the wheel. In -other positions, however, as it passes through the water, it should -present its upper edge, not towards the axle of the wheel, but -towards a point above the highest point of the wheel. The precise -point to which the edge of the paddle-board should be directed is -capable of mathematical determination. But it will vary according to -circumstances, which depend on the motion of the vessel. The -progressive motion of the vessel, independently of the wind or -current, must obviously be slower than the motion of the -paddle-boards round the axle of the wheel; since it is by the -difference of these velocities that the re-action of the water is -produced by which the vessel is propelled. The proportion, however, -between the progressive speed of the vessel and the rotative speed -of the paddle-boards is not fixed: it will vary with the shape and -structure of the vessel, and with its depth of immersion; -nevertheless it is upon this proportion that the manner in which the -paddle-boards should shift their position must be determined. If the -progressive speed of the vessel were nearly equal to the rotative -speed of the paddle-boards, the latter should so shift their -position that their upper edges should be presented to a point very -little above the highest point of the wheel. This is a state of -things which could only take place in the case of a steamer of a -small draught of water, shallop-shaped, and so constructed as to -suffer little resistance from the fluid. On the other hand, the -greater the depth of immersion, and the less fine the lines of the -[Pg476] vessel, the greater will be the resistance in passing -through the water, and the greater will be the proportion which the -rotative speed of the paddle-boards will bear to the progressive -speed of the vessel. In this latter case the independent motion of -the paddle-boards should be such that their edges, while in the -water, shall be presented towards a point considerably above the -highest point of the paddle-wheel. - -A vast number of ingenious mechanical contrivances have been -invented and patented for accomplishing the object just explained. -Some of these have failed from the circumstance of their inventors -not clearly understanding what precise motion it was necessary to -impart to the paddle-board: others have failed from the complexity -of the mechanism by which the desired effect was produced. - - -(224.) In the year 1829 a patent was granted to Elijah Galloway -for a paddle-wheel with movable paddles, which patent was -purchased by Mr. William Morgan, who made various alterations in -the mechanism, not very materially departing from the principle of -the invention. - -[Illustration: _Fig._ 133.] - -This paddle-wheel is represented in _fig._ 133. The contrivance -may be shortly stated to consist in causing the wheel which bears -the paddles to revolve on one centre, and the radial arms which -move the paddles to revolve on another centre. Let A B C D E F G H -I K L be the polygonal circumference of the paddle-wheel, formed -of straight bars, securely connected together at the extremities -of the spokes or radii of the wheel which turns on the shaft which -is worked by the engine; the centre of this wheel being at O. So -far this wheel is similar to the common paddle-wheel; but the -paddle-boards are not, as in the common wheel, fixed at A B C, -&c., so as to be always directed to the centre O, but are so -placed that they are capable of turning on axles which are always -horizontal, so that they can take any angle with respect to the -water which may be given to them. From the centres, or the line -joining the pivots on which these paddle-boards turn, there -proceed short arms K, firmly fixed to the paddle-boards at an -angle of about 120°. On a motion given to this arm K, it will -therefore give a corresponding angular motion to the paddle-board, -so as to make it turn on its pivots. At [Pg477] the extremities -of the several arms marked K is a pin or pivot, to which the -extremities of the radial arms L are severally attached, so that -the angle between each radial arm L and the short paddle-arm K is -capable of being changed by any motion imparted to L; the radial -arms are connected at the other end with a centre, round which -they are capable of revolving. Now, since the points A B C, &c., -which are the pivots on which the paddle-boards turn, are moved in -the circumference of a circle, of which the centre is O, they are -always at the same distance from that point; consequently they -will continually vary their distance from the other centre P. -Thus, when a paddle-board arrives at that point of its revolution -at which the centre round which it revolves lies precisely between -it and the centre O, its distance from the former centre is less -than in any other position. As it departs from that point, its -distance from that centre gradually increases until it arrives at -the opposite point of its revolution, where the centre O is -exactly between it and the former centre; then the distance of the -paddle-board from the former centre is greatest. [Pg478] This -constant change of distance between each paddle-board and the -centre P is accommodated by the variation of the angle between the -radial arm L and the short paddle-board arm K; as the paddle-board -approaches the centre P this gradually diminishes; and as the -distance of the paddle-board increases, the angle is likewise -augmented. This change in the magnitude of the angle, which thus -accommodates the varying position of the paddle-board with respect -to the centre P, will be observed in the figure. The paddle-board -D is nearest to P; and it will be observed that the angle -contained between L and K is there very acute; at E the angle -between L and K increases, but is still acute; at G it increases -to a right angle; at H it becomes obtuse; and at K, where it is -most distant from the centre P, it becomes most obtuse. It again -diminishes at K, and becomes a right angle between A and B. Now -this continual shifting of the direction of the short arm K is -necessarily accompanied by an equivalent change of position in the -paddle-board to which it is attached; and the position of the -second centre P is, or may be, so adjusted that this paddle-board, -as it enters the water and emerges from it, shall be such as shall -be most advantageous for propelling the vessel, and therefore -attended with less of that vibration which arises chiefly from the -alternate depression and elevation of the water, owing to the -oblique action of the paddle-boards. - - -(225.) In the year 1833, Mr. Field, of the firm of Maudslay and -Field, constructed a paddle-wheel with fixed paddle-boards, but -each board being divided into several narrow slips arranged one a -little behind the other, as represented in _fig._ 134. These -divided boards he proposed to arrange in such cycloidal curves -that they must all enter the water at the same place in immediate -succession, avoiding the shock produced by the entrance of the -common board. These split paddle-boards are as efficient in -propelling when at the lowest point as the common paddle-boards, -and when they emerge the water escapes simultaneously from each -narrow board, and is not thrown up, as is the case with common -paddle-boards.[39] - -[Illustration: _Fig._ 134.] - -[Pg479] The theoretical effect of this wheel is the same as that -of the common wheel, and experience alone, the result of which has -not yet been obtained, can prove its efficiency. The number of -bars, or separate parts into which each paddle-board is divided, -has been very various. When first introduced by Mr. Galloway each -board was divided into six or seven parts: this was subsequently -reduced, and in the more recent wheels of this form constructed -for the government vessels the paddle-boards consist only of two -parts, coming as near to the common wheel as is possible, without -altogether abandoning the principle of the split paddle. - - -(226.) To obtain an approximate estimate of the extent to which -steam-power is applicable to long sea-voyages, it would be -necessary to investigate the mutual relation which, in the -existing state of this application of steam-power, exists between -the capacity or tonnage of the vessel, the magnitude, weight, and -power, of the machinery, the available stowage for fuel, and the -average speed attainable in all [Pg480] weathers, as well as the -general purposes to which the vessel is to be appropriated, -whether for the transport of goods or merchandise, or merely for -despatches and passengers, or for both of these combined. That -portion of the capacity of the vessel which is appropriated to the -moving power consists of the space occupied by the machinery and -the fuel. The distribution of it between these must mainly depend -on the length of the voyage which the vessel must make without -receiving a fresh supply of coals. If the trips be short, and -frequent relays of fuel can be obtained, then the space allotted -to the machinery may bear a greater proportion to that assigned to -the fuel; but in proportion as each uninterrupted stage of the -voyage is increased, a greater stock of coals will be necessary, -and a proportionally less space left for the machinery. Other -things being the same, therefore, steam-vessels intended for long -sea-voyages must be less powerful in proportion to their tonnage. - -It will be apparent that every improvement which takes place in -the application of the steam-engine to navigation will modify all -these data on which such an investigation must depend. Every -increased efficiency of fuel, from whatever cause it may be -derived, will either increase the useful tonnage of the vessel, or -increase the length of the voyage of which it is capable. Various -improvements have been and are still in progress, by which this -efficiency has undergone continual augmentation, and voyages may -now be accomplished with moderate economy and profit, to which a -few years since marine engines could not be applied with permanent -advantage. The average speed of steam-vessels has also undergone a -gradual increase by such improvements. During the four years -ending June, 1834, it was found that the average rate of steaming -obtained from fifty-one voyages made by the Admiralty steamers -between Falmouth and Corfu, exclusive of stoppages, was seven -miles and a quarter an hour direct distance between port and port. -The vessels which performed this voyage varied from 350 to 700 -tons measured burden, and were provided with engines varying from -100 to 200 horse-power, with stowage for coals varying from 80 to -240 tons. The proportion of the power to the [Pg481] tonnage -varied from one horse to three tons to one horse to four tons. -Thus the MESSENGER had a power of 200 horses and measured 730 -tons; the FLAMER had a power of 120 horses, and measured 500 tons; -the COLUMBIA had a power of 120 horses, and measured 360 tons. In -general it may be assumed that for the shortest class of trips, -such as those of the Channel steamers, the proportion of the power -to the tonnage should be about one horse for every two tons; but -for the longer class of voyages, the proportion of power to -tonnage should be about one horse-power to from three to four tons -measured tonnage. These data, however, must be received as very -rough approximations, subject to considerable modifications in -their application to particular vessels. We have already stated -that the nominal horse-power is itself extremely indefinite; and -if, as is now customary in the longer class of voyages, the steam -be worked expansively, then the nominal power almost ceases to -have any definite relation to the actual performance of the -vessel. It is usual to calculate the horse-power by assuming a -uniform pressure of steam upon the piston, and, consequently, by -excluding the consideration of the effect of expansion. The most -certain test of the amount of mechanical power exerted by the -machinery would be obtained from the quantity of water actually -transmitted in the form of steam from the boiler to the cylinder. -But the effect of this would also be influenced by the extent to -which the expansive principle has been brought into operation. - -From the reported performances of the larger class of steam-ships -within the last few years, it would appear that the average speed -has been increased since the estimate above mentioned, which was -obtained in 1834; and on comparing the consumption of fuel with -the actual performance, it would appear that the efficiency of -fuel has also been considerably augmented. No extensive course of -accurate experiments or observations have, however, been obtained -from which correct inferences may be drawn of the probable limits -to which steam-navigation, in its present state, is capable of -being extended. The jealousy of rival companies has obstructed the -inquiries of those who, solicitous more [Pg482] for the general -advancement of the art than for the success of individual -enterprises, have directed their attention to this question; and -it is hardly to be expected that sufficiently correct and -extensive data can be obtained for this purpose. - - -(227.) Increased facility in the extension and application of -steam-navigation is expected to arise from the substitution of iron -for wood, in the construction of vessels. Hitherto iron steamers -have been chiefly confined to river-navigation; but there appears no -sufficient reason why their use should be thus limited. For -sea-voyages they offer many advantages; they are not half the weight -of vessels of equal tonnage constructed of wood; and, consequently, -with the same tonnage they will have less draught of water, and -therefore less resistance to the propelling power; or, with the same -draught of water and the same resistance, they will carry a -proportionally heavier cargo. The nature of their material renders -them more stiff and unyielding than timber; and they do not suffer -that effect which is called _hogging_, which arises from a slight -alteration which takes place in the figure of a timber vessel in -rolling, accompanied by an alternate opening and closing of the -seams. Iron vessels have the further advantage of being more proof -against fracture upon rocks. If a timber vessel strike, a plank is -broken, and a chasm opened in her many times greater than the point -of rock which produces the concussion. If an iron vessel strike, she -will either merely receive a dinge, or be pierced by a hole equal in -size to the point of rock which she encounters. Some examples of the -strength of iron vessels were given by Mr. Macgregor Laird, in his -evidence before the Committee of the Commons on Steam Navigation, -among which the following may be mentioned:--An iron vessel, called -the ALBURKAH, in one of their experimental trials got aground, and -lay upon her anchor: in a wooden vessel the anchor would probably -have pierced her bottom; in this case, however, the bottom was only -dinged. An iron vessel, built for the Irish Inland Navigation -Company, was being towed across Lough Derg in a gale of wind, when -the towing rope broke, and she was driven upon rocks, on which she -bumped for a considerable time [Pg483] without any injury. A wooden -vessel would in this case have gone to pieces. A further advantage -of iron vessels (which in warm climates is deserving of -consideration) is their greater coolness and perfect freedom from -vermin. - -Iron steam-vessels on a very large scale are now in preparation in -the ports of Liverpool and Bristol, intended for long sea-voyages. -The largest vessel of this description which has yet been -projected is stated to be in preparation for the voyage between -Bristol and New York, by the company who have established the -steam-ship called the Great Western, plying between these places. - -Several projects for the extension of steam-navigation to voyages -of considerable length have lately been entertained both by the -public and by the legislature, and have imparted to every attempt -to improve steam-navigation increased interest. A committee of the -House of Commons collected evidence and made a report in the last -session in favour of an experiment to establish a line of -steam-communication between Great Britain and India. Two routes -have been suggested by the committee, each being a continuation of -the line of Admiralty steam-packets already established to Malta -and the Ionian Isles. One of the routes proposed is through Egypt, -the Red Sea, and across the Indian Ocean to Bombay, or some of the -other presidencies; the other across the north part of Syria to -the banks of the Euphrates, by that river to the Persian Gulf, and -from thence to Bombay. Each of these routes will be attended with -peculiar difficulties, and in both a long sea-voyage will be -encountered. - -In the route by the Red Sea it is proposed to establish steamers -between Malta and Alexandria (eight hundred and sixty miles). A -steamer of four hundred tons' burden and one hundred horse-power -would perform this voyage, upon an average of all weathers -incident to the situation, in from five to six days, consuming ten -tons of coal per day. But it is probable that it might be found -more advantageous to establish a higher ratio between the power -and the tonnage. From Alexandria the transit might be effected by -land across the isthmus to Suez--a journey of from four to five -days--by caravan and camels; or the transit might be made either -[Pg484] by land or water from Alexandria to Cairo, a distance of -one hundred and seventy-three miles; and from Cairo to Suez, -ninety-three miles, across the desert, in about five days. At Suez -would be a station for steamers, and the Red Sea would be -traversed in three runs or more. If necessary, stations for coals -might be established at Cosseir, Judda, Mocha, and finally at Aden -or at Socatra--an island immediately beyond the mouth of the Red -Sea, in the Indian Ocean; the run from Suez to Cosseir would be -three hundred miles--somewhat more than twice the distance from -Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty -miles; from Judda to Mocha, five hundred and seventeen miles; and -from Mocha to Socatra, six hundred and thirty-two miles. It is -evident that all this would, without difficulty, in the most -unfavourable weather, fall within the present powers of -steam-navigation. If the terminus of the passage be Bombay, the -run from Socatra to Bombay will be twelve hundred miles, which -would be from six to eight days' steaming. The whole passage from -Alexandria to Bombay, allowing three days for delay between Suez -and Bombay, would be twenty-six days: the time from Bombay to -Malta would therefore be about thirty-three days; and adding -fourteen days to this for the transit from Malta to England, we -should have a total of forty-seven days from London to Bombay, or -about seven weeks. - -If the terminus proposed were Calcutta, the course from Socatra -would be one thousand two hundred and fifty miles south-east to -the Maldives, where a station for coals would be established. This -distance would be equal to that from Socatra to Bombay. From the -Maldives, a run of four hundred miles would reach the southern -point of Ceylon, called the Point de Galle, which is the best -harbour (Bombay excepted) in British India: from the Point de -Galle, a run of six hundred miles will reach Madras, and from -Madras to Calcutta would be a run of about six hundred miles. The -voyage from London to Calcutta would be performed in about sixty -days. - -At a certain season of the year there exists a powerful physical -opponent to the transit from India to Suez: from [Pg485] the -middle of June until the end of September, the south-west monsoon -blows with unabated force across the Indian Ocean, and more -particularly between Socatra and Bombay. This wind is so violent -as to leave it barely possible for the most powerful steam-packet -to make head against it, and the voyage could not be accomplished -without serious wear and tear upon the vessels during these -months. - -The attention of parliament has therefore been directed to another -line of communication, not liable to this difficulty: it is -proposed to establish a line of steamers from Bombay through the -Persian Gulf to the Euphrates. - -The run from Bombay to a place called Muscat, on the southern shore -of the gulf, would be eight hundred and forty miles in a north-west -direction, and therefore not opposed to the south-west monsoon. From -Muscat to Bassidore, a point upon the northern coast of the strait -at the mouth of the Persian Gulf, would be a run of two hundred and -fifty-five miles; from Bassidore to Bushire, another point on the -eastern coast of the Persian Gulf, would be a run of three hundred -miles; and from Bushire to the mouth of the Euphrates, would be one -hundred and twenty miles. It is evident that the longest of these -runs would offer no more difficulty than the passage from Malta to -Alexandria. From Bussora, near the mouth of the Euphrates, to Bir, a -town upon its left bank near Aleppo, would be one thousand one -hundred and forty-three miles, throughout which there are no -physical obstacles to the river-navigation which may not be -overcome. Some difficulties arise from the wild and savage character -of the tribes who occupy its banks. It is, however, thought that by -proper measures, and securing the co-operation of the pacha of -Egypt, any serious obstruction from this cause may be removed. From -Bir, by Aleppo, to Scanderoon, a port upon the Mediterranean, -opposite Cyprus, is a land-journey, said to be attended with some -difficulty, but not of great length; and from Scanderoon to Malta is -about the same distance as between the latter place and Alexandria. -It is calculated that the time from London to Bombay by the -Euphrates--supposing the passage to be successfully [Pg486] -established--would be a few days shorter than by Egypt and the Red -Sea. - -Whichever of these courses may be adopted, it is clear that the -difficulties, so far as the powers of the steam engine are -concerned, lie in the one case between Socatra and Bombay, or -between Socatra and the Maldives, and in the other case between -Bombay and Muscat. This, however, has already been encountered and -overcome on four several voyages by the HUGH LINDSAY steamer from -Bombay to Suez: that vessel encountered a still longer run on -these several trips, by going, not to Socatra, but to Aden, a -point on the coast of Arabia, near the Straits of Babel Mandeb, -being a run of one thousand six hundred and forty-one miles, which -she performed in ten days and nineteen hours. The same trip has -since been repeatedly made by other steamers; and, in the present -improved state of steam navigation, no insurmountable obstacles -are opposed to their passage. - -[Illustration] - - FOOTNOTES: - - [35] This cut is taken from the plate of the engine of the Red - Rover, manufactured by Boulton and Watt, given in the last - edition of _Tredgold on the Steam Engine_. - - [36] Appendix I., _on Marine Boilers, by J. Dinnen; Tredgold_ - _on the Steam Engine_, second edition. - - [37] _Tredgold on the Steam Engine_, Appendix, I. p. 171. - - [38] Engines on a very large scale constructed upon this - principle are said to be in process of construction for an - iron steam-vessel of great tonnage, which is in preparation - for the New York passage. It is said that the cylinders of - these engines will be one hundred and twenty inches in - diameter. - - [39] A patent was subsequently taken out for these by Mr. - Galloway. Mr. Field did not persevere in its use at the time - he invented it. It has, however, been more generally adopted - since the date of Galloway's patent. - -[Pg487] - - - - -[Illustration] - -CHAP. XIV. - -AMERICAN STEAM NAVIGATION. - - STEAM NAVIGATION FIRST ESTABLISHED IN AMERICA. -- CIRCUMSTANCES - WHICH LED TO IT. -- FITCH AND RUMSEY. -- STEVENS OF HOBOKEN. -- - LIVINGSTONE AND FULTON. -- EXPERIMENTS ON THE SEINE. -- FULTON'S - FIRST BOAT. -- THE HUDSON NAVIGATED BY STEAM. -- EXTENSION AND - IMPROVEMENT OF RIVER NAVIGATION. -- SPEED OF AMERICAN STEAMERS. - -- DIFFERENCE BETWEEN THEM AND EUROPEAN STEAMERS. -- SEA-GOING - AMERICAN STEAMERS. -- AMERICAN PADDLE-WHEELS. -- LAKE STEAMERS. - -- THE MISSISIPPI AND ITS TRIBUTARIES. -- STEAMERS NAVIGATING - IT. -- THEIR STRUCTURE AND MACHINERY. -- NEW ORLEANS HARBOUR. -- - STEAM TUGS. - - -(228.) The credit of having afforded the first practical solution -of the problem to apply the steam engine to the propulsion of -ships, undoubtedly belongs to the people of the United States of -America. The geographical character of their vast country, not -less than the sanguine and enterprising spirit of the nation, -contributed to this. A coast of four thousand miles in extent, -stretching from the Gulf of St. Lawrence to the embouchures of the -Mississippi, indented and [Pg488] serrated in every part with -natural harbours and sheltered bays, and fringed with islands -forming sounds--capes, and promontories enclosing arms of the sea, -in which the waters are free from the roll of the ocean, and take -the placid character of lakes,--rivers of imposing magnitude, -navigable for vessels of the largest class, for many hundreds and -in some instances for many thousands of miles, affording access to -the innermost population of an empire, whose area vastly exceeds -the whole European continent,--chains of lakes composed of the -most extensive bodies of fresh water in the known world,--and this -extensive continent peopled by races carrying with them the habits -and feelings together with much of the skill and knowledge of the -most civilized parts of the globe, endowed also with that -inextinguishable spirit of enterprise which ever belongs to an -emigrant people,--form a combination of circumstances more than -sufficient to account for the fact of this nation snatching from -England, the parent of the steam engine, the honour of first -bringing into practical operation one of the most important--if -indeed it be not altogether the most important--of the many -applications of that machine to the uses of life. - -The circumstances which rendered these extensive tracts of inland -and coast navigation eminently suited to the application of steam -power, formed so many obstructions and difficulties to the -application of other more ordinary means of locomotion on water. -The sheltered bays and sounds which offered a smooth and -undisturbed surface to the action of the infant steamer argued the -absence of that element which gave effect to the sails and rigging -of the wind-propelled ship, and the rapid currents of the gigantic -streams formed by the drainage of this great continent, though -facilitating access to the coast, rendered the oar powerless in -the ascent. - - -(229.) The first great discovery of Watt had scarcely been -realized in practice by the construction of the single-acting -steam-engine, when the speculative and enterprising Americans -conceived the project of applying it as a moving power in their -inland navigation. So early as the year 1783 [Pg489] Fitch and -Rumsey made attempts to apply the single-acting engine to the -propulsion of vessels, and their failure is said to have arisen -more from the inherent defects of that machine in reference to -this application of it, than from any want of ingenuity or -mechanical skill on their parts. In 1791, John Stevens of Hoboken -commenced his experiments on steam navigation, which were -continued for sixteen years; during a part of this period he was -assisted by Livingstone (who was subsequently instrumental in -advancing the views of Fulton), and by Roosevelt. These projectors -had, at that time also, the assistance and advice of Brunel, since -so celebrated for the invention of the block machinery, and the -construction of the Thames Tunnel. Their proceedings were -interrupted by the appointment of Livingstone as American Minister -at Paris, under the Consular Government. - -At Paris, Livingstone met Fulton, who had been previously engaged -in similar speculations, and being struck with his mechanical -skill, and the soundness of his views, joined him in causing a -series of experiments to be made, which were accordingly carried -on at Plombières, and subsequently on a still more extensive scale -on the Seine, near Paris. Having by this course of experiments -obtained proofs of the efficiency of Fulton's projects, sufficient -to satisfy the mind of Livingstone, he agreed to obtain for Fulton -the funds necessary to construct a steam boat on a large scale, to -be worked upon the Hudson. It was decided, in order to give the -project the best chance of success, to obtain the machinery from -Bolton and Watt. In 1803, Fulton accordingly made drawings of the -engines intended for this first steamer, which were sent to Soho, -with an order for their construction. Fulton, meanwhile, repaired -to America, to superintend the construction of the boat. The -delays incidental to these proceedings retarded the completion of -the boat and machinery until the year 1807, when all was -completed, and the first successful experiment made at New York. -The vessel was placed, for regular work, to ply between New York -and Albany, in the beginning of 1808; and, from that time to the -present, this river has been the theatre of the most [Pg490] -remarkable series of experiments on locomotion on water which has -ever been presented in the history of navigation. - - -(230.) The form and arrangement of this first marine engine was, -in many respects, similar to that which is still generally used -for marine purposes. The cold water cistern was abandoned, and an -increased condensing power obtained by enlarging the condenser. It -was usual to make the condenser half the diameter of the cylinder, -and half its length, and therefore one eighth of its capacity. The -condenser, however, was now made of the same diameter as the -cylinder, being still half its length; its capacity therefore, -instead of being only an eighth, was half of the cylinder; the -condensing jet was admitted by a pipe passing through the bottom -of the vessel. As in the present marine engines, two working beams -were provided, one at either side of the cylinder; but in order to -provide against the difficulties which might arise in the -adaptation of machinery made at Birmingham to a vessel made at New -York, beams were constructed in the form of an inverted T, the -working arms being twofold, one horizontal and the other vertical, -so that the connecting rod might be carried from the crank, either -downwards, to the end of the horizontal arm, or horizontally, to -the end of the vertical arm. In fact there was a choice, to use -either a straight beam, or a bell-crank. The latter was that which -was adopted in this instance. The paddle-shaft, driven by the -crank, passed across the vessel, and had the paddle-wheels keyed -upon it as at present; and in order to equalise the effect of the -engine spur wheels were also placed on the paddle-shaft, by which -pinions were driven, placed upon an axle, which carried a -fly-wheel. - -The speed attained by this steam boat, when it first began to ply -upon the river, did not exceed four miles an hour, but by a series -of improvements its rate of motion was soon increased to six miles -an hour. In the steam boats subsequently constructed by Fulton a -greater speed was attained; but in the latest vessels built by him -he did not exceed a speed of nine miles an hour, which he -considered to be the greatest that could be advantageously -obtained. - -While Fulton was making his plans, and engaged in the [Pg491] -construction of his first boat, Mr. Stevens of Hoboken, already -mentioned, was engaged in a like project, and completed a vessel, -to be propelled by a steam engine, within a few weeks after the -first successful voyage of Fulton. Stevens was likewise completely -successful; but the exclusive privilege of navigating the Hudson -by steam having been granted to Fulton by an act of Congress, -Stevens was compelled to select another theatre for his -operations, and he accordingly sent his steam boat by sea to -Philadelphia, to navigate the Delaware, thus securing for himself -the honour of having made the first sea voyage by steam. - -Fulton did not long retain the monopoly of the steam navigation of -the Hudson. Fortunately for the progress of steam navigation, the act -conferring upon him that privilege was declared unconstitutional; -and the navigation of that noble river was thrown open to the spirit -and enterprise of American genius. The number of passengers conveyed -upon it became enormous beyond all precedent, and inducements of the -strongest kind were accordingly held out to the improvement of its -navigation. The distance between New York and Albany, ascertained by -a late survey to be one hundred and twenty-five geographical miles by -water, had been performed by Fulton's boats occasionally in fifteen -or sixteen hours, being at the rate of about eight miles an hour, -including stoppages. It became a great object to increase the speed -of this trip, so that it might at all times of the year be performed -between sunrise and sunset. Robert L. Stevens, the son of the person -of that name already mentioned, immediately after the abolition -of Fulton's monopoly, placed on the river a vessel which had been -built for the Delaware, which easily performed the passage in twelve -hours, being at the rate of nearly ten and a half geographical miles -an hour. By this increase of speed the improved boats so entirely -monopolised the day work upon the river, that the former steamers -were either converted into steam tugs to draw barges laden with -goods, or used for night trips between New York and Albany. In the -night trips the saving of one or two hours was immaterial, it being -sufficient that the vessel which left the one port at night should -reach the other in the morning. [Pg492] - -The river Hudson rises near Lake Champlain, the easternmost of the -great chain of lakes or inland seas which extend from east to west -across the northern boundary of the United States. The river -follows nearly a straight course southwards for two hundred and -fifty miles, and empties itself into the sea at New York. The -influence of the tide is felt as far as Albany, above which the -stream begins to contract. Although this river in magnitude and -extent is by no means equal to several others which intersect the -States, it is nevertheless rendered an object of great interest by -reason of the importance and extent of its trade. The produce of -the state of New York and that of the banks of the great Lakes -Ontario and Erie are transported by it to the capital; and one of -the most extensive and populous districts of the United States is -supplied with the necessary imports by its waters. A large fleet -of vessels is constantly engaged in its navigation; nor is the -tardy but picturesque sailing vessel as yet excluded by the more -rapid steamers. The current of the Hudson is said to average -nearly three miles an hour; but as the ebb and flow of the tide -are felt as far as Albany, the passage of the steamers between -that place and New York may be regarded as equally affected by -currents in both directions, or nearly so. The passage therefore, -whether in ascending or descending the river, is made nearly in -the same time. - - -(231.) The prevalence of smooth water navigation, whether on the -surfaces of rivers or in sheltered bays and sounds, has invested -the problem of steam navigation in America with conditions so -entirely distinct and different from those under which the same -problem presents itself to the European engineer, that any -comparison of the performance of vessels, whether with regard to -speed or the absorption of power in the two cases, must be utterly -fallacious. In Europe a steamer is almost invariably a vessel -designed to encounter the agitated surface of an open sea, and is -accordingly constructed upon principles of suitable strength and -stability. It is likewise supplied with rigging and with sails, to -be used in aid of the mechanical power, and manned and commanded -by experienced seamen; in fact, it is a combination of a nautical -and mechanical structure. In America, on the other hand, [Pg493] -with the exception of the vessels which navigate the great -northern lakes, the steamers are structures exclusively -mechanical, being designed for smooth water. They require no other -strength or stability than that which is sufficient to enable them -to float and to bear a progressive motion through the water. Their -mould is conceived with an exclusive view to speed; they are -therefore slender and weak in their build, of great length in -proportion to their width, and having a very small draught of -water. In fact, they approach in their form to that of a Thames -wherry on a very large scale. - -The position and form of the machinery is likewise affected by -these conditions. Without the necessity of being protected from a -rough sea, it is placed on the deck in an elevated position. The -cylinders of large diameter and short stroke invariably used in -Europe are unknown in America, and the proportions are reversed, a -small diameter and stroke of great length being invariably -adopted. It is rarely that two engines are used. A single engine, -placed in the centre of the deck, with a cylinder from forty to -sixty inches' diameter, and from eight to ten foot stroke, drives -paddle-wheels from twenty-one to twenty-five feet in diameter, -producing from twenty-five to thirty revolutions per minute. The -great magnitude of the paddle-wheels and the velocity imparted to -them enable them to perform the office of fly-wheels, and to carry -the engine round its centres, not however without a perceptible -inequality of motion, which gives to the American steamer an -effect like that of a row boat advancing by starts with each -stroke of the piston. The length of stroke adopted in these -engines enables them to apply with great effect the expansive -principle, which is almost universally used, the steam being -generally cut off at half stroke. - -The steamers which navigate the Hudson are vessels of considerable -magnitude, splendidly fitted up for the accommodation of -passengers; they vary from one hundred and eighty to two hundred -and forty feet in length, and from twenty to thirty feet in width -of beam. In the following table is given the particulars of nine -steamers plying on this river, taken from [Pg494] the work of Mr. -Stevenson, and from the paper of Mr. Renwick, inserted in the last -edition of Tredgold:-- - - ------------------------------------------------------------- - | Length | Breadth | Draft | Drain | Length - Names. | of | of | of | of | of - | Deck. | Beam. | Water. | Wheel. | Paddles. - ------------------------------------------------------------- - | Ft. | Ft. | Ft. | Ft. | Ft. - Dewit Clinton | 230 | 28 | 5·5 | 21 | 13·7 - Champlain | 180 | 27 | 5·5 | 22 | 15 - Erie | 180 | 27 | 5·5 | 22 | 15 - North America | 200 | 30 | 5 | 21 | 13 - Independence | 148 | 26 | -- | -- | -- - Albany | 212 | 26 | -- | 24·5 | 14 - Swallow | 233 | 22·5 | 3·75 | 24 | 11 - Rochester | 200 | 25 | 3·75 | 23·5 | 10 - Utica | 200 | 21 | 3·5 | 22 | 9·5 - ------------------------------------------------------------- - - ---------------------------------------------------------------------- - | Depth | Number | Drain | Length| Number | Part of - Names. | of | of | of | of | of | Stroke - | Paddles| Engines| Cylinder| Stroke| Revs | at which - | | | | | | it is - | | | | | | cut off. - ---------------------------------------------------------------------- - | In. | | In. | Ft. | | - Dewit Clinton | 36 | 1 | 65 | 10 | 29 | 3/4 - Champlain | 34 | 2 | 44 | 10 | 27·5 | 1/2 - Erie | 34 | 2 | 44 | 10 | 27·5 | 1/2 - North America | 30 | 2 | 44·5 | 8 | 24 | 1/2 - Independence | -- | 1 | 44 | 10 | | - Albany | 30 | 1 | 65 | -- | 19 | - Swallow | 30 | 1 | 46 | -- | 27 | - Rochester | 24 | 1 | 43 | 10 | 28 | - Utica | 24 | 1 | 39 | 10 | | - ---------------------------------------------------------------------- - -None of these vessels have either masts or rigging, and -consequently never derive any propelling power except from the -engines: they are neither manned nor commanded by persons having -any knowledge of navigation: the works that are visible above -their decks are the beam and framing of the engine, and the -chimneys. - -The engines used for steamers on the Hudson, and other great -rivers and bays on the eastern coast of America, are most commonly -condensing engines, but they nevertheless work with steam of very -high pressure, being seldom less than twenty-five pounds per -square inch, and sometimes as much as fifty. By reference to the -preceding table it will be seen, that the velocity of the piston -greatly exceeds the limit generally observed in Europe. It is -customary in European marine engines to limit the speed of the -piston to about two hundred and twenty feet per minute. Even the -piston of a locomotive engine does not much exceed the rate of -three hundred feet per minute. In the American steamers, however, -the pistons commonly move at the rate of from five to six hundred -feet per minute, while the circumference of the paddle-wheels are -driven at the rate of from twenty to twenty-two miles an hour. -[Pg495] - -[Illustration: _Fig._ 135.] - -The hulls of these boats are formed with a perfectly flat bottom -and perpendicular sides, rounded at the angles, as represented in -_fig._ 135. At the bow, or cutwater, they are made very sharp, and -the deck projects to a great distance over the sides. The weight -of the machinery is distributed over an extensive surface of the -bottom of this feeble structure, by means of a frame-work of -substantial carpentry to which it is attached. - -At the height of from four to six feet above the water-line is -placed the deck, which is a platform, having the shape of a very -elongated ellipse. The extremities of its longer axis are -supported by the sternpost and the cutwater, and its sides expand -in gentle curves on either hand to a considerable distance beyond -the limits of the hull; those parts of the deck thus overhanging -the water are called the wheel guards. - -Beneath the first deck is the saloon, or dining-room, which also, -as is usual in European steamers, forms the gentlemen's -sleeping-room. It usually extends from end to end of the vessel. -The middle of the first deck is occupied by the engine, boilers, -furnaces, and chimneys, of which latter there are generally two. -Between the chimneys and the stern, above the first deck, is -constructed the ladies' cabin, which is covered by the second -deck, called the promenade deck. The great length of these boats -and the elevation of the cabins render it impossible for a -steersman at the stern to see ahead, and they are, consequently, -steered from the bow; the wheel placed there communicating with -the helm at the stern, by chains or rods carried along the sides -of the boat. Until a recent period, the wheel was connected with -the stern by ropes, but some fatal accidents, produced by fire, -[Pg496] in which these ropes were burnt, and the steersman lost -all power to guide the vessel, caused metal rods or chains to be -substituted. - - -(232.) The paddle-wheels universally used in American steam-boats -are formed, as if by the combination of two or more common -paddle-wheels, placed one outside the other, on the same axle, but -so that the paddle boards of each may have an intermediate -position between those of the adjacent one, as represented in -_fig._ 136. - -[Illustration: _Fig._ 136.] - -The spokes, which are bolted to cast-iron flanges, are of wood. -These flanges, to which they are so bolted, are keyed upon the -paddle shaft. The outer extremities of the spokes are attached to -circular bands or hoops of iron, surrounding the wheel; and the -paddle boards, which are formed of hard wood, are bolted to the -spokes. The wheels thus constructed, sometimes consist of three, -and not unfrequently four, independent circles of paddle boards, -placed one beside the other, and so adjusted in their position, -that the boards of no two divisions shall correspond. - -The great magnitude of the paddle-wheels, and the circumstance of -the navigation being carried on, for the most part, in smooth -water, have rendered unnecessary, in America, the adoption of any -of those expedients for neutralising the effects of the oblique -action of the paddles, which have been tried, but hitherto with so -little success, in Europe. - - -(233.) Sea-going steamers are not numerous in America, the chief -of them being those which ply between New York and Providence, and -between New York and Charleston. These vessels, however, do not -resemble the sea-going steamers of Europe as closely as might be -expected; and to those who are accustomed to the latter, the -sea-going [Pg497] steamers of America can hardly be regarded as -safe means of transport. - -In the following Table is given the dimensions of five of these -vessels, all plying between New York and Providence:-- - - -------------------------------------------------------------- - | Length | Breadth | Draft | Diameter | Length - Names. | of | of | | of | of - | Deck. | Beam. | | Wheel. | Paddles. - -------------------------------------------------------------- - | Ft. | Ft. | Ft. | Ft. | Ft. - Providence | 180 | 27 | 9 | -- | -- - Lexington | 207 | 21 | -- | 23 | 9 - Narragansett | 210 | 26 | 5 | 25 | 11 - Massachusetts | 200 | 29·5 | 8·5 | 22 | 10 - Rhode Island | 210 | 26 | 6·5 | 24 | 11 - -------------------------------------------------------------- - --------------------------------------------------------------------- - | Depth | Number | Diameter| Length| Number | Part of - Names. | of | of | of | of | of | Stroke - | Paddles| Engines| Cylinder| Stroke| Revs | at which - | | | | | | it is - | | | | | | cut off. - --------------------------------------------------------------------- - | In. | | In. | Ft. | | - Providence | -- | 1 | 10 | 65 | | - Lexington | 30 | 1 | 11 | 48 | 24 | - Narragansett | 30 | 1 | 60 | 12 | 2 | 1/2 - Massachusetts | 28 | 2 | 44 | 8 | 26 | - Rhode Island | 30 | 1 | 11 | 60 | 21 | - --------------------------------------------------------------------- - -The Narragansett, the finest of these vessels, is built of oak, -strengthened by diagonal straps or ties of iron, by which her -timbers are connected; she is driven by a condensing engine, and -has two boilers, exposing about three thousand square feet of -surface to the fire. The steam is maintained at a pressure of from -twenty to twenty-five lbs. per square inch: the cylinder is -horizontal. - -The cabins of these sea-boats are of great magnitude, and afford -excellent accommodation for passengers, containing generally four -hundred berths. In the Massachusetts the chief cabin is one hundred -and sixty feet long, twenty-two feet wide, and twelve feet in -height, its vast extent being uninterrupted by pillars or any other -obstruction. "I have dined," says Mr. Stevenson, "with one hundred -and seventy-five persons in this cabin, and, notwithstanding this -numerous assembly, the tables, which were arranged in two parallel -rows, extending from one end of the cabin to the other, were far -from being fully occupied, the attendance was good, and every thing -was conducted with perfect regularity and order. There are one -hundred and twelve fixed berths ranged round this cabin, and one -hundred temporary berths can be erected in the middle of the floor: -besides these there are sixty fixed berths in the ladies' cabin, and -several temporary sleeping [Pg498] places can be erected in it -also. The cabin of the Massachusetts is by no means the largest in -the United States. Some steamers have cabins upwards of one hundred -and seventy-five feet in length. Those large saloons are lighted by -Argand lamps, suspended from the ceiling, and their appearance, when -brilliantly lighted up and filled with company, is very remarkable. -The passengers generally arrange themselves in parties at the -numerous small tables into which the large tables are converted -after dinner, and engage in different amusements. The scene -resembles much more the coffee-room of some great hotel than the -cabin of a floating vessel." - - -(234.) Nothing has excited more surprise among engineers and -others interested in steam navigation in Europe, than the -statements which have been so generally and so confidently made of -the speed attained by American steamers. This astonishment is due -to several causes, the chief of which is the omission of all -notice of the great difference between the structure and operation -of the American steamers and the nature of the navigation in which -they are engaged, compared with the structure and operation of, -and the navigation in which European steamers are employed: as -well might the performance of a Thames wherry, or one of the -fly-boats on the northern canals, be compared with that of the -Great Western, or the British Queen. The statements alluded to all -have reference to steamers navigating the Hudson between New York -and Albany, the form and structure of which we have already -described; and doubtless the greatest speed ever attained on the -surface of water has been exhibited in the passages of these -vessels. - -Mr. Stevenson states, that exclusive of the time lost in -stoppages, the voyage between New York and Albany is usually made -in ten hours. Dr. Renwick, however, who has probably more -extensive opportunities of observation, states, that the average -time, exclusive of stoppages, is ten hours and a half. The -distance being 125·18 geographical miles, the average rate would -therefore be 11-9/10 miles per hour. If it be observed that the -average rate of some of the best sea-going steamers in Europe -obtained from experiments [Pg499] and observations made by -myself, more than three years ago, showed a rate of steaming -little less than ten geographical miles per hour, and that since -that time considerable improvements in steam navigation have been -made, and further, that these performances were made under -exposure to all the disadvantages of an open sea, the difference -between them and the performance of the American river steamers -will cease to create astonishment. - -Dr. Renwick states that he made, in a boat called the "New -Philadelphia," one of the most remarkable passages ever performed. -He left New York at five in the afternoon, with the first of the -flood, and landed at Catskill, distant 95·8 geographical miles -from New York, at a quarter before twelve. Passengers were landed -and taken in at seven intermediate points: the rate, including -stoppages, was therefore 14·2 miles per hour; and if half an hour -be allowed for stoppages, the actual average rate of motion would -be fifteen miles and three quarters an hour. As the current, which -in this case was with the course of the vessel, did not exceed -three miles and a half an hour, the absolute velocity through the -water would have been somewhat under twelve miles an hour. This -speed is nearly the same as the speed obtained from taking the -average time of the voyages between New York and Albany at ten -hours and a half; it would therefore appear that the great speed -attained in this trip must have been chiefly, if not altogether, -owing to the effect of the current. - - -(235.) The steamers which navigate the great northern lakes differ -so little in their construction and appearance from the European -steam-boats, that it will not be necessary here to devote any -considerable space to an account of them. These vessels were -introduced on the lakes at about the same time that steamers were -first introduced on the Clyde. These steamers are strongly built -vessels, supplied with sails and rigging, and propelled by -powerful engines. The largest in 1837, when Mr. Stevenson visited -the States, was the _James Madison_. This vessel was one hundred -and eighty-one feet in length on the deck, thirty feet in breadth -of beam, and twelve feet six inches in depth of hold: her draught -of water was ten feet, and her measured capacity seven hundred -[Pg500] tons. She plyed between Buffalo on Lake Erie and Chicago -on Lake Michigan, a distance of nine hundred and fifty miles. - -The severe storms and formidable sea encountered on the lakes -render necessary for the navigation, vessels in all respects as -strong and powerful as those which navigate the open ocean. - - -(236.) By far the most remarkable and important of all the -American rivers is the Mississippi and its tributaries. That part -of the American continent which extends from the southern shores -of the great northern lakes to the northern shores of the Gulf of -Mexico, is watered by these great streams. The main stream of the -Mississippi has its fountains in the tract of country lying north -of the Illinois and east of Lake Michigan, in latitude forty-three -degrees. At about latitude thirty-nine degrees, a little north of -St. Louis, it receives the waters of the Missouri, and further -south, at the latitude of thirty-seven degrees, the Ohio flows -into it, after traversing five degrees of longitude and four of -latitude, and winding its way from the Alleghany range through -several of the states, and forming a navigable communication with -numerous important towns of the Union, among which may be -mentioned Pittsburg, Cincinnati, Frankfort, Lexington, and -Louisville. The main stream of the Mississippi, after receiving -the waters of the Arkansas, and numerous other minor tributaries, -flows into the Gulf of Mexico by four mouths. The main stream of -the Mississippi, independently of its tributaries, forms an -unbroken course of inland navigation for a distance of nearly two -thousand three hundred miles. Its width, through a distance of one -thousand one hundred miles from its mouth, is not less than half a -mile, and its average depth a hundred feet. The Ohio, its chief -eastern tributary, flowing into it at a distance of about a -thousand miles from its mouth, traverses also about the same -extent of country, and is navigable throughout the whole of that -extent. This river also has several navigable tributaries of -considerable extent, among which may be mentioned the Muskingum, -navigable for one hundred and twenty miles; the Miami, navigable -for seventy-five miles; the Scioto, navigable for one hundred and -twenty [Pg501] miles; the Tennessee, navigable for two hundred -and fifty miles; the Cumberland, navigable for four hundred and -forty miles; the Kentucky, navigable for one hundred and thirty -miles; and the Green River, navigable for one hundred and fifty -miles. The total length of the Ohio and its tributaries is -estimated at above seven thousand miles. - - -(237.) Steam-boats were introduced on the Mississippi about the -year 1812, the period of their first introduction in Europe; and -their increase has been rapid beyond all precedent. In the year -1831 there were one hundred and ninety-eight steamers plying on -its waters; and the number in 1837 amounted to nearly four -hundred. These vessels are built chiefly on the banks of the Ohio, -at the towns of Pittsburg and Cincinnati, at distances of about -two thousand miles from the mouth of the river they are intended -to navigate. - - -(238.) These steamers, which are decidedly inferior to those which -navigate the eastern waters, are generally of a heavy build, -fitted to carry goods as well as passengers, and vary from one -hundred to seven hundred tons burthen. Their draught of water is -also greater than that of the eastern river steamers--varying from -six to eight feet. The hull, at about five feet from the water -line, is covered with a deck, under which is the hold, in which -the heavy part of the cargo is stowed. About the middle of this -deck the engines are placed, the boilers and furnaces occupying a -space nearer to the bow, near which two chimneys are placed. The -fire-doors of the furnaces are presented towards the bow, and -exposed so as to increase the draught. That part of the first deck -which extends from the machinery to the stern is the place -allotted to the crew and the deck passengers, and is described as -being filthy and inconvenient in the extreme. A second deck is -constructed, which extends from the chimneys near the bow to the -stern of the vessel. On this is formed the great cabin or saloon, -which extends from the chimneys to within about thirty feet of the -stern, where it is divided by a partition from the ladies' cabin, -which occupies the remaining space. These principal cabins are -surrounded by a gallery about three feet in width, from which, at -convenient [Pg502] places, an ascent is supplied by stairs to the -highest deck, called the hurricane or promenade deck. - - -(239.) The engines by which these boats are propelled are totally -different from the machinery already described as used in the -eastern steamers. They are invariably non-condensing engines, -worked by steam of extremely high pressure; the boilers are -therefore tubular, and the cylinders small in diameter, but -generally having a long stroke. - -The pressure of steam used in these machines is such as is never -used in European engines, even when worked on railways. A pressure -of one hundred pounds per inch is here considered extremely -moderate. The captain of one of these boats, plying between -Pittsburg and St. Louis, told Mr. Stevenson that "under ordinary -circumstances his safety valves were loaded with a pressure equal -to one hundred and thirty-eight pounds per square inch, but that -the steam was occasionally raised as high as one hundred and fifty -pounds to enable the vessel to pass parts of the river in which -there is a strong current;" and he added, by way of consolation, -that "this pressure was never exceeded except on _extraordinary -occasions_!" - -The dimensions and power of the Mississippi steamers may be -collected from those of the St. Louis, a boat which was plying on -that river in 1837. That vessel measured two hundred and fifty -feet on deck, and had twenty-eight feet breadth of beam. Her -draught of water was eight feet, and her measured capacity one -thousand tons. She was propelled by two engines with thirty-inch -cylinders, and ten feet stroke; the safety valve being loaded at -one hundred pounds per square inch. - -The paddle wheels of these vessels are attached to the paddle -shaft, in such a manner as to be thrown into and out of gear, at -discretion, by the engineer, so that the paddle shaft may revolve -without driving the wheels: by this expedient the power of the -engine is used to feed the boilers while the vessel stops at the -several stations. The vessel is therefore stopped, not, as is -usually the case, by stopping the engines, but by throwing the -wheels out of connection with the paddle shaft. The engines -continue to work, but their [Pg503] power is expended in forcing -water into the boiler. By this expedient the activity of the -engines may, within practical limits, be varied with the -resistance the vessel has to encounter. In working against a -strong current, the feed may be cut off from the boilers, and the -production of steam, and consequently the power of the engines, -thereby stimulated, while this suspension of the feed may be -compensated at the next station. - -The stoppages to take in goods and passengers, and for relays of -fuel, are frequent. "The liberty which they take with their -vessels on these occasions," says Mr. Stevenson, "is somewhat -amusing: I had a good example of this on board a large vessel, -called the Ontario. She was steered close in shore amongst stones -and stumps of trees, where she lay for some hours to take in -goods: the additional weight increased her draught of water, and -caused her to heel a good deal; and when her engines were put in -motion, she actually _crawled_ into the deep water on her paddle -wheels: the steam had been got up to an enormous pressure to -enable her to get off, and the volume of steam discharged from the -escapement pipe at every half stroke of the piston made a sharp -sound almost like the discharge of fire-arms, while every timber -in the vessel seemed to tremble, and the whole structure actually -groaned under the shocks." - -Besides the steamers used for the navigation of the Mississippi, -innumerable steam tugs are constantly employed in towing vessels -between the port of New Orleans and the open sea of the Gulf of -Mexico. Before the invention of steam navigation, this southern -capital of the United States laboured under the disadvantage of -possessing almost the only bad and inconvenient harbour in the -vast range of coast by which the country is bounded. New Orleans -lies at a distance of about one hundred miles from the Gulf of -Mexico. The force of the stream, the frequency of shoals, and the -winding course of the channel rendered it scarcely possible for a -sailing vessel to pass between the port and the sea with the same -wind. The anchorage was every where bad, and great difficulty and -risk attended the mooring of large vessels to the banks. The steam -engine has, however, overcome all [Pg504] these difficulties, and -rendered the most objectionable harbour of the Union a safe and -good seaport, perfectly easy of approach and of egress at all -times; a small steam tug will take in tow several large ships, and -carry them with safety and expedition to the offing, where it will -dismiss them on their voyage, and take back vessels which may have -arrived. - -[Illustration: GREAT WESTERN OFF NEW YORK.] - -[Pg505] - - - - -APPENDIX. - - _On the Relation between the Temperature, Pressure, and Density_ - _of Common Steam._ - - -There is a fixed relation between the temperature and pressure of -common steam, which has not yet been ascertained by theory. -Various empirical formulæ have been proposed to express it, -derived from tables of temperatures and corresponding pressures -which have been founded on experiments and completed by -interpolation. - -The following formula, proposed by M. Biot, represents with great -accuracy the relation between the temperature and pressure of -common steam, throughout all that part of the thermometric scale -to which experiments have been extended. - -Let - - a = 5·96131330259 - log. a_{1} = 0·82340688193 - 1 - log. b_{1} = -·01309734295 - log. a_{2} = 0·74110951837 - log. b_{2} = -·00212510583 - -The relation between the temperature t with reference to the -centesimal thermometer, and the pressure p in millimètres of -mercury at the temperature of melting ice, will then be expressed -by the following formula:-- - - log. p = a - a_{1}b_{1}^{20 + t} - a_{2}b_{2}^{20 + t}. (1.) - -Formulæ have, however, been proposed, which, though not applicable -to the whole scale of temperatures, are more manageable in their -practical application than the preceding. - -For pressures less than an atmosphere, Southern proposed the -following formula, where the pressure is intended to be expressed -[Pg506] in pounds per square inch, and the temperature in -reference to Fahrenheit's thermometer,-- - - p = 0·04948 + ((51·3 + t) / 155·7256)^{5·13} | - |. (2.) - t = 155·7256 ((p - 0·04948) - 51·3)^{1/(5·13)} | - - -The following formula was proposed by Tredgold, where p expresses -the pressure in inches of mercury:-- - - p = ((100 + t) / 177)^{6}. - -This was afterwards modified by Mellet, and represents with -sufficient accuracy experiments from 1 to 4 atmospheres. Let p -represent pounds per square inch, and t the temperature by -Fahrenheit's thermometer,-- - - p = ((103 + t) / 201·18)^{6} | - |. (3.) - t = 201·18 p^{1/6} - 103 | - -M. de Pambour has proposed the following formula, also applicable -through the same limits of the scale:-- - - p = ((98·806 + t) / 198·562)^{6} | - |. (4.) - t = 198·562 p^{1/6} - 98·806 | - -MM. Dulong and Arago have proposed the following formula for all -pressures between 4 and 50 atmospheres:-- - - p = (0·26793 + 0·0067585 t)^{5} | - |. (5.) - t = 147·961 p^{1/5} - 39·644 | - -It was about the year 1801, that Dalton, at Manchester, and -Gay-Lussac, at Paris, instituted a series of experiments on -gaseous bodies, which conducted them to the discovery of the law -mentioned in art. (96.), p. 171. These philosophers found that all -gases whatever, and all vapours raised from liquids by heat, as -well as all mixtures of gases and vapours, are subject to the -_same quantity of expansion_ between the temperatures of melting -ice and boiling water; and by experiments subsequently made by -Dulong and Petit, this uniformity of expansion has been proved to -extend to all temperatures which can come under practical -inquiries. - -Dalton found that 1000 cubic inches of air at the temperature of -melting ice dilated to 1325 cubic inches if raised to the -temperature of boiling water. According to Gay-Lussac, the -increased volume was 1375 cubic inches. The latter determination -has been subsequently found to be the more correct one.[40] - -[Pg507] It appears, therefore, that for an increase of temperature -from 32° to 212°, amounting to 180°, the increase of volume is 375 -parts in 1000; and since the expansion is uniform, the increase of -volume for 1° will be found by dividing this by 180, which will -give an increase of 208-1/3 parts in 100,000 for each degree of -the common thermometer. - -To reduce the expression of this important and general law to -mathematical language, let v be the volume of an elastic fluid at -the temperature of melting ice, and let nv be the increase which -that volume would receive by being raised one degree of -temperature under the same pressure. Let V be its volume at the -temperature T. Then we shall have - - V = v + nv (T - 32) = v (1 + n (T - 32)). - -If V' be its volume at any other temperature T', and under the -same pressure, we shall have, in like manner, - - V' = v (1 + n (T' - 32)). - -Hence we obtain - - V/V' = (1 + n (T - 32)) / (1 + n (T' - 32)); (6.) - -which expresses the relation between the volumes of the same gas -or vapour under the same pressure and at any two temperatures. The -co-efficient n, as explained in the text, has the same value for -the same gas or vapour throughout the whole thermometric scale. -But it is still more remarkable that this constant has the same -value for all gases and vapours. It is a number, therefore, which -must have some essential relation to the gaseous or elastic state -of fluid matter, independent of the peculiar qualities of any -particular gas or vapour. - -The value of n, according to the experiments of Gay-Lussac, is -0·002083, or 1/480. - -To reduce the law of Mariotte, explained in (97.) p. 171., to -mathematical language, let V, V' be the volumes of the same gas or -vapour under different pressures P, P', but at the same -temperature. We shall then have - - VP = V'P'. (7.) - -If it be required to determine the relation between the volumes of -the same gas or vapour, under a change of both temperature and -pressure, let V be the volume at the temperature T and under the -pressure P, and let V' be the volume at the temperature T' and -under the pressure P'. Let v be the volume at the temperature T -and under the pressure P'. - -By formula (7.) we have - - VP = vP'; - - -[Pg508] and by formula (6.) we have - - (V'/v) = (1 + n(T' - 32)) / (1 + n(T - 32)) - -Eliminating v, we shall obtain - - (V/V') = (P'/P) · (1 + n(T - 32)) / (1 + n(T' - 32)); - -or, - - (VP/V'P') = (1 + n(T - 32)) / (1 + n(T' - 32)); (8.) - -which is the general relation between the volumes, pressures, and -temperatures of the same gas or vapour in two different states. - -To apply this general formula to the case of the vapour of water, -let T' = 212°. It is known by experiment that the corresponding -value of P', expressed in pounds per square inch, is 14·706; and -that V', expressed in cubic inches, the water evaporated being -taken as a cubic inch, is 1700. If, then, we take 0·002083 as the -value of n, we shall have by (8.), - - VP = 1700 × 14·706 × (1 + 0·002083 (T - 32)) / (1 + 0·002083 × 180) - - = 18183(1 + 0·002083 (T - 32)). (9.) - -If, by means of this formula (9.), and any of the formulæ (1.), -(2.), (3.), (4.), (5.), T were eliminated, we should obtain a -formula between V and P, which would enable us to compute the -enlargement of volume which water undergoes in passing into steam -under any proposed pressure. But such a formula would not be -suitable for practical computations. By the formulæ (1.) to (5.), -a table of pressures and corresponding temperatures may be -computed; and these being known, the formula (9.) will be -sufficient for the computation of the corresponding values of V, -or the enlargement of volume which water undergoes in passing into -steam. - -In the following table, the temperatures corresponding to -pressures from 1 to 240 lbs. per square inch are given by -computation from the formulæ (2.) to (5.), and the volumes of -steam produced by an unit of volume of water as computed from the -formula (9.). - -The mechanical effect is obtained by multiplying the pressure in -pounds by the expansion of a cubic inch of water in passing into -steam expressed in feet, and is therefore the number of pounds -which would be raised one foot by the evaporation of a cubic inch -of water under the given pressure. [Pg509] - - --------------------------------------------------------- - | | Volume of | Mechanical - Total pressure| | the Steam | Effect of - in Pounds | Corresponding| compared to | a Cubic Inch - per Square | Temperature. | the Volume | of Water - Inch. | | of the | evaporated - | | Water that | in Pounds - | | has | raised One - | | produced it.| Foot. - --------------------------------------------------------- - 1 | 102·9 | 20868 | 1739 - 2 | 126·1 | 10874 | 1812 - 3 | 141·0 | 7437 | 1859 - 4 | 152·3 | 5685 | 1895 - 5 | 161·4 | 4617 | 1924 - 6 | 169·2 | 3897 | 1948 - 7 | 175·9 | 3376 | 1969 - 8 | 182·0 | 2983 | 1989 - 9 | 187·4 | 2674 | 2006 - 10 | 192·4 | 2426 | 2022 - 11 | 197·0 | 2221 | 2036 - 12 | 201·3 | 2050 | 2050 - 13 | 205·3 | 1904 | 2063 - 14 | 209·1 | 1778 | 2074 - 15 | 212·8 | 1669 | 2086 - 16 | 216·3 | 1573 | 2097 - 17 | 219·6 | 1488 | 2107 - 18 | 222·7 | 1411 | 2117 - 19 | 225·6 | 1343 | 2126 - 20 | 228·5 | 1281 | 2135 - 21 | 231·2 | 1225 | 2144 - 22 | 233·8 | 1174 | 2152 - 23 | 236·3 | 1127 | 2160 - 24 | 238·7 | 1084 | 2168 - 25 | 241·0 | 1044 | 2175 - 26 | 243·3 | 1007 | 2182 - 27 | 245·5 | 973 | 2189 - 28 | 247·6 | 941 | 2196 - 29 | 249·6 | 911 | 2202 - 30 | 251·6 | 883 | 2209 - 31 | 253·6 | 857 | 2215 - 32 | 255·5 | 833 | 2221 - 33 | 257·3 | 810 | 2226 - 34 | 259·1 | 788 | 2232 - 35 | 260·9 | 767 | 2238 - 36 | 262·6 | 748 | 2243 - 37 | 264·3 | 729 | 2248 - 38 | 265·9 | 712 | 2253 - 39 | 267·5 | 695 | 2259 - 40 | 269·1 | 679 | 2264 - 41 | 270·6 | 664 | 2268 - 42 | 272·1 | 649 | 2273 - 43 | 273·6 | 635 | 2278 - 44 | 275·0 | 622 | 2282 - 45 | 276·4 | 610 | 2287 - 46 | 277·8 | 598 | 2291 - 47 | 279·2 | 586 | 2296 - 48 | 280·5 | 575 | 2300 - 49 | 281·9 | 564 | 2304 - 50 | 283·2 | 554 | 2308 - 51 | 284·4 | 544 | 2312 - 52 | 285·7 | 534 | 2316 - 53 | 286·9 | 525 | 2320 - 54 | 288·1 | 516 | 2324 - 55 | 289·3 | 508 | 2327 - 56 | 290·5 | 500 | 2331 - 57 | 291·7 | 492 | 2335 - 58 | 292·9 | 484 | 2339 - 59 | 294·2 | 477 | 2343 - 60 | 295·6 | 470 | 2347 - 61 | 296·9 | 463 | 2351 - 62 | 298·1 | 456 | 2355 - 63 | 299·2 | 449 | 2359 - 64 | 300·3 | 443 | 2362 - 65 | 301·3 | 437 | 2365 - 66 | 302·4 | 431 | 2369 - 67 | 303·4 | 425 | 2372 - 68 | 304·4 | 419 | 2375 - 69 | 305·4 | 414 | 2378 - 70 | 306·4 | 408 | 2382 - 71 | 307·4 | 403 | 2385 - 72 | 308·4 | 398 | 2388 - 73 | 309·3 | 393 | 2391 - 74 | 310·3 | 388 | 2394 - 75 | 311·2 | 383 | 2397 - 76 | 312·2 | 379 | 2400 - 77 | 313·1 | 374 | 2403 - 78 | 314·0 | 370 | 2405 - 79 | 314·9 | 366 | 2408 - 80 | 315·8 | 362 | 2411 - 81 | 316·7 | 358 | 2414 - 82 | 317·6 | 354 | 2417 - 83 | 318·4 | 350 | 2419 - 84 | 319·3 | 346 | 2422 - 85 | 320·1 | 342 | 2425 - 86 | 321·0 | 339 | 2427 - 87 | 321·8 | 335 | 2430 - 88 | 322·6 | 332 | 2432 - 89 | 323·5 | 328 | 2435 - 90 | 324·3 | 325 | 2438 - 91 | 325·1 | 322 | 2440 - 92 | 325·9 | 319 | 2443 - 93 | 326·7 | 316 | 2445 - 94 | 327·5 | 313 | 2448 - 95 | 328·2 | 310 | 2450 - 96 | 329·0 | 307 | 2453 - 97 | 329·8 | 304 | 2455 - 98 | 330·5 | 301 | 2457 - 99 | 331·3 | 298 | 2460 - 100 | 332·0 | 295 | 2462 - 110 | 339·2 | 271 | 2486 - 120 | 345·8 | 251 | 2507 - 130 | 352·1 | 233 | 2527 - 140 | 357·9 | 218 | 2545 - 150 | 363·4 | 205 | 2561 - 160 | 368·7 | 193 | 2577 - 170 | 373·6 | 183 | 2593 - 180 | 378·4 | 174 | 2608 - 190 | 382·9 | 166 | 2622 - 200 | 387·3 | 158 | 2636 - 210 | 391·5 | 151 | 2650 - 220 | 395·5 | 145 | 2663 - 230 | 399·4 | 140 | 2675 - 240 | 403·1 | 134 | 2687 - --------------------------------------------------------- - -[Pg511] In the absence of any direct method of determining the -general relation between the pressure and volume of common steam, -empirical formulæ expressing it have been proposed by different -mathematicians. - -The late Professor Navier proposed the following:--Let S express -the volume of steam into which an unit of volume of water is -converted under the pressure P, this pressure being expressed in -kilogrammes per square mètre. Then the relation between S and P -will be - - S = a/(b + mP), - - where a = 1000, b = 0·09, and m = 0·0000484. - -This formula, however, does not agree with experiment at pressures -less than an atmosphere. M. de Pambour, therefore, proposes the -following changes in the values of its co-efficients:--Let P -express the pressure in pounds per square foot; and let - - a = 10000 b = 0·4227 m = 0·00258, - -and the formula will be accurate for all pressures. For pressures -above two atmospheres the following values give more accuracy to -the calculation:-- - - a = 10000 b = 1·421 m = 0·0023. - -In these investigations I shall adopt the following modified -formula. The symbols S and P retaining their signification, we -shall have - - S = a/(b + P) (10.) - -where - - a = 3875969 b = 164. - -These values of a and b will be sufficiently accurate for practical -purposes for all pressures, and may be used in reference to -low-pressure engines of every form, as well as for high-pressure -engines which work expansively. - -When the pressure is not less than 30 pounds per square inch, the -following values of a and b will be more accurate:-- - - a = 4347826 b = 618. - - _On the Expansive Action of Steam._ - -The investigation of the effect of the expansion of steam which -has been given in the text, is intended to convey to those who are -not conversant with the principles and language of analysis, some -notion of the nature of that mechanical effect to which the -advantages attending the expansive principle are due. We shall -now, however, explain these effects more accurately. [Pg512] - -The dynamical effect produced by any mechanical agent is expressed -by the product of the resistance overcome and the space through -which that resistance is moved. - - Let - P = the pressure of steam expressed in pounds per square foot. - S = the number of cubic feet of steam of that pressure - produced by the evaporation of a cubic foot of water. - E = the mechanical effect produced by the evaporation of a - cubic foot of water expressed in pounds raised one foot. - -Then we shall have E = PS; and if W be a volume of water -evaporated under the pressure P, the mechanical effect produced by -it will be WPS. - -By (10.) we have - - SP = a - bS. - -Hence, for the mechanical effect of a cubic foot of water -evaporated under the pressure P we have - - E = a - bS. (11.) - -Let a cubic foot of water be evaporated under the pressure P', and -let it produce a volume of steam S' of that pressure. Let this -steam afterwards be allowed to expand to the increased volume S -and the diminished pressure P; and let it be required to determine -the mechanical effect produced during the expansion of the steam -from the volume S' to the volume S. - - Let - E' = the mechanical effect produced by the evaporation of - the water under the pressure P' without expansion. - E'' = the mechanical effect produced during the expansion - of the steam. - E = the mechanical effect which would be produced by - the evaporation under the pressure P without expansion. - _E_ = the total mechanical effect produced by the evaporation - under the pressure P' and subsequent expansion. - -Thus we have - - _E_ = E' + E''. - -Let s be any volume of the steam during the process of expansion, -p the corresponding pressure, and e'' the mechanical effect -produced by the expansion of the steam. We have then by (10.) - - p = (a/s) - b; - - [because] de'' = (ads/s) - bds. - -Hence by integrating we obtain - - e'' = a log. s - bs + C; - - -[Pg513] which, taken between the limits s = S' and s = S, becomes - - E'' = a log. S/S' - b(S - S'). (12.) - -But by (11.) we have - - E' = a - bS', - E = a - bS; - [because] E' - E = b(S - S'); - [because] E'' = a log. S/S' - E' + E; - [because] _E_ = E'' + E' = a log. S/S' + E. (13.) - -Or, - - _E_ = a (1 + log. S/S') - bS. (14.) - -Hence it appears that the mechanical effect of a cubic foot of -water evaporated under the pressure P may be increased by the -quantity a log. S/S', if it be first evaporated under the greater -pressure P', and subsequently expanded to the lesser pressure P. - -The logarithms in these formulæ are hyperbolic. - -To apply these principles to the actual case of a double acting -steam engine, - - Let - L = the stroke of the piston in feet. - A = the area of the piston in square feet. - n = the number of strokes of the piston per minute. - [because] 2n AL = the number of cubic feet of space through which - the piston moves per minute. - Let cLA = the clearage, or the space between the steam valve - and the piston at each end of the stroke. - [because] The volume of steam admitted through the steam valve - at each stroke of the engine will be 2n AL(1 + c). - - Let - V = the mean speed of the piston in feet per minute, - [because] 2nL = V. - -The volume of steam admitted to the cylinder per minute will -therefore be VA (1 + c), the part of it employed in working the -piston being VA. - - Let - W = the water in cubic feet admitted per minute in the form - of steam through the steam valve. - S = the number of cubic feet of steam produced by a cubic - foot of water. - -[Pg514] Hence we shall have - - WS = VA (1 + c); - [because] S = (VA(1 + c))/W. (15.) - -Since by (10.) we have - - P = a/S - b; - [because] P = [Wa/(VA(1 + c))] - b. (16.) - -By which the pressure of steam in the cylinder will be known, when -the effective evaporation, the diameter of the cylinder, and speed -of the piston, are given. - -If it be required to express the mechanical effect produced per -minute by the action of steam on the piston, it is only necessary -to multiply the pressure on the surface of the piston by the space -per minute through which the piston moves. This will give - - VAP = W(a/(1 + c)) - VAb; (17.) - -which expresses the whole mechanical effect per minute in pounds -raised one foot. - -If the steam be worked expansively, let it be cut off after the -piston has moved through a part of the stroke expressed by e. - -The volume of steam of the undiminished pressure P' admitted per -minute through the valve would then be - - VA (e + c); - -and the ratio of this volume to that of the water producing it -being expressed by S', we should have - - S' = (VA(e + c))/W. - -The final volume into which this steam is subsequently expanded -being VA(1 + c), its ratio to that of the water will be - - S = (VA (1 + c))/W. - -The pressure P', till the steam is cut off, will be - - P' = [Wa / (VA(e + c))] - b. (18.) - -The mechanical effect E' produced per minute by the steam of full -pressure will be - - E' = P'AVe = [Wae / (e + c)] - AVbe; - -and the effect E'' per minute produced by the expansion of the -steam will by (12.) be [Pg515] - - E'' = Wa log.[(1 + c) / (e + c)] - bVA(1 - e). - -Hence the total effect per minute will be - - _E_ = Wa [(e/(e + c)) + log.([1 + c]/[e + c])] - bVA. (19.) - -If the engine work without expansion, e = 1; - - [because] _E'_ = ( Wa/(1 + c)) - bVA, (20.) - -as before; and the effect per minute gained by expansion will -therefore be - - _E_ - _E'_ = - - Wa [(e/(e + c)) - (1/(1 + c)) + log.([1 + c]/[e + c])]; (21.) - -which therefore represents the quantity of power gained by the -expansive action, with a given evaporating power. - -In these formulæ the total effect of the steam is considered -without reference to the nature of the resistances which it has to -overcome. - -These resistances may be enumerated as follows:-- - - 1. The resistance produced by the load which the engine is - required to move. - - 2. The resistance produced by the vapour which remains - uncondensed if the engine be a condensing engine, or of the - atmospheric pressure if the engine do not condense the steam. - - 3. The resistance of the engine and its machinery, consisting - of the friction of the various moving parts, the resistances - of the feed pump, the cold water pump, &c. A part of these - resistances are of the same amount, whether the engine be - loaded or not, and part are increased, in some proportion - depending on the load. - -When the engine is maintained in a state of uniform motion, the -sum of all these resistances must always be equal to the whole -effect produced by the steam on the piston. The power expended on -the first alone is the _useful effect_. - -Let R = the pressure per square foot of the piston surface, which -balances the resistances produced by the load. - -mR = the pressure per square foot, which balances that part of the -friction of the engine which is proportional to the load. - -r = the pressure per square foot, which balances the sum of all -those resistances that are not proportional to the load. - -The total resistance, therefore, being R + mR + r, which, when the -mean motion of the piston is uniform, must be equal to the mean -pressure on the piston. The total mechanical effect [Pg516] must -therefore be equal to the total resistance multiplied by the space -through which that resistance is driven. Hence we shall have - - [R(1 + m) + r]VA = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] - VAb; - - [because] RVA(1 + m) = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] - VA(b + r). - -For brevity, let - - e' = a[(e/(e + c)) + log.([1 + c]/[e + c])]; - - [because] RVA(1 + m) = We' - VA(b + r). (22.) - -By solving this for VA, we obtain - - VA = We'/(R(1 + m) + b + r); - - [because] RVA = We'R/(R(1 + m) + b + r). (23.) - -This quantity RVA, being the product of the resistance RA, of the -load reduced to the surface of the piston, multiplied by the space -through which the piston is moved, will be equal to the load -itself multiplied by the space through which it is moved. This -being, in fact, the useful effect of the engine, let it be -expressed by U, and we shall have - - U = We'R/(R(1 + m) + b + r). (24.) - -Or by (22.), - - U(1 + m) = We' - VA(b + r). (25.) - -The value of the useful effect obtained from these formulæ will be -expressed in pounds, raised one foot per minute, W being the -effective evaporation in cubic feet per minute, A the area of the -piston in square feet, and V the space per minute through which it -is moved, in feet. - -Since a resistance amounting to 33,000 pounds moved through one -foot per minute is called one-horse power, it is evident that the -horse power H of the engine is nothing more than the useful effect -per minute referred to a larger unit of weight or resistance; that -is to 33,000 pounds instead of one pound. Hence we shall have - - H = U/33000. (26.) - -Since the useful effect expressed in (24.) and (25.) is that due -to a number of cubic feet of water, expressed by W, we shall -obtain the effect due to one cubic foot of water, by dividing U by -W. If, therefore, U' be the effect produced by the effective -evaporation of a cubic foot of water, we shall have [Pg517] - - U' = U/W. (27.) - -If the quantity of fuel consumed per minute be expressed by F, the -effect produced by the unit of fuel, called the DUTY of the -engine, will, for like reason, be - - D = U/F. (28.) - -If the fuel be expressed in hundredweights of coal, then D will -express the number of pounds' weight raised one foot by a -hundredweight of coal. - -By solving (24.) and (25.) for W, we obtain - - W = [U(R(1 + m) + b + r)]/Re', (29.) - - W = (1/e')[U(1 + m) + VA(b + r)]. (30.) - -By eliminating U, by (26.), we shall have - - W = [33000 H(R(1 + m) + b + r)]/Re', (31.) - - W = (1/e')[33000 H(1 + m) + VA(b + r)]. (32.) - -The evaporation necessary per horse power per minute will be found -by putting H = 1 in these formulæ.[41] - -It will be observed that the quantities A and V, the area of the -cylinder and the speed of the piston, enter all these formulæ as -factors of the same product. Other things, therefore, being the -same, the speed of the piston will be always inversely as the area -of the cylinder. In fact, VA is the volume of steam per minute -employed in working the piston, and if the piston be increased or -diminished in magnitude, its speed must be inversely [Pg518] -varied by the necessity of being still moved through the same -number of cubic feet by the same volume of steam. - -It has been already stated in the text, that no satisfactory -experiments have yet been made, by which the numerical value of -the quantity r can be exactly known. In engines of different -magnitudes and powers, this resistance bears very different -proportions to the whole power of the machine. In general, -however, the larger and more powerful the engine, the less that -proportion will be. - -That part of this resistance which arises from the reaction of the -uncondensed vapour on the piston is very variable, owing to the -more or less perfect action of the condensing apparatus, the -velocity of the piston, and the magnitude and form of the steam -passages. M. de Pambour states, that, by experiments made with -indicators, the mean amount of this resistance in the cylinder is -2-1/2 lbs. per square inch more than in the condenser, and that -the pressure in the latter being usually 1-1/2 lb. per square -inch, the mean amount of the pressure of the condensed vapour in -the cylinder is about 4 lbs. per square inch. Engineers, however, -generally consider this estimate to be above the truth in -well-constructed engines, when in good working order. - -In condensing low pressure engines of forty horse power and -upwards, working with an average load, it is generally considered -that the resistance produced by the friction of the machine and -the force necessary to work the pumps may be taken at about 2 lbs. -per square inch of piston surface. - -Thus the whole resistance represented by r in the preceding -formulæ, as applied to the larger class of low pressure engines, -may be considered as being under 6 lbs. per square inch, or 864 -lbs. per square foot, of the piston. It is necessary, however, to -repeat, that this estimate must be regarded as a very rough -approximation; and as representing the mean value of a quantity -subject to great variation, not only in one engine compared with -another, but even in the same engine compared with itself at -different times and in different states. - -In the same class of engines, the magnitude of the clearage is -generally about a twentieth part of the capacity of the cylinder, -so that c = 0·05. - -That part of the resistance which is proportional to the load, and -on which the value of m in the preceding formulæ depends, is still -more variable, and depends so much on the form, magnitude, and the -arrangement of its parts, that no general rule can be given for -its value. It must, in fact, be determined in every particular -case. - -In the practical application of the preceding formulæ in -condensing engines we shall have [Pg519] - - a = 3875969 b = 164 c= 0·05; - - e' = 3875969([e/(e + 0·05)] + log.[1·05/(e + 0·05)]). - -In engines which work without condensation, and therefore with -high pressure steam, we shall have - - a = 4347826 b = 618 c = 0·05 - - e' = 4347826([e/(e + 0·05)] + log.[1·05/(e + 0·05)]) - -To facilitate computation, the values of e' corresponding to all -values of e, from e = ·10 to e = ·90, are given in the following -table:-- - - -------------------------------------------------------------- - |Condensing|Non-condensing|| |Condensing|Non-condensing - | Engines | Engines || | Engines | Engines - e | e'. | e'. || e | e'. | e'. - -------------------------------------------------------------- - ·10 | 10126265 | 11359029 ||·51 | 5966367 | 6692708 - ·11 | 9956867 | 11169008 ||·52 | 5903837 | 6622565 - ·12 | 9793136 | 10985344 ||·53 | 5842288 | 6553525 - ·13 | 9634926 | 10807875 ||·54 | 5781693 | 6485552 - ·14 | 9482029 | 10636364 ||·55 | 5722024 | 6418619 - ·15 | 9334219 | 10470560 ||·56 | 5663251 | 6352693 - ·16 | 9191251 | 10310186 ||·57 | 5605353 | 6287745 - ·17 | 9052888 | 10154978 ||·58 | 5548297 | 6223742 - ·18 | 8918896 | 10004675 ||·59 | 5492064 | 6160662 - ·19 | 8789043 | 9859014 ||·60 | 5436628 | 6098478 - ·20 | 8663120 | 9717760 ||·61 | 5381969 | 6037166 - ·21 | 8540918 | 9580682 ||·62 | 5328065 | 5976699 - ·22 | 8422242 | 9447559 ||·63 | 5274896 | 5917057 - ·23 | 8306916 | 9318193 ||·64 | 5222444 | 5858219 - ·24 | 8194770 | 9192396 ||·65 | 5170684 | 5800159 - ·25 | 8085644 | 9069984 ||·66 | 5119605 | 5742860 - ·26 | 7979392 | 8950796 ||·67 | 5069186 | 5686304 - ·27 | 7875870 | 8834674 ||·68 | 5019410 | 5630469 - ·28 | 7774952 | 8721468 ||·69 | 4970263 | 5575340 - ·29 | 7676514 | 8611048 ||·70 | 4921727 | 5520894 - ·30 | 7580447 | 8503284 ||·71 | 4873790 | 5467121 - ·31 | 7486640 | 8398056 ||·72 | 4826434 | 5414000 - ·32 | 7394990 | 8295250 ||·73 | 4779648 | 5361519 - ·33 | 7305407 | 8194760 ||·74 | 4733417 | 5309659 - ·34 | 7217807 | 8096496 ||·75 | 4687728 | 5258408 - ·35 | 7132097 | 8000352 ||·76 | 4642569 | 5207751 - ·36 | 7048206 | 7906249 ||·77 | 4597928 | 5157676 - ·37 | 6966058 | 7814100 ||·78 | 4553794 | 5108170 - ·38 | 6885585 | 7723832 ||·79 | 4510155 | 5059218 - ·39 | 6806720 | 7635365 ||·80 | 4466999 | 5010808 - ·40 | 6729408 | 7548642 ||·81 | 4424317 | 4962931 - ·41 | 6653578 | 7463580 ||·82 | 4382096 | 4915569 - ·42 | 6579187 | 7380132 ||·83 | 4340332 | 4868720 - ·43 | 6506174 | 7298230 ||·84 | 4299010 | 4822368 - ·44 | 6434491 | 7217822 ||·85 | 4258120 | 4776500 - ·45 | 6364099 | 7138858 ||·86 | 4217658 | 4731113 - ·46 | 6294944 | 7061285 ||·87 | 4177613 | 4686192 - ·47 | 6226989 | 6985058 ||·88 | 4137974 | 4641728 - ·48 | 6160190 | 6910126 ||·89 | 4098737 | 4597713 - ·49 | 6094510 | 6836450 ||·90 | 4059893 | 4554140 - ·50 | 6029916 | 6763992 || | | - -------------------------------------------------------------- - - -[Pg520] In engines which work without expansion we have - - e' = a/(1 + c). - -For condensing engines without expansion, we shall then have - - e' = 3875969/1·05 = 3691399; (33.) - -and for non-condensing engines, - - e' = 4347826/1·05 = 4140787. (34.) - -As the diameters of the cylinders of engines are generally -expressed in inches, the corresponding areas of the pistons -expressed in square feet are given in the following table, so that -the values of A may be readily found:-- - - ----------------------------------------------------------------------- - Diam. | Area. | Diam. | Area. | Diam. | Area. | Diam. | Area. - ----------------------------------------------------------------------- - Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet. - 10 | 0·545 | 48 | 12·566 | 86 | 40·339 | 124 | 83·863 - 11 | 0·660 | 49 | 13·095 | 87 | 41·283 | 125 | 85·221 - 12 | 0·785 | 50 | 13·635 | 88 | 42·237 | 126 | 86·590 - 13 | 0·922 | 51 | 14·186 | 89 | 43·202 | 127 | 87·970 - 14 | 1·069 | 52 | 14·748 | 90 | 44·179 | 128 | 89·361 - 15 | 1·227 | 53 | 15·321 | 91 | 45·166 | 129 | 90·763 - 16 | 1·396 | 54 | 15·904 | 92 | 46·164 | 130 | 92·175 - 17 | 1·576 | 55 | 16·499 | 93 | 47·173 | 131 | 93·599 - 18 | 1·767 | 56 | 17·104 | 94 | 48·193 | 132 | 95·033 - 19 | 1·969 | 57 | 17·721 | 95 | 49·224 | 133 | 96·479 - 20 | 2·182 | 58 | 18·348 | 96 | 50·265 | 134 | 97·935 - 21 | 2·405 | 59 | 18·986 | 97 | 51·318 | 135 | 99·402 - 22 | 2·640 | 60 | 19·635 | 98 | 52·382 | 136 | 100·880 - 23 | 2·885 | 61 | 20·295 | 99 | 53·456 | 137 | 102·369 - 24 | 3·142 | 62 | 20·966 | 100 | 54·542 | 138 | 103·869 - 25 | 3·409 | 63 | 21·648 | 101 | 55·638 | 139 | 105·380 - 26 | 3·687 | 64 | 22·340 | 102 | 56·745 | 140 | 106·901 - 27 | 3·976 | 65 | 23·044 | 103 | 57·863 | 141 | 108·434 - 28 | 4·276 | 66 | 23·758 | 104 | 58·992 | 142 | 109·977 - 29 | 4·587 | 67 | 24·484 | 105 | 60·132 | 143 | 111·532 - 30 | 4·909 | 68 | 25·220 | 106 | 61·283 | 144 | 113·097 - 31 | 5·241 | 69 | 25·967 | 107 | 62·445 | 145 | 114·674 - 32 | 5·585 | 70 | 26·725 | 108 | 63·617 | 146 | 116·261 - 33 | 5·940 | 71 | 27·494 | 109 | 64·801 | 147 | 117·859 - 34 | 6·305 | 72 | 28·274 | 110 | 65·995 | 148 | 119·468 - 35 | 6·681 | 73 | 29·065 | 111 | 67·201 | 149 | 121·088 - 36 | 7·069 | 74 | 29·867 | 112 | 68·417 | 150 | 122·719 - 37 | 7·467 | 75 | 30·680 | 113 | 69·644 | 151 | 124·361 - 38 | 7·876 | 76 | 31·503 | 114 | 70·882 | 152 | 126·013 - 39 | 8·296 | 77 | 32·338 | 115 | 72·131 | 153 | 127·676 - 40 | 8·727 | 78 | 33·183 | 116 | 73·391 | 154 | 129·351 - 41 | 9·168 | 79 | 34·039 | 117 | 74·662 | 155 | 131·036 - 42 | 9·621 | 80 | 34·907 | 118 | 75·944 | 156 | 132·732 - 43 | 10·085 | 81 | 35·785 | 119 | 77·236 | 157 | 134·439 - 44 | 10·559 | 82 | 36·674 | 120 | 78·540 | 158 | 136·157 - 45 | 11·045 | 83 | 37·574 | 121 | 79·854 | 159 | 137·886 - 46 | 11·541 | 84 | 38·485 | 122 | 81·180 | 160 | 139·626 - 47 | 12·048 | 85 | 39·406 | 123 | 82·516 | 161 | 141·377 - --------------------------------------------------------------------- - - -[Pg521] The practical application of the preceding formulæ will be -shown by the following examples. - - -EXAMPLES. - -1. _A 36-inch cylinder with 5-1/2 feet stroke is supplied by a -boiler evaporating effectively 60 cubic feet of water per hour, -and the piston makes 20 strokes per minute without expansion;-- -what is the power of the engine and the pressure of steam -in the cylinder?_ - -Let it be assumed that r = 6 × 144 = 864 and m = 0·1. Since the -engine is a condensing engine, we have b = 164 and e' = 3691399. -By the formulæ (25.) and (26.) we have - - H = [We' - VA(b + r)]/[33000(1 + m)]; - -and since by the data we have - - W = 1 A = 7·069 V = 2nL = 40 × 5·5 = 220, - -the formula, by these substitutions, becomes - - H = (3691399 - 220 × 1028 × 7·069) / (33000 × 1·1); - [because] H = 57·6. - -Since e = 1, the pressure P of steam in the cylinder, by (18.), is - - P = (We'/VA) - b. - -Therefore - - P = (3691399/1555·18) - 164 = 2210; - -which being the pressure in pounds per square foot, the pressure -per square inch will be 15-1/3 lbs. - -2. _To find the effective evaporation necessary to produce a power -of 80 horses with the same engine. Also, find the pressure of -steam in the cylinder, the speed of the piston being the same._ - -By the formula (32.), with the above substitutions, we have - - W = (33000 × 80 × 1·1 + 220 × 7069 × 1028)/3691399 = 1·22. - -The evaporating power would therefore be only increased 22 per -cent., while the working power of the engine would be increased -nearly 40 per cent. - -The pressure P in the cylinder will be given, by (18.), as before. - - P = [(1·22 × 3691399)/1555·18] - 164 = 2732; - -which is equivalent to 19 lbs. per square inch. [Pg522] - -3. _What must be the diameter of a cylinder to work with a power -of a hundred horses, supplied by a boiler evaporating effectively -70 cubic feet of water per hour, the mean speed of the piston -being 240 feet per minute, and the steam being cut off at half -stroke? Also, what will be the full pressure of steam on the -piston?_ - -Taking, as in the former examples, m = 0·1, b = 164, and r = 864, -we shall have - - H = 100 W = 7/6 V = 240, - -and by the column for condensing engines, in table, p. 519, we -have e' = 6029916, where e = 0·50. Making these substitutions in - - We' = 33000 H (1 + m) + VA (b + r), - -we shall have - - (7/6) × 6029916 = 3300000 × 1·1 + 240 × 1028 × A. - -Whence we find - - A = 13·8; - -and by the table, p. 520, the corresponding diameter of the -cylinder will be 50-1/3 inches. - -If P' be the full pressure of the steam, we shall have, by (18.), - - P' = (Wa/VA(e + c)) - b. - -Making in this the proper substitutions, we have - - P' = ((7/6) × 3875969) / (240 × 13·8 × 0·55) - 164 = 2318; - -which being in pounds per square foot, the pressure per square -inch will be 16-1/10 lbs. - - FOOTNOTES: - - [40] M. de Pambour states that the increased volume is 1364 - cubic inches. - - [41] Formulæ equivalent to some of the preceding are given, - with numerous others, by M. de Pambour, in his Theory of the - Steam Engine. These mathematical details contain nothing new - in principle, being merely the application of the known - principles of general mechanics to this particular machine. M. - de Pambour objects against the methods of calculating the - practical effects of steam engines generally adopted by - engineers in this country. Their estimates of the loss of - power by friction, imperfect condensation, and other causes, - are, as I have stated in this volume, vague, and can be - regarded at best as very rough approximations; but, subject to - the restrictions under which their methods of calculation are - always applied, they are by no means so defective as M. de - Pambour supposes. He proves what he considers to be their - inaccuracy, by applying them in cases in which they are never - intended to be applied by English engineers. Those who desire - to reduce to general algebraical formulæ the effects of the - different kinds of steam engines will, however, find the - volume of M. de Pambour of considerable use. - -[Pg523] - - - - -INDEX. - - - Air, elasticity of, 28; - May be partially expelled from a vessel by the application of - heat, 44. - - America, steam navigation first established in, 487; - Circumstances which led to it, 488; - Fitch and Rumsey, their attempts to apply the single-acting - engine to the propulsion of vessels, 489; - Stevens of Hoboken commences experiments on steam navigation, - 489; - Experiments of Livingstone and Fulton, 489; - Fulton's first boat, 490; - The Hudson navigated by steam, 491; - Extension and improvement of river navigation, 492; - American steamers, 494; - Difference between them and European steamers, 494; - Steamers on the Hudson, 494; - American paddle-wheels, 495; - Sea-going American steamers, 496; - Speed attained by American steamers, 497; - Lake steamers, 499; - The Mississippi and its tributaries, 499; - Steam-boats navigating it, 500; - Their structure and machinery, 500; - New Orleans Harbour, 503; - Steam tugs, 503. - - Atmosphere, 38; - Weight of, 39. - - Atmospheric air, mechanical properties of, 38; - Composition of, 253. - - Atmospheric engine, Thomas Newcomen the reputed inventor of, 62; - Description of, as first constructed by Newcomen, 67; - The operation of considered, 69; - Not unfrequently used in preference to the modern steam - engine, 72; - Advantages which it possessed over Savery's, 73; - Considerably improved by Beighton, 75; - John Smeaton investigates this machine, 76; - Brindley obtains a patent for improvements in, 76; - Applied by Champion of Bristol to raise water, 181; - Possessed but limited power of adaptation to a varying load, - 151; - Expedient to remedy this, 151; - Working-beam, cylinder, and piston applied to by Newcomen, - 322. - - Atmospheric pressure rendered available as a mechanic agent by - Denis Papin, 38; - Means of measuring the force of, 39; - The idea of using against a vacuum or partial vacuum to work - a piston in a cylinder, suggested by Otto Guericke, 73. - - - Barometer gauge, 272. - - Barton's piston, 248. - - Beighton, his improvement of the atmospheric engine, 75. - - Black, Dr., his doctrine of latent heat, 93. - - Blasco de Garay, his contrivance to propel vessels, 16; - The contrivance of, probably identical with that of Hero, 17. - - Blinkensop, his locomotive engine, 337. - - Blowing-box, 429. - - Blowing out, Seaward's method of, 454. - - Blow-off cocks, 452. - - Boiler, forms of, most convenient, 255; - The waggon boiler adopted by Watt, 255; - Furnace, 256; - Method of feeding, 257; - Combustion of gas in flues, 260; - Mr. Williams's method of consuming the unburned gases which - escape from the grate, and are carried through the flues, - 260; - Construction of grate and ash-pit, 261; - Magnitude of heating surface of boiler, 262; - Capacity of, must be proportioned to the quantity of water to - be evaporated, 263; - Water-space and steam-space in boiler, 263; - Proportion of water-space in the boiler, how to be regulated, - 264; - Position of flues, 264; - Method of feeding, 265; - The magnitude of the feed should be equal to the quantity of - water evaporated, 265; - Different methods for indicating the level of the water in - the boiler, 266; - Level guages, 266; - Self-regulating feeder, 267; - Another method of arranging, 269; - Steam gauge, 270; - Thermometer gauge, 271; - Barometer gauge, 272; - The indicator to measure the mean efficient force of the - piston invented by Watt, 274; - The counter contrived by Watt, 278; - Safety valve, 279; - Fusible plugs used in high pressure boilers, 280; - Self-regulating damper, 281; - Self-regulating furnace invented by Brunton, 283; - Duty of a boiler, 294; - Boilers of locomotive engines, 351; - Construction of the boiler of Gurney's steam carriage, 423; - All boilers require occasional cleansing, 427; - Gurney's method of removing crust of deposited matter in - boilers, 427; - The boiler of Dr. Church's engine formed of copper, 439; - Boilers in marine engines, 449; - Effects of sea-water in, 450; - Remedies for them, 451; - Substitution of copper for iron, 460; - Expedient of coating boilers with felt, applied by Watt, 463. - - Booth, Mr., his report on locomotive engines, 361. - - Boulton and Watt's experiments on the horse power of engines, - 288. - - Branca, Giovanni, his machine for propelling a wheel by a blast - of steam, 22. - - Brindley (James) obtains a patent for improvements in - atmospheric engine, 76; - Undertook to erect an engine at Newcastle-under-Lyne, 76; - Discouraged by the obstacles thrown in his way, 76. - - Brougham, Lord, his sketch of Watt's character, 313; - Inscription from the pen of, on Watt's monument in Westminster - Abbey, 320. - - Buffers, 404. - - Cartwright's engine to use the vapour of alcohol to work the - piston, 245; - His piston, 247. - - Cawley and Newcomen obtain a patent for the atmospheric engine, - 64. - - Champion applies atmospheric engine to raise water, 181. - - Chapman, Messrs., their locomotive engine, 337. - - Chlorine introduced in bleaching by Watt, 310. - - Church, Dr., his steam engine, 439; - The boiler formed of copper, 439. - - Coals, the virtues and powers which steam has conferred upon, - 6; - The amount of labour a bushel of performs by means of the - steam engine, compared with horse power, 7; - Constituents of, 252; - Process of combustion, 252. - - Coal mines, apprehensions as to the possibility of the - exhaustion of groundless, 8. - - Cocks, friction on, 240. - - Cocks and valves, 227. - - Combustion of gas in flues, 260. - - Condensation by injection, accidental discovery of, 69. - - Condensation in the cylinder incompatible with a due economy - of fuel, 120. - - Condensing principle, circumstance which led to Savery's - discovery of, 47. - - Condensing pipe in Savery's engine, 52. - - Condensing out of the cylinder, 120. - - Condensing jet, 191. - - Conical steam valves, 228. - - Conversion of ice into water, 103; - Of water into steam, 105. - - Copying press invented by Watt, 302. - - Cornish system of inspection, 297. - - Cornish engines, improvement of, 298; - Historical detail of the duty of, 299. - - Cylinders, Wilkinson's machine for accurately boring the - insides of, 149. - - - D valve, 230. - - Dalton and Gay-Lussac, law of, relating to the pressure of - elastic bodies, 171. - - Dixon, Mr. The substitution of brass for copper tubes in - locomotive engines ascribed to him, 370. - - Double clack-valve, 228. - - - Eccentric, 225; - Two expedients to reverse the position of, 379. - - Effect of an engine, 285. - - Elastic fluids. The law according to which the pressure of, - increases with their temperature, discovered by Dalton and - Gay-Lussac, 171. - - Evaporation of water and other liquids, physical and - mechanical principles connected with, 97. - - Expansion of common steam, effects of, 173. - - Expansive action of steam, 159; - Stated by Watt in a letter to Dr. Small, 157; - Its principle explained, 158; - Mechanical effect resulting from it, 161; - Computed effect of cutting off steam at different portions - of the stroke, 162; - Involves the condition of a variation in the intensity of - the moving power, 163; - Expedients for equalising the power, 164; - The expansive principle in the engines constructed by - Boulton and Watt, limited, 165; - Its more extensive application in the Cornish engines, 165; - Methods of equalising, 174; - Description of Hornblower's engine for this purpose, 174. - - Expansive principle, application of in marine engines, 466. - - - Farey on the steam engine, quotation from, relative to - Savery's engine, 58; - His evidence before the House of Commons, 435. - - Field, construction of his split paddle, 478. - - Fitch and Rumsey, their attempts to apply the single-acting - engine to the propulsion of vessels, 489. - - Flues, position of, 264. - - Fluids, of two kinds, 25; - Mechanical properties of, 25; - Elastic, 27; - Experimental proof that they press equally in all - directions, 41. - - Fly-wheel, 205. - - Four-way cock, 239; - Disadvantages of, 240. - - Fuel, means of economising, in marine furnaces, 463. - - Fulton and Livingstone, their experiments in steam navigation, - 489. - - Fulton's first boat, 490. - - Furnace, self-regulating, invented by Brunton, 283. - - Fusible plugs used in high-pressure boilers, 280. - - - Galloway, his paddle-wheel described, 476. - - Gas, elasticity of, 28. - - Gay-Lussac and Dalton, law of, relating to the pressure of - elastic bodies, 171. - - Governor, adaptation of, 209. - - Gradients, restrictions on, 411; - Disposition of, should be uniform, 415. - - Great Western Railway, Dr. Lardner's experiments on, 408. - - Griff, proposals to drain a colliery at, mentioned by - Desaguliers, 64. - - Gurney's steam carriage, 423; - Construction of the boiler of, 423; - His method of removing crust of deposited matter in boilers, - 427; - His experiments on common roads, 432. - - - Hall, his condensers described, 458. - - Hancock, his steam carriage, 436; - In what manner it differs from that of Gurney, 437. - - Harris, Dr., mentions Savery's engine in his "Lexicon - Technicum," 56. - - Heat, effects of upon water, 29; - Waste of in atmospheric engine, 89; - An examination of the analogous effects produced by the - continued application of, to water in the liquid state, - 102; - Radiation of, 254. - - Heating by steam brought forward by Watt, 303. - - "Hecla," experiments with the, 412. - - Hero of Alexandria, description of his machine, 12. - - High pressure engines described, 321; - One of the earliest forms of the steam engine, 322; - Obscurely described in the "Century of Inventions," 322; - Construction of the first, by Messrs. Trevethick and Vivian, - 324. - - Hooke exposes the fallacy of Papin's project, 64. - - Horse carriages compared with steam, 435. - - Horse power of steam engines, 288; - Smeaton's estimation of, 288; - Boulton and Watt's experiments on, 288. - - Howard's description of his marine engine, 464. - - Hudson, the, navigated by steam, 491. - - Hull, Jonathan, his application of the steam engine to water - wheels, 180. - - Humphrey. His marine engine described, 470. - - Huskisson, Mr., death of, 329. - - Hydrogen, 253. - - - India, steam navigation to, 483. - - Indicator invented by Watt, 274. - - - Jeffrey, Lord; his sketch of the character of Watt, 315. - - - Kinneal, description of Watt's experimental engine at, 131. - - - Lake steamers, 499. - - Lardner's, Dr., experiments on the Manchester Railway in 1832, - 357; - His experiments in 1838, 406; - Experiments on the Great Western Railway, 408. - - Leupold's engine, description of, 323. - - Level gauges, 266. - - Linen, machine for drying by steam, invented by Watt, 303. - - Liverpool and Manchester railroad, effects of the introduction - of steam transport on, 329; - Want of experience in the construction of the engines, 329; - Death of Mr. Huskisson, 329; - Proceedings of the directors, 342; - Premium offered by them for the best engine, 344; - Experimental trial, 344. - - Livingstone and Fulton, experiments of in steam navigation, - 489. - - Locomotive engine, history of, 328; - Blinkensop's engine, 337; - Chapman's engine, 337; - Walking engine, 337; - Mr. Stephenson's engine at Killingworth, 339; - Defect of, 341; - Description of the "Rocket," 345; - The "Sanspareil," 347; - The "Novelty," 349; - Superiority of the "Rocket," 350; - Subsequent improvements in the locomotive engine, 352; - Table, showing the economy of fuel gained by subdividing the - flue into tubes, 354; - Engines constructed in the form of the "Rocket" subject to - two principal defects, 354; - These defects remedied, 355; - Improved by the adoption of a more contracted blast pipe, - 356; - Dr. Lardner's experiments in 1832, 357; - Adoption of brass tubes, 361; - Mr. Booth's report, 361; - Detailed description of the most improved locomotive - engines, 364; - Substitution of brass for copper tubes ascribed to Mr. - Dixon, 370; - Mr. Stephenson constructed the driving wheels without - flanges, 383; - Pressure of steam in the boiler limited by two safety-valves, 402; - Buffers, 404; - Steam whistle, 404; - Water tank, 404; - Power of locomotive engines, 405; - Evaporation of boilers, 406; - Dr. Lardner's experiments in 1838, 406; - Resistance to railway trains, 407; - Dr. Lardner's experiments on the Great Western Railway, 408; - Restriction on gradients, 411; - Experiment with the "Hecla," 412; - Disposition of gradients should be uniform, 415; - Method of surmounting steep inclinations, 415; - Steam carriages on common roads, 419; - Difference between steam engines on railways and those used - to propel carriages on turnpike roads, 422; - Gurney's steam carriage, 423; - Construction of the boiler of, 423; - Escape of steam from the engines on the Liverpool road, 428; - Blowing-box, 429; - Separator, 430; - Difficulties in the practical working of steam carriages - upon common roads, 432; - Gurney's experiments on common roads, 432; - Prejudice against locomotive engines on common roads, 432; - Not more destructive to roads than carriages drawn by - horses, 433; - Report of the committee of the House of Commons, 433; - Weight of steam carriages, 433; - Two methods of applying locomotives upon common roads, 434; - Horse carriages compared with, 435; - Farey's evidence before the House of Commons, 435; - Risk of accident from explosion extremely slight, 435; - Hancock's steam carriage, 436; - In what manner it differs from that of Gurney, 437; - Ogle's steam carriage, 438; - His evidence before the House of Commons, 439; - Dr. Church's steam engine, 439; - The boiler of formed of copper, 439. - - Lunar Society, Boulton and Watt leading members in, 302. - - - Marine engines, form and arrangement of, 441; - Difference between marine and land engines, 443; - Engine-room, arrangement of, 446; - Boilers in, 449; - Effects of sea-water on boilers, 450; - Remedies for them, 451; - Blow-off cocks, 452; - Indicators of saltness, 452; - Seaward's indicator, 454; - His method of blowing out, 454; - Method of Maudslay and Field to preserve freshness of water - in the boiler, 456; - Brine pumps, 457; - Tubular condensers applied by Mr. Watt, 457; - Hall's condensers, 458; - Substitution of copper for iron boilers, 461; - Process of stoking, 462; - Marine furnaces, 463; - Expedient of coating boilers with felt applied by Watt, 463; - Means of economising fuel, 463; - Description of Howard's engine, 464; - Application of the expansive principle in marine engines, - 466; - Recent improvements of Messrs. Maudslay and Field, 467; - Humphrey's engine, 470; - Common paddle-wheel, 472; - Defect of, 474; - Feathering paddles, 474; - Galloway's paddle-wheel, 476; - Field's split paddle, 478; - Proportion of power to tonnage, 480; - Iron steam vessels, 482. - - Mariotte's law relating to pressure, 171. - - Maudslay and Field, their method to preserve the requisite - freshness of water in the boiler, 456; - Brine pumps, 457; - Recent improvements of in marine engines, 466. - - Metallic pistons, 244; - Cartwright's engine, 245; - An improved form given to by Barton, 248. - - Mill work, Stewart's application of the steam engine to, 182. - - Mines, the drainage of, Watt endeavours to bring to perfection - the application of the steam engine to, 178. - - Mississippi and its tributaries, 499; - Steam-boats on, 500; - Their structure and machinery, 500. - - Morland, Sir Samuel, his application of steam to raise water, - 34; - The reputed inventor of several ingenious contrivances, 34; - His work in French upon the raising of water, 35; - Extract from it, 35; - Evelyn's account of his visit to, 36. - - Murray's slide-valve, 229. - - - Newcomen, Thomas, the reputed inventor of the atmospheric - engine, 62; - His acquaintance with Dr. Hooke, 62; - Acquainted with Papin's writings, 64; - The merits of his engine ascribed principally to its - mechanism and combinations, 73; - Obtains with Cawley a patent for the atmospheric engine, 64; - Resumes the old method of raising water from mines by - ordinary pumps, 65; - The means proposed to effect this, 66; - First conception of the atmospheric engine, 66; - Description of his construction of atmospheric engine, 67; - Suggestion of a better method of condensation than the - application of cold water on the external surfaces of the - cylinder, 69; - He abandons the external cylinder, 69; - Applied the working-beam, cylinder, and piston to the - atmospheric engine, 322. - - New Orleans Harbour, 503. - - "Novelty," description of the, a locomotive engine, 349. - - - Ogle, his steam carriage, 438; - His evidence before the House of Commons, 439. - - Otto Guericke, his suggestion relative to atmospheric - pressure, 73. - - Oxley made the first attempt to drive water-wheels by the - steam engine, 182. - - - Paddle-wheel described, 472; - Defect of, 474; - Feathering paddles, 474; - Galloway's paddle-wheel, 476; - Field's split paddle, 478. - - Paddle-wheels of American steamers, 495. - - Papin, Denis, conceived the idea of rendering atmospheric - pressure available as a mechanical agent, 37; - Description of his contrivance, 37; - His discovery of condensation of steam, 45; - Quotation from his work relative to this discovery, 45; - Explanation of this important discovery, 46; - Discovers the method of producing a vacuum by the - condensation of steam, 178; - His projected applications of the steam engine, 178; - His proposition for the construction of an engine working by - atmospheric pressure, 62; - Abandons the project when informed of the principle and - structure of Savery's engine, 62; - His engine described, 62; - This project nothing more than a reproduction of the Marquis - of Worcester's engine, 63; - The fallacy of his project exposed by Hooke, 64; - His project for producing a vacuum under a piston by - condensing the steam, published in the "Actæ Eruditorum," - 64. - - Parallel motion, 195. - - Physical science, the rapid progress of, 8. - - Pistons, 242; - The common hemp-packed, 242; - Woolf's method of tightening the packing of, without - removing the lid of the cylinder, 244; - This method further simplified, 244; - Metallic, 244; - Cartwright's engine, 245; - Cartwright's piston, 247; - Invention of the indicator by Watt to measure the mean - efficient force of, 274. - - Piston rod and beam, methods of connecting in the double-acting - engine, 193. - - Pneumatic institution at Clifton, Watt one of the founders of, - 310. - - Potter, Humphrey, his contrivance for working the valves, 71; - Improved by the substitution of a plug-frame, 72. - - Power, proportion of, to tonnage in marine engines, 480. - - Power and duty of steam engines, 287. - - Priestley, Watt's letter to, relative to the composition of - water, 307. - - Pump, an illustration of force attained by a vacuum, 43. - - Puppet clacks, or button valves, 144. - - - Rack and Sector, 194. - - Railways, speed of coaches on, compared with that of stage-coaches - on a common road, 7. - - Railway transport, effects of, 328. 330. - - Railways and stone roads compared, 420. - - River navigation, extension and improvement of, 492. - - "Rocket," description of the, a locomotive engine, 345; - Engines constructed in the form of, subject to two principal - defects, 354; - These defects remedied, 355; - Improved by the adoption of a more contracted blast-pipe, 356. - - Roebuck, Dr., Watt's partnership with, 130. - - Rotatory motion, method of producing by sun and planet wheels, - 187. - - - Safety-valve not adopted by Savery, 57; - Invented by Papin, 57; - Description of, 57; - First applied to Savery's engine by Desaguliers, 58. - - "Sanspareil," description of the, a locomotive engine, 347. - - Savery, Thomas, obtains a patent for an engine to raise water, - 47; - Circumstance which led to his discovery of the condensing - principle, 47; - An account of his engine, 49; - Description of the working apparatus in which the steam is - used as a moving power, 51; - His engine described in a work entitled "The Miner's Friend," - 56; - Mentioned by Dr. Harrison in his "Lexicon Technicum," 56; - Quotation from his address to the Royal Society, 56; - Quotation from his address to the Miners of England, 57; - Mentioned by Bradley in his "Improvements of Planting and - Gardening," 57; - The safety-valve not adopted by him, 57; - The safety-valve first applied to his engine by Desaguliers, - 58; - Farey on the steam engine quoted, 58; - Further Improvements made by Desaguliers, 58; - Defects of his engine, 59; - His engine applied to the drainage of mines, 59; - Further defects of, 60; - The first to suggest the method of expressing the power of - an engine with reference to that of horses, 61; - Failure of his engine in the work of drainage, 61; - The tendency of high pressure to weaken and gradually destroy - the vessels, 72; - The power of his engines restricted, 73; - The atmospheric engine superior to, 73; - The boiler, guage-pipes, and regulator borrowed from his - engine, 73; - Proposes to apply his engine as a prime mover for all sorts - of machinery, 180. - - Scott, Sir Walter, his sketch of the character of Watt, 314. - - Sculpture, Watt's invention of machine for copying, 318. - - Sea-going American steamers, 496. - - Sea-water, effects of upon boilers, 450. - - Seaward's slides, 235; - Indicator of saltness, 454; - His method of blowing out, 454. - - Self-regulating damper, 281; - Furnace, 283. - - Separator, 430. - - Single-acting engine, description of Watt's, 133. 144. - - Single clack-valve, 227. - - Single cock, 238. - - Slide-valves, 229; - That contrived by Mr. Murray, 229. - - Smeaton, John, investigates the atmospheric engine, 76; - Applies himself to the improvement of wind and water mills, - 181; - His estimate of the horse power of engines, 288. - - Solomon De Caus, description of the apparatus of, 17; - M. Arago claims for him a share of the honour of the - invention of the steam engine, 21; - Republished, with additions, the work of Isaac De Caus, 22. - - Somerset, Edward, Marquis of Worcester. Invention of the steam - engine ascribed to him, 23; - Description of his contrivance, 23; - His "Century of Inventions," 24; - Brief account of his engine described in this work, 31; - His contrivance compared with that of De Caus, 33; - Many of his inventions have been reproduced and brought into - general use, 34. - - Steam cannot be applied _immediately_ to any useful purpose, - but requires the interposition of mechanism, 11; - Elastic force of, recognised by the ancients only in vague - and general terms, 14; - The power of, formerly made to minister to the objects of - superstition, mentioned by Arago, 15; - Anecdote showing the knowledge which the ancients had of the - mechanical force of, 15; - The discovery of the condensation of, by Papin, 45; - Mechanical power obtained from the direct pressure of the - elastic force of, suggested by De Caus and Lord Worcester, - 73; - Latent heat of, 107; - The mechanical force of considered, 115; - Watt's early experiments on, 87; - Discovery of the expansive action of, 157; - Expansive action of stated by Watt in a letter to Dr. Small, - 157; - Its principle explained, 158; - Mechanical effect resulting from it, 161; - Properties of, 168; - Common and super-heated steam, 168; - Pressure and temperature of, 171; - Relation between the temperatures of common steam and its - pressure and density, 172; - Effects of the expansion of common steam, 173; - Mechanical effects of, 173; - Methods of equalising the varying force of expanding steam, - 174; - Method of producing a vacuum by the condensation of, - discovered by Papin, 178; - Applied to move machinery, 179; - Steam guage, 270; - Heating by steam brought forward by Watt, 303; - A machine for drying linen by, invented by Watt, 303; - Mode of escape of, from the engines on the Liverpool road, - 429. - - Steam case or jacket, invented by Watt, 124. - - Steam engine, a subject of popular interest, 3; - The effects which it has produced upon the well-being of the - human race considered, 4; - Presents peculiar claims upon the attention of the people of - Great Britain, 5; - The exclusive offspring of British genius, 5; - The virtues and powers which it has conferred upon coals, 6; - Water the means of calling these powers into activity, 6; - Used in the drainage of Cornish mines, 7; - Comparison of its power with human labour, 8; - Investigation of the origin of, 10; - A combination of a great variety of contrivances and the - production of several inventions, 12; - Before the discoveries of James Watt was of extremely - limited power, 12; - Invention of, ascribed to the Marquis of Worcester, 23; - Account of Savery's, 49; - Farey quoted, 58; - Improvements made by Desaguliers, 58; - Applied to the drainage of mines, 59; - Humphrey Potter's contrivance, 72; - Advantages of the atmospheric engine over that of Savery, 73; - Progress of the atmospheric engine, 75; - Description of Papin's engine, 62; - Smeaton's improvements, 76; - First experiments of Watt and subsequent improvements, 83; - Watt's experiments on the force of steam at high pressure, - 83; - Watt discovers the great defects of the atmospheric engine, - 85; - Waste of heat in atmospheric engine, 89; - Dr. Black's theory of latent heat, 93; - Description of Watt's experimental engine at Kinneal, 131; - Description of his single-acting engine, 133; - Disadvantages of the atmospheric compared with the old - engine, 150; - Expedients to force the atmospheric engines into use, 152; - Watt's exertions to improve the manufacture of, at Soho, 155; - Efficiency of fuel in the new engines, 156; - Hornblower's engine, 175; - Woolf's engine, 176; - Watt endeavours to bring to perfection the application of, - to the drainage of mines, 178; - Papin's projected application of, 178; - Savery proposed to apply his steam engine as a prime mover - for all sorts of machinery, 180; - Jonathan Hull's application of, to water-wheels, 180; - Steam engine used for driving water wheels, 182; - First attempt of this kind made by Oxley, 182; - Stewart's application of, to mill work, 182; - Wasbrough's application of the fly-wheel and crank, 183; - Reasons why Watt's single-acting engine was not adapted to - produce continuous uniform motion of rotation, 184; - Watt's second patent, 186; - Valves of double-acting engine, 189; - Condensing jet, 191; - Methods of connecting the piston-rod and beam in the - double-acting engine, 193; - Rack and sector, 194; - Parallel motion, 195; - Connecting rod and crank, 202; - Fly-wheel, 205; - Throttle-valve, 207; - Adaptation of the governor, 209; - Double-acting engine considered as a whole, 216; - Process of its operation investigated, 217; - The eccentric, 225; - Cocks and valves, 227; - Single clack-valve, 227; - Double clack-valve, 228; - Conical steam-valves, 228; - Slide-valves, 229; - Murray's slide-valve, 229; - D valve, 230; - Seaward's slides, 235; - Single cock, 238; - Four-way cock, 239; - Pistons, 242; - Gross effect and useful effect of engines, 285; - Power and duty of, 287; - Horse power of, 288; - The means whereby mechanical power is expended in working - the engines enumerated, 290; - Common rules followed by engine makers, 292; - Duty of engines, 294; - Duty distinguished from power, 295; - Proportion of stroke to diameter of cylinder, 295; - Cornish system of inspection, 297; - Improvement of the Cornish engines, 298; - Historical detail of the duty of Cornish engines, 299; - High-pressure engines, 321; - Leupold's engine described, 323; - Construction of the first high-pressure engine by Messrs. - Trevethick and Vivian, 324; - First application of the steam engine to propel carriages - on railroads, 328; - Computation of how much corn could be saved by the - substitution of steam engines for horse power, 332; - Marine engines, form and arrangement of, 441; - Difference between marine and land engines, 443; - Mr. Howard's patent engine described, 464; - Humphrey's engine described, 470. - - Steam navigation to India, 483; - First established in America, 487; - Circumstances which led to it, 488; - Attempts of Fitch and Rumsey to apply the single-acting - engine to the propulsion of vessels, 489; - Stevens of Hoboken commences experiments in, 489; - Experiments of Livingstone and Fulton, 489; - Fulton's first boat, 490; - The Hudson navigated by steam, 491; - Extension and improvement of river navigation, 492; - American steamers, 494; - Difference between them and European steamers, 494; - Steamers on the Hudson, 494; - Sea-going American steamers, 496; - Speed attained by American steamers, 497; - Lake steamers, 499; - Steam-boats on the Mississippi, 500. - - Steam tugs, 503. - - Steep inclinations, method of surmounting, 415. - - Stephenson, his locomotive engine at Killingworth, 339; - Defect of, 341; - Constructed the driving wheels without flanges, 383. - - Stevens, of Hoboken, commences experiments on steam navigation, - 489. - - Stewart, his application of the steam engine to mill work, 182. - - Stoking, process of, 462. - - Stuffing-box, contrivance of, 147. - - Sun and planet wheels, method of producing rotatory motion, - 187. - - - Thermometers, the process of filling described, 44; - Explanation of the principle of, 98; - Construction of mercurial thermometer, 98; - Method of graduating, 99. - - Thermometer gauge, 270. - - Throttle-valve, description of, 207. - - Tredgold, his remark relative to Newcomen's engine, 73. - - Trevethick and Vivian's engine described, 325. - - - Vacuum, force obtained by a, 43; - The pump an illustration of this, 43. - - Valves of double-acting engine, 189. - - - Wasbrough, his application of the fly-wheel and crank, 183. - - Water, a pint of, the mechanical force produced by its - evaporation, 6; - The alternate decomposition and recomposition of, by - magnetism and electricity, analogous to vaporisation and - condensation, 8; - The fixed temperature which it assumes in boiling subject to - variation, 108; - Experiments to illustrate this, 109; - Table to show the temperature at which it will boil under - different pressures of the atmosphere, 113; - Mechanical force of a cubic inch of, converted into steam, - 118; - Discovery of the composition of, 303; - The merit of this discovery shared between Cavendish, - Lavoisier, and Watt, 305; - Latent heat of, 101; - Conversion of ice into, 103. - - Water tank, 404. - - Water-wheels, steam engine used for turning, 182. - - Watt (James), birth of, 77; - His infancy, 78; - Anecdotes respecting, 78; - His boyhood, 79; - Goes to London, 80; - Returns to Glasgow, 80; - Appointed mathematical instrument-maker to the university, - 81; - Adam Smith one of his earliest friends and patrons, 81; - Also Black and Robert Simson, 81; - Extract from an unpublished manuscript of Robison respecting - the character of, 82; - His first experiments on steam, 83; - Observes defects of atmospheric engine, 84; - His first attempt to improve it, by using a wooden instead - of an iron cylinder, 85; - His method to ascertain the temperatures at which water would - boil under pressures less than that of the atmosphere, 86; - His early experiments on steam, 87; - His notice of the waste of heat in atmospheric engines, 89; - His experiments to determine the extent to which water - enlarged its volume when it passed into steam, 90; - Discovers the latent heat of steam, 91; - Learns the theory of latent heat, 93; - His letter to Dr. Brewster, explaining the circumstances - which led to the error that a large share of the merit of - his discoveries were due to Black, 93; - Finds that condensation in the cylinder is incompatible with - a due economy of fuel, 120; - Conceives the notion of condensing out of the cylinder, 120; - Discovers separate condensation, 121; - Invents the air-pump, 122; - Substitutes steam pressure for atmospheric pressure, 123; - Invents the steam case or jacket, 124; - His first experiments to realise these inventions, 125; - His experimental apparatus, 126; - Difficulties of bringing the improved engines into use, 128; - Practises as a civil engineer, 129; - Makes a survey of the river Clyde, 129; - His partnership with Dr. Roebuck, 130; - His first patent, 130; - Description of his experimental engine at Kinneal, 131; - Removes to Soho, 131; - Abstract of the act of parliament for the extension of his - patent, 132; - Description of his single-acting engine, 133-144; - His condenser worked by an injection, 146; - Objections attending condensation by surface, 146; - Improvements in construction of piston, 147; - Effected by a contrivance called a stuffing-box, 147; - Method of packing, 148; - Improved methods of boring the cylinder, 149; - His letter to Smeaton on this subject, 149; - Used black-lead dust for the purpose of lubrication, 149; - This found to wear the cylinder, 149; - Disadvantages of the atmospheric compared with the old - engines, 150; - Greatly increased economy of fuel, 151; - Expedients to force the atmospheric engines into use, 152; - His correspondence with Boulton, 153; - His correspondence with Smeaton, 154; - Exertions to improve the manufacture of engines at Soho, 155; - Efficiency of fuel in the new engines, 156; - Endeavours to bring to perfection the application of the - steam engine to the drainage of mines, 178; - The reasons why his single-acting engine was not adapted to - produce continuous uniform motion of rotation, 184; - His notes upon Dr. Robison's article on the steam engine, - 184; - His second patent, 186; - His third patent, 189; - His application of the fly-wheel, 205; - His application of the throttle-valve, 207; - His adaptation of the governor, 209; - His double-acting engine considered as a whole, 216; - Investigation of the process of its operation, 217; - Eccentric, 225; - Cocks and valves, 227; - Single clack-valve, 227; - Double clack-valve, 228; - Conical steam-valve, 228; - Slide-valves, 229; - The waggon boiler adopted by him, 225; - Invents the indicator, 274; - The counter contrived by him, 278; - The Lunar Society in which Watt and Boulton were leading - members, 302; - Invents the copying press, 302; - His friends and associates at Birmingham, 302; - Method of heating by steam brought forward by him, 303; - His invention of a machine for drying linen by steam, 303; - His share in the discovery of the composition of water, 303; - His letter to Priestley on this subject, 307; - Anecdote of his inventive genius, 309; - Introduces the use of chlorine in bleaching, 310; - One of the founders of the Pneumatic institution at Clifton, - 310; - His first marriage, 310; - Private life of, 311; - Death of his first wife, 311; - His second marriage, 311; - He retires from business, 311; - Death of his younger son, 311; - Extracts from his letters, 312; - His death, 313; - Character of, by Lord Brougham, 313; - By Sir Walter Scott, 314; - By Lord Jeffrey, 315; - Occupation of his old age, 318; - Invention of machine for copying sculpture, 318; - His last days, 318; - Monuments, 319; - Inscription on the monument in Westminster Abbey from the - pen of Lord Brougham, 319; - His application of tubular condensers, 457; - His expedient for coating boilers with felt, 463. - - Wilkinson, his machine for accurately boring the insides of - cylinders, 149. - - Williams's method of consuming the unburned gases which escape - from the grate, and are carried through the flues, 260. - - Woolf's engine, 176; - Woolf's piston, 243. - - [Illustration: RICHMOND BRIDGE.] - - -LONDON: - -Printed by A. SPOTTISWOODE, -New-Street-Square. - - - - - * * * * * - - - - -Transcriber's endnote: - - Original spelling and grammar has mostly been retained. For - example, the forms "Cyclopoedia", "cyclopædia", "Encyclopædia", - "Encyclopoedia", "guage", and "gauge" are all retained. Figures - were moved from within paragraphs to between paragraphs. - Footnotes were re-indexed and moved to the ends of chapters. - - An entry for the INDEX was inserted into the Table of Contents. - - In the Table of Contents, changed "MM. Dulong and Arrago" to "MM - Dulong and Arago". Also "Blinkinsop" to "Blinkensop". Also - "Wasborough's" to "Wasbrough's". - - Figs. 4, 5 and 6 are all in one image. Two tubes in Fig. 4 - were incorrectly labeled T'; one of these has been crossed out - and changed to T. Both tubes in Fig. 6 were incorrectly - labeled G. One of these was crossed out and replaced by G'. - Note also that Figs. 4, 5, 6 are repeated in the text on - different pages; this feature has been retained. - - Page 10: "it s already" to "it is already". - - Page 43: "Thu if heat" changed to "Thus if heat". - - Page 45: "had a diameter of only one square foot" changed to - "had a diameter of only one foot". - - Page 47: "immedate" to "immediate". - - Page 51: "a a level" to "a level". Also, comma removed from "A - gauge, pipe is inserted". - - Page 53: "proportionably" to "proportionally". - - Page 79: A paragraph beginning "He was not fourteen" contains three - double quotation marks; this is presumably an error. Possibly there - should be two double quotation marks and two single quotation - marks. - - Page 80: "S'. Gravesande" is retained, although this probably - refers to a person known as "'s Gravesande". - - Page 103: "gases n general" to "gases in general". - - Page 122: comma removed from "process may, be continued". - - Page 123: "two thin pipes F G of tin" to "two thin pipes F, G - of tin". - - Page 172: "empyrical" to "empirical". - - Page 187, Fig. 32.: The text refers to "end I of the - connecting rod", but this was labeled L on the Figure. This L - has been crossed out and replaced by I. - - Page 285: In "surrounding the boiler with iron-conducting - substances", changed "iron-" to "non-". - - Page 308: "exeitement" to "excitement". - - Page 362: "acomplish" to "accomplish". - - There were several extended quotations, for example beginning on - page 312, in which each line began with a quotation mark, with - ending quotation marks at the end of each paragraph. In this - edition, these passages have been marked by indentation, and all - but the first and last quotation marks from each paragraph were - removed. - - Page 366: Figs. 97-104 appeared originally between pages 385 - and 399, as full-page prints. Numerically, however, they - belong between Figs. 96 and 105--therefore between pages - 366 and 369. Therefore, they have been moved to a location - between two paragraphs on page 367. - - Page 368: "rivetted" to "riveted". - - Page 419: "TREVITHECK'S INVENTION" changed to "TREVETHICK'S - INVENTION", in the chapter heading. However, the references to - Trevethick occur in a previous chapter, around page 324. - - Page 468: Period added to end sentence "[...] piston is at the - bottom of its stroke". - - Page 490: Period added to end sentence "[...] therefore one - eighth of its capacity". - - Pages 494, 497: large data tables were split into two pieces - each. - - Page 505: The logarithm originally given as log x = - "[=1]·82340688193", where "[=1]" represents a numeral one with - a horizontal line over it, is herein changed to log x = - "0·82340688193 - 1", as that is the meaning of this convention. - - Page 513 "formulæ are hyberbolic" to "formulæ are hyperbolic". - - In the Appendix, pp 505-522, mathematical variables such as - "a", "p", "t", etc. were originally italicized. In these text - file versions, italicized variables have been removed from this - section of the book. This rule has two unfortunate exceptions: - E, _E_, E', and _E'_ on pp 512-515 are different variables, and - have been retained. Italics have been removed from tables - throughout the work. - - The tables on page 494 and 497 were divided into two parts, - better to fit the width constraints of this format. Most of the - tables will not look good unless viewed with a monospace font, - such as Courier New or Lucida Console. - - - -***END OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND -ILLUSTRATED (SEVENTH EDITION)*** - - -******* This file should be named 42602-8.txt or 42602-8.zip ******* - - -This and all associated files of various formats will be found in: -http://www.gutenberg.org/dirs/4/2/6/0/42602 - - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. Special rules, -set forth in the General Terms of Use part of this license, apply to -copying and distributing Project Gutenberg-tm electronic works to -protect the PROJECT GUTENBERG-tm concept and trademark. 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You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at <a -href="http://www.gutenberg.org">www.gutenberg.org</a></p> -<p>Title: The Steam Engine Explained and Illustrated (Seventh Edition)</p> -<p> With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt</p> -<p>Author: Dionysius Lardner</p> -<p>Release Date: April 26, 2013 [eBook #42602]</p> -<p>Language: English</p> -<p>Character set encoding: UTF-8</p> -<p>***START OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND ILLUSTRATED (SEVENTH EDITION)***</p> <p> </p> -<h4>E-text prepared by Chris Curnow, RichardW,<br /> - and the Online Distributed Proofreading Team<br /> - (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br /> - from page images generously made available by<br /> - Internet Archive<br /> - (<a href="http://archive.org">http://archive.org</a>)</h4> <p> </p> <table class="pg" border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10"> <tr> @@ -27381,360 +27364,6 @@ split into two tables.</p> <p> </p> <p> </p> -<hr class="pg" /> -<p>***END OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND ILLUSTRATED (SEVENTH EDITION)***</p> -<p>******* This file should be named 42602-h.txt or 42602-h.zip *******</p> -<p>This and all associated files of various formats will be found in:<br /> -<a href="http://www.gutenberg.org/dirs/4/2/6/0/42602">http://www.gutenberg.org/4/2/6/0/42602</a></p> -<p> -Updated editions will replace the previous one--the old editions -will be renamed.</p> - -<p> -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - - - - -Title: The Steam Engine Explained and Illustrated (Seventh Edition) - With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt - - -Author: Dionysius Lardner - - - -Release Date: April 26, 2013 [eBook #42602] - -Language: English - -Character set encoding: ISO-646-US (US-ASCII) - - -***START OF THE PROJECT GUTENBERG EBOOK THE STEAM ENGINE EXPLAINED AND -ILLUSTRATED (SEVENTH EDITION)*** - - -E-text prepared by Chris Curnow, RichardW, and the Online Distributed -Proofreading Team (http://www.pgdp.net) from page images generously made -available by Internet Archive (https://archive.org) - - - -Note: Project Gutenberg also has an HTML version of this - file which includes the original illustrations. - See 42602-h.htm or 42602-h.zip: - (http://www.gutenberg.org/files/42602/42602-h/42602-h.htm) - or - (http://www.gutenberg.org/files/42602/42602-h.zip) - Some characters might not display in the html version. - If so, the reader should consult the iso-8859-1 (Latin-1) - text file 42602-8.txt or 42602.zip: - (http://www.gutenberg.org/files/42602/42602-8.txt) - or - (http://www.gutenberg.org/files/42602/42602-8.zip) - - - Images of the original pages are available through - Internet Archive. See - https://archive.org/details/steamengineexpla00lardrich - - -Transcriber's note: - - Text enclosed by underscores is in italics (_italics_). - - The notation "^{n}" means that n is superscript. The notation - "_{n}" means that n is subscript. - - Text that was in small capitals has been converted to all - capitals. - - The original page numbers are embedded in square brackets, - e.g. "[Pg135]". - - More detailed notes are provided in the Transcriber's endnote. - - - - - -[Illustration: WATT. - -Engraved by H. Adlard, from a Drawing by H. Corbould, taken with -the permission of James Watt, Esq. - -FROM THE STATUE BY CHANTREY. - -London: Taylor & Walton, Upper Gower Street.] - - -THE STEAM ENGINE EXPLAINED AND ILLUSTRATED; - -With an Account of Its Invention and Progressive Improvement, -and Its Application to Navigation and Railways; - -Including also a Memoir of Watt. - -by - -DIONYSIUS LARDNER, D.C.L. F.R.S. -&c. &c. - -SEVENTH EDITION, - -Illustrated by Engravings on Wood. - - - - - - - -London: -Printed for Taylor and Walton, -28. Upper Gower Street. - -MDCCCXL. - -London: -Printed by A. Spottiswoode, -New-Street-Square. - - - - - TO - - THE RIGHT HONOURABLE - - HENRY LORD BROUGHAM AND VAUX, - - FELLOW OF THE ROYAL SOCIETY, - - AND - - MEMBER OF THE NATIONAL INSTITUTE OF FRANCE, - - AS A MARK OF PUBLIC RESPECT - - AND - - A TESTIMONY OF PRIVATE REGARD, - - THIS WORK - - INSCRIBED, BY HIS ATTACHED FRIEND, - - THE AUTHOR. - - - - -ADVERTISEMENT. - - -The Drawings for several of the Cuts in this Volume have been -taken, by the permission of Mr. Weale, from the admirable Plates -annexed to the last edition of Tredgold on the Steam Engine and on -Steam Navigation. This acknowledgment is especially due for the -Illustrations which abound in this Volume. - -_London, June, 1840._ - - - - -[Illustration: LONDON ENTRANCE TO THE BIRMINGHAM RAIL-ROAD.] - - - - - CONTENTS. - - - CHAPTER I. - - PRELIMINARY MATTER. - - Page - - The Steam Engine, a Subject of popular Interest 4 - - Effects of Steam 5 - - Great Power of Steam 7 - - Object of this Work 9 - - Disputes respecting the Invention 11 - - Hero of Alexandria's Machine 13 - - Blasco De Garay's Proposition to propel Vessels - by a Machine 16 - - Solomon De Caus 17 - - Giovanni Branca proposes to work Mills by Steam 22 - - Marquis of Worcester 23 - - Mechanical Properties of Fluids 25 - - Elastic and Inelastic Fluids 25 - - Elasticity of Gases 28 - - Effects of Heat 29 - - Application of these Principles to the Engines - of Hero, De Caus, and Lord Worcester 30 - - Sir Samuel Morland 34 - - Denis Papin 36 - - Atmospheric Pressure 38 - - Weight of Air 39 - - Pressure of Air 41 - - Barometer 41 - - Elastic Force of Air and Gases 42 - - Force obtained by a Vacuum 43 - - Common Pump 43 - - Rarefaction by Heat 44 - - Process of filling Thermometers 44 - - Papin's Method of producing a Vacuum 44 - - His Discovery of the Condensation of Steam 45 - - Thomas Savery 47 - - - CHAP. II. - - ENGINES OF SAVERY AND NEWCOMEN. - - Savery's Engine 49 - - Boilers and their Appendages 50 - - Working Apparatus described 51 - - Mode of Operation 52 - - Defects of Savery's Engine 58 - - Newcomen's Engine described by Papin 62 - - Newcomen and Cawley obtain a Patent for - Atmospheric Engine 65 - - Accidental Discovery of Condensation by Injection 69 - - Potter's Discovery of the Method of working - the Valves 71 - - His Contrivance improved by the Substitution - of a Plug Frame 72 - - Advantages of the Atmospheric Engine over that - of Savery 72 - - The Power of Savery's Engine restricted 73 - - It contained no new Principle 73 - - Its practical Superiority 73 - - - CHAP. III. - - EARLY CAREER AND DISCOVERIES OF JAMES WATT. - - Atmospheric Engine improved by Beighton 75 - - Smeaton's Improvements in the Atmospheric Engine 76 - - Brindley obtains a Patent for Improvement in 76 - - Invents the Self-regulating Feeder 76 - - Infancy of James Watt 77 - - His Descent and Parentage 77 - - Anecdotes of his Boyhood 78 - - His early Acquirements 79 - - Goes to London 80 - - Returns to Glasgow 80 - - Appointed Instrument-maker to the University 81 - - Opens a Shop in Glasgow 81 - - His Friends and Patrons 81 - - Professor Robison's Remarks on Watt's personal - Character 82 - - His industrious and studious Habits 82 - - His Attention first directed to Steam 83 - - Experiments on High-pressure Engine 83 - - Repairs an Atmospheric Model 84 - - Experimental Inquiry consequent on this 84 - - Its Results 84 - - Observes great Defects in the Atmospheric Engine 85 - - His first Attempt to improve it 85 - - His early Experiments on Steam 87 - - Discovery by Experiment of the Expansion which - Water undergoes in Evaporation 90 - - Discovers the latent Heat of Steam 91 - - Informed by Dr. Black of the Theory of latent Heat 93 - - His Improvement not due to Black 93 - - - CHAP. IV. - - EXPOSITION OF PHYSICAL PRINCIPLES. - - Construction of Thermometer 98 - - Method of graduating it 99 - - Freezing and boiling Points 99 - - Latent Heat of Water 101 - - Quantity of Heat necessary to convert Ice into - Water, first noticed by Dr. Black 101 - - Examination of the analogous Effects produced - by the continued Application of Heat to Water - in the liquid State 102 - - Process of Boiling 104 - - Reconversion of Steam into Water 104 - - Conversion of Water into Steam 105 - - Latent Heat of Steam 107 - - Boiling Point varies 108 - - Different in different Places 109 - - Inquiry whether a Diminution of Pressure will - produce a corresponding Effect on the boiling - Point 112 - - Table showing the Temperature at which Water - will boil under different Pressures of the - Atmosphere 113 - - Mechanical Force of Steam 115 - - Facts to be observed in 117 - - - CHAP. V. - - FURTHER DISCOVERIES OF WATT. - - Watt finds that Condensation in the Cylinder - is incompatible with a due Economy of Fuel 119 - - Conceives the Notion of condensing out of the - Cylinder 120 - - Discovers separate Condensation 121 - - Invents the Air Pump 123 - - Substitutes Steam Pressure for Atmospheric - Pressure 123 - - Invents the Steam Case, or Jacket 124 - - His first Experiments to realise these - Inventions 125 - - His experimental Apparatus 125 - - His experimental Models fitted up at Delft - House, in Glasgow 128 - - Difficulties of bringing the improved Engines - into Use 129 - - Watt first employed by Roebuck as a Civil - Engineer 130 - - His Partnership with Roebuck 130 - - His first Patent 130 - - Experimental Engine at Kinneal 131 - - Abstract of the Act of Parliament for the - Extension of his Patent 132 - - Description of his single-acting Steam Engine 133 - - - CHAP. VI. - - WATT'S ENGINES. - - Correspondence of Watt with Smeaton 145 - - Failure of Condensation by Surface 146 - - Improvements in Construction of Piston 147 - - Method of Packing 148 - - Improvements in boring the Cylinder 149 - - Disadvantages of the new compared with the old - Engines 150 - - Greatly increased Economy of Fuel 150 - - Economy of the Engine 151 - - Expedients to force the new Engines into Use 151 - - Correspondence of Boulton 153 - - Correspondence with Smeaton 155 - - Efficiency of Fuel in the new Engines 156 - - Discovery of the expansive Action of Steam 157 - - Watt states it in a Letter to Dr. Small 157 - - Its Principle explained 158 - - Mechanical Effects resulting from it 162 - - Computed Effect of cutting off Steam at - different Portions of the Stroke 163 - - Produces a variable Power 163 - - Expedients for equalising the Power 164 - - Expansive Principle in Watt's Engines limited 165 - - Its more extensive Application in the Cornish - Engines 165 - - - CHAP. VII. - - DOUBLE-ACTING ENGINE. - - Common Steam 168 - - Superheated Steam 170 - - Laws of Dalton and Gay Lussac 171 - - Law of Mariotte 171 - - Relation between Temperature and Pressure of - common Steam 171 - - Effects of the Expansion of common Steam 173 - - Mechanical Effects of Steam 173 - - Method of equalising the expansive Force 174 - - Hornblower's Engine 175 - - Woolf's Engine 176 - - Watt's Attempts to extend the Steam Engine to - Manufactures 178 - - Papin's projected Applications of the Steam - Engine 178 - - Savery's Application of the Engine to move - Machinery 180 - - Jonathan Hull's Application to Water Wheels 180 - - Champion of Bristol applies the Atmospheric - Engine to raise Water 181 - - Stewart's Application of the Engine to - Mill-work 182 - - Wasbrough's Application of the Fly-wheel and - Crank 183 - - Reasons why Watt's single-acting Steam Engine - was not adapted to produce continuous uniform - Motion of Rotation 184 - - Watt's Second Patent 186 - - Sun-and-Planet Wheels 187 - - Valves of double-acting Engine 189 - - - CHAP. VIII. - - DOUBLE-ACTING ENGINE. - - Methods of connecting the Piston-rod and Beam - in the double-acting Engine 193 - - Rack and Sector 194 - - Parallel Motion 195 - - Connection of Piston-rod and Beam 195 - - Connecting Rod and Crank 203 - - Fly-wheel 205 - - Throttle-valve 207 - - Governor 209 - - Construction and Operation of the - double-acting Engine 216 - - Eccentric 225 - - Cocks and Valves 227 - - Single-clack Valves 227 - - Double-clack Valves 228 - - Conical Valves 228 - - Slide Valves 229 - - Murray's Slides 229 - - D Valves 230 - - Seaward's Slides 235 - - Single Cock 238 - - Two-way Cock 239 - - Four-way Cock 239 - - Pistons 242 - - Common hemp-packed Piston 242 - - Woolf's Piston 243 - - Metallic Pistons 244 - - Cartwright's Engine 245 - - Cartwright's Piston 247 - - Barton's Piston 248 - - - CHAP. IX. - - BOILERS AND FURNACES. - - Analysis of Coal 252 - - Process of Combustion 253 - - Heat evolved in it 254 - - Form and Structure of Boiler 255 - - Waggon Boiler 255 - - Furnace 256 - - Method of feeding it 257 - - Combustion of Gas in Flues 260 - - Williams's Patent for Method of consuming - unburned Gases 260 - - Construction of Grate and Ash-pit 261 - - Magnitude of heating Surface of Boiler 262 - - Steam-space and Water-space in Boiler 263 - - Position of Flues 264 - - Method of feeding Boiler 265 - - Method of indicating the Level of Water in - Boiler 266 - - Level Gauges 266 - - Self-regulating Feeders 267 - - Steam Gauge 270 - - Barometer Gauge 272 - - Watt's Invention of the Indicator 274 - - Counter 278 - - Safety-valve 279 - - Fusible Plugs 280 - - Self-regulating Damper 281 - - Brunton's Self-regulating Furnace 283 - - Gross and useful Effect of an Engine 285 - - Power and Duty of Engines 287 - - Horse-power of Steam Engines 289 - - Evaporation proportional to Horse-power 290 - - Sources of Loss of Power 292 - - Absence of good practical Rules for Power 292 - - Common Rules followed by Engine-makers 292 - - Duty distinguished from Power 294 - - Duty of Boilers 294 - - Proportion of Stroke to Diameter of Cylinder 295 - - Duty of Engines 296 - - Cornish System of Inspection 297 - - Table showing the Improvement of Cornish Engines 298 - - Beneficial Effects of Cornish Inspection 299 - - Successive Improvements on which the increased - Duty of Engines depends, traced by John Taylor - in his "Records of Mining" 299 - - - CHAP. X. - - LIFE OF WATT. - - Watt's Friends and Associates at Birmingham 302 - - His Invention of the Copying Press 302 - - Heating Apartments and Buildings by Steam 303 - - Watt's Machine for drying Linen 303 - - His Share in the Discovery of the Composition - of Water 303 - - The Merit of this Discovery shared between - Cavendish, Lavoisier, and Watt 305 - - Anecdote of Watt's Activity of Mind 309 - - His Introduction of the Use of Chlorine in - Bleaching 310 - - One of the Founders of the Pneumatic - Institution at Clifton 310 - - His first Marriage 311 - - Death of his first Wife 311 - - His second Marriage 311 - - Death of his younger Son 311 - - Extracts from his Letters 312 - - Character of Watt by Lord Brougham 313 - - Extract from Sir Walter Scott's "Monastery" on - the Character of Watt 314 - - Sketch of the Life of Watt from the "Scotsman" - by Lord Jeffrey 315 - - Occupation of his old Age 318 - - His Invention of Machine for copying Sculpture 318 - - His last Days 318 - - Monuments to his Memory 319 - - Inscription by Lord Brougham on the Pedestal - of the Monument in Westminster Abbey 320 - - - CHAP. XI. - - LOCOMOTIVE ENGINES ON RAILWAYS. - - Page - High-pressure Engines 322 - - One of the earliest Forms of the Steam Engine 322 - - Description of Leupold's Engine 323 - - Non-condensing Engine of Messrs. Trevethick - and Vivian 324 - - Construction of a Machine for moving Carriages - on Railroads 328 - - Effects of Railway Transport 329 - - Moral and political Consequences of 334 - - History of the Locomotive Engine 337 - - Construction of Locomotive Engine by Blinkensop 337 - - Messrs. Chapman's Contrivance 337 - - Walking Engine 337 - - Mr. Stephenson's Engines at Killingworth 339 - - Liverpool and Manchester Railway 342 - - The Directors offer a Prize for the best - Locomotive Engine 344 - - Experimental Trial 344 - - The "Rocket," "Sanspareil," and "Novelty" 344 - - Admirable Arrangement in the Rocket 345 - - Description of the "Sanspareil" 347 - - Description of the "Novelty" 349 - - The Superiority of the "Rocket" 350 - - Method of subdividing the Flue into Tubes 353 - - Progressive Improvement of Locomotive Engines 354 - - Dr. Lardner's Experiments in 1832 357 - - Adoption of Brass Tubes 360 - - Great Expense of Locomotive Power 361 - - Mr. Booth's Report 362 - - Detailed Description of the most improved - Locomotive Engines 365 - - Substitution of Brass for Copper Tubes - ascribed to Mr. Dixon 370 - - Power of Locomotive Engines 379 - - Position of the Eccentrics 379 - - Pressure of Steam in the Boiler 401 - - Dr. Lardner's Experiments in 1838 406 - - Resistance to Railway Trains 407 - - Dr. Lardner's Experiments on the Great Western - Railway 408 - - Experiments on Resistance 409 - - Restrictions on Gradients 410 - - Compensating Effect of Gradients 412 - - Experiment with the "Hecla" 412 - - Disposition of Gradients should be uniform 415 - - Methods of surmounting steep Inclinations 415 - - - CHAP. XII. - - LOCOMOTIVE ENGINES ON TURNPIKE ROADS. - - Railways and Stone Roads compared 420 - - Gurney's Steam Carriage 423 - - The Boiler of Gurney's Engine 423 - - His Method of cleansing Boilers 428 - - Convenience and Safety of Steam Carriages 432 - - Two Methods of applying Locomotive Engines - upon common Roads 434 - - Horse Carriages compared with Steam 435 - - Extract from Mr. Farey's Evidence before the - House of Commons 435 - - Hancock's Steam Carriage 436 - - How it differs from that of Mr. Gurney 437 - - Ogle's Locomotive Carriage 438 - - Dr. Church's Steam Engine 439 - - - CHAP. XIII. - - STEAM NAVIGATION. - - Form and Arrangement of Marine Engines 441 - - Arrangement of the Engine-room 446 - - Marine Boilers 448 - - Effects of Sea Water in Boilers 450 - - Remedies for them 451 - - Blow-off Cocks 452 - - Indicators of Saltness 453 - - Seaward's Indicator 454 - - His Method of blowing out 454 - - Field's Brine Pumps 456 - - Tubular Condensers applied by Mr. Watt 457 - - Hall's Condensers 458 - - Substitution of Copper for Iron Boilers 460 - - Process of Stoking 462 - - Watt's Expedient of attaching Felt to the - Boiler Surface 463 - - Means of economising Fuel 463 - - Number and Arrangement of Furnaces and Flues 463 - - Howard's Marine Engine 464 - - Application of the expansive Principle in - Marine Engines 466 - - Recent Improvements of Messrs. Maudslay and - Field 467 - - Humphrey's Marine Engine 470 - - Common Paddle-wheel 472 - - Feathering Paddles 474 - - Galloway's Patent for a Paddle-wheel with - movable Paddles 476 - - Split Paddle 478 - - Proportion of Power to Tonnage 480 - - Improved Efficiency of Marine Engines 482 - - Iron Steam Vessels 483 - - Steam Navigation to India 484 - - - CHAP. XIV. - - AMERICAN STEAM NAVIGATION. - - Steam Navigation first established in America 487 - - Circumstances which led to it 488 - - Attempts of Fitch and Rumsey to apply the - single-acting Engine to the Propulsion of - Vessels 489 - - Stevens of Hoboken commences Experiments in - Steam Navigation 489 - - Experiments of Livingstone and Fulton 489 - - Fulton's first Boat 490 - - The Hudson navigated by Steam 491 - - Extension and Improvement of River Navigation 492 - - American Steamers 494 - - Difference between them and European Steamers 494 - - Steamers on the Hudson 494 - - American Paddle-wheels 495 - - Sea-going American Steamers 496 - - Speed attained by American Steamers 497 - - Lake Steamers 499 - - The Mississippi and its Tributaries 499 - - Steam-boats navigating it 500 - - Their Structure and Machinery 500 - - New Orleans Harbour 503 - - Steam Tugs 503 - - - APPENDIX. - - _On the Relation between the Temperature, Pressure, and_ - _Density of Common Steam._ - - Empirical Formula of Biot, showing the - Relation between the Pressure and Temperature 505 - - Empirical formula of Southern 506 - Tredgold 506 - Mellet 506 - De Pambour 506 - MM. Dulong and Arago 506 - - Law of the Expansion of elastic Fluids, - discovered by Dalton and Gay Lussac 506 - - Formula for the Relation between the Volumes - and Temperatures 507 - - Law of Mariotte 507 - - Table of Pressures, Temperatures, Volumes, and - Mechanical Effects of Steam 509 - - Empirical Formulae for the Relation between the - Volume of Water and that of the Steam produced - by its Evaporation under given Pressures 511 - - Formula of Navier 511 - - Modified by De Pambour 511 - - _On the Expansive Action of Steam._ - - Mechanical Effect produced during a given - Extent of Expansion 511 - - Mechanical Effect produced during Evaporation - and subsequent Expansion 512 - - Application to double-acting Engines 513 - - Formula for Pressure of Steam in Cylinders 514 - - Formula for total Mechanical Effect per Minute - of Steam when cut off at any proposed Part of - the Stroke 514 - - Formulae exhibiting the Relation between the - Resistance of the Load, the Resistances of the - Engine, the Evaporation, the Speed of the - Piston, and the Magnitude of the Cylinder 515 - - Formulae showing the Relation between the Power - of the Engine, the Evaporation, and the useful - Load 516 - - Formulae for the _useful Effect_ and the _Duty_ 517 - - Estimates of the several Sources of - Resistances 518 - - Tables to facilitate the Computation of the - Effects of Expansive Engines 519 - - Table of the Areas of Pistons 520 - - EXAMPLES of the Application of these Formulae 521 - - - INDEX. 523 - - - - -[Illustration: VIADUCT, NEAR WATFORD, BIRMINGHAM RAIL-ROAD.] - -[Pg001] - - - - -THE STEAM ENGINE. - - - - -[Pg003] - - - - -[Illustration: HERO OF ALEXANDRIA.] - -CHAPTER I. - - THE STEAM ENGINE, A SUBJECT OF POPULAR INTEREST. -- THE OBJECT - OF THIS WORK. -- DISPUTES RESPECTING THE INVENTION. -- HERO. -- DE - GARAY. -- DE CAUS. -- BRANCA. -- MARQUIS OF WORCESTER. -- PHYSICAL - PRINCIPLES. -- ELASTIC AND INELASTIC FLUIDS. -- THEIR - PROPERTIES. -- APPLICATION OF THESE PRINCIPLES TO THE ENGINES - OF HERO, DE CAUS, AND LORD WORCESTER. -- SIR SAMUEL MORLAND. -- - PAPIN. -- ATMOSPHERIC PRESSURE. -- THE WEIGHT OF AIR. -- LESS AT - GREATER HEIGHTS. -- BAROMETER. -- PRESSURE OF AIR. -- ELASTIC - FORCE OF AIR AND GASES. -- FORCE PRODUCED BY A VACUUM. -- COMMON - PUMP. -- RAREFACTION BY HEAT. -- PAPIN'S METHODS OF PRODUCING A - VACUUM. -- HIS DISCOVERY OF THE CONDENSATION OF STEAM. -- - SAVERY. - - -(1.) That the history of the invention of a piece of mechanism, -and the description of its structure, operation, and [Pg004] -uses, should be capable of being rendered the subject matter of a -volume, destined not alone for the instruction of engineers or -machinists, but for the information and amusement of the public in -general, is a statement which at no very remote period would have -been deemed extravagant and incredible. - -Advanced as we are in the art of rendering knowledge popular, and -cultivated as the public taste is in the appreciation of the -expedients by which science ministers to the uses of life, there -is still perhaps but one machine of which such a proposition can -be truly predicated: it is needless to say that that machine is -the STEAM ENGINE. There are many circumstances attending this -extraordinary piece of mechanism which impart to it an interest so -universally felt. Whether we regard the details of its structure -and operation, the physical principles which it calls into play, -and the beautiful contrivances by which these physical principles -are rendered available;--or, passing over these _means_, we direct -our attention to the _ends_ which they attain, we are equally -filled with astonishment and admiration. The history of the steam -engine offers to our notice a series of contrivances which, for -exquisite and refined ingenuity, stand without any parallel in the -annals of mechanical science. These admirable inventions, unlike -other results of scientific inquiry, have also this peculiarity, -that, to understand their excellence and to perceive their beauty, -no previous or subsidiary knowledge is necessary, save what may be -imparted with facility and clearness in the progress of the -explanation and development of the machine itself. A simple and -clear exposition, divested of needless technicalities and aided by -well-selected diagrams, is all that is necessary to render the -construction and operation of the steam engine, in all its forms, -intelligible to persons of plain understanding and moderate -information. - -But if the contrivances by which this vast power is brought to bear -on the arts and manufactures, be rendered attractive by their great -mechanical beauty, how much more imposing will the subject become -when the effects which the steam engine has produced upon the -well-being of the human race are considered. It has penetrated the -crust of the earth, and drawn from beneath it boundless treasures -[Pg005] of mineral wealth, which, without its aid, would have been -rendered inaccessible; it has drawn up, in measureless quantity, -the fuel on which its own life and activity depend; it has relieved -men from their most slavish toils, and reduced labour in a great -degree to light and easy superintendence. To enumerate its present -effects, would be to count almost every comfort and every luxury of -life. It has increased the sum of human happiness, not only by -calling new pleasures into existence, but by so cheapening former -enjoyments as to render them attainable by those who before could -never have hoped to share them: the surface of the land, and the -face of the waters, are traversed with equal facility by its power; -and by thus stimulating and facilitating the intercourse of nation -with nation, and the commerce of people with people, it has knit -together remote countries by bonds of amity not likely to be -broken. Streams of knowledge and information are kept flowing -between distant centres of population, those more advanced -diffusing civilisation and improvement among those that are more -backward. The press itself, to which mankind owes in so large a -degree the rapidity of their improvement in modern times, has had -its power and influence increased in a manifold ratio by its union -with the steam engine. It is thus that literature is cheapened, -and, by being cheapened, diffused; it is thus that Reason has taken -the place of Force, and the pen has superseded the sword; it is -thus that war has almost ceased upon the earth, and that the -differences which inevitably arise between people and people are -for the most part adjusted by peaceful negotiation. - -Deep as the interest must be with which the steam engine will be -regarded in every civilised country, it presents peculiar claims -upon the attention of the people of Great Britain. Its invention -and progressive improvement are the work of our own time and our -own country; it has been produced and matured almost within the -last century, and is the exclusive offspring of British genius, -fostered and sustained by British enterprise and British capital. - -The steam engine is a mechanical contrivance, by which coal, wood, -or other fuel is rendered capable of executing any [Pg006] kind -of labour. COALS are by it made to spin, weave, dye, print and -dress silks, cottons, woollens, and other cloths; to make paper, -and print books upon it when made; to convert corn into flour; to -express oil from the olive, and wine from the grape; to draw up -metal from the bowels of the earth; to pound and smelt it, to melt -and mould it; to forge it; to roll it, and to fashion it into -every desirable form; to transport these manifold products of its -own labour to the doors of those for whose convenience they are -produced; to carry persons and goods over the waters of rivers, -lakes, seas, and oceans, in opposition alike to the natural -difficulties of wind and water; to carry the wind-bound ship out -of port; to place her on the open deep ready to commence her -voyage; to throw its arms around the ship of war, and place her -side by side with the enemy; to transport over the surface of the -deep persons and information, from town to town, and from country -to country, with a speed as much exceeding that of the ordinary -wind, as the ordinary wind exceeds that of a common pedestrian. - -Such are the virtues, such the powers, which the steam engine has -conferred upon COALS. The means of calling these powers into -activity are supplied by a substance which nature has happily -provided in unbounded quantity in every part of the earth; and -though it has no price, it has inestimable value: this substance -is WATER. - -A pint of water may be evaporated by two ounces of coals. In its -evaporation it swells into two hundred and sixteen gallons of -steam, with a mechanical force sufficient to raise a weight of -thirty-seven tons a foot high. The steam thus produced has a -pressure equal to that of common atmospheric air; and by allowing -it to expand, by virtue of its elasticity, a further mechanical -force may be obtained, at least equal in amount to the former. A -pint of water, therefore, and two ounces of common coal, are thus -rendered capable of doing as much work as is equivalent to -seventy-four tons raised a foot high. - -The circumstances under which the steam engine is worked on a -railway are not favourable to the economy of fuel. Nevertheless -a pound of coke burned in a locomotive engine [Pg007] will -evaporate about five pints of water. In their evaporation they -will exert a mechanical force sufficient to draw two tons weight -on the railway a distance of one mile in two minutes. Four horses -working in a stage-coach on a common road are necessary to draw -the same weight the same distance in six minutes. - -A train of coaches weighing about eighty tons, and transporting -two hundred and forty passengers with their luggage, has been -taken from Liverpool to Birmingham, and back from Birmingham to -Liverpool, the trip each way taking about four hours and a -quarter, stoppages included. The distance between these places by -the railway is ninety-five miles. This double journey of one -hundred and ninety miles is effected by the mechanical force -produced in the combustion of four tons of coke, the value of -which is about five pounds. To carry the same number of passengers -daily between the same places by stage-coaches on a common road, -would require twenty coaches and an establishment of three -thousand eight hundred horses, with which the journey in each -direction would be performed in about twelve hours, stoppages -included. - -The circumference of the earth measures twenty-five thousand -miles; and if it were begirt with an iron railway, such a train as -above described, carrying two hundred and forty passengers, would -be drawn round it by the combustion of about thirty tons of coke, -and the circuit would be accomplished in five weeks. - -In the drainage of the Cornish mines the economy of fuel is much -attended to, and coals are there made to do more work than -elsewhere. A bushel of coals usually raises forty thousand tons of -water a foot high; but it has on some occasions raised sixty -thousand tons the same height. Let us take its labour at fifty -thousand tons raised one foot high. A horse worked in a fast -stage-coach pulls against an average resistance of about a quarter -of a hundred weight. Against this he is able to work at the usual -speed through about eight miles daily: his work is therefore -equivalent to one thousand tons raised one foot. A bushel of coals -consequently, as used in Cornwall, performs as much labour as a -day's work of one hundred such horses. [Pg008] - -The great pyramid of Egypt stands upon a base measuring seven -hundred feet each way, and is five hundred feet high, its weight -being twelve thousand seven hundred and sixty millions of pounds. -Herodotus states, that in constructing it one hundred thousand men -were constantly employed for twenty years. The materials of this -pyramid would be raised from the ground to their present position -by the combustion of about four hundred and eighty tons of coals. - -The Menai Bridge consists of about two thousand tons of iron, and -its height above the level of the water is one hundred and twenty -feet. Its mass might be lifted from the level of the water to its -present position by the combustion of four bushels of coal. - -The enormous consumption of coals produced by the application of -the steam engine in the arts and manufactures, as well as to -railways and navigation, has of late years excited the fears of -many as to the possibility of the exhaustion of our coal-mines. -Such apprehensions are, however, altogether groundless. If the -present consumption of coal be estimated at sixteen millions of -tons annually, it is demonstrable that the coal-fields of this -country would not be exhausted for many centuries. - -But in speculations like these, the probable, if not certain -progress of improvement and discovery ought not be overlooked; and -we may safely pronounce that, long before such a period of time -shall have rolled away, other and more powerful mechanical agents -will supersede the use of coal. Philosophy already directs her -finger at sources of inexhaustible power in the phenomena of -electricity and magnetism. The alternate decomposition and -recomposition of water, by magnetism and electricity, has too -close an analogy to the alternate processes of vaporisation and -condensation, not to occur at once to every mind: the development -of the gases from solid matter by the operation of the chemical -affinities, and their subsequent condensation into the liquid -form, has already been essayed as a source of power. In a word, -the general state of physical science at the present moment, the -vigour, activity, and sagacity with which researches in it are -prosecuted in every civilised [Pg009] country, the increasing -consideration in which scientific men are held, and the personal -honours and rewards which begin to be conferred upon them, all -justify the expectation that we are on the eve of mechanical -discoveries still greater than any which have yet appeared; and -that the steam engine itself, with the gigantic powers conferred -upon it by the immortal Watt, will dwindle into insignificance in -comparison with the energies of nature which are still to be -revealed; and that the day will come when that machine, which is -now extending the blessings of civilisation to the most remote -skirts of the globe, will cease to have existence except in the -page of history. - - -(2.) The object of the present volume will be to deliver, in an -easy and familiar style, an historical view of the invention of the -steam engine, and an exposition of its structure and operation in -the various forms in which it is now used, and of its most -important applications in the arts of life, especially in transport -by land and water. It is hoped that the details of these subjects -may be rendered easily intelligible to all persons of ordinary -information, whether urged by that natural and laudable spirit of -inquiry awakened by contemplating effects on the material and -social condition of our species, so rapid and so memorable as those -which have followed the invention of the steam engine, and by the -pleasure which results from the perception of the numerous -instances of successful contrivances and beautiful applications of -science to art which it unfolds,--or impelled by the exigencies of -trade or profession to acquire an acquaintance with a machine on -which, more than any other, the prosperity of our commercial and -manufacturing interests depends. It will be our aim to afford to -the former class all the information which they can require; and, -if this work be not as comprehensive in its scope, and as minute in -its details, as some of the latter may wish, it will at least serve -as an easy and convenient introduction to other works more -voluminous, costly, and detailed, but less elementary in their -matter, and less familiar in their style. - -In explaining the different forms of steam engine which have been -proposed in the course of the progressive improvement [Pg010] of -that machine from its early rude and imperfect state to its -present comparatively perfect form, it will be necessary to advert -to various physical phenomena and mechanical principles, which, -however obvious to those who are conversant with matters of -science, must necessarily be at least imperfectly known by the -great majority of our readers. To refer for information on such -topics to other works on Mechanics and general Physics, would be -with most readers ineffectual, and with all unsatisfactory. In -former editions of the present work, we consigned these necessary -general principles of physics and mechanics to a preliminary -chapter; but it appears, on the whole, more convenient not to -remove the exposition of the principle from the place where its -application is required. We shall therefore pause as we proceed, -where these difficulties occur, to give such explanation and -illustration as may seem best suited to render them intelligible -and interesting to the unscientific reader. - -The history of the arts and manufactures affords no example of any -invention the credit for which has been claimed by so many -different nations and individuals as that of the steam engine. The -advocates of the competitors for this honour have urged their -pretensions, and pressed their claims, with a zeal which has -occasionally outstripped the bounds of discretion, and the contest -has not unfrequently been tinged with prejudices, national and -personal, and characterised by a degree of asperity altogether -unworthy of so noble a cause, and beneath the dignity of science. - -"When a question is clearly proposed, it is already half resolved." -Let us see whether a careful attention to this maxim will aid us -in the investigation of the origin of the steam engine. The source -of the power of that machine is found in the following natural -phenomena. - -_First._ When fire is applied to water, the liquid swells into -vapour, and in undergoing this change exerts, as has been already -stated, a considerable amount of mechanical force. This force may, -by proper means, be rendered applicable to any purpose for which -labour or power is needful. - -_Second._ The vapour so produced is endowed with the property of -elasticity, in virtue of which it is capable of [Pg011] swelling -or expanding into increased dimensions, exerting, as it expands, a -force, the energy of which is gradually diminished as the -dimensions of the vapour are increased. This mechanical force is -likewise capable of being applied to any useful purpose for which -labour or power is necessary. - -_Third._ This vapour is capable, by proper means, of being -reconverted into water; and when so reconverted, it shrinks into -its original dimensions, deserting the large space which it -occupied as vapour, and leaving that space a vacuum. It is known -in physics, that when a vacuum is produced, surrounding bodies -have a tendency to rush into it with a definite amount of force. -Consequently any agent which produces a vacuum, becomes a source -of a considerable amount of mechanical power. By its reconversion -into water, therefore, steam again becomes a mechanical agent. - -Such are the natural phenomena in which are found the original -sources of all steam power. In some forms of steam engine one of -these is used, and in some another, and in some the application of -all of them is combined; but in no existing form of steam engine -whatever is there any other source of mechanical power. - -Neither these nor any other natural forces can be applied -_immediately_ to any useful purpose. The interposition of -mechanism is indispensable; on the invention and contrivance of -that mechanism depends altogether the useful application of these -natural forces. - -The world owes the steam engine then partly to _discovery_, and -partly to _invention_. - -He that discovered the fact, that mechanical force was produced in -the conversion of water into steam, must be justly held to be a -sharer in the merit of the steam engine, even though he should -never have practically applied his discovery. The like may be said -of him who first discovered the source of the mechanical power -arising from the expansion of steam. - -The discoverer of the fact, that steam being reconverted into -water greatly contracted its dimensions, and thereby produced a -vacuum, is likewise entitled to a share of the credit. [Pg012] - -The mechanism by which these natural forces have been rendered so -universally available as a moving power, is very various and -complicated, and cannot be traced to one inventor. "If a -watchmaker," says M. Arago, "well instructed in the history of his -art, were required to give a categorical answer to the question, -Who has invented watches? he would remain mute; but the question -would be divested of much of its difficulty if he were required -separately to declare who discovered the use of the main spring, -the different forms of escapement, or the balance wheel." So it is -with the steam engine. It is a combination of a great variety of -contrivances, distinct from each other, which are the production -of several inventors. If, however, one name more than the rest be -entitled to special notice; if _he_ is entitled to the chief -credit of the invention who by the powers of his mechanical genius -has imparted to the steam engine that form, and conferred upon it -those qualities, on which mainly depends its present extensive -utility, and by which it has become an agent of transcendant -power, spreading its beneficial effects throughout every part of -the civilised globe, then the universal voice will, as it were by -acclamation, award the honour to one individual, whose pre-eminent -genius places him far above all other competitors, and from the -application of whose mental energies to this machine may be dated -those grand effects which render it a topic of interest to all for -whom the progress of civilisation has any attractions. Before the -era rendered memorable by the discoveries of JAMES WATT, the steam -engine, which has since become an object of such universal -interest, was a machine of extremely limited power, inferior in -importance and usefulness to most other mechanical agents used as -prime movers; but, from that epoch, it is scarcely necessary here -to state, that it became a subject not of British interest only, -but one having an important connection with the progress of the -human race. - - -HERO OF ALEXANDRIA, 120 B. C. - -[Illustration: _Fig._ 1.] - - -(3.) The discovery of the fact, that a mechanical force is -produced when water is evaporated by the application of heat, -[Pg013] must be considered as the first capital step in the -invention of the steam engine. It is recorded in a work entitled -_Spiritalia seu Pneumatica_, that Hero of Alexandria contrived a -machine, 120 years before the Christian era, which was moved by -the mechanical force of the vapour of water. The principle of this -machine admits of easy explanation: When a fluid issues from any -vessel in which it is confined, that vessel suffers a force equal -to that with which the fluid escapes from it, and in the opposite -direction. If water issues from an orifice, a pressure is produced -behind the orifice corresponding to the force with which the water -escapes. If a man discharge a gun, the gases produced by the -explosion of the powder issue with a certain force from the -muzzle, and his shoulder is driven backwards by the recoil with a -corresponding force. If the muzzle, instead of being presented -forwards, were turned at right angles to the length of the gun, -then, as the gases of explosion would escape sideways, the recoil -would likewise take place sideways, and the shooter, instead of -being driven backward, would be made to spin round as a dancer -pirouettes. This was the principle of Hero's steam engine. A small -globe or ball was placed on pivots at A and B (_fig._ 1.), on -which it was capable of revolving: steam was supplied through one -of these pivots from one of the tubes D C E F, which communicated -with the boiler. This steam filled the globe A I B K, and also the -arms I H and K G. A lateral orifice, represented at G, near the -end of these arms, allowed the steam to escape in a jet, and the -reaction, producing a recoil, had a tendency to drive the arm -round. A small orifice at H, on the other side of the tube, -produced a like effect. In the same manner, any convenient number -of arms might be provided, surrounding the globe and communicating -with its interior like the spokes of a wheel. Thus these arms, -having lateral orifices for the escape of the steam, all placed so -that the recoil may [Pg014] tend to turn the globe in the same -direction, a rotatory motion might be communicated to any -machinery which it was desired to move. - -After having been allowed to slumber for nearly two thousand -years, this machine has recently been revived, and engines -constructed similar to it are now working in these countries. In -the proper place we shall describe Avery's Rotatory Engine, which -it will be seen is, not only in its principle, but almost in its -details, the machine of HERO OF ALEXANDRIA. - -Although the elastic force of steam was not reduced to numerical -measure by the ancients, nor brought under control, nor applied to -any useful purpose, yet it appears to have been recognised in -vague and general terms. Aristotle, Seneca, and other ancient -writers, accounted for earthquakes by the sudden conversion of -water into steam within the earth. This change, according to them, -was effected by subterranean heat. Such tremendous effects being -ascribed to steam, it can scarcely be doubted that the Greeks and -Romans were acquainted with the fact, that water in passing into -vapour exercises considerable mechanical power. They were aware -that the earthquakes, which they ascribed to this cause, exerted -forces sufficiently powerful to extend the natural limits of the -ocean; to overturn from their foundations the most massive -monuments of human labour; to raise islands in the midst of seas; -and to heave up the surface of the land of level continents so as -to form lofty mountains. - -Such notions, however, resulted not as consequences of any exact -or scientific principles, but from vague analogies derived from -effects which could not fail to have been manifested in the arts, -such as those which commonly occurred in the process of casting in -metal the splendid statues which adorned the temples, gardens, and -public places of Rome and Athens. The artisan was liable to the -same accidents to which modern founders are exposed, produced by -the casual presence of a little water in the mould into which the -molten metal is poured. Under such circumstances, the sudden -formation of steam of an extreme pressure produces, as is well -known, explosions attended with destructive effects. The Grecian -[Pg015] and Roman artisans were subject to such accidents; and the -philosopher, generalising such a fact, would arrive at a solution -of the grander class of phenomena of earthquakes and volcanoes. - -Before natural phenomena are rendered subservient to purposes of -utility, they are often made to minister to the objects of -superstition. The power of steam is not an exception to this rule. -It is recorded in the Chronicles, that upon the banks of the Weser -the ancient Teutonic gods sometimes marked their displeasure by a -sort of thunderbolt, which was immediately succeeded by a cloud -that filled the temple. An image of the god _Busterich_, which was -found in some excavations, clearly explains the manner in which -this prodigy was accomplished by the priests. The head of the -metal god was hollow, and contained within it a pot of water: the -mouth, and another hole, above the forehead, were stopped by -wooden plugs; a small stove, adroitly placed in a cavity of the -head under the pot, contained charcoal, which, being lighted, -gradually heated the liquid contained in the head. The vapour -produced from the water, having acquired sufficient pressure, -forced out the wooden plugs with a loud report, and they were -immediately followed by two jets of steam, which formed a dense -cloud round the god, and concealed him from his astonished -worshippers.[1] - -Among other amusing anecdotes showing the knowledge which the -ancients had of the mechanical force of steam, it is related that -Anthemius, the architect of Saint Sophia, occupied a house next -door to that of Zeno, between whom and Anthemius there existed a -feud. To annoy his neighbour, Anthemius placed on the ground floor -of his own house several close digesters, or boilers, containing -water. A flexible tube proceeded from the top of each of these, -which was conducted through a hole made in the wall between the -houses, and which communicated with the space under the floors of -the rooms in the house of Zeno. When Anthemius desired to annoy -his neighbour, he lighted fires under his boilers, and the steam -produced by them rushed in such quantity and with [Pg016] such -force under Zeno's floors, that they were made to heave with all -the usual symptoms of an earthquake.[2] - - -BLASCO DE GARAY, A. D. 1543. - - -(4.) In the year 1826, M. de Navarrete published, in Zach's -Astronomical Correspondence, a communication from Thomas Gonzales, -Director of the royal archives of Simancas, giving an account of -an experiment reported to have been made in the year 1543, in -which a vessel was propelled by a machine having the appearance of -a steam engine. - -Blasco de Garay, a sea captain, proposed in that year to the Emperor -Charles V. to propel vessels by a machine which he had invented, -even in time of calm, without oars or sails. Notwithstanding the -apparent improbability attending this project, the Emperor ordered -the experiment to be made in the port of Barcelona, and the 17th of -June, 1543, was the day appointed for its trial. The commissioners -appointed by Charles V. to attend and witness the experiment were -Don Henry of Toledo, Don Pedro of Cardona, the treasurer Ravago, the -vice chancellor and intendant of Catalonia, and others. The vessel -on which the experiment was made was the Trinity, 200 tons burthen, -which had just discharged a cargo of corn at Barcelona. Garay -concealed the nature of his machinery, even from the commissioners. -All that could be discovered during the trial was, that it consisted -of a large boiler containing water, and that wheels were attached to -each side of the vessel, by the revolution of which it was -propelled. The commissioners having witnessed the experiment, made a -report to the king, approving generally of the invention, -particularly on account of the ease and promptitude with which the -vessel could be put about by it. - -The treasurer Ravago, who was himself hostile to the project, -reported that the machine was capable of propelling a vessel at the -rate of two leagues in three hours; but the other commissioners -stated that it made a league an hour at the least, and that it put -the vessel about as speedily as would be accomplished with a galley -worked according to the common [Pg017] method. Ravago reported that -the machinery was too complicated and expensive, and that it was -subject to the danger of the boiler bursting. - -After the experiment was made, Garay took away all the machinery, -leaving nothing but the framing of wood in the arsenals of -Barcelona. - -Notwithstanding the opposition of Ravago, the invention was -approved, and the inventor was promoted and received a pecuniary -reward, besides having all his expenses paid. - -From the circumstance of the nature of the machinery having been -concealed, it is impossible to say in what this machine consisted; -but as a boiler was used, it is probable, though not certain, that -steam was the agent. There have been various machines proposed, of -which a furnace and boiler form a part, and in which the agency of -steam is not used. The machine of Amontons furnishes an example of -this. It is most probable that the contrivance of Garay was -identical with that of Hero. The low state of the arts in Spain in -the sixteenth century would be incompatible with the construction -of any machine requiring great precision of execution. But the -simplicity of Hero's contrivance would have rendered its -construction and operation quite practicable. As to the claims to -the invention of the steam engine advanced by the advocates of De -Garay, founded on the above document, a refutation is supplied by -the admission, that though he was rewarded and promoted by the -government of the day, in consequence of the experiment, and -although the great usefulness of the contrivance in towing ships -out of port, &c., was admitted, yet it does not appear that a -second experiment was ever tried, much less that the machine was -ever brought into practical use. - - -SOLOMON DE CAUS, 1615. - - -(5.) Solomon De Caus was engineer and architect to Louis XIII., -king of France, before the year 1612. In that year he entered the -service of the Elector Palatine, who married the daughter of King -James I., with whom he came to England. He was there employed by -the Prince of Wales in ornamenting the gardens of his house at -Richmond. [Pg018] During his sojourn in England he composed and -published at London, in the same year, a Treatise on Perspective. -This person was the author of a work entitled, "_Les Raisons des -Forces Mouvantes, avec diverses Machines tant utiles que -plaisantes_," which was apparently composed at Heidelburg, but -published at Franckfort, in 1615. The same work was subsequently -republished in Paris in 1623. - -The treatise commences with definitions of what were then -considered the four elements: earth, air, fire, and water. Air is -defined to be a cold, dry, and light element, capable of -compression, by which it may be rendered very violent. He says, -"The violence will be great when water exhales in air by means of -fire, and that the said air is enclosed: as, for example, take a -ball of copper of one or two feet diameter, and one inch thick, -which being filled with water by a small hole, which shall be -strongly stopped with a peg, so that neither air nor water can -escape, it is certain that if we put the said ball upon a great -fire, so that it will become very hot, that it will cause a -compression so violent, that the ball will burst in pieces, with a -noise like a petard." - -The effect which is here described is due to the combined pressure -of the heated air contained in the ball and the high pressure -steam raised from the water, but much more to the latter than to -the former. It is evident, however, from the language of De Caus, -that he ascribes the force entirely to the air, and seems to -consider that the force of the air proceeded from the water which -exhaled in it. - -The first theorem is, "that the parts of the elements mix together -for a time, and then each returns to its place" (the elements here -referred to being apparently air and water). Upon this subject the -following is an example: "Take a round vessel of copper, soldered -close on every side, and with a tube, whereof one end approaches -nearly to the bottom of the vessel, and the other end, which -projects on the outside of the vessel, has a stop-cock; there is -also a hole in the top of the vessel, with a plug to stop it. If -this vessel will contain three pots of water, then pour in one pot -of water, and place the vessel on the fire about three or four -minutes, leaving the hole open; then take the vessel off the -[Pg019] fire, and a little after pour out the water at the hole, -and it will be found that a part of the said water has been -evaporated by the heat of the fire. Then pour in one pot of water -as before, and stop up the hole and the cock, and put the vessel -on the fire for the same time as before; then take it off, and let -it cool of itself, without opening the plug, and after it is quite -cold pour out the water, and it will be found exactly the same -quantity as was put in. Thus we see that the water which was -evaporated (the first time that the vessel was put on the fire) is -returned into water the second time when that vapour has been shut -up in the vessel, and cooled of itself." - -In the description of these experiments, the processes of -evaporation and condensation are obscurely indicated; but there is -no intimation that the author possessed any knowledge of the -elastic force of steam. His theorem is, that the parts of the -element water mix for a time with the parts of the element air; -that fire causes this mixture, and that on removing the fire, and -dissipating the heat, then the parts of the water mixed with air -return to their proper place, forming again part of the water. -There is no indication of a _change of property_ of the water in -passing into vapour. It is difficult to conceive, if De Caus had -been aware that the vapour of water possessed the same violent -force which he distinctly and in terms ascribes to air, or if he -had been aware that in effect the vapour of the water produced by -the fire was a fluid, possessing exactly the same mechanical -qualities, and producing the same mechanical effects as air, that -he would not have expressed himself clearly on the subject. - -He proceeds to give another demonstration that heat will cause the -particles of water to mix with those of air. - -"After having put the measure of water into the vessel, and shut -the vent-hole, and opened the cock, put the vessel on the fire, -and put the pot under the cock; then the water of the vessel, -raising itself by the heat of the fire, will run out through the -cock; but about one sixth or one eighth part of the water will not -run out, because the violence of the vapour which causes the water -to rise proceeds from the [Pg020] said water; which vapour goes -out through the cock after the water with great violence. There is -also another example in quicksilver, or mercury, which is a fluid -mineral, but being heated by fire, exhales in vapour, and mixes -with the air for a time; but after the said vapour is cooled, it -returns to its first nature of quicksilver. The vapour of water is -much lighter, and therefore it rises higher," &c. &c. - -In this second demonstration there appears to be some obscure -indication of the force of steam in the words "because of the -violence of the vapour which causes the water to rise," &c. - -The fifth theorem is the following:-- - -"_Water will mount by the help of fire higher than its level_," -which is explained and proved in the following terms:-- - -[Illustration: _Fig._ 2.] - -"The third method of raising water is by the aid of fire. On this -principle may be constructed various machines: I shall here -describe one. Let a ball of copper marked A; well soldered in -every part, to which is attached a tube and stop-cock marked D, by -which water may be introduced; and also another tube marked B C, -which will be soldered into the top of the ball, and the lower end -C of which shall descend nearly to the bottom of the ball without -touching it. Let the said ball be filled with water through the -tube D, then shutting the stop-cock D, and opening the stop-cock -in the vertical tube B C, let the ball be placed upon a fire the -heat acting upon the said ball will cause the water to rise in the -tube B C." - -In the apparatus as here described, the space enclosed in the -boiler above the surface of the water is filled with air. By the -action of the fire, two effects are produced: first, the air -enclosed above the water, being heated, acquires increased -elasticity, and presses with a corresponding force on the surface -of the water. By this means a column of water will be driven up -the tube A B at such a height as will balance the elasticity of -the heated air confined in the boiler; but besides [Pg021] this -the water contained in the boiler being heated, will produce -steam, which being mixed with air contained in the boiler, will -likewise press with its proper elasticity on the surface of the -water, and will combine with the air in raising a column of water -in the tube A B. In the above description of the machine, the -force which raises the water in the tube A B is ascribed to the -fire, no mention being made of the water, or of the vapour or -steam produced from it having any agency in raising the water in -the tube A B. - -Antecedently to the date of this invention, the effect of heat in -increasing the elastic force of air was known, and so far as the -above description goes, the whole operation might be ascribed to -the air by a person having no knowledge whatever of the elasticity -of steam. M. Arago, however, who, on the grounds of this passage -in the work of De Caus, claims for him a share of the honour of -the invention of the steam engine, contends that the agency of -steam in this apparatus was perfectly known to De Caus, although -no mention is made of steam in the above description, because in -the second demonstration above quoted he uses the words, "the -violence of the vapour which causes the water to rise proceeds -from the said water; which vapour goes out from the cock after the -water with great violence." By these words M. Arago considers that -De Caus expresses the quality of elasticity proper to the vapour, -and that the context justifies the inference, that to this -elasticity he ascribed the elevation of the water in the tube C B. - -There appears to be some uncertainty attending the birthplace of -De Caus. In the _Biographie Universelle_ he is said to have been -born and to have died in Normandy. M. Arago assigns Dieppe, or its -neighbourhood, as his birthplace. - -There was another engineer and architect, Isaac De Caus, a native -of Dieppe, who published a work in folio, entitled "_Nouvelle -Invention de Lever l'Eau plus haut que sa Source, avec quelque -Machines mouvantes, par le Moyen de l'Eau, et un Discours de la -Conduite d'Icelle_." This volume is without a date, but from the -nature of its contents it would appear to have been published -before the work of Solomon De Caus already cited. The drawings and -machines described in both [Pg022] are exactly the same; but the -definitions and theorems quoted above on raising water by fire are -not given in the work of Isaac. It seems, therefore, that Solomon -De Caus re-published, with additions, the work of Isaac De Caus. -From the same birthplace being assigned to both these authors, as -well as from the similarity of their pursuits, it is likely they -were members of the same family, and from their christian names -they were probably Jews. - -The work cited above, was dedicated to Louis XIII., and in the -dedication Solomon De Caus calls himself the subject of that -monarch; and in the privilege prefixed to the work he is -designated, "Our well-beloved Solomon De Caus, master engineer, -being at present in the service of our dear and well-beloved -cousin, the Prince Elector Palatine, has made known to us," -&c.--"we, desiring to gratify the said De Caus, he being our -subject," &c. - -It is therefore certain, whatever may have been the birthplace of De -Caus, that he was at least a subject of France. The circumstance of -his work being written in French, though published beyond the Rhine, -is also an argument in favour of his being a native of that country. - - -GIOVANNI BRANCA, 1629. - - -(6.) Giovanni Branca of Loretto in Italy, an engineer and -architect, proposed to work mills of different kinds by steam -issuing from a large aeolopile, and blowing against the vanes of a -wheel. Branca was the author of many ingenious mechanical -inventions, a collection of which he dedicated to M. Cenci, the -governor of Loretto. These were published in a work printed at -Rome in 1629. It is a thin quarto, entitled "_Le Machine volume -nuovo, et di molto artificio da fare effetti maravigliosi tanto -Spiritali quanto di Animale Operatione, arichito di bellissime -figure. Del Sig. Giovanni Branca, Cittadino Romano. In Roma, -1629._" The work contains sixty-three engravings, accompanied by -descriptions in Italian and Latin. Branca's steam engine, -represented in the twenty-fifth plate, consists of a wheel -furnished with flat vanes upon its rim, like the boards of a -paddle wheel. The steam is produced in a close vessel, and made to -issue with violence from the extremity [Pg023] of a pipe directed -against the vanes, and causes the wheel to revolve. This motion -being imparted by the usual mechanical contrivances, any machinery -may be impelled by it. Different useful applications of this power -are contained in the work, viz. pestles and mortars for pounding -materials to make gunpowder, and rolling stones for grinding the -same; machines for raising water by buckets, for sawing timbers, -for driving piles, &c. &c. - -This method of applying the force of steam has no analogy to any -application of steam in modern engines. - - -EDWARD SOMERSET, MARQUIS OF WORCESTER, 1663. - - -(7.) Of all the names which figure in the early annals of steam, -by far the most remarkable is that of the Marquis of Worcester, -who has left a description of a machine in a work, entitled "The -Scantling of One Hundred Inventions," which has been generally in -this country considered as giving him a right to the honour of -having been the inventor of the steam engine. - -Lord Worcester having been engaged on the side of the Royalists in -the civil wars of the revolution, lost his fortune, and went to -Ireland, where he was imprisoned. He escaped from thence, and -reached France; from that country he ventured to London, as a -secret agent of Charles II., but was detected, and imprisoned in -the Tower, where he remained until the restoration, when he was -set at liberty. Tradition has connected the invention of the steam -engine with the following anecdote:--One day, during his -imprisonment, Lord Worcester observed the lid of the pot in which -his dinner was being cooked, suddenly forced upwards by the vapour -of the water which was boiling in it. Reflecting on this, it -occurred to him that the same force which raised the cover of the -pot might be rendered, when properly applied, a useful and -convenient moving power. After he recovered his liberty, he -accordingly proceeded to carry into effect this conception. The -contrivance to which he was ultimately led is described in the -following terms in the sixty-eighth invention, in the work above -named:-- - -"I have invented an admirable and forcible way to drive [Pg024] -up water by fire; not by drawing or sucking it upwards, for that -must be, as the philosopher terms it, _infra sphoerum activitatis_, -which is but at such a distance. But this way hath no bounder if -the vessels be strong enough. For I have taken a piece of whole -cannon whereof the end was burst, and filled it three quarters -full of water, stopping and screwing up the broken end, as also -the touch-hole, and making a constant fire under it; within -twenty-four hours, it burst and made a great crack. So that, -having a way to make my vessels so that they are strengthened by -the force within them, and the one to fill after the other, I have -seen the water run like a constant fountain stream forty feet -high. One vessel of water rarefied by fire driveth up forty of -cold water, and a man that tends the work has but to turn two -cocks; that one vessel of water being consumed, another begins to -force and refill with cold water, and so successively; the fire -being tended and kept constant, which the self-same person may -likewise abundantly perform in the interim between the necessity -of turning the said cocks." - -Since the date of the publication of the "Century of Inventions" -was the year 1663, the experiments here mentioned must have been -made before that year. The description of the machine here given, -as well as others in the same work, was intended by the author, -not to convey a knowledge of the nature of the mechanism which he -used, but only to express the effects produced, and to indicate -the physical principle on which they depended. It should also be -observed, that an air of mystery was thrown by Worcester over the -accounts of all the machines which he described; and therefore any -obscurity in the above description ought not to be regarded as an -evidence against his claim to the discovery of the mechanical -agency of steam, so far as that agency is indicated by the effects -said by him to be produced. The above account is, however, -sufficiently distinct and explicit to enable any one possessing a -knowledge of the mechanical qualities of steam to perceive the -general nature of the machine described. To render this machine, -and that of De Caus, previously described, intelligible to those -who are not familiar with physical science, we must here explain -some general principles on which their agency depends. [Pg025] - - -(8.) Fluid bodies are of two kinds, inelastic fluids, or liquids, -and elastic fluids, or gases. Of the former of these classes, -water is the most familiar example, and of the latter, air. - -These two species of fluids are each distinguished by peculiar -mechanical properties. - -[Illustration: _Fig._ 3.] - - -(9.) The constituent particles of a liquid are distinguished from -those of solids by having little or no coherence; so that unless -the mass be confined by the sides of the vessel which contains it, -the particles will fall asunder by their gravity. A mass of -liquid, therefore, unlike a solid, can never retain any particular -form, but will accommodate itself to the form of the vessel in -which it is placed. It will press against the bottom of the vessel -which contains it with the whole force of its weight, and it will -press against the sides with a force proportional to the depth of -the particles in contact with the sides measured from the surface -of the liquid above. This lateral pressure also distinguishes -liquids from solids. Let us take for illustration the case of a -square or a cubical vessel, A B C D, _fig._ 3. If a solid body, -such as a piece of lead, be cut to the shape of this vessel, so as -to fit in it without pressing with any force against its sides, -the mechanical effect which would be produced by it when placed in -the vessel, would be merely a pressure upon the bottom, B C, the -amount of which would be equal to the weight of the metallic mass. -No pressure would be exerted against the sides; for the coherence -of the particles of the solid maintaining them in their position, -the removal of the sides would not subject the solid body -contained in the vessel to any change. - -Now let us suppose this solid mass of lead to be rendered liquid -by being melted. The constituent particles will then be deprived -of that cohesion by which they were held together; they will -accordingly have a tendency to separate, and fall asunder by their -gravity, and will only be prevented from actually doing so by the -support afforded to them by the sides, [Pg026] A B, D C, of the -vessel. They will therefore produce a pressure against the sides, -which was not produced by the lead in its solid state. This -pressure will vary at different depths: thus a part of the side of -the vessel at P will receive a pressure proportional to the depth -of the point P below the surface of the lead. If, for example, we -take a square inch of the inner surface of the side of the vessel -at P, it will sustain an outward pressure equal to the weight of a -column of lead having a square inch for its base, and a height -equal to P A. And, in like manner, every square inch of the sides -of the vessel will sustain an outward pressure equal to the weight -of a column of lead having a square inch for its base, and a -height equal to the depth of the point below the surface of the -lead. - - -(10.) We have here proceeded upon the supposition that no force -acts on the upper surface A D of the lead. If any force presses A -D downwards, that force would be transferred to the bottom by the -lead, and would produce a pressure on the bottom B C equal to its -own amount in addition to the weight of the lead; and if the lead -were solid, this would be the only additional mechanical effect -which such a force acting on the surface A D of the lead would -produce. But if, on the other hand, the lead were liquified, then -the force now adverted to, acting on the surface A D, would not -only produce a pressure on the bottom B C, equal to its own amount -in addition to the weight of the lead, but it would also produce a -pressure against every part of the sides of the vessel, equal to -that which it would produce upon an equal magnitude of the surface -A D. - -Thus if we suppose any mechanical cause producing a pressure on -the surface A D amounting to ten pounds on each square inch, the -effect which would be produced, if the lead were solid, would be -an additional pressure on the base B C amounting to ten pounds per -square inch. But if the lead were liquid, besides this pressure on -each square inch of the base B C, there would likewise be a -pressure of ten pounds on every square inch of the sides of the -vessel. - -All that has been here stated with respect to a square or a -cubical vessel will be equally applicable to a vessel of any other -form. [Pg027] - - -(11.) The second class of fluids are distinguished from liquids by -the particles not merely being destitute of cohesion, but having a -tendency directly the reverse, to repel each other, and fly -asunder with more or less force. Thus if a vessel, such as that -represented in _fig._ 3., were filled with a fluid of this kind, -being open at the top, and not being restrained by any pressure -incumbent upon it, the particles of the fluid would not rest in -the vessel by their gravity, as those of the liquid would do; but -they would, by their mutual repulsion, fly asunder, and rise out -of the vessel, as smoke is seen to rise from a chimney, or steam -from the spout of a kettle. Let us suppose, then, that the vessel -in which an elastic fluid is contained is closed on every side by -solid surfaces. In fact, let us imagine that the square or cubical -vessel represented in _fig._ 3. is closed by a square lid at the -top A D, having contained in it an elastic fluid, such as -atmospheric air. - -If such a cover, or lid, had been placed upon a liquid, the cover -would sustain no pressure from the fluid, nor would any mechanical -effect be produced, save those already described in the case of -the open vessel; but when the fluid contained in the vessel is -elastic, as is the case with air, then the elasticity (by which -name is expressed the tendency of the particles of the fluid to -fly asunder) will produce peculiar mechanical effects, which have -no existence whatever in the case of a liquid. - -It is true that, supposing the fluid to be air or any other gas or -vapour, a pressure will be produced upon the bottom B C of the -vessel equivalent to the weight of such fluid, and lateral -pressures will be produced on the different points of the sides by -the weight of that part of the fluid which is above these points; -but gases and vapours are bodies of such extreme levity, that -these effects due to their weight are neglected in practice. - -Putting, then, the weight of the air contained in the vessel out -of the question, let us consider the effect of its elasticity. If -the vessel, as already described, be supposed to contain -atmospheric air in its ordinary state, the tendency of the -constituent particles to fly asunder will be such as to produce on -every square inch of the inner surface of the vessel [Pg028] a -pressure amounting to fifteen pounds; this pressure being, as -already stated, quite independent of the weight of the air. In -fact, this pressure would continue to exist if the air contained -in the vessel actually ceased to have weight by being removed from -the neighbourhood of the earth, which is the cause of its gravity. - - -(12.) Different gases are endowed with different degrees of -elasticity, and the same gas may have its elasticity increased or -diminished, either by varying the space within which it is -confined, or by altering the temperature to which it is exposed. - -If the space within which an elastic fluid is enclosed be -enlarged, its elasticity is found to diminish in the same -proportion. Thus if the air contained in the vessel A B C D -(_fig._ 3.) be allowed to pass into a vessel of twice the -magnitude, the elasticity of the particles will cause them to -repel each other, so that the same quantity of air shall diffuse -itself throughout the larger vessel, assuming double its former -bulk. Under such circumstances, the pressure which it would exert -upon the sides of the larger vessel would be only half that which -it had exerted on the sides of the smaller vessel. If, on the -other hand, it were forced into a vessel of half the magnitude of -A B C D, as it might be, then its elasticity would be double, and -it would press on the inner surface of that vessel with twice the -force with which it pressed on that of the vessel A B C D. - -This power of swelling and contracting its dimensions according to -the dimensions of the vessel in which it is confined, or to the -force compressing it, is a quality which results immediately from -elasticity, and is consequently one which is peculiar to the gases -or elastic fluids, and does not at all appertain to liquids. If -the liquid contained in the vessel A B C D were transferred to a -vessel of twice the magnitude, it would only occupy half the -capacity of that vessel, and it could not by any means be -transferred, as we have supposed the air or gas to be, to a vessel -of half the dimensions, since it is inelastic and incompressible. - - -(13.) The elasticity of gases is likewise varied by varying the -temperature to which they are exposed; thus, in general, [Pg029] -if air or any other gas be augmented in temperature, it will -likewise be increased in elasticity; and if, on the other hand, it -be diminished in temperature, it will be likewise diminished in -its elastic force. The more heated, therefore, any air or gas -confined in a vessel becomes, the greater will be the force with -which it will press on the inner surface of that vessel, and tend -to burst it. - - -(14.) The same body may, by the agency of heat, be made to pass -successively through the different states of solid, liquid, and -gas, or vapour. The most familiar and obvious example of these -successive transitions is presented by water. Exposed to a certain -temperature, water can only exist as a solid; as the temperature -is increased, the ice, or solid water, is liquefied; and by the -continued application of heat, this water again undergoes a -change, and assumes the form, and acquires the mechanical -qualities, of air or gas: in such a state it is called STEAM. - -This is a common property of all liquids. If they be exposed for a -sufficient length of time to a sufficient degree of heat, they -will always be converted into elastic fluids. These are usually -distinguished from air and other permanent gases, which never are -known to exist in the liquid form, by the term _vapour_, by which, -therefore, must be understood an elastic fluid which at common -temperatures exists in the liquid or solid state; by _steam_ is -expressed the vapour of water; and by _gases_, those elastic -fluids which like air are never known--at least, under ordinary -circumstances--to exist in any other but the elastic form. - - -(15.) When a liquid is caused, by the application of heat, to take -the form of an elastic fluid, or is evaporated, besides acquiring -the property of elasticity, it always undergoes a considerable -change of bulk. The amount of this change is different with -different liquids, and even with the same liquid it varies with -the circumstances under which the change is produced. - - -(16.) When water is evaporated under ordinary circumstances,--that -is, when exposed to no other external pressure than that of the -atmosphere,--it increases its volume about seventeen-hundred-fold. -Thus a cubic inch of liquid [Pg030] water would form about -seventeen hundred cubic inches of common steam. If, however, the -water be confined by a greater pressure than that produced by the -common atmosphere, then the increase of volume which takes place -in its evaporation would be less in proportion. - -These important physical circumstances are now only indicated in a -general way. As we proceed with our account of the invention and -improvement of the steam engine, they will be developed more fully -and accurately. - - -(17.) After duly considering what has been just explained, no -difficulty will be found in comprehending the principles on which -the first rude attempts at the mechanical application of steam -already stated depend. In the apparatus ascribed to _Hero_ of -Alexandria, the elasticity of the vapour contained in the arms of -the revolving ball causes that vapour to issue from the lateral -orifices in the arms, such as that of G, _fig._ 1. As these -orifices, however, are exposed to the common atmosphere pressing -inwards with a force, the mean amount of which has been stated to -be about fifteen pounds per square inch, it follows that the steam -cannot escape from these orifices until its pressure or elasticity -exceeds this amount, and that when it does, the force with which -it will so escape will be the excess of its elasticity above that -of the atmosphere; and it is the reaction produced by this -difference of pressure, causing the arms to recoil, which will -give motion to the machine. - -In the case of the apparatus of _De Caus_ (5.), the heat of the -fire acting on the vessel D C (_fig._ 2.) will raise the -temperature of the water contained in it, and also of the air -confined within it above the surface of that water. This air, as -it is increased in temperature, will also increase in elasticity; -it will therefore press on the surface of the water with increased -force, and will gradually force the water upwards in the tube; and -this effect would continue until all the water in the vessel would -be forced up the tube. - -But at the same time that the heat acting on the vessel increases -the temperature of the air above the water, it also produces a -partial evaporation of the water, so that more or less steam is -mixed with the air in the vessel above the surface [Pg031] of the -water; and this steam possessing elasticity, unites with the air -in pressing on the surface of the water, and in raising it in the -tube. - -[Illustration: _Figs._ 4, 5, and 6.] - -Let us now revert to the brief account of the engine of the -Marquis of Worcester, described in "The Century of Inventions." We -collect from that description that the vessel in which the water -was evaporated was separate from those which contained the water -to be elevated; also that there were two vessels of the like -description, the contents of which were alternately elevated by -the pressure of the "water rarefied by the fire;" in other words -by steam; and that the water was raised in an uninterrupted -stream, by the management of two cocks communicating with these -vessels and with the boiler. The following is such an apparatus as -would answer this description. Let E (_fig._ 4.) be the vessel -containing the water to be evaporated, placed over a proper -furnace A; let S be a pipe to allow the steam produced from the -boiling water in E to pass into the vessels where its mechanical -action is required. Let R represent a cock or regulator, having in -it a curved passage, leading from S to the tube T, when the lever -or handle L is in the position represented by the cut; but leading -to the tube T', when the lever L is turned one quarter of a -revolution to the right, as represented in _fig._ 5. By the -shifting of this lever, therefore, the steam pipe S may be made to -communicate alternately with the tubes T and T'. The tubes T and -T' are carried respectively to two vessels V and V', which are -filled with the water required to be raised. In these [Pg032] -vessels tubes enter at C and C', descending nearly to the bottom: -these tubes have valves at B and B', opening upwards, by which -water will be allowed to pass into the vertical tube F, but which -will not allow it to return downwards, the valves B and B' being -then closed by the weight of the water above them. - -Let G G' be a pipe entering the sides of the vessels V and V', for -the purpose of filling them with the water to be raised: let K be -a cock having a curved passage similar to the cock R, and leading -to a tube by which water is supplied from the reservoir or other -source from which the water to be raised is drawn. When the cock K -is placed as represented in _fig._ 4., the water from the -reservoir will flow through the curved passage in the cock K into -the tube G', and thence into the vessel V'; but when this cock is -turned one quarter round, by shifting the lever to the left, it -will take the position represented in _fig._ 6., and the water -will flow through the curved passage into the tube G, and thence -into the vessel V. Let us now suppose the vessel V already filled -with water to be elevated, and the vessel V' to have discharged -its contents. The cock R is turned, so as to allow the steam -generated in the boiler E to pass into the tube T, and thence into -the upper part of the vessel V, while the cock K is turned so as -to allow the water from the reservoir to pass into the tube G', -and thence into the vessel V'. The steam collecting in the upper -part of the vessel V' presses with its elastic force on the -surface of the water therein, and forces the water upwards in the -tube C; it passes through the valve B, which it opens by the -upward pressure received from the action of the steam, and thence -into the tube F, its descent into the tube C' being prevented by -the valve V', which can only be opened upwards. As the steam is -gradually supplied from the boiler E, the water in the vessel V is -forced up the tube C, through the valve B, and into the tube F, -until all the contents of the vessel V above the lower end of the -tube C have been raised. In the meanwhile, the vessel V' has been -filled with water, through the cock K: when this has been -accomplished, the man who attends the machine shifts the cocks R -and K, so as to give them the position represented in _fig._ 5. -and _fig._ 6. [Pg033] In this position, the steam from the -boiler, being excluded from the tube T, will be conducted to the -tube T', and thence to the vessel V', while the water from the -reservoir will be excluded from the tube G', and conducted through -the tube G to the vessel V. The vessel V will thus be replenished -and, by a process similar to that already described, the contents -of the vessel V' will be forced up the tube C', through the valve -B', and into the tube F; its descent into the tube C being -prevented by the valve B, which will then be closed. After the -contents of the vessel V' have thus been raised, and the vessel V -replenished, the two cocks R and K are once more shifted, and the -contents of V raised while V' is replenished, and so on. - -[Illustration: _Fig._ 4, 5, and 6.] - -If, having comprehended the apparatus here described, the reader -refers to the description of the Marquis of Worcester's machine, -he will find that all the conditions therein laid down are -fulfilled by it. One vessel (E) of "water rarefied by fire" may by -such means "drive up forty (or more) of cold water; and the man -that tends the work has but to turn two cocks, that one vessel (V) -of water being consumed, another (V') begins to force and refill -with cold water, and so on successively, the fire being tended and -kept constant; which the self-same person may likewise abundantly -perform, in the interim between the necessity of turning the said -cocks." - -On comparing this with the contrivance previously suggested by De -Caus, it will be observed, that even if De Caus [Pg034] knew the -physical agent by which the water was driven upwards in the -apparatus described by him, still it was only a method of causing -a vessel of boiling water to empty itself; and before a repetition -of the process could be made, the vessel should be refilled, and -again boiled. In the contrivance of Lord Worcester, on the other -hand, the agency of the steam was employed in the same manner as -it is in the steam engines of the present day, being generated in -one vessel, and used for mechanical purposes in another. Nor must -this distinction be regarded as trifling or insignificant, because -on it depends the whole practicability of using steam as a -mechanical agent. Had its action been confined to the vessel in -which it was produced, it never could have been employed for any -useful purpose. - -Although many of the projects contained in Lord Worcester's work -were in the highest degree extravagant and absurd, yet the engine -above described is far from being the only practicable and useful -invention proposed in it. On the contrary, many of his inventions -have been reproduced, and some brought into general use since his -time. Among these may be mentioned, stenography, telegraphs, -floating baths, speaking statues, carriages from which horses can -be disengaged if unruly, combination locks, secret escutcheons for -locks, candle moulds, the rasping mill, the gravel engine, &c. - - -SIR SAMUEL MORLAND, 1683. - - -(18.) Sir Samuel Morland was the son of a baronet of the same name, -who had received his title at the restoration for some services to -the royalist party, performed by him during the wars of the -Commonwealth. He appears to have devoted much attention to -mechanics, in which he attained some celebrity. He was the reputed -inventor of several ingenious contrivances, such as the drum capstan -for ships, the plunger pump, &c. He also investigated various -questions in acoustics, and among others, the determination of the -best form for the speaking-trumpet. - -In 1680, Sir Samuel Morland was appointed Master [Pg035] of the -Works to Charles II., and in the following year was sent to -France, to execute some waterworks for Louis XIV. In 1683, while -in France, he wrote in the French language, a work entitled -"_Elevation des Eaux par toute sorte de Machines, reduite a la -Mesure, au Poids et a la Balance. Presentee a sa Majeste tres -Chrestienne, par le Chevalier Morland, Gentilhomme Ordinaire de la -Chambre Privee, et Maistre des Mechaniques du Roi de la Grande -Bretagne, 1683._" This book is preserved in manuscript in the -Harleian Collection in the British Museum. It is written on -vellum, and consists of only thirty-eight pages. It contains -tables of measures and weights, theorems for the calculation of -the volumes of cylinders, the weights of columns of water, the -thickness of lead for pipes, and is concluded by a chapter on -steam, consisting of four pages, of which the following is a -translation:-- - -"_The principles of the new force of fire invented by Chevalier -Morland in 1682, and presented to His Most Christian Majesty in -1683_:-- - -"'Water being converted into vapour by the force of fire, these -vapours shortly require a greater space (about 2000 times) than the -water before occupied, and sooner than be constantly confined would -split a piece of cannon. But being duly regulated according to the -rules of statics, and by science reduced to measure, weight, and -balance, then they bear their load peaceably (like good horses), and -thus become of great use to mankind, particularly for raising water, -according to the following table, which shows the number of pounds -that may be raised 1800 times per hour to a height of six inches by -cylinders half filled with water, as well as the different diameters -and depths of the said cylinders.'" - -There is nothing in the description here given which can indicate -the form of the machine by which Morland proposed to render the -force of steam a useful mover. It is, however, remarkable, that at -this early period, before experiments had been made on the -expansion which water undergoes in evaporation, he should have -given so near an approximation to [Pg036] the actual amount of -that expansion. It is scarcely supposable that such an estimate -could be obtained by him otherwise than by experiment. - -The work containing the above description was not printed; but a -work bearing nearly the same title, containing, however, no -mention of the force of steam, was published by him in Paris in -the year 1685. In this he describes various experiments made by -him at St. Germains on the weight of the water of the Seine, and -gives weights of the columns of water, the contents of cylinders, -&c. - -Soon after the publication of this work, Morland returned to -England, and resided near the court till his death. The celebrated -John Evelyn mentioned having paid a visit to him at his house at -Hammersmith, in 1695, when he had become aged and blind, but was -still remarkable for his mechanical ingenuity. "On the 25th of -October," says Evelyn, "the Archbishop and myself went to -Hammersmith to visit Sir Samuel Morland, who was entirely blind; a -very mortifying sight. He showed us his invention of writing -(short-hand), which was very ingenious; also his wooden kalendar, -which instructed him all by feeling; and other pretty and useful -inventions of mills, pumps, &c.; and the pump he had erected, that -serves water to his garden and to passengers, with an inscription, -and brings from a filthy part of the Thames near it a most perfect -and pure water."[3] - -He died at Hammersmith, in January 1696; and before his death, as -a penance for his past life, was guilty of the eccentricity of -burying in the ground six feet deep a great collection of music -which he possessed.[4] - - -DENIS PAPIN, 1688. - - -(19.) Denis Papin, a native of Blois in France, and professor of -mathematics at Marbourg, is the name which stands next recorded in -the progressive invention of the steam engine. To this philosopher -is due the discovery of one of the qualities of steam, to the -proper management of which is owing much of the efficacy of the -modern steam engine. [Pg037] - -Papin was born at Blois in France. He devoted his youth to the -study of medicine, in which he took a degree at Paris. The -revocation of the Edict of Nantes having driven him into exile, he -went to England, where the celebrated Boyle associated him in -several of his experiments with the air-pump, and caused him to be -elected a fellow of the Royal Society in 1681. Having been invited -to Germany by the Landgrave of Hesse, he discharged during several -years the duties of professor of mathematics at the university of -Marbourg, where he died in 1710. Notwithstanding his discoveries -respecting the agency of steam, he never received any mark of -distinction in his own country. The truth is, the importance and -value of these investigations were not apparent until long -afterwards. - -This philosopher conceived the idea of producing a moving power by -means of a piston working in a cylinder, in the manner which we -shall now briefly explain. - -[Illustration: _Fig._ 7.] - -Let A B (_fig._ 7.) be a cylinder open at the top, and let a -piston P be fitted into it, so as to move in it air tight. At the -bottom of the cylinder suppose an opening provided, which can be -closed at pleasure, by a stop-cock, or otherwise, so that the -communication may be opened and closed at will between the -interior of the cylinder and the external air. This stop-cock -being opened, let the piston be drawn upwards till it reach the -top of the cylinder. Let the stop-cock at the bottom be then -removed, and imagine that some means can be supplied by which the -air within the cylinder can be suddenly annihilated. The piston, -now at the top, will have above it the pressure of the atmosphere; -and having no air below, it will be resisted in its descent by no -force save that arising from its friction with the cylinder. If, -then, the force of the air above the piston be greater than the -resistance arising from this friction, the piston will descend -with the excess of this force, and will continue so to descend -until it reach the bottom of the cylinder. Having attained that -position, let us [Pg038] suppose the stop-cock in the bottom -opened, so as to allow the external air to pass freely below the -piston. The piston may now be drawn to the top of the cylinder -again, offering no resistance save that of its weight, and its -friction with the cylinder. Having reached the top of the cylinder -once more, let the stop-cock be closed, and the air included -within the cylinder once more annihilated. A second descent of the -piston will take place, with the same force as before, and in like -manner the process may be continued indefinitely. - -Now, if it should appear that means could be provided suddenly and -repeatedly to annihilate the air within the cylinder, and that the -pressure of the atmosphere above the piston should exert a force -compared with which the weight of the piston and its friction are -trifling, it is evident that a moving power would be obtained -which would be capable, by proper mechanism, of being applied to -any useful purpose, but which would more especially be applicable -to the working of pumps, the motion of which corresponds with that -which has been just ascribed to the piston in the cylinder. Such -were the first ideas of Papin. But in order to enable those who -are not conversant with physical science fully to appreciate their -importance, it will be necessary here to explain some of the -mechanical properties of atmospheric air. - - -(20.) The atmosphere is the thin, transparent, colourless, and -therefore invisible, fluid in which we live and move, which by -respiration sustains animal life, and is otherwise connected with -various important functions of organised matter. This fluid is so -light and attenuated, that it might at first be doubted whether it -be really a body at all; and, indeed, the name expressing -incorporeal beings, _spirit_, is a word in its origin signifying -_air_.[5] The air, however, is light only as compared with other -material substances, which exist in a more condensed state: it -possesses the quality of weight as absolutely as the most solid -and massive bodies in nature, and to render this quality manifest, -it is only necessary to submit a sufficient quantity of air to any -of the usual tests of gravitation. [Pg039] - -A direct demonstration of this may be given by the following -experiment:--On the mouth of a flask let a stop-cock be fastened so -as to be air-tight. The interior of the flask may then be put into -free communication with the external air, or that communication may -be cut off at pleasure, by opening or closing the stop-cock. If a -syringe be applied to the mouth of the flask, the stop-cock being -open a part of the air contained in it may be drawn out. After this, -the stop-cock being closed, and the syringe detached, let the -flask be placed in the dish of a good balance, and accurately -counterpoised by weights in the other dish. This counterpoise will -then represent the weight of the flask, and of the air which has -remained in it. If the stop-cock be now opened, air will immediately -rush in, and replace that which the syringe had withdrawn from the -flask; and immediately the dish of the balance containing the flask -will sink by the effect of the weight of the air thus admitted into -the flask. - -If the weight of quantity of air so small as to be capable of -being withdrawn by a syringe from an ordinary flask be thus of -sensible amount, it may be easily imagined that the vast mass of -atmosphere extending from the surface of the earth upwards, to a -height not ascertained with precision, but certainly not being -less than thirty miles, must be very considerable. Such a force, -pressing as it must constantly do, upon the surfaces of all -bodies, whether solid or fluid, and resisting and modifying their -movements, would play an important part in all mechanical -phenomena; and it is, therefore, not sufficient merely to have -recognised its existence, but it is most needful to measure its -amount with that degree of certainty and precision, which will -enable us to estimate its effects on those phenomena which we -shall have to investigate. - - -(21.) The amount of the pressure of the atmosphere on each square -inch of horizontal surface on which it rests, is obviously the -weight of the column of air extending from that square inch of -surface upwards to the top of the atmosphere. This force is -measured by the following means:-- - -[Illustration: _Fig._ 8.] - -[Illustration: _Fig._ 9.] - -Take a glass tube, A B (_fig._ 8.), above 32 inches long, open at -one end A, and closed at the other end B, and let it [Pg040] be -filled with mercury (quicksilver). Let a glass vessel or cistern -C, containing a quantity of mercury, be also provided. Applying -the finger at A, so as to prevent the mercury in the tube from -falling out, let the tube be inverted, and the end, stopped by the -finger, plunged into the mercury in C. When the end of the tube is -below the surface of the mercury in C (_fig._ 9.), let the finger -be removed. It will be found that the mercury in the tube will -not, as might be expected, fall to the level of the mercury in the -cistern C, which it would do were the end B open, so as to admit -the air into the upper part of the tube. On the other hand, the -level D of the mercury in the tube will be nearly 30 inches above -the level C of the mercury in the cistern. - -The cause of this effect is, that the weight of the atmosphere -rests on the surface C of the mercury in the cistern, and tends -thereby to press it up, or rather to resist its fall in the tube; -and as the fall is not assisted by the weight of the atmosphere on -the surface D (since B is closed), it follows, that as much -mercury remains suspended in the tube above the level C, as the -weight of the atmosphere is able to support. - -If the section of the tube were equal to the magnitude of a square -inch, the weight of the column of mercury in the tube above the -level C would be exactly equal to the weight of the atmosphere on -each square inch of the surface C. - - -(22.) If such an apparatus be observed from time to time, it will -be found that the column of mercury sustained in the tube will be -subject to variation between certain limits, never falling below -twenty-eight inches, and never rising above thirty-one inches. -This variation of the mercurial column is produced by a -corresponding variation in the weight of the atmosphere. - -If the apparatus be transported to any height above its ordinary -position, it will have a less quantity of atmosphere above it, and -therefore the surface of the mercury in the cistern will be -pressed by a less weight, and consequently the [Pg041] column of -mercury will fall proportionally. In virtue of this effect, such -an instrument has been rendered a means of measuring heights, such -as the heights of mountains, the ascents of balloons, &c. &c. - - -(23.) If a proper scale be attached to the tube containing the -mercurial column, showing the absolute height of the column -sustained at any time, and indicating its changes of height, the -instrument becomes a BAROMETER. - -Two cubic inches of mercury weigh very nearly one pound -avoirdupois.[6] Hence, when the barometric column measures thirty -inches, the weight of the atmosphere resting on each square inch -of surface is about fifteen pounds. - - -(24.) It is an established property of fluids, that they press -equally in all directions; and air, like every other fluid, -participates in this quality. Hence, it follows, that when the -downward pressure or weight of the atmosphere is fifteen pounds on -the square inch, the lateral, upward, and oblique pressures are of -the same amount. But, independently of the general principle, it -may be satisfactory to give experimental proof of this. - -[Illustration: _Fig._ 10.] - -Let four glass tubes, A, B, C, D (_fig._ 10.), be constructed of -sufficient length, closed at one end, A, B, C, D, and open at the -other. Let the open ends of three of them be bent, as represented -in the tubes B, C, D. Being previously filled with mercury, let -them all be gently inverted, so as to have their closed ends up, -as here represented. It will be found that the mercury will be -sustained in all, and that the difference of the levels in all -will be the same.[7] Thus, the mercury is sustained in A by the -upward pressure of the atmosphere; in B, by its horizontal or -lateral pressure; in C, by its downward pressure; [Pg042] and in -D, by its oblique pressure: and, as the difference of the levels -is the same in all, these pressures are exactly equal. - - -(25.) The same arrangement by which the pressure of the atmosphere -is measured by a mercurial column of equivalent weight, also -supplies the means of measuring the pressure or elasticity of -atmospheric air, or any other gas or vapour, whether in a more or -less compressed or rarefied state; and as instruments constructed -on this principle are of considerable use in steam engines, we -shall take this occasion to explain their principle and -application. - -In the experiments described in (21), the space D B in the top of -the barometer-tube, from which the mercury descended, is a vacuum. -If, however, it were occupied by a quantity of air in a rarefied -state, or any other gas or vapour, such gas or vapour would press -on the surface of the mercury at D, with a force determined by its -elasticity. In that case, the atmospheric pressure acting on the -surface of the mercury C in the cistern, would be balanced by the -combined forces of the weight of the mercurial column sustained in -the tube, and the elasticity of the gas or vapour in the upper -part of it. Now if we know the actual amount of the atmospheric -pressure,--that is to say, the height of the column of mercury -which it would be capable of sustaining,--we should then be able -to determine the pressure of the rarefied air in the space C D. - -For example, let us suppose that the barometric column, when B D -(_fig._ 9.) is a vacuum, measures thirty inches: the atmospheric -pressure, therefore, would be equal to the weight of a column of -mercury of that height. Let us suppose that the elasticity of the -gas or vapour occupying the upper part of the tube D B causes the -column to fall to the height of twenty-six inches: it is evident, -then, that the pressure of the air in the top of the tube would be -equal to the weight of a column of mercury of four inches. In -fine, to determine the pressure of the rarefied gas or vapour in -the top of the tube, it is only necessary to observe the -difference between the height of the column of mercury actually -sustained in the tube, and the column sustained at the same time -and [Pg043] place in a common barometer: the difference of the -two will be the column of mercury whose weight will represent the -pressure of the vapour or gas in the top of the tube. - - -(26.) Whenever the air contained in any vessel or other enclosed -space has by any means had its pressure reduced so as to be -rendered less than that of the external air, the external air will -have a tendency to rush into such vessel or enclosed space with a -force proportionate to the excess of the pressure of such external -air over that of the air within; and if any communication be -opened between the interior of such vessel or enclosed space, and -the external air, the latter will rush in until an equilibrium be -established between the pressures within and without. It is -evident that the force thus obtained by diminishing the pressure -of air within a vessel may be applied to any mechanical purpose. - -It is by such means that water is raised in an ordinary pump. A -portion of the air contained between the piston of the pump and -the surface of the water below, is withdrawn by the action of the -piston, and the pressure of the air remaining under the piston is -thereby diminished. The superior pressure of the atmosphere upon -the external surface of the water in the well then forces up a -column of water in the pump-barrel, and this is continued as the -air is more and more rarefied by the action of the piston. By -whatever means, therefore, the air can be wholly or partially -withdrawn from any space, a mechanical power will be thereby -developed, proportional in its amount and efficacy to the quantity -of air so withdrawn. If, however, such air be withdrawn by any -mechanical process, such as by a syringe, by a common pump, or by -an air-pump, the quantity of force expended in withdrawing it is -always equivalent to the amount of mechanical power obtained by -the vacuum or partial vacuum so produced. Indeed the power -expended is greater than the power so obtained, inasmuch as the -friction, leakage, &c. of the exhausting apparatus must be allowed -for. - - -(27.) There are, however, various other means by which air may be -partially expelled from a vessel besides the direct application of -mechanical force. Thus if heat be applied to [Pg044] the vessel, -the air, as has been already explained, will acquire increased -elasticity, and will rush from the vessel with a force proportionate -to the excess of its elasticity above that of the external air, and -this process may be continued by increasing the heat to which the -vessel is exposed, until a very considerable portion of the air has -been expelled. If the orifice by which the air has escaped be then -closed, and the vessel be allowed to cool, the air within, by having -its temperature reduced to that of the external air, will lose all -the elasticity which it had gained from the heat, and will be in the -same condition as if an equivalent quantity of air had been -withdrawn by any mechanical agent. The external air, therefore, will -have a tendency to rush in with a force corresponding to the -difference of pressures. - -The process of filling thermometers with mercury shows one use of -producing a high degree of rarefaction by heat. To construct the -instrument it is necessary to fill the bulb and a part of the tube -with mercury; but the bore of the tube is so small that the -mercury cannot be introduced by any ordinary means. It is -therefore held over flame until heated to a high temperature. The -air within it gradually increasing in pressure as its temperature -is raised, is forced through the small bore of the tube, until the -pressure of the air within becomes no more than equal to the -pressure of the external atmosphere; this air being so rarefied -that quantity in the bulb bears a very small proportion to its -contents at common temperatures. The mouth of the tube is then -plunged into mercury, and as the bulb cools, the air within it -loses its elasticity, and the superior pressure upon the external -surface forces the mercury into the tube. This continues until the -air remaining within the bulb has been so contracted, that its -pressure combined with the weight of the mercury, shall balance -the atmospheric pressure. The tube is then reversed, and the air -which remained rises in a bubble to the surface, and escapes. - - -(28.) Let us now return to the proceedings of Papin. How great a -power would result from such a machine as he conceived, will be -apparent, if it be considered that the unresisted atmosphere -exercises a pressure of about fifteen pounds on [Pg045] each -square inch of surface exposed to it, and that if the piston in -the cylinder imagined by Papin, had a diameter of only one foot, -its superficial magnitude would be about 114 square inches. The -pressure of the atmosphere upon it, therefore, would be 114 times -fifteen pounds, or 1710 pounds. Papin first proposed to produce -the vacuum under the piston by means of common air pumps, worked -by a water-wheel; and by such means he conceived that the power of -a river, stream, or waterfall might be conveyed by pipes to a -distance. While he was in England, in 1687, he laid his -contrivance before the Royal Society of London, but was met by -objections and difficulties, the nature of which he does not -explain. - -It is, however, apparent, from what has been already explained, -that such a method of proceeding would amount to a mere transfer -of power, and would not, properly speaking, be itself a moving -force: the moving power would, in reality, be the force of the -water by which the water-wheel would be driven; and the air-pumps, -tubes, together with the piston and cylinder, would be merely -means of conveying the power of the water-wheel to the objects to -be moved, or the machinery to be driven. Papin states, that, long -before this, he had attempted to expel the air from his cylinder -by means of gunpowder; but, notwithstanding all the precautions -which he could take, there always remained a considerable -quantity; so much, indeed, as to deprive the vacuum of more than -half its proper force. At length he adopted an expedient for the -production of a vacuum which forms a most important step in the -progressive invention of the steam engine, and which gives to -Papin's name a high place in the history of that machine. This -method is explained in the following paragraph of a work published -by Papin in 1695, at Cassel, entitled "_Recueil de diverses Pieces -touchant quelques nouvelles Machines_", p. 53. - -"I have endeavoured," says he, "to attain this end (viz. the -production of a vacuum in the cylinder) in another way. As water -has the property of elasticity, when converted into steam by heat, -and afterwards of being so completely recondensed by cold, that -there does not remain the least [Pg046] appearance of this -elasticity, I have thought that it would not be difficult to work -machines in which, by means of a moderate heat and at a small -cost, water might produce that perfect vacuum which has vainly -been sought by means of gunpowder." - -This remarkable passage is given in the work just cited, as an -extract from the "Leipsic Acts," of August, 1690. - -Let us pause here to explain more fully this important discovery. - - -(29.) We have explained that, in its conversion into vapour, by -the application of heat, water, besides acquiring the property of -elasticity, undergoes a vast enlargement of bulk, filling, under -ordinary circumstances, about 1700 times more space than it -occupied in the liquid form. This fact was known generally, though -not with numerical accuracy, by Papin, having been the foundation -of the machines previously invented and published by De Caus and -Lord Worcester; the happy idea of reversing the process occurred -to him. If water in its conversion into steam swelled into many -hundred times its original bulk, it would necessarily follow, that -steam, being reconverted into water, would shrink into its -primitive dimensions. Papin therefore saw, that if he could by any -means expel the air from his cylinder under the piston, and -replace it by the pure vapour of water, he could cause that vapour -to be reconverted into a comparatively minute quantity of water by -depriving it of the heat which sustained it in the state of steam, -and that by accomplishing this, the space in the cylinder under -the piston would become a vacuum; that by such means, the pressure -of the atmosphere above the piston would take full effect, and -would urge the piston down; that by introducing more steam under -the piston, it might be again raised by the elastic force of the -steam, the destruction of which by cold water would again produce -the descent of the piston with the same mechanical force; and that -in this way the alternate ascent and descent of the piston might -be continued indefinitely. - -In accordance with these ideas, Papin constructed a model -consisting of a small cylinder, in which was placed a solid -piston; [Pg047] and in the bottom of the cylinder under the -piston was contained in a small quantity of water. The piston -being in immediate contact with this water, so as to exclude the -atmospheric air, on applying fire to the bottom of the cylinder, -steam was produced, the elastic force of which raised the piston -to the top of the cylinder; the fire being then removed, and the -cylinder being cooled by the surrounding air, the steam was -condensed and reconverted into water, leaving a vacuum in the -cylinder into which the piston was pressed by the force of the -atmosphere. The fire being applied and subsequently removed, -another ascent and descent were accomplished; and in the same -manner the alternate motion of the piston might be continued. -Papin described no other form of machine by which this property -could be rendered available in practice; but he states generally, -that the same end may be attained by various forms of machines -easy to be imagined.[8] - - -THOMAS SAVERY, 1698. - - -(30.) The discovery of the method of making a vacuum by the -condensation of steam was reproduced, before 1698, by Captain -Thomas Savery, to whom a patent was granted in that year for a -steam engine to be applied to the raising of water, &c. Savery -proposed to combine the machine described by the Marquis of -Worcester with an apparatus for raising water by suction into a -vacuum produced by the condensation of steam. - -Savery appears to have been ignorant of the publication of Papin, -and stated that his discovery of the condensing principle arose -from the following circumstance:-- - -Having drunk a flask of Florence at a tavern, and flung the empty -flask on the fire, he called for a basin of water to wash his -hands. A small quantity which remained in the flask began to boil, -and steam issued from its mouth. It occurred to him to try what -effect would be produced by inverting the flask and plunging its -mouth in the cold water. Putting on a thick glove to defend his -hand from the heat, he seized the [Pg048] flask, and the moment -he plunged its mouth in the water the liquid immediately rushed up -into the flask and filled it. - -Savery stated that this circumstance immediately suggested to him -the possibility of giving effect to the atmospheric pressure by -creating a vacuum in this manner. He thought that if, instead of -exhausting the barrel of a pump by the usual laborious method of a -piston and sucker, it was exhausted by first filling it with -steam, and then condensing the same steam, the atmospheric -pressure would force the water from the well into the pump-barrel, -and into any vessel connected with it, provided that vessel were -not more than about thirty-four feet above the elevation of the -water in the well. He perceived also, that, having lifted the -water to this height, he might use the elastic force of steam in -the manner described by the Marquis of Worcester to raise the same -water to a still greater elevation, and that the same steam which -accomplished this mechanical effect would serve, by its subsequent -condensation, to reproduce the vacuum, and draw up more water. It -was on this principle that Savery constructed the first engine in -which steam was ever brought into practical operation. - -[Illustration: BRANCA'S ENGINE.] - - FOOTNOTES: - - [1] Arago, Eloge historique de James Watt; p. 22. - - [2] Ibid., p. 21. note. - - [3] Farey, Treatise on the Steam Engine, p. 93. - - [4] Arago, sur les Machines a Vapeur, Annuaire, 1829, p. 165 - - [5] SPIRITUS, _breath_ or _air_. - - [6] Exactly 15.68 oz. = 0.98 lb. - - [7] This experiment with the tube A requires to be very - carefully executed, and the tube should be one of small bore. - - [8] Recueil de diverses Pieces touchant quelques nouvelles - Machines, p. 38. - -[Pg049] - - - - -[Illustration: SAVERY'S ENGINE.] - -CHAP. II. - -ENGINES OF SAVERY AND NEWCOMEN. - - SAVERY'S ENGINE. -- BOILERS AND THEIR APPENDAGES. -- WORKING - APPARATUS. -- MODE OF OPERATION. -- DEFECTS OF THE ENGINE. -- - NEWCOMEN AND CAWLEY. -- ATMOSPHERIC ENGINE. -- ACCIDENTAL - DISCOVERY OF CONDENSATION BY INJECTION. -- HUMPHREY POTTER - MAKES THE ENGINE WORK ITSELF. -- ADVANTAGES OF THE ATMOSPHERIC - ENGINE OVER THAT OF SAVERY. -- IT CONTAINED NO NEW PRINCIPLE. -- - ITS PRACTICAL SUPERIORITY. - - -(31.) The steam engine contrived by Savery, like every other which -has since been constructed, consists of two parts, essentially -distinct. The first is that which is employed to [Pg050] generate -the steam, which is called the boiler; and the second, that in -which the steam is applied as a moving power. - -[Illustration: _Fig._ 11.] - -The former apparatus in Savery's engine consists of two strong -boilers, sections of which are represented at D and E in _fig._ -11.; D the greater boiler, and E the less. The tubes T and T' -communicate with the working apparatus, which we shall presently -describe. A thin plate of metal R, is applied closely to the top -of the great boiler D, turning on a centre C, so that by moving a -lever applied to the axis C on the outside of the top, the sliding -plate R can be brought from the mouth of the one tube to the mouth -of the other alternately. This sliding valve is called the -_regulator_, since it is by it that the communications between the -boiler and two steam vessels (hereafter described) are alternately -opened and closed, the lever which effects this being moved at -intervals by the hand of the attendant. - -Two _gauge cocks_ are represented at G, G', the use of which is to -determine the depth of water in the boiler. One, G, has its lower -aperture a little above the proper depth; and the other, G', a -little below it. Cocks are attached to the upper ends G, G', which -can be opened or closed at pleasure. The steam collected in the -top of the boiler pressing on the surface of the water, forces it -up in the tubes G, G', if their lower ends be immersed. Upon -opening the cocks G, G', if water be forced from both, there is -too much water in the boiler, since the mouth of G is _below_ its -level. If steam issue from both, there is too little water in the -boiler, since the mouth of G' is _above_ its level. But if steam -issue from G, and water from G', the water in the boiler is at its -proper level. This ingenious contrivance for determining the level -of the water in the boiler is the invention of Savery, and is used -in many instances at the present day. - -The mouth of the pipe G should be at a level of a little less -[Pg051] than one third of the whole depth, and the mouth of G' at -a level little lower than one third; for it is requisite that -about two thirds of the boiler should be kept filled with water. -The tube I forms a communication between the greater boiler D and -the lesser or feeding boiler E, descending nearly to the bottom of -it. This communication can be opened and closed at pleasure by the -cock K. A gauge pipe is inserted similar to G, G', but extending -nearly to the bottom. From this boiler a tube F extends, which is -continued to a cistern C (_fig._ 12.), and a cock is placed at M, -which, when opened, allows the water from the cistern to flow into -the feeding boiler E, and which is closed when that boiler is -filled. The manner in which this cistern is supplied will be -described hereafter. - -Let us now suppose that the principal boiler is filled to the -level between the gauge pipes, and that the subsidiary boiler is -nearly full of water, the cock K and the gauge cocks G G' being -all closed. The fire being lighted beneath D, and the water -boiled, steam is produced, and is transmitted through one or other -of the tubes T, T', to the working apparatus. When evaporation has -reduced the water in D below the level of G', it will be necessary -to replenish the boiler D. This is effected thus:--A fire being -lighted beneath the feeding boiler E, steam is produced in it -above the surface of the water, which, having no escape, presses -on the surface so as to force it up in the pipe I. The cock K -being then opened, the boiling water is forced into the principal -boiler D, into which it is allowed to flow until water issues from -the gauge cock G'. When this takes place, the cock K is closed, -and the fire removed from E until the great boiler again wants -replenishing. When the feeding boiler E has been exhausted, it is -replenished from the cistern C (_fig._ 12.), through the pipe F, -by opening the cock M. - - -(32.) We shall now describe the working apparatus in which the -steam is used as a moving power. - -Let V V' (_fig._ 12.) be two steam vessels communicating by the -tubes T T' (marked by the same letters in _fig._ 11.) with the -greater boiler D. - -[Illustration: _Fig._ 12.] - -Let S be a pipe, called the _suction pipe_, descending into -[Pg052] the well or reservoir from which the water is to be -raised, and communicating with each of the steam vessels through -tubes D D', by valves A A', which open upwards. Let F be a pipe -continued from the level of the engine to whatever higher level it -is intended to elevate the water. The steam vessels V V' -communicate with the _force-pipe_ F by valves B B', which open -upwards, through the tubes E E'. Over the steam vessels and on the -force-pipe is placed a small cistern C, already mentioned, which -is kept filled with cold water from the force-pipe, and from the -bottom of which proceeds a pipe terminated with a cock G. This is -called the _condensing pipe_, and can be brought alternately over -each steam vessel. From this cistern another pipe communicates -with the feeding boiler (_fig._ 11.), by the cock M.[9] - -The communication of the pipes T T' with the boiler can be opened -and closed alternately, by the regulator R (_fig._ 11.), already -described. - -Now suppose the steam vessels and tubes to be all filled with -common atmospheric air, and that the regulator be placed so that -the communication between the tube T and the boiler be opened, the -communication between the other tube T' and the boiler being -closed, steam will flow into V through T. At first, while the -vessel V is cold, the steam will be condensed, and will fall in -drops of water on the bottom and sides of the vessel. The -continued supply of steam from the boiler will at length impart -such a degree of heat to the vessel V, that it will cease to -condense it. Mixed with the heated air [Pg053] contained in the -vessel V, it will have an elastic force greater than the -atmospheric pressure, and will therefore force open the valve B, -through which a mixture of air and steam will be driven until all -the air in the vessel V will have passed out, and it will contain -nothing but the pure vapour of water. - -When this has taken place, suppose the regulator be moved so as to -close the communication between the tube T and the boiler, and to -stop the further supply of steam to the vessel V; and at the same -time let the condensing pipe G be brought over the vessel V, and -the cock opened so as to let a stream of cold water flow upon it. -This will cool the vessel V, and the steam with which it is filled -will be condensed and fall in a few drops of water, leaving the -interior of the vessel a vacuum. The valve B will be kept closed -by the atmospheric pressure. But the elastic force of the air -between the valve A and the surface of the water in the well, or -reservoir, will open A, so that a part of this air will rush in, -and occupy the vessel V. The air in the suction pipe S, being thus -allowed an increased space, will be proportionally diminished in -its elastic force, and its pressure will no longer balance that of -the atmosphere acting on the external surface of the water in the -reservoir. This pressure will, therefore, force water up in the -tube S until its weight, together with the elastic force of the -air above it, balances the atmospheric pressure. When this has -taken place, the water will cease to ascend. - -Let us now suppose that, by shifting the regulator, the -communication is opened between T and the boiler, so that steam -flows again into V. The condensing cock G being removed, the -vessel will be again heated as before, the air expelled, and its -place filled by the steam. The condensing pipe being again allowed -to play upon the vessel V, and the further supply of steam being -stopped, a vacuum will be produced in V, and the atmospheric -pressure will force the water through the valve A into the vessel -V, which it will nearly fill, a small quantity of air, however, -remaining above it. - -Thus far the mechanical agency employed in elevating the water is -the atmospheric pressure; and the power of steam is no further -employed than in the production of a vacuum. [Pg054] But, in -order to continue the elevation of the water through the force -pipe F, above the level of the steam vessel, it will be necessary -to use the elastic pressure of the steam. The vessel V is now -nearly filled by the water which has been forced into it by the -atmosphere. Let us suppose that, the regulator being shifted -again, the communication between the tube T and the boiler is -opened, the condensing cock removed, and that steam flows into V. -At first, coming in contact with the cold surface of the water and -that of the vessel, it is condensed; but the vessel is soon -heated, and the water formed by the condensed steam collects in a -sheet or film upon the surface of the water in V, so as to form a -surface as hot as boiling water.[10] The steam then being no -longer condensed, presses on the surface of the water with its -elastic force; and when that pressure becomes greater than the -atmospheric pressure, the valve B is forced open, and the water -issuing through it, passes through E into the force-pipe F; and -this is continued until the steam has forced all the water from V, -and occupies its place. - -The further admission of steam through T is once more stopped by -moving the regulator; and the condensing pipe being again allowed -to play on V, so as to condense the steam which fills it, produces -a vacuum. Into this vacuum, as before, the atmospheric pressure -will force the water, and fill the vessel V. The condensing pipe -being then closed, and steam admitted through T, the water in V -will be forced by its pressure through the valve B and tube E into -F, and so the process is continued. - -We have not yet noticed the other steam vessel V', which, as far -as we have described, would have remained filled with common -atmospheric air, the pressure of which on the valve A' would have -prevented the water raised in the suction pipe S from passing -through it. However, this is not the case; for, during the entire -process which has been described in V, similar effects have been -produced in V', which we have only omitted to notice to avoid the -confusion which the two processes might produce. It will be -remembered, that after the steam, in the first instance, having -flowed from the boiler [Pg055] through T, has blown the air out -of V through B, the communication between T and the boiler is -closed. Now the same motion of the regulator which closes this, -opens the communication between T' and the boiler; for the sliding -plate R (_fig._ 11.) is moved from the one tube to the other, and -at the same time, as we have already stated, the condensing pipe -is brought to play on V. While, therefore, a vacuum is being -formed in V by condensation, the steam, flowing through T', blows -out the air through B', as already described in the other vessel -V; and while the air in S is rushing up through A into V, followed -by the water raised in S by the atmospheric pressure, the vessel -V' is being filled with steam, and the air is completely expelled -from it. - -The communication between T and the boiler is now again opened, -and the communication between T' and the boiler closed by moving -the regulator R (_fig._ 11.) from the tube T to T'; at the same -time the condensing pipe is removed from over V, and brought to -play upon V'. While the steam once more expels the air from V -through B, a vacuum is formed by condensation in V', into which -the water in S rushes through the valve A'. In the mean time V is -again filled with steam. The communication between T and the -boiler is now closed, and that between T' and the boiler is -opened, and the condensing pipe removed from V', and brought to -play on V. While the steam from the boiler forces the water in V' -through B' into the force-pipe F, a vacuum is being produced in V, -into which water is raised by the atmospheric pressure. - -Thus each of the vessels V V' is alternately filled from S, and -the water thence forced into F. The same steam which forces the -water from the vessels into F, having done its duty, is condensed, -and brings up the water from S, by giving effect to the -atmospheric pressure. - -During this process, two alternate motions or adjustments must be -constantly made; the communication between T and the boiler must -be opened, and that between T' and the boiler closed, which is -done by one motion of the regulator. The condensing pipe at the -same time must be brought from V to play on V', which is done by -the lever placed upon it. Again [Pg056] the communication between -T' and the boiler is to be opened, and that between T and the -boiler closed; this is done by moving back the regulator. The -condensing pipe is brought from V' to V by moving back the other -lever, and so on alternately. - -For the clearness and convenience of description, some slight and -otherwise unimportant changes have been made in the position of -the parts. A perspective view of this engine is represented at the -head of this chapter. The different parts already described will -easily be recognised. - -The engine of Savery was very clearly described in a small work -published in London in 1702, entitled, _The Miner's Friend, or an -Engine to raise Water by Fire described, and the Manner of Fixing -it in Mines; with an Account of the several Uses it is applicable -unto, and an Answer to the Objection made against it; by Thomas -Savery, Gentleman_. This volume was dedicated to William III. (to -whom the engine had been exhibited at Hampton Court palace), to -the Royal Society, and to the mining adventurers of England. The -following are the uses to which Savery proposed the engine should -be applied: _First_, to raise water for turning all sorts of -mills; _second_, supplying palaces and houses with water, and -supplying means of extinguishing fire therein by the water so -raised; _third_, the supplying cities and towns with water; -_fourth_, draining fens or marshes; _fifth_, for ships; _sixth_, -the drainage of mines. - -Dr. Harris, in his _Lexicon Technicum, or Dictionary of Arts and -Sciences_, mentions a machine of Savery's for propelling a vessel -in a calm, by paddle-wheels placed at the side; but it does not -appear that Savery contemplated the application of a steam engine -to work these wheels. - -It is only from scattered passages in publications of the day that -it can be ascertained to what extent the engines of Savery were -practically applied. In his address to the Royal Society, he -speaks of the "difficulties and expense which he encountered in -instructing artisans to make engines according to his wish; but -that after much experience the workmen had become such masters of -the thing, that they bound themselves to deliver the engines -'exactly tight and fit for [Pg057] service, and such as he -(Savery) dare warrant them to every one that has occasion for -them.'" - -In his address to the miners of England he also says, "that the -frequent disorders and cumbersomeness of water engines then in use -encouraged him to invent engines to work by this new force; that -though they were obliged to encounter the oddest and almost -insuperable difficulties, yet he spared neither time, pains, nor -money, till he had conquered them." - -In Bradley's _Improvements of Planting and Gardening_, 1718, the -author thus speaks of an engine erected by Savery:-- - -"Supposing the situation of a house or garden to be a considerable -height above any pond, river, or spring, and that it has at -present no other conveniency of water than what is brought -continually by men or horses to it. In this case, the wonderful -invention of the late Mr. Savery, F.R.S., for raising water by -fire, will not only supply the defect, by flinging up as much -water as may be desired, but may be maintained with very little -trouble and very small expense. - -"It is now about six years since Mr. Savery set up one of them for -that curious gentleman Mr. Balle, at Cambden House, Kensington, -near London, which has succeeded so well that there has not been -any want of water since it has been built; and, with the -improvements since made to it, I am apt to believe will be less -subject to be out of order than any engine whatever." - -It is remarkable that, notwithstanding the high pressure steam -necessary for the operation of Savery's engine, he does not appear -to have adopted the obvious expedient of a safety valve. The -safety valve had been previously known, having been invented about -the year 1681, by Papin, for his digester, which was a close -boiler, contrived by him for stewing meat and digesting bones, by -submitting them to a higher temperature than that of water boiling -in an open vessel. - -The safety valve which has ever since been used for steam boilers -of every kind is a valve which opens outwards, and is fitted to an -aperture in the boiler, so as to be steam tight. It is pressed -down by a weight, the amount of which is regulated by the maximum -pressure to which it is intended the steam [Pg058] shall be -limited. Thus, if the magnitude of the valve be a square inch, and -the pressure of the steam be limited to 10 lbs. per square inch -above the pressure of the atmosphere, then the valve would be -loaded with a weight of 10 lbs.; but as it was found necessary to -vary from time to time the limiting pressure of the steam, or the -load of the safety valve, these valves were usually constructed so -as to be held down by the pressure of a lever having a sliding -weight upon it. By moving the weight on the arm of the lever, the -pressure on the valve could be increased or diminished at the -discretion of the engineer. This contrivance was first applied to -Savery's engines, by Desaguliers, about the year 1717, before -which year Savery died. - -It is justly observed by Mr. Farey, in his treatise on the steam -engine, that, "when a comparison is made between Captain[11] -Savery's engine and those of his predecessors, the result will be -in every respect favourable to his character as an inventor, and -as a practical engineer; all the details of his invention are made -out in a masterly style, and accidents and contingencies are -provided for, so as to render it a real working engine; whereas De -Caus, the Marquis of Worcester, Sir Samuel Morland, and Papin, -though ingenious philosophers, only produced mere outlines, which -required great labour and skill of subsequent inventors to fill -up, and make them sufficiently complete to be put in execution." - -About the year 1718 further improvements were made in the -construction of Savery's engine, by Dr. Desaguliers; but it is -probable that some of these were suggested by the proceedings of -the inventors of the atmospheric engine, which shall presently -describe. - - -(33.) In order duly to appreciate the value of improvements, it is -necessary first to perceive the defects which these improvements -are designed to remove. Savery's steam engine, considering how -little was known of the value and properties of steam, and how low -the general standard of mechanical knowledge was in his day, is -certainly highly [Pg059] creditable to his genius. Nevertheless -it had very considerable defects, and was finally found to be -inefficient for the most important purposes to which he proposed -applying it. - -At the time of this invention, the mines in England had greatly -increased in depth, and the process of draining them had become -both expensive and difficult; so much so, that it was found in -many instances that their produce did not cover the cost of -working them. The drainage of these mines was the most important -purpose to which Savery proposed to apply his steam engine. - -It has been already stated that the pressure of the atmosphere -amounts to about fifteen pounds on every square inch. Now, a -column of water, whose base is one square inch, and whose height -is thirty-four feet, weighs about fifteen pounds. If we suppose -that a perfect vacuum were produced in the steam vessels V V' -(_fig._ 12.) by condensation, the atmospheric pressure would fail -to force up the water, if the height of the top of these vessels -above the water to be raised exceeded thirty-four feet. It is -plain, therefore, that the engine cannot be more than thirty-four -feet above the water which it is intended to elevate. But in fact -it cannot be so much; for the vacuum produced in the steam vessels -V V' is never perfect. Water, when not submitted to the pressure -of the atmosphere, will vaporise at a very low temperature, as we -shall hereafter explain; and it was found that a vapour possessing -a considerable elasticity would, notwithstanding the condensation, -remain in the vessels V V' and the pipe S, and would oppose the -ascent of the water. In consequence of this, the engine could -never be placed with practical advantage at a greater height than -twenty-six feet above the level of the water to be raised. - - -(34.) When the water is elevated to the engine, and the steam -vessels filled, if steam be introduced above the water in V, it -must first balance the atmospheric pressure, before it can force -the water through the valve B. Here, then, is a mechanical -pressure of fifteen pounds per square inch expended, without any -water being raised by it. If steam of twice that elastic force be -used, it will elevate a column in F of thirty-four feet in height; -and if steam of triple the force be used, it will raise a column -of sixty-eight feet high, [Pg060] which, added to twenty-six feet -raised by the atmosphere, gives a total lift of ninety-four feet. - -In effecting this, steam of a pressure equal to three times that -of the atmosphere acts on the inner surface of the vessels V V'. -One third of this bursting pressure is balanced by the pressure of -the atmosphere on the external surface of the vessels; but an -effective pressure of thirty pounds per square inch still remains, -tending to burst the vessels. It was found that the apparatus -could not be constructed to bear more than this with safety; and, -therefore, in practice, the lift of such an engine was limited to -about ninety perpendicular feet. In order to raise the water from -the bottom of the mine by these engines, therefore, it was -necessary to place one at every ninety feet of the depth; so that -the water raised by one through the first ninety feet should be -received in a reservoir, from which it was to be elevated the next -ninety feet by another, and so on. - -Besides this, it was found that sufficient strength could not be -given to those engines, if constructed upon a large scale. - -They were, therefore, necessarily very limited in their -dimensions, and were incapable of raising the water with -sufficient speed. Hence arose a necessity for several engines at -each level, which greatly increased the expense. - - -(35.) These, however, were not the only defects of Savery's -engines. The consumption of fuel was enormous, the proportion of -heat wasted being much more than what was used in either forcing -up the water, or producing a _vacuum_. This will be very easily -understood by attending to the process of working the engine -already described. - -When the steam is first introduced from the boiler into the steam -vessels V V', preparatory to the formation of a vacuum, it is -necessary that it should heat these vessels up to the temperature -of the steam itself; for until then the steam will be condensed -the moment it enters the vessel by the cold surface. All this -heat, therefore, spent in raising the temperature of the steam -vessels is wasted. Again, when the water has ascended and filled -the vessels V V', and steam is introduced to force this water -through B B' into F, it is immediately condensed by the cold -surface in V V', and does not [Pg061] begin to act until a -quantity of hot water, formed by condensed steam, is collected on -the surface of the cold water which fills these vessels. Hence -another source of the waste of heat arises. - -When the steam begins to act upon the surface of the water in V -V', and to force it down, the cold surface of the vessels is -gradually exposed to the steam, and must be heated while the steam -continues its action; and when the water has been forced out of -the vessel, the vessel itself has been heated to the temperature -of the steam which fills it, all which heat is dissipated by the -subsequent process of condensation. It must thus be evident that -the steam used in forcing up the the water in F, and in producing -a vacuum, bears a very small proportion indeed to what is consumed -in heating the apparatus after condensation. - - -(36.) There is also another circumstance which increases the -consumption of fuel. The water must be forced through B, not only -against the atmospheric pressure, but also against a column of -sixty-eight feet of water. Steam is therefore required of a -pressure of forty-five pounds on the square inch. Consequently the -water in the boiler must be boiled under this pressure. That this -should take place, it is necessary that the water should be raised -to a temperature considerably above 212 deg., even so high as 275 -deg.; and thus an increased heat must be given to the boiler. -Independently of the other defects, this intense heat weakened and -gradually destroyed the apparatus. - -Savery was the first who suggested the method of expressing the -power of an engine with reference to that of horses. In this -comparison, however, he supposed each horse to work but eight -hours a day, while the engine works for twenty-four hours. This -method of expressing the power of steam engines will be explained -hereafter. - - -(37.) The failure of the engines proposed by Captain Savery in the -work of drainage, from the causes which have been just mentioned, -and the increasing necessity for effecting this object, arising -from the large property in mines which became every year -unproductive by being flooded, stimulated the ingenuity [Pg062] -of mechanics to contrive some means of rendering those powers of -steam exhibited in Savery's engine available. - -Thomas Newcomen, the reputed inventor of the atmospheric engine, was -an ironmonger, or, according to some, a blacksmith, in the town of -Dartmouth in Devonshire. From his personal acquaintance and -intercourse with Dr. Hooke, the celebrated natural philosopher, it -is probable that he was a person of some education, and therefore -likely to be above the position of a blacksmith. Being in the habit -of visiting the tin mines in Cornwall, Newcomen became acquainted -with the engine invented by Savery, and with the causes which led to -its inefficiency for the purposes of drainage. - -It has been stated that Papin, about the year 1690, proposed the -construction of an engine working by the atmospheric pressure -acting on one side of a piston against a vacuum produced by the -condensation of steam on the other side. Papin was not conscious -of the importance of this principle; for, so far from ever having -attempted to apply it to practical purposes, he probably never -constructed, even on a small scale, any machine illustrating it. -On the contrary, he abandoned the project the moment he was -informed of the principle and structure of the steam engine of -Savery; and he then proposed an engine for raising water, acting -by the expansive force of steam similar to Savery's, but -abandoning the method of working by a vacuum. - -This engine is described by Papin in a work published in 1707. - -[Illustration: _Fig._ 13.] - -A (_fig._ 13.) is an oval boiler, having a safety-valve B, which -limits the pressure of the steam. It is connected with a cylinder -C, by a curved pipe having a stop-cock at D. A pipe with a -stop-cock G opens from the top of the cylinder into the -atmosphere, and a safety-valve F is placed upon the cylinder. A -hollow copper piston H moves freely in the cylinder, and floats -upon the water. O is a funnel with a valve L in the bottom, -opening downwards, through which the cylinder C may be filled with -water to the level of the top of the funnel. A close air-vessel -communicates with the cylinder C by the curved tube, and has a -valve K opening upwards. The force-pipe through which the water is -raised communicates [Pg063] with the air-vessel I. If the cock D -be shut, and the cock G opened, water poured into the funnel O -will rise into the cylinder C, the air which fills the cylinder -escaping through the open pipe G. When the cylinder is thus filled -with water, let the cock G be closed, and the cock D opened. The -steam from the boiler, after heating the metal of the cylinder, -will force the piston downwards, and drive the water through the -curved tube into the vessel I, from which its return is prevented -by the valve K, which is closed by its weight. The air which -filled the vessel I will then be compressed, and by its elasticity -will drive a column of water up the pipe N. After the contents of -the cylinder have been thus discharged it may be refilled in the -same manner, and the process repeated. - -It will be perceived that this project is nothing more than a -reproduction of the engine of the Marquis of Worcester. In the -preface to the work containing this description, Papin gives an -extract from a letter addressed by him to Leibnitz in 1698, from -which it appears that he had abandoned his idea of working the -piston by the atmospheric pressure acting against a vacuum, -considering it to be a contrivance inferior [Pg064] to the engine -now described. "We now raise water," he says, "by the force of -fire, _in a more advantageous manner than that which I had -published some years before_; for besides the suction, we now also -use the pressure which the water exerts upon other bodies in -dilating itself by heat; instead of which I before employed the -suction only, the effects of which are more limited." - -From documents which have been preserved in the Royal Society, it -appears that Newcomen was acquainted with Papin's writings, and -therefore probably first derived from them the suggestion which he -subsequently realised in the atmospheric engine. Among some papers -of Dr. Hooke's have been found notes for the use of Newcomen, on -Papin's method of transmitting the force of a stream or fall of -water to a distance by pipes. Hooke dissuaded Newcomen from -attempting any machine on this principle, which, as first proposed -by Papin, was impracticable. He exposed the fallacy of Papin's -first project in several discourses before the Royal Society, and -considered his improved edition of it, though free from fallacy, -as impracticable. - -Papin's project for producing a vacuum under a piston by -condensing the steam having been published in the _Actae -Eruditorum_, in Latin, in 1690, and in French, at Cassel, in 1695, -and subsequently, in the _Philosophical Transactions_, in England -in 1697, cannot be supposed to be unknown to Dr. Hooke; and if -known to him, would probably have been communicated to Newcomen. -Dr. Hooke died in 1703, some years before the date of Newcomen's -invention. - -John Cawley, who was the associate of Newcomen in his experiments -and inquiries, was a plumber and glazier of the same town. -Newcomen and Cawley obtained a patent for the atmospheric engine -in 1705, in which Savery was associated, he having previously -obtained a patent for the method of producing a vacuum by the -condensation of steam, which was essential to Newcomen's -contrivance. It was not, however, until about the year 1711 that -any engine had been constructed under this patent. - -In the latter end of that year, according to Desaguliers, the -patentees "made proposals to drain a colliery at Griff, in -[Pg065] Warwickshire, in which work five hundred horses were -constantly employed. This proposal not being accepted, they -contracted, in the following March, to drain water for Mr. Back of -Wolverhampton, where, after many laborious attempts, they -succeeded in making their engine work; but not being either -philosophers to understand the reason, or mathematicians enough to -calculate the power and proportions of the parts, they very -luckily, by accident, found what they sought for." - -[Illustration: _Fig._ 14.] - -Newcomen resumed the old method of raising the water from the -mines by ordinary pumps, but conceived the idea of working these -pumps by some moving power less expensive than that of horses. The -means whereby he proposed effecting this, was by connecting the -end of the pump-rod D (_fig._ 14.) by a chain with the arch head A -of a [Pg066] working-beam A B, playing on an axis C. The other -arch head B of this beam was connected by a chain with the rod E -of a solid piston P, which moved air-tight in a cylinder F. If a -vacuum be created beneath the piston P, the atmospheric pressure -acting upon it will press it down with a force of fifteen pounds -per square inch; and the end A of the beam being thus raised, the -pump-rod D will be drawn up. If a pressure equivalent to the -atmosphere be then introduced below the piston, so as to -neutralise the downward pressure, the piston will be in a state of -indifference as to the rising or falling; and if in this case the -rod D be made heavier than the piston and its rod, so as to -overcome the friction, it will descend, and elevate the piston -again to the top of the cylinder. The vacuum being again produced, -another descent of the piston, and consequent elevation of the -pump-rod, will take place; and so the process may be continued. - -Such was Newcomen's first conception of the _atmospheric engine_; -and the contrivance had much, even at the first view, to recommend -it. The power of such a machine would depend entirely on the -magnitude of the piston; and being independent of highly elastic -steam, would not expose the materials to the destructive heat -which was necessary for working Savery's engine. Supposing a -perfect vacuum to be produced under the piston in the cylinder, an -effective downward pressure would be obtained, amounting to -fifteen times as many pounds as there are square inches in the -section of the piston.[12] Thus, if the base of the piston were -100 square inches, a pressure equal to 1500 pounds would be -obtained. - - -(38.) In order to accomplish this, two things were necessary: 1. -To make a speedy and effectual vacuum below the [Pg067] piston in -the descent; and, 2. To contrive a counterpoise for the atmosphere -in the ascent. - -The condensation of steam immediately presented itself as the most -effectual means of accomplishing the former; and the elastic force -of the same steam previous to condensation an obvious method of -effecting the latter. Nothing now remained to carry the design -into execution, but the contrivance of means for the alternate -introduction and condensation of the steam; and Newcomen and -Cawley were accordingly granted a patent in 1707, in which Savery -was united, in consequence of the principle of condensation for -which he had previously received a patent being necessary to the -projected machine. We shall now describe the _atmospheric engine_, -as first constructed by Newcomen:-- - -The boiler K (_fig._ 14.) is placed over a furnace I, the flue of -which winds round it, so as to communicate heat to every part of -the bottom of it. In the top, which is hemispherical, two -gauge-cocks G G' are placed, as in Savery's engine, and a _puppet -valve_ V, which opens upward, and is loaded at one pound per -square inch; so that when the steam produced in the boiler exceeds -the pressure of the atmosphere by more than one pound on the -square inch, the valve V is lifted, and the steam escapes through -it, and continues to escape until its pressure is sufficiently -diminished, when the valve V again falls into its seat. This valve -performs the office of the safety-valve in modern engines. - -The great steam-tube is represented at S, which conducts steam -from the boiler to the cylinder; and a feeding pipe T, furnished -with a cock, which is opened and closed at pleasure, proceeds from -a cistern L to the boiler. By this pipe the boiler may be -replenished from the cistern, when the gauge cock G' indicates -that the level has fallen below it. The cistern L is supplied with -hot water, by means which we shall presently explain. - - -(39.) To understand the mechanism necessary to work the piston, -let us consider how the supply and condensation of steam must be -regulated. When the piston has been forced to the bottom of the -cylinder by the atmospheric pressure acting against a vacuum, in -order to balance that pressure, [Pg068] and enable it to be drawn -up by the weight of the pump-rod, it is necessary to introduce -steam from the boiler. This is accomplished by opening the cock R -in the steam pipe S. The steam being thus introduced from the -boiler, its pressure balances the action of the atmosphere upon -the piston, which is immediately drawn to the top of the cylinder -by the weight of the pump-rod D. It then becomes necessary to -condense this steam, in order to produce a vacuum. To accomplish -this, the further supply of steam must be cut off, which is done -by closing the cock R. The supply of steam from the boiler being -thus suspended, the application of cold water on the external -surface of the cylinder becomes necessary to condense the steam -within it. This was done by enclosing the cylinder within another, -leaving a space between them.[13] Into this space cold water was -allowed to flow from a cock M placed over it, supplied by a pipe -from the cistern N. This cistern is supplied with water by a pump -O, which is worked by the engine. - -The cold water supplied from M, having filled the space between -the two cylinders, abstracts the heat from the inner one; and -condensing the steam, produces a vacuum, into which the piston is -forced by the atmospheric pressure. Preparatory to the next -descent, the water which thus fills the space between the -cylinders, and which is warmed by the heat abstracted from the -steam, must be discharged, in order to give room for a fresh -supply of cold water from M. An aperture, furnished with a cock, -is accordingly provided in the bottom of the cylinder, through -which the water is discharged into the cistern L; and being warm, -is adapted for the supply of the boiler through T, as already -mentioned. - -The cock R being now again opened, steam is admitted below the -piston, which, as before, ascends, and the descent is again -accomplished by closing the cock R, and opening the cock M, -admitting cold water between the cylinders, and thereby condensing -the steam below the piston. - -The condensed steam, thus reduced to water, will collect [Pg069] -in the bottom of the cylinder, and resist the descent of the -piston. It is therefore necessary to provide an exit for it, which -is done by a valve opening _outwards_ into a tube which leads to -the feeding cistern L, into which the condensed steam is driven. - -That the piston should continue to be air-tight, it was necessary -to keep a constant supply of water over it; this was done by a -cock similar to M, which allowed water to flow from the pipe M on -the piston. - - -(40.) Soon after the first construction of these engines, an -accidental circumstance suggested to Newcomen a much better method -of condensation than the application of cold water on the external -surface of the cylinder. An engine was observed to work several -strokes with unusual rapidity, and without the regular supply of -the condensing water. Upon examining the piston, a hole was found -in it, through which the water, which was poured on to keep it -air-tight, flowed, and instantly condensed the steam under it. - -On this suggestion Newcomen abandoned the external cylinder, and -introduced a pipe H, furnished with a cock Q, into the bottom of -the cylinder, so that, on turning the cock, the pressure of the -water in the pipe H, from the level of the water in the cistern N, -would force the water to rise as a jet into the cylinder, and -would instantly condense the steam. This method of condensing by -injection formed a very important improvement in the engine, and -is still used. - - -(41.) Having taken a general view of the parts of the atmospheric -engine, let us now consider more particularly its operation. - -When the engine is not working, the weight of the pump-rod D -(_fig._ 14.) draws down the beam A, and draws the piston to the -top of the cylinder, where it rests. Let us suppose all the cocks -and valves closed, and the boiler filled to the proper depth. The -fire being lighted beneath it, the water is boiled until the steam -acquires sufficient force to lift the valve V. When this takes -place, the engine may be started. For this purpose the regulating -valve R is opened. The steam rushes in, and is first condensed by -the cold cylinder. After a short time the cylinder acquires the -temperature of the steam, which then [Pg070] ceases to be -condensed, and mixes with the air which filled the cylinder. The -steam and heated air, having a greater force than the atmospheric -pressure, will open a valve placed at the end X of a small tube in -the bottom of the cylinder, and which opens outwards. From this -(which is called the _blowing valve_[14]) the steam and air rush -in a constant stream, until all the air has been expelled, and the -cylinder is filled with the pure vapour of water. This process is -called _blowing_ the engine preparatory to starting it. - -When it is about to be started, the engine-man closes the -regulator R, and thereby suspends the supply of steam from the -boiler. At the same time he opens the _condensing valve_ H[15]; -and thereby throws up a jet of cold water into the cylinder. This -immediately condenses the steam contained in the cylinder, and -produces the vacuum. (The atmosphere cannot enter the _blowing_ -valve, because it opens _outwards_, so that no air can enter to -vitiate the vacuum.) The atmospheric pressure above the piston now -takes effect, and forces it down in the cylinder. The descent -being completed, the engine-man closes the condensing valve H, and -opens the regulator, R. By this means he stops the play of the jet -within the cylinder, and admits the steam from the boiler. The -first effect of the steam is to expel the condensing water and -condensed steam which are collected in the bottom of the cylinder, -through the tube Y, containing a valve which opens _outwards_ -(called the _eduction valve_), which leads to the hot cistern L, -into which this water is therefore discharged. - -When the steam admitted through R ceases to be condensed, it -balances the atmospheric pressure above the piston, and thus -permits it to be drawn to the top of the cylinder by the weight of -the rod D. This ascent of the piston is also assisted by the -circumstance of the steam being somewhat stronger than the -atmosphere. - -When the piston has reached the top, the regulating valve R is -closed, and the condensing valve H opened, and another descent -produced, as before, and so the process is continued. [Pg071] - -The manipulation necessary in working this engine was, therefore, -the alternate opening and closing of two valves; the regulating -and condensing valves. When the piston reached the top of the -cylinder, the former was to be closed, and the latter opened; and, -on reaching the bottom, the former was to be opened, and the -latter closed. - - -(42.) The duty of working the engine requiring no great amount of -labour, or skill, was usually entrusted to boys, called, _cock -boys_. It happened that one of the most important improvements -which has ever been made in the working of steam engines was due -to the ingenuity of one of these boys. It is said that a lad, -named _Humphrey Potter_, was employed to work the cocks of an -atmospheric engine, and being tempted to escape from the -monotonous drudgery to which his duty confined him, his ingenuity -was sharpened so as to prompt him to devise some means by which he -might indulge his disposition to play without exposing himself to -the consequences of suspending the performance of the engine. On -observing the alternate ascending and descending motion of the -beam above him, and considering it in reference to the labour of -his own hands, in alternately raising and lowering the levers -which governed the cocks, he perceived a relation which served as -a clue to a simple contrivance, by which the steam engine, for the -first time, became an automaton. When the beam arrived at the top -of its play, it was necessary to open the steam valve by raising a -lever, and to close the injection valve by raising another. This -he saw could be accomplished by attaching strings of proper length -to these levers, and tying them to some part of the beam. These -levers required to be moved in the opposite direction when the -beam attained the lowest point of its play. This he saw could be -accomplished by strings, either connected with the outer arm of -the beam, or conducted over rods or pulleys. In short, he -contrived means of so connecting the levers which governed the two -cocks by strings with the beam, that the beam opened and closed -these cocks with the most perfect regularity and certainty as it -moved upwards and downwards. - -Besides rendering the machine independent of manual [Pg072] -superintendence, this process conferred upon it much greater -regularity of performance than any manual superintendence could -ensure. - -This contrivance of Potter was very soon improved by the -substitution of a bar, called a _plug frame_, which was suspended -from the arm of the beam, and which carried upon it pins, by which -the arms of the levers governing the cocks were struck as the -plug-frame ascended and descended, so as to be opened and closed -at the proper times. - -The engine thus improved required no other attendance except to -feed the boiler occasionally by the cock T, and to attend the -furnace. - - -(43.) However the merit of the discovery of the physical -principles on which the mechanical application of steam depends -may be awarded, it must be admitted that the engine contrived by -Newcomen and his associates, considered as a practical machine, -was immeasurably superior to that which preceded it; superior, -indeed, to such a degree, that while the one was incapable of any -permanently useful application, the other soon became a machine of -extensive utility in the drainage of mines; and, even at the -present time, the atmospheric engine is not unfrequently used in -preference to the modern steam engine, in districts where fuel is -abundant and cheap; the expense of constructing and maintaining it -being considerably less than that of an improved steam engine. The -low pressure of the steam used in working it, rendered it -perfectly safe. While Savery's engine, to work with effect, -required that the steam confined in the vessels should have a -bursting pressure amounting to about thirty pounds per square -inch, the pressure of steam in the boiler and cylinder of the -atmospheric engine required only a pressure about one pound per -square inch. The high pressure also of the steam used in Savery's -engine, was necessarily accompanied, as we shall presently -explain, by a greatly increased temperature. The effect of this -was, to weaken and gradually destroy the vessels, especially those -which, like the steam vessels V and V' (_fig._ 12.), were -alternately heated and cooled. - -Besides these defects, the power of Savery's engines was [Pg073] -also very restricted, both as to the quantity of water raised and as -to the height to which it was elevated. On the other hand, the -atmospheric engine was limited in its power only by the dimensions -of its piston. Another considerable advantage which the atmospheric -engine possessed over that of Savery, was the facility with which it -was capable of driving machinery by means of the working-beam. The -merit, however, of Newcomen's engine, regarded as an invention, and -apart from merely practical considerations, must be ascribed -principally to its mechanism and combinations. We find in it no new -principle, and scarcely even a novel application of a principle. The -agency of the atmospheric pressure acting against a vacuum, or -partial vacuum, had been long known: the method of producing a -vacuum by the condensation of steam had been suggested by Papin, and -carried into practical effect by Savery. The mechanical power -obtained from the direct pressure of the elastic force of steam, -used in the atmospheric engine to balance the atmosphere during the -ascent of the piston, was suggested by De Caus and Lord Worcester. -The boiler, gauge pipes, and the regulator, were all borrowed from -the engine of Savery. The idea of using the atmospheric pressure -against a vacuum or partial vacuum, to work a piston in a cylinder, -had been suggested by Otto Guericke, an ingenious German -philosopher, who invented the air-pump; and this, combined with the -production of a vacuum by the condensation of steam, was -subsequently suggested by Papin. The use of a working-beam could not -have been unknown. Nevertheless, the judicious combination of these -scattered principles must be acknowledged to deserve considerable -credit. In fact, the mechanism contrived by Newcomen rendered a -machine which was before altogether inefficient, highly efficient: -and, as observed by Tredgold, such a result, considered in a -practical sense, should be more highly valued than the fortuitous -discovery of a physical principle. The method of condensing the -steam by the sudden injection of water, and of expelling the air and -water from the cylinder by the injection of steam, are two -contrivances not before in use, which are quite essential to the -[Pg074] effective operation of the engine. These processes, which -are still necessary to the operation of the improved steam engine, -appear to be wholly due to the inventors of the atmospheric engine. - -[Illustration: ATMOSPHERIC ENGINE.] - - FOOTNOTES: - - [9] This pipe is represented as proceeding from the force-pipe - above the cistern C, in the perspective view of Savery's - engine at the head of this chapter. - - [10] Hot water being lighter than cold, floats on the surface. - - [11] "Captain" is a title given in Cornwall to the - superintendent of the works connected with a mine. - - [12] As the calculation of the power of an engine depends on - the number of square inches in the section of the piston, it - may be useful to give a rule for computing the number of - square inches in a circle. The following rule will always give - the dimensions with sufficient accuracy:--_Multiply the number_ - _of inches in the diameter by itself; divide the product by 14,_ - _and multiply the quotient thus obtained by 11, and the result_ - _will be the number of square inches in the circle_. Thus, if - there be 12 inches in the diameter, this multiplied by itself - gives 144, which divided by 14 gives 10-4/44, which multiplied - by 11 gives 115, neglecting fractions. There are, therefore, - 115 square inches in a circle whose diameter is 12 inches. - - [13] The external cylinder is not represented in the diagram. - - [14] Also called the _snifting_ valve, from the peculiar noise - made by the air and steam escaping from it. - - [15] Also called the _injection valve_. - -[Pg075] - - - - -[Illustration: GREENOCK, IN 1824.] - -CHAP. III. - - PROGRESS OF THE ATMOSPHERIC ENGINE. -- SMEATON'S IMPROVEMENTS. - -- BRINDLEY, ENGINEER OF THE BRIDGEWATER CANAL. -- INVENTS THE - SELF-REGULATING FEEDER. -- JAMES WATT. -- HIS DESCENT AND - PARENTAGE. -- ANECDOTES OF HIS BOYHOOD. -- HIS EARLY AQUIREMENTS. - -- GOES TO LONDON. -- RETURNS TO GLASGOW. -- IS APPOINTED - INSTRUMENT-MAKER TO THE UNIVERSITY. -- OPENS A SHOP IN GLASGOW. - -- HIS FRIENDS AND PATRONS. -- ADAM SMITH. -- DR. BLACK. -- ROBERT - SIMSON. -- PROFESSOR ROBISON. -- WATT'S PERSONAL CHARACTER. -- - INDUSTRIOUS AND STUDIOUS HABITS. -- HIS ATTENTION FIRST - DIRECTED TO STEAM. -- EXPERIMENTS ON HIGH-PRESSURE STEAM. -- - REPAIRS AN ATMOSPHERIC MODEL. -- EXPERIMENTAL INQUIRY - CONSEQUENT ON THIS. -- ITS RESULTS. -- DISCOVERS THE GREAT - DEFECTS OF THE ATMOSPHERIC ENGINE. -- DISCOVERY BY EXPERIMENT - OF THE EXPANSION WHICH WATER UNDERGOES IN EVAPORATION. -- - DISCOVERS THE LATENT HEAT OF STEAM. -- IS INFORMED BY DR. BLACK - OF THE THEORY OF LATENT HEAT. - - -(44.) The atmospheric engine was brought to a state of -considerable efficiency and improvement by Mr. Beighton, in 1718. -From that time it continued in use without any change in its -[Pg076] principle, and with little improvement in its structure, -for half a century. Although engines of this kind continued to be -extensively constructed, they were usually executed by ordinary -mechanics, incapable of applying to them the just principles of -practical science; and, consequently, little attention was paid to -their proportions. It was not until about the year 1772, that Mr. -John Smeaton, the celebrated engineer, applied the powers of his -mind to the investigation of this machine, as he had previously -done with such success to wind and water mills. Although he did -not introduce any new principle into the atmospheric engine, yet -it derived greatly augmented power from the proportions which he -established for engines of different magnitudes. - -In 1759, Mr. James Brindley, whose name is so celebrated as the -engineer of the Duke of Bridgewater's canal, obtained a patent for -some improvements in the atmospheric engine. He proposed that the -boiler should be made of wood and stone, with a stove or -fire-place of cast iron within it, so that the fire should be -surrounded on every side by water. The chimney was to be an iron -pipe or tube, conducted through the boiler; so that the heated -air, in passing from the fire, should impart a portion of its heat -to the water. He also proposed a method of feeding the boiler, -which, by self-acting machinery, would keep the water in the -boiler at a fixed level, independently of any attention on the -part of the engine-man. This was to be accomplished by a buoy or -float upon the surface of the water in the boiler, which should -communicate with a valve in the feed-pipe, so that when the level -of the water in the boiler fell, the float or buoy, falling with -it, would open the valve and supply the feed. It is stated, in the -_Biographia Britannica_, that Mr. Brindley, in 1756, undertook to -erect an engine at Newcastle-under-Lyne; but he is said to have -been discouraged by the obstacles which were thrown in his way, -and to have abandoned the steam engine. - -The interval between the invention of the atmospheric engine, and -the amelioration it received at the hands of Smeaton, has been -rendered memorable by the advent of one who was destined to work a -mighty change in the condition [Pg077] of the human race by the -application of his vast genius to the adaptation of steam power to -the uses of life. - - -(45.) JAMES WATT was born at Greenock, in Scotland, on the -nineteenth day of January, in the year 1736.[16] - -The great-grandfather of Watt, a farmer in Aberdeenshire, was killed -in one of the battles of Montrose. The victorious party, not -thinking death a sufficient expiation for the political opinions in -support of which he had fought and bled, punished him in the person -of his son, by confiscating his little property. Thomas Watt, the -son, thus deprived of support, was received by distant relations, -and, for a time, applied himself to study, by which he was enabled, -after the restoration of tranquillity, to establish himself at -Greenock as a teacher of practical mathematics and navigation. He -resided in the burgh or barony of Crawford's Dyke, and attained a -position of sufficient respectability to be elected to the office of -baron-baillie, or chief magistrate, and died in 1734, at the -advanced age of ninety-two years. - -Thomas Watt had two sons. The elder, John, adopted the profession -of his father, and was a teacher of mathematics and navigation at -Glasgow: he died in 1737, at the age of fifty years. The second -son, James, the father of the celebrated engineer, was, during a -quarter of a century, treasurer of the town council of Greenock, -and a local magistrate. He was remarked for the ardent zeal and -enlightened spirit with which he discharged his public duties. His -business was that of a ship-chandler, builder, and general -merchant; but, unhappily, notwithstanding his active industry, he -lost, in the decline of his life, by unsuccessful commercial -speculations, a part of the property which he had so honourably -acquired. He died in 1782, at the age of eighty-four years. - -JAMES WATT, to whom the world is so largely indebted for the -extension and improvement of steam power, had from his birth an -extremely delicate constitution. From his mother, [Pg078] whose -family name was Muirhead, he received his first lessons in -reading, and he learned from his father writing and arithmetic. -Although he was entered as a pupil in the grammar school of -Greenock, yet such was his delicate state of health, that his -attendance there was so interrupted by constant indisposition that -he could derive but little benefit from the opportunities of -instruction which it afforded. For a great period of the year he -was confined to his room, where he devoted himself to study -without the aid of instruction. It was in the retirement of the -sick chamber that the high intellectual faculties of Watt, which -were destined to produce such precious fruits, began to unfold -themselves. He was too sickly to be subjected to the restraints -which the business of education usually imposes on children. His -parents, therefore, found it necessary to leave him at liberty to -choose his occupations and amusements. The following anecdotes -will show the use he made of this freedom. - -A friend of his father found the boy one day stretched upon the -hearth tracing with chalk various lines and angles. "Why do you -permit this child," said he, "to waste his time so; why not send -him to school?" Mr. Watt replied, "You judge him hastily; before -you condemn us, ascertain how he is employed." On examining the -boy, then six years of age, it was found that he was engaged in -the solution of a problem of Euclid! - -Having observed the tendency of his son's mind, Mr. Watt placed at -his disposal a collection of tools. These he soon learned to use -with the greatest skill. He took to pieces and put together, again -and again, all the children's toys which he could procure; and he -was constantly employed in making new ones. Subsequently he used -his tools in constructing a little electrical machine, the sparks -proceeding from which became a great subject of amusement to all -the playfellows of the poor invalid. - -Though endowed with great retentive powers, Watt would probably -never have figured among the prodigies of a common school: he would -have been slow to commit his lessons to memory, from the repugnance -which he would feel to repeat like a parrot anything which he did -not perfectly [Pg079] understand. The natural tendency of his mind -to meditate on whatever came before it, would give him, to -superficial observers, the appearance of dullness. Happily, however, -he had a parent who was sufficiently clear-sighted, and who -entertained high hopes of the growing faculties of his son. More -distant and less sagacious relations were not so sanguine. One day -Mrs. Muirhead, the aunt of the boy, reproaching him for what she -conceived to be listless idleness, desired him to take a book and -occupy himself usefully. "More than an hour has now passed away," -said she, "and you have not uttered a single word. Do you know what -you have been doing all this time? You have taken off, and put on, -repeatedly, the lid of the tea-pot; you have been holding the -saucers and the spoons over the steam, and you have been -endeavouring to catch the drops of water formed on them by the -vapour. Is it not a shame for you to waste your time so?" - -Mrs. Muirhead was little aware that this was the first experiment -in the splendid career of discovery which was subsequently to -immortalise her little nephew. She did not see, as we now can, in -the little boy playing with the tea-pot, the great engineer -preluding to those discoveries which were destined to confer on -mankind benefits so inestimable. - -One of the social qualities of mind which was remarkable -throughout his life, was the singular felicity and grace with -which he related anecdotes. This power was manifested even in his -earliest childhood. The following is an extract from a letter -written by Mrs. Marion Campbell, his cousin, and the playfellow of -his childhood:-- - -"He was not fourteen when his mother brought him to Glasgow to -visit a friend of hers; his brother John accompanied him. On Mrs. -Watt's return to Glasgow, some weeks after, her friend said, 'You -must take your son James home; I cannot stand the degree of -excitement he keeps me in; I am worn out for want of sleep. Every -evening before ten o'clock, our usual hour of retiring to rest, he -contrives to engage me in conversation, then begins some striking -tale, and, whether humorous or pathetic, the interest is so -overpowering that the family all listen to him with breathless -attention, and hour after hour strikes unheeded.'" [Pg080] - -Watt had a younger brother, John, who was subsequently lost by -shipwreck, in a voyage from Scotland to the United States. This lad, -having determined on following the business of his father, left James -more completely at liberty to choose his own occupation. But such a -choice was difficult for a student who commanded equal success in -every thing to which he directed his attention. - -The excursions which he was in the habit of making on the Scottish -mountains surrounding Loch Lomond, naturally directed his -attention to botany and mineralogy, in each of which he attained -considerable knowledge. His love of anecdote and romance was -likewise gratified by the scenery which he enjoyed in these walks; -and the traditions and popular songs with which they made him -acquainted. When from ill-health, as constantly happened, he was -confined to the house, he devoted himself to chemistry, natural -philosophy, and even to medicine and surgery. In chemistry he -acquired some experimental skill, and studied with eager zeal the -elements of natural philosophy by S'. Gravesande. His own unhappy -maladies prompted him to read works on surgery and medicine; and -to such an extent did the activity of his mind impel him on these -subjects, that he was found one day dissecting, in his room, the -head of a child, who had died of some unknown disease, with a view -to ascertain the cause of its death. - -In 1775, at the age of nineteen, at the recommendation of Dr. -Dick, professor of natural philosophy in the university of -Glasgow, he went to London, where he employed himself in the house -of Mr. John Morgan, a mathematical instrument maker, in Finch -Lane, Cornhill, to whom he apprenticed himself for three years. He -remained, however, only a year, at the expiration of which -(probably owing to his delicate state of health) he was released -from his apprenticeship, and returned to Glasgow, with the -intention of establishing himself in business as an optician and -mathematical instrument maker. In the fulfilment of this -intention, however, he was obstructed by the interposition of the -Corporation of Trades in that town, who regarded him as an -intruder, not qualified by the necessary apprenticeship to carry -on business. All means of conciliation being [Pg081] exhausted, -the Professors of the University interfered, and gave him the use -of three apartments within the college, for carrying on his -business, and likewise appointed him mathematical instrument maker -to the University. Soon afterwards the opposition of the local -trades seems to have given way, and he opened a shop in Glasgow -for the sale of mathematical instruments. - -After the celebrity at which he has arrived, it will be easily -believed that every trace of his earlier connection with Glasgow -college is carefully cherished. There are accordingly preserved at -that place little instruments and pieces of apparatus of exquisite -workmanship, which were executed entirely by the hand of Watt, at -a time when he was not in a condition to command the aid of -workmen under him. - -At the time of obtaining this appointment in the University, Watt -was in his twenty-first year. His natural talents and winning -manners were speedily the means of gaining for him the esteem and -friendship of all those eminent persons connected at the time with -that university whose regard was most valued. Among these the -earliest of his friends and patrons were--ADAM SMITH, the author of -"The Wealth of Nations;" BLACK, afterwards celebrated for his -chemical discoveries, and more especially for his theory of latent -heat; and ROBERT SIMSON, rendered illustrious by his works on -ancient geometry. In releasing Watt from the persecution of the -Glasgow corporation, these distinguished persons first imagined that -they were conferring a benefit merely on an industrious and clever -artisan, whose engaging manners won their regard; but a short -acquaintance with him was sufficient to convince them how superior -his mind was to his position, and they conceived towards him the -most lively friendship. His shop became the common rendezvous, the -afternoon lounge, of all who were most distinguished for literary -and scientific attainments among the professors and students. There -they met to discuss the topics of the day in art, science, and -literature. Among these students, the name which afterwards attained -the highest distinctions, and among these distinctions, not the -least, the lasting personal friendship and esteem of Watt himself, -was ROBISON, [Pg082] the author of a well known work on Mechanics, -and one of the contributors to the _Encyclopoedia Britannica_. - -The following extract from an unpublished manuscript by Robison -himself will show at once the estimation in which Watt was held, -and will illustrate one of the most interesting traits of his -personal character:-- - -"I had always, from my earliest youth, a great relish for the -natural sciences, and particularly for mathematical and mechanical -philosophy, when I was introduced by Drs. Simson, Dick, and Moor, -gentlemen eminent for their mathematical abilities, to Mr. Watt. I -saw a workman, and expected no more; but was surprised to find a -philosopher as young as myself, and always ready to instruct me. I -had the vanity to think myself a pretty good proficient in my -favourite study, and was rather mortified at finding Mr. Watt so -much my superior. . . . Whenever any puzzle came in the way of any -of the young students, we went to Mr. Watt. He needed only to be -prompted, for every thing became to him the beginning of a new and -serious study, and we knew that he would not quit it till he had -either discovered its insignificancy, or had made something of it. -He learnt the German language in order to peruse Leupold's -'Theatrum Machinarum;' so did I, to know what he was about. -Similar reasons made us both learn the Italian language. * * * -When to his superiority of knowledge is added the _naive_ -simplicity and candour of Mr. Watt's character, it is no wonder -that the attachment of his acquaintances was strong. I have seen -something of the world, and am obliged to say I never saw such -another instance of general and cordial attachment to a person -whom all acknowledged to be their superior. But that superiority -was concealed under the most amiable candour, and a liberal -allowance of merit to every man. Mr. Watt was the first to ascribe -to the ingenuity of a friend things which were nothing but his own -surmises, followed out and embodied by another. I am the more -entitled to say this, as I have often experienced it in my own -case." - -Watt never permitted the inquiries which arose out of these -reunions to interfere with the discharge of the duties of his -workshop. There he passed the day, devoting the [Pg083] night to -study. Every inquiry appeared to him to be attractive in -proportion to its difficulty, and to have charms in proportion as -it was removed from the common routine of his business. As an -example of this may be mentioned the fact, that, being himself so -insensible to the charms of music that he could not distinguish -one note from another, he was actually induced to undertake the -construction of an organ, in which he was nevertheless completely -successful. The instrument he constructed, as might have been -expected, contained many improvements in its mechanism; but what -is much more remarkable, its tone and its musical qualities -commanded the admiration of all the professional musicians who -heard it. In the construction of this instrument Watt showed that -vigorous spirit of investigation which characterised all the -subsequent labours of his life. He made out the scale of -temperament by the aid of the phenomena of beats, of which he -could only obtain a knowledge by a profound but obscure work -published by Dr. Robert Smith of Cambridge. - -The earliest occasion on which the attention of Watt is said to -have been called to the agency of steam, was in the year 1759, -when his friend Robison entertained some speculations for applying -that agent as a means of propelling wheel carriages; and he -consulted Watt on the subject. No record, however, has been -preserved of any experiments which were tried on this occasion; -nor does it appear that the inquiry was carried farther than a -verbal discussion, such as habitually took place on other subjects -of science between Watt and his friends. - - -(46.) In 1762, Watt tried some experiments on the force of steam -at a high pressure, confined in a close digester; and he then -constructed a small model to show how motion could be obtained -from that power. The practicability of what has since been called -the _High Pressure Engine_, was demonstrated by him on this -occasion; but he did not pursue the inquiry, on account of the -supposed danger of working with such compressed steam as was -required. - -It is usual to provide, in the cabinets of experimental apparatus -for the instruction of the students of universities, [Pg084] -small working models of the most useful machines. In the -collection for the illustration of the lectures delivered to the -Natural Philosophy class in the University of Glasgow was a -working model of Newcomen's atmospheric engine, applied to a pump -for raising water; which, however, had never been found to work -satisfactorily. The Professor of Experimental Philosophy of that -day, Dr. John Anderson (the founder of the celebrated Andersonian -Institution), sent this model in 1763 to Watt's workshop, to be -repaired. Its defects soon disappeared, and it was made to work to -the satisfaction of the professor and students. - -This simple discharge of his duty, however, did not satisfy the -artisan; and his wonted activity of mind rendered this model a -subject of profound meditation, and led him into a course of -practical inquiry respecting it, which formed the commencement of a -most brilliant career of mechanical discovery. The improvement--we -might almost say the creation--of the steam engine, by this great -man, must not therefore be regarded, as so often happens with -mechanical discoveries, as the result of fortuitous observation, or -even of a felicitous momentary inspiration. Watt, on the other hand, -conducted his investigation by a course of deep thought, and of -experiments marked by the last refinement of delicacy and address. -If he had received a more extended and liberal education, one would -have thought that he had adopted for his guide the celebrated maxim -of Bacon:-- - -"To write, speak, meditate, or act, when we are not provided with -_facts_ to direct our thoughts, is to navigate a coast full of -dangers without a pilot, and to launch into the immensity of the -ocean without either rudder or compass." - -The model which he had repaired, had a cylinder of only two inches -diameter, and six inches stroke. After he had put it in complete -order, he found, that although the boiler was much larger in -proportion to the cylinder than those of real engines, yet, that -it was incapable of supplying the cylinder with steam in -sufficient quantity to keep it at work. To enable it to continue -to move, he found it necessary to lessen the quantity of water -raised by its pump, so as to [Pg085] reduce the load on its -piston very much below the proper standard according to the common -rules for large engines. - -He ascribed the great inferiority in the performance of the model, -compared with the performance of the large engines, to the small -size of the cylinder, and to its material. The cylinder of the -model was brass, while those of large engines were of cast iron; -and brass being a better conductor of heat than iron, he concluded -that more heat in proportion was lost from this cause in the -model, than in the larger engines. He observed that the small -cylinder was so heated when the steam was admitted into it, that -it could not be touched by the hand; but, nevertheless, that this -heat contributed nothing to its performance, inasmuch as before -the piston descended, the cylinder required to be cooled. - - -(47.) His first attempt to improve the engine, was by using a -wooden cylinder instead of an iron one. He accordingly made a -model with a cylinder of wood, soaked in linseed oil, and baked to -dryness. With this he made numerous experiments, and found that it -required a less quantity of water to be thrown into the cylinder -to condense the steam, and that it was worked with a less supply -of steam from the boiler than was necessary with the metallic -cylinder. - -Still he found that the force with which the piston descended was -considerably less than that which the atmospheric pressure ought -to supply, supposing a tolerably perfect vacuum to be produced -under the piston. This led him to suspect that the water injected -into the cylinder was not perfectly effectual in condensing the -steam. The experiments which he had previously made on the -increased temperature at which water boils under pressures greater -than that of the atmosphere, led him by analogy to the conclusion -that it would boil at lower temperatures if it were submitted to a -pressure less than the atmosphere, and he was aware that Dr. -Cullen and others had then recently discovered that in vacuo, -water would boil at so low a temperature as 100 deg. These notions -suggested the probability that the water injected into the -cylinder being heated by the condensed steam, might produce vapour -of a low temperature [Pg086] and reduced pressure under the -piston, which would account for the deficiency he observed in the -power of the engine. - -No means occurred to him by which he could ascertain, by direct -experiment, the temperatures at which water would boil under -pressures less than that of the atmosphere. He sought, however, to -determine it by the following method. Having ascertained, by -repeating and multiplying the experiments which he had tried in -1762, on high-pressure steam, he obtained a table of the -temperatures at which water boils at various pressures greater -than that of the atmosphere. These results he laid down in a -series forming a curve, of which the abscissa represented the -temperatures, and the ordinates the pressures. He then continued -this curve, backwards as it were, and obtained, by analogy, an -approximation to the boiling temperatures, corresponding to -pressures less than that of the atmosphere. In other words, having -obtained by his experiments a notion, however imperfect, of the -law or rule observed by the temperatures at which water boils at -different pressures _greater_ than that of the atmosphere, he -calculated by the same law or rule what the pressures would be at -different pressures _less_ than that of the atmosphere. - -Applying these results to the interior of the cylinder of the -atmospheric engine, he obtained an approximation to the pressure -of the vapour which would be produced from the warm water formed -by the cold water injected into the cylinder, and the steam -condensed by it; and he accordingly found that vapour, having a -pressure seriously injurious to the power of the engine would be -produced in the cylinder, unless considerably more water of -injection was thrown in than was customary. - -It was apparent that the actual quantity of steam usefully -employed in the cylinder at each stroke, was only the quantity -which filled the cylinder; and therefore, in order to ascertain -the quantity of steam lost by the imperfections of the machine, it -was necessary to compare the actual quantity of steam transmitted -by the boiler to the cylinder at each stroke, with the quantity -which would just fill the cylinder. The difference would of course -be wasted. But to determine [Pg087] the actual quantity of steam -supplied by the boiler to the cylinder, there was no other means -than by observing the quantity of water evaporated in the boiler. -That being observed, it was necessary to know the quantity of -steam which that water formed; and it was therefore necessary to -determine the quantity or volume of steam which a given volume of -water produced. - - -(48.) On considering more attentively the operation of the -machine, the following circumstances gradually unfolded themselves -to him. - -Let us suppose the piston at the top of the cylinder, and the -space in the cylinder below it, filled with steam so as to balance -the pressure of the atmosphere above the piston. Under such -circumstances the steam, as will presently be explained, must have -the temperature of boiling water. But that the steam should have, -and should maintain, this temperature, it was evidently necessary -that the inner surface of the cylinder in contact with it should -have the same temperature: for if it had a lower temperature, it -would take heat from the steam, and reduce the temperature of the -latter. Now the cylinder being a mass of metal, has a quality in -virtue of which heat passes freely through its dimensions, so that -its inner surface could not be maintained at a temperature more -elevated than that of its dimensions extending from the inner -surface to the outer surface. Therefore, to maintain the steam -contained in the cylinder at the proper temperature, it was -essential that the whole of the solid metal composing the cylinder -should be itself at that temperature. - -Things being in this state, it was required that a vacuum should -be produced under the piston to give effect to the atmospheric -pressure above it, by relieving it from the pressure below. This, -indeed, would appear to have been attained by introducing as much -cold water within the cylinder as would be sufficient to reconvert -the steam contained in it into water; but Watt found, in his -experiments on the atmospheric model, that the piston would not -descend with the proper force, unless a vastly greater quantity of -water were introduced into the cylinder than the quantity which he -had ascertained to be [Pg088] necessary for the reconversion of -the steam into water. The cause of this he perceived and fully -explained. - -If we suppose as much, and no more, cold water introduced into the -cylinder as would reconvert the steam contained in it into water, -then we should have in the bottom of the cylinder a quantity of -warm water with a vacuum above it: but the entire mass of metal -composing the cylinder itself, which was previously at the -temperature of boiling water, would still be at the same -temperature. The warm water, resting in contact with this metal in -the bottom of the cylinder, would be immediately heated by it, and -would rise in its temperature, while the metal of the cylinder -itself would be somewhat lowered in temperature by the heat which -it would thus impart to the warm water contained in it. Under -these circumstances, as we shall presently explain, steam would be -produced from the water, which would fill the cylinder; and -although such steam would not have a mechanical pressure equal in -amount to the atmosphere, and therefore would not altogether -prevent the piston from descending if it had no load to move, yet -it would deprive the engine of so great a portion of its -legitimate power as to render it altogether inefficient. But this -defect would be removed by throwing into the cylinder a sufficient -quantity of cold water, not only to destroy the steam contained in -it, but also to cool the entire mass of metal composing the -cylinder itself, until it would be reduced to such a temperature -that the vapour proceeding from the water contained in it would -have so small a pressure that it would not seriously or -injuriously obstruct the descent of the piston. - -The piston being made to descend with such force as to render the -machine practically efficient, it would then be necessary again to -make it ascend; and to accomplish this, Watt found that the boiler -should supply a quantity of steam many times greater than was -necessary to fill the cylinder. Mature reflection on the -circumstances which have been just explained, enabled him to -discover how this undue quantity of steam was rendered necessary. - -Let it be recollected, that when the piston has reached the bottom -of the cylinder, the whole mass of the cylinder, and [Pg089] the -piston itself, are reduced to so low a temperature that the vapour -of water, having the same temperature, has no pressure sufficiently -great to obstruct the action of the machine. When, in order to make -the piston ascend, steam is introduced from the boiler into the -cylinder under the piston, this steam encounters, in the first -instance, the cold surfaces of the metal forming the bottom of the -cylinder and the bottom of the piston. The first effect of this is -to convert the steam which comes from the boiler into water, an -effect which is produced by that steam imparting its heat to the -metal with which it comes into contact. This destruction of steam -continues until the metal exposed to contact with it has been heated -up to the temperature of boiling water. Then, and not till then, the -steam below the piston will have a pressure equal to that of the -atmosphere above it, and the piston will begin to ascend. As it -ascends, however, the sides of the cylinder which it exposes to the -contact of the steam are cold, and partially destroy the steam. -Steam, therefore, must be supplied from the boiler to replace the -steam thus destroyed; nor can the piston reach the top of the -cylinder until such a quantity of steam shall have flowed from the -boiler into the cylinder, as shall be sufficient not only to fill -the cylinder under the piston, but likewise, by its condensation, to -raise the whole mass of the cylinder and piston to the temperature -of boiling water. - -Such were the circumstances which forced themselves upon the -attention of Watt, in the course of repairing, and subsequently -trying, the model of the atmospheric engine, at Glasgow. Being -informed generally of the uses of the engine in the drainage of -mines, and of the vast expense attending its operation, by reason -of the quantity of fuel which it consumed, he saw how important -any improvement would be by which the extensive sources of waste -which had thus presented themselves could be removed. He saw also, -that all that portion of steam which was expended, not in filling -the cylinder under the piston, but in heating the great mass of -metal composing the cylinder and piston, from a low temperature to -that of boiling water, upon each stroke of the piston, was so much -heat lost, and that the proportion of the fuel expended in -evaporating the steam thus wasted would be saved, if by any -[Pg090] expedient _he could make the piston descend without -cooling the cylinder_. But in order to estimate the full amount of -this waste, and to discover the most effectual means of preventing -it, it was necessary to investigate the quantity of heat necessary -for the evaporation of a given quantity of water; also, the -quantity of steam which a given quantity of water would produce, -as well as other circumstances connected with the temperature and -pressure of steam. He, therefore, applied himself to make -experiments with a view to elucidate these questions; and -succeeded in obtaining results which led to the discovery of some -of the most important of those physical phenomena, on the due -application of which, the efficacy of the steam engine, which he -afterwards invented, depended, and which also form striking facts -in the general physics of heat. - - -(49.) The first question to which he directed his experiments, was -the determination of the extent to which water enlarged its -volume, or magnitude, when it passed into steam. To ascertain -this, he filled a thin Florence flask with steam, of a pressure -equal to the atmosphere, and weighed it accurately. The same flask -was then filled with water, and weighed again. Finally, the weight -of the flask itself was ascertained. It is evident, that by such -means, the exact weight of the steam which filled the flask, and -of the same bulk of water, would be obtained. He found that the -water weighed about eighteen hundred times more than the steam; -from whence he inferred that the steam which filled the flask -contained about eighteen hundred times less water than the flask -would contain.[17] - -[Pg091] Having once ascertained this point, he was able, by -observing the quantity of water evaporated in the boiler of the -atmospheric model, to compute the volume of steam which was -supplied to the cylinder. It was evident, that for every cubic -inch of water evaporated in the boiler, eighteen hundred cubic -inches of steam were supplied to the cylinder. Having accurately -observed the evaporation of the boiler for a short time, and the -number of strokes made by the piston in the same time, he found -that the quantity of water evaporated in the boiler would supply -about four times as much steam as the cylinder would require. He -consequently inferred, that about three-fourths of the steam -produced was wasted. - -The next question to which he directed his experiments, was to -ascertain the quantity of cold water necessary to be injected into -the cylinder, in order to condense the steam contained in it. To -ascertain this, he attached a pipe to a boiler, by which he was -enabled to conduct the steam from the boiler into a glass jar -containing cold water at fifty-two degrees of temperature. The -steam, as it passed from the boiler through the pipe, was -condensed by the cold water, and continued to be so condensed, -until, by the heat which it imparted to the water, the latter -began to boil, and would then condense no more steam. On comparing -the water in the glass jar, when boiling, with the water -originally contained in it at fifty-two [Pg092] degrees, the -quantity was found to be increased in the proportion of six to -seven, very nearly; from which he inferred, that to reduce one -ounce of steam to water, it was necessary to mix about six ounces -of cold water with it. - -He was further led to the conclusion, that steam contains a vast -quantity of heat, by the following experiment. He heated, in a -close digester, a quantity of water several degrees above the -common boiling point. When thus heated, by opening a stop-cock, he -allowed the compressed steam to escape into a cold vessel; in -three or four seconds, he found that the heat of the water in the -digester was reduced from a very high temperature to the common -boiling point; yet, that all the steam which escaped from it, and -which carried off with it the superabundant heat, formed only a -few drops of water when condensed; from which he inferred, that -this small quantity of water, in the form of steam, contained as -much heat as was sufficient to raise all the water in the digester -from the boiling point to the temperature at which it was before -the steam was allowed to escape. - -Having thus ascertained the exact quantity of cold water which -ought to be injected into the cylinder in order to condense the -steam which filled the cylinder, he found, on comparing the -quantity necessary to be injected in order to enable the piston to -descend, that this quantity was about four times as great as that -which was necessary to condense the steam. This led him to the -conclusion, that about four times as much heat was destroyed in -the cylinder as needed to be destroyed, if the object were the -mere condensation of the steam. This result fully corroborated the -other conclusion, deduced, from the proportion which he found -between the quantity of steam supplied by the boiler and the -actual contents of the cylinder. - - -(50.) Watt was forcibly struck with these circumstances, not only -on account of their importance in an economical point of view, -when their relation to steam power was considered, but still more -so, as indicating phenomena in the physics of heat altogether -novel to him. - -He, therefore, eagerly sought his friend Dr. Black, to whom he -communicated these results. Then, for the first time, he [Pg093] -was informed, by Black, of the theory of LATENT HEAT, which had -recently been discovered by him, and of which these very phenomena -formed the basis. - -Some passages in the works of Dr. Robison produced an erroneous -impression, that a large share of the merit of the discoveries of -Watt which have been just explained was due to Dr. Black, to whose -instructions on the subject of latent heat Watt was represented to -have owed the knowledge of those facts which led to his principal -inventions and improvements. We shall here give, in the words of -Watt himself, his explanation of the circumstances which led to -this error. This explanation is given in a letter addressed by -Watt to Dr. Brewster, in May 1814, and prefixed to the third -volume of Brewster's edition of Robison's Mechanical Philosophy:-- - - "The representations of friends whose opinions I highly value - induce me to avail myself of this opportunity of noticing an error - into which not only Dr. Robison, but apparently also Dr. Black, - has fallen, in relation to the _origin_ of my improvements upon - the steam engine, and which not having been publicly controverted - by me, has, I am informed, been adopted by almost every subsequent - writer upon the subject of latent heat. - - "Dr. Robison, in the article Steam Engine, after passing an - encomium upon me, dictated by the partiality of friendship, - qualifies me as the '_pupil_ and intimate friend of Dr. Black,'--a - description which not being there accompanied with any inference, - did not particularly strike me at the time of its first perusal. - He afterwards, in the dedication to me of his edition of Dr. - Black's lectures upon chemistry, goes the length of supposing me - to have professed to owe my improvements upon the steam engine to - the instructions and information I had received from that - gentleman, which certainly was a misapprehension; as, though I - have always felt and acknowledged my obligations to him for the - information I had received from his conversation, and particularly - for the knowledge of the doctrine of latent heat, I never did nor - _could_ consider my improvements as originating in those - communications. He is also mistaken in his assertion (p. 8. of the - preface to the above work), that 'I had attended two courses - [Pg094] of the doctor's lectures;' for, unfortunately for me, the - necessary avocations of my business prevented me from attending - his or any other lectures at college; and as Dr. Robison was - himself absent from Scotland for four years at the period referred - to, he must have been misled by erroneous information. In p. 184. - of the lectures, Dr. Black says, 'I have the pleasure of thinking - that the knowledge we have acquired concerning the nature of - elastic vapours, in consequence of my fortunate observation of - what happens in its formation and condensation, has contributed in - no inconsiderable degree to the public good by _suggesting_ to my - friend Mr. Watt of Birmingham, then of Glasgow, his improvement on - this useful engine' (meaning the steam engine of which he is then - speaking). There can be no doubt from what follows in his - description of the engine, and from the very honourable mention - which he has made of me in various parts of his lectures, that he - did not mean to lessen any merit that might attach to me as an - inventor; but, on the contrary, he was always disposed to give me - fully as much praise as I deserved. - - "And were that otherwise doubtful, it would, I think, be evident - from the following quotation from a letter of his to me, dated - 13th February 1783, where, speaking of an intended publication by - a friend of mine, on subjects connected with the history of steam, - he says, 'I think it is very proper for you to give him a short - account of your discoveries and speculations; _and particularly_ - _to assert clearly and fully your sole right to the honour of_ - _the improvements of the steam engine_.' And in a written - testimonial which he very kindly gave me, on the occasion of a - trial at law against a piracy of my invention in 1796-7, after - giving a short account of the invention, he adds, '_Mr. Watt was_ - _the sole inventor of the capital improvement and contrivance_ - _above mentioned._' - - "Under this conviction of his candour and friendship, it is very - painful to me to controvert any assertion or opinion of my revered - friend; yet, in the present case I find it necessary to say, that - he appears to me to have fallen into an error; and I hope, in - addition to my assertion, to make that appear by the short history - I have given of my invention, in my [Pg095] notes upon Dr. - Robison's essay, as well as by the following account of the state - of my knowledge previous to my receiving any explanation of the - doctrine of latent heat; and also from that of the facts which - principally guided me in the invention. - - "It was known very long before my time, that steam was condensed - by coming into contact with cold bodies, and that it communicated - heat to them; witness the common still, &c. &c. - - "It was known, by some experiments of Dr. Cullen and others, that - water and other liquids boiled in vacuo at very low heats; water - below 100 deg. - - "It was known to some philosophers that the capacity or - equilibrium of heat, as we then called it, was much smaller in - mercury and tin than in water. - - "It was also known that evaporation caused the cooling of the - evaporating liquid, and bodies in contact with it. - - "I had myself made experiments to determine the following facts:-- - - "First, the capacities of heat for iron, copper, and some sorts of - wood, comparatively with water. - - "Second, the bulk of steam compared with that of water. - - "Third, the quantity of water evaporated in a certain boiler by a - pound of coals. - - "Fourth, the elasticities of steam at various temperatures greater - than that of boiling water, and an approximation to the law which - it followed at other temperatures. - - "Fifth, how much water in the form of steam was required every - stroke by a small Newcomen's engine, with a wooden cylinder six - inches diameter, and twelve inches stroke. - - "Sixth, the quantity of cold water required in every stroke to - condense the steam in that cylinder, so as to give it a working - power of about 7 lb. on the inch. - - "Here I was at a loss to understand how so much cold water could - be heated so much by so small a quantity of water in the form of - steam; and I accordingly applied to Dr. Black, and then first - understood what was called latent heat. - - "But this theory, though useful in determining the quantity of - injection necessary where the quantity of water [Pg096] - evaporated by the boiler, and used by the cylinder, was known, and - in determining, by the quantity and heat of the hot water emitted - by Newcomen's engines, the quantity of steam required to work them - did not lead to the improvements I afterwards made in the engine. - These improvements proceeded upon the old established fact, that - steam was condensed by the contact of cold bodies; and the later - known one, that water boiled in vacuo at heats below 100 deg., and - consequently that a vacuum could not be obtained unless the - cylinder and its contents were cooled every stroke to below that - heat." - -[Illustration: LOCH LOMOND.] - - FOOTNOTES: - - [16] We are indebted for many of the anecdotes of the life of - Watt to the _Eloge Historique_, recently published by M. Arago, - who was furnished with all the documents and circumstances - relating to this celebrated person which were considered proper - for publication, by his son, the present James Watt, Esq., of - Aston Hall, near Birmingham, and to the notes added to this - memoir by Mr. Muirhead, a relative of Mr. Watt. - - [17] The following is the account of these experiments given - in Watt's own words:-- - - "It being evident that there was a great error in Dr. - Desagulier's calculations of Mr. Beighton's experiments on the - bulk of steam, a Florence flask, capable of containing about a - pound of water, had about one ounce of distilled water put - into it; a glass tube was fitted into its mouth, and the - joining made tight by lapping that part of the tube with - packthread covered with glazier's putty. When the flask was - set upright, the tube reached down near to the surface of the - water, and in that position the whole was placed in a tin - reflecting oven before a fire until the water was wholly - evaporated, which happened in about an hour, and might have - been done sooner, had I not wished the heat not much to exceed - that of boiling water. As the air in the flask was heavier - than the steam, the latter ascended to the top, and expelled - the air through the tube. When the water was all evaporated, - the oven and flask were removed from the fire, and a blast of - cold air was directed against one side of the flask, to - collect the condensed steam in one place. When all was cold, - the tube was removed, the flask and its contents were weighed - with care; and the flask being made hot, it was dried by - blowing into it by bellows, and when weighed again was found - to have lost rather more than four grains, estimated at 4-1/3 - grains. When the flask was filled with water, it was found to - contain about 17-1/8 ounces avoirdupois of that fluid which - gave about 1800 for the expansion of water converted into - steam of the heat of boiling water. - - "This experiment was repeated with nearly the same result, and - in order to ascertain whether the flask had been wholly filled - with steam, a similar quantity of water was for the third time - evaporated; and, while the flask was still cold, it was placed - inverted with its mouth (contracted by the tube) immersed in a - vessel of water, which it sucked in as it cooled, until in the - temperature of the atmosphere it was filled to within half an - ounce measure of water. - - "In repetitions of this experiment at a later date, I - simplified the apparatus by omitting the tube, and laying the - flask upon its side in the oven, partly closing its mouth by a - cork, having a notch on one side, and otherwise proceeding as - has been mentioned." - -[Pg097] - - - - -[Illustration: GLASGOW.] - -CHAP. IV. - - EXPOSITION OF PHYSICAL PRINCIPLES. -- THERMOMETER. -- METHOD OF - GRADUATING IT. -- FREEZING AND BOILING POINTS. -- LATENT HEAT OF - WATER. -- QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER. - -- QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO - ICE. -- PROCESS OF BOILING. -- OF RECONVERSION OF STEAM INTO - WATER. -- QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO - STEAM. -- BOILING POINT OF WATER. -- DIFFERENT IN DIFFERENT - PLACES. -- DEPENDS ON THE BAROMETER. -- VARIES WITH THE - PRESSURE. -- EXPERIMENTAL PROOF OF THIS. -- BOILS AT LOWER - TEMPERATURES THAN 212 DEG. UNDER PRESSURES LESS THAN THE - ATMOSPHERE. -- SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS - THE SAME. -- THE FUEL NECESSARY TO EVAPORATE WATER THE SAME, - WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS - EVAPORATED. -- MECHANICAL FORCE OBTAINED BY EVAPORATION. -- THIS - FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES. - - -(51.) We shall pause here to put the reader in possession of the -physical and mechanical principles connected with the evaporation -of water and other liquids, which are necessary to enable him to -understand the full extent of the value and the merit of the -discoveries of Watt, and to comprehend the [Pg098] structure and -operation of the steam engine in its improved form, as it has -passed to us from his hands. - -As we shall frequently have occasion to refer to the indications -of a thermometer, we shall first explain the principle of that -instrument as it is commonly used in this country. - -The thermometer is an instrument used for the purpose of measuring -and indicating the temperature or sensible heat of material -substances. - -Heat, like all other physical agents, can only be measured by its -effects. One of these effects best suited for this purpose, is the -change of dimension which all bodies undergo in consequence of -their change of temperature. In general, when heat is applied to a -material substance, that substance undergoes an enlargement of -bulk; and if heat be abstracted from it, it suffers a diminution -of bulk. This variation of magnitude is not always in the same -proportion as the increase or diminution of temperature; but it is -so when applied to certain substances and between certain limits. -One of the substances whose expansion and contraction through an -extensive range of temperature has been found to be nearly -uniform, and which is attended with other convenient qualities for -a thermometer, is the liquid called _mercury_ or _quicksilver_. A -mercurial thermometer is constructed in the following way:-- - -A glass tube is made with a small and uniform bore: upon the end -of this tube, a bulb is blown, having a magnitude very great -compared with the bore of the tube. Let us suppose this bulb and a -part of the tube to be filled with mercury. If the mercury -contained in the bulb be heated, it will expand, and being more -susceptible of expansion than the glass which contains it, the -bulb will be too small for its augmented volume: the mercury in -the bulb can only, therefore, obtain room for its increased bulk -by pressing the mercury in the tube upwards, which it will -accordingly do. The increase of volume which the mercury in the -bulb therefore undergoes, will be exhibited by the increased -length of the column in the tube. Since the bore of the tube is -made so exceedingly minute compared with the magnitude of the -bulb, a very small quantity of mercury forced [Pg099] from the -bulb into the tube, will cause a considerable increase of the -length of the column. Small degrees of expansion will therefore be -rendered very apparent, and may be accurately measured. The -following is the method by which the thermometer called -_Fahrenheit's thermometer_ is graduated. - -The tube and bulb being prepared and supplied with mercury, as -already explained, let the instrument be plunged in a vessel of -melting ice. It will be found that the mercury will stand in the -tube at a certain point, from which it will not vary so long as -any ice remains not completely melted in the vessel. Let a mark be -made on the tube, or on a scale attached to the tube, at the point -corresponding to the top of the column: the point thus marked is -called the _freezing point_. - -Now let the instrument be immersed in a vessel of boiling water, -the barometer at the time having the height of thirty inches. It -will be found that so long as the water is kept boiling, the -column of mercury in the tube will remain stationary. Let the -point corresponding with the top of the column be marked on the -tube, or on the scale attached to it. This is called the _boiling -point_. Let the space on the scale between the freezing and -boiling points be now divided into 180 equal parts: each of these -parts is called a _degree_. Let the same divisions be continued -upon the scale below the freezing point, until thirty-two -divisions be taken; let the lowest division be then marked 0, and -let the successive divisions upwards from that be numbered 1, 2, -3, &c. In like manner, let the same divisions be continued above -the boiling point, as far as the tube will admit. - -It is evident that, under these circumstances, the freezing point -will be marked by 32, and the boiling point by 212. It is usual to -express the degrees of a thermometer in the same manner as the -degrees of a circle, by placing a small [circle or degree sign] -above the number. Thus the freezing point is expressed by 32 deg., -and the boiling point by 212 deg. - -The reason the degrees were commenced at 32 deg. below the freezing -point was, because, when the thermometer was invented, that -temperature was supposed to be the lowest degree of cold possible, -being that of a certain mixture of [Pg100] snow and salt. This, -however, has since been found to be an error, very much lower -temperatures being obtained by various physical expedients. - -The temperature of a body is, then, that elevation to which the -thermometer would rise when immersed in that body. Thus, if in -plunging the thermometer in water we found the mercury to rise or -fall to the division marked 100, we should then say, the -temperature of the water was 100 deg. - -Let us suppose a spirit lamp, or other regular source of heat, -applied to a bath of mercury, so as to maintain the mercury at a -fixed temperature of 200 deg., and let another vessel, containing a -quantity of ice at a temperature of 20 deg. be immersed in the -mercury. Let a thermometer be placed in the mercury, and another -in the ice. The following effects will then ensue. The thermometer -immersed in the ice will be observed gradually to rise from 20 deg. -upwards, until it indicates the temperature of 32 deg. It will then -become stationary, and the ice which had hitherto remained in a -solid state will begin to melt and be converted into water. This -process of liquefaction will continue for a considerable time, -during which the thermometer immersed in the ice will constantly -be maintained at 32 deg. At the moment, however, when the last -portion of ice is liquefied, the thermometer will begin again to -rise. The coincidence of this ascent of the thermometer with the -completion of the liquefaction of the ice, may be very easily -observed, because the ice being lighter, bulk for bulk, than -water, will float on the surface, and so long as a particle of it -remains unmelted it will be distinctly seen. - -Now it cannot be doubted that, during the whole of this process, -the mercury, supposed to be maintained at 200 deg., constantly -imparts heat to the ice; yet, from the moment the liquefaction begins, -until it is completed, no increased temperature is exhibited by -the thermometer immersed in the melting ice. If during this part -of the process no heat were received by the ice from the mercury, -the consequence would be, that the application of the lamp would -cause the temperature of the mercury to rise above 200 deg., which -may be easily demonstrated by withdrawing the vessel of ice from the -mercurial bath during the process of liquefaction. The moment -[Pg101] it is withdrawn, the thermometer immersed in the mercury, -instead of remaining fixed at 200 deg., will begin to rise, although -the action of the lamp remains the same as before; from which it -is evident that the heat which now causes the mercury to rise -above 200 deg. was before received by the melting ice. - -The heat which thus enters ice in the process of liquefaction, and -which is not indicated by the thermometer, is for this reason -called _latent heat_. It will be perceived that this phrase is the -name of a fact, and not of an hypothesis. That heat really enters -the water, and is contained in it, has been established by the -experiments; and to declare that it is present there, is to -declare an established fact. To call it by the name _latent_ heat, -is to declare another established fact, viz., that it is not -sensible to the thermometer. - -These facts show us that heat is capable of existing in bodies in -two distinct states, in one of which it is sensible to the -thermometer, and in the other not. Heat which is sensible to the -thermometer is called, for distinction, _sensible_ or _free heat_. -It may be here observed, that heat which is sensible to the -thermometer is also perceptible by the senses, and heat not -sensible to the thermometer is not perceptible by the senses. -Thus, ice at 32 deg. and water at 32 deg. _feel_ equally cold, and -yet we have seen that the latter contains considerably more heat -than the former. - -Dr. Black, who first noticed the remarkable fact to which we have -now alluded, inferred that ice is converted into water by -communicating to it a certain quantity or dose of heat, which -enters into combination with it in a manner analogous to that -which takes place when bodies combine chemically. The heat, thus -combined with the solid ice, loses its property of affecting the -senses or the thermometer, and the effects therefore bear a -resemblance to those cases of chemical combination in which the -constituent elements change their sensible properties when they -form the compound. - -The fact that the thermometer immersed in the ice remains stationary -only as long as the process of liquefaction is going on, shows that -this absorption of heat is necessarily connected with that process, -and that, were it not for the conversion of [Pg102] the solid ice -into liquid water, the heat which is so received would be sensible, -and would cause the thermometer immersed in the ice to rise. Before -the time of Black it was supposed that the slightest addition of -heat would cause solid ice to be converted into water, and that the -thermometer would immediately pass from the freezing temperature to -higher degrees. The experiments above described, however, show the -falsehood of such a supposition. If, while the mercurial bath, in -which the ice is immersed, is maintained at the temperature of 200 deg., -the length of time necessary to complete the liquefaction of the ice -be observed, it would be found that that time is about twenty-eight -times the length of time which it would take to raise the liquid -water from 32 deg. to 37 deg.; and if it be assumed that the same quantity -of heat is imparted to the ice, during the process of liquefaction, -during each minute, as is imparted to the water, during each minute, -in rising from 32 deg. to 37 deg., it will follow, that to liquefy the -ice requires twenty-eight times as much heat as is necessary to raise -the water from 32 deg. to 37 deg. It appears, therefore, that, instead of a -small quantity of heat being necessary to melt the ice, a very -considerable portion is absorbed in that process. - -Having ascertained the remarkable fact, that heat is absorbed in a -large quantity in the conversion of ice into water, without -rendering the body so absorbing it warmer, let us now inquire what -the exact quantity of heat so absorbed is. We have already stated -that, if the quantity communicated in equal times be the same, the -heat necessary to liquefy a given weight of ice would be -twenty-eight times as much as would be necessary to raise the same -weight of water from 32 deg. to 37 deg.; or, if the heat necessary to -raise water through every 5 deg. be the same, that quantity of heat -would be sufficient to raise water from 32 deg. to 172 deg.: and hence -we infer, that as much heat is absorbed in the liquefaction of a -given quantity of ice as would raise the same quantity of water -through 140 degrees of the thermometric scale. - - -(52.) Let us now examine the analogous effects produced by the -continued application of heat to water in the liquid state. - -Let a small quantity of water be placed in a glass flask of -considerable size, and then closed so as to prevent the escape -[Pg103] of any vapour. Let this vessel be now placed over the -flame of a spirit lamp, so as to cause the water it contains to -boil. For a considerable time the water will be observed to boil, -and apparently to diminish in quantity, until at length all the -water disappears, and the vessel is apparently empty. If the -vessel be now removed from the lamp, and suspended in a cool -atmosphere, the whole of the interior of its surface will -presently appear to be covered with a dewy moisture; and at length -a quantity of water will collect in the bottom of it, equal to -that which had been in it at the commencement of the process. That -no water has at any period of the experiment escaped from it, may -be easily determined, by performing the experiment with the glass -flask suspended from the arm of a balance, counterpoised by a -sufficient weight suspended from the other arm. The equilibrium -will be preserved throughout, and the vessel will be found to have -the same weight, when to all appearance it is empty, as when it -contains the liquid water. It is evident, therefore, that the -water exists in the vessel in every stage of the process, but that -it becomes invisible when the process of boiling has continued for -a certain length of time; and it may be shown that it will -continue to be invisible, provided the flask be exposed to a -temperature considerably elevated. Thus, for example, if it be -suspended in a vessel of boiling water, the water which it -contains will continue to be invisible; but the moment it is -withdrawn from the boiling water, and exposed to the cold air, the -water will again become visible, as above mentioned, forming a dew -on the inner surface, and finally collecting in the bottom, as in -the commencement of the experiment. - -In fact, the liquid has, by the process of boiling, been converted -into _vapour_, or _steam_, which is a body similar in its leading -properties to common air, and, like it, is invisible. It will -hereafter appear that it likewise possesses the property of -elasticity, and other mechanical qualities enjoyed by gases in -general. - - -(53.) Again, let an open vessel be filled with water at 60 deg., and -placed in a mercurial bath, which is maintained, by a fire or lamp -applied to it, at the temperature of 230 deg. Place a thermometer -in the water, and it will be observed gradually to [Pg104] rise as -the temperature of the water is increased by the heat which it -receives from the mercury in which it is immersed. The water will -steadily rise in this manner until it attains the temperature of -212 deg.; but here the thermometer immersed in it will become -stationary. At the same time the water contained in the vessel -will become agitated, and its surface will present the same -appearance as if bubbles of air were rising from the bottom, and -issuing at the top. A cloudy vapour will be given off in large -quantities from its surface. This process is called _ebullition_ -or _boiling_. If it be continued for any considerable time, the -quantity of water in the vessel will be sensibly diminished; and -at length every particle of it will disappear, and the vessel will -remain empty. During the whole of this process, the thermometer -immersed in the water will remain stationary at 212 deg. - -Now, it will be asked, what has become of the water? It cannot be -imagined that it has been annihilated. We shall be able to answer -this by adopting means to prevent the escape of any particle of -matter from the vessel containing the water, into the atmosphere -or elsewhere. Let us suppose that the top of the vessel containing -the water is closed, with the exception of a neck communicating -with a tube, and let that tube be carried into another close -vessel removed from the cistern of heated mercury, and plunged in -another cistern of cold water. Such an apparatus is represented in -_fig._ 15. - -[Illustration: _Fig._ 15.] - -A is a cistern of heated mercury, in which the glass vessel B, -containing water, is immersed. From the top of the vessel B -proceeds a glass tube C, inclining downwards, and entering a glass -vessel D, which is immersed in a cistern E of cold water. If the -process already described be continued until the water by constant -ebullition has disappeared, as already mentioned, [Pg105] from -the vessel B, it will be found that a quantity of water will be -collected in the vessel D; and if this water be weighed, it will -be found to have exactly the same weight as the water had which -was originally placed in the vessel B. It is, therefore, quite -apparent that the water has passed by the process of boiling from -the one vessel to the other; but, in its passage, it was not -perceptible by the sight. The tube C and the upper part of the -vessel B, had the same appearance, exactly, as if they had been -filled with atmospheric air. That they are not merely filled with -atmospheric air may, however, be easily proved. When the process -of boiling first commences, it will be found that the tube C is -cold, and the inner surface dry. When the process of ebullition -has continued a short time, the tube C will become gradually -heated, and the inner surface of it covered with moisture. After a -time, however, this moisture disappears, and the tube attains the -temperature 212 deg. In this state it continues until the whole of -the water is discharged from the vessel B to the vessel D. - - -(54.) These effects are easily explained. The water in the vessel -B is incapable of receiving any higher temperature than 212 deg., -consistently with its retaining the liquid form. Small portions, -therefore, are constantly converted into steam by the heat -received from the surrounding mercury, and bubbles of steam are -formed on the bottom and sides of the vessel B. These bubbles, -being very much lighter, bulk for bulk, than water, rise rapidly -through the water, just in the same manner as bubbles of air -would, and produce that peculiar agitation at its surface which -has been taken as the external indication of boiling. They escape -from the surface, and collect in the upper part of the vessel. The -steam thus collected, when it first enters the tube C, is cooled -below the temperature of 212 deg. by the surface of the tube; and -consequently, being incapable of remaining in the state of vapour -at any lower temperature than 212 deg., it is reconverted into water, -and forms the dewy moisture which is observed in the commencement -of the process on the interior of the tube C. At length, however, -the whole of the tube C is heated to the temperature of 212 deg., and -the moisture which was previously collected upon its inner -[Pg106] surface is again converted into steam. As the quantity of -steam evolved from the water in B increases, it drives before it -the steam previously collected in the tube C, and forces it into -the vessel B. Here it encounters the inner surface of this vessel, -which is kept constantly cold by being surrounded with the cold -water in which it is immersed; and the vapour, being thus -immediately reduced below the temperature of 212 deg., is reconverted -into water. At first it collects in a dew on the surface of the -vessel D; but as this accumulates, it drops into the bottom of the -vessel, and forms a more considerable quantity. As the quantity of -water is observed to be gradually diminished in the vessel B, the -quantity will be found to be gradually increased in the vessel D; -and if the operation be suspended at any stage of the process, and -the water in the two vessels weighed, it will be found that the -weight of the water in D is exactly equal to the weight which the -water in B has lost. - - -(55.) The demonstration is, therefore, perfect, that the gradual -diminution of the boiling water in the vessel B is produced by the -conversion of that water into steam by the heat. In the process -first described, when the top of the vessel B was supposed to be -open, this steam made its escape into the air, where it was first -dispersed, and subsequently cooled in separate particles, and was -deposited in minute globules of moisture on the ground and on -surrounding objects. - - -(56.) In reviewing this process, we are struck by the fact, that -the continued application of heat to the vessel B is incapable of -raising the temperature of the water contained in it above 212 deg. -This presents an obvious analogy to the process of liquefaction, -and leads to inquiries of a similar nature, which are attended -with a like result. We must either infer, that the water, having -arrived at 212 deg., received no more heat from the mercury; or that -such heat, if received, is incapable of affecting the thermometer; -or, finally, that the steam which passes off carries this heat -with it. That the water receive heat from the mercury, will be -proved by the fact, that, if the vessel B be removed from the -mercury, other things remaining as before, the temperature of the -mercury will rapidly rise, and if the fire be continued, it will -even boil; but so long as the [Pg107] vessel B remains immersed, -it prevents the mercury from increasing in temperature. It -therefore receives that heat which would otherwise raise the -temperature of the quicksilver. - -[Illustration: _Fig._ 16.] - -If a thermometer be immersed in the steam which collects in the -upper part of the vessel B, it will show the same temperature (of -212 deg.) as the water from which it is raised. The heat, therefore, -received from the mercury, is clearly not imparted in a sensible -form to the steam, which has the same temperature in the form of -steam as it had in the form of water. What has been already -explained respecting liquefaction would lead us, by analogy, to -suspect that the heat imparted by the mercury to the water has -become latent in the steam, and is instrumental to the conversion -of water into steam, in the same manner as heat has been shown to -be instrumental to the conversion of ice into water. As the fact -was in that case detected by mixing ice with water, so we shall, -in the present instance, try it by a like test, viz. by mixing -water with steam. Let about five ounces and a half of water, at -the temperature of 32 deg., be placed in a vessel A (_fig._ 16.), and -let another vessel B, in which water is kept constantly boiling at -the temperature of 212 deg., communicate with A by a pipe C proceeding -from the top, so that the steam may be conducted from B, and -escape from the mouth of the pipe at some depth below the surface -of the water in A. As the steam issues from the pipe, it will be -immediately reconverted into water by the cold water which it -encounters; and, by continuing this process, the water in A will -be gradually heated by the steam combined with it and received -through the pipe C. If this process be continued until the water -in A is raised to the temperature of 212 deg., it will boil. Let it -then be weighed, and it will be found to weigh six ounces and a -half: from whence we infer, that one ounce of water has been -received from the vessel B in the form of steam, and has been -reconverted into water by the inferior temperature of the water in -A. Now, this ounce of water received in the form of steam into the -vessel A had, when in that form, the temperature of 212 deg. It is -now [Pg108] converted into the liquid form, and still retains the -same temperature of 212 deg.; but it has caused the five ounces and -a half of water with which it has been mixed, to rise from the -temperature of 32 deg. to the temperature of 212 deg.,--and this, -_without losing any temperature itself_. It follows, therefore, that, -in returning to the liquid state, it has parted with as much heat as -is capable of raising five times and a half its own weight of -water from 32 deg. to 212 deg. This heat was combined with the -steam, though not sensible to the thermometer; and was, therefore, -_latent_. Had it been sensible in the water in B, it would have -caused the water to have risen through a number of thermometric -degrees, amounting to five times and a half the excess of 212 deg. -above 32 deg.; that is, through five times and a half 180 deg.; for it -has caused five times and a half its own weight of water to receive an -equal increase of temperature. But five times and a half 180 deg. is -990 deg., or, to use round numbers (for minute accuracy is not here -our object), 1000 deg. It follows, therefore, that an ounce of water, -in passing from the liquid state at 212 deg. to the state of steam at -212 deg., receives as much heat as would be sufficient to raise it -through 1000 thermometric degrees, if that heat, instead of -becoming latent, had been sensible. - - -(57.) In order to derive all the knowledge from these experiments -which they are capable of imparting, it will be necessary to -examine very carefully how water comports itself under a variety -of different circumstances. - -If water be boiled in an open vessel, with a thermometer immersed, -on different days, it will be observed that the fixed temperature -which it assumes in boiling will be subject to a variation within -certain small limits. Thus, at one time, it will be found to boil -at the temperature of 210 deg.; while, at others, the thermometer -immersed in it will rise to 213 deg.; and, on different occasions, it -will fix itself at different points within these limits. It will -also be found, if the same experiment be performed at the same -time in distant places, that the boiling points will be subject to -a like variation. Now, it is natural to inquire what cause -produces this variation; and we shall be led to the discovery of -the cause, by examining what other physical effects undergo a -simultaneous change. [Pg109] - -If we observe the height of the barometer at the time of making -each experiment, we shall find a very remarkable correspondence -between it and the boiling temperature. Invariably, whenever the -barometer stands at the same height, the boiling temperature will -be the same. Thus, if the barometer stands at 30 inches, the -boiling temperature will be 212 deg. If the barometer fall to 29-1/2 -inches, the thermometer stands at a small fraction above 211 deg. If -the barometer rise to 30-1/2 inches, the boiling temperature rises -to nearly 213 deg. The variation in the boiling temperature is, then, -accompanied by a variation in the pressure of the atmosphere -indicated by the barometer; and it is constantly found that the -boiling point will remain unchanged, so long as the atmospheric -pressure remains unchanged, and that every increase in the one -causes a corresponding increase in the other. - - -(58.) From these facts it must be inferred, that the pressure -excited on the surface of the water has a tendency to resist its -ebullition, and to make it necessary, before it can boil, that it -should receive a higher temperature; and, on the contrary, that -every diminution of pressure on the surface of the water will give -an increased facility to the process of ebullition, or will cause -that process to take place at a lower temperature. As these facts -are of the utmost importance in the theory of heat, it may be -useful to verify them by direct experiment. - -If the variable pressure excited on the surface of the water by -the atmosphere be the cause of the change in the boiling -temperature, it must happen, that any change of pressure produced -by artificial means on the surface of the water must likewise -change the boiling point, according to the same law. Thus, if a -pressure considerably greater than the atmospheric pressure be -excited on a liquid, the boiling point may be expected to rise -considerably above 212 deg.; and, on the other hand, if the surface -of the water be relieved from the pressure of the atmosphere, and be -submitted to a considerably diminished pressure, the water would -boil below 212 deg. - -[Illustration: _Fig._ 17.] - -Let B (_fig._ 17.) be a strong spherical vessel of brass, supported -on a stand S, under which is placed a large spirit lamp L, or other -means of heating it. In the top of this vessel are three apertures, -in two of which are screwed a [Pg110] thermometer T, the bulb of -which enters the hollow brass sphere, and a stop-cock C, which may -be closed or opened at pleasure, to confine the steam, or allow it -to escape. In the third aperture at the top, is screwed a long -barometer tube, open at both ends. The lower end of this tube -extends nearly to the bottom of the spherical vessel B. In the -bottom of this vessel is placed a quantity of mercury, the surface -of which rises to some height above the lower end of the tube A. -Over the mercury is poured a quantity of water, so as to half fill -the vessel B. Matters being thus arranged, the screws are made -tight, so as to confine the water, and the lamp is allowed to act on -the vessel; the temperature of the water is raised, and steam is -produced, which, being confined within the vessel, exerts its -pressure on the surface of the water, and resists its ebullition. -The pressure of the steam acting on the surface of the water is -communicated to the surface of the mercury, and it forces a portion -of the mercury into the tube A, which presently rises above the -point where the tube is screwed into the top of the vessel B. As the -action of the lamp continues, the thermometer T exhibits a gradually -increasing temperature; while the column of mercury in A shows the -force with which the steam presses on the surface of the water in -B,--this column being balanced by the pressure of the steam. Thus, -the temperature and pressure of the steam at the same moment may -always be observed by inspecting the thermometer T and the tube A. -When the column in the tube A has risen to the height of 30 inches -above the level of the mercury in the vessel B, then the pressure of -the steam will be equivalent to double the pressure of the -atmosphere, because, the tube A being open at the top, the -atmosphere presses on the [Pg111] surface of the mercury in it. The -thermometer T will be observed gradually to rise until it attains -the temperature of 212 deg.; but it will not stop there, as it would do -if immersed in water boiled in an open vessel. It will, on the other -hand, continue to rise; and when the column of mercury in A has -attained the height of 30 inches, the thermometer T will have risen -to 251 deg.,--being 39 deg. above the ordinary boiling point. - -During the whole of this process, the surface of the water being -submitted to a constantly increasing pressure, its ebullition is -prevented, and it continues to receive heat without boiling. That -it is the increased pressure which resists its ebullition, and -causes it to receive a temperature above 212 deg., may be easily -shown. Let the stop-cock C be opened; immediately the steam in B, -having a pressure considerably greater than that of the -atmosphere, will rush out, and will continue to issue from C, -until its pressure is balanced by the atmosphere. At the same time -the column of mercury in A will be observed rapidly to fall, and -to sink below the orifice by which it is inserted in the vessel B. -The thermometer T will also fall until it attains the temperature -of 212 deg. At that point, however, it will remain stationary; and -the water will now be distinctly heard to be in a state of rapid -ebullition. If the stop-cock C be once more closed, the -thermometer will begin to rise, and the column of mercury -ascending in A will be again visible. - -If, instead of a stop-cock being at C, the aperture were made to -communicate with a valve, like the safety-valve of a steam engine, -loaded with a certain weight,--say at the rate of 15 lbs. on the -square inch,--then the thermometer T, and the mercury in the tube -A, would not rise indefinitely as before. The thermometer would -continue to rise till it attained the temperature of 251 deg.; and -the mercury in the tube A would rise to the height of 30 inches. At -this limit the resistance of the valve would be balanced by the -pressure of the steam; and as fast as the water would have a -tendency to produce steam of a higher pressure, the valve would be -raised and the steam suffered to escape; the thermometer T and the -column of mercury in A remaining stationary during this process. -If the valve were loaded more heavily, the phenomena would be -[Pg112] the same, only that the mercury in T and A would become -stationary at certain heights. But, on the other hand, if the -valve were loaded at a less pressure than 15 lbs. on the square -inch, then the mercury in the two tubes would become stationary at -lower points. - - -(59.) These experiments show that every increase of pressure above -the ordinary pressure of the atmosphere causes an increase in the -temperature at which water boils. We shall now inquire whether a -diminution of pressure will produce a corresponding effect on the -boiling point. - -This may be easily accomplished by the aid of an air pump. Let -water at the temperature of 200 deg. be placed in a glass vessel -under the receiver of an air pump, and let the air be gradually -withdrawn. After a few strokes of the pump, the water will boil; -and if the mercurial gauge of the pump be observed, it will be -found that its altitude will be about 23-1/2 inches. Thus the -pressure to which the water is submitted has been reduced from the -ordinary pressure of the atmosphere expressed by the column of 30 -inches of mercury, to a diminished pressure expressed by 23-1/2 -inches; and we find that the temperature at which the water boils -has been lowered from 212 deg. to 200 deg. Let the same experiment -be repeated with water at the temperature of 180 deg., and it will -be found that a further rarefaction of the air is necessary, but -the water will at length boil. If the gauge of the pump be now -observed, it will be found to stand at about fifteen inches, -showing, that at the temperature of 180 deg. water will boil under -half the ordinary pressure of the atmosphere. These experiments -may be varied and repeated; and it will be always found, that, as -the pressure is diminished or increased, the temperature at which -the water will boil will be also diminished or increased. - - -(60.) The same effects may be exhibited in a striking manner -without an air pump, by producing a vacuum by the condensation of -steam. Let a small quantity of water be placed in a thin glass -flask, and let it be boiled by holding it over a spirit lamp. When -the steam is observed to issue abundantly from the mouth of the -flask, let it be quickly corked and removed from the lamp. The -process of boiling will then cease, and the water will become -quiescent; but if the flask be plunged [Pg113] in a vessel of -cold water, the water it contains will again pass into a state of -violent ebullition, thus exhibiting the singular fact of water -being boiled by cooling it. This effect is produced by the cold -medium in which the flask is immersed, causing the steam above the -surface of the water in it to be condensed, and therefore -relieving the water from its pressure. The water, under these -circumstances, boils at a lower temperature than when submitted to -the pressure of the uncondensed vapour. - - -(61.) There is no limit to the temperature to which water may be -raised, if it be submitted to a sufficient pressure to resist its -tendency to take the vaporous form. If a strong metallic vessel be -nearly filled with water, so as to prevent the liquid from -escaping by any force which it can exert, the water thus inclosed -may be heated to any temperature whatever without boiling; in -fact, it may be made red-hot; and the temperature to which it may -be raised will have no limit, except the strength of the vessel -containing it, or the point at which the metal of which it is -formed may begin to soften or to be fused. - - -(62.) The following table will show the temperature at which water -will boil under different pressures of the atmosphere corresponding -to the altitudes of the barometer between 26 and 31 inches. - - Barometer. Boiling Point. - 26 inches 204 deg..91 - 26.5 205 deg..79 - 27 206 deg..67 - 27.5 207 deg..55 - 28 208 deg..43 - 28.5 209 deg..31 - 29 210 deg..19 - 29.5 211 deg..07 - 30 212 deg. - 30.5 212 deg..88 - 31 213 deg..76 - -From this table it appears, that, for every tenth of an inch which -the barometric column varies between these limits, the boiling -temperature changes by the fraction of a degree expressed by the -decimal .176, or nearly by the vulgar fraction 1/6. - - -(63.) In the experiment already described, by which the latent -[Pg114] heat of steam was determined, the water was supposed to be -boiled under the ordinary pressure of the atmosphere. Having seen, -however, that water may boil at different temperatures, under -different pressures, the inquiry presents itself, whether the heat -absorbed in vaporisation at different temperatures, and under -different pressures, is subject to any variation? Experiments of -the same nature as those already described, instituted upon water -in a state of ebullition at different temperatures, as well below -as above 212 deg., have led to the discovery of a very remarkable fact -in the theory of vapour. It has been found that the heat absorbed -by vaporisation is always less, the higher the temperature at -which the ebullition takes place; and less, by the same amount as -the temperature of ebullition is increased. Thus, if water boil at -312 deg., the heat absorbed in ebullition will be less by 100 deg. -than if it boiled at 212 deg.; and again, if water be boiled under a -diminished pressure, at 112 deg., the heat absorbed in vaporisation -will be 100 deg. more than the heat absorbed by water boiled at 212 -deg. It follows, therefore, that the actual consumption of heat in the -process of vaporisation must be the same, whatever be the -temperature at which the vaporisation takes place; for whatever -heat is saved in the sensible form, is consumed in the latent -form, and _vice versa_. - -Let us suppose a given weight of water at the temperature of 32 deg. -to be exposed to any regular source by which heat may be supplied -to it. If it be under the ordinary atmospheric pressure, the first -180 deg. of heat which it receives will raise it to the boiling point, -and the next 1000 deg. will convert it into steam. Thus, in addition -to the heat which it contains at 32 deg., the steam at 212 deg. -contains 1180 deg. of heat. But if the same water be submitted to a -pressure equal to half the atmospheric pressure, then the first 148 -deg. of heat which it receives will cause it to boil, and the next 1032 -deg. will convert it into vapour. Thus, steam at the temperature of -180 deg. contains a quantity of heat more than the same quantity of -water at 32 deg., by 1032 deg. added to 148 deg., which gives a sum of -1180 deg. Steam, therefore, raised under the ordinary pressure of the -atmosphere at 212 deg., and steam raised under half that pressure at -180 deg., contain the same quantity of heat,--with this difference -[Pg115] only--that the one has more latent heat, and less sensible -heat, than the other. - -From this fact, that the sum of the latent and sensible heats of -the vapour of water is constant, it follows that the same quantity -of heat is necessary to convert a given weight of water into -steam, at whatever temperature, or under whatever pressure, the -water may be boiled. It follows, also, that, in the steam engine, -equal weights of high-pressure and low-pressure steam are produced -by the same consumption of fuel; and that, in general, the -consumption of fuel is proportional to the quantity of water -vaporised, whatever the pressure of the steam may be.[18] - - -(64.) Having explained the conditions under which, by supplying -heat to water, it is converted into steam, and, by abstracting -heat from steam, it may be reconverted into water, let us now -consider the mechanical force which is developed in these -phenomena. - -[Illustration: _Fig._ 18.] - -Let A B (_fig._ 18.) be a tube, or cylinder, the base of which is -equal to a square inch, and let a piston P move in it so as to be -steam-tight. Let it be supposed, that under this piston there is, -in the bottom of the cylinder, a cubic inch of water between the -bottom of the piston and the bottom of the tube; let the piston be -counterbalanced by a weight W acting over a pulley, which will be -just sufficient to counterpoise the weight of the piston, so as -leave no force tending to keep the piston down, except the force -of the atmosphere acting above it. Under the circumstances here -supposed, the piston being in contact with the water, and all air -being excluded, it will be pressed down by the weight of the -atmosphere, which we will suppose to be fifteen pounds, the -magnitude of the piston being a square inch. [Pg116] - -Now let the flame of a lamp be applied at the bottom of the tube; -the water under the piston having its temperature thereby -gradually raised, and being submitted to no pressure save that of -the atmosphere above the piston, it will begin to be converted -into steam when it has attained the temperature of 212 deg. According -as it is converted into steam, it will cause the piston to ascend -in the tube until all the water has been evaporated. If the tube -were constructed of sufficient length, the piston then would be -found to have risen to the height of about seventeen hundred -inches, or one hundred and forty-two feet; since, as has been -already explained, water passing into steam under the ordinary -pressure of the atmosphere undergoes an increase of bulk in the -proportion of about seventeen hundred to one. - -Now in this process, the air above the piston, which presses on it -with a force equal to fifteen pounds, has been raised one hundred -and forty-two feet. It appears, therefore, that, by the -evaporation of a cubic inch of water under a pressure equal to -fifteen pounds per square inch, a mechanical force of this amount -is developed. - -It is evident that fifteen pounds raised one hundred and forty-two -feet successively, is equivalent to one hundred and forty-two -times fifteen pounds raised one foot. Now, one hundred and -forty-two times fifteen is two thousand one hundred and thirty, -and therefore the force thus obtained is equal to two thousand one -hundred and thirty pounds raised one foot high. This being within -about 110 pounds of a ton, it may be stated, in round numbers, -that, by the evaporation of a cubic inch of water under these -circumstances, a force is obtained equal to that which would raise -a ton weight a foot high. - -The augmentation of volume which water undergoes in passing into -steam under the pressure here supposed, may be easily retained in -the memory, from the accidental circumstance that a cubic inch of -water is converted into a cubic foot of steam, very nearly. A -cubic foot contains one thousand seven hundred and twenty-eight -cubic inches,--which is little different from the proportion which -steam bears to water, when raised under the atmospheric pressure. -[Pg117] - - -(65.) It will, therefore, be an advantage to retain in memory the -following general facts:-- - -1. _A cubic inch of water evaporated under the ordinary -atmospheric pressure, is converted into a cubic foot of steam._ - -2. _A cubic inch of water evaporated under the atmospheric -pressure, gives a mechanical force equal to what would raise about -a ton weight a foot high._ - - -(66.) Let us, again, suppose the piston P (_fig._ 23.) to be -restored to its original position, with the liquid water beneath -it; and, in addition to the weight of the atmosphere which before -pressed it down, let us suppose another weight of fifteen pounds -laid upon it, so that the water below shall be pressed by double -the weight of the atmosphere. If the lamp were now applied, and at -the same time a thermometer were immersed in the water, it would -be found that the water would not begin to be converted into steam -until it attained the temperature of about 250 deg. The piston would -then begin, as before, to ascend, and the water to be gradually -converted into vapour. The water being completely evaporated, it -would be found that the piston would be raised to a height little -more than half its former height, or 72 feet. The mechanical -effect, therefore, thus obtained, will be equivalent to double the -former weight raised half the former height. - -In like manner, if the piston were loaded with thirty pounds in -addition to the atmosphere, the whole pressure on the water being -then three times the pressure first supposed, the piston would be -raised to somewhat more than one third of its first height by the -evaporation of the water. This would give a mechanical force -equivalent to three times the original weight raised a little more -than one third of the original height. - -In general, as the pressure on the piston is increased, the height -to which the piston would be raised by the evaporation of the -water will be diminished in a proportion somewhat less than the -proportion in which the pressure on the piston is increased. If -the temperature at which the water is converted into steam under -these different pressures were the same, then the height to which -the piston would be raised by the evaporation of the water would -be diminished in precisely [Pg118] the same proportion as the -pressure on the piston is increased; and, in that case, the whole -mechanical force developed by the evaporation of the water would -remain exactly the same under whatever pressure the water might be -boiled. We shall explain hereafter the extent to which the -variation of temperature in the water and steam corresponding to -the variation of pressure modifies this law; but, as the effect of -the difference of temperatures is not considerable, it will be -convenient to register in the memory the following important -practical conclusion:-- - - -(67.) _A cubic inch of water converted into steam will supply a -mechanical force very nearly equal to a ton weight raised a foot -high; and this force will not be subject to considerable -variation, whatever be the temperature or pressure at which the -water may be evaporated._ - -[Illustration: GLASGOW.] - - FOOTNOTES: - - [18] The preceding paragraphs, and some other parts of the - present volume on the general properties of Heat, are taken - from my Treatise on Heat, in the _Cabinet Cyclopoedia_, to - which those who desire more detailed explanation and more - copious illustration should refer. - -[Pg119] - - - - -[Illustration: GLASGOW COLLEGE.] - -CHAP. V. - - WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE - WITH A DUE ECONOMY OF FUEL. -- CONCEIVES THE NOTION OF - CONDENSING OUT OF THE CYLINDER. -- DISCOVERS SEPARATE - CONDENSATION. -- INVENTS THE AIR-PUMP. -- SUBSTITUTES STEAM - PRESSURE FOR ATMOSPHERIC PRESSURE. -- INVENTS THE STEAM CASE, - OR JACKET. -- HIS FIRST EXPERIMENTS TO REALISE THESE - INVENTIONS. -- HIS EXPERIMENTAL APPARATUS. -- DIFFICULTIES OF - BRINGING THE IMPROVED ENGINES INTO USE. -- WATT PRACTISES AS A - CIVIL ENGINEER. -- HIS PARTNERSHIP WITH ROEBUCK. -- HIS FIRST - PATENT. -- DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE. - - -(68.) At the period to which we have now brought the history of -the invention of the steam engine, Watt had obtained, chiefly by -his own experiments, a sufficient knowledge of the phenomena which -have been just explained, to enable him to arrive at the -conclusion that a very small proportion of the whole mechanical -effect attending the evaporation was really rendered available by -the atmospheric engine; and that, [Pg120] therefore, extensive -and injurious sources of waste existed in its machinery. - -He perceived that the principal source of this wasteful -expenditure of power consisted in the quantity of steam which was -condensed at each stroke of the piston, in heating the cylinder -previous to the ascent of the piston. Yet, as it was evident that -that ascent could not be accomplished in a cold cylinder, it was -apparent that this waste of power must be inevitable, unless some -expedient could be devised, by which _a vacuum could be maintained -in the cylinder, without cooling it_. But, to produce such a -vacuum, the steam must be condensed; and, to condense the steam, -its temperature must be lowered to such a point that the vapour -proceeding from it shall have no injurious pressure; yet, if -condensed steam be contained in a cylinder at a high temperature, -it will return to the temperature of the cylinder, recover its -elasticity, and resist the descent of the piston. - -Having reflected on these circumstances, it became apparent to -Watt, that a vice was inherent in the structure of the atmospheric -engine, which rendered a large waste of power inevitable; this -vice arising from the fact, that the condensation of the steam was -incompatible with the condition of maintaining the elevated -temperature of the cylinder in which that condensation took place. -It followed, therefore, either that the steam must be imperfectly -condensed, or that the condensation could not take place in the -cylinder. It was in 1765, that, pondering on these circumstances, -the happy idea occurred to him, that the production of a vacuum -could be equally effected, though _the place_ where the -condensation of the steam took place were not the cylinder itself. -He saw, that if a vessel in which a vacuum was produced were put -into communication with another containing an elastic fluid, the -elastic fluid would rush into the vacuum, and diffuse itself -through the two vessels; but if, on rushing into such vacuum, this -elastic fluid, being vapour, were there condensed, or restored to -the liquid form, that then the space within the two vessels would -be equally rendered a vacuum;--that, under such circumstances, one -of the vessels might be maintained at any temperature, however -high, while [Pg121] the other might be kept at any temperature, -however low. This felicitous conception formed the first step in -that splendid career of invention and discovery which has -conferred immortality on the name of Watt. He used to say, that -the moment the idea of separate condensation occurred to -him,--that is, of condensing, in one vessel kept cold, the steam -coming from another vessel kept hot,--all the details of his -improved engine rushed into his mind in such rapid succession, -that, in the course of a day, his invention was so complete that -he proceeded to submit it to experiment. - -[Illustration: _Fig._ 19.] - -To explain the first conception of this memorable invention; let a -tube or pipe, S (_fig._ 19.), be imagined to proceed from the -bottom of the cylinder A B to a vessel, C, having a stop-cock, D, -by which the communication between the cylinder and the vessel C -may be opened or closed at pleasure. If we suppose the piston P at -the top of the cylinder, and the space below it filled with steam, -the cylinder and steam being at the usual temperature, while the -vessel C is a vacuum, and maintained at a low temperature. Then, -on opening the cock D, the steam will rush from the cylinder A B -through the tube S, and, passing into the cold vessel C, will be -condensed by contact with its cold sides. This process of -condensation will be rendered instantaneous if a jet of cold water -is allowed to play in the vessel C. When the steam thus rushing -into C, has been destroyed, and the space in the cylinder A B -becomes a vacuum, then the pressure of the atmosphere being -unobstructed, the piston will descend with the force due to the -excess of the pressure of the atmosphere above the friction. When -it has descended, suppose the stop-cock D closed, and steam -admitted from [Pg122] the boiler through a proper cock or valve -below the piston, the cylinder and piston being still at the same -temperature as before. The steam on entering the cylinder, not -being exposed to contact with any surface below its own -temperature, will not be condensed, and therefore will immediately -cause the piston to rise, and the piston will have attained the -top of the cylinder when as much steam shall have been supplied by -the boiler as will fill the cylinder. When this has taken place, -suppose the communication with the boiler cut off, and the cock D -once more opened: the steam will again rush through the pipe S -into the vessel C, where encountering the cold surface and the jet -of cold water, it will be condensed, and the vacuum, as before, -will be produced in the cylinder A B; that cylinder still -maintaining its temperature, the piston will again descend, and so -the process may be continued. - - -(69.) Having carried the invention to this point, Watt saw that -the vessel C would gradually become heated by the steam which -would be continually condensed in it. To prevent this, as well as -to supply a constant jet of cold water, he proposed to keep the -vessel C submerged in a cistern of cold water, from which a pipe -should conduct a jet to play within the vessel, so as to condense -the steam as it would pass from the cylinder. - -But here a difficulty presented itself, against which it was -necessary to provide. The cold water admitted through the jet to -condense the steam, mixed with the condensed steam itself, would -gradually collect in the vessel C, and at length choke it. To -prevent this, Watt proposed to put the vessel C in communication -with a pump F, which might be wrought by the engine itself, and by -which the water, which would collect in the bottom of the vessel -C, would be constantly drawn off. This pump would be evidently -rendered the more necessary, since more or less atmospheric air, -always combined with water in its common state, would enter the -vessel C by the condensing jet. This air would be disengaged in -the vessel C by the heat of the steam condensed therein; and it -would rise through the tube S, and vitiate the vacuum in the -cylinder;--an effect which would be rendered the more injurious, -[Pg123] inasmuch as, unlike steam, this elastic fluid would be -incapable of being condensed by cold. The pump F, therefore, by -which Watt proposed to draw off the water from the vessel C, might -also be made to draw off the air, or the principal part of it. - -The vessel C was subsequently called a _condenser_; and, from the -circumstances just adverted to, the pump F has been called the -_air-pump_. - -These--namely, the cylinder, the condenser, and the air-pump--were -the three principal parts in the invention, as it first presented -itself to the mind of Watt--and even before it was reduced to a -model, or submitted to experiment. But, in addition to these, -other two improvements offered themselves in the very first stage -of its progress. - -In the atmospheric engine, the piston was maintained steam-tight -in the cylinder by supplying a stream of cold water above it, by -which the small interstices between the piston and cylinder would -be stopped. It is evident that the effect of this water as the -piston descended would be to cool the cylinder, besides which any -portion of it which might pass between the piston and cylinder and -which would pass below the piston, would boil the moment it would -fall into the cylinder, which itself would be maintained at the -boiling temperature. This water, therefore, would produce steam, -the pressure of which would resist the descent of the piston. - -Watt perceived, that even though this inconvenience were removed -by the use of oil or tallow upon the piston, still, that as the -piston would descend in the cylinder, the cold atmosphere would -follow it; and would, to a certain extent, lower the temperature -of the cylinder. On the next ascent of the piston, this -temperature would have to be again raised to 212 deg. by the steam -coming from the boiler, and would entail upon the machine a -proportionate waste of power. - -If the atmosphere of the engine-house could be kept heated to the -temperature of boiling water, this inconvenience would be removed. -The piston would then be pressed down by air as hot as the steam -to be subsequently introduced into it. On further consideration, -however, it occurred to Watt that it would be still more -advantageous if the cylinder itself could be [Pg124] worked in an -atmosphere of steam, having only the same pressure as the -atmosphere. Such steam would press the piston down as effectually -as the air would; and it would have the further advantage over -air, that if any portion of it leaked through between the piston -and cylinder, it would be condensed, which could not be the case -with atmospheric air. He therefore determined on surrounding the -cylinder by an external casing, the space between which and the -cylinder he proposed to be filled with steam supplied from the -boiler. The cylinder would thus be enclosed in an atmosphere of -its own, independent of the external air, and the vessel so -enclosing it would only require to be a little larger than the -cylinder, and to have a close cover at the top, the centre of -which might be perforated with a hole to admit the rod of the -piston to pass through, the rod being made smooth, and so fitted -to the perforation that no steam should escape between them. This -method would be attended also with the advantage of keeping the -cylinder and piston always heated, not only inside but outside; -and Watt saw that it would be further advantageous to employ the -pressure of steam to drive the piston in its descent instead of -the atmosphere, as its intensity or force would be much more -manageable; for, by increasing or diminishing the heat of the -steam in which the cylinder was enclosed, its pressure might be -regulated at pleasure, and it might be made to urge the piston -with any force that might be required. The power of the engine -would therefore be completely under control, and independent of -all variations in the pressure of the atmosphere. - - -(70.) This was a step which totally changed the character of the -machine, and which rendered it a STEAM ENGINE instead of an -ATMOSPHERIC ENGINE. Not only was the vacuum below the piston now -produced by the property of steam, in virtue of which it is -reconverted into water by cold; but the pressure which urged the -piston into this vacuum was due to the elasticity of steam. - -The external cylinder, within which the working cylinder was -enclosed, was called THE JACKET, and is still very generally used. - -[Illustration: _Fig._ 20.] - - -(71.) The first experiment in which Watt attempted to [Pg125] -realise, on a small scale, his conceptions, was made in the -following manner. The cylinder of the engine was represented by a -brass syringe A B (_fig._ 20.) an inch and a third in diameter, and -ten inches in length, to which a top and a bottom of tin plate was -fitted. Steam was conveyed by a pipe, S, from a small boiler into -the lower end of this syringe, a communication being made with the -upper end of the syringe by a branch pipe D. For the greater -convenience of the experiment, it was found desirable to invert the -position of the cylinder, so that the steam should press the piston -P upwards instead of downwards. The piston-rod R therefore was -presented downwards. An eduction pipe E was also inserted in the top -of the cylinder, which was carried to the condenser. The piston-rod -was made hollow, or rather a hole was drilled longitudinally through -it, and a valve was fitted at its lower end, to carry off the water -produced by the steam, which [Pg126] would be condensed in the -cylinder in the commencement of the process. The condenser used in -this experiment operated without injection, the steam being -condensed by the contact of cold surfaces. It consisted of two thin -pipes F, G of tin, ten or twelve inches in length, and the sixth of -an inch in diameter, standing beside each other perpendicularly, and -communicating at the top with the eduction pipe, which was provided -with a valve opening upwards. At the bottom these two pipes -communicated with another tube I of about an inch in diameter, by a -horizontal pipe, having in it a valve, M, opening towards I, fitted -with a piston K, which served the office of the air-pump, being -worked by the hand. This piston, K, had valves in it opening -upwards. These condensing pipes and air-pump were immersed in a -small cistern, filled with cold water. The steam was conveyed by the -steam-pipe S to the bottom of the cylinder, a communication between -the top and bottom of the cylinder being occasionally opened by a -cock, C, placed in the branch pipe. The eduction pipe leading to the -condenser also had a cock, L, by which the communication between the -top of the cylinder and the condenser might be opened and closed at -pleasure. In the commencement of the operation, the cock N admitting -steam from the boiler, and the cock L opening a communication -between the cylinder and the condenser, and the cock C opening a -communication between the top and bottom of the cylinder, being all -open, steam rushed from the boiler, passing through all the pipes, -and filling the cylinder. A current of mixed air and steam was thus -produced through the eduction pipe E, through the condensing pipes F -and G, and through the air-pump I, which issued from the valve H in -the eduction pipe, and from the valve in the air-pump piston, all of -which opened upwards. The steam also in the cylinder passed through -the hole drilled in the piston-rod, and escaped, mixed with air, -through the valve in the lower end of that rod. This process was -continued until all the air in the cylinder, pipes, and condenser, -was blown out, and all these spaces filled with pure steam. The -cocks L, C, and N, were then closed, and the atmospheric pressure -closed the valve H and the valves in the air-pump piston. The cold -surfaces condensing the steam in [Pg127] the pipes F and G, and in -the lower part of the air-pump, a vacuum was produced in these -spaces. The cock C being now closed, and the cocks L and N being -open, the steam in the upper part of the cylinder rushed through the -pipe E into the condenser, where it was reduced to water, so that a -vacuum was left in the upper part of the cylinder. The steam from -the boiler passing below the piston, pressed it upwards with such -force, that it lifted a weight of eighteen pounds hung from the end -of the piston-rod. When the piston reached the top of the cylinder, -the cocks L and N were closed, and the cock C opened. All -communication between the cylinder and the boiler, as well as -between the cylinder and the condenser, were now cut off, and the -steam in the cylinder circulated freely above and below the piston, -by means of the open tube D. The piston, being subject to equal -forces upwards and downwards, would therefore descend by its own -weight, and would reach the bottom of the cylinder. The air-pump -piston meanwhile being drawn up, the air and the condensed steam in -the tubes F and G were drawn into the air-pump I, through the open -horizontal tube at the bottom. Its return was stopped by the valve -M. By another stroke of the air-pump, this water and air were drawn -out through valves in the piston, which opened upwards. The cock C -was now closed, and the cocks L and N opened, preparatory to another -stroke of the piston. The steam in the upper part of the cylinder -rushed, as before, into the tubes F and G, and was condensed by -their cold surfaces, while steam from the boiler coming through the -pipe S, pressed the piston upwards. The piston again ascended with -the same force as before, and in the same manner the process was -continually repeated. - - -(72.) The quantity of steam expended in this experimental model in -the production of a given number of strokes of the piston was -inferred from the quantity of water evaporated in the boiler; and -on comparing this with the magnitude of the cylinder and the -weight raised by the pressure of the steam, the contrivance was -proved to affect the economy of steam, as far as the imperfect -conditions of such a model could have permitted. A larger model -was next constructed, having an outer cylinder, or steam case, -surrounding the working cylinder, and [Pg128] the experiments -made with it fully realised Watt's expectations, and left no doubt -of the great advantages which would attend his invention. The -weights raised by the piston proved that the vacuum in the -cylinder produced by the condensation was almost perfect; and he -found that when he used water in the boiler which by long boiling -had been well cleared of air, the weight raised was not much less -than the whole amount of the pressure of the steam upon the -piston. In this larger model, the cylinder was placed in the usual -position, with a working lever and other apparatus similar to that -employed in the Atmospheric Engine. - - -(73.) It was in the beginning of the year 1765, Watt being then in -the twenty-ninth year of his age, that he arrived at these great -discoveries. The experimental models just described, by which his -invention was first reduced to a rude practical test, were fitted -up at a place called Delft House, in Glasgow. It will doubtless, -at the first view, be a matter of surprise that improvements of -such obvious importance in the economy of steam power, and capable -of being verified by tests so simple, were not immediately adopted -wherever atmospheric engines were used. At the time, however, -referred to, Watt was an obscure artisan, in a provincial town, -not then arrived at the celebrity to which it has since attained, -and the facilities by which inventions and improvements became -public were much less than they have since become. It should also -be considered that all great and sudden advances in the useful -arts are necessarily opposed by the existing interests with which -their effects are in conflict. From these causes of opposition, -accompanied with the usual influence of prejudice and envy, Watt -was not exempt, and was not therefore likely suddenly to -revolutionise the arts and manufactures of the country by -displacing the moving powers employed in them, and substituting an -engine, the efficacy and power of which depended mainly on -physical principles, then altogether new and but imperfectly -understood. - -Not having the command of capital, and finding it impracticable to -inspire those who had, with the same confidence in the advantages -of his invention which he himself felt, he was [Pg129] unable to -take any step towards the construction of engines on a large -scale. Soon after this, he gave up his shop in Glasgow, and -devoted himself to the business of a Civil Engineer. In this -capacity he was engaged to make a survey of the river Clyde, and -furnished an elaborate and valuable Report upon its projected -improvements. He was also engaged in making a plan of the canal, -by which the produce of the Monkland Colliery was intended to be -carried to Glasgow, and in superintending the execution of that -work. Besides these, several other engineering enterprises -occupied his attention, among which may be mentioned, the -navigable canal across the isthmus of Crinan, afterwards completed -by Rennie; improvements proposed in the ports of Ayr, Glasgow, and -Greenock; the construction of the bridges at Hamilton, and at -Rutherglen; and the survey of the country through which the -celebrated Caledonian canal was intended to be carried. - -"If, forgetful of my duties as the organ of this academy," says M. -Arago (whose eloquent observations on the delays of this great -invention, addressed to the assembled members of the National -Institute of France, we cannot forbear to quote), "I could think -of making you smile, rather than expressing useful truths, I would -find here matter for a ludicrous contrast. I would call to your -recollection the authors, who at our weekly sittings demand with -all their might and main (_a cor et a cris_) an opportunity to -communicate some little remark--some small reflection--some -trifling note, conceived and written the night before; I would -represent them to you cursing their fate, when according to your -rules, the reading of their communication is postponed to the next -meeting, although during this cruel week, they are assured that -their important communication is deposited in our archives in a -sealed packet. On the other hand, I would point out to you the -creator of a machine, destined to form an epoch in the annals of -the world, undergoing patiently and without murmur, the stupid -contempt of capitalists,--conscious of his exalted genius, yet -stooping for eight years to the common labour of laying down -plans, taking levels, and all the tedious calculations connected -with the routine of common engineering. While in this conduct you -cannot fail to recognise the serenity, [Pg130] the moderation, -and the true modesty of his character, yet such indifference, -however noble may have been its causes, has something in it not -altogether blameless. It is not without reason that society visits -with severe reprobation those who withdraw gold from circulation -and hoard it in their coffers. Is he less culpable who deprives -his country, his fellow citizens, his age, of treasures a thousand -times more precious than the produce of the mine; who keeps to -himself his immortal inventions, sources of the most noble and -purest enjoyment of the mind, who abstains from conferring upon -labour those powers, by which would be multiplied in an infinite -proportion the products of industry, and by which, with advantage -to civilisation and human nature, he would smooth away the -inequalities of the conditions of man."[19] - - -(74.) Although Watt was thus attracted by pursuits foreign to his -recent investigations respecting the improvement of steam power, -he never lost sight of that object. It was not until the year -1768, three years after his great discoveries, that any step was -taken to enable him to carry them into effect on a large scale. At -that time his friends brought him into communication with Dr. -Roebuck, the proprietor of the Carron Iron Works, who rented -extensive coal works at Kinneal from the Duchess of Hamilton. Watt -was first employed by Roebuck as a civil engineer; but when he -made known to him the improvements he had projected in the steam -engine, Roebuck proposed to take out a patent for an engine on the -principle of the model which had been fitted up at Delft House, -and to join Watt in a partnership, for the construction of such -engines. Sensible of the advantages to be derived from the -influence of Roebuck, and from his command of capital, Watt agreed -to cede to him two thirds of the advantages to be derived from the -invention. A patent was accordingly taken out on the fifth of -January, 1769, nearly four years after the invention had been -completed; and an experimental engine on a large scale was -constructed by him, and fitted up at Kinneal House. In the first -trial this machine more than fulfilled Watt's anticipations. Its -[Pg131] success was complete. In the practical details of its -construction, however, some difficulties were still encountered, -the greatest of which consisted in packing the piston, so as to be -steam-tight. The principle of the new engine did not admit of -water being kept upon the piston, to prevent leakage, as in the -old engines; he was therefore obliged to have his cylinders much -more accurately bored, and more truly cylindrical, and to try a -great variety of soft substances for packing the piston, which -would make it steam-tight without great friction, and maintain it -so in a situation perfectly dry, and at the temperature of boiling -water. - -While Watt was endeavouring to overcome these and other -difficulties, in the construction of the machine, his partner, Dr. -Roebuck, became embarrassed, by the failure of his undertaking in -the Borrowstowness coal and salt works; and he was unable to -supply the means of prosecuting with the necessary vigour the -projected manufacture of the new engines. - -The important results of Watt's labours having happily at this -time become more publicly known, Mr. Matthew Boulton, whose -establishment at Soho, near Birmingham, was at that time the most -complete manufactory for metal-work in England, and conducted with -unexampled enterprise and spirit, proposed to purchase Dr. -Roebuck's interest in the patent. This arrangement was effected in -the year 1773, and in the following year Mr. Watt removed to Soho, -where a portion of the establishment was allotted to him, for the -erection of a foundery, and other works necessary to realise his -inventions on a grand scale. - -The patent which had been granted in 1769 was limited to a period -of fourteen years, and would consequently expire about the year -1783. From the small progress which had hitherto been made in the -construction of engines upon the new principle, and from the many -difficulties still to be encountered, and the large expenditure of -capital which must obviously be incurred before any return could -be obtained, it was apparent that unless an extension of the -patent right could be obtained, Boulton and Watt could never -expect any advantage adequate to the risk of their great [Pg132] -enterprise. In the year 1774 an application was accordingly made -to parliament for an extension of the patent, which was supported -by the testimony of Dr. Roebuck, Mr. Boulton, and others, as to -the merits and probable utility of the invention. An Act was -accordingly passed, in 1775, extending the term of the patent -until the year 1800. - - -(75.) The following abstract of this Act may not be uninteresting -at this time, when the anticipations expressed in it have been so -successfully and extensively realised:-- - -"An Act for vesting in James Watt, engineer, his executors, -administrators, and assigns, the sole use and property of certain -steam engines, commonly called fire engines, of his invention, -throughout his majesty's dominions, for a limited time: - -"And whereas the said James Watt hath employed many years, and a -considerable part of his fortune, in making experiments upon steam -engines, commonly called fire engines, with a view to improve -those very useful machines, by which several very considerable -advantages over the common steam engines are acquired; but upon -account of the many difficulties which always arise in the -execution of such large and complex machines, and of the long time -requisite to make the necessary trials, he could not complete his -intention before the end of the year 1774, when he finished some -large engines as specimens of his construction, which have -succeeded, so as to demonstrate the utility of the said invention: - -"And whereas, in order to manufacture these engines with the -necessary accuracy, and so that they may be sold at moderate -prices, a considerable sum of money must be previously expended in -erecting mills and other apparatus; and as several years and -repeated proofs will be required before any considerable part of -the public can be fully convinced of the utility of the invention, -and of their interest to adopt the same, the whole term granted by -the said letters patent may probably elapse before the said James -Watt can receive an advantage adequate to his labour and -invention: - -"And whereas, by furnishing mechanical power at much less expense, -and in more convenient forms, than has hitherto been done, his -engines may be of great utility, in facilitating [Pg133] the -operations in many great works and manufactures of this kingdom; -yet it will not be in the power of the said James Watt to carry -his invention into that complete execution which he wishes, and so -as to render the same of the highest utility to the public of -which it is capable, unless the term granted by the said letters -patent be prolonged, and his property in the said invention -secured for such time as may enable him to obtain an adequate -recompense for his labour, time, and expense: - -"To the end, therefore, that the said James Watt may be enabled -and encouraged to prosecute and complete his said invention, so -that the public may reap all the advantages to be derived -therefrom in their fullest extent: it is enacted, - -"That from and after the passing of this Act, the sole privilege -and advantage of making, constructing, and selling the said -engines hereinbefore particularly described, within the kingdom of -Great Britain, and his majesty's colonies and plantations abroad, -shall be, and are hereby declared to be, vested in the said James -Watt, his executors, administrators, and assigns, for and during -the term of twenty-five years," &c. &c. - - -(76.) Thus protected and supported, Watt now directed the whole -vigour of his mind to perfect the practical details of his -invention, and the result was, the construction on a large scale -of the engine which has since been called his SINGLE ACTING STEAM -ENGINE. - -It is necessary to recollect, that notwithstanding the extensive and -various application of steam power in the arts and manufactures, at -the time to which our narrative has now reached, the steam engine -had never been employed for any other purpose save that of raising -water by working pumps. The motion, therefore, which was required -was merely an upward force, such as was necessary to elevate the -piston of a pump, loaded with the column of water which it raised. -The following then is a description of the improved engine of Watt, -by which such work was proposed to be performed:-- - -[Illustration: _Fig._ 21.] - -In the cylinder represented at C (_fig._ 21.), the piston P moves -steam-tight. It is closed at the top, and the piston [Pg134] rod, -being accurately turned, runs in a steam-tight collar, B, -furnished with a stuffing-box, and is constantly lubricated with -melted tallow. A funnel is screwed into the top of the cylinder, -through which, by opening a stop-cock, melted [Pg135] tallow is -permitted from time to time to fall upon the piston within the -cylinder, so as to lubricate it, and keep it steam-tight. Two -boxes, A A, called the upper and lower steam boxes, contain valves -by which steam from the boiler may be admitted and withdrawn. -These steam boxes are connected by a tube of communication T, and -they communicate with the cylinder at the top and bottom by short -tubes represented in the figure. The upper steam box A contains -one valve, by which a communication with the boiler may be opened -or closed at pleasure. The lower valve box contains two valves. -The lower valve I communicates with the tube T', leading to the -condenser D, which being opened or closed, a communication is made -or cut off at pleasure, between the cylinder C and the condenser -D. A second valve, or upper valve H, which is represented closed -in the figure, may be opened so as to make a free communication -between the cylinder C and the tube T, and by that means between -the cylinder C, below the piston and the space above the piston. -The condenser D is submerged in a cistern of cold water. At the -side there enters it a tube, E, governed by a cock, which being -opened or closed to any required extent, a jet of cold water may -be allowed to play in the condenser, and may be regulated or -stopped, at pleasure. This jet, when playing, throws the water -upwards in the condenser towards the mouth of the tube T', as -water issues from the rose of a watering pot. The tube S proceeds -from the boiler, and terminates in the steam box A, so that the -steam supplied from the boiler constantly fills that box. The -valve G is governed by levers, whose pivots are attached to the -framing of the engine, and is opened or closed at pleasure, by -raising or lowering the lever G'. The valve G, when open, will -therefore allow steam to pass from the boiler through the short -tube to the top of the piston, and this steam will also fill the -tube T. If the lower valve H be closed, its circulation beyond -that point will be stopped; but if the valve H be open, the valve -I being closed, then the steam will circulate equally in the -cylinder, above and below the piston. If the valve I be open, then -steam will rush through the tube T' into the condenser; but this -escape of the steam will be [Pg136] stopped, if the valve I be -closed. The valve H is worked by the lever H', and the valve I by -the lever I'. - -The valve G is called the upper steam valve, H the lower steam -valve, I the exhausting valve, and E the condensing valve. - -From the bottom of the condenser D proceeds a tube leading to the -air-pump, which is also submerged in the cistern of cold water. In -this tube is a valve M, which opens outwards from the condenser -towards the air-pump. In the piston of the air-pump N is a valve -which opens upwards. The piston-rod Q of the air-pump is attached -to a beam of wood called a plug frame, which is connected with the -working beam by a flexible chain playing on the small arch-head -immediately over the air-pump. From the top of the air-pump barrel -above the piston proceeds a pipe or passage leading to a small -cistern, B, called the hot well. The pipe which leads to this -well, is supplied with a valve, K, which opens outwards from the -air pump barrel towards the well. From the nature of its -construction, the valve M admits the flow of water from the -condenser towards the air-pump, but prevents its return; and, in -like manner, the valve K admits the flow of water from the upper -part of the air-pump barrel into the hot well B, but obstructs its -return. - -Let us now consider how these valves should be worked in order to -move the piston upwards and downwards with the necessary force. It -is in the first place necessary that all the air which fills the -cylinder, the tubes and the condenser shall be expelled. To -accomplish this it is only necessary to open at once the three -valves G, H, and I. The steam then rushing from the boiler through -the steam-pipe S, and the open valve G will pass into the cylinder -above the piston, will fill the tube T, pass through the lower -steam valve H, will fill the cylinder C below the piston, and will -pass through the open valve I into the condenser. If the valve E -be closed so that no jet shall play in the condenser, the steam -rushing into it will be partially condensed by the cold surfaces -to which it will be exposed; but if the boiler supply it through -the pipe S in sufficient abundance, it will rush with violence -through the cylinder and all the passages, and its pressure in -the [Pg137] condenser D, combined with that of the heated air -with which it is mixed, will open the valve M, and it will rush -through mixed with the air into the air-pump barrel N. It will -press the valves in the air-pump piston upwards, and, opening -them, will rush through, and will collect in the air-pump barrel -above the piston. It will then, by its pressure, open the valve K, -and will escape into the cistern B. - -Throughout this process the steam, which mixed with the air fills -the cylinder, condenser and air-pumps will be only partially -condensed in the last two, and it will escape mixed with air -through the valve K, and this process will continue until all the -atmospheric air which at first filled the cylinder, tubes, -condenser and air-pump barrel shall be expelled through the valve -K, and these various spaces shall be filled with pure steam. When -that has happened let us suppose all the valves closed. In closing -the valve I the flow of steam to the condenser will be stopped, -and the steam contained in it will speedily be condensed by the -cold surface of the condenser, so that a vacuum will be produced -in the condenser, the condensed steam falling in the form of water -to the bottom. In like manner, and for like reasons, a vacuum will -be produced in the air-pump. The valve M, and the valves in the -air-pump piston will be closed by their own weight. - -By this process, which is called _blowing through_, the -atmospheric air, and other permanent gases, which filled the -cylinder, tubes, condenser and air-pump are expelled, and these -spaces will be a vacuum. The engine is then prepared to be -started, which is effected in the following manner:--The upper -steam valve G is opened, and steam allowed to flow from the boiler -through the passage leading to the top of the cylinder. This steam -cannot pass to the bottom of the cylinder, since the lower steam -valve H is closed. The space in the cylinder below the piston -being therefore a vacuum, and the steam pressing above it the -piston will be pressed downwards with a corresponding force. When -it has arrived at the bottom of the cylinder the steam valve G -must be closed, and at the same time the valve H opened. The valve -I leading to the condenser being also closed, the steam [Pg138] -which fills the cylinder above the piston is now admitted to -circulate through the open valve H below the piston, so that the -piston is pressed equally upwards and downwards by steam, and -there is no force to resist its movement save its friction with -the cylinder. The weight of the pump rods on the opposite end of -the beam being more than equivalent to overcome this the piston is -drawn to the top of the cylinder, and pushes before it the steam -which is drawn through the tube T, and the open valve H, and -passes into the cylinder C below the piston. - -[Illustration: _Fig._ 22.] - -When the piston has thus arrived once more at the top of the -cylinder, let the valve H be closed, and at the same time the -valves G and I opened, and the condensing cock E also opened, so -as to admit the jet to play in the condenser. The steam which -fills the cylinder C below the piston, will now rush through the -open valve I into the condenser which has been hitherto a vacuum, -and there encountering the jet, will be instantly converted into -water, and a mixture of condensed steam and injected water will -collect in the bottom of the condenser. At the same time, the -steam proceeding from the boiler by the steam pipe S to the upper -steam box A, will pass through the open steam valve G to the top -of the piston, but cannot pass below it because of the lower steam -valve H being closed. The piston, thus acted upon above by the -pressure of the steam, and the space in the cylinder below it -being a vacuum, its downward motion is resisted by no force but -the friction, and it is therefore driven to the bottom of the -cylinder. During its descent the valves G, I, and E remained open. -At the moment it arrives at the bottom of the cylinder, all these -three valves are closed, and the valve H opened. The steam which -fills the cylinder above the piston is now permitted to circulate -below it, by the open valve H, and the piston being consequently -pressed equally upwards and downwards will be drawn upwards as -before by the preponderance of the pump rods at the opposite end -of the beam. The weight of these rods must also be sufficiently -great to draw the air-pump piston N upwards. As this piston rises -in the air-pump, it leaves a vacuum below it into which the water -and air collected in the condenser will be drawn through the valve -M, which opens outwards. When the [Pg139] air-pump piston has -arrived at the top of the barrel, which it will do at the same -time that the steam piston arrives at the top of the cylinder, the -water and the chief part of the air or other fluids which may have -been in the condenser will be drawn into the barrel of the -air-pump, and the valve M being closed by its own weight, assisted -by the pressure of these fluids they cannot return into the -condenser. At the moment the steam piston arrives at the top of -the cylinder, the valve H is closed, and the three valves G, I, -and E are opened. The effect of this change is the same as was -already described in the former case, and the piston will in the -same manner and from the same causes be driven downwards. The -air-pump piston will at the same time descend by the force of its -own weight, aided by the weight of the plug-frame attached to its -rod. As it descends, the air below it will be gradually compressed -above the surface of the water in the bottom of the barrel, until -its pressure becomes sufficiently great to open the valves in the -air-pump piston. When this happens, the valves in the air-pump -piston, as represented on a large scale in _fig._ 22., will be -opened, and the air will pass through them above the piston. When -the piston comes in contact with the water in the bottom of the -barrel, this water will likewise pass through the open valves. -When the piston has arrived at the bottom of the air-pump barrel, -the valves in it will be closed by the pressure of the fluids -above them. The next ascent of the steam piston will draw up the -air-pump piston, and with it the fluids in the pump barrel above -it. As the air-pump [Pg140] piston approaches the top of its -barrel, the air and water above it will be drawn through the valve -K into the hot cistern B. The air will escape in bubbles through -the water in that cistern, and the warm water will be deposited in -it. - -The magnitude of the opening in the condensing valve E, must be -regulated by the quantity of steam admitted to the cylinder. As -much water ought to be supplied through the injection valve as -will be sufficient to condense the steam contained in the -cylinder, and also to reduce the temperature of the water itself, -when mixed with the steam, to a sufficiently low degree to prevent -it from producing vapour of a pressure which would injuriously -affect the working of the piston. It has been shown, that five and -a half cubic inches of ice-cold water mixed with one cubic inch of -water in the state of steam would produce six and a half cubic -inches of water at the boiling temperature. If then the cylinder -contained one cubic inch of water in the state of steam, and only -five and a half cubic inches of water were admitted through the -condensing jet, supposing this water, when admitted, to be at the -temperature of 32 deg., then the consequence would be that six and -a half cubic inches of water at the boiling temperature would be -produced in the condenser. Steam would immediately arise from -this, and at the same time the temperature of the remaining water -would be lowered by the amount of the latent heat taken up by the -steam so produced. This vapour would rise through the open -exhausting valve I, would fill the cylinder below the piston, and -would impair the efficiency of the steam above pressing it down. -The result of the inquiries of Watt respecting the pressure of -steam at different temperatures, showed, that to give efficiency -to the steam acting upon the piston it would always be necessary -to reduce the temperature of the water in the condenser to 100 deg. - -Let us then see what quantity of water at the common temperature -would be necessary to produce these effects. - -If the latent heat of steam be taken at 1000 deg., a cubic inch of -water in the state of steam may be considered for the purposes of -this computation, as equivalent to one cubic inch of water at -1212 deg. Now the question is, how many cubic inches of water at 60 -deg. must be mixed with this, in order that the [Pg141] mixture may -have the temperature of 100 deg.? This will be easily computed. As the -cubic inch of water at 1212 deg. is to be reduced to 100 deg., it must -be deprived of 1112 deg. of its temperature. On the other hand, as -many inches of water at 60 deg. as are to be added, must be raised in -the same mixture to the temperature of 100 deg., and therefore each of -these must receive 40 deg. of temperature. The number of cubic inches -of water necessary to be added will therefore be determined by -finding how often 40 deg. are contained in 1112 deg. If 1112 be divided -by 40, the quotient will be 27.8. Hence it appears, that to reduce -the water in the condenser to the temperature of 100 deg., supposing -the temperature of the water injected to be 60 deg., it will be -necessary to supply by the injection cock very nearly twenty-eight -times as much water as passes through the cylinder in the state of -steam; and therefore if it be supposed that all the water -evaporated in the boiler passes through the cylinder, it follows -that about twenty-eight times as much water must be thrown into -the condenser as is evaporated in the boiler. - -From these circumstances it will be evident that the cold cistern -in which the condenser and air-pump are submerged, must be -supplied with a considerable quantity of water. Independently of -the quantity drawn from it by the injection valve, as just -explained, the water in the cistern itself must be kept down to a -temperature of about 60 deg. The interior of the condenser and -air-pump being maintained by the steam condensed in them at a -temperature not less than 100 deg.; the outer surfaces of these -vessels consequently impart heat to the water in the cold cistern, -and have therefore a tendency to raise the temperature of that -water. To prevent this, a pump called the _cold pump_, represented -at L in _fig._ 21., is provided. By this pump water is raised from -any convenient reservoir, and driven through proper tubes into the -cold cistern. This cold pump is wrought by the engine, the rod -being attached to the beam. Water being, bulk for bulk, heavier -the lower its temperature, it follows that the water supplied by -the cold pump to the cistern will have a tendency to sink to the -bottom, pressing upwards the warmer water contained in it. A -waste-pipe is provided, by which this [Pg142] water is drained -off, and the cistern therefore maintained at the necessary -temperature. - -From what has been stated, it is also evident that the hot well B, -into which the warm water is thrown by the air-pump, will receive -considerably more water than is necessary to feed the boiler. A -waste-pipe, to carry off this, is also provided; and the quantity -necessary to feed the boiler is pumped up by a small pump, O, the -rod of which is attached to the beam, as represented in _fig._ -21., and which is worked by the engine. The water raised by this -pump is conducted to a reservoir from which the boiler is fed, by -means which will be hereafter explained. - -We shall now explain the manner in which the machine is made to -open and close the valves at the proper times. By referring to the -explanation already given, it will be perceived that at the moment -the piston reaches the top of the cylinder, the upper steam valve -G must be open, to admit the steam to press it down; while the -exhausting valve I must be opened, to allow the steam to pass to -the condenser; and the condensing valve E must be opened, to let -in the water necessary for the condensation of the steam; and at -the same time the lower steam valve H must be closed, to prevent -the passage of the steam which has been admitted through G. The -valves G, I, and E must be kept open, and the valve H kept closed, -until the piston arrives at the bottom of the cylinder, when it -will be necessary to close all the three valves, G, I, and E, and -to open the valve H, and the same effects must be produced each -time the piston arrives at the top and bottom of the cylinder. All -this is accomplished by a system of levers, which are exhibited in -_fig._ 21. The pivots on which these levers play are represented -on the framing of the engine, and the arms of the levers G', H', -and I', communicating with the corresponding valves G, H, and I, -are represented opposite a bar attached to the rod of the -air-pump, called the _plug frame_. This bar carries certain pegs -and detents, which act upon the arms of the several levers in such -a manner that, on the arrival of the beam at the extremities of -its play upwards and downwards, the levers are so struck that the -valves are opened and closed at the proper [Pg143] times. It is -needless to explain all the details of this arrangement. Let it be -sufficient, as an example of all, to explain the method of working -the upper steam valve G. When the piston reaches the top of the -cylinder, a pin strikes the arm of the lever G', and throws it -upwards: this, by means of the system of levers, pulls the arm of -the valve G downwards, by which the upper steam valve is raised -out of its seat, and a passage is opened from the steam pipe to -the cylinder. The valve is maintained in this state until the -piston reaches the bottom of the cylinder, when the arm G' is -pressed downwards, by which the arm G is pressed upwards, and the -valve restored to its seat. By similar methods the levers -governing the other three valves, H, I, and E, are worked. - -[Illustration: _Fig._ 23.] - -[Illustration: _Fig._ 24.] - -The valves used in these engines were of the kind called _spindle -valves_. They consisted of a flat circular plate of bell metal, A -B, _fig._ 23., with a round spindle passing perpendicularly -through its centre, and projecting above and below it. This valve, -having a conical form, was fitted very exactly, by grinding into a -corresponding circular conical seat, A B C D, _fig._ 24., which -forms the passage which it is the office of the valve to open and -close. When the valve falls into its seat, it fits the aperture -like a plug, so as entirely to stop it. The spindle plays in -sockets or holes, one above and the other below the aperture which -the valve stops; these holes keep the valve in its proper -position, so as to cause it to drop exactly into its place. - -In the experimental engine made by Mr. Watt at Kinneal, he used -cocks, and sometimes sliding covers, like the regulator described -in the old engines; but these he found very soon to become leaky. -He was, therefore, obliged to change them for the spindle valves -just described, which, being truly [Pg144] ground, and accurately -fitted in the first instance, were not so liable to go out of -order. These valves are also called _puppet clacks_, or _button -valves_. - -In the earlier engines constructed by Watt, the condensation was -produced by the contact of cold surfaces, without injection. The -reason of rejecting the method of condensing by injection was, -doubtless, to avoid the injurious effects of the air, which would -always enter the condenser, in combination with the water of -condensation, and vitiate the vacuum. It was soon found, however, -that a condenser acting by cold surfaces without injection, being -necessarily composed of narrow pipes or passages, was liable to -incrustation from bad water, by which the conducting power of the -material of the condenser was diminished; so that, while its outer -surface was kept cold by the water of the cold cistern, the inner -surface might, nevertheless, be so warm that a very imperfect -condensation would be produced. - -[Illustration: SOHO, BIRMINGHAM.] - - FOOTNOTES: - - [19] Eloge, p. 308. - -[Pg145] - - - - -[Illustration: BIRMINGHAM.] - -CHAP. VI. - - CORRESPONDENCE OF WATT WITH SMEATON. -- FAILURE OF CONDENSATION - BY SURFACE. -- IMPROVEMENTS IN CONSTRUCTION OF PISTON. -- METHOD - OF PACKING. -- IMPROVEMENTS IN BORING THE CYLINDERS. -- - DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES. -- - GREATLY INCREASED ECONOMY OF FUEL. -- EXPEDIENTS TO FORCE THE - NEW ENGINES INTO USE. -- CORRESPONDENCE WITH SMEATON. -- - EFFICIENCY OF FUEL IN THE NEW ENGINES. -- DISCOVERY OF THE - EXPANSIVE ACTION OF STEAM. -- WATT STATES IT IN A LETTER TO DR. - SMALL. -- ITS PRINCIPLE EXPLAINED. -- MECHANICAL EFFECT - RESULTING FROM IT. -- COMPUTED EFFECT OF CUTTING OFF STEAM AT - DIFFERENT PORTIONS OF THE STROKE. -- PRODUCES A VARIABLE POWER. - -- EXPEDIENTS FOR EQUALISING THE POWER. -- LIMITATION OF THE - EXPANSIVE PRINCIPLE IN WATT'S ENGINES. -- ITS MORE EXTENSIVE - APPLICATION IN THE CORNISH ENGINES. - - -(77.) In a letter addressed by Watt to Smeaton, dated April, 1766, -Watt refers to some of these practical difficulties which he had -to encounter. "I have been," says he, "tormented with exceedingly -bad health, resulting from the operation of an anxious mind, the -natural consequence of staking everything [Pg146] upon the cast -of a die; for in that light I look upon every project which has -not received the sanction of repeated success. - -"I have made considerable alterations in our engine lately, -particularly in the condenser. That which I used at first was -liable to be impaired, from incrustations from bad water; -therefore we have substituted one which works by an injection. In -pursuing this idea I have tried several kinds, and have at last -come to one, which I am not inclined to alter. It consists of a -jack-head pump, shut at bottom, with a common clack bucket, and a -valve in the cover of the pump, to discharge the air and water. -The eduction steam pipe, which comes from the cylinder, -communicates with this pump both above and below the bucket, and -has valves to prevent anything from going back from the pump to -the eduction pipe. The bucket descends by its own weight, and is -raised by the engine when the great piston descends, being hung to -the outer end of the great lever: the injection is made both into -the upper part of this pump and into the eduction pipe, and -operates beyond my ideas in point of quickness and perfection." - -Besides the difficulty arising from incrustation, Watt found the -tubulated condensers, and indeed all other expedients for -condensing by cold surfaces, subject to a fatal objection. They -did not condense instantaneously, and although they were capable -of ultimately effecting the condensation, yet that process was not -completed until a great part of the stroke of the piston was made. -Thus during more or less of the stroke the uncondensed steam -resisted the piston, and robbed the moving power of a part of its -effect. This objection has ever attended condensation by surface. - -[Illustration: _Fig._ 25.] - -[Illustration: _Fig._ 26.] - - -(78.) Another source of difficulty arose from the necessity of -constructing the piston and cylinder with greater precision than -had been usual in the old engines. To fit the cover to the -cylinder so as to be steam-tight; to construct the piston rod so -as to move through it without allowing the escape of steam, and -yet at the same time without injurious friction; to connect the -piston rod with the piston, so as to drive the [Pg147] latter -through the cylinder with a perfectly straight and parallel -motion; to make such connection perfectly centrical and firm, and -yet to allow the piston in its ascent to come nearly into contact -with the cover of the cylinder--were all difficulties peculiar to -the new engine. In the atmospheric engine the shank of the piston -rod was rough and square, and the rod was secured to the piston by -two or four branches or stays, as represented in _fig._ 25. It is -evident that such a construction would be inadmissible in an -engine in which the piston in its ascent must be brought nearly -into contact with the close cover of the cylinder. Besides this -the piston rod of an atmospheric engine might throughout its whole -length have any form which was most convenient, and required no -other property than the strength necessary to work the beam. In -the new engine, on the contrary, it was necessary that it should -be accurately turned and finely polished, so as to pass through -the hole in the top of the cylinder, and be maintained in it -steam-tight. This was effected by a contrivance called a -_stuffing-box_ B, represented in _fig._ 26. A hole is made in the -cover of the cylinder very little greater in magnitude than the -diameter of the piston rod. Above this hole is a cup in which, -around the piston, is placed a stuffing of hemp or tow, which is -saturated with oil or melted tallow. This collar of hemp is -pressed down by another piece, also perforated with a hole through -which the piston rod plays, and which is screwed down on the said -collar of hemp. - - -(79.) Although the imperfect manner in which the interior of the -cylinders was then formed impaired the efficiency of the [Pg148] -new engines, yet such imperfections were not so injurious as in -the old atmospheric engines. Any imperfection of form of the inner -surface of the cylinder would necessarily cause more or less steam -or air to escape between the piston and cylinder. In the improved -engine this steam passing into the vacuum below the piston would -rush into the condenser, and be there condensed, so that its -effect in resisting the motion of the piston would necessarily be -trifling. But on the other hand, any escape of air between the -piston and cylinder of an atmospheric engine would introduce an -elastic fluid under the piston, which would injuriously affect the -action of the machine. - -[Illustration: _Fig._ 27.] - -To make the pistons move sufficiently steam-tight in these early -imperfect cylinders, Watt contrived a packing formed of a collar -of hemp, or tow, as represented in _fig._ 27. The bottom of the -piston was formed of a circular plate of a diameter nearly, but -not altogether equal to the interior diameter of the cylinder. The -part of the piston above this was considerably less in diameter, -so that the piston was surrounded by a circular groove or channel -two inches wide, into which hemp or soft rope, called _gasket_, -was run, so as to form the packing. The top of the piston was -placed over this, having a rim or projecting part, which entered -the circular groove and pressed upon the packing, the cover being -pressed downwards by screws passing through the piston. The lower -part of the groove round the piston was rounded with a curve, so -that the pressure on the packing might force the latter against -the inner surface of the cylinder. This packing was kept supplied -with melted tallow, as already described, from the funnel, screwed -into the top of the cylinder. The metallic edges of the piston -were by this means prevented from coming into contact with the -surface of the cylinder, which was only pressed upon by the -stuffing or packing projecting beyond these. - - -(80.) Improved methods of boring soon, however, relieved [Pg149] -the engine from a part of these imperfections, and Watt writes to -Mr. Smeaton in the letter above quoted as follows:-- - -"Mr. Wilkinson has improved the art of boring cylinders; so that I -promise, upon a 72 inch cylinder, being not further distant from -absolute truth than the thickness of a thin sixpence in the worst -part. I am labouring to improve the regulators; my scheme is to -make them acute conical valves, shut by a weight, and opened by -the force of the steam. They bid fair for success, and will be -tried in a few days." - -The person here alluded to was Mr. John Wilkinson, of Bersham near -Chester, who, about the year 1775, contrived a new machine for -accurately boring the insides of cylinders. The cylinder being -first obtained from the foundery with a surface as accurate as the -process of casting would admit, had its inner surface reduced to -still greater accuracy by this machine, which consists of a -straight central bar extended along the axis of the cylinder, -which was made to revolve slowly round it. During the operation of -boring, the borer or cutter was fitted to slide along this bar, -which being perfectly straight, served as a sort of ruler to guide -the borer or cutter in its progress through the cylinder. In this -manner the interior surface of the cylinder was rendered not only -true and straight in its longitudinal direction, but also -perfectly circular in its cross section. - -The grease found to be most eligible for lubrication was the -tallow of beef or mutton; but in the earlier cylinders this was -soon consumed by reason of the imperfection of the boring, and the -piston being left dry ceased to be steam-tight. To prevent this, -Watt sought for some substance, which while it would thicken the -tallow, and detain it around the piston, would not be subject to -decomposition by heat. Black lead dust was used for this purpose, -but was soon found to wear the cylinder. In the mean while, -however, the improved method of boring supplied cylinders which -rendered this expedient unnecessary. - -When the inner surface of the cylinder is perfectly true and -smooth, the packing of the piston is soon rendered solid and hard, -being moulded to the cylinder by working, so as to fit it -perfectly. When by wear it became loose, it was [Pg150] only -necessary to tighten the screws by which the top and bottom of the -piston were held together. The packing being compressed by those -means, was forced outwards towards the surface of the cylinder, so -as to be rendered steam-tight. - - -(81.) It was not until about the year 1778, nine years after the -date of the patent, and thirteen after the invention of separate -condensation, that any impression was produced on the mining -interests by the advantages which were presented to them by these -vast improvements. This long interval, however, had not elapsed -without considerable advantage; for although all the great leading -principles of the contrivance were invented so early as the year -1765, yet the details of construction had been in a state of -progressive and continued improvement from the time Watt joined -Dr. Roebuck, in 1769, to the period now adverted to. - -The advantages which the engine offered in the form in which it -has been just described, were numerous and important, as compared -even with the most improved form of the atmospheric engine; and it -should be remembered, that that machine had also gone on -progressively improving, and was probably indebted for some of its -ameliorations to hints derived from the labours of Watt, and to -the adoption of such of his expedients as were applicable to this -imperfect machine, and could be adopted without an infraction of -his patent. - -In the most improved forms to which the atmospheric engine had -then attained, the quantity of steam wasted at each stroke of the -piston was equal to the contents of the cylinder. Such engines, -therefore, consumed twice the fuel which would be requisite, if -all sources of waste could have been removed. In Watt's engines, -the steam consumed at each stroke of the piston amounted only to -1-1/4 times the contents of the cylinder. The waste steam, -therefore, per stroke, was only a quarter of what was usefully -employed. The absolute waste, therefore, of the best atmospheric -engines was four times that of the improved engine, and -consequently the saving of fuel in the improved engines amounted -to about three eighths of all the fuel consumed in atmospheric -engines of the same power. [Pg151] - - -(82.) But independently of this saving of steam, which would -otherwise be wasted, the power of Watt's engine, as compared with -the atmospheric engine, was so much augmented that the former -would work against a resistance of ten pounds on the square inch -under the same circumstances in which the latter would not move -against more than seven pounds. The cause of this augmentation of -power is easily explained. In the atmospheric engine the -temperature of the condensed steam could not be reduced below 152 -deg. without incurring a greater loss than would be compensated by -the advantage to be obtained from any higher degree of condensation. -Now steam raised from water at 152 deg. has a pressure of nearly four -pounds per square inch. This pressure, therefore, acted below the -piston resisting the atmospheric pressure above. In Watt's engine, -however, the condenser was kept at a temperature of about 100 deg., -at which temperature steam has a pressure of less than one pound per -square inch. A resisting force upon the piston of three pounds per -square inch was therefore saved in Watt's engine as compared with -the atmospheric engine. - - -(83.) Besides these direct sources of economy, there were other -advantages incidental to Watt's engine. An atmospheric engine -possessed very limited power of adaptation to a varying load. The -moving power being the atmospheric pressure, was not under -control, and, on the other hand, was subject to variations from -day to day and from hour to hour, according to the changes of the -barometer. In the first construction of such an engine, therefore, -its power being necessarily adapted to the greatest load which it -would have to move, whenever the load upon its pumps was -diminished, the motion of the piston in descending would be -rapidly accelerated in consequence of the moving power exceeding -the resistance. By this the machinery would be subject to sudden -shocks, which were productive of rapid wear, and exposed the -machinery to the danger of fracture. To remedy this inconvenience, -the following expedient was provided in the atmospheric engine: -whenever the load on the engine was materially diminished, the -quantity of water admitted through the injection valve to condense -the steam was proportionally [Pg152] diminished. An imperfect -condensation being therefore produced, vapour remained in the -cylinder under the piston, the pressure of which resisted the -atmosphere, and mitigated the force of the machine. Besides this, -a cock was provided in the bottom of the cylinder, called an _air -cock_, by which atmospheric air could be admitted to resist the -piston whenever the motion was too rapid. - -These expedients, however, were all attended with a waste of fuel -in relation to the work done by the engine; for it is evident that -the consumption of steam was necessarily the same, whether the -engine was working against its full load or against a reduced -resistance. - -On the other hand, in the improved engine of Watt, when the load, to -work against which the engine exerted its full power, was -diminished, a cock or valve was provided in the steam pipe leading -from the boiler, which was called a _throttle valve_, by adjusting -which the passage in that pipe could be more or less contracted. By -regulating this cock the supply of steam from the boiler was -checked, and the quantity transmitted to the cylinder diminished, so -that its effect upon the piston might be rendered equal to the -amount of the diminished resistance. By this means the quantity of -steam transmitted to the cylinder was rendered exactly proportional -to the work which the engine had to perform. If, under such -circumstances, the boiler was worked to its full power, so as to -produce steam as fast as it would when the engine was working at -full power, then no saving of fuel would be effected, since the -surplus steam produced in the boiler would necessarily escape at the -safety valve. But in such case the fireman was directed to limit the -fuel of the furnace until the discharge at the safety valve ceased. - -By these expedients, the actual consumption of fuel in one of -these improved engines was always in the exact proportion of the -work which it performed, whether it worked at full power or at any -degree under its regular power. - - -(84.) Notwithstanding these and other advantages attending the new -engines, Boulton and Watt experienced difficulties all but -insurmountable in getting them into use. No manufactory existed in -the country possessing machinery capable of [Pg153] executing -with the necessary precision the valves and other parts which -required exact execution, and the patentees were compelled to -construct machinery at Soho for this purpose; and even after they -succeeded in getting the cylinders properly bored, the piston rods -exactly turned and polished, the spindle valves constructed so as -to be steam-tight, and every other arrangement completed which was -necessary for the efficiency of the machine, the novelty of the -engine, and the difficulty which was supposed to attend its -maintenance in good working order, formed strong objections to its -adoption. - -To remove such objections, great sacrifices were necessary on the -part of Boulton and Watt; and they accordingly resolved to -undertake the construction of the new engines without any profit, -giving them to the parties requiring their use at first cost, on -the condition of being remunerated by a small share of what they -would save in fuel. - -"We have no objection," writes Mr. Boulton, "to contract with the -Carron Company to direct the making of an engine to return the -water for their mills. * * * * We do not aim at profits in engine -building, but shall take our profits out of the saving of fuel; so -that if we save nothing, we shall take nothing. Our terms are as -follows: we will make all the necessary plans, sections, and -elevations for the building, and for the engine with its -appurtenances, specifying all cast and forged iron work, and every -other particular relative to the engine. We will give all -necessary directions to your workmen, which they must implicitly -obey. We will execute, for a stipulated price, the valves, and all -other parts which may.require exact execution, at Soho; we will -see that all the parts are put together, and set to work, -properly; we will keep our own work in repair for one year, and we -have no other objection to seven years than the inconvenience of -the distance. We will guarantee that the engine so constructed -shall raise at least 20,000 cubic feet of water twenty-four feet -high with each hundred weight of coals burnt. - -"When all this is done, a fair and candid comparison shall be made -between it, and your own engine, or any other engine in Scotland, -from which comparison the amount of savings in fuel shall be -estimated, and that amount being [Pg154] divided into three -parts, we shall be entitled to one of those parts, in recompense -for our patent licence, our drawings, &c. &c. Our own share of -savings shall be estimated in money, according to the value of -your coals delivered under the boiler, and you shall annually pay -us that sum, during twenty-five years from the day you begin to -work; provided you continue the use of the engine so long. And in -case you sell the engine, or remove it to any other place, you -must previously give us notice, for we shall then be entitled to -our third of the savings of fuel, according to the value of coals -at such new place. This is a necessary condition, otherwise the -engine which we make for you at an expense of two thousand pounds -may be sold in Cornwall for ten thousand pounds. - -"Such parts of the engine as we execute at Soho we will be paid -for at a fair price; I conclude, from all the observations I have -had an opportunity of making, that our engines are four times -better than the common engines. In boilers, which are a very -expensive article, the savings will be in proportion to the -savings of coal. If you compare our engine with the common engine -(not in size, but in power), you will find the original expense of -erecting one to be nearly the same. - -"Mr. Wilkinson has bored us several cylinders, almost without -error; that of fifty inches diameter, which we put up at Tipton, -does not err the thickness of an old shilling in any part; so that -you must either improve your method of boring, or we must furnish -the cylinder to you." - -The reluctance of mining companies to relinquish the old engines, -even on these terms, led them to propose to Mr. Watt to grant them a -licence for the use of his condenser, to be applied to the -atmospheric engine, without the introduction of other improvements. -Such a proposition was made to him by Mr. Smeaton, in the year 1778, -to which he returned the following answer:-- - -"I have several times considered the propriety of the application -of my condensers to common engines, and have made experiments with -that view upon our engine at Soho, but have never found such -results as would induce me to try [Pg155] it any where else; and, -in consequence, we refused to make that application to Wheal -Virgin engines in Cornwall, and to some others; our reasons were, -that though it might have enabled them to have gone deeper with -their present engines, yet, the savings of fuel would not have -been great, in comparison to the complete machine. By adding -condensers to engines that were not in good order, our engine -would have been introduced into that country (which we look upon -as our richest mine) in an unfavourable point of view, and without -such profits as would have been satisfactory either to us or to -the adventurers; and if we had granted the use of condensers to -one, we must have done so to all, and thereby have curtailed our -profits, and perhaps injured our reputation. Besides, where a new -engine is to be erected, and to be equally well executed in point -of workmanship and materials, an engine of the same power cannot -be constructed materially cheaper on the old plan than on ours; -for our boiler and cylinder are much smaller, and the building, -the lever, the chains, together with all the pump and pit work, -are only the same. * * * * - -"We charge our profits in proportion to the saving made in fuel by -our engine, when compared with a common one which burns the same -kind of coals; we ask one third of these savings to be paid us -annually, or half yearly; the payment being redeemable in the -option of our employer, at ten years' purchase; and when the coals -are low priced, we should also make some charge as engineers. In -all these comparisons our own interest has made us except your -(Mr. Smeaton) improved engines, unless we were allowed a greater -proportion of the savings." - -Their exertions to improve the manufacture of engines at Soho is -shown by the following letter from Mr. Boulton, in the same -correspondence to Mr. Smeaton:-- - -"We are systematising the business of engine making, as we have -done before in the button manufactory; we are training up workmen, -and making tools and machines to form the different parts of Mr. -Watt's engines with more accuracy, and at a cheaper rate than can -possibly be done by the ordinary methods of working. Our workshop -and apparatus will be of [Pg156] sufficient extent to execute all -the engines which are likely to be soon wanted in this country; -and it will not be worth the expense for any other engineers to -erect similar works, for that would be like building a mill to -grind a bushel of corn. - -"I can assure you from experience, that our small engine at Soho -is capable of raising 500,000 cubic feet of water 1 foot high with -every 112 lbs. of coals, and we are in hopes of doing much more. -Mr. Watt's engine has a very great advantage in mines, which are -continually working deeper: suppose, for instance, that a mine is -50 fathoms deep, you may have an engine which will be equal to -draining the water when the mine is worked, to 100 fathoms deep, -and yet you can constantly adapt the engine to its load, whether -it be 50 or 100 fathoms, or any intermediate depth; and the -consumption of coals will be less in proportion when working at -the lesser than at the greater depths; supposing it works, as our -engines generally do, at 11 lbs. per square inch, when the mine -becomes 100 fathoms deep." - - -(85.) The great improvement which has been introduced within the -last half century, in the details of Watt's steam engine, will be -rendered manifest by comparing the effects of a given weight of -fuel here supplied by Mr. Boulton with the effects which the same -weight of fuel is now known to produce in the best pumping engines -worked in Cornwall. One of these engines, in good working order, -has been known to raise 125,000,000 lbs. 1 foot high, by the -combustion of a bushel of coals. But the average performance of -even the best engines is below this amount. If we take it at -90,000,000, this will be equivalent to the weight of about 1-1/2 -million cubic feet of water, a bushel of coals being 3/4 cwt. It -will therefore follow that, with the present engines, one hundred -weight of coals is capable of raising about two million cubic feet -of water one foot high, being a duty four times that assigned to -the early engines by Mr. Boulton. - - -(86.) At the time that Watt, in conjunction with Dr. Roebuck, -obtained the patent for his improved engine, the idea occurred to -him, that the steam which had impelled the piston in its descent -rushed from the cylinder with a mechanical force much more than -sufficient to overcome any resistance [Pg157] which it had to -encounter in its passage to the condenser; and that such force -might be rendered available as a moving power, in addition to that -already obtained from the steam during the stroke of the piston. -This notion involved the whole principle of the expansive action -of steam, which subsequently proved to be of such importance in -the performance of steam engines. Watt was, however, so much -engrossed at that time, and subsequently, by the difficulties he -had to encounter in the construction of his engines, that he did -not attempt to bring this principle into operation. It was not -until after he had organised that part of the establishment at -Soho which was appropriated to the manufacture of steam engines, -that he proceeded to apply the expansive principle. Since the date -of the patent which he took out for this (1782), was subsequent to -the application of the same principle by another engineer, named -Hornblower, it is right to state, that the claim of Mr. Watt to -this important step in the improvement of the steam engine, is -established by a letter addressed by him to Dr. Small, of -Birmingham, dated Glasgow, May, 1769:-- - -"I mentioned to you a method of still doubling the effect of the -steam, and that tolerably easy, by using the power of steam -rushing into a vacuum, at present lost. This would do little more -than double the effect, but it would too much enlarge the vessels -to use it all: it is peculiarly applicable to wheel engines, and -may supply the want of a condenser, where the force of steam only -is used; for open one of the steam valves, and admit steam until -one fourth of the distance between it and the next valve is filled -with steam, then shut the valve, and the steam will continue to -expand, and to press round the wheel, with a diminishing power, -ending in one fourth of its first exertion. The sum of the series -you will find greater than one half, though only one fourth of -steam was used. The power will indeed be unequal, but this can be -remedied by a fly, or by several other means." - -In 1776 the engine, which had been then recently erected at Soho, -was adapted to act upon the principle of expansion. When the -piston had been pressed down in the cylinder for a certain portion -of the stroke, the further supply of steam [Pg158] from the -boiler was cut off, by closing the upper steam valve, and the -remainder of the stroke was accomplished by the expansive power of -the steam which had already been introduced into the cylinder. - - -(87.) To make this method of applying the force of steam -intelligible, some previous explanation of mechanical principles -will be necessary. - -If a body which offers a certain resistance be urged by a certain -moving force, the motion which it will receive will depend on the -relation between the energy of the moving force and the amount of -the resistance opposed to it. If the moving force be precisely -equal to the resistance, the motion which the body will receive -will be perfectly uniform. - -If the energy of the moving force be greater than the resistance, -then its surplus or excess above the amount of resistance will be -expended in imparting momentum to the mass of the body moved, and -the latter will, consequently, continually acquire augmented -speed. The motion of the body will, therefore, be in this case -accelerated. - -If the energy of the moving force be less in amount than the -resistance, then all that portion of the resistance which exceeds -the amount of the moving force will be expended in depriving the -mass of the body of momentum, and the body will therefore be moved -with continually diminished speed until it be brought to rest. - - -(88.) Whenever, therefore, a uniform motion is produced in a body, -it may be taken as an indication of the equality of the moving -force to the resistance; and, on the other hand, according as the -speed of the body is augmented or diminished, it may be inferred -that the energy of the moving force has been greater or less than -the resistance. - -It is an error to suppose that rest is the only condition possible -for a body to assume when under the operation of two or more -mechanical forces which are in equilibrium. By the laws of motion -the state of a body which is not under the operation of any -external force must be either in a state of rest or of uniform -motion. Whichever be its state, it will suffer no change if the -body be brought under the operation of two or more forces which -are in equilibrium; for to suppose [Pg159] such forces to produce -any change in the state of the body, whether from rest to motion, -or _vice versa_, or in the velocity of the motion which the body -may have previously had, would be equivalent to a supposition that -the forces applied to the body being in equilibrium were capable -of producing a dynamical effect, which would be a contradiction in -terms. This, though not always clearly understood by mere -practical men, or by persons superficially informed, is, in fact, -among the fundamental principles of mechanical science. - - -(89.) When the piston is at the top of the cylinder, and about to -commence its motion downwards, the steam acting upon it will have -not only to overcome the resistance arising from the friction of -the various parts of the engine, but will also have to put in -motion the whole mass of matter of the piston pump rods, pump -pistons, and the column of water in the pump barrels. Besides -imparting to this mass the momentum corresponding to the velocity -with which it will be moved, it will also have to encounter the -resistance due to the preponderance of the weight of the water and -pump rods over that of the steam piston. The pressure of steam, -therefore, upon the piston at the commencement of the stroke must, -in accordance with the mechanical principles just explained, have -a greater force than is equal to all the resistances which it -would have to overcome, supposing the mass to be moving at a -uniform velocity. The moving force, therefore, being greater than -the resistance, the mass, when put in motion, will necessarily -move with a gradually augmented speed, and the piston of the -engine which has been described in the last chapter would -necessarily move from the top to the bottom of the cylinder with -an accelerated motion, having at the moment of its arrival at the -bottom a greater velocity than at any other part of the stroke. As -the piston and all the matter which it has put in motion must at -this point come to rest, the momentum of the moving mass must -necessarily expend itself on some part of the machinery, and would -be so much mechanical force lost. It is evident, therefore, -independently of any consideration of the expansive principle, to -which we shall presently refer, that the action of the [Pg160] -moving power in the descent of the piston ought to be suspended -before the arrival of the piston at the bottom of the cylinder, in -order to allow the momentum of the mass which is in motion to -expend itself, and to allow the piston to come gradually to rest -at the termination of the stroke. - -Thus, if we were to suppose that after the piston had descended -through three fourths of the whole length of the cylinder, and had -acquired a certain velocity, the steam above it were suddenly -condensed, so as to leave a vacuum both above and below it, the -piston, being then subject to no impelling force, would still move -downwards, in virtue of the momentum it had acquired, until the -resistance would deprive it of that momentum, and bring it to -rest; and if the remaining fourth part of the cylinder were -necessary for the accomplishment of this, then it is evident that -that part of the stroke would be accomplished without further -expenditure of the moving power. - -In fact, this part of the stroke would be made by the expenditure -of that excess of moving power, which, at the commencement of the -stroke, had been employed in putting the machinery and its load in -motion, and in subsequently accelerating that motion. - -Although under such circumstances the resistance, during the -operation of the moving power, shall not have been at any time -equal to the moving power, since while the motion was accelerated -it was less, and while retarded greater than that power, yet as -the whole moving power has been expended upon the resistance, the -mechanical effect which the moving power has produced under such -circumstances will be equal to the actual amount of that power. If -in an engine of this kind the steam was not cut off till the -conclusion of the stroke, a part of the moving power would be lost -upon those fixed points in the machinery which would sustain the -shock produced by the instantaneous cessation of motion at the end -of the stroke. - -Independently, therefore, of any consideration of the expansive -principle, it appears that, in an engine of this kind, the steam -ought to be cut off before the completion of the stroke. [Pg161] - -[Illustration: _Fig._ 28.] - - -(90.) To render the expansive action of steam intelligible, let A -B (_fig._ 28.) represent a cylinder whose area we will suppose, -for the sake of illustration, to be a square foot, and whose -length, A B, shall also be a foot. If steam of a pressure equal to -the atmosphere be supplied to this cylinder, it will exert a -pressure of about one ton on the piston; and if such steam be -uniformly supplied from the boiler, the piston will be moved from -A to B with the force of one ton, and that motion will be uniform -if the piston be opposed throughout the same space by a resistance -equal to a ton. When the piston has arrived at B, let us suppose -that the further supply of steam from the boiler is stopped by -closing the upper steam valve, and let us also suppose the -cylinder to be continued downwards so that B C shall be equal to A -B, and suppose that B C has been previously in communication with -the condenser, and is therefore a vacuum. The piston at B will -then be urged with a force of one ton downwards, and as it -descends the steam above it will be diffused through an increased -volume, and will consequently acquire a diminished pressure. We -shall, for the present, assume that this diminution of pressure -follows the law of elastic fluids in general; that it will be -decreased in the same proportion as the volume of the steam is -augmented. While the piston, therefore, moves from B downwards it -will be urged by a continually decreasing force. Let us suppose, -that by some expedient, it is also subject to a continually -decreasing resistance, and that this resistance decreases in the -same proportion as the force which urges the piston. In that case -the motion of the piston would continue uniform. When the piston -would arrive at P', the middle of the second cylinder, then the -space occupied by the steam being increased in the proportion of 2 -to 3, the pressure on the piston would be diminished in the -proportion of 3 to 2, and the pressure at B being one ton, it -would be two-thirds of a ton at P'. In like manner when the piston -would arrive at C, the space occupied by the steam being double -that which [Pg162] it occupied when the piston was at B, the -pressure of the steam would be half its pressure at B, and -therefore at the termination of the stroke, the pressure on the -piston would be half a ton. - -If the space from B to C, through which the steam is here supposed -to act expansively, be divided into ten equal parts, the pressure -on the piston at the moment of passing each of those divisions -would be calculated upon the same principle as in the cases now -mentioned. After moving through the first division, the volume of -the steam would be increased in the proportion of 10 to 11, and -therefore its pressure would be diminished in the proportion of 11 -to 10. The pressure, therefore, driving the piston at the end of -the first of these ten divisions would be 10/11ths of a ton. In -like manner, its pressure at the second of the divisions would be -10/12ths of a ton, and the third 10/13ths of a ton; and so on, as -indicated in the figure. - -Now if the pressure of the steam through each of these divisions -were to continue uniform, and, instead of gradually diminishing, -to suffer a sudden change in passing from one division to another, -then the mechanical effect produced from B to C would be obtained -by taking a mean or average of the several pressures throughout -each of the ten divisions. In the present case it has been -supposed that the force on the piston at B was 2240 pounds. To -obtain the pressure in pounds corresponding to each of the -successive divisions, it will therefore only be necessary to -multiply 2240 by 10, and to divide it successively by 11, 12, 13, -&c. The pressures, therefore, in pounds, at each of the ten -divisions, will be as follows:-- - - 1st 2036.3 - 2d 1866.6 - 3d 1723.1 - 4th 1600.0 - 5th 1493.3 - 6th 1400.0 - 7th 1317.6 - 8th 1244.4 - 9th 1179.0 - 10th 1120.0 - -If the mean of these be taken by adding them together [Pg163] and -dividing by 10, it will be found to be 1498 pounds. It appears, -therefore, that the pressures through each of the ten divisions -being supposed to be uniform (which however, strictly, they are -not,) the mechanical effect of the steam from B to C would be the -same as if it acted uniformly throughout that space upon the -piston with a force of about 1500 pounds, being rather less than -three-fourths of its whole effect from A to B. - -But it is evident that this principle will be equally applicable -if the second cylinder had any other proportion to the first. Thus -it might be twice the length of the first; and in that case, a -further mechanical effect would be obtained from the expansion of -the steam. - -The more accurate method of calculating the effect of the -expansion from B to C, would involve more advanced mathematical -principles than could properly be introduced here; but the result -of such a computation would be that the actual average effect of -the steam from B to C would be equal to a uniform pressure through -that space, amounting to one thousand five hundred and forty-five -pounds, being greater than the result of the above computation, -the difference being due to the expansive action through each of -the ten divisions, which was omitted in the above computation. - - -(91.) It is evident that the expansive principle, as here explained, -involves the condition of a variation in the intensity of the moving -power. Thus, if the steam act with a uniform energy on the piston so -long as its supply from the boiler continues, the moment that supply -is stopped, by closing the steam valve, the steam contained in the -cylinder will fill a gradually increasing volume by the motion of -the piston, and therefore will act above the piston with a gradually -decreasing energy. If the resistance to the moving power produced by -the load, friction, &c. be not subject to a variation corresponding -precisely to such variation in the moving power, then the -consequence must be that the motion imparted to the load will cease -to be uniform. If the energy of the moving power at any part of the -stroke be greater than the resistance, the motion produced will be -accelerated; if it be less, the motion will be retarded; and if it -be at one time greater, and another [Pg164] time less, as will -probably happen, then the motion will be alternately accelerated and -retarded. This variation in the speed of the body moved will not, -however, affect the mechanical effect produced by the power, -provided that the momentum imparted to the moving mass be allowed to -expend itself at the end of the stroke, so that the piston may be -brought to rest as nearly as possible by the resistance of the load, -and not by any shock on any fixed points in the machine. This is an -object which, consequently, should be aimed at with a view to the -economy of power, independently of other considerations connected -with the wear and tear of the machinery. So long as the engine is -only applied to the operation of pumping water, great regularity of -motion is not essential, and, therefore, the variation of speed -which appears to be an almost inevitable consequence of any -extensive application of the expansive principle, is of little -importance. In the patent which Watt took out for the application of -the expansive principle, he specified several methods of producing a -uniform effect upon a uniform resistance, notwithstanding the -variation of the energy of the power which necessarily attended the -expansion of the steam. This he proposed to accomplish by various -mechanical means, some of which had been previously applied to the -equalisation of a varying power. One consisted in causing the piston -to act on a lever, which should have an arm of variable length, the -length increasing in the same proportion as the energy of the moving -power diminished. This was an expedient which had been already -applied in mechanics for the purpose of equalising a varying power. -A well-known example of it is presented in the main-spring and fuzee -of a watch. According as the watch goes down, the main-spring -becomes relaxed, and its force is diminished; but, at the same time, -the chain by which it drives the fuzee acts upon a wheel or circle, -having a diameter increased in the same proportion as the energy of -the spring is diminished. - -Another expedient consisted in causing the moving power, when -acting with greatest energy, to lift a weight which should be -allowed to descend again, assisting the piston when the energy of -the moving force was diminished. [Pg165] - -Another method consisted in causing the moving force, when acting -with greatest energy, to impart momentum to a mass of inert -matter, which should be made to restore the same force when the -moving power was more enfeebled. We shall not more than allude -here to these contrivances proposed by Watt, since their -application has never been found advantageous in cases where the -expansive principle is used. - - -(92.) The application of the expansive principle in the engines -constructed by Boulton and Watt, was always very limited, by -reason of their confining themselves to the use of steam having a -pressure not much exceeding that of the atmosphere. If the -principle of expansion, as above explained, be attentively -considered, it will be evident that the extent of its application -will mainly depend on the density and pressure of the steam -admitted from the boiler. If the density and pressure be not -considerable when the steam is cut off, the extent of its -subsequent expansion will be proportionally limited. It was in -consequence of this, that this principle from which considerable -economy of power has been derived, was applied with much less -advantage by Mr. Watt than it has since been by others, who have -adopted the use of steam of much higher pressure. In the engines -of Boulton and Watt, where the expansive principle was applied, -the steam was cut off after the piston had performed from one half -to two thirds of the stroke, according to the circumstances under -which the engine was worked. The decreasing pressure produced by -expansion was, in this case, especially with the larger class of -engines, little more than would be necessary to allow the momentum -of the mass moved to spend itself, before the arrival of the -piston at the end of the stroke. - -Subsequently, however, boilers producing steam of much higher -pressure were applied, and the steam was cut off when the piston -had performed a much smaller part of the whole stroke. The great -theatre of these experiments and improvements has been the mining -districts in Cornwall, where, instead of working with steam of a -pressure not much exceeding that of the atmosphere, it has been -found advantageous to use steam whose pressure is at least four -times as great as [Pg166] that of the atmosphere; and instead of -limiting its expansion to the last half or fourth of the stroke, -it is cut off after the piston has performed one fourth part of -the stroke or less, all the remainder of the stroke being -accomplished by the expansive power of the steam, and by momentum. - -[Illustration: BRIDGE OVER THE CLYDE AT HAMILTON, DESIGNED BY -WATT.] - -[Pg167] - - - - -[Illustration: DOUBLE-ACTING ENGINE, ZINC WORKS, CITY ROAD, -LONDON.] - -CHAP. VII. - - PROPERTIES OF STEAM. -- COMMON STEAM. -- SUPERHEATED STEAM. -- - LAW OF DALTON AND GAY LUSSAC. -- LAW OF MARIOTTE. -- RELATION - BETWEEN TEMPERATURE AND PRESSURE OF COMMON STEAM. -- EFFECTS OF - THE EXPANSION OF COMMON STEAM. -- MECHANICAL EFFECTS OF STEAM. - -- METHOD OF EQUALISING THE EXPANSIVE FORCE. -- HORNBLOWER'S - ENGINE. -- WOOLF'S ENGINE. -- WATT'S ATTEMPTS TO EXTEND THE - STEAM ENGINE TO MANUFACTURES. -- PAPIN'S PROJECTED APPLICATIONS - OF THE STEAM ENGINE. -- SAVERY'S APPLICATIONS OF THE ENGINE TO - MOVE MACHINERY. -- JONATHAN HULL'S APPLICATION TO WATER WHEELS. - -- STEWART'S APPLICATION OF THE ENGINE TO MILL WORK. -- - WASHBOROUGH'S APPLICATION OF THE FLY WHEEL AND CRANK. -- WATT'S - SECOND PATENT. -- DOUBLE-ACTION VALVES. - - -(93.) Since the application of the expansive action of steam -involves the consideration of its properties when it ceases to be -in contact with the water from which it was produced, and likewise -the variation of its pressure in different states of [Pg168] -density and at different temperatures, it is necessary here to -explain some of the most important of these properties of vapour. - -Steam may exist in two states, distinguished from each other by -the following circumstances:-- - -1st. It may be such that the abstraction from it of any portion of -heat, however small, will cause its partial condensation. - -2d. It may be such as to admit of the abstraction of heat from it -without undergoing any other change than that which air would -undergo under like circumstances, viz. a diminution of temperature -and pressure. - - -(94.) We shall call, for distinction, the former _Common Steam_, -and the latter _Superheated Steam_. - -[Illustration: _Fig._ 29.] - -To explain the circumstances out of which these properties arise, -let B (_fig._ 29.) be imagined to be a vessel filled with water, -communicating by a pipe and stopcock with another vessel A, which -in the commencement of the process may be conceived to be filled -with air. Let D be a pipe and stopcock at the top of this vessel. -If the vessel B be heated, and the two cocks be opened, the steam -proceeding from the water in B will blow the air out of the vessel -A through the open stopcock D, in the same manner as air is blown -from a steam engine. When the vessel A by these means has been -filled with pure steam, let both stopcocks be closed. If the steam -in A, under these circumstances, have a pressure of 15 lbs. per -square inch, its temperature will be found to be 213 deg. Now, if any -heat be abstracted from this steam, its temperature will fall, and -a portion of it will be reconverted into water. - -Again, suppose the vessel A to be filled with pure steam which has -been produced from the heated water in B, the stopcock C being -open. Let the stopcock C be then closed, and the water in B be -heated to a higher temperature, the temperature and pressure of -the steam in A being observed. If the stopcock C be now opened, -the steam in A will be immediately observed to rise to the more -elevated temperature which has been imparted to the water in B, -and at the same time it will acquire an increased pressure. -[Pg169] - -The increase of temperature which it has received would of itself -produce an increased pressure; but that this is not the sole cause -of the augmented pressure in the present case might be proved by -weighing the vessel A. It would be found to have increased weight, -which could only arise from its having received from the water in -B an additional quantity of vapour. The increased pressure -therefore, which the steam in A has acquired, is due conjointly to -its increased density and its increased temperature. In general, -if the water in the vessel B be raised or lowered in temperature, -the steam in the vessel A will rise and fall in temperature in a -corresponding manner, always having the same temperature as the -water in B. If the weight of the vessel A were observed, it would -be found to increase with every increase of temperature, and to -diminish with every diminution of temperature, proving that the -augmented temperature of the water in B produces an augmented -density of the steam in A. The same pressure would be found always -to correspond to the same temperature and density, so that if the -numerical amount of any one of the three quantities, the -temperature, the pressure, or the density, were known, the other -two must necessarily be determined, the same temperature always -corresponding to the same pressure, and _vice versa_. And in like -manner, steam produced under these circumstances of the same -density cannot have different pressures. It must be observed that -the steam here produced receives all the heat which it possesses -from the water from which it is raised. Now it is easily -demonstrable, that this is the least quantity of heat which is -compatible with the steam maintaining the vaporous form; for if -the stopcock C be closed so as to separate the steam in A from the -water in B, and that any portion of heat, however small, be then -abstracted from the steam in A, some portion of the steam will be -reconverted into water. - -This then, according to the definition already given, is _Common -Steam_. - - -(95.) Let us now suppose that the vessel A, being in communication -with the vessel B by the open stopcock, has been filled with pure -steam of any given temperature. The steam which it thus contains -will be common steam, and, as has been [Pg170] shown (94.), it -cannot lose any portion of heat, however small, without being -partially condensed; but let the stopcock C be closed, and let the -steam in A be then exposed to any source of heat by which its -temperature may be raised any required number of degrees. From the -steam thus obtained heat may be abstracted without producing any -condensation; and such abstraction of heat may be continued -without producing condensation, until the steam is cooled down to -that temperature at which it was raised from the water in B, when -the stopcock C was opened. Any further reduction of temperature -would be attended with condensation. - -If after increasing the temperature of the steam in A, the -stopcock C being shut so as to render it superheated steam, its -pressure be observed, the pressure will be found to be increased, -but not to that amount which it would have been increased had the -steam in A been raised to the same temperature by heating the -water in B to that temperature, and keeping the stopcock open. In -fact, its present augmented pressure will be due only to its -increased temperature, since its density remains unchanged. But if -in these circumstances the stopcock C be suddenly opened, the -pressure of the steam in A will as suddenly rise to that pressure -which in common steam corresponds to its temperature; and if the -vessel A were weighed, it would be found to have increased in -weight, proving that the steam contained in it has received -increased density by an increased quantity of vapour proceeding -from the water in A. In fact, by opening the stopcock the steam -which was before superheated steam, has become common steam. It -has the greatest density which steam of that temperature can have; -and consequently, if any heat be abstracted from it, a partial -condensation will ensue. - -To render these general principles more intelligible, let us -suppose that the water in B is raised to the temperature of 213 deg., -the stopcock C being open; the vessel A will then be filled with -steam of the same temperature, and having a pressure of 15 lbs. -per square inch. This will be common steam. If the stopcock be now -closed, and the whole apparatus be exposed to the temperature of -243 deg.; the steam in A will preserve the same density, but its -pressure will be [Pg171] increased from 15 lbs. to a little more -than 16 lbs. per square inch. Let the stopcock C be then opened -and while the temperature of the steam in A shall continue to be -243 deg., the pressure will suddenly rise from 16 lbs. to about 26 -lbs. per square inch. The weight of the steam in A will be at the -same time increased in the same proportion of 16 to 26 as its -pressure. The steam thus produced in A will then be common steam, -and any abstraction of heat from it would be attended with partial -condensation. - - -(96.) The law, according to which the pressure of elastic fluids -in general, whether gases or vapours, increases with their -temperature, was simultaneously discovered by Dalton and Gay -Lussac. If the pressure which the gas or vapour would have at the -temperature of melting ice, were expressed by 10,000, then the -increase of pressure which it would receive for every degree of -temperature by which it would be raised, its volume being supposed -to be preserved, would be expressed by 208-1/3. Thus, if the -pressure of gas, or vapour, on a surface of a certain magnitude at -the temperature of 32 deg. were 10,000 ounces, then the same gas or -vapour would acquire an additional pressure of 208-1/3 ounces for -every degree of temperature which would be imparted to it above -32 deg. This law is common to all gases and vapours. - -It may be objected that water cannot exist in the state of vapour -under the usual pressures at so low a temperature as melting ice. -This, however, does not hinder the application of the above law, -for that law will equally hold good by computing the pressure -which the vapour would have if it were a permanent gas, and if it -could therefore exist in the elastic form at that low temperature. - - -(97.) Another law, common to all elastic fluids, and of equal -importance with the former, was discovered by Mariotte. By this -law it appears that every gas or vapour, so long as its -temperature is unchanged, will have a pressure directly -proportional to its density. If therefore, while we compress steam -into half its volume, we could preserve its temperature unaltered, -we should increase its pressure in a two-fold proportion; but if -the process of compression should cause its temperature to -increase, [Pg172] then its increase of pressure will be greater -than its increase of density, since it will be due conjointly to -the increase of density and to the increase of temperature. In -this case the increased pressure may be deduced from the combined -application of the two laws just explained; that of Mariotte will -determine that increase of pressure which is due to the increase -of density, and that of Dalton and Gay Lussac will determine the -further increase of pressure which will be due to the increase of -temperature. The full investigation of these effects, and the -formulae expressing them, will be found in the Appendix to this -volume. - - -(98.) The fixed relations which exist between the temperatures of -common steam and its pressure and density, have never been -discovered from any general physical principles. The pressures and -the densities however, which correspond to a great variety of -temperatures throughout the thermometric scale, have been -ascertained by extensive series of experiments instituted by -philosophers of this and other countries. From a comparison of the -temperatures and pressures thus found by experiment, empirical -formulae have been constructed, which exhibit, with an approximation -sufficiently close for practice, this relation; and these formulae -may accordingly be used for the computation of tables exhibiting the -pressures, temperatures, and densities of common steam; and such -tables will have sufficient numerical accuracy for all practical -purposes. These formulae, and the tables resulting from them, will be -found in the Appendix to this volume. - - -(99.) It has been explained, that to effect the conversion of -water into steam, it is only necessary to impart to it as much -heat as, added to the temperature which it has, would, if it -continued in the liquid form, raise it to the temperature of -1212 deg. This condition is necessary, and sufficient to effect the -transition of water into vapour. If, for example, as much heat -were imparted to the water evaporated, as would maintain it in the -liquid state to 1300 deg., then the steam so produced would be -superheated steam, having 80 deg. of heat more than is necessary to -maintain it in the vaporous form. From such steam, therefore, 80 deg. -of heat may be abstracted without producing any condensation. -[Pg173] - - -(100.) Common steam being raised from water at any pressure and -temperature, and being afterwards separated from the water, if the -same steam be compressed into a small volume, or allowed to expand -into a greater volume, it will still maintain its quality of -common steam, and will have the same pressure and temperature, -whatever volume it may assume, as it would have if immediately -raised from water at that pressure. Thus if steam be raised from -water under a pressure of 30 lbs. per square inch, and, being -separated from the water, be allowed to dilate, until its pressure -is reduced to 15 lbs. per square inch, its temperature will then -be reduced to 213 deg., which is that temperature which it would have -if immediately raised from water under a pressure of 15 lbs. per -square inch; and if any heat be abstracted from such steam, -whether under its original pressure, or under the diminished -pressure of 15 lbs. per square inch, a condensation will be -produced, the amount of which will be the same, if the same -quantity of heat be abstracted from the steam. These are -consequences which immediately flow from the fact, that the sum of -the latent and sensible heats of steam is always the same.[20] - -It appears, therefore, that supposing the steam used in an engine -to receive no additional heat after it leaves the boiler, however -it may be changed in its density by subsequent expansion, it will -still retain its character of common steam, and cannot lose any -portion of heat, however small, without suffering partial -condensation. The mechanical force also exerted by such steam, -after expansion, must be computed in the same manner as if it were -raised immediately. - - -(101.) If the law of Mariotte were strictly applicable to steam, -its mechanical effect would be the same as has been already -explained in all states of density; but since its temperature will -rise and fall as its density is increased or diminished, a -corresponding change will be produced in its [Pg174] mechanical -efficacy. It is therefore necessary in the calculation of the -mechanical effect of steam, whether it be used at a uniform -pressure without the principle of expansion, or with the -application of that principle to any given extent, to take into -account the combined operation of the laws of Mariotte and Dalton. -Formulae exhibiting the relation between the temperatures, -pressures, volumes, and densities of steam, and the mechanical -effect produced by the evaporation of water, whether acting with -or without expansion, together with the tables necessary for the -practical application of these, will be found in the Appendix. - - -(102.) One of the methods of equalising the varying force of -expanding steam, would be to work it at the same time in two -cylinders connected with the same beam; so that while its force in -one would be augmented, its force in the other would be -diminished, the combination of the two producing a uniform effect. -Soon after the expansive principle was promulged by Mr. Watt, this -expedient was accordingly resorted to by an engineer named -Hornblower. - -[Illustration: _Fig._ 30.] - -In the year 1781, Hornblower conceived the notion of working an -engine with two cylinders of different sizes, by allowing the -steam to flow freely from the boiler until it fills the smaller -cylinder, and then permitting it to expand into the greater one, -employing it thus to press down two pistons in the following -manner. - -Let C, _fig._ 30., be the centre of the great working-beam, -carrying two arch heads, on which the chains of the piston rods -play. The distances of these arch heads from the centre C must be -in the same proportion as the length of the cylinders, in order -that the same play of the beam may correspond to [Pg175] the -plays of both pistons. Let F be the steam-pipe from the boiler, -and G a valve to admit the steam above the lesser piston. H is a -tube by which a communication may be opened by the valve I, -between the top and bottom of the lesser cylinder B. K is a tube -communicating by the valve L, between the bottom of the lesser -cylinder B and the top of the greater cylinder A. M is a tube -communicating, by the valve N, between the top and bottom of the -greater cylinder A; and P a tube leading to the condenser by the -exhausting valve O. - -At the commencement of the operation, suppose all the valves -opened, and steam allowed to flow through the engine until the air -be completely expelled, and then let all the valves be closed. To -start the engine, let the exhausting valve O and the steam valves -G and L be opened, as in _fig._ 30. The steam will flow freely -from the boiler, and press upon the lesser piston, and at the same -time the steam below the greater piston will flow into the -condenser, leaving a vacuum in the greater cylinder. The valve L -being opened, the steam which is under the piston in the lesser -cylinder will flow through K, and press on the greater piston, -which, having a vacuum beneath it, will consequently descend. At -the commencement of the motion, the lesser piston is as much -resisted by the steam below it, as it is urged by the steam above -it; but after a part of the descent has been effected, the steam -below the piston, in the lesser cylinder, passing into the -greater, expands into an increased space, and therefore loses part -of its elastic force. The steam above the lesser piston retaining -its full force by having a free communication with the boiler by -the valve G, the lesser piston will be urged by a force equal to -the excess of the pressure of this steam above the diminished -pressure of the expanded steam below it. As the pistons descend, -the steam which is between them is continually increasing in its -bulk, and therefore decreasing in its pressure, from whence it -follows, that the force which resists the lesser piston is -continually decreasing, while that which presses it down remains -the same, and therefore the effective force which impels it must -be continually increasing. [Pg176] - -On the other hand, the force which urges the greater piston is -continually decreasing, since there is a vacuum below it, and the -steam which presses it is continually expanding into an increased -bulk. - -[Illustration: _Fig._ 31.] - -Impelled in this way, let us suppose the pistons to have arrived -at the bottoms of the cylinders, and let the valves G, L, and O, -be closed, and the valves I and N opened. No steam is allowed to -flow from the boiler, G being closed, nor any allowed to pass into -the condenser, since O is closed, and all communication between -the cylinders is stopped by closing L. By opening the valve I, a -free communication is made between the top and bottom of the -lesser piston through the tube H, so that the steam which presses -above the lesser piston will exert the same pressure below it, and -the piston is in a state of indifference. In the same manner the -valve N being open, a free communication is made between the top -and bottom of the greater piston, and the steam circulates above -and below the piston, and leaves it free to rise. A counterpoise -attached to the pump-rods, in this case, draws up the piston, as -in Watt's single engine; and when they arrive at the top, the -valves I and N are closed, and G, L, and O, opened, and the next -descent of the pistons is produced in the manner already -described, and so the process is continued. - -The valves are worked by the engine itself, by means similar to -some of those already described. By computation, we find the power -of this engine to be nearly the same as a similar engine on Watt's -expansive principle. It does not, however, appear, that any -adequate advantage was gained by this modification of the -principle, since no engines of this construction are now made. - - -(103.) The use of two cylinders was revived by Arthur Woolf in 1804, -who, in this and the succeeding year, obtained patents for the -application of steam raised under a high pressure to double-cylinder -engines. The specification of his patent states, that he has proved -by experiment that steam raised [Pg177] under a safety-valve loaded -with any given number of pounds upon the square inch will, if -allowed to expand into as many times its bulk as there are pounds of -pressure on the square inch, have a pressure equal to that of the -atmosphere. Thus, if the safety-valve be loaded with four pounds on -the square inch, the steam, after expanding into four times its -bulk, will have the atmospheric pressure; if it be loaded with 5, 6, -or 10 lbs. on the square inch, it will have the atmospheric pressure -when it has expanded into 5, 6, or 10 times its bulk, and so on. It -was, however, understood in this case, that the vessel into which it -was allowed to expand should have the same temperature as the steam -before it expands. - -It is very unaccountable how a person of Mr. Woolf's experience in -the practical application of steam could be led into errors so -gross as those involved in the averments of this patent; and it is -still more unaccountable how the experiments could have been -conducted which led him to conclusions not only incompatible with -all the established properties of elastic fluids, but even -involving in themselves palpable contradiction and absurdity. If -it were admitted that every additional pound avoirdupois which -should be placed upon the safety-valve would enable steam, by its -expansion into a proportionally enlarged space, to attain a -pressure equal to the atmosphere, the obvious consequence would -be, that a physical relation would subsist between the atmospheric -pressure and the pound avoirdupois! It is wonderful that it did -not occur to Mr. Woolf, that, granting his principle to be true at -any given place, it would necessarily be false at another place, -where the barometer would stand at a different height! Thus, if -the principle were true at the foot of a mountain, it would be -false at the top of it; and if it were true in fair weather, it -would be false in foul weather, since these circumstances would be -attended by a change in the atmospheric pressure, without making -any change in the pound avoirdupois.[21] - -[Pg178] - - -(104.) For several years after the extension of Watt's first -patent had been obtained from parliament, he was altogether -engrossed by the labour of bringing to perfection the application -of the steam-engine to the drainage of mines, and in surmounting -the numerous difficulties which presented themselves to its -general adoption, even after its manifold advantages were -established and admitted. When, however, these obstacles had been -overcome, and the works for the manufacture of engines for pumping -water, at Soho, had been organised and brought into active -operation, he was relieved from the pressure of these anxieties, -and was enabled to turn his attention to the far more extensive -and important uses of which he had long been impressed with the -conviction that the engine was capable. His sagacious mind enabled -him to perceive that the machine he had created was an infant -force, which by the fostering influence of his own genius would -one day extend its vast power over the arts and manufactures, the -commerce and the civilisation of the world. Filled with such -aspirations, he addressed his attention about the year 1779, to -the adaptation of the steam-engine to move machinery, and thereby -to supersede animal power, and the natural agents, wind and water. - -The idea that steam was capable of being applied extensively as a -prime mover, had prevailed from a very early period; and now that -we have seen its powers so extensively brought to bear, it will -not be uninteresting to revert to the faint traces by which its -agency was sketched in the crude speculations of the early -mechanical inventors. - - -(105.) Papin, to whom the credit of discovering the method of -producing a vacuum by the condensation of steam is due, was the -earliest and most remarkable of those projectors. With very limited -powers of practical application, he was, nevertheless, peculiarly -happy in his mechanical conceptions; and had his experience and -opportunities been proportionate to the clearsighted character of -his mind, he would doubtless have anticipated some of the most -memorable of his successors in the progressive improvement of the -steam engine. - -In his work already cited, after describing his method of -imparting an alternate motion to a piston by the atmospheric -[Pg179] pressure acting against a vacuum produced by the -condensation of steam, he stated that his invention, besides being -applicable to pumping water, could be available for rowing vessels -against wind and tide, which he proposed to accomplish in the -following manner. - -Paddle-wheels, such as have since been brought into general use, -were to be placed at the sides, and attached to a shaft extending -across the vessel. Within the vessel, and under this shaft, he -proposed to place several cylinders supplied with pistons, to be -worked by the atmospheric pressure. On the piston-rods were to be -constructed racks furnished with teeth: these teeth were to work in -the teeth of wheels or pinions, placed on the shaft of the -paddle-wheels. These pinions were not to be fixed on the shaft, but -to be connected with it by a ratchet; so that when they turned in -one direction, they would revolve without causing the shaft to -revolve; but when driven in the other direction, the catch of the -ratchet-wheel would act upon the shaft so as to compel the shaft and -paddle-wheels to revolve with the motion of the pinion or wheel upon -it. By this arrangement, whenever the piston of any cylinder was -forced down by the atmospheric pressure, the rack descending would -cause the corresponding pinion of the paddle-shaft to revolve; and -the catch of the ratchet wheel, being thus in operation, would cause -the paddle-shaft and paddle-wheels also to revolve; but whenever the -piston would rise, the rack driving the pinion in the opposite -direction, the catch of the ratchet wheel would merely fall from -tooth to tooth, without driving the paddle-shaft. - -It is evident that by such an arrangement a single cylinder and -piston would give an intermitting motion to the paddle-shaft, the -motion of the wheel being continued only during the descent of the -piston; but if several cylinders were provided, then their motion -might be so managed, that when one would be performing its -ascending stroke, and therefore giving no motion to the -paddle-shaft, another should be performing its descending stroke, -and therefore driving the paddle-shaft. As the interval between -the arrival of the piston at the bottom of the cylinder and the -commencement [Pg180] of its next descent would have been, in the -imperfect machine conceived by Papin, much longer than the time of -the descent, it was evident that more than two cylinders would be -necessary to insure a constantly acting force on the paddle-shaft, -and, accordingly, Papin proposed to use several cylinders. - -In addition to this, Papin proposed to construct a boiler having a -fireplace surrounded on every side by water, so that the heat -might be imparted to the water with such increased rapidity as to -enable the piston to make four strokes per minute. These projects -were promulged in 1690, but it does not appear that they were ever -reduced to experiment. - - -(106.) Savery proposed, in his original patent, in 1698, to apply -his steam engine as a general prime mover for all sorts of -machinery, by causing it to raise water to make an artificial -fall, by which overshot water-wheels might be driven. This -proposal was not acted on during the lifetime of Savery, but it -was at a subsequent period partially carried into effect. Mr. -Joshua Rigley erected several steam engines on this principle at -Manchester, and other parts of Lancashire, to impel the machinery -of some of the earliest manufactories and cotton mills in that -district. The engines usually raised the water from sixteen to -twenty feet high, from whence it was conveyed to an overshot -wheel, to which it gave motion. The same water was repeatedly -elevated by the engine, so that no other supply was necessary, -save what was sufficient to make good the waste. These engines -continued in use for some years, until superseded by improved -machines.[22] - - -(107.) In 1736, Jonathan Hulls obtained a patent for a method of -towing ships into or out of harbour against wind and tide. This -method was little more than a revival of that proposed by Papin in -1690. The motion, however, was to be communicated to the -paddle-shaft by a rope passing over a pulley fixed on an axis, and -was to be maintained during the returning stroke of the piston by -the descent of a weight which was elevated during the descending -stroke. There is no record, however, of this plan, any more than -that of Papin, ever having been reduced to experiment. - - -(108.) During the early part of the last century the [Pg181] -manufactures of this country had not attained to such an extent as -to render the moving power supplied by water insufficient or -uncertain to any inconvenient degree; and accordingly mills, and -other works in which machinery required to be driven by a moving -power, were usually built along the streams of rivers. About the -year 1750 the general extension of manufactures, and their -establishment in localities where water power was not accessible, -called the steam engine into more extensive operation. In the year -1752, Mr. Champion, of Bristol, applied the atmospheric engine to -raise water, by which a number of overshot wheels were driven. -These were applied to move extensive brass-works in that -neighbourhood, and this application was continued for about twenty -years, but ultimately given up on account of the expense of fuel -and the improved applications of the steam engine. - -About this time Smeaton applied himself with great activity and -success to the improvement of wind and water mills, and succeeded -in augmenting their useful effect in a twofold proportion with the -same supply of water. From the year 1750 until the year 1780 he -was engaged in the construction of his improved water mills, which -he erected in various parts of the country, and which were -imitated so extensively that the improvement of such mills became -general. In cases where a summer drought suspended the supply of -water, horse machinery was provided, either to work the mill or to -throw back the water. These improvements necessarily obstructed -for a time the extension of steam power to mill work; but the -increase of manufactures soon created a demand for power greatly -exceeding what could be supplied by such limited means. - -In the manufacture of iron, it is of great importance to keep the -furnaces continually blown, so that the heat may never be abated -by day or night. In the extensive ironworks at Colebrook Dale, -several water-wheels were used in the different operations of the -manufacture of iron, especially in driving the blowers of the iron -furnaces. These wheels were usually driven by the water of a -river, but in the summer months the supply became so short that it -was insufficient to work them all. Steam engines were accordingly -erected to [Pg182] return the water for driving these wheels. -This application of the engine as an occasional power for the -supply of water-wheels having been found so effectual, returning -engines were soon adopted as the permanent and regular means of -supplying water-wheels. The first attempt of this kind is recorded -to have been made by Mr. Oxley, in 1762, who constructed a machine -to draw coals out of a pit at Hartley colliery, in Northumberland. -It was originally intended to turn the machine by a continuous -circular motion received from the beam of the engine; but that -method not being successful, the engine was applied to raise water -for a wheel by which the machine was worked. This engine was -continued in use for several years, and though it was at length -abandoned, on account of its defective construction, it -nevertheless established the practicability of using steam power -as a means of driving water wheels.[23] - - -(109.) In the year 1777, Mr. John Stewart read a paper before the -Royal Society, describing a method for obtaining a continued -circular motion for turning all kinds of mills from the -reciprocating motion of a steam engine. He proposed to accomplish -this by means of two endless chains passing over pulleys, which -should be moved upwards and downwards by the motion of the engine, -in the manner of a window sash. The joint pins of the links of the -two chains worked in teeth at the opposite sides of a cog wheel, -to which they imparted a circular motion, first by one chain, and -then by the other, acting alternately on opposite sides of the -wheel. One chain impelled it during the descent of the piston, and -the other during the ascent; but one of these chains always passed -over its pulleys so as to produce no effect on one side of the cog -wheel, whilst the other chain worked on the opposite side to turn -it round. For this purpose each chain was provided with a catch, -to prevent its circulating over its pulleys in one direction, but -to allow it free motion in the other. The cog wheel thus kept in -revolution might be applied to the axis of any mill which the -engine was required to work. Thus, if it were applied to a -flour-mill, the millstone itself would perform the office of a -fly-wheel to regulate the intermission of [Pg183] the power, and -in other mills a fly-wheel might be added for this purpose. - -The hints obtained by Mr. Stewart from Papin's contrivance, before -mentioned, will not fail to be perceived. In Mr. Stewart's paper -he notices indirectly the method of obtaining a continued circular -motion from a reciprocating motion by means of a crank or winch, -which, he says, occurs naturally in theory, but in practice would -be impossible, from the nature of the motion of the engine, which -depends on the force of the steam, and cannot be ascertained in -its length. Therefore, on the first variation, the machine would -be either broken in pieces or turned back. Such an opinion, -pronounced by a man of considerable mechanical knowledge and -ingenuity, against a contrivance which, as will presently appear, -proved in practice, not less than in theory, to be the most -effectual means of accomplishing the end here pronounced to be -impossible, is sufficiently remarkable. It might cast some doubt -on the extent of Mr. Stewart's practical knowledge, if it did not -happen to be in accordance with a judgment so generally -unimpeachable as that of Mr. Smeaton. This paper of Mr. Stewart's -was referred by the council of the Royal Society to Mr. Smeaton, -who remarked upon the difficulty arising from the absolute -stopping of the whole mass of moving power, whenever the direction -of the motion is changed; and observed, that although a fly-wheel -might be applied to regulate the motion, it must be such a large -one as would not be readily controlled by the engine itself; and -he considered that the use of such a fly-wheel would be a greater -incumbrance to a mill than a water-wheel to be supplied by water -pumped up by the engine. This engineer, illustrious as he was, not -only fell into the error of Mr. Stewart in respect of the crank, -but committed the further blunder of condemning the very expedient -which has since rendered the crank effectual. It will presently -appear that the combination of the crank and fly-wheel have been -the chief means of establishing the dominion of the steam engine -over manufactures. - - -(110.) In 1779, Mr. Matthew Wasbrough, an engineer at Bristol, -took out a patent for the application of a steam engine [Pg184] -to produce a continuous circular motion by means of ratchet -wheels, similar to those previously used by Mr. Oxley, at Hartley -colliery; to which, however, Mr. Wasbrough added a fly-wheel to -maintain and regulate the motion. Several machines were -constructed under this patent; and among others, one was erected -at Mr. Taylor's saw-mills and block manufactory at Southampton. In -1780, one was erected at Birmingham, where the ratchet work was -found to be subject to such objections, that one of the persons -about the works substituted for it the simple crank, which has -since been invariably used. A patent was taken out for this -application of the crank in the same year, by Mr. James Pickard, -of Birmingham. It will presently appear, however, that the -suggestion of this application of the crank was derived from the -proceedings of Watt, who was at the same time engaged in similar -experiments. - - -(111.) The single-acting steam engine, as constructed by Watt, was -not adapted to produce continuous uniform motion of rotation, for -the following reasons:-- - -_First._ The effect required was that of an uniformly acting -force. The steam engine, on the other hand, supplied an -intermitting force. Its operation was continued during the -descending motion of the piston, but it was suspended during the -ascent of the piston. To produce the continued effect now -required, either its principle of operation should be altered, or -some expedient should be devised for maintaining the motion of the -revolving shaft during the ascent of the piston, and the -consequent suspension of the moving power. - -_Secondly._ The action of the steam engine was rectilinear. It was -a power which acted in a straight line, viz., in the direction of -the cylinder. The motion, however, required to be produced, was a -circular motion--a motion of rotation around the axis or shaft of -the mill. - -The steps by which Watt proceeded to accomplish these objects have -been recorded by himself as follows, in his notes upon Dr. -Robison's article on the steam engine:-- - -"I had very early turned my mind to the producing of continued -motion round an axis; and it will be seen, by reference to my -first specification in 1769, that I there described [Pg185] a -steam wheel, moved by the force of steam, acting in a circular -channel against a valve on one side, and against a column of -mercury, or some other fluid metal, on the other side. This was -executed upon a scale of about six feet diameter at Soho, and -worked repeatedly, but was given up, as several practical -objections were found to operate against it; similar objections -lay against other rotative engines, which had been contrived by -myself and others, as well as to the engines producing rotatory -motions by means of ratchet wheels. - -"Having made my single reciprocating engines very regular in their -movements, I considered how to produce rotative motions from them -in the best manner; and amongst various schemes which were -subjected to trial, or which passed through my mind, none appeared -so likely to answer the purpose as the application of the crank, -in the manner of the common turning lathe; but as the rotative -motion is produced in that machine by impulse given to the crank -in the descent of the foot only, it requires to be continued in -its ascent by the energy of the wheel, which acts as a fly; being -unwilling to load my engine with a fly-wheel heavy enough to -continue the motion during the ascent of the piston (or with a -fly-wheel heavy enough to equalise the motion, even if a -counterweight were employed to act during that ascent), I proposed -to employ two engines, acting upon two cranks fixed on the same -axis, at an angle of 120 deg. to one another, and a weight placed upon -the circumference of the fly-wheel at the same angle to each of -the cranks, by which means the motion might be rendered nearly -equal, and only a very light fly-wheel would be requisite. - -"This had occurred to me very early; but my attention being fully -employed in making and erecting engines for raising water, it -remained _in petto_ until about the year 1778 or 1789, when Mr. -Wasbrough erected one of his ratchet-wheel engines at Birmingham, -the frequent breakages and irregularities of which recalled the -subject to my mind, and I proceeded to make a model of my method, -which answered my expectations; but having neglected to take out a -patent, the invention was communicated by a workman employed -to [Pg186] make the model, to some of the people about Mr. -Wasbrough's engine, and a patent was taken out by them for the -application of the crank to steam engines. This fact the said -workman confessed, and the engineer who directed the works -acknowledged it; but said, nevertheless, that the same idea had -occurred to him prior to his hearing of mine, and that he had even -made a model of it before that time; which might be a fact, as the -application to a single crank was sufficiently obvious. - -"In these circumstances, I thought it better to endeavour to -accomplish the same end by other means, than to enter into -litigation; and if successful, by demolishing the patent, to lay -the matter open to every body. Accordingly, in 1781, I invented -and took out a patent for several methods of producing rotative -motions from reciprocating ones; amongst which was the method of -the sun-and-planet wheels. This contrivance was applied to many -engines, and possesses the great advantage of giving a double -velocity to the fly-wheel; but is perhaps more subject to wear, -and to be broken under great strains, than a simple crank, which -is now more commonly used, although it requires a fly-wheel of -four times the weight, if fixed upon the first axis; my -application of the double engine to these rotative machines -rendered the counterweight unnecessary, and produced a more -regular motion." - - -(112.) Watt's second patent here referred to, was dated 25th -October, 1781, and was entitled "A patent for certain new methods -of applying the vibrating or reciprocating motions of steam or -fire engines to produce a continued rotative or circular motion -round an axis or centre, and thereby to give motion to the wheels -of mills and other machines." - -All the methods specified in this patent were intended to be -worked by the single-acting engine, already described, a -counterweight being applied to impel the machinery during the -returning stroke of the engine, which weight would be elevated -during the descent of the piston. There were five different -expedients proposed in the specification for producing a rotatory -motion; but, of these five, two only were ever applied in -practice. [Pg187] - - -(113.) Suppose a rod or bar attached by a pin or joint at the -upper extremity to the working end of the beam of the engine, and -by a similar pin or joint at the lower extremity to an iron wheel -fixed on the extremity of the axis of the fly-wheel. One half of -this wheel is formed of a solid semicircle of cast iron, while the -other half is constructed of open spokes, so as to be as light as -is consistent with strength. The position of the wheel on the axis -is such that during the returning stroke of the piston, when the -operation of the steam is suspended, the heavy semicircle of the -wheel will be descending, and by its weight will draw down the -connecting bar, and thereby draw down the working end of the beam, -and draw up the piston in the cylinder. When the piston descends -and is driven by the power of the steam, the heavy semicircle of -the above-mentioned wheel will be drawn upwards, and in the same -way the motion will be continued. - -[Illustration: _Fig._ 32.] - - -(114.) The second method of producing a rotatory motion, which was -subsequently continued for many years in practical operation, was -that which was called the _Sun-and-planet Wheels_. A toothed wheel -A (_fig._ 32.), called the sun wheel, was fixed on the axle of the -fly-wheel, to which rotation was to be imparted. The wheel B, -called the planet wheel, having an equal diameter, was fastened on -the end I of the connecting rod H I, so as to be incapable of -revolving. During the descent of the piston, the working end of -the beam was drawn upwards, and the end I of the connecting rod -travelled from C to D, through the dotted semicircle C I D. The -wheel B not being capable of revolving on the centre I, would, -during this motion, drive the sun wheel A. During the ascent of -the steam piston, the working end of the beam would descend, and -the centre I of [Pg188] the planet wheel B would be driven -downwards from D to C, through the other dotted semicircle, and -would consequently continue to drive the sun wheel round in the -same direction. - -This contrivance, although in the main inferior to the more simple -one of the crank, is not without some advantages; among others, it -gives to the sun wheel double the velocity which would be -communicated by the crank; for in the crank one revolution only on -the axle is produced by one revolution of the crank, but in the -sun-and-planet wheel, two revolutions of the sun wheel are -produced by one of the planet wheel; thus a double velocity is -obtained from the same motion of the beam. This will be evident -from considering that when the planet wheel is in its highest -position, its lowest tooth is engaged with the highest tooth of -the sun wheel; as the planet wheel passes from the highest -position, its teeth drive those of the sun wheel before them, and -when it comes into the lowest position, the highest tooth of the -planet wheel is engaged with the lowest of the sun wheel: but then -half of the sun wheel has _rolled off_ the planet wheel, and, -therefore, the tooth which was engaged with it in its highest -position, must now be distant from it by half the circumference of -the wheel, and must, therefore, be again in the highest position; -so that while the planet wheel has been carried from the top to -the bottom, the sun wheel has made a complete revolution. - -This advantage of giving an increased velocity may be obtained -also by the crank, by placing toothed wheels on its axle. -Independently of the greater expense attending the construction of -the sun-and-planet wheel, its liability to go out of order, and -the rapid wear of the teeth, and other objections, rendered it -inferior to the crank, which has entirely superseded it. - - -(115.) Although by these contrivances Watt succeeded in obtaining -a continuous circular motion from the reciprocating motion of -the steam engine, the machine was still one of intermitting, -instead of continuous action. The expedient of a counterweight, -elevated during the descending stroke, and giving back the power -expended on it in the interval of the returning stroke, did not -satisfy the [Pg189] fastidious mechanical taste of Watt. He -soon perceived that all which he proposed to accomplish by the -application of two cylinders and pistons working alternately, -could be attained with greater simplicity and effect by a single -cylinder, if he could devise means by which the piston might be -impelled by steam upwards as well as downwards. To accomplish -this, it was only necessary to throw the lower end of the cylinder -into alternate communication with the boiler, while the upper end -would be put into communication with the condenser. If, for -example, during the descent of the piston, the upper end of the -cylinder communicated with the boiler, and the lower end with the -condenser; and, on the other hand, during the ascent of the -piston, the lower end communicated with the boiler, and the upper -end with the condenser; then the piston would be driven -continually, whether upwards or downwards, by the power of steam -acting against a vacuum. Watt obtained his third patent for this -contrivance, on the 12th of March, 1782. - -This change in the principle of the machine involved several other -changes in the details of its mechanism. - -[Illustration: _Fig._ 33.] - - -(116.) It was necessary, in the first place, to provide means for -admitting and withdrawing the steam at either end of the cylinder. -For this purpose let B and B' (_fig._ 33.) be two steam-boxes, B -the upper, and B' the lower, communicating respectively with the -top and bottom of the cylinder by proper passages D D'. Let two -valves be placed in B, one, S, above the passage D, and the other, -C, below it; and in like manner two other valves in the lower -valve-box, B', one, S', above the passage D', and the other, C', -below it. Above the valve S in the upper steam-box is an opening -at which the steam-pipe from the boiler enters, and below the -valve C is another opening, at which enters the exhausting-pipe -leading to the condenser. In like manner, above the valve S' in -the lower steam-box enters a steam-pipe leading from the boiler, -and below the valve C' enters an exhausting-pipe leading to -[Pg190] the condenser. It is evident, therefore, that steam can -always be admitted above the piston by opening the valve S, and -below it by opening the valve S'; and, in like manner, steam can -be withdrawn from the cylinder above the piston, and allowed to -pass to the condenser, by opening the valve C, and from below it -by opening the valve C'. - -[Illustration: _Fig._ 34.] - -Supposing the piston P to be at the top of the cylinder, and the -cylinder below the piston to be filled with pure steam, let the -valves S and C' be opened, the valves C and S' being closed as -represented in _fig._ 34. Steam from the boiler will, therefore, -flow in through the open valve S, and will press the piston -downwards, while the steam that has filled the cylinder below the -piston will pass through the open valve C' into the exhausting-pipe -leading to the condenser, and being condensed will leave the -cylinder below the piston a vacuum. The piston will, therefore, be -pressed downwards by the action of the steam above it, as in the -single-acting engine. Having arrived at the bottom of the cylinder, -let the valves S and C' be both closed, and the valves S' and C be -opened, as represented in _fig._ 34. Steam will now be admitted -through the open valve S' and through the passage D' below the -piston, while the steam which has just driven the piston downwards, -filling the cylinder above the piston, will be drawn off through the -open valve C, and the exhausting-pipe, into the condenser, leaving -the cylinder above the piston a vacuum. The piston will, therefore, -be pressed upwards by the action of the steam below it, against the -vacuum above it, and will ascend with the same force as that with -which it had descended. - -This alternate action of the piston upwards and downwards may -evidently be continued by opening and closing the valves alternately -in pairs. Whenever the piston is at the top of the cylinder, as -represented in _fig._ 33., the valves S and C', that is, the upper -steam-valve and the lower exhausting-valve, are opened, and the -valves C and S', that is, the upper exhausting-valve and the lower -steam-valve, are closed; and [Pg191] when the piston has arrived at -the bottom of the cylinder, as represented in _fig._ 34., the valves -C and S', that is, the upper exhausting-valve and the lower -steam-valve, are opened, and the valves S and C', that is, the upper -steam-valve and the lower exhausting-valve, are closed. - -If these valves, as has been here supposed, be opened and closed -at the moments at which the piston reaches the top and bottom of -the cylinder, it is evident that they may be all worked by a -single lever connected with them by proper mechanism. When the -piston arrives at the top of the cylinder, this lever would be -made to open the valves S and C', and at the same time to close -the valves S' and C; and when it arrives at the bottom of the -cylinder, it would be made to close the valves S and C', and to -open the valves S' and C. - -If, however, it be desired to cut off the steam before the arrival -of the piston at the termination of its stroke, whether upwards or -downwards, then the steam-valves must be closed before the arrival -of the piston at the end of its stroke; and as the exhausting-valve -ought to be left open until the stroke is completed, these valves -ought to be moved at different times. In that case separate levers -should be provided for the different valves. We shall, however, -return again to the subject of the valves which regulate the -admission of steam to the cylinder and its escape to the condenser. - - -(117.) It will be remembered that in the single-acting engine the -process of condensation was suspended while the piston ascended in -the cylinder, and therefore the play of the jet of cold water in the -condenser was stopped during this interval. In the double-acting -engine, however, the flow of steam from the cylinder to the -condenser is continued, whether the piston ascends or descends, and -therefore a constant condensation of steam must be produced. The -condensing jet, therefore, does not in this case, as in the former, -play with intervals of intermission. A constant jet of cold water -must be maintained in the condenser. - -It will presently appear that in the double-acting engine applied -to manufactures, the motion of the piston was subject to more or -less variation of speed, and the quantity of steam [Pg192] -admitted to the cylinder was subject to a corresponding change. -The quantity of steam, therefore, drawn into the condenser was -subject to variation, and required a considerable change in the -quantity of cold water admitted through the jet to condense it. To -regulate this, the valve or cock by which the water was admitted -into the condenser was worked in the double-acting engine by a -lever furnished with an index, by which the quantity of condensing -water admitted into the condenser could be regulated. This index -played upon a graduated arch, by which the engine-man was enabled -to regulate the supply. - -[Illustration: HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF -WATT.] - - FOOTNOTES: - - [20] These effects are explained in my Treatise on Heat; and - they have lately been verified by experiments made with - locomotive engines by M. de Pambour, who found that the steam - raised from the boiler of a locomotive engine, under a - pressure of above 50 lbs. per square inch, was in the state of - common steam as it issued from the chimney at a very diminished - pressure and at a lower pressure. - - [21] It is strange that this absurdity has been repeatedly - given as unquestionable fact in various encyclopaedias, as well - as in by far the greater number of treatises expressly on the - subject. - - [22] Farey, Treatise on the Steam Engine, p. 122. - - [23] Farey on the Steam Engine, p. 297. - -[Pg193] - - - - -[Illustration: DOUBLE-ACTING ENGINE.--CITY SAW-MILLS.] - -CHAP. VIII. - - METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE - DOUBLE-ACTING ENGINE. -- RACK AND SECTOR. -- PARALLEL MOTION. -- - CONNECTING ROD AND CRANK. -- FLY-WHEEL. -- THROTTLE-VALVE. -- - GOVERNOR. -- CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING - ENGINE. -- ECCENTRIC. -- COCKS AND VALVES. -- SINGLE-CLACK VALVE. - -- DOUBLE-CLACK VALVE. -- CONICAL VALVES. -- SLIDE VALVES. -- - MURRAY'S SLIDES. -- THE D VALVE. -- SEAWARD'S SLIDES. -- SINGLE - COCK. -- FOUR-WAY COCK. -- PISTONS. -- COMMON HEMP-PACKED PISTON. - -- WOOLFE'S PISTON. -- METALLIC PISTONS. -- CARTWRIGHT'S ENGINE. - -- CARTWRIGHT'S PISTON. -- BARTON'S PISTON. - - -(118.) In the single-acting engine, the force of the piston acted -on the beam only during its descent; and this force was -transmitted from the piston to the beam, as we have seen, by a -flexible chain, extended from the end of the piston-rod, [Pg194] -and playing upon the arch head of the beam. In the double-acting -engine, however, the force of the steam pressing the piston -upwards must likewise be transmitted to the beam, so as to drive -the latter upwards while the piston ascends. This action could not -be accomplished by a chain connecting the piston with the arch -head of the beam. - -Where the mechanical action to be transmitted is a _pull_, and not -a _push_, a flexible chain, cord, or strap, is sufficient; but if -a _push_ or _thrust_ is required to be transmitted, then the -flexibility of the medium of mechanical communication afforded by -a chain renders it inapplicable. In the double-acting engine, -during the descent, the piston-rod still pulls the beam down; and -so far a chain connecting the piston-rod with the beam would be -sufficient to transmit the action of the one to the other; but in -the ascent, the beam no longer pulls up the piston-rod, but is -pushed up by it. A chain from the piston-rod to the arch head, as -described in the single-acting engine, would fail to transmit this -force. If such a chain were used with the double engine, where -there is no counterweight on the opposite end of the beam, the -consequence would be, that in the ascent of the piston the chain -would slacken, and the beam would still remain depressed. It is -therefore necessary that some other mechanical connection be -contrived between the piston-rod and the beam, of such a nature -that in the _descent_ the piston-rod may _pull_ the beam down, and -may _push_ it up in the _ascent_. - -[Illustration: _Fig._ 35.] - -Watt first proposed to effect this by attaching to the end of the -piston-rod a straight rack, faced with teeth, which should work in -corresponding teeth raised on the arch head of the beam, as -represented in _fig._ 35. If his improved steam engines required no -further precision of operation and construction than the atmospheric -engines, this might have been sufficient; but in these engines it -was indispensably necessary that the piston-rod should be guided -with a smooth and even motion through the stuffing-box in the top of -the cylinder, otherwise any shake or irregularity would cause it to -work loose in the stuffing-box, and either to admit the air, or to -let the steam escape. Under these circumstances, the motion of -[Pg195] the rack and toothed arch head were inadmissible, since it -was impossible by such means to impart to the piston-rod that smooth -and equable motion which was requisite. Another contrivance which -occurred to Watt was, to attach to the top of the piston-rod a bar, -which should extend above the beam, and to use two chains or straps, -one extending from the top of the bar to the lower end of the arch -head, and the other from the bottom of the bar to the upper end of -the arch head. By such means the latter strap would pull the beam -down when the piston would descend, and the former would pull the -beam up when the piston would ascend. These contrivances, however, -were superseded by the celebrated mechanism since called the -_Parallel Motion_, one of the most ingenious mechanical combinations -connected with the history of the steam engine. - - -(119.) It will be observed that the object was to connect by some -inflexible means the end of the piston-rod with the extremity of -the beam, and so to contrive the mechanism, that while the end of -the beam would move alternately up and down in part of a circle, -the end of the piston-rod connected with the beam should move up -and down in a straight line. If the end of the piston-rod were -fastened upon the end of the beam by a pivot without any other -connection, it is evident that, being moved up and down in the -arch of a circle, it would be drawn to the left and the right -alternately, and would consequently either be broken or bent, or -would work loose in the stuffing-box. Instead of connecting the -end of the rod immediately with the end of the beam by a pivot, -Watt proposed to connect them by certain moveable rods, so -arranged that, as the end of the beam would move up and down in -the circular arch, the rods would so accommodate themselves to -that motion, that the end connected with the piston-rod should not -be disturbed from its rectilinear course. - -To explain the principle of the mechanism called the parallel -motion, let us suppose that O P (_fig._ 36.) is a rod or lever -moveable on a centre O, and that the end P of this rod shall move -through a circular arch P P' P'' P''' a vertical plane, and let its -play be limited by two stops S, which shall prevent its ascent -above the point P, and its descent below [Pg196] the point P'''. -Let the position of the rod and the limitation of its play be such -that the straight line A B drawn through P and P''', the extreme -positions of the lever O P, shall be a vertical line. - -[Illustration: _Fig._ 36.] - -Let _o_ be a point on the other side of the vertical line A B, and -let the distance of O to the right of A B be the same as the -distance of _o_ to the left of A B. Let _o p_ be a rod equal in -length to O P, moving like O P on the centre _o_, so that its -[Pg197] extremity _p_ shall play upwards and downwards through the -arch _p p' p'' p'''_, its play being limited in like manner by stops -_s_. - -Now, let us suppose that the ends P _p_ of these two rods are joined -by a link P _p_, the connection being made by a pivot, so that the -angles formed by the link and the rods shall be capable of changing -their magnitude. This link will make the motion of one rod depend on -that of the other, since it will preserve their extremities P _p_ -always at the same distance from each other. If, therefore, we -suppose the rod O P to be moved to the position O P''', its extremity -P tracing the arch P P' P'' P''', the link connecting the rods will at -the same time drive the extremity _p_ of the rod _o p_ through the -arch _p p' p'' p'''_ so that when the extremity of the one rod arrives -at P''', the extremity of the other rod will arrive at _p'''_. By this -arrangement, in the simultaneous motion of the rods, whether upwards -or downwards, through the circular arches to which their play is -limited, the extremities of the link joining them will deviate from -the vertical line A B in opposite directions. At the limits of their -play, the extremities of the link will always be in the line A B; -but in all intermediate positions, the lower extremity of the link -will be to the right of A B, and its upper extremity to the left of -A B. So far as the derangement of the lower extremity of the link is -concerned, the matter composing the link would be transferred to the -right of A B, and so far as the upper extremity of the link is -concerned, the matter composing it would be transferred to the left -of A B. - -By the combined effects of these contrary derangements of the -extremities of the link from the vertical line, it might be -expected that a point would exist, in the middle of the link, -where the two contrary derangements would neutralise each other, -and which point would therefore be expected to be disturbed -neither to the right nor to the left, but to be moved upwards and -downwards in the vertical line A B. Such is the principle of the -parallel motion; and in fact the middle point of the link will -move for all practical purposes accurately in the vertical line A -B, provided that the angular play of the levers O P and _o p_ does -not exceed a certain [Pg198] limit, within which, in practice, -their motion may always be restrained. - -To trace the motion of the middle point of the link more minutely, -let P P' P'' P''' be four positions of the lever O P, and let _p_ _p'_ -_p''_ _p'''_ be the four corresponding positions of the lever _o p_. In -the positions O P _o p_, the link will take the position P _p_, in -which the entire link will be vertical, and its middle point _x_ -will therefore be in the vertical line A B. - -When the one rod takes the position O P', the other rod will have -the position _o p'_; and the link will have the position P' _p'_. -The middle point of the link will be at _x'_, which will be found -to be on the vertical line A B. Thus one half of the link P' _x'_ -will be to the left of the vertical line A B; while the other -half, _p' x'_, will be to the right of the vertical line; the -derangement from the vertical line affecting each half of the link -in contrary directions. - -Again, taking the one rod in the position O P'', the corresponding -position of the other rod will be _o p''_, and the position of the -link will be P'' _p''_. If the middle point of the link in this -position be taken, it will be found to be at _x''_, on the vertical -line A B; and, as before, one half of the link P'' _x''_ will be -thrown to the left of the vertical line, while the other half _p'' -x''_, will be thrown to the right of the vertical line. - -Finally, let the one rod be in its lowest position, O P''', while -the other rod shall take the corresponding position, _o p'''_. The -direction of the link P''' _p'''_ will now coincide with the vertical -line; and its middle point _x'''_ will therefore be upon that line. -The previous derangement of the extremities of the rod, to the -right and to the left, are now redressed, and all the parts of the -rod have assumed the vertical position. - -It is plain, therefore, that by such means the alternate motion of -a point such as P or _p_, upwards and downwards in a circular -arch, may be made to produce the alternate motions of another -point _x_, upwards and downwards in a straight line. - - -(120.) Although the guidance of the air-pump rod in a true -vertical line is not so necessary as that of the steam piston, -[Pg199] and as the air-pump piston is always brought down by its -own weight and that of its rod, the connection of the air-pump -piston-rod with the beam, by any contrivance of the kind now -described, was not so necessary. Nevertheless, by a slight -addition to the mechanical contrivance which has been just -described, Watt obtained the means of at once preserving the true -rectilinear motion of both piston-rods. - -[Illustration: _Fig._ 37.] - -Let the lever represented by O P in _fig._ 36. be conceived to be -prolonged to twice its length, as represented in _fig._ 37., so -that O P' shall be twice O P. Let the points P _p_ be connected by -a link as before. Let a link P' _x'_, equal in length to the link -P _p_ be attached to the point P', and let the extremity _x'_ of -this link be connected with the point _p_ by another link, equal -in length to P P', by pivots at _x'_ and _p_, so that the figure P -P' _x' p_ shall be a jointed parallelogram, the angles of which -will be capable of altering their magnitude with every change of -position of the rods _o p_ and O P. Thus, when the rod O P -descends, the angles of the parallelogram at P and _x'_ will be -diminished in magnitude, while the angles at P' and _p_ will be -increased in magnitude. Now, let a line be conceived to be drawn -from O to _x'_. It is evident that that line will pass through the -middle point of the link _p_ P, for the triangle O P _x_ is in all -respects similar to the greater triangle O P' _x'_ only on half -the scale, so that every side of the one is [Pg200] half the -corresponding side of the other. Therefore P _x_ is half the -length of P' _x'_; but P' _x'_ was made equal to P _p_, and -therefore _p x_ is half of P _p_, that is to say, _x_ is the -middle point of P _p_. - -It has been already shown, that in the alternate motion of the -rods _o p_, O P in ascending and descending, the point _x_ is -moved upwards and downwards in a true vertical line. Now since the -triangle O P _x_ is in all respects similar to O P' _x'_, and -subject to a similar motion during the ascent and descent of the -rods, it is apparent that the point _x'_ must be subject to a -motion in all respects similar to that which affects the points -_x_, except that the point _x'_ will move through double the -space. In fact, the principle of the mechanism is precisely -similar to that of the common pantograph, where two rods are so -connected as that the motion of the one governs the motion of the -other, so that whatever line or figure may be described by one, a -similar line or figure must be described by the other. Since, -then, the point _x_ is moved upwards and downwards in a vertical -straight line, the point _x'_ will also be moved in a vertical -straight line of double the length. - -If such an arrangement of mechanism as has been here described can -be connected with the beam of the steam engine, so that while the -point _x'_ is attached to the top of the steam piston, and the -space through which it ascends and descends shall be equal to the -length of the stroke of that piston, the point _x_ shall be -attached to the rod of the air-pump piston, the stroke of the -latter being half that of the steam piston, then the points _x'_ -and _x_ will guide the motion of the two pistons so as to preserve -them in true vertical straight lines. - -The manner in which these ideas are reduced to practice admits of -easy explanation: let the point O be the centre of the great -working beam, and let O P' be the arm of the beam on the side of -the steam cylinder. Let P be a pivot upon the beam, at the middle -point between its centre O and its extremity P'; and let the links -P p, P' _x'_, and P _p_ be jointed together, as already described. -Let the point or pivot _o_ be attached to some part of the fixed -framing of the engine or engine house, and let the rod _o p_, -equal to half the arm of the beam, be attached by a pivot to the -corner of the parallelogram at [Pg201] _p_. Let the end of the -steam piston-rod be attached to the corner of the parallelogram -_x'_, and let the end of the air-pump be attached to the middle -point _x_ of the link P _p_; by which arrangement it is evident -that the rectilinear motion of the two piston-rods will be -rendered compatible with the alternate circular motions of the -points P' and P on the beam. - -Among the many mechanical inventions produced by the fertile -genius of Watt, there is none which has excited such universal, -such unqualified, and such merited admiration as that of the -parallel motion. It is indeed impossible, even for an eye -unaccustomed to view mechanical combinations, to behold the beam -of a steam engine moving the pistons, through the instrumentality -of the parallel motion, without an instinctive feeling of pleasure -at the unexpected fulfilment of an end by means having so little -apparent connection with it. When this feeling was expressed to -Watt himself, by those who first beheld the performance of this -exquisite mechanism, he exclaimed with his usual vivacity, that he -himself, when he first beheld his own contrivance in action, was -affected by the same sense of pleasure and surprise at its -regularity and precision. He said, that he received from it the -same species of enjoyment that usually accompanies the first view -of the successful invention of another person. - -"Among the parts composing the steam engine, you have doubtless," -says M. Arago, "observed a certain articulated parallelogram. At -each ascent and descent of the piston, its angles open and close -with the sweetness--I had almost said with the grace--which charms -you in the gestures of a consummate actor. Follow with your eye -alternately the progress of its successive changes, and you will -find them subject to the most curious geometrical conditions. You -will see, that of the four angles of the jointed parallelogram, -three describe circular arches, but the fourth which holds the -piston-rod is moved nearly in a straight line. The immense utility -of this result strikes mechanicians with even less force than the -simplicity of the means by which Watt has attained it." - -The parallel motion, of which there are several other varieties, -depending, however, generally upon the same [Pg202] principle, -formed part of a patent which Mr. Watt obtained in the year 1784, -another part of which patent was for a locomotive engine, by which -a carriage was to be propelled on a road. In a letter to Mr. -Smeaton dated 22d October, in the same year, Watt says,-- - -"I have lately contrived several methods of getting entirely rid -of all the chains and circular arches about the great levers of -steam engines, and nevertheless making the piston-rods ascend and -descend perpendicularly, without any sliding motions or -right-lined guides, merely by combinations of motions about -centres; and with this further advantage, that they answer equally -well to push upwards as to pull downwards, so that this method is -applicable to our double engines which act both in the ascent and -descent of their pistons. - -"A rotative engine of this species with the new motion which is -now at work in our manufactory (but must be sent away very soon) -answers admirably. It has cost much brain work to contrive proper -working gear for these double engines, but I have at last done it -tolerably well, by means of the circular valves, placed in an -inverted position, so as to be opened by the force of the steam; -and they are kept shut by the working gear. We have erected an -engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield, -London." - -[Illustration: _Fig._ 38.] - - -(121.) By the contrivance which has been explained above, the -force of the piston in ascending and descending would be conveyed -to the working end of the beam; and the next problem which Watt -had to solve was, to produce by the force exerted by the working -end of the beam in ascending and descending a continuous motion of -rotation. In the first instance he proposed to accomplish this by -a crank placed upon the axle to which rotation was to be imparted, -and driven by a rod connecting it with the working end of the -beam. Let K (_fig._ 38.) be the centre, to which motion is to be -imparted by the working end H of the beam. On the axle K suppose a -short lever K I to be fixed so that when K I is turned round the -centre K, the axle must turn with it. Let an iron rod, the weight -of which shall balance the piston and piston-rod at the other end -of the beam, be connected by joints with the working end H of the -beam, and the extremity I of the [Pg203] lever K I. As the end H -of the beam is moved upwards and downwards, the lever K I will be -turned round the centre K, taking successively the positions -represented by faint lines in the figure; and thus a motion of -continued rotation will be imparted to the axle K. - -This simple and effectual expedient of producing a continued -rotatory motion by a crank was abandoned by Watt, as already -explained, by reason of a patent having been obtained upon -information of his experiments surreptitiously procured. To avoid -litigation, he therefore substituted for the crank the sun and -planet wheel already described; but at the expiration of the -patent, which restricted the use of the crank, the sun and planet -wheel was discontinued in Watt's engine, and the crank restored. - - -(122.) Whether the crank or the sun and planet wheel be used, -there is still a difficulty in the maintenance of a regular motion -of rotation. In the various positions which the crank and -connecting rod assume throughout a complete revolution, there are -two in which the moving power loses all influence in impelling the -crank. These positions are those which the crank assumes when the -piston is at the top and bottom of the [Pg204] cylinder, and is -just about to change the direction of its motion. When the piston -is at the bottom of the cylinder, the pivot I (_fig._ 38.), by -which the connecting rod H I is attached to the end of the crank, -is immediately over the axle K of the crank, and under the pivot -H, which joins the upper end of the connecting rod with the beam. -In fact, in this position the connecting rod and crank are in the -same straight line, extending from the end of the beam to the axle -of the crank. The steam, on entering the cylinder below the -piston, and pressing it upwards, would produce a corresponding -downward force on the connecting rod at H, which would be -continued along the connecting rod and crank to the axle K. It is -evident that such a force could have no tendency to turn the crank -round, but would expend its whole energy in pressing the axle K -downwards. - -The other position in which the power loses its effect upon the -crank is when the piston is at the top of the cylinder. In this -case, the working end of the beam will be at the lowest point of -its play, and the crank-pin I will be immediately below the axle -K; so that K will be placed immediately between H and I. When the -steam presses on the top of the piston, it will expend its force -in drawing the end H of the connecting rod upwards, by which the -crank-pin I will likewise be drawn upwards. It is evident that -this force can have no effect in turning the crank round, but will -expend its whole energy in producing an upward strain on the axle -K. - -If the crank were absolutely at rest in either of the positions -above described, it is apparent that the engine could not be put -in motion by the steam; but if the engine has been previously in -motion, then the mass of matter forming the crank, and the axle on -which the crank is formed, having already had a motion of -rotation, will have a tendency to preserve the momentum it has -received, and this tendency will be sufficient to throw the crank -K I out of either of those critical positions which have been -described. Having once escaped these dead points, then the -connecting rod forming an angle, however obtuse or acute, with the -crank, the pressure or pull upon the former will have a tendency -to produce rotation in the latter. As the crank revolves, however, -the influence [Pg205] of the connecting rod upon it will vary -according to the angle formed by the connecting rod and crank. -When that angle is a right angle, then the effect of the -connecting rod on the crank is greatest, since the force upon it -has the advantage of the whole leverage of the crank; but -according as the angle formed by the crank and connecting rod -becomes more or less acute or obtuse in the successive attitudes -which they assume in the revolution of the crank, the influence of -the connecting rod over the crank varies, changing from nothing at -the two dead points already described, to the full effect produced -in the two positions where they are at right angles. In -consequence of this varying leverage, by which the force with -which the connecting rod is driven by the steam is transmitted to -the axle on which the crank revolves, a corresponding variation of -speed would necessarily be produced in the motion imparted to the -crank. The speed at the dead points would be least, being due -altogether to the momentum already imparted to the revolving mass -of the crank and axle; and it would gradually increase and be -greatest at the points where the effect of the crank on the -connecting rod is greatest. Although this change of speed would -not affect the actual mechanical efficacy of the machine, and -although the same quantity of steam would perform the same work at -the varying velocity as it would do if the velocity were -regulated, yet this variation of speed would be incompatible with -the purposes to which it was now proposed that the steam engine -should be applied in manufactures. In these a regular uniform -motion should be imparted to the main axle. - - -(123.) One of the expedients which Watt proposed for the -attainment of this end was, by placing two cranks on the same -axle, in different positions, to be worked by different cylinders, -so that while one crank should be at its dead points, the other -should be in the attitude most favourable for its action. This -expedient has since, as we shall see, been carried into effect in -steam vessels; but one more simple and efficient presented itself -in the use of a _fly-wheel_. - -On the main axle driven by the crank Watt placed a large wheel of -metal, as represented in _fig._ 43., called a _fly-wheel_. This -wheel being well constructed, and nicely balanced on its [Pg206] -axle, was subject to very little resistance from friction; any -moving force which it would receive it would therefore retain, and -would be ready to impart such moving force to the main axle -whenever that axle ceased to be driven by the power. When the -crank, therefore, is in those positions in which the action of the -power upon it is most efficient, a portion of the energy of the -power is expended in increasing the velocity of the mass of matter -composing the fly-wheel. As the crank approaches the dead points, -the effect of the moving power upon the axle and upon the crank is -gradually enfeebled, and at these points vanishes altogether. The -momentum which has been imparted to the fly-wheel then comes into -play, and carries forward the axle and crank out of the dead -points with a velocity very little less than that which it had -when the crank was in the most favourable position for receiving -the action of the moving power. - -By this expedient, the motion of revolution received by the axle -from the steam piston is subject to no other variation than just -the amount of change of momentum in the great mass of the -fly-wheel, which is sufficient to extricate the crank twice in -every revolution from the mechanical dilemma to which its peculiar -form exposes it; and this change of velocity may be reduced to as -small an amount as can be requisite by giving the necessary weight -and magnitude to the fly-wheel. - - -(124.) By such arrangements the motion imparted to the main axle K -would be uniform, provided that the moving power of the engine be -always proportionate to the load which it drives. But in the -general application of the steam engine to manufactures it was -evident that the amount of the resistance to which any given -machine would be subject must be liable to variation. If, for -example, the engine drive a cotton-mill, it will have to impart -motion to all the spinning frames in that mill. The operation of -one or more of these may from time to time be suspended, and the -moving power would be relieved from a corresponding amount of -resistance. If, under such circumstances, the energy of the moving -power remained the same, the velocity with which the machines -would be driven would be subject to variation, being increased -whenever the operation of any portion of the machines usually -[Pg207] driven by it is suspended; and, on the other hand, -diminished when any increased number of machines are brought into -operation. In fine, the speed would vary nearly in the inverse -proportion of the load driven, increasing as the load is -diminished, and _vice versa_. - -On the other hand, supposing that no change took place in the -amount of the load driven by the engine, and that the same number -of machines of whatever kind would have to be continually driven, -the motion imparted to the main axle would still be subject to -variation by the changes inevitable to the moving power. The -piston of the engine being subject to an unvaried resistance, a -uniform motion could only be imparted to it, by maintaining a -corresponding uniformity in the impelling power. This would -require a uniform supply of steam from the boiler, which would -further imply a uniform rate of evaporation in the boiler, unless -means were provided in the admission of steam from the boiler to -the cylinder to prevent any excess of steam which might be -produced in the boiler from reaching the cylinder. - -[Illustration: _Fig._ 39.] - -[Illustration: _Fig._ 40.] - -This end was attained by a contrivance afterwards called the -_throttle-valve_. An axis A B (_figs._ 39, 40.) was placed across -the steam pipe in a ring of cast-iron D E, of proper thickness. On -this axis was fastened a thin circular plate T, of nearly the same -diameter as the steam pipe. On the outer end B of this axle was -placed a short lever or handle B C, by which it could be turned. -When the circular plate T was turned into such a position as to be -at right angles to the length of the tube, it stopped the passage -within the tube altogether, so that no steam could pass from the -boiler to the engine. On the other hand, when the handle was -turned through a fourth of a revolution from this position, then -the circular plate T had its plane in the direction of the length -of the tube, so that its edge would be presented towards the -current of steam flowing from the boiler to the cylinder. In that -position the passage within the tube [Pg208] would be necessarily -unobstructed by the throttle-valve. In intermediate positions of -the valve, as that represented in _figs._ 39, 40., the passage -might be left more or less opened, so that steam from the boiler -might be admitted to the cylinder in any regulated quantity -according to the position given to the lever B C. - -A view of the throttle-valve taken by a section across the steam -pipe is exhibited in _fig._ 40., and a section of it through the -axis of the steam pipe is represented in _fig._ 39. The form of -the valve is such, that, if accurately constructed, the steam in -passing from the boiler would have no effect by its pressure to -alter any position which might be given to the valve; and any -slight inaccuracy of form which might give a tendency to the steam -to alter the position would be easily counteracted by the friction -of the valve upon its axle. The latter might be regulated at -pleasure. - -By this expedient, however the evaporation of water in the boiler -might vary within practical limits, the supply of steam to the -cylinder would be rendered regular and uniform. If the boiler became -too active, and produced more steam than was necessary to move the -engine with its load at the requisite speed, then the throttle-valve -was shifted so as to contract the passage and limit the supply of -steam. If, on the other hand, the process of evaporation in the -boiler was relaxed, then the throttle-valve was placed with its edge -more directed towards the steam. Independently of the boiler, if the -load on the engine was lightened, then the same supply of steam to -the cylinder would unduly accelerate the motion. In this case, -likewise, the partial closing of the throttle-valve would limit the -supply of steam and regulate the motion; and if, on the other hand, -the increase of load upon the engine rendered necessary an increased -supply of steam, then the opening of the throttle-valve would -accomplish the purpose. By these means, therefore, a uniform motion -might be maintained, provided the vigilance of the engine-man -was sufficient for the due management of the lever B C, and -provided that the furnace under the boiler was kept in sufficient -activity to supply the greatest amount of steam which would be -necessary [Pg209] for the maintenance of a uniform motion with the -throttle-valve fully opened. - - -(125.) Watt, however, soon perceived that the proper manipulation -of the lever B C would be impracticable with any degree of -vigilance and skill which could be obtained from the persons -employed to attend the engine. He, therefore, adapted to this -purpose a beautiful application of a piece of mechanism, which had -been previously used in the regulation of mill-work, and which has -since been well known by the name of the _Governor_, and has -always been deservedly a subject of much admiration. - -The governor is an apparatus by which the axle of the fly-wheel is -made to regulate the throttle-valve, so that the moment that the -axle begins to increase its velocity, it shifts the position of -the throttle-valve, so as to limit the supply of steam from the -boiler, and thereby to check the increase of speed. And on the -other hand, whenever the velocity of the axle is diminished, the -lever B C is moved in the contrary direction, so as to open more -fully the passage for the steam, and accelerate the motion of the -engine. - -A small grooved wheel A B (_fig._ 41.) is attached to a vertical -spindle supported in pivots or sockets C and D, in which it is -capable of revolving. An endless cord works in the groove A B, and -is carried over proper pulleys to the axle of the fly-wheel, where -it likewise works in a groove. When this cord is properly -tightened the motion of the fly-wheel will give motion to the -wheel A B, so that the velocity of the one will be subject to all -the changes incidental to the velocity of the other. By this means -the speed of the grooved wheel A B may be considered as -representing the speed of the fly-wheel, and of the machinery -which the axle of the fly-wheel drives. - -[Illustration: _Fig._ 41.] - -It is evident that the same end might be attained by substituting -for the grooved wheel A B a toothed wheel, which might be -connected by other toothed wheels, and proper shafts, and axles -with the axle of the fly-wheel. - -A ring or collar E is placed on the upright spindle, so as to be -capable of moving freely upwards and downwards. To this ring are -attached by pivots two short levers, E F, the [Pg210] pivots or -joints at E allowing these levers to play upon them. At F these -levers are joined by pivots to other levers F G, which cross each -other at H, where an axle or pin passes through them, and attaches -them to the upright spindle C D. These intersecting levers are -capable, however, of playing on this axle or pin H. To the ends G -of these levers are attached two heavy balls of metal I. The -levers F G pass through slits in a metallic arch attached to the -upright spindle, so as to be capable of revolving upon it. If the -balls I are drawn outwards from the vertical axis, it is evident -that the ends F of the levers will be drawn down, and therefore -the pivots E likewise drawn down. In fact, the angles E F H will -become more acute, and the angle F E F more obtuse. By these means -the sliding ring E will be drawn down. To this sliding ring E, and -immediately above it, is attached a grooved collar, which slides -on the vertical spindle upwards and downwards with the ring E. In -the grooved collar are inserted the prongs of a fork K, formed at -the end of the lever K L, the fulcrum or pivot of the lever being -at L. By this arrangement, when the divergence of the balls I -causes the collar E to be drawn down, the fork K, whose prongs are -inserted in the groove of that collar, is likewise drawn down; -and, on the other hand, when, by reason of the balls I falling -towards the [Pg211] vertical spindle, the collar E is raised, the -fork K is likewise raised. - -The ascent and descent of the fork K necessarily produce a -contrary motion in the other end N of the lever. This end is -connected by a rod, or system of rods, with the end M of the short -lever which works the throttle-valve T. By such means the motion -of the balls I, towards or from the vertical spindle, produces in -the throttle-valve a corresponding motion; and they are so -connected that the divergence of the balls I will cause the -throttle-valve to close, while their descent towards the vertical -spindle will cause it to open. - -These arrangements being comprehended, let us suppose that, either -by reason of a diminished load upon the engine or an increased -activity of the boiler, the speed has a tendency to increase. This -would impart increased velocity to the grooved wheel A B, which -would cause the balls I to revolve with an accelerated speed. The -centrifugal force which attends their motion would therefore give -them a tendency to move from the axle, or to diverge. This would -cause, by the means already explained, the throttle-valve T to be -partially closed, by which the supply of steam from the boiler to -the cylinder would be diminished, and the energy of the moving -power, therefore, mitigated. The undue increase of speed would -thereby be prevented. - -If, on the other hand, either by an increase of the load, or a -diminished activity in the boiler, the speed of the machine was -lessened, a corresponding diminution of velocity would take place -in the grooved wheel A B. This would cause the balls I to revolve -with less speed, and the centrifugal force produced by their -circular motion would be diminished. This force being thus no -longer able fully to counteract their gravity, they would fall -towards the spindle, which would cause, as already explained, the -throttle-valve to be more fully opened. This would produce a more -ample supply of steam to the cylinder, by which the velocity of -the machine would be restored to its proper amount. - -[Illustration: _Fig._ 42.] - - -(126.) The principle which renders the governor so perfect a -regulator of the velocity of the machine is difficult to be -[Pg212] explained without having recourse to the aid of the -technical language of mathematical physics. As, however, this -instrument is of such great practical importance, and has -attracted such general admiration, it may be worth while here to -attempt to render intelligible the mechanical principles which -govern its operation. Let S (_fig._ 42.) be the point of -suspension of a common pendulum S P, and let P O P' be the arch of -its vibration, so that the ball P shall swing or vibrate -alternately to the east and to the west of the lowest point O, -through the arches O P' and O P. It is a property of such an -instrument that, provided the arch in which it vibrates be not -considerable in magnitude, the time of its vibration will be the -same whether the arch be long or short. Thus, for example, if the -pendulum, instead of vibrating in the arch P P', vibrated in the -arch _p p'_, the time which it would take to perform its -vibrations would be the same. If, however, the magnitude of the -arch of vibration be increased, then a variation will take place -in the time of vibration; but unless the arch of vibration be -considerably increased, this variation will not be great. - -Now let it be supposed that while the pendulum P P' continues to -vibrate east and west through the arch P P', it shall receive such -an impulse from north and south as would, if it were not in a -state of previous vibration, cause it to vibrate between north and -south, in an arch similar to the arch P P'. This second vibration -between north and south [Pg213] would not prevent the continuance -of the other vibration between east and west; but the ball P would -be at the same time affected by both vibrations. While, in virtue -of the vibration from east to west, the ball would swing from P to -P', it would, in virtue of the other vibration, extend its motion -towards the north to a distance from the line W E equal to half a -vibration, and will return from that distance again to the -position P'. While returning from P' to P, its second vibration -will carry it towards the south to an equal distance on the -southern side of W E, and it will return again to the position P. -If the combination of these two motions or vibrations be -attentively considered, it will be perceived that the effect on -the ball will be a circular motion, precisely similar to the -circular motion of the balls of the governor already described. - -Now the time of vibration of the pendulum S P between east and -west will not in any way be affected by the second vibration, -which it is supposed to receive between north and south, and -therefore the time the pendulum takes in moving from P to P' and -back again from P' to P will be the same whether it shall have -simultaneously or not the other vibration between north and south. -Hence it follows that the time of revolution of the circular -pendulum will be equal to the time of similar vibrations of the -same pendulum, if, instead of having a circular motion, it were -allowed to vibrate in the manner of a common pendulum. - -If this point be understood, and if it also be remembered that the -time of vibration of a common pendulum is necessarily the same -whether the arch of vibration be small or great, it will be easily -perceived that the revolving pendulum or governor will have nearly -the same time of revolution whether it revolve in a large circle -or a small one: in other words, whether the balls revolve at a -greater or a less distance from the central spindle or axis. This, -however, is to be understood only approximately. When the angle of -divergence of the balls is as considerable as it usually is in -governors, the time of revolution at different distances from the -axis will therefore be subject to some variation, but to a very -small one. [Pg214] - -The centrifugal force (which is the name given in mechanics to -that influence which makes a body revolving in a circle fly from -the centre) depends conjointly on the velocity of revolution, and -on the distance of the revolving body from the centre of the -circle. If the velocity of revolution be the same, then the -centrifugal force will increase in the same proportion as the -distance of the revolving body from the centre. If, on the other -hand, the distance of the revolving body from the centre remain -the same, the centrifugal force will increase in the same -proportion as the square of the time of vibration diminishes, or, -in other words, it will increase in the same proportion as the -square of the number of revolutions per minute. It follows from -this, therefore, that the greater is the divergence of the balls -of the governor, and the more rapidly they revolve, the greater -will be their centrifugal force. Now this centrifugal force, if it -were not counterbalanced, would give the balls a constant tendency -to recede from the centre; but from the construction of the -apparatus, the further they are removed from the centre the -greater will be the effect of their gravitation in resisting the -centrifugal force. - -It is evident that the ball at P will have a greater tendency to -fall by gravitation towards O than it would have at _p_, because -the acclivity of the arch descending towards O at P is greater -than its acclivity at _p_. The gravitation, therefore, or tendency -of the ball to fall towards the central axis being greater at P -than at _p_ it will be able to resist a greater centrifugal force. -This increased centrifugal force, which the ball would have -revolving at the distance P above what it would have at the -distance _p_, is produced partly by the greater distance of the -ball from the central axis, and partly by the greater velocity of -its motion. But it will be evident that the time of its revolution -may nevertheless be the same, or nearly the same, at both -distances. If it should appear that the actual velocity of its -motion of revolution at P be greater than its velocity at _p_, in -the same proportion as the circles in which they revolve, then it -is evident that the time of revolution would be as much increased -by the greater space which P will have to travel over, as it will -have to be [Pg215] diminished by the greater speed with which -that space is traversed. The time of revolution, therefore, may be -the same, or nearly the same, in both cases. - -If this explanation be comprehended, it will not be difficult to -apply it to the actual case of the governor. If a sudden increase -of the energy of the moving power, or a diminution of the load, -should give the machine an increased velocity, then the increased -speed of the balls of the governor will give them an increased -centrifugal force, which for the moment will be greater than the -tendency of their gravitation to make them fall towards the -vertical axis. This centrifugal force, therefore, prevailing, the -balls will recede from the axis; but as they recede, their -gravitation towards the vertical axis will, as has been already -explained, be increased, and will become equal to the centrifugal -force produced by the increased velocity, provided that velocity -do not exceed a certain limit. When the balls, by diverging, get -such increased gravitation as to balance the centrifugal force, -then they will continue to revolve at a fixed distance from the -vertical axis. When this happens, the time of the revolution must -be nearly the same as it was before their increased divergence; in -other words, the proportion of the moving power to the load will -be so restored by the action of the levers of the governor on the -throttle-valve that the machine will move at its former velocity, -or nearly so. - -The principle on which the governor acts, as just explained, -necessarily supposes temporary disarrangements of the speed. In -fact, the governor, strictly speaking, does not maintain a uniform -velocity, but restores it after it has been disturbed. When a -sudden change of motion of the engine takes place, the governor -being immediately affected will cause a corresponding alteration -in the throttle-valve; and this will not merely correct the change -of motion, but it will, as it were, overdo it, and will cause a -derangement of speed of the opposite kind. Thus if the speed be -suddenly increased to an undue amount, then the governor being -affected will first close the throttle-valve too much, so as to -reduce the speed below the proper limit. This second error will -again affect the governor in the contrary way, and the speed -[Pg216] will again be increased rather too much. In this way a -succession of alterations of effect will ensue until the governor -settles down into that position in which it will maintain the -engine at the proper speed. - -To prevent the inconvenience which would attend any excess of such -variations, the governor is made to act with great delicacy on the -throttle-valve, so that even a considerable change in the -divergence of the balls shall not produce too much alteration in -the opening of that valve: the steam in the boiler should have at -least 2 lbs. per square inch pressure more than is generally -required in the cylinder. This excess is necessary to afford scope -for that extent of variation of the power which it is the duty of -the throttle-valve to regulate. - -The governor is usually so adjusted as to make thirty-six -revolutions per minute, when in uniform motion; but if the motion -is increased to the rate of thirty-nine revolutions, the balls -will fly to the utmost extent allowed them, being the limitation -of the grooves in which their rods move; and if, on the other -hand, the speed be diminished to thirty-four revolutions per -minute, they will collapse to the lowest extent of their play. The -duty of the governor, therefore, is to correct smaller casual -derangements of the velocity; but if any permanent change to a -considerable extent be made either in the load driven by the -machine or in the moving power supplied to it from the boiler, -then a permanent change is necessary to be made in the connection -between the governor and the throttle-valve, so as to render the -governor capable of regulating those smaller changes to which the -speed of the machine is liable. - - -(127.) Having thus explained the principal mechanical contrivances -provided by Watt for the maintenance and regulation of the -rotatory motion to be produced by his double-acting steam engine, -let us now consider the machine as a whole, and investigate the -process of its operation. A section of this engine is represented -in _fig._ 43. - -[Illustration: _Fig._ 43.] - -Steam is supplied from the boiler to the cylinder by the steam -pipe S. The throttle-valve T in that pipe, near the cylinder, is -regulated by a system of levers connected with [Pg217] the -governor. The piston P is accurately fitted in the steam cylinder -C by packing, as already described in the single-acting engine. -This piston, as it moves, divides the cylinder into two -compartments, between which there is no communication by which -steam or any other elastic fluid can pass. The upper steam box B -is divided into three compartments by the two valves. Above the -upper steam valve V is a compartment communicating with the steam -pipe; below the upper exhausting valve E is another compartment -communicating with the eduction pipe which leads to the condenser. -By the valves V and E a communication may be opened or closed -between the boiler on the one hand, or the condenser on the other, -and the top of the cylinder. The continuation S' of the steam pipe -leads to the lower box B', which, like the upper, is divided into -three compartments by two valves V' and E'. The upper compartment -communicates with the steam pipe, and thereby with the boiler; and -the lower compartment communicates with the eduction pipe, and -thereby with the condenser. By means of the two valves V' and E', -a communication may be opened or closed between the steam pipe on -the one hand, or the exhausting pipe on the other, and the lower -part of the cylinder. The four valves V, E, V', and E' are -connected by a system of levers with a handle or spanner _m_, -which, being driven downwards or upwards, is capable of opening or -closing the valves in pairs, in the manner already described -(116.). The condensers, the air-pump, and the hot-water pump, are -in all respects similar to those already described in the -single-acting engine, except that the condensing jet is governed -by a lever I, by which it is allowed to play continually in the -condenser, and by which the quantity of water admitted through it -is regulated. The cold-water pump N is worked by the engine as -already described in the single-acting engine, and supplies the -cistern in which the air-pump and condenser are submerged, so as -to keep down its temperature to the proper limit. On the air-pump -rod R are two pins properly placed, so as to strike the spanner -_m_, upwards and downwards, at the proper times, when the piston -approaches the termination of the stroke at the top or bottom of -the cylinder. The pump L [Pg218] conducts the warm water drawn by -the air-pump from the condenser to a proper reservoir for feeding -the boiler. The vertical motion of the piston-rod in a straight -line is rendered compatible with the circular motion of the end of -the beam by the parallel motion already described. The point _b_, -on the beam, moves upwards and downwards in a circular arch, of -which the axis of the beam is the centre. In like manner the point -_d_ of the rod _d c_ moves upwards and downwards, in a similar -arch of which the fixed pivot _c_ is the centre. The joint or bar -_d b_, which joins these two pivots, will be moved so that its -middle point _e_ will ascend and descend nearly in a straight -line, as has been already explained (120.); [Pg219] opposite this -point _e_ is attached the piston-rod of the air-pump, which is -accordingly guided upwards and downwards by this means. The -jointed parallelogram _b d g f_ is attached to the beam by pivots; -and, as has been explained (120.), the point _g_ will be moved -upwards and downwards in a straight line, through twice the space -through which the point _e_ is moved. To the point _g_ the rod of -the steam piston is attached. Thus, the rods of the steam piston -and air-pump are moved by the same system of jointed bars, and -moved through spaces which are in the proportion of two to one. - -Although this system of jointed rods forming the parallel motion, -appears in the figure to consist only of one parallelogram _b d g -f_, and one rod _c d_, called the _radius rod_, it is, in fact, -double, a similar parallelogram and radius rod being attached to -corresponding points, and in the same manner on the other side of -the beam; but from the view given in the cut, the one set of rods -hides the other. The two systems of rods thus attached to opposite -sides of the beam at several inches asunder, are connected by -cross rods, the ends of which form the pivots or joints, and -extend between the parallelograms. The ends of these rods are only -visible in the figure. It is to the middle of one of these rods, -the end of which is represented at _e_, that the air-pump -piston-rod is attached; and it is to the middle of another, the -end of which is represented at _g_, that the steam piston-rod is -attached. These two piston-rods, therefore, are driven, not -immediately by either of the parallelograms forming the parallel -motion, but by the bars extending between them. - -To the working end of the beam H is attached a rod of cast-iron O, -called the _connecting rod_, the lower end of which is attached to -the crank by a pivot. The weight of the connecting rod is so made, -that it shall balance the weight of the piston-rods of the air-pump -and cylinder on the other side of the beam; and the weight of the -piston-rod of the cold-water pump N nearly balances the weight of -the piston-rod of the hot-water pump L. Thus, so far as the weights -of the machinery are concerned, the engine is in equilibrium, and -the piston would rest in any position indifferently in the cylinder. - -The axis of the fly-wheel on which the crank is formed is [Pg220] -square in the middle part, where the fly-wheel is attached to it, -but has cylindrical necks at each end, which rest in sockets or -bearings supported by the framing of the machine, in which sockets -the axis revolves freely. On the axle of the crank is placed the -fly-wheel, and connected with its axle is the governor Q, which -regulates the throttle-valve T in the manner already described. - -Let us now suppose the engine to be in full operation. The piston -being at the top of the cylinder, the spanner _m_ will be raised -by the lower pin on the air-pump rod, and the upper steam valve V, -and the lower exhausting valve E', will be opened, while the upper -exhausting valve E and the lower steam valve V' are closed. Steam -will, therefore, be admitted above the piston, and the steam which -filled the cylinder below it will be drawn off to the condenser, -where it will be converted into water. The piston will, therefore, -be urged by the pressure of the steam above it to the bottom of -the cylinder. As it approaches that limit, the spanner _m_ will be -struck downwards by the upper pin on the air-pump rod, and the -valves V and E' will be closed, and at the same time the lower -steam valve V' and the upper exhausting valve E will be opened. -Steam will, therefore, be admitted below the piston, while the -steam above it will be drawn off into the condenser, and converted -into water. The pressure of the steam, therefore, below the piston -will urge it upwards, and in the same manner the motion will be -continued. - -While this process is going on in the cylinder and the condenser, -the water formed in the condenser will be gradually drawn off by -the operation of the air-pump piston, in the same manner as -explained in the single-acting engine; and at the same time the -hot water thrown into the hot well by the air-pump piston will be -carried off by the hot-water pump L. - -Such are the chief circumstances attending the continuance of the -operation of the double-acting engine. It is only necessary here to -recall what has been already explained respecting the operation of -the fly-wheel. The commencement of the motion of the piston from the -top and bottom of the cylinder is produced, not by the pressure of -the steam upon it upwards or downwards, which must, for the reasons -[Pg221] already explained, be entirely inefficient; but by the -momentum of the fly-wheel, which extricates the crank from those -positions in which the moving power cannot affect it. - -The manner in which the motion of the crank affects the connecting -rod at the dead points produces an effect of great importance in -the operation of the engine. When the crank-pin is approaching the -lowest point of its play, and therefore the piston approaching the -top of the cylinder, the motion of the crank-pin becomes nearly -horizontal, and consequently its effect in drawing the connecting -rod and the working end of the beam downwards and the piston -upwards, is extremely small. The consequence of this is, that as -the piston approaches the top of the cylinder, its motion becomes -very rapidly retarded; and as the motion of the crank-pin at its -lowest point is actually horizontal, the piston is brought to a -state of rest by this gradually retarded motion at the top of the -cylinder. In like manner, when the crank-pin moves from its dead -point upwards, its motion at first is very nearly horizontal, and -consequently its effect in driving the working end of the beam -upwards, and the piston downwards, is at first very small, but -gradually accelerated. The effect of this upon the piston is, that -it arrives at and departs from the top of the stroke with a very -slow motion, being absolutely brought to rest at that point. - -The same effect is produced when the piston arrives at the bottom -of the cylinder. This retardation and suspension of the motion of -the piston at the termination of the stroke affords time for the -process of condensation to be effected, so that when the moving -power of the steam upon the piston can come into action, the -condensation shall be sufficiently complete. As the piston -approaches the top of the cylinder, and its motion becomes slow, -the working gear is made to open the lower exhausting valve; the -steam enclosed in the cylinder below the piston, and which has -just driven the piston upwards, presses with an elastic force of -17 lbs. per square inch on every part of the interior of the -cylinder, while the uncondensed vapour in the condenser presses -with a force of about 2 lbs. per square inch. The steam, -therefore, will have a tendency to rush from the cylinder to the -[Pg222] condenser through the open exhausting valve, with an -excess of pressure amounting to 15 lbs. per square inch, while the -piston pauses at the top of the cylinder. This process goes on, -and when the piston has descended by the motion of the fly-wheel, -a sufficient distance from the top of the cylinder to call the -moving force of the steam into action, the exhaustion will be -complete, and the pressure of the uncondensed vapour in the -cylinder will become the same as in the condenser. - -The pressure of steam in the cylinder, and of uncondensed vapour -in the condenser, varies, within certain limits, in different -engines, and therefore the amount here assigned to them must be -taken merely as an example. - -The size of the valves by which the steam is allowed to pass from -the cylinder to the condenser should be such as to cause the -condensation to take place in a sufficiently short time, to be -completed when the steam impelling the piston is called into -action. - -Watt, in the construction of his engines, made the -exhaustion-valves with a diameter which was one fifth of the -diameter of the cylinder, and therefore the actual magnitude of -the aperture for the escape of the steam was one twenty-fifth of -the magnitude of the cylinder; but the spindle of the valve -diminished this so that the available space for the escape of -steam did not exceed one twenty-seventh of the magnitude of the -cylinder. This was found to produce a sufficiently rapid -condensation. - -It was usual to make the steam valves of the same magnitude as the -exhausting valves, but the flow of steam through the former was -resisted by the throttle-valve, while no obstruction was opposed -to its passage through the latter. - -The rapidity with which the cylinder must be exhausted by the -condenser will, however, depend upon the velocity with which the -piston is moved in it. The magnitude, therefore, of the exhausting -valves which would be sufficient for an engine which acts with a -slow motion would be too small where a rapid motion is required. - -In the single-acting steam engine, where the moving force always -acted downwards on the piston, the pressure upon [Pg223] all the -joints of the machinery by which the force of the piston was -conveyed to the working parts, always took place in the same -direction, and consequently whatever might be the mechanical -connection by which the several joints were formed, the pins by -which they were connected, must always come to a bearing in their -respective sockets, however loosely they may have been fitted. For -the same reason, however, that the arch head and chain were -abandoned as a means of connecting the steam piston with the beam, -and the parallel motion substituted, it was also necessary in the -double-acting engine, where all joints whatever were driven -alternately in opposite directions, to fit the connecting pins -with the greatest accuracy in their sockets, and to abandon all -connection of the parts by chains. If any sensible looseness was -left in the joints, a violent jerk would be produced every time -the motion of the piston was reversed. Any looseness either in the -pivots or joints of the parallel motion of the working beam, the -connecting rod, or crank, would, at every change of stroke, be so -accumulated as to produce upon the machinery the effects of -percussion, and would consequently be attended with the danger of -straining and breaking the moveable parts of the mechanism. - -To secure, therefore, the necessary accuracy of the joints, Watt -contrived that every joint in the engine should admit of the size of -the socket being exactly adapted to the size of the pin, so as -always to make a good fitting by closing the socket upon the pin, -when any looseness would be produced by wear. With this view, all -the joints were fitted with sockets made of brass or gun-metal, -capable of adjustment. Each socket was composed of two pieces, -accurately fitted into a cell or groove, in which one of the brasses -can be moved towards the other by means of a wedge or screw. Each -brass has in it a semi-cylindrical cavity, and the two cavities -being opposed to each other, form a socket for the joint-pin. One of -the two brasses can always be tightened round that pin, so as to -enclose it tight between the two semi-cylindrical cavities, and to -prevent any looseness taking place. The brasses, and other parts of -such a joint, are represented [Pg224] in _fig._ 44. These joints -still continue to be used in the engines as now constructed. - -[Illustration: _Fig._ 44.] - -The motion of the working beam, and the pump-rods which it drives, -and of the connecting rod, ought, if the whole were constructed -with perfect precision, to take place in the same or parallel -vertical planes; but this supposes a perfection of execution which -could hardly have been expected in the early manufacture of such -engines, whatever may have been attained by improvements which -have been since made. In the details of construction, Watt saw -that there would be a liability to lateral strain, owing to the -planes of the different motions not being truly vertical and truly -parallel, and that if a provision were not made for such lateral -motion, the machinery would be subject to constant strain in its -joints and rapid wear. He provided against this by constructing -the main joints by which the great working lever was connected -with the pistons and connecting rod, so as to form universal -joints, giving freedom of motion laterally as well as vertically. - -The great lever, or working beam, was so called from being -originally made from a beam of oak. It is now, however, -universally constructed of cast-iron. The connecting rod is also -made of cast-iron, and attached to the beam and to the crank by -axles or pivots. - -The mechanism by which the four valves are opened and closed, is -subject to considerable variation in different engines. They have -been described above as being opened and closed simultaneously by -a single lever. Sometimes, however, they are opened alternately in -pairs by two distinct levers driven by two pins attached to the -air-pump rod. One pin strikes the lever, which opens and closes -the upper steam valve, and lower exhausting valve; the other -strikes that which opens and closes the lower steam valve and -upper exhausting valve. - -Since the date of the earlier double-acting engines, constructed -by Boulton and Watt, a great variety of mechanical expedients have -been practised for working the valves, by which the steam is -admitted to and withdrawn from the [Pg225] cylinder. We shall -here describe a few of these methods:-- - - -(128.) The method of working the valves by pins on the air-pump -rod driving levers connected with the valves has been, in almost -all modern double-acting machines, superseded by an apparatus -called an _eccentric_, by which the motion of the axle of the -fly-wheel is made to open and close the valves at the proper -times. - -[Illustration: _Fig._ 45.] - -An eccentric is a metallic circle attached to a revolving axle, so -that the centre of the circle shall not coincide with the centre -round which the axle revolves. Let us suppose that G (_fig._ 45.), -is a square revolving shaft. Let a circular plate of metal B D, -having its centre at C, have a square hole cut in it, corresponding -to the shaft G, and let the shaft G pass through this square -aperture, so that the circular plate B D shall be fastened upon the -shaft, and capable of revolving with it as the shaft revolves. The -centre C of the circular plate B D will be carried round the centre -G of the revolving shaft, and will describe round it a circle, the -radius of which will be the distance of the centre C of the circular -plate from the centre of the shaft. Such circular plate so placed -upon a shaft, and revolving with it, is _an eccentric_. - -Let E F be a metallic ring, formed of two semicircles of metal -screwed together at H, so as to be capable, by the adjustment of -the screws, of having the circular aperture formed by the ring -enlarged and diminished within certain [Pg226] small limits. Let -this circular aperture be supposed to be equal to the magnitude of -the eccentric B D. To the circular ring E F let an arm L M be -attached. If the ring E F be placed around the eccentric B D, and -that the screws H be so adjusted as to allow the eccentric B D to -revolve within the ring E F, then while the eccentric revolves, -the ring not partaking of its revolution, the arm L M will be -alternately driven to the right and to the left, by the motion of -the centre C of the eccentric as it revolves round the centre G of -the axle. When the centre C of the eccentric is in the same -horizontal line with the centre G, and to the left of it, then the -position of L M will be that which is represented in _fig._ 45.; -but when, after half a revolution of the main axle, the centre C -of the eccentric is thrown on the other side of the centre G, then -the point M will be transferred to the right, to a distance equal -to twice the distance C G. Thus as the eccentric B D revolves -within the ring E F, that ring, together with the arm L M, will be -alternately driven, right and left, through a space equal to twice -the distance between the centre of the eccentric and the centre of -the revolving shaft. - -If we suppose a notch formed at the extremity of the arm L M, which -is capable of embracing a lever N M, moveable on a pivot at N, the -motion of the eccentric would give to such a lever an alternate -motion from right to left, and _vice versa_. If we suppose another -lever N O connected with N M, and at right angles to it, forming -what is called a bell-crank, then the alternate motion received by -M, from right to left, would give a corresponding motion to the -extremity O of the lever N O, upwards and downwards. If this last -point O were attached to a vertical arm or shaft, it would impart to -such arm or shaft an alternate motion upwards and downwards, the -extent of which would be regulated by the length of the levers -respectively. - -By such a contrivance the revolution of the fly-wheel shaft is -made to give an alternate vertical motion of any required extent -to a vertical shaft placed near the cylinder, which may be so -connected with the valves as to open and close them. Since the -upward and downward motion of this vertical shaft is governed by -the alternate motion of the centre [Pg227] C to the right and to -the left of the centre G, it is evident that by the adjustment of -the eccentric upon the fly-wheel shaft, the valves may be opened -and closed at any required position of the fly-wheel and crank, -and therefore at any required position of the piston in the -cylinder. - -Such is the contrivance by which the valves, whatever form may be -given to them, are now almost universally worked in double-acting -steam engines. - - * * * * * - -Having described the general structure and operation of the steam -engine as improved by Watt, we shall now explain, in a more -detailed manner, some parts of its machinery which have been -variously constructed, and in which more or less improvements have -been made. - - -OF THE COCKS AND VALVES. - - -(129.) In the steam engine, as well as in every other machine in -which fluids act, it is necessary to open or close, occasionally, -the tubes or passages through which these fluids move. The -instruments by which this is accomplished are called cocks or -valves. - -Cocks or valves may be classified by the manner in which they are -opened: 1st, they may be opened by a motion similar to the lid of -a box upon its hinges; 2d, they may be opened by being raised -directly upwards, in the same manner as the lid of a pot or -kettle; 3d, they may be opened by a sliding motion, like that of -the sash of a window or the lid of a box which slides in grooves; -4th, they may be opened by a motion of revolution, in the same -manner as the cock of a beer-barrel is opened or closed. The term -_valve_ is more properly applied to the first and second of these -classes; the third class are usually called _slides_, and the -fourth _cocks_. - - -(130.) The single clack valve is the most simple example of the -first class. It is usually constructed by attaching to a plate of -metal larger than the aperture which the valve is intended to -stop, a piece of leather, and to the under side of this leather -another piece of metal smaller than the aperture. The leather -[Pg228] extending on one side beyond the larger metallic plate, -and being flexible, forms the hinge on which the valve plays. Such -a valve is usually closed by its own weight, and opened by the -pressure of the fluid which passes through it. It is also held -closed more firmly by the pressure of the fluid whose return it is -intended to obstruct. An example of this valve occurs in the steam -engine, in the passage between the condenser and the air-pump. The -aperture which it stops is there a seat inclined at an angle whose -inclination is such as to render the weight of the valve -sufficient to close it. In cases where the valve is exposed to -heat, as in the example just mentioned, where it is continually in -contact with the hot water flowing from the condenser to the -air-pump, the use of leather is inadmissible, and in that case the -metallic surface of the valve is ground smooth to fit its seat. - -The extent to which such a valve should be capable of opening, -ought to be such that the aperture produced by it shall be equal -to the aperture which it stops. This will be effected if the angle -through which it rises be about 30 deg. - -[Illustration: _Fig._ 46.] - -The valve by which the air and water collected in the bottom of -the air-pump are admitted to pass through the air-pump piston is a -double clack, consisting of two semicircular plates, having the -hinges on the diameters of these semicircles, as represented in -_fig._ 46. - - -(131.) Of the valves which are opened by a motion perpendicular to -their seat, the most simple is a flat metallic plate, made larger -than the orifice which it is intended to stop, and ground so as to -rest in steam-tight contact with the surface surrounding the -aperture. Such a valve is usually guided in its perpendicular -motion by a spindle passing through its centre, and sliding in -holes made in cross bars extending above and below the seat of the -valve. - -The conical steam-valves, which have been already described -(116.), usually called spindle-valves, are the most common of this -class. The best angle to be given to the conical seat is found in -practice to be 45 deg. With a less inclination the valve has a -tendency to be fastened in its seat, and a greater inclination -would cause the top of the valve to occupy [Pg229] unnecessary -space in the valve-box. The area, or transverse section of the -valve-box, should be rather more than double the magnitude of the -upper surface of the valve, in order to allow a sufficiently free -passage for the steam, and the play of the valve should be such as -to allow it to rise from its seat to a height not less than one -fourth of the diameter of its upper surface. - -The valves coming under this class are sometimes formed as spheres -or hemispheres resting in a conical seat, and in such cases they -are generally closed by their own weight, and opened by the -pressure of the fluid which passes through them. - - -(132.) One of the advantages attending the use of slides, compared -with the other form of valves, is the simplicity with which the -same slide may be made to govern several passages, so that a -single motion with a slide may perform the office of two or more -motions imparted to independent valves. - -In most modern engines the passage of the steam to and from the -cylinder is governed by slides of various forms, some of which we -shall now explain. - -[Illustration: _Fig._ 47.] - - -(133.) In _figs._ 47. and 48. is represented a slide-valve -contrived by Mr. Murray of Leeds. A B is a steam-tight case -attached to the side of the cylinder; E F is a rod, which receives -an alternate motion, upwards and downwards, from the eccentric, or -from whatever other part of the engine is intended to move the -slide. This rod, passing through a stuffing-box, moves the slide G -upwards and downwards. S is the mouth of the steam pipe coming -from the boiler; T is the mouth of a tube or pipe leading to the -condenser; H is a passage leading to the top, and I to the bottom, -of the cylinder. In the position of the slide represented in -_fig._ 47., the steam coming from the boiler through S passes -through the space H to the top of the cylinder, while the steam -from the bottom of the cylinder passes through the space I into -the tube T, and goes to the condenser. When the rod [Pg230] E F -is raised to the position represented in _fig._ 48., then the -passage H is thrown into communication with the tube T, while the -passage I is made to communicate with the tube S. Steam, -therefore, passes from the boiler through I below the piston, -while the steam which was above the piston, passing through H into -T, goes to the condenser. Thus the single slide G performs the -office of the four valves described in (116.). - -[Illustration: _Fig._ 48.] - - -(134.) The slide G has always steam of a full pressure behind it, -while the steam in front of it escaping to the condenser, exerts -but little pressure upon it. It is therefore always forcibly -pressed against the surfaces in contact with which it moves, and -is thereby maintained steam-tight. Indeed this pressure would -rapidly wear the rubbing surfaces, unless they were made -sufficiently extensive, and hardened so as to resist the effects -of the friction. Where fresh water is used, as in land boilers, -the slide may be made of hardened steel; and in the case of marine -boilers, it may be constructed of gun-metal. In this and all other -contrivances in which the apertures by which the steam is admitted -to and withdrawn from the piston are removed to any considerable -distance from the top and bottom of the cylinder, there is a waste -of steam, for the steam consumed at each stroke of the piston is -not only that which would fill the capacity of the cylinder, but -also the steam which fills the passage between the slide G and the -top or bottom of the cylinder. Any arrangement which would throw -the passages H and I on the other side of the slide G, that is, -between S and G, instead of being, as they are, between G and the -top and bottom of the cylinder, would remove this defect. This is -accomplished by a slide, which is usually called the D valve, -because, being semi-cylindrical in its form, and hollow, its cross -section resembles the letter D. This slide, which is that which at -present is in most general use, is represented in _figs._ 49, 50.; -E is the rod by which the slide is moved, passing [Pg231] through -a stuffing-box F; G G is the slide represented by a vertical -section, _a a_ being a passage in it extending from the top to the -bottom; S is the mouth of the great steam pipe coming from the -boiler; P is the pipe leading to the condenser; T H is a hollow -space formed in the slide always in communication with the steam -pipe S, and consequently always filled with steam from the boiler. -A transverse section of the slide and cylinder is represented in -_fig._ 51., where _a_ represents the top of the passage marked _a_ -in _fig._ 49. In the position of the slide represented in _fig._ -49., the steam filling the space T H has access to the top of the -cylinder, but is excluded from the bottom. The steam which was -below the piston, passing up the passage _a_, escapes through the -tube P to the condenser. When the piston has descended, the rod E -moves the slide downwards, so as to give it the position -represented in FIG. 50. The steam in T H has now access to the -bottom of the cylinder, while the steam above the piston passing -through P escapes to the condenser. In this way the operation of -the piston is continued and the steam consumed at each stroke only -exceeds the capacity of the cylinder by what is necessary to fill -the passages between the slide and the cylinder. - -[Illustration: _Fig._ 49.] - -[Illustration: _Fig._ 50.] - -[Illustration: _Fig._ 51.] - -In a slide constructed in this manner, the steam filling the space -T H has a tendency to press the slide back, so as to break the -contact of the rubbing surfaces, and thereby to cause the steam to -leak from the space T H to the back of the slide. This is -counteracted by the packing _x_, at the back of the slide. - -In engines of very long stroke, the extent of the rubbing surfaces -of slides of this kind renders it difficult to keep [Pg232] them -in steam-tight contact and to insure their uniform wear. In such -cases, therefore, separate slides, upon the same principle, are -provided at the top and bottom of the cylinder, moved, however, by -a single rod of communication. - - -(135.) In slides, as we have here described them, the same motion -which admits steam to either end of the cylinder, withdraws it -from the other end. Such an arrangement is only compatible with -the operation of a cylinder which works without expansion; for in -such a cylinder the full flow of steam to the piston is only -interrupted for a moment during the change of position of the -slide. But if the steam act expansively, it would be necessary to -move the slide, so as to stop its flow to one end of the cylinder, -without at the same time obstructing the escape of steam from the -other end to the condenser. It would therefore be necessary that -the slide should close the passage leading to the cylinder at one -end, without at the same time obstructing the communication -between the passage from the cylinder to the condenser at the -other end. On the arrival of the piston, however, at the bottom of -the cylinder, it would be necessary immediately to put the lower -passage to the cylinder in communication with the steam pipe, and -the upper passage in communication with the condenser. This would -necessarily suppose two motions of the slide as well as some -modifications in its length. Let the length of the slide be such -that when the passage to the top of the cylinder is stopped, the -lower part of the slide shall not reach the passage to the lower -part of the cylinder; and let such a provision be made in the -mechanism by which the rod E governing the slide is driven that it -shall receive two motions during the descent of the piston, the -first to be imparted to it at the moment the steam is to be cut -off, and the second just before the termination of the stroke. Let -the position of the slide, at the commencement of the stroke, be -represented in _fig._ 52., and let it be required that the steam -shall be cut off at one half of the stroke. When the piston has -made half the stroke, the rod governing the slide is moved -downwards, so as to throw the slide into the position represented -in _fig._ 53. The passage between the steam pipe and the cylinder -is [Pg233] now stopped at both ends; but the passage from the -bottom of the cylinder to the condenser remains open. During the -remainder of the stroke, therefore, the steam in the cylinder -works expansively. As the piston approaches the bottom of the -cylinder, another motion is imparted to the rod governing the -slide, by which the latter is thrown into the position represented -in _fig._ 54. Steam now flows below the piston while the steam -above it passes to the condenser. In a similar manner, by two -motions successively imparted to the slide during the ascent of -the piston, the steam may be cut off at half stroke; and it is -evident that by regulating the time at which these motions are -given to the slide, the steam may be worked expansively, to any -required extent. - -[Illustration: _Fig._ 52.] - -[Illustration: _Fig._ 53.] - -[Illustration: _Fig._ 54.] - -It is easy to conceive various mechanical means by which, in the -same engine, the point at which the steam is cut off may be -regulated at pleasure. - -In cases where the motion of the piston is very rapid, as in -locomotive engines, it is desirable that the passages to and from -the cylinder should be opened very suddenly. This is difficult to -be accomplished with any form of slide consisting of a single -aperture; but if, instead of admitting the steam to the cylinder -by a single aperture, the same magnitude of opening were divided -among several apertures, then a proportionally less extent of -motion in the slide would clear the passage for the steam, and -consequently greater suddenness of opening would be effected. -[Pg234] - -The great advantages in the economy of fuel resulting from the -application of the expansive principle have, of late years forced -themselves on the attention of engineers, and considerable -improvements have been made in its application, especially in the -case of marine engines used for long voyages, in which the economy -of fuel has become an object of the last importance. The mechanism -by which expansive slides are moved, is made capable of adjustment, -so that the part of the stroke at which the steam is cut off, can be -altered at pleasure. The working power of the engine, therefore, -instead of being controlled by the throttle-valve, is regulated by -the greater or less extent to which the expansive principle is -applied. Steam of the same pressure is admitted to the cylinder in -all cases; but it is cut off at a greater or less portion of the -stroke, according to the power which the engine is required to -exert. - -The last degree of perfection has been conferred on this principle -by connecting the governor with the mechanism by which the slide -is moved, so that the governor instead of acting on the -throttle-valve, is made to act upon the slide. By this means when, -by reason of any diminution of the resistance, the motion of the -engine is accelerated, the balls of the governor diverging shift -the cam or lever which governs the slide, so that the steam is cut -off after a shorter portion of the stroke, the expansive principle -is brought into greater play, and the quantity of steam admitted -to the cylinder at each stroke is diminished. If, on the other -hand, the resistance to the machine be increased, so as to -diminish the velocity of the engine, then the balls collapsing the -levers of the governor shift the cam which moves the slides, so as -to increase the portion of the stroke made by the piston before -the steam is cut off, and thereby to increase the amount of -mechanical power developed in the cylinder at each stroke. The -extent to which the expansive principle is capable of being -applied, more especially in marine engines, has been hitherto -limited by the necessity of using steam of very high pressure, -whenever the steam is cut off after the piston has performed only -a small part of the stroke. A method, however, is now (March, -1840) under experimental trial, by [Pg235] Messrs. Maudsley and -Field, by which the expansive principle may be applied to any -required extent without raising the steam in the boiler above the -usual pressure of from three to five pounds per square inch. This -method consists in the use of a piston of great magnitude. The -force urging the piston is thus obtained not by an excessive -pressure on a limited surface, but by a moderate pressure diffused -over a large surface. The entire moving force acting on the piston -before the steam is cut off, is considerably greater than the -resistance; but during the remainder of the stroke this force is -gradually enfeebled until the piston is brought to the extremity -of its play. - -[Illustration: _Fig._ 55.] - - -(136.) Mr. Samuel Seaward, of the firm of Messrs. Seawards, -engineers, has contrived an improved system of slides, for which -he has obtained a patent. A section of Seaward's slides is -represented in _fig._ 55. The steam pipe proceeding from the -boiler to the cylinder is represented at A A, and it communicates -with passages S and S' leading to the top and bottom of the -cylinder. These passages are formed in nozzles of iron or other -hard metal cast upon the side of the cylinder. These nozzles -present a smooth face outwards, upon which the slides B B', also -formed with smooth faces, play. The slides B B' are attached by -knuckle-joints to rods E E', which move through stuffing-boxes, -and the [Pg236] connection of these rods with the slides is such -that the slides have play so as to detach their surfaces easily -from the smooth surfaces of the nozzles when not pressed against -these surfaces. The steam in the steam pipe A A will press against -the backs of the slides B B', and keep their faces in steam-tight -contact with the smooth surfaces of the nozzles. These slides may -be opened or closed by proper mechanism at any point of the -stroke. When steam is to be admitted to the top of the cylinder, -the upper slide is raised and the passage S opened; and when it is -to be admitted to the bottom of the cylinder, the lower slide is -raised and the passage S' opened; and its communication to the top -or bottom of the cylinder is stopped by the lowering of these -slides respectively. On the other side of the cylinder are -provided two passages C C' leading to a pipe G, which is continued -to the condenser. On this pipe are cast nozzles of iron or other -metal presenting smooth faces towards the cylinder, and having -passages D D' communicating between the top and bottom of the -cylinder respectively and the pipe G G leading to the condenser. -Two slides _b b'_, having smooth faces turned from the cylinder, -and pressing upon the faces of the nozzles D D', are governed by -rods playing through stuffing-boxes, in the same manner as already -described. The faces of these slides being turned from the -cylinder, the steam in the cylinder having free communication with -them, has a tendency to keep them by its pressure in steam-tight -contact with the surfaces in which the apertures leading to the -condenser are formed. These two slides may be opened or closed -whenever it is necessary. - -When the piston commences its descent, the upper steam slide is -raised, so as to open the passage S, and admit steam above the -piston; and the lower exhausting slide _b'_ is also raised, so as -to allow the steam below the piston to escape through G to the -condenser, the other two passages S' and C being closed by their -respective slides. The slide which governs S is lowered at that -part of the stroke at which the steam is intended to be cut off, -the other slides remaining unchanged; and when the piston has -reached the bottom of the cylinder, the lower steam slide opens -the passage S', and [Pg237] the upper exhausting slide opens the -passage C; and at the same time the lower exhausting slide closes -the passage C'. Steam being admitted below the piston through S', -and at the same time the steam above it being drawn away to the -condenser through the open passage C and the tube G, the piston -ascends. When it has reached that point at which the steam is -intended to be cut off, the slide which governs S' is lowered, the -other slides remaining unaltered, and the upward stroke is -completed in the same manner as the downward. - -These four slides may be governed by a single lever, or they may -be moved by separate means. From the small spaces between the -several slides and the body of the cylinder, it will be evident -that the waste of steam by this contrivance will be very small. - -In the slide valves commonly used, the packing of hemp at the back -of the slide, by which the pressure necessary to keep the slide in -steam-tight contact is obtained, requires constant attention from -the engine-man while the engine is at work. Any neglect of this -will produce a corresponding loss in the power of the engine; and -accordingly it is found that in many cases where engines work -inefficiently, the defect is owing either to ignorance or want of -attention on the part of the engine-man in the packing of the -slides. In Seaward's slides no hemp packing is used, nor is any -attention on the part of the engine-man required after the slides -are first adjusted. The slides receive the pressure necessary to -keep them in steam-tight contact with the surfaces of the nozzles -from the steam itself, which acts behind them. - -The eduction and steam slides being independent of each other, -they may be adjusted so that the engine shall work expansively in -any required degree; and this may be accomplished either by -working the slides by separate mechanism, or by a single -eccentric. - -One of the advantages claimed by the patentees for these slides -is, that the engines are secured from the accidents which arise -from the accumulation of water within the steam cylinder. If such -a circumstance should occur, the action of the piston will press -the water against the faces of the steam [Pg238] slides, and the -play allowed to them by their connection with the rods which move -them permits their faces to be raised from the surfaces of the -nozzles, so that the water collected in the cylinder shall be -driven into the steam pipe, and sent back from thence to the -boiler. - -[Illustration: _Fig._ 56.] - -[Illustration: _Fig._ 57.] - -[Illustration: _Fig._ 58.] - - -(137.) Of the cocks or valves which are opened and closed by the -motion of an axis passing through their centre, the throttle-valve, -whether worked by hand or by the governor, is an example. But the -most common form for cocks is that of a cylindrical or slightly -conical plug (_fig._ 56.), inserted in an aperture of corresponding -magnitude passing across the pipe or passage which the cock is -intended to open or close. One or more holes are pierced -transversely in the cock, and when the cock is turned so that these -holes run in the direction of the tube, the passage through the tube -is opened; but when the passage through the cock is placed at right -angles to the tube, then the sides of the tube stop the ends of the -passage in the cock, and the passage through the tube is obstructed. -The simple cock is designed to open or close the passage through a -single tube. When the cock is turned, as in _fig._ 57., so that the -passage through the cock shall be at right angles to the length of -the tube, then the passage through the tube is stopped; but when the -cock is turned from that position through a quarter of a revolution, -as in _fig._ 58., then the passage through the cock takes the -direction of the passage through the tube, and the cock is opened, -and the passage through the tube unobstructed. In such a cock the -passage may be more or less _throttled_ by [Pg239] adjusting the -position of the cock, so that a part of the opening in it shall be -covered by the side of the tube. - - -(138.) It is sometimes required to put one tube or passage -alternately in communication with two others. This is accomplished -by a _two-way cock_. In this cock the passage is curved, opening -usually at points on the surface of the cock, at right angles to -each other. Such a cock has already been described, and its use -illustrated in the description of the Marquis of Worcester's -engine (17.); the two-way cock, as represented at K and R (_fig._ -4.), being the means by which steam and water are alternately -supplied to the two forcing vessels. - -[Illustration: _Fig._ 59.] - -[Illustration: _Fig._ 60.] - - -(139.) When it is required to put four passages alternately in -communication by pairs, a _four-way cock_ is used. Such a cock has -two curved passages (_fig. 59._), each similar to the curved -passage in the two-way cock. Let S C B T be the four tubes which -it is required to throw alternately into communication by pairs. -When the cock is in the position _fig._ 59., the tube S -communicates with T, and the tube C with B. By turning the cock -through a quarter of a revolution, as in _fig._ 60., the tube S is -made to communicate with B, and the tube C with T; and if the cock -continue to be turned at intervals through a quarter of a -revolution, these changes of communication will continue to be -alternately made. It is evident that this may be accomplished by -turning the cock continually in the same direction. - -The four-way cock is sometimes used as a substitute for the valves -or slides in a double-acting steam engine to conduct the steam to -and from the cylinder. If S represent a pipe conducting steam from -the boiler, C that which leads to the condenser, T the tube which -leads to the top of the cylinder, and B that which leads to the -bottom, then when the cock is in the position (_fig._ 59.), steam -would flow from the boiler to [Pg240] the top of the piston, -while the steam below it would be drawn off to the condenser; and -in the position (_fig._ 60.), steam would flow from the boiler to -the bottom of the piston, while the steam above it would be drawn -off to the condenser. Thus by turning the cock through a quarter -of a revolution towards the termination of each stroke, the -operation of the machine would be continued. - -One of the disadvantages which is inseparable from the use of a -four-way cock for this purpose is the loss of the steam at each -stroke, which fills the tubes between the cock and the ends of the -cylinder. This disadvantage could only be avoided by the -substitution of two two-way cocks (138.) instead of a four-way -cock. A two-way cock at the top of the cylinder would open an -alternate communication between the cylinder and steam pipe, and -the cylinder and condenser, while a similar office would be -performed by another two-way cock at the other end. - -The friction on cocks of this description is more than on other -valves; but this is in some degree compensated by the great -simplicity of the instrument. When the cock is truly ground into -its seat, being slightly conical in its form, the pressure of the -steam has a tendency to keep the surfaces in contact; but this -pressure also increases the friction, and has a tendency to wear -the seat of the cock into an elliptical shape. Consequently, such -cocks require to be occasionally ground and refitted. - - -(140.) The four-way cock, as above described, admits the steam to -one end of the piston at the same moment that it stops it at the -other end. It would therefore be inapplicable where steam is -worked expansively. A slight modification, however, analogous to -that already described in the slides, will adapt it to expansive -action. This will be accomplished by giving to one of the passages -through the cock one aperture larger than the other, and working -the cock so that this passage shall always be used to conduct -steam to the cylinder; also by enlarging both apertures of the -other passage, and using it always to conduct steam from the -cylinder. The effect of such an arrangement will be readily -understood. - -[Illustration: _Fig._ 61.] - -[Illustration: _Fig._ 62.] - -[Illustration: _Fig._ 63.] - -[Illustration: _Fig._ 64.] - -Let the position of the cock at the commencement of the [Pg241] -descending stroke be represented in _fig._ 61. Steam flows from S -through T to the top of the cylinder, while it escapes from B -through C from the bottom of the cylinder. When the piston has -arrived at that point at which the steam is to be cut off, let the -cock be shifted to the position represented in _fig._ 62. The -passage of steam from the boiler is now stopped, but the escape of -steam from the bottom of the cylinder through C continues, and the -cock is maintained in this position until the piston approaches -the bottom of the cylinder, when it is further shifted to the -position represented in _fig._ 63. Steam now flows from S through -B to the bottom of the cylinder, while the steam from the top of -the cylinder escapes through C to the condenser. When the piston -has arrived at that point where the steam is to be cut off, the -cock is shifted to the position represented in _fig._ 64. The -communication between the steam and the bottom of the piston is -now stopped, while the communication between the top of the -cylinder and the condenser is still open. During the next double -stroke of the piston the position of the cock is similarly -changed, but in the contrary direction, and in the same way the -motion is continued. Under these circumstances the cock, instead -[Pg242] of being moved constantly in the same direction, as in the -case of the common four-way cock, will require to be moved -alternately in opposite directions. - - -PISTONS. - - -(141.) The office of a piston being to divide a cylinder into two -compartments by a movable partition which shall obstruct the passage -of any fluid from one compartment to the other, it is evident that -the two conditions which such an instrument ought to fulfil are, -_first_, that the contact of its sides with the surface of the -cylinder shall be so close and tight throughout its entire play that -no steam or other fluid can pass between them; _secondly_, that it -shall be so free from friction, notwithstanding this necessary -tightness, that it shall not absorb any injurious quantity of the -moving power. - -Since, however accurately the surfaces of the piston and cylinder -may be constructed, there will always be in practice more or less -imperfection of form, it is evident that the contact of the -surface of the piston with the cylinder throughout the stroke can -only be maintained by giving to the circumference of the piston -sufficient elasticity to accommodate itself to such inequalities -of form. The substance, whatever it may be, used for this purpose, -and by which the piston is surrounded, is called _packing_. - -In steam pistons the material used for packing must be such as is -capable of resisting the united effects of heat and moisture. -Hence leather and other animal substances are inapplicable. - -The packing used for steam pistons is therefore of two kinds, -_vegetable packing_, usually hemp, or _metallic packing_. - -The common hemp-packed piston has been already in part described -(79.). The bottom of the piston is a circular plate just so much -less in diameter than the cylinder as is sufficient to allow its -free motion in ascending and descending. A little above its lowest -point this plate begins gradually to diminish in thickness, until -its diameter is reduced to from one to two inches less than that -of the cylinder, leaving therefore around [Pg243] it a hollow -space, as represented in _fig._ 65. The cover of the piston is a -plate similarly formed, being in like manner gradually reduced in -thickness downwards, so as to correspond with the lower plate. In -the hollow space which thus surrounds the piston a packing of -unspun hemp or soft rope, called _gasket_, is introduced by -winding it round the piston so as to render it an even and compact -mass. When the space is thus filled up, the top of the piston is -attached to the bottom by screws. The curved form of the space -within which the hempen packing is confined is such that when the -screws are tightened, that part of the packing which is nearest to -the top and bottom of the piston is forced against the cylinder, -so as to produce upon the two parallel rings as much pressure as -is necessary to render it steam-tight. When by use the packing is -worn down so as to produce leakage, the cover of the cylinder must -be removed, and the screws connecting the top and bottom of the -piston tightened: this will force out the packing and render the -piston steam-tight. This packing is lubricated by melted tallow -let down upon the piston from the funnel inserted in the top of -the cylinder, furnished with a stop-cock to prevent the escape of -steam. The lower end of the piston-rod is formed slightly conical, -the thickest part of the cone being downward. It is passed up -through the piston, and a nut or wedge between the top and bottom -is inserted so as to secure the piston in its position upon the -rod. - -[Illustration: _Fig._ 65.] - -The process of removing the top of the cylinder for the purpose of -tightening the screws in the piston is one of so laborious a -nature, that the men entrusted with the superintendence of these -machines are tempted to allow the engine to work notwithstanding -injurious leakage at the piston, rather than incur the labour of -tightening the screws as often as it is necessary to do so. - -To avoid this inconvenience, the following method of [Pg244] -tightening the packing of the piston without removing the lid of -the cylinder, was contrived by Woolf. The head of each of the -screws was formed into a toothed pinion, and as these screws were -placed at equal distances from the centre of the piston, these -several pinions were driven by a large toothed wheel, revolving on -the piston-rod as an axis. By such an arrangement it is evident -that if any one of the screws be turned, a like motion will be -imparted to all the others through the medium of the large central -wheel. Woolf accordingly formed, on the head of one of the screws, -a square end. When the piston was brought to the top of the -cylinder, this square end entered an aperture made in the under -side of the cover of the cylinder. This aperture was covered by a -small circular piece screwed into the top of the cylinder, which -was capable of being removed so as to render the square head of -the screw accessible. When this was done, a proper key being -applied to the square head of the screw, it was turned; and by -being turned, all the other screws were in like manner moved. In -this way, instead of having to remove the cover of the cylinder, -which in large cylinders was attended with great labour and loss -of time, the packing was tightened by merely unscrewing a piece in -the top of the cylinder not much greater in magnitude than the -head of one of the screws. - -This method was further simplified by causing the great circular -wheel already described to move upon the piston-rod, not as an -axis, but as a screw, the thread being cut upon a part of the -piston-rod which worked in a corresponding female screw cut upon -the central plate. By such means, the screw whose head was let -into the cover of the cylinder which turned, would cause this -circular plate to be pressed downwards by the force of the screw -constructed on the piston-rod. This circular plate thus pressed -downwards, acted upon pins or plugs which pressed together the top -and bottom of the cylinder in the same manner as they were pressed -together by the screws connecting them as already described. - - -METALLIC PISTONS. - - -(142.) The notion of constructing a piston so as to move -steam-tight in the cylinder without the use of packing of -vegetable [Pg245] matter was first suggested by the Rev. Mr. -Cartwright, a gentleman well known for other mechanical -inventions. A patent was granted in 1797 for a new form of steam -engine, in which he proposed to use the vapour of alcohol to work -the piston instead of the steam of water: and since the principle -of the engine excluded the use of lubrication by oil or tallow, he -substituted a piston formed of metallic rings pressed against the -surface of the cylinder by springs, so as to be maintained in -steam-tight contact with it, independently either of packing or -lubrication. Although the engine for which this form of piston was -intended never came into practical use, yet it is so simple and -elegant in its structure, and forms a link so interesting in the -history of the steam engine, that some explanation of it ought not -to be omitted in this work. - -The steam-pipe from the boiler is represented cut off at B (_fig._ -66.); T is a spindle-valve, for admitting steam above the piston, -and R is a spindle-valve in the piston; D is a curved pipe forming -a communication between the cylinder and the condenser, which is -of very peculiar construction. Cartwright proposed effecting a -condensation without a jet, by exposing the steam to contact with -a very large quantity of cold surface. For this purpose, he formed -his condenser by placing two cylinders nearly equal in size, one -within the other, allowing the water of the cold cistern in which -they were placed to flow through the inner cylinder, and to -surround the outer one. Thus, the thin space between the two -cylinders formed the condenser. - -[Illustration: _Fig._ 66.] - -The air-pump is placed immediately under the cylinder, and the -continuation of the piston-rod works its piston, which is solid -and without a valve. F is the pipe from the condenser to the -air-pump, through which the condensed steam is drawn off through -the valve G on the ascent of the piston, and on the descent this -is forced through a tube into a hot well H, for the purpose of -feeding the boiler through the feed-pipe I. In the top of the hot -well H is a valve which opens inwards, and is kept closed by a -ball floating on the surface of the liquid. The pressure of the -condensed air above the surface of the liquid in H forces it -through I into the boiler. When the air accumulates in too great a -degree [Pg246] in H, the surface of the liquid is pressed so low -that the ball falls and opens the valve, and allows it to escape. -The air in H is that which is pumped from the condenser with the -liquid, and from which it was disengaged. - -Let us suppose the piston at the top of the cylinder: it strikes -the tail of the valve T, and raises it, while the stem of the -piston-valve R strikes the top of the cylinder, and is pressed -into its seat. A free communication is at the same time open -between the cylinder, below the piston and the condenser, through -the tube D. The pressure of the steam [Pg247] thus admitted above -the piston acting against the vacuum below it, will cause its -descent. On arriving at the bottom of the cylinder, the tail of -the piston-valve R will strike the bottom, and it will be lifted -from its seat, so that a communication will be opened through it -with the condenser. At the same moment, a projecting spring K, -attached to the piston-rod, strikes the stem of the steam-valve T, -and presses it into its seat. Thus while the further admission of -steam is cut off, the steam above the piston flows into the -condenser, and the piston being relieved from all pressure, is -drawn up by the momentum of the fly-wheel, which continues the -motion it received from the descending force. On the arrival of -the piston again at the top of the cylinder, the valve T is opened -and R closed, and the piston descends as before, and so the -process is continued. - -The mechanism by which motion is communicated from the piston to -the fly-wheel is peculiarly elegant. On the axis of the fly-wheel -is a small wheel with teeth, which work in the teeth of another -larger wheel L. This wheel is turned by a crank, which is worked -by a cross-piece attached to the end of the piston-rod. Another -equal-toothed wheel M is turned by a crank, which is worked by the -other end of the cross-arm attached to the piston-rod. - -One of the peculiarities of this engine is, that the liquid which -is used for the production of steam in the boiler circulates -through the machine without either diminution or admixture with -any other fluid, so that the boiler never wants more feeding than -what can be supplied from the hot well H. This circumstance forms -an important feature in the machine, as it allows of ardent -spirits being used in the boiler instead of water, which, since -they boil at low heats, promised a saving of fuel. The inventor -proposed that the engine should be used as a still, as well as a -mechanical power, in which case the whole of the fuel would be -saved. - -[Illustration: _Fig._ 67.] - -[Illustration: _Fig._ 68.] - - -(143.) That part of Cartwright's piston which in the common piston -is occupied by the packing of gasket, already explained (141.), -was filled by a number of rings, one placed within and above -another, and divided into three or four [Pg248] segments. Two -rings of brass were made of the full size of the cylinder, and so -ground as to fit the cylinder nearly steam-tight. These were cut -into several segments A A A (_fig._ 67.), and were placed one -above the other, so as to fill the space between the top and -bottom plates of the piston. The divisions of the segments of the -one ring were made to fit between the divisions of the other. -Within these another series of rings, B B B, were placed, -similarly constructed, so as to fit within the first series in the -same manner as the first series were made to fit within the -cylinder. The joints of the upper series of each set of rings are -exhibited in the plan (_fig._ 67.); the places of the joints of -the lower series are shown by dotted lines; the position of the -rings of each series one above the other is shown in the section -(_fig._ 68.). The joints of the inner series of rings are so -placed as to lie between those of the outer series, to prevent the -escape of steam which would take place by one continued joint from -top to bottom of the packing. The segments into which the rings -are divided are pressed outwards by steel springs in the form of -the letter V, the springs which act upon the outer series of -segments abutting upon the inner series, and those which act on -the inner series abutting upon the solid centre of the piston: -these springs are represented in _fig._ 67. - -[Illustration: _Fig._ 69.] - -[Illustration: _Fig._ 70.] - - -(144.) An improved form was given to the metallic piston by -Barton. Barton's piston consists of a solid cylinder of cast iron, -represented at A in section in _fig._ 69., and in plan in [Pg249] -_fig._ 70. In the centre of this is a conical hole, increasing in -magnitude downwards, to receive the piston-rod, in which the -latter is secured by a cross-pin B. A deep groove, square in its -section, is formed around the piston, so that while the top and -bottom of the piston form circles equal in magnitude to the -section of the cylinder, the intermediate part of the body of the -piston forms a circle less than the former by the depth of the -groove. Let a ring of brass, cast iron, or cast steel, be made to -correspond in magnitude and form with this groove, and let it be -divided as represented in _fig._ 70., into four segments C C C C, -and four corresponding angular pieces D D D D. Let the groove -which surrounds the piston be filled by the four segments with the -four wedge-like angular pieces within them, and let the latter be -urged against the former by eight spiral springs, as represented -in _fig._ 69. and _fig._ 70. These springs will abut against the -solid centre by the piston, and will urge the segments C against -the cylinder. The spiral springs which urge the wedges are -confined in their action by steel pins which pass through their -centre, and by being [Pg250] confined in cylindrical cavities -worked into the wedges and into corresponding parts of the solid -centre of the piston, as the segments C wear, the springs urge the -wedges outwards, and the points of the latter protruding, are -gradually worn down so as to fill up the spaces left between the -segments, and thus to complete the outer surface of the piston. - -Various other forms of metallic pistons have been proposed, but as -they do not differ materially in principle from those we have just -described, it will not be necessary here to describe them. - -[Illustration: ENGINE AT THE CITY SAW MILLS.] - -[Pg251] - - - - -[Illustration: FURNACE AT THE CITY SAW MILLS.] - -CHAP. IX. - - CONSTITUENTS OF COAL. -- PROCESS OF COMBUSTION. -- HEAT EVOLVED - IN IT. -- FORM AND STRUCTURE OF BOILER. -- WAGGON BOILER. -- - FURNACE. -- METHOD OF FEEDING IT. -- COMBUSTION OF GAS IN FLUES. - -- CONSTRUCTION OF GRATE AND ASH-PIT. -- MAGNITUDE OF HEATING - SURFACE OF BOILER. -- STEAM-SPACE AND WATER-SPACE IN BOILER. -- - POSITION OF FLUES. -- METHOD OF FEEDING BOILER. -- LEVEL GAUGES. - -- SELF-REGULATING FEEDERS. -- STEAM-GAUGE. -- BAROMETER-GAUGE. -- - INDICATOR. -- COUNTER. -- SAFETY-VALVE. -- FUSIBLE PLUGS. -- - SELF-REGULATING DAMPER. -- SELF-REGULATING FURNACE. -- POWER AND - DUTY OF ENGINES. -- HORSE-POWER OF STEAM ENGINES. -- EVAPORATION - PROPORTIONAL TO HORSE-POWER. -- SOURCES OF LOSS OF POWER. -- - ABSENCE OF GOOD PRACTICAL RULES FOR POWER. -- COMMON RULES - FOLLOWED BY ENGINE MAKERS. -- DUTY DISTINGUISHED FROM POWER. -- - DUTY OF BOILERS. -- PROPORTION OF STROKE TO DIAMETER OF - CYLINDER. -- DUTY OF ENGINES. -- CORNISH SYSTEM OF INSPECTION. -- - ITS GOOD EFFECTS. -- HISTORICAL DETAIL OF THE DUTY OF CORNISH - ENGINES. - - -(145.) The machinery which has been explained in the preceding -chapters, consisting of the cylinder with its passages and valves, -the piston-rod, parallel motion, beam, connecting-rod and crank, -together with the condenser, air-pump, and other appendages, -having no source of moving power in themselves, must be regarded -as mere instruments by which the mechanical effect developed by -the furnace and the boiler is transmitted to the working point and -so [Pg252] modified as to be adapted to the uses to which the -machine is applied. The boiler is at once a magazine in which the -moving power is stored in sufficient quantity to supply the -demands of the engine and an apparatus in which that power is -fabricated. The mechanical effect evolved in the conversion of -water into steam by heat, is the process by which the power of the -steam-engine is produced, and space is provided in the boiler, -capacious enough to contain as much steam as is necessary for the -engine, besides a sufficient quantity of water to continue that -supply undiminished, notwithstanding the constant drafts made upon -it by the cylinder: even the water itself, from the evaporation of -which the mechanical power is produced, ought to be regarded as an -instrument by which the effect of the heat of the combustible is -rendered mechanically efficient, inasmuch as the same heat, -applied not only to other liquids but even to solids, would -likewise be productive of mechanical effects. The boiler and its -furnace are therefore parts of the steam-engine, the construction -and operation of which are entitled to especial attention. - - -(146.) COAL, the combustible almost universally used in -steam-engines, is a substance, the principal constituents of which -are _carbon_ and _hydrogen_, occasionally mixed with sulphur in a -small proportion, and earthy incombustible matter. In different -sorts of coal the proportions of these constituents vary, but in -coal of good quality about three quarters of the whole weight of -the combustible is carbon. - -When carbon is heated to a temperature of about 700 deg. in an -atmosphere of pure oxygen, it will combine chemically with that -gas, and the product will be the gas called _carbonic acid_. The -volume of carbonic acid produced by this combination, will be -exactly equal to that of the oxygen combined with the carbon, and -therefore the weight of a given volume of the gas will be -increased by the weight of carbon which enters the combination. It -is found that two parts by weight of oxygen combined with three of -carbon, form carbonic acid. The weight of the carbonic acid, -therefore, produced in the combustion, will be greater than the -weight of the oxygen, bulk for bulk, in the proportion of five to -two, the volume being the same and the gases being [Pg253] -compared at the same temperatures and under equal pressures. In -this combination heat is evolved in very large quantities. This -effect arises from the heat previously latent in the carbon and -oxygen being rendered sensible in the process of combustion. The -carbonic acid proceeding from the combustion is by such means -raised to a very high temperature, and the carbon during the -process acquires a heat so intense as to become luminous; no -flame, however, is produced. - -Hydrogen, heated to a temperature of about 1000 deg., in contact with -oxygen will combine with the latter, and a great evolution of heat -will attend the process; the gases will be rendered luminous, and -flame will be produced. The product of this process will be water, -which being exposed to the intense heat of combustion, will be -immediately converted into steam. Hydrogen combines with eight -times its own weight of oxygen, producing nine times its own -weight of water. - -Hydrogen gas is, however, not usually disengaged from coal in a -simple form, but combined chemically with a certain portion of -carbon, the combination being called carburetted hydrogen. Pure -hydrogen burns with a very faintly luminous blue flame, but -carburetted hydrogen gives that bright flame occasionally having -an orange or reddish tinge, which is seen to issue from burning -coals: this is the gas used for illumination, being expelled from -the coal by the process of coking, and conducted to the various -burners through proper pipes. - -The sulphur, which in a very small proportion is contained in -coals, is also combustible, and combines in the process of -combustion with oxygen, forming sulphurous acid: it is also -sometimes evolved in combination with hydrogen, forming -sulphuretted hydrogen. - -Atmospheric air consists of two gases, azote and oxygen, mixed -together in the proportion of four to one; five cubic feet of -atmospheric air consisting of four cubic feet of azote and one of -oxygen. Any combustible will combine with the oxygen contained in -atmospheric air, if raised to a temperature somewhat higher than -that which is necessary to cause its combustion in an atmosphere -of pure oxygen. - -If coals, therefore, or other fuel exposed to atmospheric [Pg254] -air, be raised to a sufficiently high temperature, their -combustible constituents will combine with the oxygen of the -atmospheric air, and all the phenomena of combustion will ensue. -In order, however, that the combustion should be continued, and -should be carried on with quickness and activity, it is necessary -that the carbonic acid, and other products, should be removed from -the combustible as they are produced, and fresh portions of -atmospheric air brought into contact with it; otherwise the -combustible would soon be surrounded by an atmosphere composed -chiefly of carbonic acid to the exclusion of atmospheric air, and -therefore of uncombined oxygen, and consequently the combustion -would cease, and the fuel be extinguished. To maintain the -combustion, therefore, a current of atmospheric air must be -constantly carried through the fuel: the quantity and force of -this current must depend on the quantity and quality of the fuel -to be consumed. It must be such that it shall supply sufficient -oxygen to the fuel to maintain the combustion, and not more than -sufficient, since any excess would be attended with the effect of -absorbing the heat of combustion, without contributing to the -maintenance of that effect. - -Heat is communicated from body to body in two ways, by radiation -and by contact. - -Rays of heat issue from a heated body, and are dispersed through -the surrounding space in a manner, and according to laws, similar -to those which govern the radiation of light. The heat thus -radiated meeting other bodies is imparted to them, and penetrates -them with more or less facility according to their physical -qualities. - -A heated body also brought into contact with another body of lower -temperature, communicates heat to that other body, and will -continue to do so until the temperature of the two bodies in -contact shall be equalised. Heat proceeds from fuel in a state of -combustion in both these ways: the heated fuel radiates heat in -all directions around it, and the heat thus radiated will be -imparted to all parts of the furnace which are exposed to the -fuel. - -The gases, which are the products of the combustion, escape from -the fuel at a very high temperature, and consequently, in -acquiring that temperature they absorb a considerable [Pg255] -quantity of the heat of combustion. But besides the gases actually -formed in the process of combustion, the azote forming four fifths -of the air carried through the fuel to support the combustion, -absorbs heat from the combustible, and rises into the upper part -of the furnace at a high temperature. These various gases, if -conducted directly to the chimney, would carry off with them a -considerable quantity of the heat. Provision should therefore be -made to keep them in contact with the boiler such a length of time -as will enable them to impart such a portion of the heat which -they have absorbed from the fuel, as will still leave them at a -temperature sufficient, and not more than sufficient, to produce -the necessary draft in the chimney. - - -(147.) The forms of boiler which have been proposed as the most -convenient for the attainment of all these requisite purposes have -been very various. If strength alone were considered, the -spherical form would be the best; and the early boilers were very -nearly hemispheres, placed on a slightly concave base. The form -adopted by Watt, called the waggon boiler, consists of a -semi-cylindrical top, flat perpendicular sides, flat ends, and a -slightly concave bottom. The steam intended to be used in boilers -of this description did not exceed the pressure of the external -atmosphere by more than from 3 to 5 lbs. per square inch; and the -flat sides and ends, though unfavourable to strength, could be -constructed sufficiently strong for this purpose. In a boiler of -this sort, the air and smoke passing through the flues that are -carried round it, are in contact at one side only with the boiler. -The brickwork, or other materials forming the flue, must therefore -be non-conductors of heat, that they may not absorb any -considerable portion of heat from the air passing in contact with -them. A boiler of this form is represented in _fig._ 71. - -The grate and a part of the flues are rendered visible by the -removal of a portion of the surrounding masonry in which the -boiler is set. The interior of the boiler is also shown by cutting -off one half of the semi-cylindrical roof. A longitudinal vertical -section is shown in _fig._ 72., and a cross section in _fig._ 73. -A horizontal section taken above the level of the grate, and below -the level of the water in the boiler, showing [Pg256] the course -of the flues, is given in _fig._ 74. The corresponding parts in -all the figures are marked by the same letters. - -[Illustration: _Fig._ 71.] - -The door by which fuel is introduced upon the grate is represented -at A, and the door leading to the ash-pit at B. The fire bars at C -slope downwards from the front at an angle of about 25 deg., giving -a tendency to the fuel to move from the front towards the back of -the grate. The ash-pit D is constructed of such a magnitude, form, -and depth, as to admit a current of atmospheric air to the -grate-bars, sufficient to sustain the combustion. The form of the -ash-pit is usually wide below, contracting towards the top. - -[Illustration: _Fig._ 72.] - -The fuel when introduced at the fire-door A, should be laid on that -part of the grate nearest to the fire-door, called the dead plates: -there it is submitted to the process of coking, by which the gases -and volatile matter which it contains are expelled, and being -carried by a current of air, admitted [Pg257] through small -apertures in the fire-door over the burning fuel in the hinder part -of the grate, they are burnt. When the fuel in front of the grate -has been thus _coked_, it is pushed back, and a fresh feed -introduced in front. The coal thus pushed back soon becomes vividly -ignited, and by continuing this process, the fuel spread over the -grate is maintained in the most active state of combustion at the -hinder part of the grate. By such an arrangement, the smoke produced -by the combustion of the fuel may be burnt before it enters the -flues. The flame and heated air proceeding from the burning fuel -arising from the grate, and rushing towards the back of the furnace, -passes over the _fire-bridge_ E, and is carried through the flue F -which passes under the boiler. This flue (the cross section of which -is shown in _fig._ 73., by the dark shade put under the boiler) is -very nearly equal in width to the bottom of the boiler, the space at -the bottom of the boiler, near the corners, being only what is -sufficient to give the weight of the boiler support on the masonry -forming the [Pg258] sides of the flue. The bottom of the boiler -being concave, the flame and heated air as they pass along the flue -rise to the upper part by the effects of their high temperature, and -_lick_ the bottom of the boiler from the fire-bridge at E to the -further end G. - -[Illustration: _Fig._ 73.] - -[Illustration: _Fig._ 74.] - -At G the flue rises to H, and turning to the side of the boiler at -I I, conducts the flame in contact with the side from the back to -the front; it then passes through the flue K across the front, and -returns to the back by the other side [Pg259] flue L. The side -flue is represented, stripped of the masonry, in _fig._ 71., and -also appears in the plan in _fig._ 74., and in the cross section -in _fig._ 73. The course of the air is represented in _fig._ 74. -by the arrows. From the flue L the air is conducted into the -chimney at M. - -By such an arrangement, the flame and heated air proceeding from -the grate are made to circulate round the boiler, and the length -and magnitude of the flues through which it is conducted should be -such, that when it shall arrive at the chimney its temperature -shall be reduced, as nearly as is consistent with the maintenance -of draught in the chimney, to the temperature of the water with -which it is in contact. - -The method of feeding the furnace, which has been described above, -is one which, if conducted with skill and care, would produce a -much more perfect combustion of the fuel than would attend the -common method of filling the grate from the back to the front with -fresh fuel, whenever the furnace is fed. This method, however, is -rarely observed in the management of the furnace. It requires the -constant attention of the stokers (such is the name given to those -who feed the furnaces). The fuel must be supplied, not in large -quantities, and at distant intervals, but in small quantities and -more frequently. On the other hand, the more common practice is to -allow the fuel on the grate to be in a great degree burned away, -and then to heap on a large quantity of fresh fuel, covering over -with it the burning fuel from the back to the front of the grate. -When this is done, the heat of the ignited coal acting upon the -fresh fuel introduced, expels the gases combined with it and, -mixed with these, a quantity of carbon, in a state of minute -division, forming an opaque black smoke. This is carried through -the flues and drawn up the chimney. The consequence is, that not -only a quantity of solid fuel is sent out of the chimney -unconsumed, but the hydrogen and other gases also escape unburned, -and a proportional waste of the combustible is produced; besides -which, the nuisance of an atmosphere filled with smoke ensues. -Such effects are visible to all who observe the chimneys of -steam-vessels, while the engine is in operation. When the furnaces -are thus filled with fresh fuel, a large volume of [Pg260] dense -black smoke is observed to issue from the chimney. This gradually -subsides as the fuel on the grate is ignited, and does not -reappear until a fresh feed is introduced. - -This method of feeding, by which the furnace would be made to -consume its own smoke, and the combustion of the fuel be rendered -complete, is not however free from counteracting effects. In -ordinary furnaces the feed can only be introduced by opening the -fire-doors, and during the time the fire-doors are opened a volume -of cold air rushes in, which passing through the furnace is -carried through the flues to the chimney. Such is the effect of -this in lowering the temperature of the flues, that in many cases -the loss of heat occasioned is greater than any economy of fuel -obtained by the complete consumption of smoke. Various methods, -however, may be adopted by which fuel may be supplied to the grate -without opening the fire-doors, and without disturbing the supply -of air to the fire. A hopper built into the front of the furnace, -with a moveable bottom or valve, by which coals may be allowed to -drop in from time to time upon the front of the grate, would -accomplish this. - - -(148.) In order to secure the combustion of the gases evolved from -the coals placed in the front of the grate, it is necessary that a -supply of atmospheric air should be admitted with them over the -burning fuel. This is effected by small apertures or regulators, -provided in the fire-doors, governed by sliding-plates, by which -they may be opened or closed to any required extent. - -A patent has recently been granted to Mr. Williams, one of the -directors of the City of Dublin Steam Navigation Company, for a -method of consuming the unburned gases which escape from the -grate, and are carried through the flues. This method consists in -introducing into the flue tubes placed in a vertical position, the -lower ends of which being inserted in the bottom of the flue are -made to communicate with the ash-pit, and the upper ends of which -are closed. The sides and tops of these tubes are pierced with -small holes, through which atmospheric air drawn from the ash-pit -issues in jets. The oxygen supplied by this air immediately -combines with the carburetted hydrogen, which [Pg261] having -escaped from the furnace unburned is carried through the flues at -a sufficient temperature to enter into combination with the oxygen -admitted through holes in the tubes. A number of jets of flame -thus proceed from these holes, having an appearance similar to the -flame of a gas-lamp. - -It is evident that such tubes must be inefficient unless they are -placed in the flues so near the furnace, that the temperature of -the unburned gases shall be sufficiently high to produce their -combustion. - - -(149.) The magnitude of the grate and ash-pit must be determined by -the rate at which the evaporation is required to be conducted in the -boiler and the quality of the fuel. It must be a matter of regret, -that the proportions of the various parts of steam-engines, with -their boilers and furnaces, have not been determined by any exact or -satisfactory experiments; and those who project and manufacture the -engines themselves, are not less in ignorance on those points than -others. With coals of the common quality a certain average -proportion must exist between the necessary magnitude of the -grate-surface and the quantity of water to be evaporated in a given -time in the boiler. But what that proportion is for any given -quality of fuel, is at present unascertained. Each engine-maker -follows his own rule, and the rule thus followed is in most cases a -matter of bare conjecture, unsupported by any experimental evidence. -Some engine-makers will allow a square foot of grate-surface for -every cubic foot of water per hour, which is expected to be -evaporated in the boiler; others allow only half a square foot: and -practice varies between these limits. Bituminous coals which melt -and cake, and which burn with much flame and smoke, must be spread -more thinly on the grate than other descriptions of fuel, otherwise -a considerable quantity of combustible gases would be dismissed into -the flues unburnt. Such coals therefore, other circumstances being -the same, require a larger portion of grate-surface; and the same -may be said of coals which produce clinkers in their combustion, and -form lumps of vitrified matter on the grate, by which the spaces -between the grate-bars are speedily closed up. When such fuel is -used, the grate-bars require to be frequently raked [Pg262] out, -otherwise the spaces between them being obstructed, the draught -would become insufficient for the due combustion of the fuel. - -To facilitate the raking out of the grate, the bars are placed -with their ends towards the fire-door: they are usually made of -cast-iron, from two to two inches and a half wide on the upper -surface, with intervals of nearly half an inch between them. The -bars taper downwards, their under surfaces being much narrower -than their upper, the spaces between them thus widening, to -facilitate the fall of the ashes between them. The grate-bars -slope downwards from the front to the back. The height of the -centre of the bottom of the boiler, above the front of the grate, -is usually about two feet, and about three feet above the back of -it. The concave bottom of the boiler, however, brings its surfaces -at the slide closer to the grate. - - -(150.) Between the evaporating power of the boiler, and the -magnitude of surface it exposes to the action of the furnace, -there is a relation which, like that of the grate surface, has -never been ascertained by any certain or satisfactory experimental -investigation; much less have the different degrees of efficiency -attending different parts of the boiler-surface been determined. -That part of the surface of the boiler immediately over and around -the grate, is exposed to the immediate radiation of the burning -fuel, and is therefore probably the most efficient in the -production of steam. The tendency of flame and heated air to rise, -would naturally bring them in the flues into closer contact with -those parts of the boiler-surface which are horizontal in their -position, and which form the tops of the flues, than with those -which are lateral or vertical in their position, and which form -the sides of the flues. In a boiler constructed like that already -described, the flue-surface therefore, which would be most -efficient, would be the concave bottom of the boiler extending -from the fire-bridge to its remote end. In some boilers, -especially those in which steam of high pressure is produced, the -form is cylindrical, the middle flue being formed into an -elliptical tube the greater axis of which is horizontal from end -to end of the boiler. It seems doubtful, however, [Pg263] whether -in such a boiler the heat produces any useful effect on the water -below the flue, the water above being always at a higher -temperature, and therefore lighter than that below, and -consequently no currents being established between the upper and -lower strata of the water. - -It was considered by Mr. Watt, but we are not aware on what -experimental grounds, that from eight to ten square feet of -heating surface were sufficient to produce the evaporation of one -cubic foot of water per hour. The practice of engine-makers since -that time has been to increase the allowance of heating surface -for the same rate of evaporation. Engine-builders have varied very -much in this respect, some allowing twelve, fifteen, and even -eighteen square feet of surface for the same rate of evaporation. -It must, however, still be borne in mind, that whether this -increased allowance did or did not produce the actual evaporation -imputed to it, has not been, as far as we are informed, ever -accurately ascertained. The production of a given rate of -evaporation by a moderate heat diffused over a larger surface, -rather than by a fiercer temperature confined to a smaller -surface, is attended with many practical advantages. The plates of -the boiler acted upon by the fire are less exposed to oxydisation, -and the boiler will be proportionally more durable. - - -(151.) Besides presenting to the action of the fire a sufficient -surface to produce steam at the required rate, the capacity of the -boiler must be proportioned to the quantity of water to be -evaporated. The space within the boiler is appropriated to a -twofold purpose: 1st, To contain the water to be evaporated; 2dly, -To contain a quantity of ready made steam for the supply of the -cylinder. If the space appropriated to the steam did not bear a -considerable proportion to the magnitude of the cylinder, the -momentary expansion of the steam passing to the cylinder from the -boiler at each stroke would reduce the pressure of the steam in a -great proportion, and unless the pressure in the boiler were -considerably greater than that which the steam is intended to have -in the cylinder, the pressure in the latter would be reduced below -the proper amount. The proportion of the [Pg264] steam space in -the boiler to the magnitude of the cylinder has been very -variously estimated, nor can it be said that any practical rule of -a general kind has been adopted. It is held by some that the -steam-space will be sufficient if it contain five times the -quantity of steam consumed at each stroke, while others maintain -that it should contain at least ten times that quantity, and -opinions vary between these limits. - - -(152.) The proportion of water-space in the boiler to its -evaporating power should also be regulated, so that the -introduction of the feed at a comparatively low temperature may -not unduly chill the water in the boiler. Supposing the feed to be -introduced in a low pressure boiler at the temperature of 100 deg., -and that the necessary temperature within the boiler be 225 deg., -the quantity of water it contains should be about five times the -quantity evaporated, and therefore also five times the quantity -introduced through the feed per hour. For every cubic foot of -water per hour therefore, intended to be evaporated by the boiler, -water-space for five cubic feet should be provided. It is, -however, right to repeat that this (like almost every other so -called rule) is the result not of any exact general calculation, -but one deduced from the custom which has obtained among the -manufacturers of steam-engines. - - -(153.) The surface of the water in the boiler should always be -above the range of the flues. When the heated air in the flues -acts upon a part of the boiler within which water is contained, -the water within receiving an increased temperature becomes, bulk -for bulk, lighter than the strata of water above it, and ascends. -It is replaced by the descending strata, which, in their turn -receiving increased temperature, rise to the surface; or if the -action of the heat convert the water into steam, the bubbles of -steam rise to the surface, fresh portions of water continually -coming into contact with the boiler-plates on which the heated air -or flame acts. By this process the boiler-plates are continually -cooled, either by being successively washed by water at a lower -temperature, or by the heat taken from them becoming latent in the -steam bubbles formed in contact with them. But if the heat act -[Pg265] upon a part of the boiler containing steam within it, -which steam being a slow recipient of heat, and no currents being -established, nor any phenomenon produced in which heat is rendered -latent, the heat of the fire communicated to the boiler-plates -accumulates in them, and raises their temperature to an injurious -degree. The plates may by this means be softened, so as to cause -the boiler to burst, or the difference between the expansion of -the highly heated plates thus exposed to fire in contact with -steam and that of the plates which are cooled by contact with -water, may cause the joinings of the boiler-plates to open, and -the boiler to leak. By whatever means, therefore, the boiler be -fed, care should be taken that the evaporation should not be -allowed to reduce the level of the water in it below the highest -flue. - - -(154.) As the water by which the boiler is fed must always have a -much lower temperature than that at which the boiler is -maintained, the supply of the feed will have a constant tendency -to lower the temperature of the water, and this tendency will be -determined by the proportion between the magnitude of the feed and -the quantity of water in the boiler. - -Since it is requisite that the level of the water in the boiler -shall not suffer any considerable change, it is evident that the -magnitude of the feed must be equal to the quantity of water -evaporated. If it were less, the level of the water would -continually fall by reason of the excess of the evaporation over -the feed; and if it were greater, the level would rise by the -accumulation of water in the boiler. If therefore the quantity of -water-space allowed in the boiler be five times the volume of -water evaporated per hour, the quantity introduced by the feed per -hour, whether continuously or at intervals, must be of the same -amount. Since the process of evaporation is continuous, the -variation of level of water in the boiler will be entirely -dependent on the intervals between the successive feeds. If the -feed be continuous, and always equal to the evaporation, then the -level of the water in the boiler will undergo no change; but if -while the evaporation is continuous the feed be made at intervals, -then the change of level of water in the boiler as [Pg266] well -as its change of temperature, will be subject to a variation -proportional to the intervals between the successive feeds. It is -manifest, therefore, that the feed should either be uninterrupted -or be supplied at short intervals, so that the change of level and -temperature of the water in the boiler should not be considerable. - - -(155.) Different methods have been, from time to time, suggested -for indicating the level of the water in the boiler. We have -already mentioned the two gauge-pipes used in the earlier -steam-engines (31.), and which are still generally continued. -There are, however, some other methods which merit our attention. - -[Illustration: _Fig._ 75.] - -A weight F (_fig._ 75.), half immersed in the water in the boiler, -is supported by a wire, which, passing steam-tight through a small -hole in the top, is connected by a flexible string, or chain, -passing over a wheel W, with a counterpoise A, which is just -sufficient to balance F when half immersed. If F be raised above the -water, _A_ being lighter will no longer balance it, and F will -descend pulling up A, and turning the wheel W. If, on the other -hand, F be plunged deeper in the water, A will more than balance it, -and will pull it up, so that the only position in which F and A will -balance each other is, when F is half immersed. The wheel W is so -adjusted, that when two pins placed on its rim are in the horizontal -position, the water is at its proper level. Consequently it follows, -that if the water rise above this level, the weight F is lifted and -A falls, so that the pins come into another position. If, on the -other hand, the level of the water fall, F falls and A rises, so -that the pins assume a different position. Thus, in general, the -position of the pins becomes an indication of the quantity of water -in the boiler. - - -(156.) Another method is to place a glass tube (_fig._ 76.), with -one end T entering the boiler above the proper level, and the -other end T' entering it below the proper level. It must [Pg267] -be evident that the water in the tube will always stand at the -same level as the water in the boiler, since the lower part has a -free communication with that water, while the surface is submitted -to the pressure of the same steam as the water in the boiler. This -and the last-mentioned gauge have the advantage of addressing the -eye of the engineer at once, without any adjustment; whereas the -gauge-cocks must be both opened, whenever the depth is to be -ascertained. - -[Illustration: _Fig._ 76.] - -These gauges, however, require the frequent attention of the -engine-man; and it becomes desirable either to find some more -effectual means of awakening that attention, or to render the -supply of the boiler independent of any attention. In order to -enforce the attention of the engine-man to replenish the boiler -when partially exhausted by evaporation, a tube was sometimes -inserted at the lowest level to which it was intended that the -water should be permitted to fall. This tube was conducted from -the boiler into the engine-house, where it terminated in a -mouth-piece or whistle, so that whenever the water fell below the -level at which this tube was inserted in the boiler, the steam -would rush through it, and issuing with great velocity at the -mouth-piece, would summon the engineer to his duty with a call -that would rouse him even from sleep. - -[Illustration: _Fig._ 77.] - - -(157.) In the most effectual of these methods, the task of -replenishing the boiler should still be executed by the engineer; -and the utmost that the boiler itself was made to do, was to give -due notice of the necessity for the supply of water. The -consequence was, among other inconveniences, that the level of the -water was subject to constant variation. - -To remedy this a method has been invented, by which [Pg268] the -engine is made to feed its own boiler. The pipe G (_fig._ 77.), -which leads from the hot water pump, terminates in a small cistern -C in which the water is received. In the bottom of this cistern, a -valve V is placed, which opens upwards, and communicates with a -feed-pipe, which descends into the boiler below the level of the -water in it. The stem of the valve V is connected with a lever -turning on the centre D, and loaded with a weight F dipped in the -water in the boiler in a manner similar to that described in -_fig._ 75., and balanced by a counterpoise A in exactly the same -way. When the level of the water in the boiler falls, the float F -falls with it, and pulling down the arm of the lever raises the -valve V, and lets the water descend into the boiler from the -cistern C. When the boiler has thus been replenished, and the -level raised to its former place, F will again be raised, and the -valve V closed by the weight A. In practice, however, the valve V -adjusts itself by means of the effect of the water on the weight -F, so as to permit the water from the feeding-cistern C to flow in -a continued stream, just sufficient in quantity to supply the -consumption from evaporation, and to maintain the level of the -water in the boiler constantly the same. - -By this arrangement the boiler is made to replenish itself, or, -more properly speaking, it is made to receive such a supply, as -that it never wants replenishing, an effect which no effort of -attention on the part of an engine-man could produce. But this is -not the only good effect produced by this contrivance. A part of -the steam which originally left the boiler, and having discharged -its duty in moving the piston, was condensed and reconverted into -water, and lodged by the air-pump in the hot well (_fig._ 77.), is -here again restored to the source from which it came, bringing -back all the unconsumed portion of its heat preparatory to being -once more put in circulation through the machine. - -The entire quantity of hot water pumped into the cistern C, is not -always necessary for the boiler. A waste-pipe may be provided for -carrying off the surplus, which may be turned to any purpose for -which it may be required; or it may be discharged into a cistern -to cool, preparatory to [Pg269] being restored to the cold -cistern, in case water for the supply of that cistern be not -sufficiently abundant. - -[Illustration: _Fig._ 78.] - - -(158.) Another method of arranging a self-regulating feeder is -shown in _fig._ 78. A is a hollow ball of metal attached to the -end of a lever, whose fulcrum is at B. The other arm of the lever -C is connected with the stem of a spindle-valve, communicating -with a tube which receives water from the feeding-cistern. Thus, -when the level of the water in the boiler subsides, the ball A -preponderating over the weight of the opposite arm, the lever -falls, the arm C rises and opens the valve, and admits the feeding -water. This apparatus will evidently act in the same manner and on -the same principles as that already described. [Pg270] - -The mouth of the tube by which the feed is introduced should be -placed at that part of the boiler which is nearest the end of the -flues which issue into the chimney. By such means the temperature -of the water in contact with those flues will be lowest at the -place where the temperature of the heated air intended to act upon -it is also lowest. The difference of the temperatures will -therefore be greater than it would be if the point of the boiler -containing water of a higher temperature was left in contact with -this part of the flue. - -[Illustration: _Fig._ 79.] - - -(159.) It is necessary to have a ready method of ascertaining at -all times the pressure of the steam which is used in working the -engine. For this purpose a bent tube containing mercury is -inserted into some part of the apparatus, which has free -communication with the steam. Let A B C (_fig._ 79.) be such a -tube. The pressure of the steam forces the mercury down in the leg -A B, and up in the leg B C. If the mercury in both legs be at -exactly the same level, the pressure of the steam must be exactly -equal to that of the atmosphere; because the steam pressure on the -mercury in A B balances the atmospheric pressure on the mercury in -B C. If, however, the level of the mercury in B C be above the -level of the mercury in B A, the pressure of the steam will exceed -that of the atmosphere. The excess of its pressure above that of -the atmosphere may be found by observing the difference of the -level of the mercury in the tubes B C and B A, allowing a pressure -of one pound on each square inch for every two inches in the -difference of the levels. - -If, on the contrary, the level of the mercury in B C should fall -below its level in A B, the atmospheric pressure will [Pg271] -exceed that of the steam, and the quantity of the excess may be -ascertained exactly in the same way. - -If the tube be glass, the difference of levels of the mercury -would be visible; but it is most commonly made of iron; and in -order to ascertain the level, a thin wooden rod with a float is -inserted in the open end of B C, so that the portion of the stick -within the tube indicates the distance of the level of the mercury -from its mouth. A bulb or cistern of mercury might be substituted -for the leg A B, as in the common barometer. This instrument is -called the _steam-gauge_. - -If the steam-gauge be used as a measure of the strength of the -steam which presses on the piston, it ought to be on the same side -of the throttle-valve (which is regulated by the governor) as the -cylinder; for if it were on the same side of the throttle-valve -with the boiler, it would not be affected by the changes which the -steam may undergo in passing through the throttle-valve, when -partially closed by the agency of the governor. - -For boilers in which steam of very high pressure is used, as in -those of locomotive engines, a steam-gauge, constructed on the above -principle, would have inconvenient or impracticable length. In such -boilers the pressure of the steam is equal to four or five times -that of the atmosphere, to indicate which the column of mercury in -the steam-gauge would be four or five feet in height. In such cases -a thermometer-gauge may be used with advantage. The principle of -this gauge is founded on the fact, that between the pressure and -temperature of steam produced in contact with water there is a fixed -relation, the same temperature always corresponding to the same -pressure. If, therefore, a thermometer be immersed in the boiler -which shall show the temperature of the steam, a scale may be -attached to it, on which shall be engraved the corresponding -pressures. Such gauges are now very generally used on locomotive -engines. - -[Illustration: _Fig._ 80.] - - -(160.) The force with which the piston is pressed depends on two -things, 1st, the actual strength of the steam which presses on it; -and, 2dly, on the actual strength of the vapour which resists it. -For although the vacuum produced by the method of separate -condensation be much more perfect than [Pg272] what had been -produced in the atmospheric engines, yet still some vapour of a -small degree of elasticity is found to be raised from the hot -water in the bottom of the condenser before it can be extracted by -the air-pump. One of these pressures is indicated by the -steam-gauge already described; but still, before we can estimate -the force with which the piston descends, it is necessary to -ascertain the force of the vapour which remains uncondensed, and -resists the motion of the piston. Another gauge, called the -barometer-gauge, is provided for this purpose. A glass tube A B -(_fig._ 80.), more than thirty inches long and open at both ends, -is placed in an upright or vertical position, having the lower end -B immersed in a cistern of mercury C. To the upper end is attached -a metal tube, which communicates with the condenser, in which a -constant vacuum, or rather high degree of rarefaction, is -sustained. The same vacuum must therefore exist in the tube A B, -above the level of the mercury, and the atmospheric pressure on -the surface of the mercury in the cistern C will force the mercury -up in the tube A B, until the column which is suspended in it is -equal to the difference between the atmospheric pressure and the -pressure of the uncondensed steam. The difference between the -column of mercury sustained in this instrument and in the common -barometer, will determine the strength of the uncondensed steam, -allowing a force proportional to one pound per square inch for -every two inches of mercury in the difference of the two columns. -In a well-constructed engine which is in good order, there is very -little difference between the altitude in the barometer-gauge and -the common barometer. - -To compute the force with which the piston descends, thus becomes a -very simple arithmetical process. First, ascertain the difference of -the levels of the mercury in the steam-gauge; this gives the excess -of the steam pressure above the atmospheric pressure. Then find the -height of the mercury in the barometer-gauge; this gives the excess -of the atmospheric pressure above the uncondensed steam. Hence, if -these two heights be added together, we shall obtain the [Pg273] -excess of the impelling force of the steam from the boiler, on the -one side of the piston, above the resistance of the uncondensed -steam on the other side: this will give the effective impelling -force. Now, if one pound be allowed for every two inches of mercury -in the two columns just mentioned, we shall have the number of -pounds of impelling pressure on every square inch of the piston. -Then, if the number of square inches in the section of the piston be -found, and multiplied by the number of pounds on each square inch, -the force with which it moves will be obtained. - -From what we have stated it appears that, in order to estimate the -force with which the piston is urged, it is necessary to refer to -both the barometer and the steam-gauge. This double computation -may be obviated by making one gauge serve both purposes. If the -end C of the steam-gauge (_fig._ 79.), instead of communicating -with the atmosphere were continued to the condenser, we should -have the pressure of the steam acting upon the mercury in the tube -B A, and the pressure of the uncondensed vapour which resists the -piston acting on the mercury in the tube B C. Hence the difference -of the levels of the mercury in the tubes would at once indicate -the difference between the force of the steam and that of the -uncondensed vapour, which is the effective force with which the -piston is urged. - - -(161.) But these methods of determining the effective force by -which the piston is urged, can only be regarded as approximations, -and not very perfect ones. If the condensation of steam on one -side of the piston were instantaneously effected, or the -uncondensed vapour were of the same tension during the whole -stroke; and if, besides this, the pressure of steam on the piston -were of uniform intensity from the beginning to the end of the -stroke, then the steam and barometer gauges taken together would -become an accurate index of the effective force of steam on the -piston: but such is not the case. When the steam is first admitted -through the steam-valve it acts on the piston with a pressure -which is first slightly diminished, and afterwards a little -increased, until it arrives at that part of the stroke at which -the steam-valve is closed, after which the pressure is diminished. -The [Pg274] pressure, therefore, urging the piston is subject to -variation; but the pressure of the uncondensed vapour on the other -side of the piston is subject to still greater change. At the -moment the exhausting-valve is opened, the piston is relieved from -the pressure upon it by the commencement of the condensation; but -this process during the descent of the piston is gradual, and the -vacuum is rendered more and more perfect, until the piston has -nearly attained the limit of its play. These variations, both as -well of the force urging the piston as of the force resisting it, -are such as not to be capable of being accurately measured by a -mercurial column, since they would produce oscillations in such a -column, which would render any observations of its mean height -impracticable. - -To measure the mean efficient force of the piston, taking into -account these circumstances, Mr. Watt invented an instrument, -which, like all his mechanical inventions, has answered its -purpose perfectly, and is still in general use. This instrument, -called an _indicator_, consists of a cylinder of about 1-3/4 inch -in diameter, and 8 inches in length. It is bored with great -accuracy, and fitted with a solid piston moving steam-tight in it -with very little friction. The rod of this piston is guided in the -direction of the axis of the cylinder through a collar in the top, -so as not to be subject to friction in any part of its play. At -the bottom of the cylinder is a pipe governed by a stop-cock and -turned in a screw, by which the instrument may be screwed on the -top of the steam-cylinder of the engine. In this position, if the -stop-cock of the indicator be opened, a free communication will be -made between the cylinder of the indicator and that of the engine. -The piston-rod of the indicator is attached to a spiral spring, -which is capable of extension and compression, and which by its -elasticity is capable of measuring the force which extends or -compresses it in the same manner as a spring steel-yard or -balance. If a scale be attached to the instrument at any point on -the piston-rod to which an index might be attached, then the -position of that index upon the scale would be governed by the -position of the indicator-piston in its cylinder. If any force -pressed the indicator-piston upwards, so as to compress the -spring, [Pg275] the index would rise upon the scale; and if, on -the other hand, a force pressed the indicator-piston downwards, -then the spiral spring would be extended, and the index on the -piston-rod descend upon the scale. In each case the force of the -spring, whether compressed or extended, would be equal to the -force urging the indicator-piston, and the scale might be so -divided as to show the amount of this force. - -Now, let the instrument be supposed to be screwed upon the top of -the cylinder of a steam-engine, and the stop-cock opened so as to -leave a free communication between the cylinder of the indicator -below its piston and the cylinder of the steam-engine above the -steam-piston. At the moment the upper steam-valve is opened, the -steam rushing in upon the steam-piston will also pass into the -indicator, and press the indicator-piston upwards: the index upon -its piston-rod will point upon the scale to the amount of pressure -thus exerted. As the steam-piston descends, the indicator-piston -will vary its position with the varying pressure of the steam in -the cylinder, and the index on the piston-rod will play upon the -scale, so as to show the pressure of the steam at each point -during the descent of the piston. - -If it were possible to observe and record the varying position of -the index on the piston-rod of the indicator, and to refer each of -these varying positions to the corresponding point of the descending -stroke, we should then be able to declare the actual pressure of the -steam at every point of the stroke. But it is evident that such an -observation would not be practicable. A method, however, was -contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt, -by which this is perfectly effected. A square piece of paper, or -card, is stretched upon a board, which slides in grooves formed in a -frame. This frame is placed in a vertical position near the -indicator, so that the paper may be moved in a horizontal direction -backwards and forwards, through a space of fourteen or fifteen -inches. Instead of an index a pencil is attached to the indicator of -the piston-rod: this pencil is lightly pressed by a spring against -the paper above mentioned, and as the paper is moved in a horizontal -direction [Pg276] under the pencil, would trace upon the paper a -line. If the pencil were stationary this line would be straight and -horizontal, but if the pencil were subject to a vertical motion, the -line traced on the paper moved under the pencil horizontally would -be a curve, the form of which would depend on the vertical motion of -the pencil. The board thus supporting the paper is put into -connexion by a light cord carried over pulleys with some part of the -parallel motion, by which it is alternately moved to the right and -to the left. As the piston ascends or descends, the whole play of -the board in the horizontal direction will therefore represent the -length of the stroke, and every fractional part of that play will -correspond to a proportional part of the stroke of the steam-piston. - -[Illustration: _Fig._ 81.] - -The apparatus being thus arranged, let us suppose the steam-piston -at the top of the cylinder commencing its descent. As it descends, -the pencil attached to the indicator piston-rod varies its height -according to the varying pressure of the steam in the cylinder. At -the same time the paper is moved uniformly under the pencil, and a -curved line is traced upon it from right to left. When the piston -has reached the bottom of the cylinder, the upper exhausting-valve -is opened, and the steam drawn off to the condenser. The -indicator-piston being immediately relieved from a part of the -pressure acting upon it descends, and with it the pencil also -descends; but at the same time the steam-piston has begun to ascend, -and the paper to return from left to right under the pencil. While -the steam-piston continues to ascend, the condensation becomes more -and more perfect, and the vacuum in the cylinder, and therefore also -in the indicator, being gradually increased in power, the -atmospheric pressure above the indicator-piston presses it downwards -and stretches the spring. The pencil meanwhile, with the paper -moving under it from right to left, traces a second curve. As the -former curve showed the actual pressure of the steam impelling the -piston in its descent, this latter will show the pressure of the -uncondensed steam raising the piston in its ascent, and a comparison -of the two will exhibit the effective force on the piston. _Fig._ -81. represents such a diagram as would be [Pg277] produced by this -instrument. A B C is the curve traced by the pencil during the -descent of the piston, and C D E that during its ascent. A is the -position of the pencil at the moment the piston commences its -descent, B is its position at the middle of the stroke, and C at the -termination of the stroke. On closing the upper steam-valve and -closing the exhausting-valve, the indicator-piston being gradually -relieved from the pressure of the steam the pencil descends, and at -the same time the paper moving from left to right, the pencil traces -the curve C D E, the gradual descent of this curve showing the -progressive increase of the vacuum. As the atmospheric pressure -constantly acts above the piston of the indicator, its position will -be determined by the difference between the atmospheric pressure and -the pressure of the steam below it; and therefore the difference -between the heights of the pencil at corresponding points in the -ascending and descending stroke, will express the difference between -the pressure of the steam impelling the piston in the ascent and -resisting it in the descent at these points. Thus at the middle of -the stroke, the line B D will express the extent to which the spring -governing the indicator-piston would be stretched by the difference -between the force of steam impelling the piston at the middle of the -descending stroke, and the force of steam resisting it at the middle -of the ascending stroke. The force therefore measured by the line B -D will be the effective force on the piston at that point; and the -same may be said of every part of the diagram produced by the -indicator. - -The whole mechanical effect produced by the stroke of the piston -being composed of the aggregate of all its varying effects -throughout the stroke, the determination of its amount [Pg278] is -a matter of easy calculation by the measurement of the diagram -supplied by the indicator. Let the horizontal play of the pencil -from A to C be divided into any proposed number of equal parts, -say ten: at the middle of the stroke, B D expresses the effective -force on the piston, and if this be considered to be uniform -through the tenth part of the stroke, as from _f_ to _g_, then the -number of pounds expressed by B D multiplied by the tenth part of -the stroke expressed in parts of a foot, will be the mechanical -effect through that part of the stroke expressed in pounds' weight -raised one foot. In like manner _m n_ will express the effective -force on the piston after three fourths of the stroke have been -performed, and if this be multiplied by a tenth part of the stroke -as before, the mechanical effect similarly expressed will be -obtained; and the same process being applied to any successive -tenth part of the stroke, and the numerical results thus obtained -being added together, the whole effect of the stroke will be -obtained, expressed in pounds' weight raised one foot. - - -(162.) By means of the indicator, the actual mechanical effect -produced by each stroke of the engine can be obtained, and if the -actual number of strokes made in any given time be known, the -whole effect of the moving power would be determined. An -instrument called a _counter_ was also contrived by Watt, to be -attached either to the working beam or to any other reciprocating -part of the engine. This instrument consisted of a train of -wheel-work with governing hands or indices moved upon divided -dials, like the hand of a clock. A record of the strokes was -preserved by means precisely similar to those by which the hands -of a clock or time-piece indicated and recorded the number of -vibrations of the pendulum or balance-wheel. - - -(163.) To secure the boiler from accidents arising from the steam -contained in it acquiring an undue pressure, a safety-valve is -used, similar in principle to those adopted in the early engines. -This valve is represented in _fig._ 71. at N. It is a conical -valve, kept down by a weight sliding on a rod upon it. When the -pressure of the steam overcomes the force of this weight, it -raises the valve and escapes, being carried off through the tube. -[Pg279] - -With a view to the economy of heat, this waste steam tube is -sometimes conducted into the feeding cistern, where the steam -carried off by it is condensed, and heats the feeding water. - -The magnitude of the safety-valve should be such that, when open, -steam should be capable of passing through it as rapidly as it is -generated in the boiler. The superficial magnitude, therefore, of -such valves must be proportional to the evaporating power of the -boiler. In low pressure boilers the steam is generally limited to -five or six pounds' pressure per square inch, and consequently the -load over the safety-valve in pounds would be found by multiplying -the superficial magnitude of its smallest part by these numbers. -In boilers in which the steam is maintained at a higher pressure, -it would be inconvenient to place upon the safety-valve the -necessary weight. In such cases a lever is used, the shorter arm -of which presses down the valve, and the longer arm is held down -by a weight capable of adjustment, so that the pressure on the -valve may be regulated at discretion. Two safety-valves should be -provided on all boilers, one of which should be locked up, so that -the persons in care of the engine should have no power to increase -the load upon it. In such case, however, it is necessary that a -handle connected with the valve should project outside the box -containing it, so that it may always be possible for the engineer -to ascertain that the valve is not locked in its seat, a -circumstance which is liable to happen. - -Sometimes also two safety-valves are provided, one loaded a little -heavier than the other. The escape of steam from the lighter valve -in this case gives notice to the engine-man of the growing -increase of pressure, and warns him to check the production of -steam. The lever by which the safety-valve is held down is -sometimes acted on by a spiral spring, capable of being so -adjusted as to produce any required pressure on the valve. This -arrangement is adopted in locomotive engines, where steam of very -high pressure is used; and in such cases also there are always -provided two such valves, one of which cannot be increased in its -pressure. - -The pipe by which the boiler is fed with water will [Pg280] -necessarily act as a safety-valve, for when the pressure of the -steam increases in an undue degree, it will press the water in the -boiler up through the feed-pipe, so as to discharge it into the -feed-cistern, a circumstance which would immediately give notice -of the internal state of the boiler. The steam-gauge, already -described (_fig._ 79.), would also act as a safety-valve; for if -the pressure of steam in the boiler should be so augmented as to -blow the mercury out of the steam-gauge, the steam would then -issue through the gauge, and the pressure of the boiler be -reduced, provided that the magnitude of the tube forming the -steam-gauge were sufficient for this purpose. - - -(164.) In high pressure boilers which are exposed to extreme -temperatures and pressures, and which are therefore subject to -danger of explosion, a plug of metal is sometimes inserted, which is -capable of being fused at a temperature above which the boiler -should not be permitted to be raised. If the pressure of steam -increase beyond the proper limit, the temperature of the water and -steam will undergo a corresponding increase; and if the metal of the -plug be capable of being fused at such a temperature, the plug will -fall out of the boiler, and the steam and water will issue from it. -Various alloys of metal are fusible at temperatures sufficiently low -for this purpose. An alloy composed of one part of lead, three of -tin, and five of bismuth, will fuse at the common temperature of -boiling water; and alloys of the same metals, in various -proportions, will fuse at different temperatures from 200 deg. to -400 deg. - -Although fusible plugs may be used, in addition to other means of -insuring safety, they ought not to be exclusively relied on at the -ordinary working pressure of the boiler. The fusible plug ought to -be capable of more than resisting the pressure; but if it be so, -its point of fusion would be one at which the steam would have a -pressure of at least two atmospheres above its working pressure. -The plug would therefore be capable of being fused only as soon as -the steam would acquire a pressure of 30 lbs. per inch above its -regular working pressure. - -When a boiler ceases to be worked, and the furnace has been -extinguished, the space within it appropriated to steam [Pg281] -will be left a vacuum by the condensation of the steam with which -it was previously filled. The external pressure of the atmosphere -acting on the boiler would, under such circumstances, have a -tendency to crush it inwards. To prevent this, a safety-valve is -provided, opening inwards, and balanced by a weight sufficient to -keep it closed until it be relieved from the pressure of the steam -below. - -A large aperture closed by a flange secured with screws, -represented at O in _fig._ 71., called the _man-hole_, is provided -to admit persons into the boiler for the purpose of cleaning or -repairing its interior. - - -(165.) The manner in which the governor regulates the supply of -steam from the boiler to the cylinder, proportioning the quantity -to the work to be done, and thereby sustaining a uniform motion, -has been already explained (p. 125.). Since then the _consumption_ -of steam in the engine is subject to variation, owing to the -various quantities of work it may have to perform, it is evident -that the _production_ of steam in the boiler should be subject to -a proportional variation. For otherwise, one of two effects would -ensue: the boiler would either fail to supply the engine with -steam, or steam would accumulate in the boiler from being produced -in too great abundance, and would escape at the safety-valve, and -thus be wasted. - -In order to vary the production of steam in proportion to the -demands of the engine, it is necessary to stimulate or mitigate -the furnace, as the evaporation is to be augmented or diminished. - -The activity of the furnace must depend on the current of air -which is drawn through the grate-bars, and this will depend on the -magnitude of the space afforded for the passage of that current -through the flues. A plate called a _damper_ is accordingly placed -with its plane at right angles to the flue, so that by raising and -lowering it in the same manner as the sash of a window is raised -or lowered, the space allowed for the passage of air through the -flue may be regulated. This plate might be regulated by the hand, -so that by raising or lowering it the draught might be increased -or diminished, and a corresponding effect produced on the [Pg282] -evaporation in the boiler: but the force of the fire is rendered -uniformly proportional to the rate of evaporation by the following -arrangement, without the intervention of the engineer. The column -of water sustained in the feed-pipe (_figs._ 71, 72.) represents -by its weight the difference between the pressure of steam within -the boiler and that of the atmosphere. If the engine consumes -steam faster than the boiler produces it, the steam contained in -the boiler acquires a diminished pressure, and consequently the -column of water in the feed-pipe will fall. If, on the other hand, -the boiler produce steam faster than the engine consumes it, the -accumulation of steam in the boiler will cause an increased -pressure on the water it contains, and thereby increase the height -of the column of water sustained in the feed-pipe. This column -therefore necessarily rises and falls with every variation in the -rate of evaporation in the boiler. A hollow float P is placed upon -the surface of the water of this column; a chain connected with -this float is carried upwards, and passed over two pulleys, after -which it is carried downwards through an aperture leading to the -flue which passes beside the boiler: to this chain is attached the -damper. By such an arrangement it is evident that the damper will -rise when the float P falls, and will fall when the float P rises, -since the weight of the damper is so adjusted, that it will only -balance the float P when the latter rests on the surface of the -water. - -Whenever the evaporation of the boiler is insufficient, it is -evident from what has been stated, that the float P will fall and -the damper will rise, and will afford a greater passage for air -through the flue. This will stimulate the furnace, will augment -its heating power, and will therefore increase the rate of -evaporation in the boiler. If, on the other hand, the production -of steam in the boiler be more than is requisite for the supply of -the engine, the float will be raised and the damper let down, so -as to contract the flue, to diminish the draught, to mitigate the -fire, and therefore to check the evaporation. In this way the -excess, or defect, of evaporation in the boiler is made to act -upon the fire, so as to render the heat proceeding from the -combustion as nearly as possible proportional to the wants of the -engine. [Pg283] - - -(166.) The method of feeding the furnace by hand through the -fire-door being subject to the double objection of admitting more -cold air over the fuel than is necessary for its combustion, and -the impracticability of insuring that regular attendance on the -part of the stokers, directed the attention of engineers to the -construction of self-regulating furnaces. The most effectual of -these, and that which has come into most general use, was invented -by Mr. William Brunton of Birmingham. - -The advantages proposed to be attained by him were those expressed -in his patent:-- - -"First, I put the coal upon the grate by small quantities, and at -very short intervals, say every two or three seconds. 2dly, I so -dispose of the coals upon the grate, that the smoke evolved must -pass over that part of the grate upon which the coal is in full -combustion, and is thereby consumed. 3dly, As the introduction of -coal is uniform in short spaces of time, the introduction of air -is also uniform, and requires no attention from the fireman. - -"As it respects economy: 1st, The coal is put upon the fire by an -apparatus driven by the engine, and so contrived that the quantity -of coal is proportioned to the quantity of work which the engine -is performing; and the quantity of air admitted to consume the -smoke is regulated in the same manner. 2dly, The fire-door is -never opened, excepting to clean the fire; the boiler, of course, -is not exposed to that continual irregularity of temperature which -is unavoidable in the common furnace, and which is found -exceedingly injurious to boilers. 3dly, The only attention -required is to fill the coal-receiver every two or three hours, -and clean the fire when necessary. 4thly, The coal is more -completely consumed than by the common furnace, as all the effect -of what is termed stirring up the fire (by which no inconsiderable -quantity of coal is passed into the ash-pit), is attained without -moving the coal upon the grate." - -A circular grate is placed on a vertical revolving shaft; on the -lower part of this shaft, under the ash-pit, is placed a toothed -wheel driven by a pinion. This pinion is placed on another vertical -shaft, which ascends above the boiler; and [Pg284] on the other end -of this is placed a bevelled wheel driven by a pinion. This pinion -is attached to a shaft, which takes its motion from the axis of the -fly-wheel, or any other revolving shaft connected with the engine. A -constant motion of revolution is therefore imparted to the circular -grate, and its velocity being proportional to that of the engine, -will necessarily be also proportional to the quantity of fuel which -ought to be consumed. Through that part of the boiler which is over -the fire-grate a vertical tube or opening is made directly over that -part of the furnace which is most distant from the flues. Over this -opening a hopper is placed, which contains the fuel by which the -boiler is to be fed; and in the bottom of this hopper is a sliding -valve, capable of being opened or closed, so as to regulate the -quantity of fuel supplied to the fire-grate. The fuel dropping in in -small quantities through this open valve falls on the grate, and is -carried round by it, so as to leave a fresh portion of the grate to -receive succeeding feeds. The coals admitted through the hopper are -previously broken to a proper size; and in some forms of this -apparatus there are two rollers, at a regulated distance asunder, -the surfaces of which are formed into blunt angular points, and -which are kept in slow revolution by the engine. Between these -rollers the coals must pass before they reach the valve through -which the furnace is fed, and they are thus broken and reduced to a -regulated size. The valve which regulates the opening through which -the feed is admitted, is connected by chains and pulleys with the -self-regulating damper already described, so that in proportion as -the damper is raised, the valve governing the feed may be opened. -Thus, while the quantity of air admitted by the damper is increased -according to the demands of the engine, the quantity of fuel -admitted for the feed is increased by opening the valve in the -bottom of the hopper in the same proportion. Apertures are also -provided in the front of the grate, governed by regulators, by which -the quantity of air necessary and sufficient to produce the -combustion of the gas evolved from the fuel is admitted, these -openings being also connected with the self-regulating damper. - -A considerable portion of the heat imparted to the water [Pg285] -in the boiler escapes by radiation from the surface of the boiler, -steam-pipes, and other parts of the machinery in contact with the -steam and hot water. The effects of this are rendered very -apparent in marine engines, where a large quantity of water is -found to be condensed in the great steam-pipes leading from the -boiler to the cylinder. In stationary land boilers this loss of -heat is usually diminished, and in some cases in a great degree -removed, by surrounding the boiler with non-conducting substances. -In some cases the boiler is built round in brick work. In -Cornwall, where the economy is regarded perhaps to a greater -extent than elsewhere, the boiler and steam-pipes are surrounded -with a packing of sawdust, which being almost a non-conductor of -heat, is impervious to the heat proceeding from the surfaces with -which it is in contact, and consequently confines all the heat -within the boiler. In marine boilers it has been the practice -recently to clothe the boiler and steam-pipes with a coating of -felt, which is attended with a similar effect. When these remedies -are properly applied, the loss of heat proceeding from the -radiation of the boiler is reduced to an extremely small amount. -The engine-houses of some of the Cornish engines, where the boiler -generates steam at a very high temperature, are nevertheless -frequently maintained at a lower temperature than the external -air, and on entering them they have in a great degree the effect -of a cave. - - -(167.) All mechanical action is measured by the amount of force -exercised, or resistance overcome, and the space through which -that force has acted, or through which the resistance has been -moved. - -The gross amount of mechanical action developed by the moving -power of an engine, is expended partly on moving the engine -itself, and partly on overcoming the resistance on which the -engine is intended to act. That part of the mechanical energy of -the moving power which is expended on the resistance or load which -the engine moves exclusively, and of the power expended on moving -the engine itself, is called _the useful effect_ of the machine. - -The _gross effect_, therefore, exceeds the _useful effect_ by the -[Pg286] amount of power spent in moving the engine, or which may -be wasted or destroyed in any way by the engine. - -It is usual to express and estimate all mechanical effect whatever -by nature of the resistance overcome, by an equivalent weight -raised a certain height. Thus, if an engine exerts a certain power -in driving a mill, in drawing a carriage on a road, or in -propelling a vessel on water, the resistance against which it has -to act must be equal to a definite amount of weight. If a carriage -be drawn, the traces are stretched by the tractive power, by the -same tension that would be given to them if a certain weight were -appended to them. If the paddle-wheels of a boat are made to -revolve, the water opposes to them a resistance equal to that -which would be produced, if instead of moving the water the wheel -had to raise some certain weight. In any case, therefore, weight -becomes the exponent of the energy of the resistance against which -the moving power acts. - -But the amount of mechanical effect depends conjointly on the -amount of resistance, and the space through which that resistance -is moved. The quantity of this effect, therefore, will be -increased in the same proportion, whether the quantity of -resistance or the space through which that resistance is moved be -augmented. Thus, a resistance of one hundred pounds, moved through -two feet, is mechanically equivalent to a resistance of two -hundred pounds moved through one foot, or of four hundred pounds -moved through six inches. To simplify, therefore, the expression -of mechanical effect, it is usual to reduce it invariably to a -certain weight raised one foot. If the resistance under -consideration be equivalent to a certain weight raised through ten -feet, it is always expressed by ten times the amount of that -weight raised through one foot. - -It has also been usual in the expression of mechanical effect, to -take the pound weight as the unit of weight, and the foot as the -unit of length, so that all mechanical effect whatsoever is -expressed by a certain number of pounds raised one foot. - - -(168.) The gross effect of the moving power in a steam-engine, is -the whole mechanical force developed by the evaporation [Pg287] -of water in the boiler. A part of this effect is lost by the -partial condensation of the steam before it acts upon the piston, -and by the imperfect condensation of it subsequently: another -portion is expended on overcoming the friction of the different -moving parts, and in acting against the resistance which the air -opposes to the machine. If the motion be subject to sudden shocks, -a portion of the power is then lost by the destruction of momentum -which such shocks produce. But if those parts of the machine which -have a reciprocating motion be, as they ought to be, brought -gradually to rest at each change of direction, then no power is -absorbed in this way. - - -(169.) The useful effect of an engine is variously denominated -according to the relation under which it is considered. If it be -referred to the time during which it is produced, it is called -POWER. - - -(170.) If it be referred to the fuel, by the combustion of which -the evaporation has been effected, it is called DUTY. - - -(171.) When steam-engines were first brought into use, they were -commonly applied to work pumps for mills which had been previously -worked or driven by horses. In forming their contracts, the first -steam-engine builders found themselves called upon to supply -engines capable of executing the same work as was previously -executed by some certain number of horses. It was therefore -convenient, and indeed necessary, to be able to express the -performance of these machines by comparison with the animal power -to which manufacturers, miners, and others, had been so long -accustomed. When an engine, therefore, was capable of performing -the same work in a given time as any given number of horses of -average strength usually performed, it was said to be an engine of -so many horses' power. Steam-engines had been in use for a -considerable time before this term had acquired any settled or -uniform meaning, and the nominal power of engines was accordingly -very arbitrary. At length, however, the use of steam-engines -became more extended, and the confusion and inconvenience arising -out of all questions respecting the performance of engines, -rendered it necessary that some fixed [Pg288] and definite -meaning should be assigned to the terms by which the powers of -this machine were expressed. To have abandoned the term -_horse-power_, which had been so long in use, would have been -obviously inconvenient; nor could there be any objection to its -continuance, provided all engine-makers, and all those who used -engines, could be brought to agree upon some standard by which the -unit of horse-power might be defined. The performance of a horse -of average strength working for eight hours a day was therefore -selected as a standard, or unit, of steam-engine power. Smeaton -estimated that such an animal, so working, was capable of -performing a quantity of work equal in its mechanical effect to -22,916 lbs. raised one foot per minute, while Desaguliers -estimated the same power at 27,500 lbs. raised through the same -height in the same time. The discrepancy between these estimates -probably arose from their being made from the performances of -different classes of horses. Messrs. Boulton and Watt caused -experiments to be made with the strong horses used in the -breweries in London, and from the result of these trials they -assigned 33,000 lbs. raised one foot per minute, as the value of a -horse's power. This is the unit of engine-power now universally -adopted; and when an engine is said to be of so many horses' -power, what is meant is, that that engine, in good working order -and properly managed, is capable of moving a resistance equal to -33,000 lbs. through one foot per minute. Thus an engine of ten -horse-power is one that would raise 330,000 lbs. weight one foot -per minute. - -Whether this estimate of an average horse's power be correct or -not, in reference to the actual work which the animal is capable -of executing, is a matter of no present importance in its -application to steam-power. The steam-engine is no longer used to -replace the power of horses, and therefore no contracts are based -upon such a comparison. The term horse-power, therefore, as -applied to steam-engines, must be understood to have no reference -whatever to the actual animal power, but must be taken as a term -having no other meaning than the expression of the ability of the -[Pg289] machine to move the amount of resistance above mentioned -through one foot per minute. - - -(172.) It has been already explained (67.) that the conversion of -a given volume of water into steam is productive of a certain -definite amount of mechanical force, this amount depending on the -pressure under which the water is evaporated, and the extent to -which the expansive principle is used in working the steam. It is -evident that this amount of mechanical effect is a major limit, -which cannot be exceeded by the power of the engine. - -If the steam be not worked expansively, then the whole power of -the water, transmitted in the form of steam from the boiler to the -working machinery, will be a matter of easy calculation, when the -pressure at which the steam is worked is known. A table, -exhibiting the mechanical power of a cubic foot of water converted -into steam at various pressures, expressed in an equivalent number -of pounds' weight raised one foot high, is given in the Appendix -to this volume. Where much accuracy is sought for, the pressure at -which the steam is used must be taken into account; but by -reference to the table it will be seen, that when steam is worked -without expansion, its mechanical effect varies very little with -the pressure. It may therefore be assumed, as has been already -stated, that for every cubic inch of water transmitted in the form -of steam to the cylinders, a force is produced, represented by a -ton weight raised a foot high. Now, as 33,000 lbs. is very nearly -15 tons, it follows that 15 cubic inches of water converted into -steam per minute, or 900 cubic inches per hour, will produce a -mechanical force equal to one horse. If, therefore, to 900 cubic -inches be added the quantity of water per hour necessary to move -the engine itself, independently of its load, we shall obtain the -quantity of water per hour which must be supplied by the boiler to -the engine for each horse-power, and this will be the same -whatever may be the magnitude or proportions of the cylinder. - - -(173.) The quantity of power expended in working the engine -itself, independently of that required to move its load, will be -less in proportion to the degree of perfection which [Pg290] may -be attained in the construction of the engine, and to the order in -which it is kept while working. Engines vary one from another so -much in these respects, that it is scarcely possible to lay down -any general rules for the quantity of power to be allowed over and -above what is necessary to move the load. The means whereby -mechanical power is expended in working the engine may be -enumerated as follows:-- - -_First._ Steam in passing from the boiler to the cylinder is -liable to lose its temperature by the radiation of the steam-pipes -and other passages through which it is conducted. Since the steam -produced in the boiler is in contact with water, it will be common -steam (94.), and consequently the least loss of heat will cause a -partial condensation. To whatever extent this condensation may be -carried, a proportional loss of power, in reference to the heat -obtained from the fuel, will be entailed upon the engine. - -It has been said that the force necessary to move the steam from -the boiler to the cylinder through passages more or less -contracted, subject to the friction of the pipes and tubes through -which it moves, should be taken into account in estimating the -power, and a corresponding deduction made. This, however, is not -the case: the steam having passed into the cylinder remains common -steam, its pressure being diminished by reason of the force -expended in thus moving it from the boiler to the cylinder. But -its mechanical efficacy at the reduced pressure is not sensibly -different from the efficacy which it had in the boiler. If at the -reduced pressure its volume were the same, then a loss of effect -would be sustained equivalent to the difference of the pressures; -but its volume being augmented in very nearly the same proportion -as its pressure is diminished, the mechanical efficacy of a given -weight of steam in the cylinder will be sensibly the same as in -the boiler. - -_Second._ The radiation of heat from the cylinder and its -appendages, will cause a partial condensation of steam, and -thereby produce a diminished mechanical effect. - -_Third._ The steam, which at each stroke of the piston fills the -passages between the steam-valves and the piston, at the [Pg291] -moment the latter commences the stroke will be inefficient. If it -were possible for the piston to come into steam-tight contact with -each end of the cylinder, and that the steam-valve should be in -immediate contact with the side or top of the piston, then the -whole of the steam which would pass through the steam-valve would -be efficient; but as some space, however small, must remain -between the piston and the ends of the cylinder, and between the -side of the cylinder and the steam-valve, there will always be a -volume of steam bearing a sensible proportion to the magnitude of -the cylinder, which at each stroke of the piston will be -inefficient. This volume of steam is called the _clearance_. - -_Fourth._ Since the piston must move in steam-tight contact with -the cylinder, it must have a definite amount of friction with the -sides of the cylinder by whatever means it may be packed. This -friction will produce a corresponding resistance to the moving -power. - -_Fifth._ The various joints of the machinery where steam is -contained are subject to leakage, and whatever amount of steam -shall thus escape must be placed to the account of power lost. - -_Sixth._ When the eduction-valve is opened to admit the steam to -the condenser, a certain force is required to expel the steam from -the cylinder. This force reacts upon the piston, and counteracts -to a proportional extent the moving power of the steam on the -other side. Besides this the water in the condenser cannot be -conveniently reduced below the temperature of about 100 deg., and -at this temperature steam has a pressure of about 1 lb. per square -inch. This vapour will continue to fill the cylinder, and will -resist the moving power which impels the piston. - -_Seventh._ Power must be provided for opening and closing the -valves or slides, for working the air-pump, hot-water pump, and -cold-water pump, and finally to overcome the friction on the -journals and centres of the parts of the parallel motion, the main -axle of the beam, the connecting rod, crank, and fly-wheel axle. - -It will be apparent how very much these sources of resistances -must vary in different engines, and how rough [Pg292] an -approximation any general estimate must be of their gross amount. - - -(174.) There are many circumstances which obstruct the practical -application of any standard of engine-power: the magnitude of -furnace, and the extent of heating surface necessary to produce -any required rate of evaporation in the boiler, are unascertained; -each engine-maker has his own rule in these matters, and all the -rules are equally unsupported by any experimental test entitled to -respect. Thus the circumstances that govern the rate of -evaporation in the boiler may be regarded as almost wholly -unknown. But supposing the rate of evaporation to be ascertained, -the amount of power absorbed by the condensation of steam on its -passage to the cylinder, the imperfect condensation of the same -steam after it has worked the piston, the friction of the various -moving parts of the machinery, and, above all, the difference of -effect of these losses of power in engines constructed on -different scales of magnitude, are absolutely unknown. We are, -therefore, not placed in a condition to assign any thing more than -a general account of what has been the practice of engine-makers -in constructing engines which are nominally of a certain power. - -In common low-pressure engines of the larger kind, to which class -alone we at present refer, it has been usual, with the same fuel -and under like circumstances, to allow from 10 to 18 square feet -of heating surface in the boiler for every nominal horse-power of -the engine. Within these wide limits the practice of engine-makers -has varied. It is not, however, to be supposed, that the boiler -with 18 square feet of surface per horse-power has the same -evaporating power as that which has but 10. This difference, -therefore, amounts to nothing more than different manufacturers of -steam-engines putting into circulation boilers having powers -_really_ different while they are _nominally_ the same. The -magnitude of the cylinder is regulated by the nominal power of the -engine, and it is usual so to regulate the evaporating power of -the boiler, that the piston shall move at the average rate of 200 -feet per minute. This being assumed, it is customary to allow -about 22 square inches of piston [Pg293] surface for every -nominal horse-power of the engine. If this power were in -conformity to the standard already defined, this amount of surface -moved at 200 feet per minute would be impelled by a pressure -amounting to 7-1/2 lbs. per square inch. The safety-valve of the -boiler of such engines is usually loaded at from 4 to 5 lbs. per -square inch, and consequently the steam in the boiler will have a -pressure of from 19 to 20 lbs. per square inch. If, therefore, the -effective pressure on the piston be really only 7-1/2 lbs. per -square inch, the pressure expended in overcoming the friction of -the engine, and the loss consequent on the partial condensation of -steam on one side and its imperfect condensation on the other, -would amount to from 12 to 13 lbs. per square inch, or nearly -double the assumed useful effect of the engine. - -Messrs. Maudslay and Field are accustomed to allow an evaporation -of ten gallons, or 1.6 cubic feet of water per hour, for each -nominal horse-power of the engine. They also allow about 22 square -inches of piston surface per nominal horse-power, the piston being -supposed to move at the rate of 200 feet per second.[24] - -The quantity of grate surface necessary in proportion to the power -of the engine, has been equally unascertained, and engine-makers -vary in their practice from half a square foot to one square foot -per nominal horse-power. - -The proportion which the magnitude of the heating surface of the -boiler, and the fire surface of the grate bears to the evaporating -power of the boiler, has not been determined by experiment, nor, -so far as we are informed, by any well-ascertained practical -results. - -The estimates or rather conjectures of engine-makers, of the -evaporation necessary to produce one horse-power, vary from one to -two cubic feet of water per hour. It has been [Pg294] already -shown that the evaporation of 900 cubic inches, or little more -than half a cubic foot per hour, evolves a gross mechanical effect -representing one horse-power; from which it appears, that if the -evaporation of the boilers of steam engines were what engineers -suppose them to be, the gross mechanical power produced in them -for every nominal horse-power of the engine varies in actual -amount from the power of two to that of four horses. - -The above estimates must be understood as referring to -double-acting steam engines above thirty-horse power. The -circumstances attending the performance of single-acting engines -applied to the drainage of mines, have been ascertained with much -greater precision. This has been mainly owing to a spirited system -of general inspection, which has been established in Cornwall, to -which we shall hereafter more particularly advert. - - -(175.) In expressing the duty of engines, it would have been -desirable that the duty of the boiler should have been separated -from that of the engine. - -The duty of a boiler is estimated by the volume of water -evaporated by a given quantity of fuel, independently of the time -which such evaporation may take. The duty, therefore, will be -expressed by the number of cubic feet of water evaporated, divided -by the number of bushels of coal necessary for that evaporation, -supposing the bushel of coal to be the unit of fuel. It will be -observed that the _duty_ of an engine or boiler is entirely -distinct from, and independent of, its _power_. One boiler may be -greater than another in power to any extent, while it may be equal -to or less than it in duty. A bushel of coals may evaporate the -same number of cubic feet of water under two boilers, but may take -twice as great a time to produce such evaporation under one than -under the other. In such a case the power of one boiler will be -double that of the other, while their duty will be the same. - -In like manner, a bushel of coals consumed in working two engines -may produce the same useful effect, but it may produce that useful -effect in the one in half the time it takes to produce it in the -other. In that case the _duty_ of the engines will be the same, -but the _power_ of the one will be double that of the other. -[Pg295] - -In fine, _power_ has reference to _time_,--_duty_, to _fuel_. The -more rapidly the engine produces its mechanical effect, the -greater its power will be, whatever may be the fuel consumed in -working it. And, on the other hand, the greater the useful effect -produced by a given weight of fuel, the greater will be the duty, -however long the time may be which the fuel may take to produce -the useful effect. - - -(176.) The proportion of the stroke to the diameter of the -cylinder must be determined by the velocity intended to be given -to the piston. With the same capacity of cylinder, and the same -evaporation in the boiler, the velocity of the piston will augment -as the magnitude of its diameter is diminished. - -The proportion of the diameter to the stroke of the cylinder is -very various. In engines used for steam-vessels the length of the -cylinder very little exceeds its diameter. In land engines, -however, the proportion of the length to the diameter is greater. -It is maintained by some that the proportion of the diameter and -length of the cylinder should be such as to render its surface -exposed to the cooling of the external air, the smallest possible. -Tredgold has maintained that since, during the stroke, the steam -is gradually exposed to contact with the surface of the cylinder -from the top to the bottom, the mean surface exposed in contact -with steam being half that of the entire cylinder, the proportion -of the diameter to the stroke should be such that the surface of -half the length of the cylinder, added to the magnitude of the top -and bottom, shall be a minimum. If this principle be admitted, -then the best proportion of the diameter to the stroke would be -that of one to two, the length of the stroke being twice the -diameter of the cylinder; but since the whole surface of the -cylinder is constantly exposed to the cooling effects of the air, -and since in the intervals of the stroke there is no sensible -change of the temperature of the surface, the loss of heat by -cooling will in effect be the same, especially in double-acting -engines, as if the cylinder were constantly filled with steam. If -this be admitted, then the object should be to give the cylinder -such a proportion, that its entire surface, including the top and -bottom, shall be a minimum. [Pg296] The proportion given by this -condition would be very nearly that which is observed in the -cylinders of marine engines, viz. that the length of the cylinder -should be equal to its diameter. - -If in a low-pressure engine the pressure of steam in the cylinder -be taken at 17 lbs. per square inch, then the volume of steam will -be about fifteen hundred times that of the water which produces -it. For every cubic foot of water, therefore, in the effective -evaporation of the boiler, 1500 cubic feet of steam will be passed -through the cylinder. If it be intended that the motion of the -piston shall be at the rate of 25 strokes per minute, or 1500 -strokes per hour, then the capacity of that portion of the -cylinder between the steam-valve and the piston at the end of the -stroke, must consist of half as many cubic feet as there are cubic -feet per hour evaporated in the boiler. If the steam, therefore, -be cut off at half stroke, the number of cubic feet of space in -the cylinder will be equal to the number of cubic feet of water -effectively evaporated by the boiler; and if a cubic foot of water -effectively evaporated be taken as the measure of a horse-power, -then there would be as many cubic feet in the capacity of the -cylinder as is equal to the nominal power of the engine. - - -(177.) The duty of engines varies according to their form and -magnitude, the circumstances under which they are worked, and the -purposes to which they are applied. In double-acting engines -working without expansion, the coal consumed per nominal -horse-power per hour varies from 7 to 12 lbs. An examination of -the steam-logs of several government steamers made by me a few -years since, gave, as the average of consumption of fuel at that -time of the best class of marine engines, about 8 lbs. per nominal -horse-power per hour. Since, however, no account could be obtained -of the actual evaporation of water in the boiler, nor, with the -necessary degree of precision, of the quantity and pressure of the -steam which passed through the cylinders, this estimate must be -regarded as an approximation subject to several causes of error. -The question of the duty of boilers and engines applied to the -[Pg297] general purposes of manufactures and navigation, is one -which has not yet been satisfactorily investigated; and it were -much to be desired that the proprietors of such engines should -combine to establish a strict analysis of their performance in -reference to their consumption of fuel, their evaporation of -water, and their useful effects. The results of such an -investigation, if properly conducted, would perhaps tend more to -the improvement of the steam engine than any discoveries in -science, or inventions in mechanical detail likely to be made in -the present stage of the progress of that machine. - - -(178.) A strict investigation of this kind has been for many years -carried on respecting the performance of the steam engines used -for the drainage of the mines in Cornwall; and it has been -attended with effects the most beneficial to the interests of -those concerned in them. The engines to which this important -inquiry has been applied being used for the purpose of pumping, -are generally single-acting engines, in which steam is used -expansively to a great extent. The steam is produced under a very -high pressure in the boiler, and being admitted to the cylinder is -cut off after a small portion of the entire stroke has been made, -the remainder of the stroke being produced by the expansion of the -steam. - -About the year 1811, a number of the proprietors of the principal -Cornish mines agreed to establish this system of inspection, under -the management and direction of Captain Joel Lean, and to publish -monthly reports. In these reports were stated the following -particulars:--1. The load per square inch on the piston; 2. The -consumption of coal in bushels; 3. The number of strokes made by -the engine; 4. The length of the strokes in the pumps; 5. The load -in pounds; 6. The duty of the engine, expressed by the number of -pounds raised one foot high by the consumption of a bushel of -coals; 7. The number of strokes per minute; 8. The diameter and -stroke of the cylinder, and a general description of the engine. -When these reports were commenced, the number of engines brought -under inspection was twenty-one. In the year 1813 it increased -to twenty-nine; in 1814 to thirty-two; in 1820 the number -reported upon increased [Pg298] to forty; in 1828 the number was -fifty-seven; and in 1836 it was sixty-one. This gradual increase -in the number of engines brought under this system of inspection, -was produced by the good effects which attended it. These -beneficial consequences were manifested, not only in the improved -performance of the same engines, but in the gradually improved -efficiency of those which were afterwards constructed. - -The following table taken from the statement of the duty of -Cornish engines by Thomas Lean and brother, lately published by -the British Association, will show in a striking manner the -improvement of the Cornish engines, from the commencement of this -system of inspection to the present time. The duty is expressed by -the number of pounds raised one foot high by the consumption of a -bushel of coals. - - --------------------------------------------------------------- - | No. of | Average Duty of the | Average Duty of the best - Years.|Engines.| Whole. | Engine. - --------------------------------------------------------------- - 1812 | 21 | 19,300,000 | - 1813 | 29 | 19,500,000 | 26,400,000 - 1814 | 32 | 20,600,000 | 32,000,000 - 1815 | 35 | 20,500,000 | 28,700,000 - 1816 | 35 | 23,000,000 | 32,400,000 - 1817 | 35 | 26,500,000 | 41,600,000 - 1818 | 36 | 25,400,000 | 39,300,000 - 1819 | 40 | 26,300,000 | 40,000,000 - 1820 | 46 | 28,700,000 | 41,300,000 - 1821 | 45 | 28,200,000 | 42,800,000 - 1822 | 52 | 28,900,000 | 42,500,000 - 1823 | 52 | 28,200,000 | 42,100,000 - 1824 | 49 | 28,300,000 | 43,500,000 - 1825 | 56 | 32,000,000 | 45,400,000 - 1826 | 51 | 30,500,000 | 45,200,000 - 1827 | 51 | 32,100,000 | 59,700,000 - 1828 | 57 | 37,100,000 | 76,800,000 - 1829 | 53 | 41,700,000 | 77,000,000 - 1830 | 56 | 43,300,000 | 78,000,000 - 1831 | 58 | 43,400,000 | 71,100,000 - 1832 | 59 | 45,000,000 | 85,000,000 - 1833 | 56 | 46,600,000 | 84,300,000 - 1834 | 52 | 47,800,000 | 90,900,000 - 1835 | 51 | 47,800,000 | 91,700,000 - 1836 | 61 | 46,600,000 | 85,400,000 - 1837 | 58 | 47,000,000 | 87,200,000 - 1838 | 61 | 48,700,000 | 84,200,000 - --------------------------------------------------------------- - -[Pg299] As an example of the beneficial effects produced upon the -efficiency of an individual engine by the first application of -this system of inspection, the case of the Stray Park engine may -be mentioned. This engine, constructed by Boulton and Watt, had a -sixty inch cylinder, and when first reported in 1811, its duty -amounted to 16,000,000 pounds. After having been reported on for -three years, its duty was found to have increased to 32,000,000; -this estimate being taken from the average result of twelve -months' performance. Its duty was doubled in less than three -years. - -It will appear, by inspection of the duties registered in the -preceding table, that the augmentation of the efficiency of the -engines has not been the effect of any great or sudden improvement, -but has rather resulted from the combination of a great number of -small improvements in the details of the operation of these -machines. In these improvements more is due to the successful -application of practical experience than to any new principles -developed by scientific research. Mr. John Taylor, in his "Records -of Mining," has traced the successive improvements on which the -increased duty of engines depends, and has connected these -improvements with their causes in the order of their dates. The -following results, abridged from his estimates, may not be -uninteresting:-- - -In 1769, soon after the date of the earliest discoveries of Mr. -Watt, but before they had come into practical application, Smeaton -computed that the average duty of fifteen atmospheric engines, -working at Newcastle-on-Tyne, was 5,590,000. The duty of the best -of these engines was 7,440,000, and that of the worst 3,220,000. - -In 1772, Smeaton commenced his improvements on the atmospheric -engine, and raised the duty to 9,450,000. - -In 1776, Watt obtained a duty of 21,600,000. - -At this time Smeaton acknowledged that Watt's engines gave a duty -amounting to double that of his own. - -In 1778-79, Watt reported a duty of 23,400,000. - -From 1779 to 1788, Watt introduced the application of expansion, -and raised the duty to 26,600,000. [Pg300] - -In 1798, an engine by Boulton and Watt, erected at Herland, was -reported as giving a duty of 27,000,000. - -This engine, which was probably the best which at that time had -ever been erected, attracted the particular attention of Mr. Watt, -who, on visiting Cornwall, went to see it, and had many -experiments tried with it. It was under the care of Mr. Murdock, -the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt -inspected it he pronounced it perfect, and that further -improvement could not be expected. How singular an instance this -of the impossibility, even of the most sagacious, to foresee the -results of mechanical improvement! In twenty years afterwards the -average duty of the best engine was nearly 40,000,000, and in -forty years it was above 84,000,000. - -[Illustration: BOILER MANUFACTORY.] - - FOOTNOTES: - - [24] If 22 square inches of piston surface be allowed to - represent a horse-power, the power of an engine may always be - computed by dividing the square of the diameter of the piston - expressed in inches by 28. And, on the other hand, to find the - diameter of piston which would correspond to any given power, - multiply the number of horses' power by 28, and take the - square root of the product. These rules, however, cannot be - applied if the piston be supposed to move with any other - velocity; since, in that case, the same amount of piston - surface would cease to represent a horse-power, unless the - effective pressure on the piston were at the same time - changed. - -[Pg301] - - - - -[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.] - -CHAP. X. - - NOTICE OF THE LIFE OF MR. WATT. -- HIS FRIENDS AND ASSOCIATES - AT BIRMINGHAM. -- INVENTION OF THE COPYING PRESS. -- HEATING BY - STEAM. -- DRYING LINEN BY STEAM. -- THEORY OF THE COMPOSITION OF - WATER. -- FIRST MARRIAGE OF WATT. -- DEATH OF HIS FIRST WIFE. -- - HIS SECOND MARRIAGE. -- DEATH OF HIS YOUNGER SON. -- EXTRACTS - FROM HIS LETTERS. -- CHARACTER OF WATT BY LORD BROUGHAM. -- BY - SIR WALTER SCOTT. -- BY LORD JEFFREY. -- OCCUPATION OF HIS OLD - AGE. -- INVENTION OF MACHINE FOR COPYING SCULPTURE. -- HIS LAST - DAYS. -- MONUMENTS. - - -(179.) Having brought this historical analysis of the invention -and application of the steam engine to the date of the decease of -the illustrious man, to the powers of whose mind the world stands -indebted for the benefits conferred upon [Pg302] mankind by that -machine, it will perhaps not be deemed an improper digression in -this work, to devote some pages to a notice of the principal -labours of the same mind in other departments of art and science, -and to circumstances connected with his personal history and the -close of his life, which cannot fail to possess general interest. - -At the period when Watt, having connected himself in partnership -with Boulton, went to reside at Soho, near Birmingham, a number of -persons, some of whom have since attained great celebrity by their -discoveries and their works, and all of whom were devoted to -inquiries connected with the arts and sciences, resided in that -neighbourhood. Among these may be mentioned PRIESTLEY, whose -discoveries in physical science have rendered his name immortal; -DARWIN, the philosopher and poet; WITHERING, a distinguished -physician and botanist; KEIR, a chemist, who published a translation -of Macquer, with annotations; GALTON, the ornithologist; and -EDGEWORTH, whose investigations respecting wheeled carriages and -other subjects, have rendered him well known. A society was formed -by these and other individuals, of which Boulton and Watt were -leading members, the meetings of which were held monthly on the -evening of full moon, and which was thence called the _Lunar -Society_. At the meetings of this society, subjects connected with -the arts and sciences were discussed, and out of those discussions -occasionally arose suggestions not unattended with important and -advantageous consequences. At one of these meetings, Darwin stated -that he had discovered a pen formed with two quills, by means of -which, at a single operation, an original and a copy of a letter -might be produced. Watt almost instantly observed that he thought he -could find a better expedient, and that he would turn it in his mind -that night. By the next morning the COPYING PRESS was invented, for -which he afterwards obtained a patent. - -This machine, which is now so generally used in counting-houses, -consists of a rolling-press, by which a leaf of thin paper, -previously damped, is pressed upon the letter to be copied. The -writing, of which the ink is not yet quite dry, leaves its -impression upon the thin paper thus pressed upon [Pg303] it, and -the copy taken in this manner is read through the semi-transparent -paper. If a letter be written with ink suitable for this purpose, -a copy may be taken at any time within several hours after the -letter is written. - -The method of heating apartments and buildings by steam, which has -since been improved and brought into extensive use, was likewise -brought forward by Watt. Although this contrivance had been -previously pointed out by Sir Hugh Platt about the middle of the -seventeenth century, and by Colonel Cooke in 1745, yet these -suggestions remained barren. Mr. Watt gave detailed methods of -heating buildings by steam[25]; and also invented a machine for -drying linen by steam, a description of which he communicated to -Dr. Brewster, which was read in December, 1824, before the Society -for promoting Useful Arts in Scotland.[26] - -But the circumstance, exclusive of those connected with the -invention of the steam engine, which is by far the most memorable -in the career of Watt, is the share which he had in the discovery -of the composition of water. As this circumstance has recently -excited much interest, and led to some controversy, we shall here -state, as distinctly as possible, the leading facts connected with -it. - -Water, which was so long held to be a simple element, has, in -modern times, been proved to be a substance consisting of two -aeriform bodies or gases chemically combined. These two gases are -those called in chemistry _oxygen_ and _hydrogen_. If eight grains -weight of oxygen be mixed with one grain weight of hydrogen, and -the mixture be submitted to such effects as would cause the -chemical combination of these two airs, it would be converted into -nine grains weight of pure water. - -If, on the other hand, nine grains weight of pure water be -submitted to any conditions which would separate its constituent -parts, the result would be eight grains weight of oxygen gas, and -one grain weight of hydrogen gas. There are a variety of methods -in physics by which these effects would be [Pg304] produced. It -will be sufficient here to state one method of producing each of -the above changes. - -If eight grains weight of oxygen be inclosed in a strong vessel -with one grain weight of hydrogen, all other substances being -excluded, and the mixture be inflamed, an explosion will take -place, the gases will disappear, and a small quantity of water -will be the only substance remaining in the vessel. If this water -be weighed, it will be found to weigh exactly nine grains. - -It is known that the metals have a strong attraction for oxygen -gas, and this attraction is promoted by elevating their -temperature. If a glass tube be filled with iron wire heated to -redness, and to one end of this tube a small vessel of boiling -water be attached, the steam evolved from the water will force its -way through the spaces between the red-hot wires in the tube, and -would be expected to issue from the remote end; but if the -substance issuing from the remote end of the tube be examined, it -will be found to be not steam, but hydrogen gas. If the quantity -of this gas be ascertained by weight, and also the quantity of -weight lost by the vessel of water at the other end of the tube, -it will be found that the loss of weight of the water by -evaporation will be nine times the weight of the hydrogen which -has issued from the remote end of the tube. If the weight of the -tube with the wire contained in it be next ascertained, it will be -found to be increased by eight times the weight of the hydrogen -which has issued from its remote end. From this it follows that -the weight of the hydrogen which has escaped from the tube, added -to the increase of weight which has been given to the wire in the -tube, makes up the whole weight of the water evaporated. If the -wire in the tube be next examined, it will be found that it has -suffered oxydation, or, in other words, that a new substance has -been formed in it called the oxyde of iron,--such substance being -a chemical compound formed of oxygen gas and iron. - -It follows, therefore, that in this process the vapour of the -water, in passing through the tube, has been decomposed, and that, -having given up to the iron its oxygen, the hydrogen [Pg305] -alone escaped from the other end; and for every nine grains weight -of steam which passed through the tube, eight grains of oxygen -have been combined with the iron, and one grain of hydrogen has -escaped from the end of the tube. - -Such are the class of effects on which the modern discovery of the -composition of water has been based. The merit of that discovery -has been shared between the celebrated English chemist, CAVENDISH, -and the not less celebrated French chemist, LAVOISIER, the chief -merit, however, being ascribed to the former. - -We shall now briefly state the facts which led to this discovery, -with their dates, which will necessarily show the share which Watt -had in it. - -When pure hydrogen gas is burned in an atmosphere of common air, -the process which takes place is now known to be nothing more than -the chemical combination of the hydrogen with eight times its own -weight of oxygen taken from the atmosphere, and the product of the -combustion is a quantity of water nine times the weight of the -hydrogen consumed. In the year 1776, Macquer, a well-known chemist -of that day, having held a saucer of white porcelain over a flame -of hydrogen which was burning at the mouth of a bottle, observed -that no smoke was produced and no soot deposited on the saucer. On -the other hand, he found that after the lapse of some time drops -of a clear pellucid liquid were perceptible on the saucer: this -liquid he submitted to analysis, and found it to be pure water. -Macquer mentioned this fact without comment or inference. It did -not occur to him that the water thus produced upon the saucer was -a substance which contained the hydrogen, which disappeared upon -combustion from the bottle. - -On the 18th of April, 1781, Mr. Warltire addressed a letter to Dr. -Priestley, dated Birmingham, which letter is published in Dr. -Priestley's _Experiments on Air_, printed at Birmingham in 1781, -in which Warltire informs Priestley that he had fired a mixture of -hydrogen and common air in close glass vessels, and that, although -previously to firing the mixture the vessels were clean and dry, a -dewy deposit was [Pg306] observed afterwards on their sides. In -fact, water was present which was not present before. - -The mixture was in this case fired by passing an electric spark -through the vessel; and it is now known that the effect produced -was the combination of the hydrogen, which formed part of the -mixture of airs in the vessel with the oxygen, which also formed -part of the same mixture. - -It appears, from expressions in Warltire's letter, that the same -experiment had been previously made by Priestley, and the same -result observed by him. - -The inference deduced from this by Warltire, and apparently -acquiesced in by Priestley, was, that whenever hydrogen was fired -in atmospheric air, the moisture, which is always more or less -sustained in the latter, was deposited; but neither of these -chemists perceived the real cause of the production of the water. - -In the beginning of 1783, and not later than the 21st of April, -this experiment of Warltire and Priestley was repeated by -Cavendish, with this difference, that, instead of exploding the -mixture of hydrogen and common air, Cavendish exploded a mixture -of hydrogen and oxygen. He observed that water was present after -the explosion, but _inferred nothing_. - -In a published paper dated April, 1783, Priestley announced a -further and most important result of his experiments. This was, -that in examining the weight of water produced by the explosion of -a mixture of oxygen and hydrogen, _that weight was found to be -precisely equal to the sum of the weights of the two gases_, which -disappeared in the process. - -Immediately on observing this, Priestley, being then, as has been -already stated, Watt's near neighbour, communicated to the latter -what he had observed; upon which Watt immediately, viz. by a letter -dated the 26th of the same month, declared that the inevitable -consequence which followed from Priestley's observations was, that -water was a substance compounded of oxygen and hydrogen deprived of -[Pg307] a quantity of heat which was previously latent in them.[27] -The letter containing this inference was communicated immediately by -Priestley to Sir Joseph Banks, then President of the Royal Society, -to be laid before that body; and it is accordingly printed with its -proper date in the 74th volume of the _Philosophical Transactions_. - -About two months after the date of Mr. Watt's letter just quoted, -Lavoisier made experiments on the combustion of oxygen and -hydrogen, and read a memoir before the Academy of Sciences in -Paris, in which his views of the formation of water by the -combination of these gases were developed. This paper, by -Lavoisier, was afterwards printed in the Memoirs of the Academy in -the year 1784. The experiments are there stated to have been made -in the month of June, 1783; and it is stated that Sir Charles -Blagden, who was present at the experiments, told Lavoisier that -Mr. Cavendish had already burned the same gases in close vessels, -and obtained a very sensible quantity of water. - -On the 15th of January, 1784, the celebrated paper by Cavendish, -entitled "Experiments on Air," was read before the Royal Society, -and in this paper the composition of water by the union of oxygen -and hydrogen is explained. - -In a controversy which afterwards ensued on the respective -[Pg308] claims of Cavendish and Lavoisier to credit for the -discovery of the composition of water, Sir Charles Blagden stated -that he had told Lavoisier, in June, 1783, more than Lavoisier -acknowledged, that he had not only told him that water was -produced by the combustion of the gases, but that his information -embraced the whole theory of the composition of water. This -declaration of Blagden was subsequent in date to January, 1784, -and there is no evidence of any explanation of this theory, verbal -or otherwise, having been given by Cavendish, or any other person, -antecedent to April, 1783. - -From this brief statement of the facts and dates it will appear -that the merit of the discovery of the FACT, that the weight of -water resulting from the combustion of oxygen and hydrogen, is -equal to the sum of the weights of the oxygen and hydrogen which -disappear in the combustion, is due to Priestley; and that the -merit of the INFERENCE from that fact, that water is a compound -body, whose constituents are oxygen and hydrogen, is due to -Watt.[28] Whether those who subsequently deduced the same -inference, and promulgated the same theory, were or were not -informed of Mr. Watt's solution of the phenomenon, or what credit -may be due to any person, however eminent, who at any time -posterior to Mr. Watt's letter to Priestley, asserted that they -had, at a time antecedently to that, made the same inference -without having published it, or communicated it in such a manner -as to establish their claim upon rational and credible evidence, -are questions which we shall not here discuss, being contented -with establishing the right of Mr. Watt to the merit of the -discovery of the THEORY which explained the FACT discovered by -Priestley. - -Even in his declining years, after he had withdrawn from the -active pursuits of his business, the least excitement was -sufficient to call into play the slumbering powers of his -inventive genius. No object could present itself to his notice -[Pg309] without receiving from that genius adaptation in form and -construction to useful purposes. As an example of this restless -activity of mind the following anecdote may be mentioned:-- - -A company at Glasgow had erected on the right bank of the Clyde -extensive buildings and powerful engines for supplying water to -the town. After this expense it was found that a source of water, -of very superior quality, existed on the left bank of the river. -To change the site of the establishment, after the expense which -had been incurred in its erection could not be contemplated, and -they therefore proposed to carry across the bottom of the river a -flexible suction pipe, the mouth of which should terminate in the -source from which the pure water was to be derived. This pipe was -to be supported by a flooring constructed upon the bed of the -river; but it was soon apparent that the construction of such a -flooring on a shifting and muddy bottom, full of inequalities, and -under several feet depth of water would require a greater -expenditure of capital than could with propriety be afforded. In -this difficulty the aged mechanician, for whom Glasgow itself had -been the earliest stage of professional labour, was applied to, -and instantly solved the problem. His attention is said to have -been attracted by a lobster which had been served at table: he set -himself about to contrive how, by mechanism, he could make an -apparatus of iron with joints which should have all the -flexibility of the tail of the lobster. He therefore proposed that -an articulated suction-pipe, capable of accommodating itself to -all the inequalities and to the possible changes of the bed of the -river, should be carried across it; that this flexible pipe should -be two feet in diameter, and one thousand feet in length. This -project the company accordingly caused to be executed after the -plans and drawings of Watt with the most complete success.[29] - -[Pg310] Among the less prominent, though not less useful services -rendered by Watt to his country, may be mentioned the introduction -of the use of chlorine in bleaching. That invention of Berthollet -was introduced into England by Watt after his visit to Paris at -the close of the year 1786. He constructed all the necessary -apparatus for it, directed its erection, and superintended its -first performances. He then left it to his wife's father, Mr. -Macgregor, to carry on the processes. - -When the properties of the gases began to occupy the attention of -chemists, attempts were made to apply them as a means of curing -diseases of the lungs. Dr. Beddoes pursued this inquiry with great -activity, and established, through the means of private -subscription, at Clifton, an institution in which this method of -cure was carefully investigated. The Pneumatic Institution (for so -it was called) has been rendered celebrated for having at its head -Humphry Davy, just then commencing his scientific career. Among -its founders was also numbered James Watt. Not content, however, -with affording the institution the sanction of his name, he -designed and caused to be constructed, at Soho, the apparatus used -for making the gases and administering them to the patients. - -As the exalted powers of the mind of Watt, unfolded in his -numerous mechanical and philosophical inventions and discoveries, -have commanded the admiration and respect of his species, the -affection and love of his fellow men would not have been less -conciliated, had the qualities of his heart, as developed in his -private and personal relations, been as well known as the products -of his genius. - -In the year 1764, Watt being then in the twenty-ninth year of his -age, married his cousin, Miss Miller. At this time he had fallen -into a state of despondency from his disappointments, which -produced a serious attack of nervous illness. The accomplishments -and superior understanding, the mildness of temper and goodness of -disposition of his wife, soon restored him to health. Of this -marriage four children, two sons and two daughters, were the -issue. Two of these children died in infancy; another, a daughter, -was married to Mr. Miller of Glasgow; and the fourth is the -[Pg311] present Mr. James Watt. In September, 1773, while her -husband was engaged in the design of the Caledonian canal in the -North of Scotland, Mrs. Watt died in child-bed of a fifth child, -who was still-born: "Would that I might here transcribe," says M. -Arago, "in all their simple beauty, some lines of the journal in -which he daily recorded his inmost thoughts, his fears, his hopes! -Would that you could see him, after this heavy affliction, pausing -on the threshold of that home, where 'HIS KIND WELCOMER' awaited -him no more; unable to summon courage to enter those rooms where -he was never more to meet 'THE COMFORT OF HIS LIFE!' Possibly, so -faithful a picture of a very deep sorrow might at last put to -silence those obstinate theorists, who, without being struck by -the thousands of instances to the contrary, do yet refuse -qualities of the heart to every man whose intellect has been -fostered by the fertile, sublime, and imperishable truths of the -exact sciences!" - -After the lapse of some years Watt married Miss Macgregor, a -person who is represented to have possessed qualities of mind -which rendered her a companion every way suitable to her husband. -This lady survived Watt, and died in 1832 at an advanced age. Two -children were the issue of this second marriage. - -In the year 1800 the extended patent right, which had been granted -to Boulton and Watt for their improved engine, expired, and at -this time Mr. Watt retired altogether from business. He was -succeeded by his two sons, the present Mr. James Watt, and -Gregory, one of the children of his second marriage. The works at -Soho continued to be conducted by the present Mr. Boulton, the son -of the partner of Mr. Watt, and the two Messrs. Watt. In 1804 -Gregory Watt died at the age of twenty-seven, of a disease of the -chest. This afflicting event was deeply felt by Mr. Watt; but he -did not sink under it into that state of despondency in which he -has been represented to have fallen by M. Arago. On the contrary, -he continued to show the same activity of mind which had -characterised his whole [Pg312] life; nor did he lose that -interest which he always took in the pursuit of literature and in -society. The state of his feelings under this affliction is shown -by the following extracts from letters written by him at that -time, which have been published by Mr. Muirhead. - - - "Heathfield, January 26th, 1805. - - * * "I, perhaps, have said too much to you and Mrs. Campbell - on the state of my mind. I, therefore, think it necessary to - say that _I am not low spirited_; and were you here, you would - find me as cheerful in the company of my friends as usual; my - feelings for the loss of poor Gregory are not passion, but a - deep regret that such was his and my lot. - - "I know that all men must die, and I submit to the decrees - of nature, I hope with due reverence to the Disposer of - Events. Yet one stimulus to exertion is taken away, and, - somehow or other, I have lost my relish for my usual avocations. - Perhaps time may remedy that in some measure; meanwhile, - I do not neglect the means of amusement which are in my power." - - "Heathfield, April 8th, 1805. - - * * "It is rather mortifying to see how easily the want of - even the best of us is dispensed with in the world; but it is - very well it should be so. We here, however, cannot help - feeling a terrible blank in our family. When I look at my - son's books, his writings and drawings, I always say to - myself, where are the mind that conceived these things, and - the hands that executed them? In the course of nature, he - should have said so of mine; but it was otherwise ordered, and - our sorrow is unavailing. As Catullus says:-- - - -- 'Nunc it, per iter tenebricosum, - Illuc, unde negant redire quemquam. - At vobis male sit, malae tenebrae - Orci, quae omnia bella devoratis!' - - "But Catullus was a heathen; let us hope that he (G.) is now - rejoicing in another and a better world, free from our cares, - griefs, and infirmities. Some one has said, I shall not wholly - die; and Gregory's name, his merits and virtues, will live at - least as long as those do who knew him. You are not, from - this, to conceive that we give way to grief; on the contrary, - you will find us as cheerful as we ought to be, and as much - disposed to enjoy the friends we have left as ever; but we - should approach to brutes if we had no regrets." - -Mr. Watt, at the date of these letters, had entered on his -seventieth year, a period after which great mental exertions are -rarely made. [Pg313] - -In the summer of 1819, symptoms of indisposition manifested -themselves which soon rendered Watt aware of his approaching -dissolution. "I am very sensible," said he to his afflicted -friends, "of the attachment you show me, and I hasten to thank you -for it, as I am now come to my last illness." He died on the 25th -of August, 1819. His remains were deposited in the church of -Handsworth, near his estate of Heathfield. His son has raised over -his grave a Gothic chapel, in the centre of which is placed a -statue by Chantrey. - -The personal character of Watt could not fail to excite the -admiration and the love of those distinguished persons, whose -pride and happiness it was to be admitted to a share in the -friendship of the great engineer. Among these were reckoned some -of the men who will leave upon the present age the deepest and -most lasting impressions of their genius, and such persons have -bequeathed to posterity the sentiments with which he inspired -them. We cannot here do more justice to the personal character of -the subject of this notice than by repeating the portraiture of it -which has been given by three of the most distinguished of his -friends, and of the most illustrious men of the present age. - -At a meeting convened in 1824, for erecting a monument to Watt, -Lord Brougham pronounced a speech, from which we extract the -following observations:-- - - "I had the happiness of knowing Mr. Watt, for many years, in the - intercourse of private life; and I will take upon me to bear a - testimony in which all who had that gratification I am sure will - join, that they who only knew his public merit, prodigious as - that was, knew but half his worth. Those who were admitted to - his society will readily allow that anything more pure, more - candid, more simple, more scrupulously loving of justice, than - the whole habits of his life and conversation, proved him to be, - was never known in society. One of the most astonishing - circumstances in this truly great man, was the versatility of - his talents. His accomplishments were so various, the powers of - his mind were so vast, and yet of such universal application, - that it was hard to say whether we should most admire the - extraordinary grasp of his understanding, or the accuracy of - nice research with which he could bring it to bear upon the most - minute objects of investigation. I forget of whom it was said, - that his mind resembled the trunk of an elephant, which can pick - up [Pg314] straws, and tear up trees by the roots. Mr. Watt, in - some sort, resembled the greatest and most celebrated of his own - inventions, of which we are at a loss whether most to wonder at - the power of grappling with the mightiest objects, or of - handling the most minute; so that, while nothing seems too large - for its grasp, nothing seems too small for the delicacy of its - touch, which can cleave rocks, and pour forth rivers from the - bowels of the earth, and, with perfect exactness, though not - with greater ease, fashion the head of a pin, or strike the - impress of some curious die. Now, those who knew Mr. Watt, had - to contemplate a man whose genius could create such an engine, - and indulge in the most abstruse speculations of philosophy, and - could at once pass from the most sublime researches of geology - and physical astronomy, the formation of our globe, and the - structure of the universe, to the manufacture of a needle or a - nail; who could discuss, in the same conversation, and with - equal accuracy, if not with the same consummate skill, the most - forbidding details of art and the elegances of classical - literature, the most abstruse branches of science and the - niceties of verbal criticism. - - "There was one quality in Mr. Watt which most honourably - distinguished him from too many inventors, and was worthy of - all imitation--he was not only entirely free from jealousy, - but he exercised a careful and scrupulous self-denial, and was - anxious not to appear, even by accident, as appropriating to - himself that which he thought belonged to others. I have heard - him refuse the honour universally ascribed to him, of being - the inventor of the steam engine, and call himself simply its - improver; though, in my mind, to doubt his right to that - honour, would be as inaccurate as to question Sir Isaac - Newton's claim to his greatest discoveries, because Descartes - in mathematics, and Galileo in astronomy and mechanics, had - preceded him; or to deny the merits of his illustrious - successor, because galvanism was not his discovery, though, - before his time, it had remained as useless to science as the - instrument called a steam engine was to the arts before Mr. - Watt. The only jealousy I have known him to betray, was with - respect to others, in the nice adjustment he was fond of - giving to the claims of inventors. Justly prizing scientific - discovery above all other possessions, he deemed the title to - it so sacred, that you might hear him arguing by the hour to - settle disputed rights; and if you ever perceived his temper - ruffled, it was when one man's invention was claimed by, or - given to another; or when a clumsy adulation pressed upon - himself that which he knew to be not his own." - -In the preface to the _Monastery_ Sir Walter Scott speaks of Watt -in the following terms:-- - - "There were assembled about half a score of our northern - lights. * * Amidst this company stood Mr. Watt, the man whose - genius discovered the means of multiplying our national - resources to a degree, perhaps, even beyond his own stupendous - powers of calculation and combination; bringing the treasures - of the abyss to the summit of the earth--giving the feeble arm - of man the momentum of an Afrite--commanding manufactures to - arise as the rod of the prophet produced water in the - desert--affording the means of dispensing with that time and - tide which wait for no man--and of sailing without that wind - which defied the command and threats of Xerxes himself. This - potent commander of the elements--this abridger of time and - space--this magician, whose cloudy [Pg315] machinery has - produced a change on the world, the effects of which, - extraordinary as they are, are, perhaps, only now beginning to - be felt--was not only the most profound man of science--the - most successful combiner of powers, and calculator of numbers, - as adapted to practical purposes--was not only one of the most - generally well informed, but one of the best and kindest of - human beings. - - "There he stood, surrounded by the little band I have - mentioned of northern literati, men not less tenacious, - generally speaking, of their own fame and their own opinions, - than the national regiments are supposed to be jealous of the - high character which they have won upon service. Methinks I - yet see and hear what I shall never see or hear again. In his - eighty-second year, the alert, kind, benevolent old man, had - his attention alive to every one's question, his information - at every one's command. - - "His talents and fancy overflowed on every subject. One gentleman - was a deep philologist--he talked with him on the origin of the - alphabet, as if he had been coeval with Cadmus; another a - celebrated critic--you would have said the old man had studied - political economy and belles lettres all his life. Of science it is - unnecessary to speak--it was his own distinguished walk. And yet, - Captain Clutterbuck, when he spoke with your countryman, Jedediah - Cleishbotham, you would have sworn he had been coeval with Claverse - and Burley, with the persecutors and persecuted, and could number - every shot the dragoons had fired at the fugitive Covenanters. In - fact, we discovered that no novel of the least celebrity escaped - his perusal, and that the gifted man of science was as much - addicted to the productions of your native country, in other words, - as shameless and obstinate a peruser of novels, as if he had been a - very milliner's apprentice of eighteen." - -In the Edinburgh newspaper, called the _Scotsman_, of the 4th -September, 1819, immediately after the decease of Watt, the -following sketch was published from the pen of Lord Jeffrey:-- - - "This name fortunately needs no commemoration of ours; for he - that bore it survived to see it crowned with undisputed and - unenvied honours; and many generations will probably pass away - before it shall have gathered 'all its fame.' We have said - that Mr. Watt was the great _improver_ of the steam engine; - but, in truth, as to all that is admirable in its structure, - or vast in its utility, he should rather be described as its - _inventor_. It was by his inventions, that its action was so - regulated as to make it capable of being applied to the finest - and most delicate manufactures, and its power so increased, as - to set weight and solidity at defiance. By his admirable - contrivance, it has become a thing stupendous alike for its - force and its flexibility--for the prodigious power which it - can exert, and the ease, and precision, and ductility with - which it can be varied, distributed, and applied. The trunk of - an elephant, that can pick up a pin or rend an oak, is as - nothing to it. It can engrave a seal, and crush masses of - obdurate metal before it--draw out, without breaking, a thread - as fine as gossamer, and lift a ship of war like a bauble in - the air. It can embroider muslin, and forge anchors--cut steel - into ribands, and impel loaded vessels against the fury of the - winds and waves. [Pg316] - - "It would be difficult to estimate the value of the benefits - which these inventions have conferred upon this country. There - is no branch of industry that has not been indebted to them; - and, in all the most material, they have not only widened most - magnificently the field of its exertions, but multiplied a - thousand fold the amount of its productions. It is our - improved steam engine that has fought the battles of Europe, - and exalted and sustained, through the late tremendous - contest, the political greatness of our land. It is the same - great power which now enables us to pay the interest of our - debt, and to maintain the arduous struggle in which we are - still engaged (1819), with the skill and capital of countries - less oppressed with taxation. But these are poor and narrow - views of its importance. It has increased indefinitely the - mass of human comforts and enjoyments, and rendered cheap and - accessible all over the world the materials of wealth and - prosperity. It has armed the feeble hand of man, in short, - with a power to which no limits can be assigned; completed the - dominion of mind over the most refractory qualities of matter; - and laid a sure foundation for all those future miracles of - mechanic power which are to aid and reward the labours of - after generations. It is to the genius of one man, too, that - all this is mainly owing; and certainly no man ever bestowed - such a gift on his kind. The blessing is not only universal, - but unbounded; and the fabled inventors of the plough and the - loom, who were deified by the erring gratitude of their rude - contemporaries, conferred less important benefits on mankind - than the inventor of our present steam engine. - - "This will be the fame of Watt with future generations; and it is - sufficient for his race and his country. But to those to whom he - more immediately belonged, who lived in his society and enjoyed his - conversation, it is not, perhaps, the character in which he will be - most frequently recalled--most deeply lamented--or even most highly - admired. Independently of his great attainments in mechanics, Mr. - Watt was an extraordinary, and in many respects a wonderful man. - Perhaps no individual in his age possessed so much and such varied - and exact information--had read so much, or remembered what he had - read so accurately and well. He had infinite quickness of - apprehension, a prodigious memory, and a certain rectifying and - methodising power of understanding, which extracted something - precious out of all that was presented to it. His stores of - miscellaneous knowledge were immense; and yet less astonishing than - the command he had at all times over them. It seemed as if every - subject that was casually started in conversation with him, had - been that which he had been last occupied in studying and - exhausting;--such was the copiousness, the precision, and the - admirable clearness of the information which he poured out upon it - without effort or hesitation. Nor was this promptitude and compass - of knowledge confined in any degree to the studies connected with - his ordinary pursuits. That he should have been minutely and - extensively skilled in chemistry and the arts, and in most of the - branches of physical science, might perhaps have been conjectured; - but it could not have been inferred from his usual occupations, and - probably is not generally known, that he was curiously learned in - many branches of antiquity, metaphysics, medicine, and etymology; - and perfectly at home in all the details of architecture, music, - and law. He was well acquainted, too, with most of the modern - languages, and familiar with their most recent literature. Nor was - it at all extraordinary to hear the great mechanician and engineer - detailing and expounding, for hours together, [Pg317] the - metaphysical theories of the German logicians, or criticising the - measures or the matter of the German poetry. - - "His astonishing memory was aided, no doubt, in a great measure, - by a still higher and rarer faculty--by his power of digesting, - and arranging in its proper place, all the information he - received; and of casting aside and rejecting, as it were - instinctively, whatever was worthless or immaterial. Every - conception that was suggested to his mind seemed instantly to - take its place among its other rich furniture, and to be - condensed into the smallest and most convenient form. He never - appeared, therefore, to be at all incumbered or perplexed with - the _verbiage_ of the dull books he perused, or to the idle talk - to which he listened; but to have at once extracted, by a kind - of intellectual alchemy, all that was worthy of attention, and - to have reduced it, for his own use, to its true value and to - its simplest form. And thus it often happened, that a great deal - more was learned from his brief and vigorous account of the - theories and arguments of tedious writers, than an ordinary - student could ever have derived from the most painful study of - the originals; and that errors and absurdities became manifest - from the mere clearness and plainness of his statement of them, - which might have deluded and perplexed most of his hearers - without that invaluable assistance. - - "It is needless to say that, with those vast resources, his - conversation was at all times rich and instructive in no ordinary - degree: but it was, if possible, still more pleasing than wise; and - had all the charms of familiarity with all the substantial - treasures of knowledge. No man could be more social in his spirit, - less assuming or fastidious in his manners, or more kind and - indulgent toward all who approached him. He rather liked to - talk,--at least in his latter years; but though he took a - considerable share of the conversation, he rarely suggested the - topics on which it was to turn, but readily and quietly took up - whatever was presented by those around him, and astonished the idle - and barren propounders of an ordinary theme by the treasures which - he drew from the mine they had unconsciously opened. He generally - seemed, indeed, to have no choice or predilection for one subject - of discourse rather than another; but allowed his mind, like a - great cyclopaedia, to be opened at any letter his associates might - choose to turn up, and only endeavoured to select from his - inexhaustible stores, what might be best adapted to the taste of - his present hearers. As to their capacity he gave himself no - trouble; and indeed such was his singular talent for making all - things plain, clear, and intelligible, that scarcely any one could - be aware of such a deficiency in his presence. His talk, too, - though overflowing with information, had no resemblance to - lecturing or solemn discoursing, but, on the contrary, was full of - colloquial spirit and pleasantry. He had a certain quiet and grave - humour which ran through most of his conversation; and a vein of - temperate jocularity, which gave infinite zest and effect to the - condensed and inexhaustible information which formed its main - staple and characteristic. There was a little air of affected - testiness, and a tone of pretended rebuke and contradiction, with - which he used to address his younger friends, that was always felt - by them as an endearing mark of his kindness and familiarity; and - prized, accordingly, far beyond all the solemn compliments that - ever proceeded from the lips of authority. His voice was deep and - powerful, though he commonly spoke in a low and somewhat monotonous - tone, which harmonised admirably with the weight and brevity of his - observations, and set off to the greatest advantage the pleasant - [Pg318] anecdotes, which he delivered with the same grave brow, and - the same calm smile playing soberly on his lips. There was nothing - of effort, indeed, or impatience, any more than of pride or levity, - in his demeanour; and there was a finer expression of reposing - strength, and mild self-possession in his manner, than we ever - recollect to have met with in any other person. He had in his - character the utmost abhorrence for all sorts of forwardness, - parade, and pretensions; and, indeed, never failed to put all such - impostures out of countenance, by the manly plainness and honest - intrepidity of his language and deportment. - - "In his temper and dispositions, he was not only kind and - affectionate, but generous, and considerate of the feelings of - all around him; and gave the most liberal assistance and - encouragement to all young persons who showed any indications of - talent, or applied to him for patronage or advice. His health, - which was delicate from his youth upwards, seemed to become - firmer as he advanced in years; and he preserved, up almost to - the last moment of his existence, not only the full command of - his extraordinary intellect, but all the alacrity of spirit and - the social gaiety which had illumined his happiest days. His - friends in this part of the country never saw him more full of - intellectual vigour and colloquial animation--never more - delightful or more instructive--than in his last visit to - Scotland in autumn 1817. Indeed, it was after that time that he - applied himself, with all the ardour of early life, to the - invention of a machine for mechanically copying all sorts of - sculpture and statuary; and distributed among his friends some - of its earliest performances, as the productions of a young - artist just entering on his eighty-third year. - - "This happy and useful life came, at last, to a gentle close. - He had suffered some inconvenience through the summer; but was - not seriously indisposed till within a few weeks of his death. - He then became perfectly aware of the event which was - approaching; and with his usual tranquillity and benevolence - of nature, seemed only anxious to point out to the friends - around him, the many sources of consolation which were - afforded by the circumstances under which it was about to take - place. He expressed his sincere gratitude to Providence for - the length of days with which he had been blessed, and his - exemption from most of the infirmities of age; as well as for - the calm and cheerful evening of life that he had been - permitted to enjoy, after the honourable labours of the day - had been concluded. And thus, full of years and honours, in - all calmness and tranquillity, he yielded up his soul without - pang or struggle; and passed from the bosom of his family to - that of his God." - -The English nation has ever shown itself insensible to the claims -of genius and high intellectual endowments, except where the -results have been brought directly to bear in statesmanship or -war. Of this inability to appreciate the highest order of -intellectual excellence Watt affords a striking example. When it -was suggested to the British government by those better capable -than that government was of appreciating the genius of this great -man, that the nation would do itself honour by erecting a splendid -monument at his own [Pg319] cost to him to whom it was so deeply -indebted for the extension of its resources and the augmentation -of its power, the reply was that such a measure could not be -adopted as it might be drawn into a precedent in like cases -thereafter! A precedent in like cases!! When will the time arrive -when the world will produce a like case? The monument which has -been erected in Westminster Abbey was in fact raised by private -subscription, the nation having thus stigmatised itself through -the act of its government with the everlasting disgrace of -refusing the honour proposed to it. The other statues and -monuments which have been erected to this great man, have been for -the most part raised by the filial piety and the never-dying -affection and veneration of the present Mr. James Watt. A statue -has been presented by him to the University of Glasgow, and placed -in one of the halls of that college. The inhabitants of Greenock -have also erected a marble statue of Watt, for which, and for a -library, a building has been erected at the expense of about -3,500_l._ which has been defrayed by Mr. James Watt. A colossal -bronze statue has been erected on a handsome granite pedestal, -standing at one of the corners of George Square, Glasgow. The -monument in Westminster Abbey, erected by the subscription raised -at the public meeting already alluded to, is a colossal statue of -Carrara marble, by Chantrey. - -Watt was elected a fellow of the Royal Society of Edinburgh in -1784; of the Royal Society of London in 1785; a member of the -Batavian Society in 1787; and a corresponding member of the -Institut of France in 1808. The degree of Doctor of Laws was -conferred upon him by the University of Glasgow, in 1806; and in -1814, the highest scientific honour which can be attained by a -philosopher, was conferred on him by the Academy of Sciences of -the Institut of France, who nominated him one of its eight foreign -associates. - -On the pedestal of the monument in Westminster Abbey is engraved -the following inscription from the pen of Lord Brougham:-- -[Pg320] - - NOT TO PERPETUATE A NAME WHICH MUST ENDURE WHILE THE PEACEFUL - ARTS FLOURISH, BUT TO SHOW THAT MANKIND HAVE LEARNED TO HONOUR - THOSE WHO BEST DESERVE THEIR GRATITUDE, THE KING HIS MINISTERS, - AND MANY OF THE NOBLES AND COMMONERS OF THE REALM RAISED THIS - MONUMENT TO - - JAMES WATT, - - WHO DIRECTING THE FORCE OF AN ORIGINAL GENIUS, EARLY EXERCISED - IN PHILOSOPHIC RESEARCH TO THE IMPROVEMENT OF THE STEAM - ENGINE, ENLARGED THE RESOURCES OF HIS COUNTRY, INCREASED THE - POWER OF MAN, AND ROSE TO AN EMINENT PLACE AMONG THE MOST - ILLUSTRIOUS FOLLOWERS OF SCIENCE AND THE REAL BENEFACTORS OF - THE WORLD. BORN AT GREENOCK MDCCXXXVI. DIED AT HEATHFIELD IN - STAFFORDSHIRE MDCCCXIX. - -[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.] - - FOOTNOTES: - - [25] See Buchanan on the Economy of Fuel and Management of - Heat, especially as it relates to heating and drying by means - of Steam. - - [26] See Brewster's Edinburgh Encyclopaedia, article - STEAM-DRYING MACHINE. - - [27] The following are the words in which Watt makes this - remarkable announcement to Priestley:-- - - "Let us now consider what obviously happens in the deflagration - of the inflammable (hydrogen) and dephlogisticated air (oxygen). - These two kinds of air unite with violence; they become red hot, - and upon cooling, totally disappear. When the vessel is cooled, - a quantity of water is found in it equal to the weight of the - air employed. This water is then the only remaining product of - the process; and water, light, and heat are all the products. - - "Are we not then authorised to conclude, that water is composed - of dephlogisticated air (oxygen) and phlogiston (hydrogen), - deprived of part of their latent or elementary heat; that - dephlogisticated or pure air (oxygen) is composed of water - deprived of its phlogiston (hydrogen), and united to elementary - heat and light; and that the latter are contained in it in a - latent state, so as not to be sensible to the thermometer or to - the eye; and if light be only a modification of heat, or a - circumstance attending it, or a component part of the - inflammable air (hydrogen), then pure or dephlogisticated air - (oxygen) is composed of water deprived of its phlogiston - (hydrogen), and united to elementary heat." - - [28] Those who desire to investigate this controversy more in - detail will find very full information on the subject in the - Translation of Arago's Eloge, with notes and appendix by J. P. - Muirhead, Esq. Murray, London, 1839. - - [29] An account of this remarkable apparatus, accompanied by an - engraving made from a drawing supplied by Watt, was communicated - by Sir John Robison to the _Edinburgh Philosophical Journal_ in - 1820. _See_ vol. iii, p. 60. - -[Pg321] - - - - -[Illustration] - -CHAP. XI. - -LOCOMOTIVE ENGINES ON RAILWAYS. - - NON-CONDENSING ENGINES. -- LEUPOLD'S ENGINE.-TREVETHICK AND - VIVIAN. -- EFFECTS OF RAILWAY TRANSPORT. -- HISTORY OF THE - LOCOMOTIVE ENGINE. -- BLENKINSOP. -- MESSRS. CHAPMAN. -- WALKING - ENGINE. -- MR. STEPHENSON'S ENGINES AT KILLINGWORTH. -- - LIVERPOOL AND MANCHESTER RAILWAY. -- EXPERIMENTAL TRIAL. -- THE - ROCKET. -- THE SANSPAREIL. -- THE NOVELTY. -- SUBSEQUENT - IMPROVEMENTS IN THE LOCOMOTIVE ENGINE. -- LARDNER'S EXPERIMENTS - IN 1832. -- ADOPTION OF BRASS TUBES. -- MR. BOOTH'S REPORT. -- - DETAILED DESCRIPTION OF THE MOST IMPROVED LOCOMOTIVE ENGINES. - -- POWER OF LOCOMOTIVE ENGINES. -- EVAPORATION OF BOILERS. -- - LARDNER'S EXPERIMENTS IN 1838. -- RESISTANCE TO RAILWAY TRAINS. - -- RESTRICTIONS ON GRADIENTS. -- COMPENSATING EFFECT OF - GRADIENTS. -- EXPERIMENT WITH THE HECLA. -- METHODS OF - SURMOUNTING STEEP INCLINATIONS. - - -(180.) In the various modifications of the steam engine which we -have hitherto considered, the pressure introduced on one side of the -piston derives its efficacy either wholly or partially from the -vacuum produced by condensation on the other side. This always -requires a condensing apparatus, and a constant and abundant supply -of cold water. An engine of this kind must therefore necessarily -have considerable dimensions and weight, and is inapplicable to uses -in which a small and light machine only is admissible. If the -condensing apparatus be dispensed with, the piston will always be -resisted by a force equal to the atmospheric [Pg322] pressure, and -the only part of the steam pressure which will be available as a -moving power, is that part by which it exceeds the pressure of the -atmosphere. Hence, in engines which do not work by condensation, -steam of a much higher pressure than that of the atmosphere is -indispensably necessary, and such engines are therefore called -_high-pressure engines_. - -We are not, however, to understand that every engine, in which -steam is used of a pressure exceeding that of the atmosphere, is -what is meant by an _high-pressure engine_; for in the ordinary -engines in common use, constructed on Watt's principle, the -safety-valve is loaded with from 3 to 5 lbs. on the square inch; -and in Woolf's engines, the steam is produced under a pressure of -40 lbs. on the square inch. These would therefore be more properly -called _condensing engines_ than _low-pressure engines_; a term -quite inapplicable to those of Woolf. In fact, by _high-pressure -engines_ is meant engines in which no vacuum is produced, and, -therefore, in which the piston works against a pressure equal to -that of the atmosphere. - -In these engines the whole of the condensing apparatus, viz. the -cold-water cistern, condenser, air-pump, cold-water pump, &c., are -dispensed with, and nothing is retained except the boiler, -cylinder, piston, and valves. Consequently, such an engine is -small, light, and cheap. It is portable also, and may be moved, if -necessary, along with its load, and is therefore well adapted to -locomotive purposes. - - -(181.) High-pressure engines were one of the earliest forms of the -steam engine. The contrivance, which is obscurely described in the -article already quoted (7.), from the Century of Inventions, is a -high-pressure engine; for the power there alluded to is the -elastic force of steam working against the atmospheric pressure. -Newcomen, in 1705, applied the working-beam, cylinder, and piston -to the atmospheric engine; and Leupold, about 1720, combined the -working-beam and cylinder with the high-pressure principle, and -produced the earliest high-pressure engine worked by a cylinder -and piston. The following is a description of Leupold's engine:-- -[Pg323] - -[Illustration: _Fig._ 82.] - -A (_fig._ 82.) is the boiler, with the furnace beneath it; C C are -two cylinders with solid pistons P P', connected with the -working-beams B B', to which are attached the pump-rods R R', of -two forcing pumps F F', which communicate with a great force-pipe -S; G is a _four-way cock_ (66.) already described. In the position -in which it stands in the figure, the steam issues from below the -piston P into the atmosphere, and the piston is descending by its -own weight; steam from the boiler is at the same time pressing up -the piston P', with a force equal to the difference between the -pressure of the steam and that of the atmosphere. Thus the piston -R of the forcing-pump is being drawn up, and the piston P' is -forcing the piston R' down, and thereby driving water into the -force-pipe [Pg324] S. On the arrival of the piston P at the -bottom of the cylinder C, and P' at the top of the cylinder C', -the position of the cock is changed as represented in _fig._ 83. -The steam, which has just pressed up the piston P', is allowed to -escape into the atmosphere, while the steam, passing from the -boiler below the piston P, presses it up, and thus P ascends by -the steam pressure, and P' descends by its own weight. By these -means the piston R is forced down, driving before it the water in -the pump-cylinder into the force-pipe S, and the piston R' is -drawn up to allow the other pump-cylinder to be re-filled; and so -the process is continued. - -[Illustration: _Fig._ 83.] - -A valve is placed in the bottom of the force-pipes, to prevent the -water which has been driven into it from returning. This valve -opens upwards; and, consequently, the weight of the water pressing -upon it only keeps it more effectually closed. On each descent of -the piston, the pressure transmitted to the valve acting upwards -being greater than the weight of the water resting upon it, forces -it open, and an increased quantity of water is introduced. - - -(182.) From the date of the improvement of Watt until the -commencement of the present century, non-condensing engines were -altogether neglected in these countries. In the year 1802, Messrs. -Trevethick and Vivian constructed the first non-condensing engine -of this kind which was ever brought into extensive practical use -in this kingdom. A section of this machine, made by a vertical -plane, is represented in _fig._ 84. - -The boiler A B is a cylinder with flat circular ends. The -fire-place is constructed in the following manner:--A tube enters -the cylindrical boiler at one end; and, proceeding onwards near -the other extremity, is turned and recurved, so as to be carried -back parallel to the direction in which it entered. It is thus -conducted out of the boiler, at another part of the same end at -which it entered. One of the ends of this tube communicates with -the chimney E, which is carried upwards as represented in the -figure. The other mouth is furnished [Pg325] with a door; and in -it is placed the grate, which is formed of horizontal bars, -dividing the tube into two parts; the upper part forming the -fire-place, and the lower the ash-pit. The fuel is maintained in a -state of combustion, on the bars, in that part of the tube -represented at C D; and the flame is carried by the draught of the -chimney round the curved flue, and issues at E into the chimney. -The flame is thus conducted through the water, so as to expose the -latter to as much heat as possible. - -[Illustration: _Fig._ 84.] - -A section of the cylinder is represented at F, immersed in the -boiler, except a few inches of the upper end, where the four-way -cock G is placed for regulating the admission of the steam. A tube -is represented at H, which leads from this four-way cock into the -chimney; so that the waste steam, after working the piston, is -carried off through this tube, and passes into the chimney. The -upper end of the piston-rod is furnished with a cross-bar, which -is placed in a direction at right angles to the length of the -boiler, and also to the [Pg326] piston-rod. This bar is guided in -its motion by sliding on two iron perpendicular rods fixed to the -sides of the boiler, and parallel to each other. To the ends of -this cross-bar are joined two connecting rods, the lower ends of -which work two cranks fixed on an axis extending across and -beneath the boiler, and immediately under the centre of the -cylinder. This axis is sustained in bearings formed in the legs -which support the boiler, and upon its extremity is fixed the -fly-wheel as represented at B. A large-toothed wheel is placed on -this axis; which, being turned with the cranked axle, communicates -motion to other wheels; and through them, to any machinery which -the engine may be applied to move. - -[Illustration: _Fig._ 85.] - -As the four-way cock is represented in the figure, the steam -passes from the boiler through the curved passage G above the -piston, while the steam below the piston is carried off through a -tube which does not appear in the figure, by which it is conducted -to the tube H, and thence to the chimney. The steam, therefore, -which passes above the piston presses it downwards; while the -pressure upwards does not exceed that of the atmosphere. The -piston will therefore descend with a force depending on the excess -of the pressure of the steam produced in the boiler above the -atmospheric pressure. When the piston has arrived at the bottom of -the cylinder, the cock is made to assume the position represented -in _fig._ 85. This effect is produced by the motion of the -piston-rod. The steam now passes from above the piston, through -the tube H, into the chimney, while the steam from the boiler is -conducted through another tube below the piston. The pressure -above the piston, in this case, does not exceed that of the -atmosphere; while the pressure below it will be that of the steam -in the boiler. The piston will therefore ascend with the -difference of these pressures. On the arrival of the piston at the -top of the cylinder, the four-way cock is again turned to the -position represented in _fig._ 85., and the piston again descends; -and in the same manner the process is continued. A safety-valve is -placed on the boiler at V, loaded with a weight W, proportionate -[Pg327] to the strength of the steam with which it is proposed to -work. - -In the engines now described, this valve was frequently loaded at -the rate of from 60 to 80 lbs. on the square inch. As the boilers of -high-pressure engines were considered more liable to accidents from -bursting than those in which steam of a lower pressure was used, -greater precautions were taken against such effects. A second -safety-valve was provided, which was not left in the power of the -engine-man. By this means he had a power to diminish the pressure of -the steam, but could not increase it beyond the limit determined by -the valve which was removed from his interference. The greatest -cause of danger, however, arose from the water in the boiler being -consumed by evaporation faster than it was supplied; and therefore -falling below the level of the tube containing the furnace. To guard -against accidents arising from this circumstance, a hole was bored -in the boiler, at a certain depth, below which the water should not -be allowed to fall; and in this hole a plug of metal was soldered -with lead, or with some other metal, which would fuse at that -temperature which would expose the boiler to danger. Thus, in the -event of the water being exhausted, so that its level would fall -below the plug, the heat of the furnace would immediately melt the -solder, and the plug would fall out, affording a vent for the steam, -without allowing the boiler to burst. The mercurial steam-gauge, -already described, was also used as an additional security. When the -force of the steam exceeded the length of the column of mercury -which the tube would contain, the mercury would be blown out, and -the tube would give vent to the steam. The water by which the boiler -was replenished was forced into it by a pump worked by the engine. -In order to economise the heat, this water was contained in a tube -T, which surrounded the pipe H. As the waste steam, after working -the piston, passed off through H, it imparted a portion of its heat -to the water contained in the tube T, which was thus warmed to a -certain temperature before it was forced into the boiler by the -pump. Thus a part of the heat, which was originally [Pg328] carried -from the boiler in the form of steam, was returned again to the -boiler with the water with which it was fed. - -It is evident that engines constructed in this manner may be -applied to all the purposes to which the condensing engines are -applicable. - - -(183.) Two years after the date of the patent of this engine, its -inventor constructed a machine of the same kind for the purpose of -moving carriages on railroads; and applied it successfully, in the -year 1804, on the railroad at Merthyr Tydvil, in South Wales. It -was in principle the same as that already described. The cylinder -however was in a horizontal position, the piston-rod working in -the direction of the line of road: the extremity of the -piston-rod, by means of a connecting rod, worked cranks placed on -the axletree, on which were fixed two cogged wheels: these worked -in others, by which their motion was communicated finally to -cogged wheels fixed on the axle of the hind wheels of the -carriage, by which this axle was kept in a state of revolution. -The hind wheels being fixed on the axletree, and turning with it, -were caused likewise to revolve; and so long as the weight of the -carriage did not exceed that which the friction of the road was -capable of propelling, the carriage would thus be moved forwards. -On this axle was placed a fly-wheel to continue the rotatory -motion at the termination of each stroke. The fore wheels are -described as being capable of turning like the fore wheels of a -carriage, so as to guide the vehicle. The projectors appear to -have contemplated, in the first instance, the use of this carriage -on common roads; but that notion seems to have been abandoned, and -its use was only adopted on the railroad before mentioned. On the -occasion of its first trial, it drew after it as many carriages as -contained ten tons of iron a distance of nine miles; which stage -it performed without any fresh supply of water, and travelled at -the rate of five miles an hour. - - -(184.) Capital and skill have of late years been directed with -extraordinary energy to the improvement of inland transport; and -this important instrument of national wealth and civilisation has -received a proportionate impulse. Effects are now witnessed, -which, had they been narrated a few years [Pg329] since, could -only have been admitted into the pages of fiction or volumes of -romance. Who could have credited the possibility of a ponderous -engine of iron, loaded with some hundred passengers, in a train of -carriages of corresponding magnitude, and a large quantity of -water and coal, taking flight from Manchester and arriving at -Liverpool, a distance of above thirty miles, in little more than -an hour? And yet this is a matter of daily and almost hourly -occurrence. The rapidity of transport thus attained is not less -wonderful than the weights transported. Its capabilities in this -respect far transcend the exigencies even of the two greatest -commercial marts in Great Britain. Loads, varying from fifty to -one hundred and fifty tons, are transported at the average rate of -fifteen miles an hour; and in one instance we have seen a load--we -should rather say a _cargo_--of waggons, conveying merchandise to -the amount of two hundred and thirty tons gross, transported from -Liverpool to Manchester at the average rate of twelve miles an -hour. - -The astonishment with which such performances must be viewed, -might be qualified, if the art of transport by steam on railways -had been matured, and had attained that full state of perfection -which such an art is always capable of receiving from long -experience, aided by great scientific knowledge, and the unbounded -application of capital. But such is not the present case. The art -of constructing locomotive engines, so far from having attained a -state of maturity, has not even emerged from its infancy. So -complete was the ignorance of its powers which prevailed, even -among engineers, previous to the opening of the Liverpool railway, -that the transport of heavy goods was regarded as the chief object -of the undertaking, and its principal source of revenue. The -incredible speed of transport, effected even in the very first -experiments in 1830, burst upon the public, and on the scientific -world, with all the effect of a new and unlooked-for phenomenon. -On the unfortunate occasion which deprived this country of Mr. -Huskisson, the wounded body of that statesman was transported a -distance of about fifteen miles in twenty-five minutes, being at -the rate of thirty-six miles an hour. The revenue of the road -arising from passengers since its opening, [Pg330] has, contrary -to all that was foreseen, been nearly double that which has been -derived from merchandise. So great was the want of experience in -the construction of engines, that the company was at first -ignorant whether they should adopt large steam engines fixed at -different stations on the line, to pull the carriages from station -to station, or travelling engines to drag the loads the entire -distance. Having decided on the latter, they have, even to the -present moment, laboured under the disadvantage of the want of -that knowledge which experience alone can give. The engines have -been constantly varied in their weight and proportions, in their -magnitude and form, as the experience of each successive month has -indicated. As defects became manifest they were remedied; -improvements suggested were adopted; and each year produced -engines of such increased power and efficiency, that their -predecessors were abandoned, not because they were worn out, but -because they had been outstripped in the rapid march of -improvement. Add to this, that only one species of travelling -engine has been effectively tried; the capabilities of others -remain still to be developed; and even that form of engine which -has received the advantage of a course of experiments on so grand -a scale to carry it towards perfection, is far short of this -point, and still has defects, many of which, it is obvious, time -and experience will remove. - -If, then, the locomotive engine, subject thus to all the -imperfections inseparable from a novel contrivance--with the -restrictions on the free application of skill and capital, arising -from the nature of the monopolies granted to railway companies--with -the disadvantage of very limited experience, the great parent of -practical improvement, having been submitted to experiments hitherto -only on a limited scale, and confined almost to one form of -machine;--if, under such disadvantages, such effects have been -produced as are now daily witnessed by the public, what may not be -looked for from this extraordinary power when the enterprise of the -country shall be more unfettered--when greater fields of experience -are opened--when time, ingenuity, and capital have removed or -diminished existing imperfections, and have brought to light new and -more powerful principles? This is not mere speculation [Pg331] on -abstract possibilities, but refers to what is in actual progress. -The points of greatest wealth and population--the centres of largest -capital and most active industry throughout the country--will soon -be connected by lines of railway; and various experiments are -proposed, with more or less prospect of success, for the application -of steam engines on stone roads where the intercourse is not -sufficient to render railways profitable. - -The important commercial and political effects attending such -increased facility and speed in the transport of persons and goods, -are too obvious to require any very extended notice here. A part of -the price (and in many cases a considerable part) of every article -of necessity or luxury, consists of the cost of transporting it from -the producer to the consumer; and consequently every abatement or -saving in this cost must produce a corresponding reduction in the -price of every article transported; that is to say, of every thing -which is necessary for the subsistence of the poor, or for the -enjoyment of the rich--of every comfort, and of every luxury of -life. The benefit of this will extend, not to the consumer only, but -to the producer: by lowering the expense of transport of the -produce, whether of the soil or of the loom, a less quantity of that -produce will be spent in bringing the remainder to market, and -consequently a greater surplus will reward the labour of the -producer. The benefit of this will be felt even more by the -agriculturist than by the manufacturer; because the proportional -cost of transport of the produce of the soil is greater than that of -manufactures. If two hundred quarters of corn be necessary to raise -four hundred, and one hundred more be required to bring the four -hundred to market, then the net surplus will be one hundred. But if -by the use of steam carriages the same quantity can be brought to -market with an expenditure of fifty quarters, then the net surplus -will be increased from one hundred to one hundred and fifty -quarters; and either the profit of the farmer, or the rent of the -landlord, must be increased by the same amount. - -But the agriculturist would not merely be benefited by an -increased return from the soil already under cultivation. Any -[Pg332] reduction in the cost of transporting the produce to -market would call into cultivation tracts of inferior fertility, -the returns from which would not at present repay the cost of -cultivation and transport. Thus land would become productive which -is now waste, and an effect would be produced equivalent to adding -so much fertile soil to the present extent of the country. It is -well known, that land of a given degree of fertility will yield -increased produce by the increased application of capital and -labour. By a reduction in the cost of transport, a saving will be -made which may enable the agriculturist to apply to tracts already -under cultivation the capital thus saved, and thereby increase -their actual production. Not only, therefore, would such an effect -be attended with an increased extent of cultivated land, but also -with an increased degree of cultivation in that which is already -productive. - -It has been said, that in Great Britain there are above a million -of horses engaged in various ways in the transport of passengers -and goods, and that to transport each horse requires as much land -as would, upon an average, support eight men. If this quantity of -animal power were displaced by steam engines, and the means of -transport drawn from the bowels of the earth, instead of being -raised upon its surface, then, supposing the above calculation -correct, as much land would become available for the support of -human beings as would suffice for an additional population of -eight millions; or, what amounts to the same, would increase the -means of support of the present population by about one third of -the present available means. The land which now supports horses -for transport would then support men, or produce corn for food. - -The objection that a quantity of land exists in the country -capable of supporting horses alone, and that such land would be -thrown out of cultivation, scarcely deserves notice here. The -existence of any considerable quantity of such land is extremely -doubtful. What is the soil which will feed a horse and not feed -oxen or sheep, or produce food for man? But even if it be admitted -that there exists in the country a small portion of such land, -that portion cannot exceed, nor indeed equal, what would be -sufficient for the number of horses [Pg333] which must after all -continue to be employed for the purposes of pleasure, and in a -variety of cases where steam must necessarily be inapplicable. It -is to be remembered, also, that the displacing of horses in one -extensive occupation, by diminishing their price must necessarily -increase the demand for them in others. - -The reduction in the cost of transport of manufactured articles, -by lowering their price in the market, will stimulate their -consumption. This observation applies of course not only to home -but to foreign markets. In the latter we already in many branches -of manufactures command a monopoly. The reduced price which we -shall attain by cheapness and facility of transport will still -further extend and increase our advantages. The necessary -consequence will be, an increased demand for manufacturing -population; and this increased population again reacting on the -agricultural interests, will form an increased market for that -species of produce. So interwoven and complicated are the fibres -which form the texture of the highly civilised and artificial -community in which we live, that an effect produced on any one -point is instantly transmitted to the most remote and apparently -unconnected parts of the system. - -The two advantages of increased cheapness and speed, besides -extending the amount of existing traffic, call into existence new -objects of commercial intercourse. For the same reason that the -reduced cost of transport, as we have shown, calls new soils into -cultivation, it also calls into existence new markets for -manufactured and agricultural produce. The great speed of transit -which has been proved to be practicable, must open a commerce -between distant points in various articles, the nature of which -does not permit them to be preserved so as to be fit for use -beyond a certain time. Such are, for example, many species of -vegetable and animal food, which at present are confined to -markets at a very limited distance from the grower or feeder. The -truth of this observation is manifested by the effects which have -followed the intercourse by steam on the Irish Channel. The -western towns of England have become markets for a prodigious -quantity of Irish produce, which it had been previously [Pg334] -impossible to export. If animal food be transported alive from the -grower to the consumer, the distance of the market is limited by -the power of the animal to travel, and the cost of its support on -the road. It is only particular species of cattle which bear to be -carried to market on common roads and by horse carriages. But the -peculiar nature of a railway, the magnitude and weight of the -loads which may be transported on it, and the prodigious speed -which may be attained, render the transport of cattle, of every -species, to almost any distance, both easy and cheap. In process -of time, when the railway system becomes extended, the metropolis -and populous towns will therefore become markets, not as at -present to districts within limited distances of them, but to the -whole country. - -The moral and political consequences of so great a change in the -powers of transition of persons and intelligence from place to place -are not easily calculated. The concentration of mind and exertion -which a great metropolis always exhibits, will be extended in a -considerable degree to the whole realm. The same effect will be -produced as if all distances were lessened in the proportion in -which the speed and cheapness of transit are increased. Towns at -present removed some stages from the metropolis, will become its -suburbs; others, now at a day's journey, will be removed to its -immediate vicinity; business will be carried on with as much ease -between them and the metropolis, as it is now between distant points -of the metropolis itself. Let those who discard speculations like -these as wild and improbable, recur to the state of public opinion, -at no very remote period, on the subject of steam navigation. Within -the memory of persons who have not yet passed the meridian of life, -the possibility of traversing by the steam engine the channels and -seas that surround and intersect these islands, was regarded as the -dream of enthusiasts. Nautical men and men of science rejected such -speculations with equal incredulity, and with little less than scorn -for the understanding of those who could for a moment entertain -them. Yet we have witnessed steam engines traversing not these -channels and seas alone, but sweeping the face of the waters round -every coast in Europe. The [Pg335] seas which interpose between our -Asiatic dominions and Egypt, and those which separate our own shores -from our West Indian possessions, have offered an equally -ineffectual barrier to its powers, and the establishment of a -regular steam communication between the capitals of the Old and New -World has ceased to be a question of practicability, having become -merely one of commercial profit. If steam be not used as the only -means of connecting the most distant points of our planet, it is not -because it is inadequate to the accomplishment of that end, but -because the supply of the material, from which at the present moment -it derives its powers, is restricted by local and accidental -circumstances.[30] - -We propose in the present chapter to lay before our readers some -account of the means whereby the effects above referred to have -been produced; of the manner and degree in which the public have -availed themselves of these means; and of the improvements of -which they seem to us to be susceptible. - - -(185.) It is a singular fact, that in the history of this -invention considerable time and great ingenuity were vainly -expended in attempting to overcome a difficulty, which in the end -turned out to be purely imaginary. To comprehend distinctly the -manner in which a wheel carriage is propelled by steam, suppose -that a pin or handle is attached to the spoke of the wheel at some -distance from its centre, and that a force is applied to this pin -in such a manner as to make the wheel revolve. If the tire of the -wheel and the surface of the road were absolutely smooth and free -from friction, so that the face of the tire would slide without -resistance upon the road, then the effect of the force thus -applied would be merely to cause the wheel to turn round, the -carriage being stationary, the surface of the tire slipping or -sliding upon the road as the wheel is made to revolve. But if, on -the other hand, the pressure of the face of the tire upon the road -is such as to produce between them such a degree of adhesion as -will render it impossible for the wheel to slide or slip upon the -road by [Pg336] the force which is applied to it, the consequence -will be, that the wheel can only turn round in obedience to the -force which moves it by causing the carriage to advance, so that -the wheel will roll upon the road, and the carriage will be moved -forward, through a distance equal to the circumference of the -wheel, each time it performs a complete revolution. - -It is obvious that both of these effects may be partially produced; -the adhesion of the wheel to the road may be insufficient to prevent -slipping altogether, and yet it may be sufficient to prevent the -wheel from slipping as fast as it revolves. Under such circumstances -the carriage would advance and the wheel would slip. The progressive -motion of the carriage during one complete revolution of the wheel -would be equal to the difference between the complete circumference -of the wheel and the portion through which in one revolution it has -slipped. - -When the construction of travelling steam engines first engaged -the attention of engineers, and for a considerable period -afterwards, a notion was impressed upon their minds that the -adhesion between the face of the wheel and the surface of the road -must necessarily be of very small amount, and that in every -practical case the wheels thus driven would either slip -altogether, and produce no advance of the carriage, or that a -considerable portion of the impelling power would be lost by the -partial slipping or sliding of the wheels. It is singular that it -should never have occurred to the many ingenious persons who for -several years were engaged in such experiments and speculations, -to ascertain by experiment the actual amount of adhesion in any -particular case between the wheels and the road. Had they done so, -we should probably now have found locomotive engines in a more -advanced state than that to which they have attained. - -To remedy this imaginary difficulty, Messrs. Trevethick and Vivian -proposed to make the external rims of the wheels rough and uneven, -by surrounding them with projecting heads of nails or bolts, or by -cutting transverse grooves on them. They proposed, in cases where -considerable elevations were to be ascended, to cause claws or -nails to project from the surface during the ascent, so as to take -hold of the road. [Pg337] - -In seven years after the construction of the first locomotive -engine by these engineers, another locomotive engine was -constructed by Mr. Blinkensop, of Middleton Colliery, near Leeds. -He obtained a patent, in 1811, for the application of a rack-rail. -The railroad thus, instead of being composed of smooth bars of -iron, presented a line of projecting teeth, like those of a -cog-wheel, which stretched along the entire distance to be -travelled. The wheels on which the engine rolled were furnished -with corresponding teeth, which worked in the teeth of the -railroad, and, in this way, produced a progressive motion in the -carriage. - -The next contrivance for overcoming this fictitious difficulty, -was that of Messrs. Chapman, who, in the year 1812, obtained a -patent for working a locomotive engine by a chain extending along -the middle of the line of railroad, from the one end to the other. -This chain was passed once round a grooved wheel under the centre -of the carriage; so that, when this grooved wheel was turned by -the engine, the chain being incapable of slipping upon it, the -carriage was consequently advanced on the road. In order to -prevent the strain from acting on the whole length of the chain, -its links were made to fall upon upright forks placed at certain -intervals, which between those intervals sustained the tension of -the chain produced by the engine. Friction-rollers were used to -press the chain into the groove of the wheel, so as to prevent it -from slipping. This contrivance was soon abandoned, for the very -obvious reason that a prodigious loss of force was incurred by the -friction of the chain. - -The following year, 1813, produced a contrivance of singular -ingenuity, for overcoming the supposed difficulty arising from the -want of adhesion between the wheels and the road. This was no -other than a pair of mechanical legs and feet, which were made to -walk and propel in a manner somewhat resembling the feet of an -animal. - -[Illustration: _Fig._ 86.] - -A sketch of these propellers is given in _fig._ 86. A is the -carriage moving on the railroad, L and L' are the legs, F and F' -the feet. The foot F has a joint at O, which corresponds to the -ankle; another joint is placed at K, which corresponds to the -knee; and a third is placed at L, which corresponds to [Pg338] -the hip. Similar joints are placed at the corresponding letters in -the other leg. The knee-joint K is attached to the end of the -piston of the cylinder. When the piston, which is horizontal, is -pressed outwards, the leg L presses the foot F against the ground, -and the resistance forces the carriage A onwards. As the carriage -proceeds, the angle K at the knee becomes larger, so that the leg -and thigh take a straighter position; and this continues until the -piston has reached the end of its stroke. At the hip L there is a -short lever L M, the extremity of which is connected by a cord or -chain with a point S, placed near the shin of the leg. When the -piston is pressed into the cylinder, the knee K is drawn towards -the engine, and the cord M S is made to lift the foot F from the -ground; to which it does not return until the piston has arrived -at the extremity of the cylinder. On the piston being again driven -out of the cylinder, the foot F, being placed on the road, is -pressed backwards by the force of the piston-rod at K; but the -friction of the ground preventing its backward motion, the -re-action causes the engine to advance: and in the same manner -this process is continued. - -Attached to the thigh at N, above the knee, by a joint, is a -horizontal rod N R, which works a rack R. This rack has beneath it -a cog-wheel. This cog-wheel acts in another rack below it. By -these means, when the knee K is driven _from_ the engine, the rack -R is moved _backwards_; but the cog-wheel acting on the other rack -beneath it, will move the latter _in the contrary direction_. The -rack R being then moved _in the_ [Pg339] _same direction with the -knee_ K, it follows that the other rack will always be moved _in a -contrary direction_. The lower rack is connected by another -horizontal rod with the thigh of the leg L F', immediately above -the knee at N'. When the piston is forced _inwards_, the knee K' -will thus be forced _backwards_; and when the piston is forced -_outwards_, the knee K' will be drawn _forwards_. It therefore -follows, that the two knees K and K' are pressed _alternately -backwards_ and _forwards_. The foot F', when the knee K' is drawn -forward, is lifted by the means already described for the foot F. - -It will be apparent, from this description, that the piece of -mechanism here exhibited is a contrivance derived from the motion -of the legs of an animal, and resembling in all respects the fore -legs of a horse. It is however to be regarded rather as a specimen -of great ingenuity than as a contrivance of practical utility. - - -(186.) It was about this period that the important fact was first -ascertained that the adhesion or friction of the wheels with the -rails on which they moved was amply sufficient to propel the -engine, even when dragging after it a load of great weight; and -that in such case, the progressive motion would be effected -without any slipping of the wheels. The consequence of this fact -rendered totally useless all the contrivances for giving wheels a -purchase on the road, such as racks, chains, feet, &c. The -experiment by which this was determined appears to have been first -tried on the Wylam railroad; where it was proved, that when the -road was level, and the rails clean, the adhesion of the wheels -was sufficient, in all kinds of weather, to propel considerable -loads. By manual labour it was first ascertained how much weight -the wheels of a common carriage would overcome without slipping -round on the rail, and having found the proportion which that bore -to the weight, they then ascertained that the weight of the engine -would produce sufficient adhesion to drag after it on the railroad -the requisite number of waggons.[31] - -In 1814, an engine was constructed at Killingworth, by Mr. -Stephenson, having two cylinders with a cylindrical [Pg340] -boiler, and working two pair of wheels, by cranks placed at right -angles; so that when the one was in full operation, the other was -at its dead points. By these means the propelling power was always -in action. The cranks were maintained in this position by an -endless chain, which passed round two cogged wheels placed under -the engine, and which were fixed on the same axles on which the -wheels were placed. The wheels in this case were fixed on the -axles, and turned with them. - -[Illustration: _Fig._ 87.] - -This engine is represented in _fig._ 87., the sides being open, to -render the interior mechanism visible. A B is the cylindrical -boiler; C C are the working cylinders; D E are the cogged wheels -fixed on the axle of the wheels of the engine, and surrounded by -the endless chain. These wheels being equal in magnitude, perform -their revolutions in the same time; so that, when the crank F -descends to the lowest point, the crank G rises from the lowest -point to the horizontal position D; and, again, when the crank F -rises from the lowest point to the horizontal position E, the -other crank rises to the highest point; and so on. A very -beautiful contrivance was adopted in this engine, by which it was -suspended on springs of steam. Small cylinders, represented at H, -are screwed by flanges to one side of the boiler, and project -within it a few inches; they have free communication at the top -with the water or steam of the boiler. Solid pistons are -represented at I, which move steam-tight in these [Pg341] -cylinders; the cylinders are open at the bottom, and the -piston-rods are screwed on the carriage of the engine, over the -axle of each pair of wheels, the pistons being presented upwards. -As the engine is represented in the figure, it is supported on -four pistons, two at each side. The pistons are pressed upon by -the water or steam which occupies the upper chamber of the -cylinder; and the latter being elastic in a high degree, the -engine has all the advantage of spring suspension. The defect of -this method of supporting the engine is, that when the steam loses -that amount of elasticity necessary for the support of the -machine, the pistons are forced into the cylinders, and the -bottoms of the cylinders bear upon them. All spring suspension is -then lost. This mode of suspension has consequently since been -laid aside. - -In an engine subsequently constructed by Mr. Stephenson, for the -Killingworth railroad, the mode adopted of connecting the wheels -by an endless chain and cog-wheels was abandoned; and the same -effect was produced by connecting the two cranks by a straight -rod. All such contrivances, however, have this great defect, that, -if the fore and hind wheels be not constructed with dimensions -accurately equal, there must necessarily be a slipping or dragging -on the road. The nature of the machinery requires that each wheel -should perform its revolution exactly in the same time; and -consequently, in doing so, must pass over exactly equal lengths of -the road. If, therefore, the circumference of the wheels be not -accurately equal, that wheel which has the lesser circumference -must be dragged along so much of the road as that by which it -falls short of the circumference of the greater wheel; or, on the -other hand, the greater wheel must be dragged in the opposite -direction, to compensate for the same difference. As no mechanism -can accomplish a perfect equality in four, much less in six, -wheels, it may be assumed that a great portion of that dragging -effect is a necessary consequence of the principle of this -machine; and even were the wheels, in the first instance, -accurately constructed, it is not possible that their wear could -be so exactly uniform as to continue equal. - - -(187.) The next stimulus which the progress of this [Pg342] -invention received, proceeded from the great national work -undertaken at Liverpool, by which that town and the extensive -commercial mart of Manchester were connected by a double line of -railway. When this project was undertaken, it was not decided what -moving power it might be most expedient to adopt as a means of -transport on the proposed road: the choice lay between horse -power, fixed steam engines, and locomotive engines; but the first, -for many obvious reasons, was at once rejected in favour of one or -other of the last two. - -The steam engine may be applied, by two distinct methods, to move -waggons either on a turnpike road or on a railway. By the one -method the steam engine is fixed, and draws the carriage or train -of carriages towards it by a chain extending the whole length of -road on which the engine works. By this method the line of road -over which the transport is conducted is divided into a number of -short intervals, at the extremity of each of which an engine is -placed. The waggons or carriages, when drawn by any engine to its -own station, are detached, and connected with the extremity of the -chain worked by the next stationary engine; and thus the journey -is performed, from station to station, by separate engines. By the -other method the same engine draws the load the whole journey, -travelling with it. - -The Directors of the Liverpool and Manchester railroad, when that -work was advanced towards its completion, employed, in the spring -of the year 1829, Messrs. Stephenson and Lock, and Messrs. Walker -and Rastrick, experienced engineers, to visit the different -railways, where practical information respecting the comparative -effects of stationary and locomotive engines was likely to be -obtained; and from these gentlemen they received reports on the -relative merits, according to their judgment of the two methods. -The particulars of their calculations are given at large in the -valuable work of Mr. Nicholas Wood on railways; to which we refer -the reader, not only on this, but on many other subjects connected -with the locomotive steam engine, into which it would be foreign -to our object to enter. The result of the comparison of the two -systems was, that the capital [Pg343] necessary to be advanced to -establish a line of stationary engines was considerably greater -than that which was necessary to establish an equivalent power in -locomotive engines; that the annual expense by the stationary -engines was likewise greater; and that, consequently, the expense -of transport by the latter was greater, in a like proportion. The -subjoined table exhibits the results numerically:-- - - ----------------------------------------------------------------- - | | | Expense of - | | | taking a - | | Annual | Ton of Goods - | Capital. | Expense. | a Mile. - ----------------------------------------------------------------- - | L s. d.| L s. d.| - Locomotive engines | 58,000 0 0 | 25,517 8 2 | 0.164 penny - Stationary engines | 121,496 7 0 | 42,031 16 5 | 0.269 - |--------------------------------------------- - Locomotive system | | | - less | 63,496 7 0 | 16,514 8 3 | 0.105 - ----------------------------------------------------------------- - -On the score of economy, therefore, the system of locomotive -engines was entitled to a preference; but there were other -considerations which conspired with this to decide the choice of -the Directors in its favour. An accident occurring in any part of -a road worked by stationary engines must necessarily produce a -total suspension of work along the entire line. The most vigilant -and active attention on the part of every workman, however -employed, in every part of the line, would therefore be necessary; -but, independently of this, accidents arising from the fracture or -derangement of any of the chains, or from the suspension of the -working of any of the fixed engines, would be equally injurious, -and would effectually stop the intercourse along the line. On the -other hand, in locomotive engines an accident could only affect -the particular train of carriages drawn by the engine to which the -accident might occur; and even then the difficulty could be -remedied by having a supply of spare engines at convenient -stations along the line. It is true that the _probability_ of -accident is, perhaps, less in the stationary than in the -locomotive system; but the _injurious consequences_, when accident -_does_ happen, are prodigiously greater in the former. "The one -system," says Mr. Walker, "is like a chain extending from -Liverpool to Manchester, the failure [Pg344] of a single link of -which would destroy the whole; while the other is like a number of -short and unconnected chains," the destruction of any one of which -does not interfere with the effect of the others, and the loss of -which may be supplied with facility. - -The decision of the Directors was, therefore, in favour of -locomotive engines; and their next measure was to devise some -means by which the inventive genius of the country might be -stimulated to supply them with the best possible form of engines -for this purpose. With this view, it was proposed and carried into -effect to offer a prize for the best locomotive engine which might -be produced under certain proposed conditions, and to appoint a -time for a public trial of the claims of the candidates. A premium -of five hundred pounds was accordingly offered for the best -locomotive engine to run on the Liverpool and Manchester railway; -under the condition that it should produce no smoke; that the -pressure of the steam should be limited to fifty pounds on the -inch; and that it should draw at least three times its own weight, -at the rate of not less than ten miles an hour; that the engine -should be supported on springs, and should not exceed fifteen feet -in height. Precautions were also proposed against the consequences -of the boiler bursting; and other matters not necessary to mention -more particularly here. This proposal was announced in the spring -of 1829, and the time of trial was appointed in the following -October. The engines which underwent the trial were, the Rocket, -constructed by Mr. Stephenson; the Sanspareil, by Hackworth; and -the Novelty, by Messrs. Braithwaite and Ericson. Of these, the -Rocket obtained the premium. A line of railway was selected for -the trial, on a level piece of road about two miles in length, -near a place called Rainhill, between Liverpool and Manchester; -the distance between the two stations was a mile and a half, and -the engine had to travel this distance backwards and forwards ten -times, which made altogether a journey of thirty miles. The Rocket -performed this journey twice: the first time in 2 hours 14 minutes -and 8 seconds; and the second time in 2 hours 6 minutes and 49 -seconds. Its speed at different parts of the journey varied: its -greatest rate of motion was [Pg345] rather above 29 miles an -hour; and its least, about 11-1/2 miles an hour. The average rate -of the one journey was 13-4/10 miles an hour; and of the other, -14-2/20 miles. This was the only engine which performed the -complete journey proposed, the others having been stopped from -accidents which occurred to them in the experiment. The Sanspareil -performed the distance between the stations eight times, -travelling 22-1/2 miles in 1 hour 37 minutes and 16 seconds. The -greatest velocity to which this engine attained was something less -than 23 miles per hour. The Novelty had only passed twice between -the stations when the joints of the boiler gave way, and put an -end to the experiment. - - -(188.) The great object to be attained in the construction of -these engines was, to combine with sufficient lightness the -greatest possible heating power. The fire necessarily acts on the -water in two ways: first, by its radiant heat; and second, by the -current of heated air which is carried by the draught through the -flues, and finally passes into the chimney. To accomplish this -object, therefore, it is necessary to expose to both these sources -of heat the greatest possible quantity of surface in contact with -the water. These ends were attained by the following admirable -arrangement in the Rocket:-- - -[Illustration: _Fig._ 88.] - -[Illustration: _Fig._ 89.] - -This engine is represented in _fig._ 88. It is supported on four -wheels; the principal part of the weight being thrown on one pair, -which are worked by the engine. The boiler consists of a cylinder -six feet in length, with flat ends; the chimney issues from one -end, and to the other end is attached a square box B, the bottom -of which is furnished with the grate on which the fuel is placed. -This box is composed of two casings of iron, one contained within -the other, having between them a space about three inches in -breadth; the magnitude of the box being three feet in length, two -feet in width, and three feet in depth. The casing which surrounds -the box communicates with the lower part of the boiler by a pipe -marked C; and the same casing at the top of the box communicates -with the upper part of the boiler by another pipe marked D. When -water is admitted into the boiler, therefore, it flows freely, -through the pipe C, into the casing which [Pg346] surrounds the -furnace or fire-box, and fills this casing to the same level as -that which it has in the boiler. When the engine is at work, the -boiler is kept about half filled with water; and, consequently, -the casing surrounding the furnace is completely filled. The steam -which is generated in the water contained in the casing finds its -exit through the pipe D, and escapes into the upper part of the -boiler. A section of the engine, taken at right angles to its -length, is represented at _fig._ 89. Through the lower part of the -boiler pass a number of copper tubes of small size, which -communicate at one end with the fire-box, and at the other with -the chimney, and form a passage for the heated air from the -furnace to the chimney. The ignited fuel spread on the grate at -the bottom of the fire-box disperses its heat by radiation, and -acts in this manner on the whole surface of the casing surrounding -the fire-box; and thus raises the temperature of the thin shell of -water contained in that casing. The chief [Pg347] part of the -water in the casing, being lower in its position than the water in -the boiler, acquires a tendency to ascend when heated, and passes -into the boiler; so that a constant circulation of the heated -water is maintained, and the water in the boiler must necessarily -be kept at nearly the same temperature as the water in the casing. -The air which passes through the burning fuel, and which fills the -fire-box, is carried by the draught through the tubes which extend -through the lower part of the boiler; and as these tubes are -surrounded on every side with the water contained in the boiler, -this air transmits its heat through these tubes to the water. It -finally issues into the chimney, and rises by the draught. The -power of this furnace must necessarily depend on the power of -draught in the chimney; and to increase this, and at the same time -to dispose of the waste steam after it has worked the piston, this -steam is carried off by a pipe L, which passes from the cylinder -to the chimney, and escapes there in a jet which is turned -upwards. By the velocity with which it issues from this jet, and -by its great comparative levity, it produces a strong current -upwards in the chimney, and thus gives force to the draught of the -furnace. In _fig._ 89. the grate-bars are represented at the -bottom of the fire-box at F. There are two cylinders, one of which -works each wheel; one only appearing in the drawing _fig._ 88., -the other being concealed by the engine. The spokes which these -cylinders work are placed at right angles on the wheels; the -wheels being fixed on a common axle, with which they turn. - -In this engine, the surface of water surrounding the fire-box, -exposed to the action of radiant heat, amounted to twenty square -feet, which received heat from the surface of six square feet of -burning fuel on the bars. The surface exposed to the action of the -heated air amounted to 118 square feet. The engine drew after it -another carriage, containing fuel and water; the fuel used was -coke, for the purpose of avoiding the production of smoke. - - -(189.) The Sanspareil of Mr. Hackworth is represented in _fig._ -90.; the horizontal section being exhibited in _fig._ 91. - -[Illustration: _Fig._ 90.] - -[Illustration: _Fig._ 91.] - -The draught of the furnace is produced in the same manner as in -the Rocket, by ejecting the waste steam coming from [Pg348] the -cylinder into the chimney; the boiler, however, differs -considerably from that of the Rocket. A recurved tube passes -through the boiler, somewhat similar to that already described in -the early engine of Messrs. Trevethick and Vivian. In the -horizontal section (_fig._ 91.), D expresses the opening of the -furnace at the end of the boiler, beside the chimney. The -grate-bars appear at A, supporting the burning fuel; and a curved -tube passing through the boiler, and terminating in the chimney, -is expressed at B, the direction [Pg349] of the draught being -indicated by the arrow; C is a section of the chimney. The -cylinders are placed, as in the Rocket, on each side of the -boiler; each working a separate wheel, but acting on spokes placed -at right angles to each other. The tube in which the grate and -flue are placed diminishes in diameter as it approaches the -chimney. At the mouth where the grate was placed, its diameter was -two feet; and it was gradually reduced, so that, at the chimney, -its diameter was only fifteen inches. The grate-bars extended five -feet into the tube. The surface of water exposed to the radiant -heat of the fire was sixteen square feet; and that exposed to the -action of the heated air and flame was about seventy-five square -feet. The magnitude of the grate, or sheet of burning fuel which -radiated heat, was ten square feet. - - -(190.) The Novelty, of Messrs. Braithwaite and Ericson, is -represented in _fig._ 92.; and a section of the generator and -boiler is exhibited in _fig._ 93.; the corresponding parts in the -two figures are marked by the same letters. - -[Illustration: _Fig._ 92.] - -A is the generator or receiver containing the steam which works -the engine; this communicates with a lower generator B, which -extends in a horizontal direction the entire length of the -carriage. Within the generator A is contained the furnace F, which -communicates in a tube C, carried up through the generator, and -terminated at the top by sliding shutters, which exclude the air, -and which are only opened to supply fuel to the grate F. Below the -grate the furnace is not open, as usual, to the atmosphere, but -communicates, [Pg350] by a tube E, with a bellows D; which is -worked by the engine, and which forces a constant stream of air, -by the tube E, through the fuel on F, so as to keep that fuel in -vivid combustion. The heated air contained in the furnace F is -driven on, by the same force, through a small curved tube marked -_e_, which circulates like a worm (as represented in _fig._ 93.) -through the horizontal generator or receiver; and, tapering -gradually, until reduced to very small dimensions, it finally -issues into the chimney G. The air in passing along this tube, -imparts its heat to the water by which the tube is surrounded, and -is brought to a considerably reduced temperature when discharged -into the chimney. The cylinder, which is represented at K, works -one pair of wheels, by means of a bell-crank, the other pair, when -necessary, being connected with them. - -[Illustration: _Fig._ 93.] - -In this engine, the magnitude of the surface of burning fuel on -the grate-bars is less than two square feet; the surface exposed -to radiant heat is nine and a half square feet; and the surface of -water exposed to heated air is about thirty-three square feet. - -The superiority of the Rocket may be attributed chiefly to the -greater quantity of surface of the water which is exposed to the -action of the fire. With a less extent of grate-bars than the -Sanspareil, in the proportion of three to five, it exposes a -greater surface of water to radiant heat, in the proportion of -four to three; and a greater surface of water to heated air, in -the proportion of more than three to two. It was found that -the Rocket, compared with the Sanspareil, consumed fuel, in -the evaporation of a given quantity of water, [Pg351] in the -proportion of eleven to twenty-eight. The suggestion of using the -tubes to conduct through the water the heated air to the chimney -is due to Mr. Booth, treasurer of the Liverpool and Manchester -Railway Company. - - -(191.) The object to be effected in the boilers of these engines -is, to keep a small quantity of water at an excessive temperature, -by means of a small quantity of fuel kept in the most active state -of combustion. To accomplish this, it is necessary, first, so to -shape the boiler, furnace, and flues, that the water shall be in -contact with as extensive a surface as possible, every part of -which is acted on, either immediately, by the heat radiating from -the fire, or mediately, by the air which has passed through the -fire, and which finally rushes into the chimney: and, secondly, -that such a forcible draught should be maintained in the furnace, -that a quantity of heat shall be extricated from the fuel, by -combustion, sufficient to maintain the water at the necessary -temperature, and to produce the steam with sufficient rapidity. To -accomplish these objects, therefore, the chamber containing the -grate should be completely surrounded by water, and should be -below the level of the water in the boiler. The magnitude of the -surface exposed to radiation should be as great as is consistent -with the whole magnitude of the machine. The comparative advantage -which the Rocket possessed in these respects over the other -engines will be evident on inspection. In the next place, it is -necessary that the heat, which is absorbed by the air passing -through the fuel, and keeping it in a state of combustion, should -be transferred to the water before the air escapes into the -chimney. Air being a bad conductor of heat, to accomplish this it -is necessary that the air in the flues should be exposed to as -great an extent of surface in contact with the water as possible. -No contrivance can be less adapted for the attainment of this end -than one or two large tubes traversing the boiler, as in the -earliest locomotive engines: the body of air which passed through -the centre of these tubes had no contact with their surface, and, -consequently, passed into the chimney at nearly the same -temperature as that which it had when it quitted the fire. The -only portion of air which imparted its heat to the water [Pg352] -was that portion which passed next to the surface of the tube. - -Several methods suggest themselves to increase the surface of -water in contact with a given quantity of air passing through it. -This would be accomplished by causing the air to pass between -plates placed near each other, so as to divide the current into -thin strata, having between them strata of water, or it might be -made to pass between tubes differing slightly in diameter, the -water passing through an inner tube, and being also in contact -with the external surface of the outer tube. Such a method would -be similar in principle to the steam-jacket used in Watt's steam -engines, or to the condenser of Cartwright's engine already -described. But, considering the facility of constructing small -tubes, and of placing them in the boiler, that method, perhaps, -is, on the whole, the best in practice; although the shape of a -tube, geometrically considered, is most unfavourable for the -exposure of a fluid contained in it to its surface. The air which -passes from the fire-chamber, being subdivided as it passes -through the boiler by a great number of very small tubes, may be -made to impart all its excess of heat to the water before it -issues into the chimney. This is all which the most refined -contrivance can effect. The Rocket engine was traversed by -twenty-five tubes, each three inches in diameter; and the -principle has since been carried to a much greater extent. - -The abstraction of a great quantity of heat from the air before it -reaches the chimney is attended with one consequence, which, at -first view, would present a difficulty apparently insurmountable; -the chimney would, in fact, lose its power of draught. This -difficulty, however, was removed by using the waste steam, which -had passed from the cylinder after working the engine, for the -purpose of producing a draught. This steam was urged through a jet -presented upwards in the chimney, and driven out with such force -in that direction as to create a sufficient draught to work the -furnace. - -It will be observed that the principle of draught in the Novelty -is totally distinct from this: in that engine the draught is -produced by a bellows worked by the engine. The question, as far -as relates to these two methods, is, whether more power [Pg353] -is lost in supplying the steam through the jet, as in the Rocket, -or in working the bellows, as in the Novelty. The force requisite -to impel the steam through the jet must be exerted by the -returning stroke of the piston, and, consequently, must rob the -working effect to an equivalent amount. On the other hand, the -power requisite to work the bellows in the Novelty must be -subducted from the available power of the engine. The former -method has been hitherto found to be the more effectual and -economical. - -The importance of these details will be understood, when it is -considered that the only limit to the attainment of speed by -locomotive engines is the power to produce, in a given time, a -certain quantity of steam. Each stroke of the piston causes one -revolution of the wheels, and consumes four cylinders full of -steam: consequently, a cylinder of steam corresponds to a certain -number of feet of road travelled over: hence it is that the -production of a rapid and abundant supply of heat, and the -imparting of that heat quickly and effectually to the water, is -the key to the solution of the problem to construct an engine -capable of rapid motion. - -The method of subdividing the flue into tubes was carried much -further by Mr. Stephenson after the construction of the Rocket; -and, indeed, the principle was so obvious, it is only surprising -that, in the first instance, tubes of smaller diameter than three -inches were not used. In engines since constructed, the number of -tubes vary from ninety to one hundred and twenty, the diameter -being reduced to two inches or less; and in some instances tubes -have been introduced, even to the number of one hundred and fifty, -of one and a half inch diameter. In the Meteor, twenty square feet -are exposed to radiation, and one hundred and thirty-nine to the -contact of heated air; in the Arrow, twenty square feet to -radiation, and one hundred and forty-five to the contact of heated -air. The superior economy of fuel gained by this means will be -apparent by inspecting the following table, which exhibits the -consumption of fuel which was requisite to convey a ton weight a -mile in each of four engines, expressing also the rate of the -motion:-- [Pg354] - - ------------------------------------------------------------ - | Average Rate of | Consumption of Coke - Engines. | Speed in Miles per | in Pounds per Ton - | Hour. | per Mile. - ------------------------------------------------------------ - No. 1. Rocket | 14 | 2.41 - 2. Sanspareil | 15 | 2.47 - 3. Phoenix | 12 | 1.42 - 4. Arrow | 12 | 1.25 - ------------------------------------------------------------ - -(192.) Since the period at which this railway was opened for the -actual purposes of transport, the locomotive engines have been in -a state of progressive improvement. Scarcely a month has passed -without suggesting some change in the details, by which fuel might -be economised, the production of steam rendered more rapid, the -wear of the engine rendered slower, the proportionate strength of -the different parts improved, or some other desirable end -obtained. - -Engines constructed in the form of the Rocket, were subject to two -principal defects. The cylinders, being placed outside the engine, -were exposed to the cold of the atmosphere, which produced a waste -of heat more or less considerable by condensation. The points at -which the power of the steam to turn the wheels was applied, being -at the extremities of the axle and on the exterior of the wheel, a -considerable strain was produced, owing to the distance of the -point of application of the power from the centre of resistance. -If it were possible that the impelling power could act in drawing -the train at all times with equal energy on both sides of the -engine, then no injurious strain would be produced; but from the -relative position of the points on the opposite wheels to which it -was necessary to attach the connecting rods, it was inevitable -that, at the moment when one of the pistons exerts its full power -in driving the wheel, the other piston must be altogether -inactive. The impelling power, therefore, at alternate moments of -time, acted on opposite wheels, and on each of them at the -greatest possible distance from the centre of the axle. - -[Illustration: _Fig._ 94.] - - -(193.) The next step in the improvement of the machine was made -with a view to remove these two defects. The cylinders were -transferred from the exterior of the engine to the [Pg355] -interior of the casing called the smoke-box, B, _fig._ 94., which -supports the chimney, and which receives the heated air issuing -from the tubes which traverse the boiler. Thus placed, the -cylinders are always maintained as hot as the air which issues -from the flues, and all condensation of steam by their exposure is -prevented. The piston-rods are likewise brought closer together, -and nearer the centre of the engine: the connecting rods, no -longer attached to the wheels, are made to act upon two cranks -constructed upon the axle of the wheels, and placed at right -angles to each other. From the position of these cranks, one would -always be at its dead point when the other is in full action. The -action of the steam upon them would, therefore, be generally -unequal; but this would not produce the same strain as when the -connecting rods are attached to points upon the exterior of the -wheels, owing to the cranks being constructed on the axle at -points so much nearer its centre. By this means it was found that -the working of the machine was more even, and productive of much -less strain, than in the arrangement adopted in the Rocket, and -the earlier engines. On the other hand, a serious disadvantage was -incurred by a double-cranked axle. The weakness necessarily -arising from such a form of axle could only be removed by great -thickness [Pg356] and weight of metal; and even this precaution, -at first, did not prevent their occasional fracture. The forging -of them was, however, subsequently much improved: the cranks, -instead of being formed by bending the metal when softened by -heat, were made by cutting the square of the crank out of the -solid metal; and now it rarely happens that one of these axles -fails. - -The adoption of smaller tubes, and a greater number of them, with -a view more perfectly to extract the heat from the air in passing -to the chimney, rendered a more forcible draft necessary. This was -accomplished by the adoption of a more contracted blast-pipe -leading from the eduction-pipes of the cylinders and presented up -the chimney. A representation of such a blast-pipe, with the two -tubes leading from the cylinders and uniting together near the -point, which is presented up the chimney, is given at _p p_ in -_fig._ 104. The engine thus improved is represented in _fig._ 94. - -A represents the cylindrical boiler, the lower half of which is -traversed by tubes, as described in the Rocket. They are usually -from eighty to one hundred in number, and about an inch and a half -in diameter; the boiler is about seven feet in length; the -fire-chamber is attached to one end of it, at F, as in the Rocket, -and similar in construction: the cylinders are inserted in a -chamber at the other end, immediately under the chimney. The -piston-rods are supported in the horizontal position by guides; -and connecting rods extend from them, under the engine, to the two -cranks placed on the axle of the large wheels. The effects of an -inequality in the road are counteracted by springs, on which the -engine rests; the springs being below the axle of the great -wheels, and above that of the less. The steam is supplied to the -cylinders, and withdrawn, by means of the common sliding valves, -which are worked by an eccentric wheel placed on the axle of the -large wheels of the carriage. The motion is communicated from this -eccentric wheel to the valve by sliding rods. The stand is placed -for the attendant at the end of the engine, next the fire-place F; -and two levers L project from the end which communicate with the -valves by means of rods, by which the engine is governed so as to -reverse the motion. [Pg357] - -The wheels of these engines have been commonly constructed of wood -with strong iron ties, furnished with flanges adapted to the -rails. But Mr. Stephenson afterward substituted, in some -instances, wheels of iron with hollow spokes. The engine draws -after it a tender carriage containing the fuel and water; and, -when carrying a light load, is capable of performing the whole -journey from Liverpool to Manchester without a fresh supply of -water. When a heavy load of merchandise is drawn, it is usual to -take in water at the middle of the trip. - - -(194.) In reviewing all that has been stated, it will be perceived -that the efficiency of the locomotive engines used on this railway -is mainly owing to three circumstances: 1st, The unlimited power -of draft in the furnace, by projecting the waste steam into the -chimney; 2d, The almost unlimited abstraction of heat from the air -passing from the furnace, by arrangement of tubes traversing the -boiler; and, 3d, Keeping the cylinders warm, by immersing them in -the chamber under the chimney. There are many minor details which -might be noticed with approbation, but these constitute the main -features of the improvements. - -The successive introduction of improvements in the engines, some -of which we have mentioned, was accompanied by corresponding -accessions to their practical power, and to the economy of fuel. -In the spring of the year 1832, I made several experiments on the -Manchester Railway, with a view to determine, in the actual state -of the locomotive engines at that time, their powers with respect -to the amount of load and the economy of fuel, from which I select -the following as examples:-- - - - I. - - On Saturday, the 5th of May, the engine called the "Victory" - took 20 waggons of merchandise, weighing gross 92 tons 19 cwt. - 1 qr., together with the tender containing fuel and water, of - the weight of which I have no account, from Liverpool to - Manchester (30 miles), in 1 h. 34 min. 45 sec. The train - stopped to take in water half-way, for 10 minutes, [Pg358] - not included in the above-mentioned time. On the inclined - plane rising 1 in 96, and extending 1-1/2 mile, the engine was - assisted by another engine called the "Samson," and the ascent - was performed in 9 minutes. At starting, the fire-place was - well filled with coke, and the coke supplied to the tender - accurately weighed. On arriving at Manchester, the fire-place - was again filled, and the coke remaining in the tender - weighed. The consumption was found to amount to 929 pounds net - weight, being at the rate of one third of a pound per ton per - mile. - - Speed on the level was 18 miles an hour; on a fall of 4 feet - in a mile, 21-1/2 miles an hour; fall of 6 feet in a mile, - 25-1/2 miles an hour; on the rise over Chatmoss, 8 feet in a - mile, 17-5/8 miles an hour; on level ground sheltered from the - wind, 20 miles an hour. The wind was moderate, but direct - ahead. The working wheels slipped three times on Chatmoss, and - the train was retarded from 2 to 3 minutes. - - The engine, on this occasion, was not examined before or after - the journey, but was presumed to be in good working order. - - - II. - - On Tuesday, the 8th of May, the same engine performed the same - journey, with 20 waggons, weighing gross 90 tons 7 cwt. 2 - qrs., exclusive of the unascertained weight of the tender. The - time of the journey was 1 h. 41 min. The consumption of coke - 1040 lbs. net weight, estimated as before. Rate of speed:-- - - Level 17-5/8 miles per hour. - Fall of 4 feet in a mile 22 - -- 6 22-1/2 - Rise of 8 15 - - On this occasion there was a high wind ahead on the quarter, - and the connecting rod worked hot, owing to having been keyed - too tight. On arriving at Manchester, I caused the cylinders - to be opened, and found that the pistons were [Pg359] so - loose, that the steam blew through the cylinders with great - violence. By this cause, therefore, the machine was robbed of - a part of its power during the journey; and this circumstance - may explain the slight decrease in speed, and increase in the - consumption of fuel, with a lighter load, in this journey, - compared with that performed on the 5th of May. - - The Victory weighs 8 tons 2 cwt., of which 5 tons 4 cwt. rest - on the drawing wheels. The cylinders are 11 inches diameter, - and 16 inches stroke, and the diameter of the drawing wheels - is 5 feet. - - - III. - - On the 29th of May, the engine called the "Samson" (weighing - 10 tons 2 cwt., with 14-inch cylinders, and 16-inch stroke; - wheels 4 feet 6 inches diameter, both pairs being worked by - the engine; steam 50 lbs. pressure, 130 tubes) was attached to - 50 waggons, laden with merchandise; net weight about 150 tons; - gross weight, including waggons, 223 tons 6 cwt. The tender - weighed 7 tons, making a gross load (including the engine) of - 240 tons 8 cwt. The engine with this load travelled from - Liverpool to Manchester (30 miles) in 2 hours and 40 min., - exclusive of delays upon the road for watering, &c.; being at - the rate of nearly 12 miles an hour. The speed varied - according to the inclinations of the road. Upon a level, it - was 12 miles an hour; upon a descent of 6 feet in a mile, it - was 16 miles an hour; upon a rise of 8 feet in a mile, it was - about 9 miles an hour. The weather was calm, the rails very - wet; but the wheels did not slip, even in the slowest speed, - except at starting, the rails being at that place soiled and - greasy with the slime and dirt to which they are always - exposed at the stations. The coke consumed in this journey, - exclusive of what was raised in getting up the steam, was 1762 - lbs., being at the rate of a quarter of a pound per ton per - mile. - - -(195.) The great original cost, and the heavy expense of keeping -the engines used on the railway in repair, have pressed severely -on the resources of the undertaking. One of the best [Pg360] -constructed of the later engines costs originally 1500_l._ and -sometimes more. The original cost, however, is far from being the -principal source of expense: the wear and tear of these machines, -and the occasional fracture of those parts on which the greatest -strain has been laid, have greatly exceeded what the directors had -anticipated. Although this source of expense must be in part -attributed to the engines not having yet attained that state of -perfection, in the proportion and adjustment of their parts, of -which they are susceptible, and to which experience alone can -lead, yet there are some obvious defects which demand attention. - -The heads of the boilers are flat, and formed of iron, similar to -the material of the boilers themselves. The tubes which traverse -the boiler were, until recently, copper, and so inserted into the -flat head or end as to be water-tight. When the boiler was heated, -the tubes were found to expand in a greater degree than the other -parts of the boiler; which frequently caused them either to be -loosened at the extremities, so as to cause leakage, or to bend -from want of room for expansion. The necessity of removing and -refastening the tubes caused, therefore, a constant expense. - -It will be recollected that the fire-place is situated at one end -of the boiler, immediately below the mouths of the tubes: a -powerful draft of air, passing through the fire, carries with it -ashes and cinders, which are driven violently through the tubes, -and especially the lower ones, situated near the fuel. These tubes -are, by this means, subject to rapid wear, the cinders continually -acting upon their interior surface. After a short time it becomes -necessary to replace single tubes, according as they are found to -be worn, by new ones; and it not unfrequently happens, when this -is neglected, that tubes burst. After a certain length of time the -engines require new tubing. This wear of the tubes might possibly -be avoided by constructing the fire-place in a lower position, so -as to be more removed from their mouths; or, still more -effectually, by interposing a casing of metal, which might be -filled with water, between the fire-place and those tubes which -are the most exposed to the cinders and ashes. The unequal -expansion of the tubes [Pg361] and boilers appears to be an -incurable defect, if the present form of the engine be retained. -If the fire-place and chimney could be placed at the same end of -the boiler, so that the tubes might be recurved, the unequal -expansion would then produce no injurious effect; but it would be -difficult to clean the tubes, if they were exposed, as they are at -present, to the cinders. The next source of expense arises from -the wear of the boiler-heads, which are exposed to the action of -the fire. - -A considerable improvement was subsequently introduced into the -method of tubing, by substituting brass for copper tubes. I am not -aware that the cause of this improvement has been discovered; but -it is certain, whatever be the cause, that brass tubes are subject -to considerably slower wear than copper ones. - - -(196.) The expense of locomotive power having so far exceeded what -was anticipated at the commencement of the undertaking, it was -thought advisable, about the beginning of the year 1834, to -institute an inquiry into the causes which produced the -discrepancy between the estimated and actual expenses, with a view -to the discovery of some practical means by which they could be -reduced. The directors of the company, for this purpose, appointed -a sub-committee of their own body, assisted by Mr. Booth, their -treasurer, to inquire and report respecting the causes of the -amount of this item of their expenditure, and to ascertain whether -any and what measures could be devised for the attainment of -greater economy. A very able and satisfactory report was made by -this committee, or, to speak more correctly, by Mr. Booth. - -It appears that, previous to the establishment of the railway, -Messrs. Walker and Rastrick, engineers, were employed by the -company to visit various places where steam power was applied on -railways, for the purpose of forming an estimate of the probable -comparative expense of working the railway by locomotive and by -fixed power. These engineers recommended the adoption of -locomotive power; and their estimate was, that the transport might -be effected at the rate of .278 of a penny, or very little more -than a farthing per ton per mile. In the year [Pg362] 1833, five -years after this investigation took place, it was found that the -actual cost was .625 of a penny, or something more than a -halfpenny, per ton per mile, being considerably above double the -estimated rate. Mr. Booth very properly directed his inquiries to -ascertain the cause of this discrepancy, by comparing the various -circumstances assumed by Messrs. Walker and Rastrick, in making -their estimate, with those under which the transport was actually -effected. The first point of difference which he observed was the -_speed_ of transport: the estimate was founded on an assumed speed -of ten miles an hour, and it was stated that a four-fold speed -would require an addition of 50 per cent. to the power, without -taking into account wear and tear. Now, the actual speed of -transport being double the speed assumed in the statement, Mr. -Booth holds it to be necessary to add 25 per cent. on that score. - -The next point of difference is in the amount of the loads: the -estimate is founded upon the assumption, that every engine shall -start with its full complement of load, and that with this it -shall go the whole distance. "The facts, however, are," says Mr. -Booth, "that, instead of a _full load_ of profitable carriage -_from_ Manchester, about half the waggons _come back empty_; and, -instead of the tonnage being conveyed the whole way, many thousand -tons are conveyed only half the way; also, instead of the daily -work being uniform, it is extremely fluctuating." It is further -remarked, that in order to accomplish the transport of goods from -the branches and from intermediate places, engines are despatched -several times a-day, from both ends of the line, _to clear the -road_; the object of this arrangement being rather to lay the -foundation of a beneficial intercourse in future, than with a view -to any immediate profit. Mr. Booth makes a rough estimate of the -disadvantages arising from these circumstances, by stating them at -33 per cent. in addition to the original estimate. - -The next point of difference is the fuel. In the original -estimate, _coal_ is assumed as the fuel, and it is taken at the -price of five shillings and ten-pence per ton: now the act of -parliament forbids the use of coal which would produce smoke; the -company have, therefore, been obliged to use _coke_, at [Pg363] -seventeen shillings and sixpence a ton.[32] Taking coke, then, to -be equivalent to coal, ton for ton, this would add .162 to the -original estimate. - -These several discrepancies being allowed for, and a proportional -amount being added to the original estimate, the amount would be -raised to .601 of a penny per ton per mile, which is within one -fortieth of a penny of the actual cost. This difference is -considered to be sufficiently accounted for by the wear and tear -produced by the very rapid motion, more especially when it is -considered that many of the engines were constructed before the -engineer was aware of the great speed that would be required. - -"What, then," says Mr. Booth, in the Report already alluded to, "is -the result of these opposite and mutually counteracting -circumstances? and what is the present position of the company in -respect of their moving power? Simply, that they are still in a -course of experiment, to ascertain practically the best -construction, and the most durable materials, for engines required -to transport greater weights, and at greater velocities, than had, -till very recently, been considered possible; and which, a few years -ago, it had not entered into the imagination of the most daring and -sanguine inventor to conceive: and farther, that these experiments -have necessarily been made, not with the calm deliberation and quiet -pace which a salutary caution recommends,--making good each step in -the progress of discovery before advancing another stage,--but -amidst the bustle and responsibilities of a large and increasing -traffic; the directors being altogether ignorant of the time each -engine would last before it would be laid up as inefficient, but -compelled to have engines, whether good or bad; being aware of -various defects and imperfections, which it was impossible at the -time to remedy, yet obliged to keep the machines in motion, under -all the disadvantages of heavy repairs, constantly going on during -the night, in order that the requisite number of engines might be -ready for the morning's work. Neither is this great experiment yet -complete; it is still going forward. But the most prominent -difficulties have been in a great measure surmounted, [Pg364] and -your committee conceive that they are warranted in expecting, that -the expenditure in this department will, ere long, be materially -reduced,--more especially when they consider the relative -performances of the engines at the _present time_, compared with -what it was two years ago." - -In the half year ending 31st December, 1831, the six best engines -performed as follows:-- - - Miles. - Planet 9,986 - Mercury 11,040 - Jupiter 11,618 - Saturn 11,786 - Venus 12,850 - Etna 8,764 - ------ - Making in all 66,044 - ------ - -In the half year ending 31st December, 1833, the six best engines -performed as follows:-- - - Miles. - Jupiter 16,572 - Saturn 18,678 - Sun 14,552 - Etna 17,763 - Ajax 11,678 - Firefly 15,608 - ------ - Making in all 95,851 - ------ - - -(197.) Since the date to which the preceding observations refer, -the locomotive engine has undergone several improvements in detail -of considerable importance; among which, the addition of a third -pair of wheels deserves to be particularly mentioned. An engine -supported on three pair of wheels has great security in the event -of the fracture of any one of the axles,--the remaining axles and -wheels being sufficient for the support of the machine. Connected -with this change is another, recommended by Mr. Robert Stephenson, -by which the flanges are removed from the driving wheels, those -upon the remaining pairs of wheels being sufficient to keep the -engine in its position upon the rails. We shall now describe a -locomotive engine similar in construction to those almost [Pg365] -universally used at present on railroads, as well in this kingdom -as in other countries.[33] - -The external appearance of the engine and tender is shown in the -engraving at the head of this chapter. In _fig._ 97. is exhibited -a vertical section of the engine made by a plane carried through -its length; and in _fig._ 98. is exhibited a corresponding section -of its tender,--the tender being supposed to be joined on to the -engine at the part where the connecting points appear to be broken -in the drawing. In _fig._ 99. is exhibited the plan of the working -machinery, including the cylinders, pistons, eccentrics, &c. which -are under the boiler, by the operation of which the engine is -driven. _Fig._ 100. represents the tender, also taken in plan. - -In _fig._ 101. is represented an elevation of the hinder end of -the engine next the fire-box; and in _fig._ 102. is represented a -cross vertical section through the fire-box, and at right angles -to the length of the engine, showing the interior of the boiler -above and beside the fire-box, the rivets and bolts connecting the -internal and external fire-boxes, the regulator, steam funnel, and -steam dome. - -In _fig._ 103. is represented an elevation of the front of the -engine next the smoke-box, showing the cylinder covers W, buffers -T, &c.; and in _fig._ 104. is represented a section of the -interior of the smoke-box, made by a vertical plane at right -angles to the engine, showing the tube plate forming the foremost -end of the boiler, the branches S of the steam-pipe leading to the -cylinders, the blast-pipe _p_, the cylinders H, and the chimney G. - -The same letters of reference are placed at corresponding parts in -the different figures. - -The boiler, as has been explained in the engines already -described, is a cylinder placed upon its side, the section of -which is exhibited at A, _fig._ 97. The fire-box consists of two -casings of metal, one within the other. The fire-grate is -represented at D. The tubes by which the products of combustion -are [Pg366] drawn from the fire-box to the smoke-box F are -represented at E. Upon the smoke-box is erected the chimney G. In -the engine from which this drawing has been taken, and which was -used on the London and Birmingham Railway, the boiler is a -cylinder 7-1/2 feet long, and 3-1/2 feet in diameter. It is formed -of wrought-iron plates 5/16 of an inch in thickness, overlapping -each other, and bound together by iron rivets 7/8 of an inch in -diameter and 1-3/4 inch apart. One of these rivets, as it joins -two plates, is represented in _fig._ 95. The boiler is clothed -with a boarding of wood _a_, an inch in thickness, and bound round -by iron hoops screwed together at the bottom. Wood being a slow -conductor of heat, this covering has the effect of keeping the -boiler warm, and checking the condensation of steam which would -otherwise be produced by the rapid motion of the engine through -the cold air. - -[Illustration: _Fig._ 95.] - -[Illustration: _Fig._ 96.] - -The external fire-box, B B, is a casing nearly square in its plan, -being four feet wide outside, and three feet seven and a half -inches long, measured in the direction of the boiler. It is -constructed of wrought-iron plates, similar to those of the -boiler. This box descends about two feet below the boiler, the top -being semi-cylindrical, as seen in _fig._ 102., of a somewhat -greater diameter than the boiler, and concentrical with it. The -front of the fire-box next the end of the boiler has a circular -opening equal in size to the end of the boiler. To the edge of -this opening the boiler is fastened by angle irons, and rivets in -the manner represented in _fig._ 96. These rivets are seen in -section in _fig._ 97. - -The internal fire-box C, _fig._ 97., is similar in shape to the -external, only it is flat at the top, and close every where except -at the bottom. Between it and the external fire-box an open space -of three inches and a half is left all round, and on the side next -the boiler this space is increased to four inches. This internal -fire-box is made of copper plates, 7/16 [Pg367] of an inch in -thickness, every where except next the boiler, where the thickness -is 7/8. - -As the sides and front of the external fire-box, and all the -surfaces bounding the internal fire-box, are flat, their form is -unfavourable for the resistance of pressure. Adequate means are, -therefore, provided for strengthening them. The plates forming the -internal fire-box are bent outwards near the bottom, until they -are brought into contact with those of the external fire-box, to -which they are attached by copper rivets, as represented at _f_ in -_fig._ 97. The plates forming the bounding surfaces of the two -fire-boxes are fastened together by stays represented at _k_ in -_figs._ 97. and 102. These stays, which are of copper, have a -screw cut upon them through their whole length, and holes are made -through the plates of both fire-boxes tapped with corresponding -threads. The copper screws are then passed through them, and -rivets formed on their heads within and without, as seen in _fig._ -102. These screw rivets connect all parts of the plating of the -two fire-boxes which are opposed to each other: they are placed at -about four inches apart over the sides and back of the internal -fire-place and that part of the front which is below the boiler. - -[Illustration: _Fig._ 97. - -LONGITUDINAL VERTICAL SECTION OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 98. - -LONGITUDINAL VERTICAL SECTION OF THE TENDER.] - -[Illustration: _Fig._ 99. - -PLAN OF THE WORKING MACHINERY OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 100. - -PLAN OF THE TENDER.] - -[Illustration: _Fig._ 101. - -ELEVATION OF THE HINDER END OF A LOCOMOTIVE ENGINE.] - -[Illustration: _Fig._ 102. - -CROSS VERTICAL SECTION OF THE ENGINE THROUGH THE FIRE-BOX.] - -[Illustration: _Fig._ 103. - -ELEVATION OF THE FOREMOST END OF THE ENGINE.] - -[Illustration: _Fig._ 104. - -CROSS VERTICAL SECTION OF ENGINE THROUGH THE SMOKE-BOX.] - -As the top of the internal fire-box cannot be strengthened by -stays of this kind, ribs of wrought-iron, which are seen in their -length at _l_, in _fig._ 97., and of which an end view is seen in -_fig._ 102., are attached by bolts to it. These ribs are hollowed -out, as seen in _fig._ 97., between bolt and bolt, in order to -break their contact with the roof of the fire-box, and allow a -more free passage to the heat through it. If they were in -continuous contact with the fire-box, the metal composing them -would become more highly heated, and would soon wear out, besides -intercepting heat from the water. This part of the fire-box is -subject to rapid wear, unless care be taken that the level of the -water be preserved at its proper height in the boiler. Even when -the boiler is properly filled, the depth of water above the roof -of the fire-box is not considerable, and on the least neglect the -roof may be exposed to the contact of steam, in which case it will -soon be destroyed. - -To prevent accidents arising from this cause, a leaden plug, -[Pg368] represented at _m_, _figs._ 97. and 102., is inserted in -the roof of the internal fire-box. If the water be allowed to -subside, this plug will melt out before the copper is very -injuriously heated, and the steam rushing out at the aperture will -cause the fire to be extinguished. - -Copper fire-boxes are almost universally used; but sometimes, from -the consideration of cheapness, the internal fire-box is -constructed of iron. - -In the plating which forms the back of the external fire-box, an -oval aperture is formed, as represented in the back view of the -engine, _fig._ 101., for the fire-door _g_. The plating of the -internal fire-box around this aperture is bent at right angles to -meet that of the external fire-box, to which it is fastened by a -row of copper rivets. The fire-door is formed of two plates of -wrought-iron, riveted together with a space of nine inches and a -half between them. The air between these plates being an imperfect -conductor of heat, keeps the outer plate of the fire-door at a -moderate temperature. - -In that part of the surface of the internal fire-box which forms -the end of the boiler, holes are made to receive the extremities -of the tubes, by which the air proceeding from the fire is drawn -to the smoke-box at the remote end of the boiler. These tubes are -represented in longitudinal section at E, _fig._ 97., and their -ends are seen in the surface of the internal fire-box in _fig._ -102., and in the remote end of the boiler where they terminate in -the smoke-box in _fig._ 104. These tubes are formed of the best -rolled brass, and their thickness in the engine, to which we now -refer, is 1/13 of an inch. After the brass plating is bent into -the form of a tube, and being overlapped, is properly soldered -together, and the edges smoothed off, the tubes are made perfectly -cylindrical by being drawn through a circular steel die. - -[Illustration: _Fig._ 105.] - -The tube-plates (as those parts of the boiler ends in which the -tubes are inserted are called) are bored with holes in corresponding -positions, truly cylindrical, and corresponding in magnitude to the -tubes, so that the tubes, when passed into them, will be just in -contact with them. The length of the tubes is so regulated, that -when extending from end to end of the boiler, and passing through -the holes, they shall [Pg369] project at each end a little beyond -the holes. The manner of fastening them so as to be water-tight is -as follows:--A steel hoop or ferrule, made slightly conical, a -section of which is exhibited at C. _fig._ 105., the smaller end of -which is a little less than the internal diameter of the tube, but -which increases towards the outer end, is driven in as represented -in the figure. It acts as a wedge, and forces the tube into close -contact with the edges of the hole in the tube-plate. - -When particular tubes in a boiler are worn out, and require to be -replaced, their removal is easily effected. It is only necessary -to cut the steel ferrule on the inside, and to bend it off from -contact with the tube, by which means it can be loosened and -withdrawn, and the tube removed. - -In the engine to which this description refers there were one -hundred and twenty-four tubes, the external diameter of which was -1-5/8 inch. The distance between tube and tube was 3/4 of an inch. -The number of tubes vary in different engines, some having so many -as one hundred and fifty, while the number in some is less than -ninety. The evaporating power of an engine greatly depends on the -proper number and magnitude of its tubes; and the experience which -engineers have had on railways have led them gradually to increase -the number of tubes, and diminish their magnitude. In the Rocket, -already mentioned as having gained the prize on the opening of the -Liverpool and Manchester Railway, the number of tubes was -twenty-four, and their diameter three inches; but in all the -engines subsequently made their number was augmented, and their -diameter diminished. The practical inconvenience which limits the -size of the tubes is their liability to become choked by cinders -and ashes, which get wedged in them when they are too small, and -thereby obstruct the draft, and diminish the evaporating power of -the boiler. The tubes now in use, of about an inch and a [Pg370] -half internal diameter, not only require to be cleared of the -ashes and cinders, which get fastened in them after each journey, -but it is necessary throughout a journey of any length that the -tubes should be picked and cleaned by opening the fire door at -convenient intervals. - -The substitution of brass for copper tubes, which has been already -mentioned as so great an improvement in the construction of -locomotive engines, is ascribed to Mr. Dixon, who suggested them -in 1833, being then the resident engineer of the Liverpool and -Manchester Railway. They are said to last six or eight times as -long as copper tubes of the same dimensions. - -When tubes fail, they are usually destroyed by the pressure of the -water crushing them inwards: the water enters through the rent made -in the tube, and flowing upon the fire extinguishes it. When a -single tube thus fails upon a journey, the engine, notwithstanding -the accident, may generally be made to work to the end of its -journey by plugging the ends of the broken tube with hard wood; the -water in contact with which will prevent the fire from burning it -away. - -Tubes of the dimensions here referred to weigh about sixteen -pounds, and lose from six to seven pounds before they are worn -out. Their cost is about one pound each. - -The tubes act as stays, connecting the ends of the boiler to -strengthen them. Besides these, there are rods of wrought iron -extended from end to end of the boiler above the roof of the -internal fire-place. These rods are represented at _o_ in their -length in _fig._ 97., and an end view of them is seen in _fig._ -102. The smoke-box F, _fig._ 97. 104., containing the cylinders, -steam-pipe, and blast-pipe, is four feet wide, and two feet long. -It is formed of wrought iron plates, half an inch thick on the -side next the boiler, and a quarter of an inch elsewhere. The -plates are riveted in the same manner as those of the fire-box -already described. From the top of the smoke-box, which, like the -fire-box, is semi-cylindrical, as seen in elevation in _fig._ -103., and in section _fig._ 104., rises the chimney G, fifteen -inches diameter, and formed of 1/8 inch iron plates, riveted and -bound round by hoops. It is flanged to the top of the [Pg371] -smoke-box, as represented in _fig._ 104. Near the bottom of the -smoke-box the working cylinders are placed, side by side, in a -horizontal position, with the slide valves upwards. In the top of -the external fire-box a circular aperture is formed fifteen inches -in diameter, and upon this aperture is placed the steam-dome T -(_figs._ 97. 101, 102.) two feet high, and attached around the -circular aperture by a flange and screw secured by nuts. This -steam dome is made of brass 3/8 inch thick. In stationary boilers, -where magnitude is not limited, it has been already explained, -that the space allowed for steam is sufficiently large to secure -the complete separation of the vapour from the spray which is -mixed with it when it issues immediately from the water. In -locomotive boilers sufficient space cannot be allowed for this, -and the separation of the water from the steam is effected by the -arrangement here represented. A funnel-shaped tube _d'_ (_figs._ -97. 102.), with its wide end upwards, rises into the steam-dome, -and reaches nearly to the top of it. This funnel bends towards the -back of the fire-box, and is attached by a flange and screws to -the great steam-pipe S, which traverses the whole length of the -boiler. The steam rising from the boiler fills the steam-dome T, -and descends in the funnel-shaped tube _d'_. The space it has thus -to traverse enables the steam to disengage itself almost -completely from the priming. The wider part of the great -steam-pipe _a_ is flanged and screwed at the hinder end to a -corresponding aperture in the back plate of the fire-box. This -opening is covered by a circular plate, secured by screws, having -a stuffing-box in its centre, of the same kind as is used for the -piston-rods of steam-cylinders. Through this stuffing-box the -spindle _a''_ of the regulator passes, and to its end is attached a -winch _h'_, by which the spindle _a''_ is capable of being turned. -This winch is limited in its play to a quarter of a revolution. -The other end of the spindle _a''_ is attached to a plate _e'_ seen -edgeways in _fig._ 97., and the face of which is seen in _fig._ -102.: this circular plate _e_ is perforated with two apertures -somewhat less than quadrants. That part of the plate, therefore, -which remains not pierced forms two solid pieces somewhat greater -than quadrants. This plate is ground so as to move in steam-tight -[Pg372] contact with a fixed plate under it, which terminates at -the wide end of the conical mouth of the steam-pipe S. This fixed -circular plate is likewise pierced with two nearly quadrantal -apertures, corresponding with those in the movable plate _e'_. -When the movable plate _e'_ is turned round by the winch _h'_, the -apertures in it may be made to correspond with those of the fixed -circular plate on which it moves, in which position the steam-pipe -S communicates with the funnel _d'_ by the two quadrantal -apertures thus open. If, on the other hand, the winch _h'_ be -moved from this position through a quarter revolution, then the -quadrantal openings in the movable plate will be brought over the -solid parts of the fixed plate on which it moves, and these solid -parts being a little more than quadrants, while the openings are a -little less, all communication between the steam-pipe S and the -funnel _d'_ will be stopped, for in this case the quadrantal -openings in the fixed and movable plates respectively will be -stopped by the solid parts of these plates. It will be evident -that as the winch _h'_ of the regulator is moved from the former -position to the latter, in every intermediate position the -aperture communicating between the funnel _d'_ and the steam-pipe -S will be less in magnitude than the complete quadrant. It will in -fact be composed of two openings having the form of _sectors_ of a -circle less than a quadrant, and these sectors may be made of any -magnitude, however small, until the opening is altogether closed. - -By such means the admission of steam from the boiler to the -steam-pipe S may be regulated by the winch _h'_. - -The steam being admitted to the steam-pipe passes through it to -the front end of the boiler, and the pipe being enclosed within -the boiler the temperature of the steam is maintained. The -steam-pipe passing through the tube-plate at the front end of the -boiler is carried to a small distance from the tube-plate in the -same direction, where it is flanged on to a cross horizontal pipe -proceeding to the right and to the left as represented in _fig._ -104. This cross pipe is itself flanged to two curved steam-pipes S -(_fig._ 104.), by which the steam is conducted to the valve-boxes -V V. The lower ends of these curved arms are flanged on to the -valve-boxes of the two cylinders [Pg373] at the ends nearest to -the boiler. The opening of one of these is exhibited in the right -hand cylinder in _fig._ 99. By these pipes the steam is conducted -into the valve-boxes or steam-chests, from which it is admitted by -slide-valves to the cylinders to work the pistons in the same -manner as has been already described in the large stationary -engines. - -On the upper sides of the cylinders are formed the steam-chests or -valve-boxes, which are exhibited at U (_figs._ 97. 99. 104.). -These are made of cast-iron half an inch thick, and are bolted to -the upper side of each cylinder. At the front end they are also -secured by bolts to the smoke-box, and at the hinder end are -attached to the tube-plate. These valve-boxes communicate with the -passages _m_ and _n_ _fig._ 99. leading to the top and bottom of -the cylinder: these are called the steam-ports. They also -communicate with a passage _o_ leading to the mouth of a curved -horizontal pipe _p'_ connecting the front ends of the two -cylinders, as seen in _figs._ 99. 104. These curved pipes unite in -a single vertical pipe _p_, called the _blast-pipe_, seen in -_figs._ 97. 104.: this vertical pipe becomes gradually small -towards the top, and terminates a little above the base of the -funnel or chimney G. In the valve-box is placed the slide-valve -_v_ to which is attached the spindle _l'_. This spindle moves -through a stuffing-box _k'_, and is worked by gearing, which will -be described hereafter. According to the position given to the -slide, a communication may be opened between the steam-chest, or -the waste-port, and either end of the cylinders. Thus when the -slide is in the position represented in _fig._ 97. the steam-chest -communicates with the front end of the cylinder, while the -waste-port communicates with the hinder end. If, on the other -hand, the spindle _l'_ being pressed forward, move the slide to -its extreme opposite position, the steam-port _n_ would -communicate with the waste-port _o_, while the steam-chest would -communicate with the steam-port _m_, steam would, therefore, be -admitted to the hinder end of the cylinder, while the foremost end -would communicate with the waste-port. It will be perceived that -this arrangement is precisely similar to that of the slide-valves -already described (133.). The slide-valve is represented on a -larger scale in _fig._ 106., where A is the hinder steam-port, -[Pg374] B the foremost steam-port, and C the waste-port. The -surfaces D, separating the steam-ports from the waste-ports, are -called the bars: they are planed perfectly smooth, so that the -surfaces F and G of the slide-valve, also planed perfectly smooth, -may move in steam-tight contact with them. These surfaces are kept -in contact by the pressure of the steam in the steam-chest, by -which the slide-valve is always pressed down. In its middle -position, as represented by the dotted lines in the figure, both -the steam-ports are stopped by the slide-valve, so that at that -moment no steam is admitted to either end of the cylinder. On -either side of this intermediate position the slide has an inch -and a half play, which is sufficient to open successively the two -steam-ports. - -[Illustration: _Fig._ 106.] - -The cylinders are inserted at one end in the plate of the -smoke-box, and at the other in the tube-plate of the boiler. They -are closed at either end by cast iron covers, nearly an inch -thick, flanged on by bolts and screws. In the cover of the -cylinder attached to the tube-plate is a stuffing-box, in which -the piston rod plays. The metallic pistons used in locomotive -engines do not differ materially from those already described, and -therefore need not be here particularly noticed. From their -horizontal position they have a tendency to wear unequally in the -cylinders, their weight pressing them on one side only; but from -their small magnitude this effect is found to be imperceptible in -practice. In the engine here described the stroke of the piston is -eighteen inches, and this is the most usual length of stroke in -locomotive engines. The piston, in its play, comes at either end -within about half an inch of the inner surface of the covers of -the cylinders, this space being allowed to prevent collision. In -the foremost cover of the cylinder is inserted a cock _q'_ (_fig._ -97. 99.), by which any water which may collect in the cylinder by -condensation or priming may be discharged. A cock _r'_ (_fig._ -97.), communicating with a small tube proceeding from the branches -of the waste pipe _p'_ (_fig._ 104.), is likewise provided to -discharge from that pipe any water which may be [Pg375] collected -in it. After the steam has been admitted to work the piston -through the slide-valve, and has been discharged through the -waste-port by shifting that valve, it passes through the pipe _p'_ -into the blast-pipe _p_, from the mouth of which it issues, with -great force, up the funnel G. When the motion of the engine is -rapid, the steam from the two cylinders proceeds in an almost -uninterrupted current from the blast-pipe, and causes a strong -draft up the chimney. The heated air which passes from the mouths -of the tubes into the smoke-box is drawn up by this current, and a -corresponding draft is produced in the fire-box. - -[Illustration: _Fig._ 107.] - -The piston-rods Y terminate in a fork, by which they are attached -to cross heads Z, the ends of which are confined by guide-bars A', -in which they are allowed to play backwards and forwards through a -space equal to the stroke of the piston. To these cross heads Z, -between the prongs of the fork in which the piston terminates, are -attached the foremost ends of the connecting rods B'. These rods -are, therefore, driven backwards and forwards by the motion -imparted to the cross head Z by the piston-rods Y. The connecting -rods B' are attached at the hinder ends to two cranks formed upon -the axles C' of the driving wheels D'. These two cranks are formed -upon the axles precisely at right angles to each other. The -left-hand crank is represented in its horizontal position, in -_fig._ 99., and the right-hand crank is seen in its vertical -position. A cranked axle is represented on a larger scale in -_fig._ 107., and the two cranks are seen in a position oblique to -the plane of the figure. As this axle is the instrument by which -the impelling force is conveyed to the load, and as it has to -support a great portion of the weight of the engine, it is -constructed with great strength and precision. It is made all in -one [Pg376] piece, and of the best wrought iron called Back -Barrow, or scrap iron. In the engine here described its extreme -length is six feet and a half, and its diameter is five inches. At -the centre part A it is cylindrical, and is increased to five -inches and a quarter at C, where the cranks are formed. The sides -D of the cranks are four inches thick, and the crank pins B, which -are truly cylindrical, are five inches diameter, and three inches -in length, the brasses at the extremities of the connecting rods -which play upon them having a corresponding magnitude. The -distance from the centre of the crank-pins B to the centre of the -axle A must be exactly equal to half the stroke of the piston, and -is, therefore, in this case precisely nine inches. Upon the parts -F, which are seven inches and a half long, the great driving -wheels are firmly fastened, so as to be prevented from turning or -shaking upon the axle. The axle projects beyond the wheels at G, -where it is reduced to three inches and an eighth diameter. These -projecting parts G are five inches long, having collars at the -outer ends. Brasses are fixed at the outside frame of the engine -which rest upon these projections G of the axle, and upon these -brasses the weight of the engine is supported. The entire axle is -accurately turned in a lathe, and each of the crank-pins B is -likewise turned by suspending the axle on centres corresponding -with the centres of the crank-pins, and made on strong cast iron -arms, which are firmly fixed on the ends of the axle, and project -beyond the cranks so as to balance the axle, and enable it to turn -round on the centre of the crank-pin. The axle is by such means -made perfectly true, and the cranks are made of exactly the proper -length, and precisely at right angles to each other. The corners -of the cranks are champered off, as shown in the figure, and the -ends of the cylindrical parts well rounded out. - -The strength and accuracy of construction indispensable in these -cranked axles, in order to make them execute their work, render -them very expensive. Those which are here described cost about -50_l._ each. When properly constructed, however, they are seldom -broken, but are sometimes bent when the engine escapes from the -rails. - -The proper motion to admit and withdraw the steam from [Pg377] -either end of the cylinder is imparted to the slide-valves by -eccentrics, in a manner and on a principle so similar to that -already described in large stationary engines, that it will not be -necessary here to enter into any detailed explanation of the -apparatus for communicating this motion, which is exhibited in -plan and section in _figs._ 97. 99. The eccentrics are attached to -the cranked axles at E' E''. The eccentric E' imparts motion by a -rod _e''_ to a lever _h''_, formed on an axle extending across the -frame of the engine. This conveys motion to another lever _l''_, -projecting from the same axle. This lever _l''_ is jointed to -horizontal links _m''_, which at the foremost ends are attached to -the spindle _l'_, by which the slide is driven. By these means the -motion received by the eccentric from the great working axle -conveys to the spindle _l'_ an alternate movement backwards and -forwards, and the points at which it is reversed will be regulated -by the position given to the eccentric upon the great axle. The -eccentric is formed in two separate semicircles, and is keyed on -to the great axle, and consequently any position may be given to -it which may be required. The position to be given to the -eccentrics should be such that they shall be at right angles to -their respective cranks, and they should be fixed a quarter of a -revolution behind the cranks so as to move the slides to that -extent in advance of the pistons, since by the position of the -levers _h''_ and _l''_, the motion of the eccentric becomes reversed -before it reaches the valve spindle. - -The performance of the engine is materially affected by the -position of the eccentrics on the working axle. The slide should -begin to uncover the steam-port a little before the commencement -of the stroke of the piston, in order that the steam impelling the -piston should be shut off, and the steam about to impel it in the -contrary direction admitted before the termination of the stroke. -Through this small space the steam, therefore, must act in -opposition to the motion of the piston. This is called the _lead_ -of the slide, and the extent generally given to it is about a -quarter of an inch. This is accomplished by fixing the eccentrics -not precisely at right angles to the respective cranks, but a -little in advance of that position. The introduction of the steam -to [Pg378] the piston before the termination of the stroke has -the effect of bringing it gradually to rest at the end of the -stroke, and thereby diminishing the jerk or shock produced by the -rapid change of motion. In stationary engines, where the -reciprocations of the engine are slow, the necessity for this -provision does not arise; but in locomotive engines in which the -motion of the piston is changed from four to six times in a -second, it becomes necessary. The steam admitted to the piston -before the termination of the stroke acts as a spring-cushion to -assist in changing its motion, and if it were not applied, the -piston could not be kept tight upon the piston-rod. Another -advantage which is produced by allowing some lead to the slide is -that the waste steam which has just impelled the piston begins to -make its escape through the waste-port before the commencement of -the next stroke, so that when the impelling steam begins to -produce the returning stroke, there is less waste steam on the -other side of the piston to resist it. - -When the motion of the engine is very rapid, the resistance of the -waste steam, as it escapes from the blast-pipe to the piston, has -been generally supposed to be very considerable, though we are not -aware of any direct experiments by which its amount has been -ascertained. In the account of the locomotive engine which has -been here described, supplied by Mr. Stephenson for the last -edition of Tredgold on the Steam Engine, he states, that the -average resisting pressure of the waste steam throughout the -stroke is 6 lbs. per square inch, when running at the usual rate -of from 25 to 28 miles an hour, and that at greater velocities -this negative pressure has been found to increase to more than -double that amount. No experiments are, however, cited from which -this inference has been drawn. - -It has been also thought that the pressure of steam upon the -piston in the cylinder, at high velocities, is considerably below -the pressure of steam in the boiler; but this has not been, so far -as we are informed, ascertained by any satisfactory experimental -test. Mr. Stephenson likewise states, that this loss of pressure, -causes the negative pressure or resistance of the waste steam to -amount to [Pg379] from 30 to 40 per cent. of the positive -pressure upon the piston when the engine is running very fast, and -that therefore the power of the engine is diminished nearly one -half. - -But it will be perceived that besides the uncertainty which -attends the estimate of the actual amount of pressure on the -piston compared with the pressure of steam in the boiler, the -inference here drawn does not appear to be compatible with what -has been already proved respecting the mechanical effect of steam. -No change of pressure which may take place between the boiler and -the cylinder can affect the practical efficacy of the steam. As -the steam passes through the engine, whatever change of pressure -it may be subject to, it still remains common steam; and though -its pressure may be diminished, its volume being increased in a -nearly equal proportion, its mechanical effect will remain the -same. The power of the engine, therefore, estimated as it ought to -be, by the whole mechanical effect produced, will not be altered -otherwise than by the effect of the increased resistance produced -by the blast-pipe. What that resistance is, we repeat, has not, so -far as we know, been ascertained by direct experiment, and there -are circumstances attending it which render it probable that, even -at high velocities, it is less in amount than Mr. Stephenson's -estimate. - -The position of the eccentrics which is necessary to make the -pistons drive the engine forward must be directly the reverse of -that which would cause them to drive the engine backwards. To be -able, therefore, to reverse the motion of the engine, it would -only be necessary to be able to reverse the position of the -eccentrics, which may be accomplished by either of two expedients. - -_First_, The eccentrics may be capable of revolving on the great -working axle, and also of sliding upon it through a small space. -Their revolution on the axle may be checked by letting a pin -attached to a collar fastened on the axle fall into a hole on the -side of the eccentric. Such a pin will drive the eccentric round -with the axle, and the position of this pin and the hole will -determine the position of the eccentric with reference to the -crank. At a short distance [Pg380] on the other side of the -eccentric may be a corresponding collar with a pin in the opposite -position. By moving the eccentric longitudinally on the axle, the -former pin may be withdrawn from the hole, and the latter allowed -to fall into the hole on the other side. Proper mechanism may be -provided by which the position of the eccentric may thus be -reversed in reference to the crank, and by such means the motion -of the engine may be reversed. - -_Secondly_, Supposing the eccentrics which drive the engine -forward to be immovably fixed upon the axle, two other eccentrics -may be provided attached to other parts of the same axle, and -having a position exactly the reverse with reference to the -cranks. Proper mechanism may be provided, by which either or both -pairs of eccentrics may be thrown in or out of gear. Such are the -means adopted in the engine which has been already described. The -eccentrics for driving the engine backwards are placed outside the -cranks at F' F''. A hand lever _w''_ _fig._ 101. is provided, by -which the engine man may throw either pair of eccentrics into or -out of gear, so as to make the engine work either backwards or -forwards. - -[Illustration: 108.] - -[Illustration: 109.] - -[Illustration: 110.] - -As all the moving parts of the engine require to be constantly -lubricated with oil to diminish the friction, and keep them cool, -oil-cups for this purpose are fixed upon them. In some engines -these oil-cups are attached separately to all the moving parts: in -others they are placed near each other in a row on the boiler, and -communicate by small tubes with the several parts required to be -lubricated. One of these is requisite for each end of the -connecting rods, for each of the guides of the piston-rods, for -the piston-rod itself, the spindle of the slide-valve, and other -parts. An elevation of one of these oil-cups is shown in _fig._ -108., a vertical section in _fig._ 109., and horizontal plan in -_fig._ 110. The cup A is made of brass with a cover B. This cover -has a piece projecting from it turning upon a pin in a socket C at -the side of the cup A, and square at the end, resting upon a small -spring at the bottom of the socket to hold it either open or shut. -In the bottom of the [Pg381] cup is inserted an iron tube D -extending nearly to the top. This tube projects from the bottom of -the cup, where it is tapped for the purpose of fixing the cup on -the part of the engine which it is intended to lubricate. The hole -into which the cup is screwed communicates with the rubbing -surface, and some cotton thread is passed through the tube dipping -into the oil in the cup at the one end and touching the moving -part at the other. This thread acts as a siphon, and constantly -drops oil on the rubbing surface. - -[Illustration: _Fig._ 111.] - -The tender is a carriage attached behind the engine and close to it, -carrying coke for the supply of the furnace, and water for the -boiler. The coke is contained in the space R'', (_fig._ 98. 100.) -surrounded by a tank I'' containing water to feed the boiler. The -feed for the boiler is conducted from the tank through a pipe -descending downwards and in a curved direction, P'' Q'', _fig._ 98., -and connected with a horizontal pipe K, _fig._ 97. A cock is -provided at P'', by which the supply of water to this pipe may be cut -off at pleasure. Another cock is provided at _t'_, _fig._ 97., where -the curved pipe joins the horizontal pipe by which the quantity of -water supplied to K may be regulated by opening the cock more or -less fully. The handle of this cock rises through the floor of the -engine, so that the engineer may regulate it at discretion. The pipe -K being conducted under the engine, as represented in _fig._ 97., -terminates in a vertical pipe, of greater diameter, containing two -valves, both of which open upwards, and between these valves to this -vertical pipe is attached a force-pump, by which the water is drawn -from the horizontal pipe K into the vertical pipe K', and from the -latter is driven into a delivery-pipe by which it is forced into the -boiler. The details of the interior of this feed-pump are -represented on a larger scale in _fig._ 111. The extremity of the -horizontal pipe K' is represented in section at H, where it is -joined on by a screw to the bottom of the vertical pipe which is -represented in _fig._ 97. at K, and which is here represented in -section. The vertical pipe, represented in _fig._ 97. consists of -several parts screwed together by nuts and bolts passing through -flanges. The lowest piece I is attached by a flange to the piece L: -within these is contained the valve Q resting in a seat made -conical, so that the ball [Pg382] which forms the valve shall rest -in water-tight contact with it. The ball is turned and ground to an -accurate sphere, and whatever position it assumes upon its seat its -contact will be perfect. It is guided in its upward and downward -motion by several vertical bars which confine it, and which are -united at the top, so as to limit the upward motion of the ball. A -screw V' is inserted in the bottom of the piece I, by removing which -access can be obtained to the valve. The piece L is secured to the -short pipe G by nuts and bolts passed through a flange. The pipe G -is cast upon the end of the feed-pump A. On the foremost end of this -feed-pump is constructed a stuffing-box C of the usual form, having -a gland D forced against packing by nuts and screws E. The plunger B -is turned so as to be truly cylindrical, and moves in water-tight -contact through the gland D. The plunger not being in contact with -the inner surface of the pump-barrel A, the latter need not be -ground. The horizontal rod by which the plunger B is driven is -attached at its foremost extremity to an arm which projects from the -rod of the steam-piston, and consequently this plunger is moved -through a space equal to the stroke of the steam-piston. In this -case that space is eighteen inches. The [Pg383] upper end of the -vertical tube G is attached by screws and a flange to a piece P -containing a valve R similar in all respects to the lower valve Q, -and like it opening upwards. A screw V is introduced at the top by -which access may be obtained to this valve. This screw also presses -on the crown of the guides of the valve, so as to hold it down by -regulated pressure. At the side of this upper piece P is inserted a -horizontal tube M connected with the end of the delivery-pipe N. -This latter is continued to the boiler with which it communicates at -the fire-box. When the plunger B is drawn out of the pump-barrel A, -the spherical valve Q being relieved from its downward pressure is -raised, and water passes from the pipe H through the valve Q into -the vertical pipe G; the lower valve Q then closes and stops the -return of the water. The plunger B returning into the pump-barrel A -then forces the water against the upper valve R and drives it -through the delivery-tube N, from which its return is prevented by -the valve R. When the delivery-tube N is filled with water -throughout its whole length, every stroke of the plunger will -evidently drive into the boiler a volume of water equal to the -magnitude of a part of the plunger eighteen inches in length. - -Until within the last few years, locomotive engines were supported -on only four wheels; they are, however, now almost universally -supported on six, the driving wheels being in the middle. To give -greater security to the position of the engine between the rails -it is usual to construct flanges on the tires of all the six -wheels. Mr. Stephenson, however, has been in the practice of -constructing the driving wheels without flanges, and with tires -truly cylindrical, depending on the flanges of the two pairs of -smaller wheels to maintain the engine between the rails. The -wheels of the engine here described are constructed in this -manner. The driving wheels D' are fixed on the cranked axle C', -and are five feet in diameter. The other wheels L' M', the one -being placed immediately behind the smoke-box, and the other -immediately behind the fire-box, are each three feet six inches in -diameter, and have a flange upon their tires, which running on -the [Pg384] inside of each rail keeps the engine between the -rails. Each pair of these small wheels, like the driving-wheels, -is fixed upon their axle. The axles are 3-5/8 inches diameter, and -project beyond the wheels, the projecting part supporting the -frame of the engine and turning in brasses. Upon these brasses -rest springs, which bear the whole weight of the engine. These -springs having nothing between them and the road but the wheels -and axles intercept and equalise the sudden shocks produced by the -rapid motion upon the road. - -When an engine is required for the transport of very heavy loads, -such as those of merchandise, the adhesion of one pair of working -wheels is found to be insufficient, and, in such cases, one of the -two pairs of wheels L' M' is made of the same diameter as the -wheels which are placed upon the working axle, and a bar is -attached to points on the outside of the wheels at equal distances -from their centre, connecting them in such a manner that any force -applied to make one pair of wheels revolve must necessarily impart -the same motion to the other pair. By such means the force of the -steam is made to drive both pairs of wheels, and consequently a -proportionally increased adhesion is obtained. - -The velocity which an engine is capable of imparting to the load -which it draws depends upon the rate at which the pistons are -capable of being moved in the cylinders. By every motion of each -piston backwards and forwards one revolution of the driving wheels -is produced, and by each revolution of the driving wheels, -supposing them not to slip upon the rails, the load is driven -through a distance upon the road equal to their circumference. As -the two cylinders work together, it follows, that a quantity of -steam sufficient to fill four cylinders supplied by the boiler to -the engine will move the train through a distance equal to the -circumference of the driving wheels; and in accomplishing this, -each piston must move twice from end to end of the cylinder; each -cylinder must be twice filled with steam from the boiler; and that -steam must be twice discharged from the cylinder through the -blast-pipe into the chimney. - -[Pg401] If the driving wheels be five feet in diameter their -circumference will be fifteen feet seven inches. To drive a train -with a velocity of thirty miles an hour, it will be necessary that -the engine should be propelled through a space of forty-five feet -per second. To accomplish this with five-feet wheels they must be -therefore made to revolve at the rate of very nearly three -revolutions per second; and as each revolution requires two -motions of the piston in the cylinder, it follows that each piston -must move three times forwards and three times backwards in the -cylinder in a second; that steam must be admitted six times per -second from the steam-chest to each cylinder, and discharged six -times per second from each cylinder into the blast-pipe. The -motion, therefore, of each piston, supposing it to be uniform, -must divide a second into six equal parts, and the puffs of the -blast-pipe in the chimney must divide a second into twelve equal -parts. The motion of the slides and other reciprocating parts of -the machinery must consequently correspond. - -This motion of the reciprocating parts of the machinery being -found to be injurious to it, and to produce very rapid wear, -attempts have been made to remedy the defect, and to obtain -greater speed with an equal or diminished rate of motion of the -piston, by the adoption of driving wheels of greater diameter, and -on several of the great lines of railway the magnitude of the -wheels for the passenger-engines have been increased to five feet -and a half and six feet diameter; but such engines have not been -sufficiently long in use to afford grounds for forming a practical -estimate of their effects. Experiments of a much bolder -description have, however, been tried on one of the great lines of -railway by the adoption of driving wheels of much greater -diameter. In some cases their magnitude has been increased even to -ten feet; but from various experiments to which these engines have -been submitted by myself and others, as well as from the -experience which appears to be obtained from the results of their -ordinary work, it does not appear that any advantages have -attended them, and they have been accordingly for the most part -abandoned. - -The pressure of steam in the boiler is limited by two -safety-valves, [Pg402] represented in _fig._ 97. at N and O. The -valve at N is under the control of the engineer, but the valve at -O is inaccessible to him. The structure of the safety-vale -represented at N is exhibited on a larger scale in _fig._ 112., -which represents its section, and _fig._ 113., which shows a plan -of the valve-seat with the valve removed. The valve A, which is -made of brass, is mitred round the edge at an angle of 45 deg., and -has a spindle, or stalk B, cast upon it, projecting downwards from -the middle of it. The valve-seat C is also made of brass, and cast -with a flange at the bottom to attach it to the boiler. The mitred -surface of the valve is ground into the valve-seat, so as to rest -in steam-tight contact with it. Across the valve-seat, which is -two and a half inches in diameter, is cast a thin piece D, seen in -plan in _fig._ 113. and in section in _fig._ 112. which extends -from the top to the bottom, and has a longitudinal hole through -it, in which the spindle B of the valve works: by this hole it is -guided when it rises from its seat. A projection E is cast upon -the seat of the valve, in which a standard F is inserted. This -standard is forked at the top, and receives the end of a lever G, -which turns in it upon a centre. A rod H is jointed to this lever -by another pin at three inches from the former, and the lower end -of this rod, ground to a point, presses upon the centre of the -valve A. At the other end of the lever, which is broken off in -_fig._ 112., at a distance of three feet from the centre pin, -inserted in the fork of the pillar F, the rod of a common -spring-balance _w_, _fig._ 101., is attached by a finger-nut _n_. -The bottom of this spring-balance is secured on to the fire-box. -This balance is screwed up by the finger-nut on the valve-lever -until the required pressure on the lever is produced through the -medium of the rod H, this pressure being generally fifty pounds -per square inch above the atmosphere. When the pressure of the -steam in the boiler exceeds this, the valve A is raised from its -seat, and the steam escapes. - -[Illustration: _Fig._ 112.] - -[Illustration: _Fig._ 113.] - -It is evident that the sliding weight by which the pressure -[Pg403] of the safety-valve is sometimes regulated in stationary -engines would not be admissible in a locomotive engine, since the -motion of the engine would constantly jolt it up and down, and -cause the steam to escape. One of the disadvantages attending the -use of the spring-valve is that it cannot be opened to let the -steam escape without increasing its force, so that the steam, when -escaping, must really have a greater pressure than that to which -the valve has been previously adjusted. The longer the lever is, -the greater will be this difference of pressure, inasmuch as a -given elevation of the pin governing the rod H would cause a -proportionally greater motion in that end of the lever attached to -the spring. - -The second safety-valve O is enclosed in a case, so that it is -inaccessible, and its purpose is to limit the power of the -engineer to increase the pressure of steam in the boiler. This -valve is similar in construction to the former, but instead of -being held down by a lever, is pressed upon by several small -elliptical springs placed one above another over the valve, and -held down by a screw which turns in a frame Y, fixed into the -valve-seat. By this screw the pressure on the valve can be -adjusted to any required degree; and if the open safety-valve be -screwed down to a greater pressure, the steam will begin to escape -from this second valve. - -Also in the case where the boiler produces surplus steam faster -than its escape can be effected at the valve N, the pressure will -sometimes be increased until the valve O is opened, and its escape -will take place from both valves. - -The whole weight of the engine bears upon those parts of the six -axles R', _fig._ 99., which project beyond the wheels. Boxes are -formed in which these parts of the axles turn, and through the -medium of which the weight of the engine rests upon them. Over -these boxes are constructed oil or grease cups, by means of which -the axles are constantly lubricated. It is usual to lubricate the -axles of the engine itself with oil: the axles of the tender, and -other coaches and waggons, are lubricated with a mixture of oil -and tallow. In the middle of the box in which the axle turns, and -between the two oil-cups, is cast a socket, in which the end of -the spindle on [Pg404] which the spring presses rests. The -springs are composed of a number of steel-plates, laid, in the -usual manner, one above the other, increasing in length upwards. -In the engine here described, the plates forming the springs of -the driving wheels are thirteen in number, each of which is four -inches in width, and 5/16ths of an inch in thickness. The springs -upon the other wheels are three inches in width. The springs of -the driving wheels are below the axle, while those of the smaller -wheels are above it. - -Buffers D'' are placed behind the tender, which act upon a spring C -(_fig._ 100.), to break the collision, when the waggons or -carriages strike upon the tender, and similar buffers are attached -to all passenger-coaches. Some of these buffers are constructed -with a system of springs similar to C, but more elastic, and -combined in greater number under the framing of the carriage, so -that a considerable play is allowed to them. In some cases the -rods of the buffers are made to act upon strong spiral springs -inserted in the sides of the framing of the carriage. This -arrangement gives greater play to the buffers; and as every coach -in a train has several buffers, the combined effect of these is -such, that a considerable shock given to either end of the train -may be rendered harmless by being spent upon the elasticity of -these several systems of springs. - -In order to give notice of the approach of a train, a -steam-whistle Z', _fig._ 97. 101., is placed immediately above the -fire-box at the back of the engine. This is an apparatus composed -of two small hemispheres of brass, separated one from the other by -a small space. Steam is made to pass through a hollow space -constructed in the lower hemisphere, and escapes from a very -narrow circular opening round the edge of that hemisphere, rushing -up with a force proportionate to its pressure. The edge of the -upper hemisphere presented downwards encounters this steam, and an -effect is produced similar to the action of air in organ pipes. A -shrill whistle is produced, which can be heard at a very -considerable distance, and, differing from all ordinary sounds, it -never fails to give timely notice of the approach of a train. - -The water tank I'', _fig._ 98. 100., which is constructed on the -tender, is formed of wrought-iron plates 1/8 of an inch thick, -[Pg405] riveted at the corners by angle iron already described. -This tank is 9 feet long, 6-3/4 feet wide, and 2-1/4 feet deep. -The top is covered with a board K'', and a raised platform N'' is -constructed behind, divided into three parts, covered with leads, -which open on hinges. The middle lid covers an opening to the tank -by which water is let in: the lids at either side cover boxes in -which are contained the tools necessary to be carried with the -engine. The curved pipe P'', _fig._ 98., leading from the bottom of -the tank to the pipe Q'', is of copper. The pipe Q'', connecting the -latter with the feed-pipe K', _fig._ 99., is sometimes formed of -leather or India-rubber cloth, having a spiral spring on the -inside to prevent it from collapsing. It is necessary that this -pipe Q'' should have a power of yielding to a sufficient degree to -accommodate itself to the inequalities of motion between the -engine and tender. A metal pipe is sometimes used, supplied with a -double ball and socket, and a telescopic joint, having sufficient -play to allow for the lateral and longitudinal inequalities of -motion of the engine and tender. The weight of an engine, such as -that here described, supplied with its proper quantity of water -and fuel, is about 12 tons: the tender, when empty, weighs about -3-1/4 tons; and when filled with water and fuel its weight is 7 -tons. The tank contains 700 gallons of water, and the tender is -capable of carrying about 800 weight of coke. This supply is -sufficient for a trip of from thirty to forty miles with an -ordinary load. - - -(198.) It is not usual to express the power of locomotive engines -in the same manner as that of other engines by the term -horse-power. Indeed, until the actual amount of resistance opposed -to these machines, under the various circumstances in which they -are worked, shall be ascertained with some degree of precision, it -is impossible that their power or efficiency can be estimated with -any tolerable degree of approximation. The quantity of water -evaporated, and passed in steam through the cylinders, supplies a -major limit to the power exerted; but even this necessary element -for the calculation of the efficacy of these machines has not been -ascertained by a sufficiently extensive course of observation and -experiment. Mr. Stephenson states, that the engine which [Pg406] -has been here described is capable of evaporating 77 cubic feet of -water per hour, while the early locomotives could only evaporate -16 cubic feet per hour. This evaporation, however, is inferior to -that which I have ascertained myself to be produced by engines in -regular operation on some of the northern railways. In an -experiment made in July, 1839, with the Hecla engine, I found that -the evaporation in a trip of ninety-five miles, from Liverpool to -Birmingham, was at the rate of 93.2 cubic feet per hour, and in -returning the same distance it was at the rate of 85.7 cubic feet -per hour, giving a mean of 89 cubic feet per hour nearly. The -Hecla weighed 12 tons; and its dimensions and proportions -corresponded very nearly with those of the engine above described. - -In a course of experiments which I made upon the engines then in -use on the Grand Junction Railway in the autumn of 1838 I found -that the ordinary evaporating power of these engines varied from -eighty to eighty-five cubic feet per hour. - -Engines of much greater dimensions, and consequently of greater -evaporating power, are used on the Great Western Railway. In the -autumn of 1838 experiments were made upon these engines by Mr. -Nicholas Wood and myself, when we found that the most powerful -engine on that line, the North Star, drawing a load of 110-1/2 -tons gross, engine and tender inclusive, at 30-1/2 miles an hour, -evaporated 200 cubic feet of water per hour. The same engine -drawing a load of 194-1/2 tons at 18-1/2 miles an hour evaporated -141 cubic feet per hour, and when drawing 45 tons at 38-1/2 miles -an hour evaporated 198 cubic feet of water per hour. - -It has been already shown that a cubic foot of water evaporated -per hour produces a gross amount of mechanical force very little -less than two-horse power, and consequently the gross amount of -mechanical power evolved in these cases by the evaporation of the -locomotive boilers will be very nearly twice as many horse-power -as there are cubic feet of water evaporated per hour. Thus the -evaporation of the Hecla, in the experiments made in July, 1839, -gave a gross power of about one hundred and eighty horses, while -the evaporation of the North Star gave a power of about four -hundred horses. In stationary engines about half the gross -[Pg407] power evolved in the evaporation is allowed for waste, -friction, and other sources of resistance not connected with the -load. What quantity should be allowed for this in locomotive -engines is not yet ascertained, and therefore it is impossible to -state what proportion of the whole evaporation is to be taken as -representing the useful horse-power. - - -(199.) The great uniformity of resistance produced by the traction -of carriages upon a railway is such as to render the application -of steam power to that purpose extremely advantageous. So far as -this resistance depends on mechanical defects, it is probably -rendered as uniform as is practicable, and in proportion to the -quantity of load carried is reduced to as small an amount as it is -likely to attain under any practicable circumstances. Until a -recent period this resistance was ascribed altogether, or nearly -so, to mechanical causes. The inequalities of the road-surface, -the friction of the axles of the wheels in their bearings, and the -various sources of resistance due to the machinery of the engine, -being the principal of these resistances, were for the most part -independent of the speed with which the train was moved; and it -was accordingly assumed in all calculations respecting the power -of locomotive engines that the resistance would be practically the -same whatever might be the speed of the train. It had been well -understood that so far as the atmosphere might offer resistance to -the moving power this would be dependent on the speed, and would -increase in a very high ratio with the speed; but it was -considered that the part of the resistance due to this cause -formed a fraction of the whole amount so insignificant that it -might be fairly disregarded in practice, or considered as a part -of the actual computed resistance taken at an average speed. - -It has been, until a late period, accordingly assumed that the -total amount of resistance to railway trains which the locomotive -engines have had to overcome was about the two hundred and -fiftieth part of the gross weight of the load drawn: some -engineers estimated it at a two hundred and twentieth; others at a -two hundred and fiftieth; others at a three hundred and thirtieth -part of the load; and the two hundred and fiftieth part of the -gross load drawn may perhaps be [Pg408] considered as a mean -between these much varying estimates. What the experiments were, -if any, on which these rough estimates were based, has never -appeared. Each engineer formed his own valuation of this effect, -but none produced the experimental grounds of their opinion. It -has been said that the trains run down the engine, or that the -drawing chains connecting the engine slacken in descending an -inclination of sixteen feet in a mile, or 1/330. Numerous -experiments, however, made by myself, as well as the constant -experience now daily obtained on railways, show that this is a -fallacious opinion, except at velocities so low as are never -practised on railways. - - -(200.) In the autumn of 1838 a course of experiments was commenced -at the suggestion of some of the proprietors of the Great Western -Railway Company, with a view to determine various points connected -with the structure and the working of railways. A part of these -experiments were intended to determine the mean amount of the -resisting force opposed to the moving power, and this part was -conducted by me. After having tried various expedients for -determining the mean amount of resistance to the moving power, I -found that no method gave satisfactory results except one founded on -observing the motion of trains by gravity down steep inclined -planes. When a train of waggons or coaches is placed upon an -inclined plane so steep that it shall descend by its gravity without -any moving power, its motion, when it proceeds from a state of rest, -will be gradually accelerated, and if the resistance to that motion -was, as it has been commonly supposed to be, uniform and independent -of the speed, the descent would be uniformly accelerated: in other -words, the increase of speed would be proportional to the time of -the motion. Whatever velocity the train would gain in the first -minute, it would acquire twice that velocity at the end of the -second minute, three times that velocity at the end of the third -minute, and so on; and this increase of velocity would continue to -follow the same law, however extended the plane might be. That such -would be the law which the descending motion of a train would follow -had always been supposed, up to the time of the experiments now -referred to; and it was even maintained by some that [Pg409] such a -law was in strict conformity with experiments made upon railways and -duly reported. The first experiments instituted by me at the time -just referred to afforded a complete refutation of this doctrine. It -was found that the acceleration was not uniform, but that with every -increase of speed the acceleration was lessened. Thus if a certain -speed were gained by a train in one second when moving at five miles -an hour, a much less speed was gained in one second when moving ten -miles an hour, and a comparatively small speed was gained in the -same time when moving at fifteen miles an hour, and so on. In fact, -the augmentation of the rate of acceleration appeared to diminish in -a very rapid proportion as the speed increased: this suggested to me -the probability that a sufficiently great increase of speed would -destroy all acceleration, and that the train would at length move at -a uniform velocity. In effect, since the moving power which impels a -train down an inclined plane of uniform inclination is that fraction -of the gross weight of the train which acts in the direction of the -plane, this moving power must be necessarily invariable; and as any -acceleration which is produced must arise from the excess of this -moving power over the resistance opposed to the motion of the train, -from whatever causes that resistance may arise, whenever -acceleration ceases, the moving force must necessarily be equal to -the resistance; and therefore, when a train descends an inclined -plane with a uniform velocity, the gross resistance to the motion of -the train must be equal to the gross weight of the train resolved in -the direction of the plane; or, in other words, it must be equal to -that fraction of the whole weight of the train which is expressed by -the inclination of the plane. Thus if it be supposed that the plane -falls at the rate of one foot in one hundred, then the force -impelling the train downwards will be equal to the hundredth part of -the weight of the train. So long as the resistance to the motion of -the train continues to be less than the hundredth part of its -weight, so long will the motion of the train be accelerated; and the -more the hundredth part of the weight exceeds the resistance, the -more rapid will the acceleration be; and the less the hundredth part -of the weight [Pg410] exceeds the resistance, the less rapid will -the acceleration be. If it be true that the amount of resistance -increases with the increase of speed, then a speed may at length be -attained so great that the amount of resistance to the motion of the -train will be equal to the hundredth part of the weight. When that -happens, the moving power of a hundredth part of the weight of the -train being exactly equal to the resistance to the motion, there is -no excess of power to produce acceleration, and therefore the motion -of the train will be uniform. - -Founded on these principles, a vast number of experiments were -made on planes of different inclinations, and with loads of -various magnitudes; and it was found, in general, that when a -train descended an inclined plane, the rate of acceleration -gradually diminished, and at length became uniform; that the -uniform speed thus attained depended on the weight, form, and -magnitude of the train and the inclination of the plane; that the -same train on different inclined planes attained different uniform -speeds--on the steeper planes a greater speed being attained. From -such experiments it followed, contrary to all that had been -previously supposed, that the amount of resistance to railway -trains had a dependence on the speed; that this dependence was of -great practical importance, the resistance being subject to very -considerable variation at different speeds, and that this source -of resistance arises from the atmosphere which the train -encounters. This was rendered obvious by the different amount of -resistance to the motion of a train of coaches and to that of a -train of low waggons of equal weight. - -The former editions of this work having been published before the -discovery which has resulted from these experiments, the average -amount of resistance to railway trains, there stated, and the -conclusions deduced therefrom, were in conformity with what was -then known. It was stated that the resistance to the moving power -was practically independent of the speed, and on level rails was -at the average rate of about seven pounds and a half per ton. This -amount would be equivalent to the gravitation of a load down an -inclined plane falling 1/300, and consequently in ascending such a -plane the moving power would have to encounter twice [Pg411] the -resistance opposed to it on a level. As it was generally assumed -that a locomotive-engine could not advantageously vary its -tractive power beyond this limit, it was therefore inferred that -gradients (as inclinations are called) ought not to be constructed -of greater steepness than 1/300. It was supposed that in -descending gradients more steep than this the train would be -accelerated and would require the use of the brake to check its -motion, while in ascending such planes the engine would be -required to exert more than twice the ordinary tractive power -required on level rails. As the resistance produced by the air was -not taken into consideration, no distinction was made between -heavy trains of goods presenting a frontage and magnitude bearing -a small proportion to their gross weight and lighter trains of -passenger-coaches presenting great frontage and great magnitude in -proportion to their weight. The result of the experiments above -explained leads to inferences altogether at variance with those -which have been given in former editions of the present work, and -which were then universally admitted by railway engineers. The -tendency of the results of these experiments show that low -gradients on railways are not attended with the advantageous -effects which have been hitherto ascribed to them; that, on the -contrary, the resistance produced by steeper gradients can be -compensated by slackening the speed, so that the power shall be -relieved from as much atmospheric resistance by the diminution of -velocity as is equal to the increased resistance produced by the -gravity of the plane which is ascended. And, on the other hand, in -descending the plane the speed may be increased until the -resistance produced by the atmosphere is increased to the same -amount as that by which the train is relieved of resistance by the -declivity down which it moves. Thus, on gradients, the inclination -of which is confined within practical limits, the resistance to -the moving-power may be preserved uniform, or nearly so, by -varying the velocity. - - -(201.) The series of experiments which have established these -general conclusions have not yet been sufficiently extended -and varied to supply a correct practical estimate of the limit -which it would be most advantageous to impose upon the [Pg412] -gradients of railways; but it is certain that railways may be laid -down, without practical disadvantage, with gradients considerably -steeper than those to which it has been hitherto the practice to -recommend as a limit. - -The principle of compensation by varied speed being admitted, it -will follow that the time of transit between terminus and terminus -of a line of railway laid down with gradients, varying from twenty -to thirty feet a mile, will be practically the same as it would be -on a line of the same length constructed upon a dead level; and -not only will the time of transport be equal, but the quantity of -moving power expended will not be materially different. The -difference between the circumstances of the transport in the two -cases will be merely that, on the undulating line, a varying -velocity will be imparted to the train and a varying resistance -opposed to the moving power; while on the level line the train -would be moved at a uniform speed, and the engine worked against a -uniform resistance. These conclusions have been abundantly -confirmed by the experiments made in last July with the Hecla -engine above referred to. The line of railway between Liverpool -and Birmingham on which the experiment was made extended over a -distance of ninety-five miles, and the gradients on which the -effects were observed varied from a level to thirty feet per mile, -a great portion of the line being a dead level. The following -table shows the uniform speed with which the train ascended and -descended the several gradients, and also the mean of the ascent -and descent in each case, as well as the speed upon the level -parts of the line:-- - - -------------------------------------------------- - | Speed. | - |---------------------------------------- - Gradient.| Ascending. | Descending. | Mean. - -------------------------------------------------- - One in |Miles per hour.|Miles per hour.| - 177 | 22.25 | 41.32 | 31.78 - 265 | 24.87 | 39.13 | 32.00 - 330 | 25.26 | 37.07 | 31.16 - 400 | 26.87 | 36.75 | 31.81 - 532 | 27.35 | 34.30 | 30.82 - 590 | 27.37 | 33.16 | 30.21 - 650 | 29.03 | 32.58 | 30.80 - | | |-------- - Level | | | 30.93 - -------------------------------------------------- - -[Pg413] From this table it is apparent that the gradients do -possess the compensating power with respect to speed already -mentioned. The discrepancies existing among the mean values of the -speed are only what may be fairly ascribed to casual variations in -the moving power. The experiment was made under favourable -circumstances: little disturbance was produced from the -atmosphere; the day was quite calm. In the same experiment it was -found that the water evaporated varied very nearly in proportion -to the varying resistance, and the amount of that evaporation may -be taken as affording an approximation to the mean amount of -resistance. Taking the trip to and from Birmingham over the -distance of 190 miles, the mean evaporation per mile was 3.36 -cubic feet of water. The volume of steam produced by this quantity -of water will be determined approximately by calculating the -number of revolutions of the driving wheels necessary to move the -engine one mile. The driving wheels being 5 feet in diameter, -their circumference was 15.7 feet, and consequently in passing -over a mile they would have revolved 336.3 times. Since each -revolution consumes four cylinders full of steam, the quantity of -steam supplied by the boiler to the cylinders per mile will be -found by multiplying the contents of the cylinder by four times -336.3, or 1345.2. - -The cylinders of the Hecla were 12-1/2 inches diameter, and 18 -inches in length, and consequently their contents were 1.28 cubic -feet for each cylinder: this being multiplied by 1345.2 gives -1721.86 or 1722 cubic feet of steam per mile. It appears, -therefore, that supposing the priming either nothing or -insignificant, which was considered to be the case in these -experiments, 3.36 cubic feet of water produced 1722 cubic feet of -steam, of the density worked in the cylinders. The ratio, -therefore, of the volume of this steam to that of the water -producing it, was 1722 to 3.36, or 512.5 to 1. The pressure of -steam of this density would be 54.5 pounds per square inch.[34] -Such, therefore, was the limit of the average total pressure of -the steam in the cylinders. In this experiment the safety-valve of -the boiler was screwed down to 60 pounds per square [Pg414] inch -above the atmospheric pressure, which was therefore the major -limit of the pressure of steam in the boiler; but as the actual -pressure in the boiler must have been less than this amount, the -difference between the pressure in the cylinder and boiler could -not be ascertained. This difference, however, would produce no -effect on the moving power of the steam, since the pressure of -steam in the cylinders obtained by the above calculation is quite -independent of the pressure in the boiler, or of any source of -error except what might arise from priming. The pressure of 54.5 -pounds per square inch, calculated above, being the total pressure -of the steam on the pistons, let 14.5 pounds be deducted from it, -to represent the atmospheric pressure against which the piston -must act, and the remaining 40 pounds per square inch will -represent the whole available force drawing the train and -overcoming all the resistances arising from the machinery of the -engine, including that of the blast-pipe. The magnitude of a -12-1/2 inch piston being 122.7 square inches, the total area of -the two pistons would be 245.2 square inches, and the pressure -upon each of 40 pounds per inch would give a total force of 9816 -on the two pistons. Since this force must act through a space of -three feet, while the train is impelled through a space of 15.7 -feet, it must be reduced in the proportion of 3 to 15.7, to obtain -its effect at the point of contact of the wheels upon the rails: -this will give 1875 pounds as the total force exerted in the -direction of the motion of the train. The gross weight of the -train being 80 tons, including the engine and tender, this would -give a gross moving force along the road of about 23.4 pounds per -ton of the gross load, this force being understood to include all -the resistances due to the engine. This resistance corresponds to -the gravitation of a plane rising at the rate of 1/95, and -therefore it appears that such would be the inclination of the -plane by the gravitation of which the gross resistance would be -doubled, instead of such inclination being about 1/300, as has -been hitherto supposed. - -Since the remarkable and unexpected results of this series of -experiments became known various circumstances were brought to -light, which were before unnoticed, and which [Pg415] abundantly -confirm them. Among these may be mentioned the fact, that in -descending the Madeley plane, on the Grand Junction Railway, which -falls for above three miles at the rate of twenty-nine feet a -mile, the steam can never be entirely cut off. But, on the other -hand, to maintain the necessary speed in descending, the power of -the engine is always necessary. As this plane greatly exceeds that -which would be sufficient to cause the free motion of the train -down it, the power of the engine expended in descending it, -besides all that part of the gravitating power of the plane which -exceeds the resistance due to friction and other mechanical causes -must be worked against the atmosphere. - -This estimate of the resistance is also in conformity with the -results of a variety of experiments made by me with trains of -different magnitudes down inclined planes of various inclinations. - - -(202.) In laying out a line of railway the disposition of the -gradients should be such as to preserve among them as uniform a -character as is practicable, for the weight and power of the -engine must necessarily be regulated by the general steepness of -the gradients. Thus if upon a railway which is generally level, -like that between Liverpool and Manchester, one or two inclined -planes of a very steep character occur, as happens upon that line, -then the engine which is constructed to work upon the general -gradients of the road is unfit to draw the same load up those -inclinations which form an exception to the general character of -the gradients. In such cases some extraordinary means must -generally be provided for surmounting those exceptionable -inclinations. Several expedients have been proposed for this -purpose, among which the following may be mentioned:-- - -1. Upon arriving at the foot of the plane the load is divided, and -the engine carries it up in several successive trips, descending -the plane unloaded after each trip. The objection to this method -is the delay which it occasions--a circumstance which is -incompatible with a large transport of passengers. From what has -been stated, it would be necessary, when the engine is fully -loaded on a level, to divide its load into two or more parts, to -be successively [Pg416] carried up when the incline rises 52 feet -per mile. This method has been practised in the transport of -merchandise occasionally, when heavy loads were carried on the -Liverpool and Manchester line, upon the Rainhill incline. - -2. A subsidiary or assistant locomotive engine may be kept in -constant readiness at the foot of each incline, for the purpose of -aiding the different trains, as they arrive, in ascending. The -objection to this method is the cost of keeping such an engine -with its boiler continually prepared, and its steam up. It is -necessary to keep its fire continually lighted, whether employed -or not; otherwise, when the train would arrive at the foot of the -incline, it should wait until the subsidiary engine was prepared -for work. In cases where trains would start and arrive at stated -times, this objection, however, would have less force. This method -is at present generally adopted on the Liverpool and Manchester -line. - -3. A fixed steam-engine may be erected on the crest of the -incline, so as to communicate by ropes with the train at the foot. -Such an engine would be capable of drawing up one or two trains -together, with their locomotives, according as they would arrive, -and no delay need be occasioned. This method requires that the -fixed engine should be kept constantly prepared for work, and the -steam continually up in the boiler. - -4. In working on the level, the communication between the boiler -and the cylinder in the locomotives may be so restrained by -partially closing the throttle-valve, as to cause the pressure -upon the piston to be less in a considerable degree than the -pressure of steam in the boiler. If under such circumstances a -sufficient pressure upon the piston can be obtained to draw the -load on the level, the throttle-valve may be opened on approaching -the inclined plane, so as to throw on the piston a pressure -increased in the same proportion as the previous pressure in the -boiler was greater than that upon the piston. If the fire be -sufficiently active to keep up the supply of steam in this manner -during the ascent, and if the rise be not greater in proportion -than the power thus obtained, the locomotive will draw the load up -the incline without further assistance. It is, however, to be -observed, that in this case [Pg417] the load upon the engine must -be less than the amount which the adhesion of its working wheels -with the railroad is capable of drawing; for this adhesion must be -adequate to the traction of the same load up the incline, -otherwise, whatever increase of power might be obtained by opening -the throttle-valve, the drawing wheels would revolve without -causing the load to advance. This method has been generally -practised upon the Liverpool and Manchester line in the transport -of passengers; and, indeed, it is the only method yet discovered -which is consistent with the expedition necessary for that species -of traffic. - -In the practice of this method considerable aid may be derived -also by suspending the supply of feeding water to the boiler -during the ascent. It will be recollected that a reservoir of cold -water is placed in the tender which follows the engine, and that -the water is driven from this reservoir into the boiler by a -forcing pump, which is worked by the engine itself. This pump is -so constructed that it will supply as much cold water as is equal -to the evaporation, so as to maintain constantly the same quantity -of water in the boiler. But it is evident, on the other hand, that -the supply of this water has a tendency to check the rate of -evaporation, since in being raised to the temperature of the water -with which it mixes it must absorb a considerable portion of the -heat supplied by the fire. With a view to accelerate the -production of steam, therefore, in ascending the inclines, the -engine man may suspend the action of the forcing pump, and thereby -stop the supply of cold water to the boiler; the evaporation will -go on with increased rapidity, and the exhaustion of water -produced by it will be repaid by the forcing pump on the next -level, or still more effectually on the next descending incline. -Indeed the feeding pump may be made to act in descending an -incline, if necessary, when the action of the engine itself is -suspended, and when the train descends by its own gravity, in -which case it will perform the part of a brake upon the descending -train. - -5. The mechanical connexion between the piston of the cylinder and -the points of contact of the working wheels with the road may be -so altered, upon arriving at the incline, as to [Pg418] give the -piston a greater power over the working wheels. This may be done -in an infinite variety of ways, but hitherto no method has been -suggested sufficiently simple to be applicable in practice; and -even were any means suggested which would accomplish this, unless -the intensity of the impelling power were at the same time -increased, it would necessarily follow that the speed of the -motion would be diminished in exactly the same proportion as the -power of the piston over the working wheels would be increased. -Thus, on the inclined plane, which rises fifty-five feet per mile, -upon the Liverpool line, the speed would be diminished to nearly -one fourth of its amount upon the level. - -[Illustration] - - FOOTNOTES: - - [30] Some of the preceding observations on inland transport, - as well as other parts of the present chapter, appeared in - articles written by me in the _Edinburgh Review_ for October, - 1832, and October, 1834. - - [31] Wood on Railroads, 2d edit. - - [32] The cost of coke has risen considerably since the date of - this report. - - [33] I am indebted to the enlarged edition of Tredgold on the - Steam Engine, published by Mr. Weale, for the drawings of this - engine. The details of the machine are very fully given in - that work, the description of them being supplied by Mr. - Stephenson himself. - - [34] See Table of Pressures, Temperatures, and Volumes, in - appendix. - -[Pg419] - - - - -[Illustration] - -CHAP. XII. - -LOCOMOTIVE ENGINES ON TURNPIKE ROADS. - - RAILWAYS AND STONE ROADS COMPARED. -- MR. GURNEY'S STEAM - ENGINE. -- CONVENIENCE AND SAFETY OF STEAM CARRIAGES. -- - HANCOCK'S STEAM ENGINE. -- OGLE'S STEAM ENGINE. -- TREVETHICK'S - INVENTION. -- DR. CHURCH'S STEAM ENGINE. - - -(203.) We have hitherto confined our observations on steam-power, -as a means of transport by land, to its application on railways. -But modern speculation has not stopped there; various attempts -have been made, and attended with more or less success, to work -steam-carriages on common roads. The mere practicability of this -project had long been regarded as very questionable; but enough -has been done to show that the only doubt which can attend it, is -as to whether it can be profitably resorted to, as a means of -transport, and this question [Pg420] has been materially affected -by the recent extension of railways. In comparing the effect of a -stone road with an iron railway, there are two circumstances which -give great superiority and advantage to the latter: first, the -resistance opposed by a railway to the moving power, no matter -what that moving power may be, is considerably less in proportion -to the load than on a stone road. The average resistance on a good -level stone road, to the motion of carriages drawn at the speed -usually attained by the application of horse-power, may be taken -at about a thirty-sixth part of the load, while the resistance to -a load drawn upon a railway _at the same speed_ probably does not -amount to a tenth part of this resistance. Thus the moving power, -whatever it may be, would produce on a railway ten times the -useful effect which it would produce on a stone road; secondly, -the resistance which is opposed to the moving power on a level -railway is much more uniform than on a stone road, and, -consequently, the moving power is less subjected to jerks and -inequalities. This renders the application of inanimate power more -easy on the railway. Those inequalities of surface which increase -the amount of resistance on stone roads as compared with railways -also produce a jolting motion in the carriage, to counteract -which, the use of springs become necessary. These springs render -the motion of that part of the carriage which rests upon them -different from that part of the carriage which supports them; and -in the application of steam-machinery it becomes necessary so to -connect the moving power with the wheels that the machinery may -have one motion, and the wheels which are put in mechanical -connexion with that machinery, and driven by it, shall have -another motion. This, it is true, is the case with locomotive -engines on railways; but owing to the greater smoothness and -equality of the railway surface the difference between the motion -of the carriage body suspended on springs and that of the wheels -is much less than it would be on a stone road. - -But besides the greater smoothness of railways compared with stone -roads, the latter have another disadvantage, the effects of which -have probably been exaggerated by those who are opposed to this -application of steam-power. One of the [Pg421] laws of adhesion -long since developed by experiment, and established as a principle -of practical science, is that the adhesion is greater between -surfaces of the same than between surfaces of a different kind. Thus -between two metals of the same kind, the adhesion corresponding to -any given pressure is greater than between two metals of different -kinds; between two metals of any sort the adhesion is greater than -between metal and stone, or between metal and wood. Hence, the -wheels of steam-carriages running on a railroad have a greater -adhesion with the road, and therefore offer a greater resistance to -slip round without the advance of the carriage, than wheels would -offer on a turnpike road; for on a railroad the iron tire of the -wheel rests in contact with the iron rail, while on a common road -the iron tire rests in contact with the surface of stone, or -whatever material the road may be composed of. Besides this, the -dust and loose matter which necessarily collect on a common road, -when pressed between the wheels and the solid base of the road, act -somewhat in the manner of rollers, and give the wheels a greater -facility to slip than if the road were swept clean, and the wheels -rested in immediate contact with its hard surface. The truth of this -observation is illustrated on the railroads themselves, where the -adhesion is found to be diminished whenever the rails are covered -with any extraneous matter, such as dust or moist clay. Although the -adhesion of the wheels of a carriage with a common road, however, be -less than those of the wheels of a steam-carriage with a railroad, -yet still the actual adhesion on turnpike roads is greater in amount -than has been generally supposed, and is quite sufficient to propel -carriages drawing after them loads of large amount. - -The relative facility with which carriages are propelled on -railroads and turnpike-roads equally affects any moving power, -whether that of horses or steam engines; and whether loads be -propelled by the one power or the other, the railroad, as compared -with the turnpike-road, will always possess the same proportionate -advantage; and a given amount of power, whether of the one kind or -the other, will always perform a quantity of work less in the same -proportion on a [Pg422] turnpike-road than on a rail-road. But, on -the other hand, the expense of original construction, and of -maintaining the repairs of a rail-road, is to be placed against the -certain facility which it offers to draught. - -In the attempts which have been made to adapt locomotive engines to -turnpike-roads, the projectors have aimed at the accomplishment of -two objects: first, the construction of lighter and smaller engines; -and, secondly, increased power. These ends, it is plain, can only be -attained, with our present knowledge, by the production of steam of -very high temperature and pressure, so that the smallest volume of -steam shall produce the greatest possible mechanical effect. The -methods of propelling the carriage have been in general similar to -that used in the railroad engines, viz. either by cranks placed on -the axles, the wheels being fixed upon the same axles, or by -connecting the piston rods with the spokes of the wheels. In some -carriages, the boiler and moving power, and the body of the carriage -which bears the passengers, are placed on the same wheels. In -others, the engine is placed on a separate carriage, and draws after -it the carriage which transports the passengers, as is always the -case on railways. - -The chief difference between the steam engines used on railways, and -those adapted to propel carriages on turnpike roads, is in the -structure of the boiler. In the latter it is essential that, while -the power remains undiminished, the boiler should be lighter and -smaller. The accomplishment of this has been attempted by various -contrivances for so distributing the water as to expose a -considerable quantity of surface in contact with it to the action of -the fire: spreading it in thin layers on flat plates; inserting it -between plates of iron placed at a small distance asunder, the fire -being admitted between the intermediate plates; dividing it into -small tubes, round which the fire has play; introducing it between -the surfaces of cylinders placed one within another, the fire being -admitted between the alternate cylinders,--have all been resorted to -by different projectors. - - -(204.) First and most prominent in the history of the application of -steam to the propelling of carriages on turnpike roads stands the -name of Mr. Goldsworthy Gurney, a medical [Pg423] gentleman, and -scientific chemist, of Cornwall. In 1822, Mr. Gurney succeeded Dr. -Thompson as lecturer on chemistry at the Surrey Institution; and, in -consequence of the results of some experiments on heat, his -attention was directed to the project of working steam-carriages on -common roads; and he subsequently devoted his exertions in -perfecting a steam-engine capable of attaining the end he had in -view. - -The mistake which so long prevailed in the application of -locomotives on railroads, and which, as we have shown, materially -retarded the progress of that invention, was shared by Mr. Gurney. -Without reducing the question to the test of experiment, he took for -granted, in his first attempts, that the adhesion of the wheels with -the road was too slight to propel the carriage. He was assured, he -says, by eminent engineers, that this was a point settled by actual -experiment. It is strange, however, that a person of his quickness -and sagacity did not inquire after the particulars of these "actual -experiments." So, however, it was; and, taking for granted the -inability of the wheels to propel, he wasted much labour and skill -in the contrivance of levers and propellers, which acted on the -ground in a manner somewhat resembling the feet of horses, to drive -the carriage forward. After various fruitless attempts of this kind, -the experience acquired in the trials to which they gave rise at -last forced the truth upon his notice, and he found that the -adhesion of the wheels was not only sufficient to propel the -carriage heavily laden on level roads, but was capable of causing it -to ascend all the hills which occur on ordinary turnpike-roads. In -this manner it ascended all the hills between London and Barnet, -London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old -Highgate Hill, the last at one point rising one foot in nine. - -[Illustration: _Fig._ 114.] - -[Illustration: _Fig._ 115.] - -The boiler of Mr. Gurney's engine is so constructed, that there is -no part of it in which metal exposed to the action of the fire is -out of contact with water. If it be considered how rapidly the -action of an intense furnace destroys metal when water is not -present to prevent the heat from accumulating, the advantage of -this circumstance will be appreciated. In the boiler of Mr. -Gurney, the grate-bars [Pg424] themselves are tubes filled with -water, and form, in fact, a part of the boiler itself. This boiler -consists of three strong metal cylinders placed in a horizontal -position one above the other. A section, made by a perpendicular -or vertical plane, is represented in _fig._ 114. The ends of the -three cylinders just mentioned are represented at D, H, and I. In -the side of the lowest cylinder D are inserted a row of tubes, a -ground plan of which is represented in _fig._ 115. These tubes, -proceeding from the side of the lowest cylinder D, are inclined -[Pg425] slightly upwards, for a reason which I shall presently -explain. From the nature of the section, only one of these tubes -is visible in _fig._ 114. at C. The other extremities of these -tubes at A are connected with the same number of upright tubes, -one of which is shown at E. The upper extremities G of these -upright tubes are connected with another set of tubes K, equal in -number, proceeding from G, inclining slightly upwards, and -terminating in the second cylinder H. - -[Illustration: _Fig._ 116.] - -An end view of the boiler is exhibited in _fig._ 116., where the -three cylinders are expressed by the same letters. Between the -cylinders D and H there are two tubes of communication B, and two -similar tubes between the cylinders H and I. From the nature of -the section these appear only as a single tube in _fig._ 114. From -the top of the cylinder I proceeds a tube N, by which steam is -conducted to the engine. - -It will be perceived that the space F is enclosed on every side by -a grating of tubes, which have free communication with the -cylinders D and H, which cylinders have also a free communication -with each other by the tubes B. It follows, [Pg426] therefore, -that if water be supplied to the cylinder I, it will descend -through the tubes, and first filling the cylinder D and the tubes -C, will gradually rise in the tubes B and E, will next fill the -tubes K and the cylinder H. The grating of water-pipes C E K forms -the furnace, the pipes C being the fire-bars, and the pipes E and -K being the back and roof of the stove. The fire-door, for the -supply of fuel, appears at M, fig. 116. The flue issuing between -the tubes F is conducted over the tubes K, and the flame and hot -air are carried off through a chimney. That portion of the heat of -the burning fuel, which in other furnaces destroys the bars of the -grate, is here expended in heating the water contained in the -tubes C. The radiant heat of the fire acts upon the tubes K, -forming the roof of the furnace, on the tube E at the back of it, -and partially on the cylinders D and H, and the tubes B. The draft -of hot air and flame passing into the flue at A acts upon the -posterior surfaces of the tubes E, and the upper sides of the -tubes K, and finally passes into the chimney. - -As the water in the tubes C E K is heated, it becomes specifically -lighter than water of a less temperature, and consequently -acquires a tendency to ascend. It passes, therefore, rapidly into -H. Meanwhile the colder portions descend, and the inclined -positions of the tubes C and K give play to this tendency of the -heated water, so that a prodigiously rapid circulation is -produced, when the fire begins to act upon the tubes. When the -water acquires such a temperature that steam is rapidly produced, -steam-bubbles are constantly formed in the tubes surrounding the -fire; and if these remained stationary in the tubes, the action of -the fire would not only decompose the steam, but render the tubes -red hot, the water not passing through them to carry off the heat. -But the inclined position of the tubes, already noticed, -effectually prevents this injurious consequence. A steam-bubble, -which is formed either in the tubes C or K, having a tendency to -ascend proportional to its lightness as compared with water, -necessarily rushes upwards; if in C towards A, and if in K towards -H. But this motion of the steam is also aided by the rapid -circulation of the water which is continually maintained [Pg427] -in the tubes, otherwise it might be possible, notwithstanding the -levity of steam compared with water, that a bubble might remain in -a narrow tube without rising. To bring the matter to the test of -experiment, I have connected two cylinders, such as D and H, by a -system of glass tubes, such as represented at C E K. The rapid and -constant circulation of the water was then made evident: bubbles -of steam were formed in the tubes, it is true; but they passed -with great rapidity into the upper cylinder, and rose to the -surface, so that the glass tubes never acquired a higher -temperature than that of the water which passed through them. - -Every part of the boiler being cylindrical, it has the form which, -mechanically considered, is most favourable to strength, and -which, within given dimensions, contains the greatest quantity of -water. It is also free from the defects arising from unequal -expansion, which are found to be most injurious in tubular -boilers. The tubes C and K can freely expand in the direction of -their length, without being loosened at their joints, and without -straining any part of the apparatus; the tubes E, being short, are -subject to a very slight degree of expansion; and it is obvious -that the long tubes, with which they are connected, will yield to -this without suffering a strain, and without causing any part of -the apparatus to be loosened. - -When water is converted into steam, any foreign matter which may -be combined with it is disengaged, and is deposited on the bottom -of the vessel in which the water is evaporated. All boilers, -therefore, require occasional cleansing, to prevent the crust thus -formed from accumulating; and this operation, for obvious reasons, -is attended with peculiar difficulty in tubular boilers. In the -case before us, the crust of deposited matter would gather and -thicken in the tubes C and K, and if not removed, would at length -choke them. But besides this, it would be attended with a still -worse effect; for, being a bad conductor, it would intercept the -heat in its transit from the fire to the water, and would cause -the metal of the tube to become unduly heated. Mr. Gurney of -course foresaw this inconvenience, and contrived an ingenious -chemical method of removing it, by occasionally injecting [Pg428] -through the tubes such an acid as would combine with the deposit, -and carry it away. This method was effectual; and although its -practical application was found to be attended with difficulty in -the hands of common workmen, Mr. Gurney was persuaded to adhere to -it by the late Dr. Wollaston, until experience proved the -impossibility of getting it effectually performed, under the -circumstances in which boilers are commonly used. Mr. Gurney then -adopted a method of removing the deposit by mechanical means. -Opposite the mouths of the tubes, and on the other side of the -cylinders D and H, are placed a number of holes, which, when the -boiler is in use, are stopped by pieces of metal screwed into -them. When the tubes require to be cleaned, these stoppers are -removed, and an iron scraper is introduced through the holes into -the tubes, which, being passed backwards and forwards, removes the -deposit. - -In these engines the draught through the furnace was produced by -projecting the waste steam up the chimneys as is practised in -railway engines; a method so perfectly effectual, that it is -unlikely to be superseded by any other. The objection which has -been urged against it in locomotive engines, working on -turnpike-roads, is, that the noise which it produces has a -tendency to frighten horses. - -In the engines on the Liverpool road, the steam is allowed to pass -directly from the eduction pipe of the cylinder to the chimney, -and it there escapes in puffs corresponding with the alternate -motion of the pistons, and produces a noise, which, although -attended with no inconvenience on the railroad, would perhaps be -objectionable on turnpike-roads. In the engine used in Mr. -Gurney's steam-carriage, the steam which passes from the cylinders -is conducted to a receptacle, which he calls a blowing box. This -box serves the same purpose as the upper chamber of a smith's -bellows. It receives the steam from the cylinders in alternate -puffs, but lets it escape into the chimney in a continued stream -by a number of small jets. Regular draught is by this means -produced, and no noise is perceived. Another exit for the steam is -also provided, by which the conductor is enabled to increase or -diminish, or to suspend altogether, the draught [Pg429] in the -chimney, so as to adapt the intensity of the fire to the -exigencies of the road. This is a great convenience in practice; -because on some roads a draught is scarcely required, while on -others a powerful blast is indispensable. - -Connected with this blowing box is another apparatus of -considerable practical importance. The pipe through which the -feeding water is conducted from the tank is carried through this -blowing box, within which it is coiled in a spiral form, so that -an extensive thread of the water is exposed to the heat of the -waste steam which has escaped from the cylinders, and which is -enclosed in this blowing box. In passing through this pipe the -feeding water is raised from the ordinary temperature of about 60 deg. -to the temperature of 212 deg. Fuel is thus economised and weight -diminished; but there is another still greater advantage attending -this process. The feeding water in the worm just mentioned, while -it takes up the heat from the surrounding steam in the blowing -box, condenses a part of the waste steam, which is thence -conducted to the tank, from which the feeding water is pumped. - -When steam is generated so rapidly as is necessarily the case in -locomotive boilers, it rises with great violence in numerous -bubbles from the bottom of the boiler to the surface of the water, -and puts the liquid into a state of foaming turbulence not unlike -the sea in a storm. As the steam rushes from the surface into the -upper part of the boiler, under these circumstances, it carries -with it a spray by which water is scattered in minute subdivision -among the steam, and floats there like the spray which rises from -the base of a cascade. If the steam be conducted immediately to -the cylinder from the boiler in this state, it will carry with it -the water which is thus suspended in it, which will pass through -the cylinder, and finally be driven into the atmosphere upon the -returning stroke of the piston. The hot water thus carried off -possesses none of the mechanical properties of steam, and is -wholly inefficient as a moving power, and is therefore an -extensive source of the waste of heat. In every boiler, some means -should be provided for the separation of the water thus suspended -in the steam, before the steam is conducted to the cylinder. In -ordinary boilers, the large space which [Pg430] remains above the -surface of the water serves this purpose. The steam being there -subject to no agitation or disturbance, the water mechanically -suspended in it descends by its own gravity, and leaves pure steam -in the upper part. In the small tubular boilers, this has been a -matter, however, of greater difficulty. The contracted space in -which the ebullition takes place causes the water to be mixed with -the steam in a greater quantity than could happen in common -boilers; and the want of the same steam-room renders the -separation of the water from the steam a matter of some -difficulty. These inconveniences have been attempted to be -overcome by various contrivances. I have already described the -rapid and regular circulation effected by the arrangement of the -tubes. By this a regularity in the currents is established, which -has a tendency to diminish the mixture of water with the steam. In -addition to this, a method of separation is provided in the vessel -I, which is a strong iron cylinder of some magnitude, placed out -of the immediate influence of the fire. A partial separation of -the steam from the water takes place in the cylinder H; and the -steam with the water mechanically suspended in it, technically -called moist steam, rises into the _separator_ I. Here, being free -from all agitation and currents, and being, in fact, quiescent, -the particles of water fall to the bottom, while the pure steam -remains at the top. This separator, therefore, serves all the -purposes of the steam-room above the surface of the water in the -large plate boilers. The dry steam is thus collected and ready for -the supply of the engine through the tube N, while the water, -which is disengaged from it, is collected at the bottom of the -separator, and is conducted through the tube T to the lowest -vessel D, to be again circulated through the boiler. - -The pistons of the engine work on the axles of the hind wheels of -the carriage which bears the engine, by cranks, as in the -locomotives on the Manchester railway, so that the axle is kept in -a constant state of rotation while the engine is at work. The -wheels placed on this axle are not permanently fixed or keyed upon -it, as in the Manchester locomotives; but they are capable of -turning upon it in the same manner as ordinary carriage wheels. -Immediately within [Pg431] these wheels there are fixed upon the -axles two projecting spokes or levers, which revolve with the -axle, and which take the position of two opposite spokes of the -wheel. These may be occasionally attached to the wheel or detached -from it; so that they are capable of compelling the wheels to turn -with the axle, or leaving the axle free to turn independently of -the wheel, or the wheel independent of the axle, at the pleasure -of the conductor. It is by these levers that the engine is made to -propel either or both of the wheels. If both pairs of spokes are -thrown into connexion with the wheels, the crank shaft or axle -will cause both wheels to turn with it, and in that case the -operation of the carriage is precisely the same as those of the -locomotives already described upon the Liverpool and Manchester -line; but this is rarely found to be necessary, since the adhesion -of one wheel with the road is generally sufficient to propel the -carriage, and consequently only one pair of these fixed levers are -used, and the carriage propelled by only one of the two hind -wheels. The fore wheels of the carriage turn upon a pivot similar -to those of a four-wheeled coach. The position of these wheels is -changed at pleasure by a pinion and circular rack, which is moved -by the conductor, and in this manner the carriage is guided with -precision and facility. - -The force of traction necessary to propel a carriage upon common -roads must vary with the variable quality of the road, and -consequently the propelling power, or the pressure upon the pistons -of the engine, must be susceptible of a corresponding variation; but -a still greater variation becomes necessary from the undulations and -hills which are upon all ordinary roads. This necessary change in -the intensity of the impelling power is obtained by restraining the -steam in the boiler by the throttle-valve, as already described in -the locomotive engines on the railroad. This principle, however, is -carried much further in the present case. The steam in the boiler -maybe at a pressure of from 100 to 200 lbs. on the square inch; -while the steam on the working piston may not exceed 30 or 40 lbs. -on the inch. Thus an immense increase of power is always at the -command of the conductor; so that when a hill is encountered, or a -rough piece of road, [Pg432] he is enabled to lay on power -sufficient to meet the exigency of the occasion. - -The two difficulties which have been always apprehended in the -practical working of steam-carriages upon common roads are, first, -the command of sufficient power for hills and rough pieces of -road; and, secondly, the apprehended insufficiency of the adhesion -of the wheels with the road to propel the carriage. The former of -these difficulties has been met by allowing steam of very great -pressure to be constantly maintained in the boiler with perfect -safety. As to the second, all experiments tend to show that there -is no ground for the supposition that the adhesion of the wheels -is in any case insufficient for the purposes of propulsion. Mr. -Gurney states, that he has succeeded in driving carriages thus -propelled, up considerable hills on the turnpike roads about -London. He made a journey to Barnet with only one wheel attached -to the axle, which was found sufficient to propel the carriage up -all hills upon that road. The same carriage, with only one -propelling wheel, also went to Bath, and surmounted all the hills -between Cranford Bridge and Bath, going and returning. - -A double stroke of the piston produces one revolution of the -propelling wheels, and causes the carriage to move through a space -equal to the circumference of those wheels. It will therefore be -obvious, that the greater the diameter of the wheels, the better -adapted the carriage is for speed; and, on the other hand, wheels -of smaller diameter are better adapted for power. In fact, the -propelling power of an engine on the wheels will be in the inverse -proportion of their diameter. In carriages designed to carry great -weights at a moderate speed, smaller wheels will be used; while in -those intended for the transport of passengers at considerable -velocities, wheels of at least 5 feet diameter are most -advantageous. - - -(205.) Among the numerous popular prejudices to which this new -invention has given rise, one of the most mischievous in its -effects and most glaring in its falsehood, is the notion that -carriages thus propelled are more injurious to roads than -carriages drawn by horses. This error has been successfully -exposed in the evidence taken before the committee of the [Pg433] -House of Commons upon steam carriages. It is there demonstrated, -not only that carriages thus propelled do not wear a turnpike road -more rapidly than those drawn by horses, but that, on the other -hand, the wear by the feet of horses is far more rapid and -destructive than any which could be produced by the wheels of -carriages. Steam carriages admit of having the tires of the wheels -broad, so as to act upon the road more in the manner of rollers, -and thereby to give consistency and firmness to the material of -which the road is composed. The driving wheels being proved not to -slip upon the road, do not produce any effects more injurious than -the ordinary rolling wheels; consequently the wear occasioned by a -steam carriage upon a road, is not more than that produced by a -carriage drawn by horses, of an equivalent weight and the same or -equal tires; but the wear produced by the pounding and digging of -horses' feet in draught is many times greater than that produced -by the wear of any carriage. Those who still have doubts upon this -subject, if there be any such persons, will be fully satisfied by -referring to the evidence which accompanies the report of the -committee of the House of Commons, printed in October, 1831. - -The weight of machinery necessary for steam carriages is sometimes -urged as an objection to their practical utility. Mr. Gurney -states, that, by successive improvements in the details of the -machinery, the weight of his carriages, without losing any of the -propelling power, may be reduced to 35 cwt., exclusive of the -load, and fuel and water: but thinks that it is possible to reduce -the weight still further. - -A steam carriage constructed by Mr. Gurney, weighing 35 cwt., -working for 8 hours, is found, according to his statement, to do -the work of about 30 horses. He calculates that the weight of his -propelling carriage, which would be capable of drawing 18 persons, -would be equal to the weight of 4 horses; and the carriage in -which these persons would be drawn would have the same weight as a -common stage coach capable of carrying the same number of persons. -Thus the weight of the whole--the propelling carriage and the -carriage for passengers taken together--would be the same [Pg434] -with the weight of a common stage coach, with 4 horses inclusive. - -There are two methods of applying locomotives upon common roads to -the transport of passengers or goods; the one is by causing the -locomotive to carry, and the other to draw the load; and different -projectors have adopted the one and the other method. Each is -attended with its advantages and disadvantages. If the same -carriage transport the engine and the load, the weight of the -whole will be less in proportion to the load carried; also a -greater pressure may be produced on the wheels by which the load -is propelled. It is also thought that a greater facility in -turning and guiding the vehicle, greater safety in descending the -hills, and a saving in the original cost, will be obtained. On the -other hand, when the passengers are placed in the same carriage -with the engine, they are necessarily more exposed to the noise of -the machinery and to the heat of the boiler and furnace. The -danger of explosion is so slight, that, perhaps, it scarcely -deserves to be mentioned; but still _the apprehension_ of danger -on the part of the passengers, even though groundless, should not -be disregarded. This apprehension will be obviously removed or -diminished by transferring the passengers into a carriage separate -from the engine; but the greatest advantage of keeping the engine -separate from the passengers is the facility which it affords of -changing one engine for another in case of accident or derangement -on the road, in the same manner as horses are changed at the -different stages: or, if such an accident occur in a place where a -new engine cannot be procured, the load of passengers may be -carried forward by horses, until it is brought to some station -where a locomotive may be obtained. There is also an advantage -arising from the circumstance, that when the engines are under -repair, or in process of cleaning, the carriages for passengers -are not necessarily idle. Thus the same number of carriages for -passengers will not be required when the engine is used to draw as -when it is used to carry. - -In case of a very powerful engine being used to carry great loads, -it would be quite impracticable to place the engine [Pg435] and -loads on four wheels, the pressure being such as no turnpike road -could bear. In this case it would be indispensably necessary to -place a part of the load at least upon separate carriages to be -drawn by the engine. - -In the comparison of carriages propelled by steam with carriages -drawn by horses, there is no respect in which the advantage of the -former is so apparent as the safety afforded to the passenger. -Steam power is under the most perfect control, and a carriage thus -propelled is capable of being guided with the most admirable -precision. It is also capable of being stopped almost suddenly, -whatever be its speed: it is capable of being turned within a -space considerably less than that which would be necessary for -four-horse coaches. In turning sharp corners, there is no danger, -with the most ordinary care on the part of the conductor. On the -other hand, horse power, as is well known, is under very imperfect -control, especially when horses are used adapted to that speed -which at present is generally considered necessary for the -purposes of travelling. "The danger of being run away with and -overturned," says Mr. Farey, in his evidence before the House of -Commons, "is greatly diminished in a steam coach. It is very -difficult to control four such horses as can draw a heavy stage -coach ten miles an hour, in case they are frightened or choose to -run away; and, for such quick travelling, they must be kept in -that state of courage that they are always inclined to run away, -particularly down hill, and at sharp turns in the road. Steam -power has very little corresponding danger, being perfectly -controllable, and capable of having its power reversed, to retard -in going down hill. It must be carelessness that would occasion -the overturning of a steam carriage. The chance of breaking down -has been hitherto considerable, but it will not be more than in -stage coaches when the work is truly proportioned and properly -executed. The risk from explosion of the boiler is the only new -cause of danger, and that I consider not equivalent to the danger -from horses." - -That the risk of accident from explosion is extremely slight, may -be proved by the fact that the railway between Liverpool and -Manchester has now been in operation for about ten [Pg436] years, -and that other railways more extensive in length have been worked -for a considerable time, and that no instance has ever yet -occurred of an accident to passengers from the explosion of a -boiler. Generally these machines, when they fail, are attended -with no other effect than the extinction of the fire, by the water -of the boiler flowing in upon it. I am not aware of more than one -instance, in which a serious accident has been produced by -explosion; and in that instance, the sufferers were only the -engineer and stoker. In the steam-engine of Mr. Gurney, the -carriage is drawn after the engine, as represented in _fig._ 117. - -[Illustration: _Fig._ 117.] - -[Illustration: _Fig._ 118.] - - -(206.) In the boiler to be used in the steam carriage projected by -Mr. Walter Hancock, the subdivision of the water is accomplished -by dividing a case or box by a number of [Pg437] thin plates of -metal, like a galvanic battery, the water being allowed to flow -between every alternate pair of plates, at E, _fig._ 118., and the -intermediate spaces H forming the flue through which the flame and -hot air are propelled. - -In fact, a number of thin plates of water are exposed on both -sides to the most intense action of flame and heated air; so that -steam of a high pressure is produced in great abundance and with -considerable rapidity. The plates forming the boiler are bolted -together by strong iron ties, extending across the boiler, at -right angles to the plates, as represented in the figure. The -distance between the plates is two inches. - -There are ten flat chambers of this kind for water, and -intermediately between them ten flues. Under the flues is the -fire-place, or grate, containing six square feet of fuel in vivid -combustion. The chambers are all filled to about two thirds of -their depth with water, and the other third is left for steam. The -water chambers, throughout the whole series, communicate with each -other both at top and bottom, and are held together by two large -bolts. By releasing these bolts, at any time, the chambers fall -asunder; and by screwing them up they may be all made tight again. -The water is supplied to the boiler by a forcing-pump, and the -steam issues from the centre of one of the flues at the top. - -These boilers are constructed to bear a pressure of 400 or 500 -lbs. on the square inch; but the average pressure of the steam on -the safety valve is from 60 to 100. There are 100 square feet of -surface in contact with the water exposed to the fire. The stages -which such an engine performs are eight miles, at the end of which -a fresh supply of fuel and water are taken in. It requires about -two bushels of coke for each stage. - -The steam carriage of Mr. Hancock differs from that of Mr. Gurney -in this--that in the former the passengers and engine are all -placed on the same carriage. The boiler is placed behind the -carriage; and there is an engine-house between the boiler and the -passengers, the latter being placed in the fore part of the -vehicle; so that all the machinery is behind them. The carriages -are adapted to carry 14 [Pg438] passengers, and weigh, exclusive -of their load, about 3-1/2 tons, the tires of the wheels being -about 3-1/2 inches in breadth. Mr. Hancock states, that the -construction of his boiler is of such a nature, that, even in the -case of bursting, no danger is to be apprehended, nor any other -inconvenience than the stoppage of the carriage. He states that, -while travelling about nine miles an hour, and working with a -pressure of about 100 lbs. on the square inch, loaded with -thirteen passengers, the carriage was suddenly stopped. At first -the cause of the accident was not apparent; but, on opening one of -the cocks of the boiler, it was found that it contained neither -steam nor water. Further examination proved that the boiler had -burst. On unscrewing the bolts, it was found that there were -several large holes in the plates of the water-chamber, through -which the water had flowed on the fire, but neither noise nor -explosion, nor any dangerous consequences, ensued. - - -(207.) Mr. Nathaniel Ogle of Southampton obtained a patent for a -locomotive carriage, and worked it for some time experimentally; -but as his operations do not appear to have been continued, I -suppose he was unsuccessful in fulfilling those conditions, -without which the machine could not be worked with economy and -profit. In his evidence before a committee of the House of -Commons, he has thus described his contrivance:-- - -"The base of the boiler and the summit are composed of cross -pieces, cylindrical within and square without; there are holes -bored through these cross pieces, and inserted through the whole -is an air tube. The inner hole of the lower surface, and the under -hole of the upper surface, are rather larger than the other ones. -Round the air tube is placed a small cylinder, the collar of which -fits round the larger aperture on the inner surface of the lower -frame, and the under surface of the upper frame-work. These are -both drawn together by screws from the top; these cross pieces are -united by connecting pieces, the whole strongly bolted together; -so that we obtain, in one tenth of the space, and with one tenth -of the weight, the same heating surface and power as is now -obtained in other and low-pressure boilers, with incalculably -[Pg439] greater safety. Our present experimental boiler contains -250 superficial feet of heating surface in the space of 3 feet 8 -inches high, 3 feet long, and 2 feet 4 inches broad, and weighs -about 8 cwt. We supply the two cylinders with steam, communicating -by their pistons with a crank axle, to the ends of which either -one or both wheels are affixed as may be required. One wheel is -found to be sufficient, except under very difficult circumstances, -and when the elevation is about one foot in six to impel the -vehicle forward. - -"The cylinders of which the boiler is composed are so small as to -bear a greater pressure than could be produced by the quantity of -fire beneath the boiler; and if any one of these cylinders should -be injured by violence, or any other way, it would become merely a -safety valve to the rest. We never, with the greatest pressure, -burst, rent, or injured our boiler; and it has not once required -cleaning, after having been in use twelve months." - -Dr. Church of Birmingham has obtained a succession of patents for -contrivances connected with a locomotive engine for stone roads; -and a company, consisting of a considerable number of individuals, -possessing sufficient capital, has been formed in Birmingham, for -carrying into effect his designs, and working carriages on his -principle. The present boiler of Dr. Church is formed of copper. -The water is contained between two sheets of copper, united -together by copper nails, in a manner resembling the way in which -the cloth forming the top of a mattress or cushion is united with -the cloth which forms the bottom of it, except that the nails or -pins, which bind the sheets of copper, are much closer together. -The water, in fact, seems to be "quilted" or "padded" in between -two sheets of thin copper. This double sheet of copper is formed -into an oblong rectangular box, the interior of which is the -fire-place and ash-pit, and over the end of which is the -steam-chest. The great extent of surface exposed to the immediate -action of the fire causes steam to be produced with great -rapidity. - -Various other projects for the application of steam engines on -common roads were in a state of progressive improvement, [Pg440] -when the greater advantages attending railways were considered so -manifest, that considerable doubts were raised, whether, supposing -the problem of the application of the steam engine on common roads -to be successfully solved, it could ever be attended with the same -economy and effect, as by the adoption of a railway. Among the -projects which promised a successful issue, may be mentioned the -locomotive engines contrived by Messrs. Maudslay and Field, by -Colonel Maceroni, and by Mr. Scott Russell. These and others have, -however, been abandoned, mainly, we believe, from the impression, -that wherever traffic can exist, sufficiently extensive to render -the application of steam power profitable, a railway must always -supersede a common road; and that, even in the limited traffic to -be expected on branches to the great railways, horse power applied -to railways would be attended with more economy than steam power -applied on stone roads. - -[Illustration] - -[Pg441] - - - - -[Illustration] - -CHAP. XIII. - -STEAM NAVIGATION. - - FORM AND ARRANGEMENT OF MARINE ENGINES. -- EFFECTS OF SEA WATER - IN BOILERS. -- REMEDIES FOR THEM. -- BLOWING OUT. -- INDICATORS - OF SALTNESS. -- SEAWARD'S INDICATOR. -- HIS METHOD OF BLOWING - OUT. -- FIELD'S BRINE PUMPS. -- TUBULAR CONDENSERS APPLIED BY - MR. WATT. -- HALL'S CONDENSERS. -- COPPER BOILERS. -- PROCESS OF - STOKING. -- MARINE BOILERS. -- MEANS OF ECONOMISING FUEL. -- - COATING MARINE BOILERS WITH FELT. -- NUMBER AND ARRANGEMENT OF - FURNACES AND FLUES. -- HOWARD'S ENGINE. -- APPLICATION OF THE - EXPANSIVE PRINCIPLE IN MARINE ENGINES. -- RECENT IMPROVEMENTS - OF MESSRS. MAUDSLAY AND FIELD. -- HUMPHRYS' ENGINE. -- COMMON - PADDLE-WHEEL. -- FEATHERING PADDLES. -- MORGAN'S WHEELS. -- THE - SPLIT PADDLE. -- PROPORTION OF POWER TO TONNAGE. -- IMPROVED - EFFICIENCY OF MARINE ENGINES. -- IRON STEAM-VESSELS. -- - STEAM-NAVIGATION TO INDIA. - - -(208.) Among the many ways in which the steam-engine has -ministered to the advancement of civilisation and the social -progress of the human race, there is none more [Pg442] important -or more interesting than its application to navigation. Before it -lent its giant powers to the propulsion of ships, locomotion over -the waters of the deep was attended with so much danger and -uncertainty that, as a common proverb, it became the type and the -representative of every thing which was precarious and perilous. -The application, however, of steam to navigation has rescued the -mariner and the voyager from many of the dangers of wind and -water; and even in its present state, putting out of view its -probable improvement, it has rendered all voyages of moderate -length as safe, and very nearly as regular, as journeys over-land. -As a means of transport by sea, the application of this power may -be considered as established; and it is now receiving improvements -by which its extension to the longest class of ocean voyages is a -question not of practicability, but merely of profit. - -The manner in which the steam-engine is rendered an instrument for -the propulsion of vessels must in its general features be so -familiar to every one as to require but short explanation. A shaft -is carried across the vessel, being continued on either side -beyond the timbers: to the extremities of this shaft, on the -outside of the vessel, are fixed a pair of wheels constructed like -undershot water-wheels, having attached to their rims a number of -flat boards called _paddle-boards_. As the wheels revolve, these -paddle-boards strike the water, driving it in a direction contrary -to that in which it is intended the vessel should be propelled. -The moving force imparted to the water thus driven backwards is -necessarily accompanied by a re-action upon the vessel through the -medium of the paddle-shaft, by which the vessel is propelled -forwards. On the paddle-shaft two cranks are constructed, similar -to the cranks already described on the axle of the driving wheels -of a locomotive engine. These cranks are placed at right angles to -each other, so that when either is in its highest or lowest -position the other shall be horizontal. They are driven by two -steam-engines, which are placed in the hull of the vessel below -the paddle-shaft. In the earlier steam-boats a single steam-engine -was used, and in that case the unequal action of the engine on the -crank was equalised by a fly-wheel. This, however, has been long -[Pg443] since abandoned in European vessels, and the use of two -engines is now almost universal. By the relative position of the -cranks it will be seen, that when either crank is at its dead -points, the other will be in the positions most favourable to its -action, and in all intermediate positions the relative efficiency -of the cranks will be such as to render their combined action very -nearly uniform. - -The steam-engines used to impel vessels may be either condensing -engines, similar to those of Watt, and such as are used in -manufactures generally, or they may be non-condensing and -high-pressure engines, similar in principle to those used on -railways. Low-pressure condensing engines are, however, universally -used for marine purposes in Europe and to some extent in the United -States. In the latter country, however, high-pressure engines are -also in pretty general use, on rivers where lightness is a matter of -importance. - -The arrangement of the parts of a marine engine differs in some -respects from that of a land engine. The limitation of space, -which is unavoidable in a vessel, renders greater compactness -necessary. The paddle-shaft on which the cranks to be driven by -the engine are constructed being very little below the deck of the -vessel, the beam and connecting rod could not be placed in the -position in which they usually are in land engines, without -carrying the machinery to a considerable elevation above the deck. -This is done in the steam-boat engines used on the American -rivers; but it would be inadmissible in steam-boats in general, -and more especially in sea-going steamers. The connecting rods, -therefore, instead of being presented downwards towards the cranks -which they drive, must, in steam-vessels, be presented upwards, -and the impelling force received from below. If, under these -circumstances, the beam were in the usual position above the -cylinder and piston-rod, it must necessarily be placed between the -engine and the paddle-shaft. This would require a depth for the -machinery which would be incompatible with the magnitude of the -vessel. The beam, therefore, of marine engines, instead of being -above the cylinder and piston, is placed below them. To the top of -the [Pg445] piston-rods cross pieces are attached of greater -length than the diameter of the cylinders, so that their -extremities shall project beyond the cylinders. To the ends of -these cross pieces are attached by joints the rods of a parallel -motion: these rods are carried downwards, and are connected with -the ends of two beams below the cylinder, and placed on either -side of it. The opposite ends of these beams are connected by -another cross piece, to which is attached a connecting rod, which -is continued upwards to the crank-pin, to which it is attached, -and which it drives. Thus the beam, parallel motion, and -connecting rod of a marine engine, is similar to that of a land -engine, only that it is turned upside down; and in consequence of -the impossibility of placing the beam directly over the -piston-rod, two beams and two systems of parallel motion are -provided, one on each side of the engine, acted upon by, and -acting on the piston-rod and crank by cross pieces. - -The proportion of the cylinders differs from that usually observed -in land engines, for like reasons. The length of the cylinder of -land engines is generally greater than its diameter, in the -proportion of about two to one. The cylinders of marine engines -are, however, commonly constructed with a diameter very little -less than their length. In proportion, therefore, to their power -their stroke is shorter, which infers a corresponding shortness of -crank and a greater limitation of play of all the moving parts in -the vertical direction. The valves and the gearing by which they -are worked, the air-pump, the condenser, and other parts of the -marine engines, do not materially differ from those already -described in land engines. - -[Illustration: _Fig._ 119.] - -These arrangements of a marine engine will be more clearly -understood by reference to _fig._ 119.[35], in which is -represented a longitudinal section of a marine engine with its -boiler as placed in a steam-vessel. The sleepers of oak, -supporting the engine, are represented at X, the base of the -engine being secured to these by bolts passing through them -[Pg446] and the bottom timbers of the vessel; S is the steam-pipe -leading from the steam-chest in the boiler to the slides _c_, by -which it is admitted to the top and bottom of the cylinder. The -condenser is represented at B, and the air-pump at E. The hot well -is seen at F, from which the feed is taken for the boiler; L is -the piston-rod connected by the parallel motion _a_ with the beam -H, working on a centre K, near the base of the engine. The other -end of the beam I drives the connecting rod M, which extends -upwards to the crank which it works upon the paddle-shaft O. Q R -is the framing by which the engine is supported. The beam here -exhibited is shown on dotted lines as being on the further side of -the engine. A similar beam similarly placed, and moving on the -same axis, must be understood to be at this side connected with -the cross head of the piston in like manner by a parallel motion, -and with a cross piece attached to the lower end of the connecting -rod and to the opposite beam. The eccentric which works the slides -is placed upon the paddle shaft O, and the connecting arm which -drives the slides may be easily detached when the engine requires -to be stopped. The section of the boiler, grate, and flues, is -represented at W U. The safety-valve _y_ is enclosed beneath a -pipe carried up beside the chimney, and is inaccessible to the -engine-man; _h_ are the cocks for blowing the salted water from -the boiler; and I I the feed-pipe. - -The general arrangement of the engine-room of a steam-vessel is -represented in _fig._ 120. - -The nature of the effect required to be produced by marine engines -does not render either necessary or possible that great regularity -of action which is indispensable in a steam-engine applied to the -purposes of manufacture. The agitation of the surface of the sea -will cause the immersion of the paddle-wheels to be subject to -great variation, and the resistance produced by the water to the -engine will undergo a corresponding change. The governor, -therefore, and other parts of the apparatus, contrived for giving -to the engine that great regularity required in manufactures, are -omitted in nautical engines, and nothing is introduced save what -is [Pg447] necessary to maintain the machine in its full working -efficiency. - -[Illustration: _Fig._ 120.] - -[Illustration: _Fig._ 121.] - -To save space, marine boilers are constructed so as to produce the -necessary quantity of steam within the smallest possible -dimensions. With this view a more extensive surface in proportion -to the capacity of the boiler is exposed to the action of the -fire. The flues, by which the flame and heated air are conducted -to the chimney, are so constructed that the heat may act upon the -water on every side in thin oblong shells or plates. This is -accomplished by constructing the flues so as to traverse the -boiler backwards and forwards several times before they terminate -[Pg448] in the chimney. Such an arrangement renders the expense of -the boilers greater, but their steam-producing power is -proportionally augmented, and experiments made by Mr. Watt, at -Birmingham, have proved that such boilers with the same -consumption of fuel will produce, as compared with common land -boilers, an increased evaporation in the proportion of about three -to two. - -[Illustration: _Fig._ 122.] - -[Illustration: _Fig._ 123.] - -The form and arrangement of the water-spaces and flues in marine -boilers may be collected from the sections of the boilers used in -some of the government steamers, exhibited in _figs._ 121, 122, -123. A section made by a horizontal plane passing through the -flues is exhibited in _fig._ 121. The furnaces F communicate in -pairs with the flues E, the air following the course through the -flues represented by the arrows. The flue E passes to the back of -the boiler, then returns to the front, then to the back again, and -is finally carried back to the front, where it communicates at C -with the curved flue B, represented in the transverse vertical -section, _fig._ 122. This curved flue B finally terminates in the -chimney A. There are in this case three independent boilers, each -worked by two furnaces communicating with the same system of -flues; and in the curved flues B, _fig._ 122., by which the air is -finally conducted through the chimney, are placed three -independent [Pg449] dampers, by means of which the furnace of -each boiler can be regulated independently of the other, and by -which each boiler may be separately detached from communication -with the chimney. The letters of reference in the horizontal -section, _fig._ 121., correspond with those in the transverse -vertical section, _fig._ 122., E representing the commencement of -the flues, and C their termination. - -[Illustration: _Fig._ 124.] - -A longitudinal section of the boiler made by a vertical plane -extending from the front to the back is given in _fig._ 123., -where F, as before, is the furnace, G the grate-bars sloping -downwards from the front to the back, H the fire-bridge, C the -commencement of the flues, and A the chimney. An elevation of the -front of the boiler is represented in _fig._ 124., showing two of -the fire-doors closed, and the other two removed, displaying the -position of the grate-bars in front. Small openings are also -provided, closed by proper doors, by which access can be had to -the under side of the flues between the foundation timbers of the -engine for the purpose of cleaning them. - -Each of these boilers can be worked independently of the others. -By this means, when at sea, the engine may be worked by any two of -the three boilers, while the third is being cleaned and put in -order. In all sea-going steamers multiple boilers are at present -provided for this purpose. - -In the boilers here represented the flues are all upon the same -level, winding backwards and forwards without passing one above -the other. In other boilers, however, the flues, [Pg450] after -passing backwards and forwards near the bottom of the boiler, turn -upwards and pass backwards and forwards through a level of the -water nearer its surface, finally terminating in the chimney. More -heating surface is thus obtained with the same capacity of boiler. - -The most formidable difficulty which has been encountered in the -application of the steam-engine to sea-voyages has arisen from the -necessity of supplying the boiler with sea-water instead of pure -fresh water. The sea-water is injected into the condenser for the -purpose of condensing the steam, and it is thence, mixed with the -condensed steam, conducted as feeding water into the boiler. - - -(209.) Sea-water holds, as is well known, certain alkaline -substances in solution, the principal of which is muriate of soda, -or common salt. Ten thousand grains of pure sea-water contain two -hundred and twenty grains of common salt, the remaining -ingredients being thirty-three grains of sulphate of soda, -forty-two grains of muriate of magnesia, and eight grains of -muriate of lime. The heat which converts pure water into steam -does not at the same time evaporate those salts which the water -holds in solution. As a consequence it follows, that as the -evaporation in the boiler is continued, the salt, which was held -in solution by the water which has been evaporated, remains in the -boiler, and enters into solution with the water remaining in it. -The quantity of salt contained in sea-water being considerably -less than that which water is capable of holding in solution, the -process of evaporation for some time is attended with no other -effect than to render the water in the boiler a stronger solution -of salt. If, however, this process be continued, the quantity of -salt retained in the boiler having constantly an increasing -proportion to the quantity of water, it must at length render the -water in the boiler a saturated solution--that is, a solution -containing as much salt as at the actual temperature it is capable -of holding in solution. If, therefore, the evaporation be -continued beyond this point, the salt disengaged from the water -evaporated instead of entering into solution with the water -remaining in the boiler will be precipitated in the form of -sediment; and if the process be continued in the [Pg451] same -manner, the boiler would at length become a mere salt-pan. - -But besides the deposition of salt sediment in a loose form, some -of the constituents of sea-water having an attraction for the iron -of the boiler, collect upon it in a scale or crust in the same -manner as earthy matters held in solution by spring-water are -observed to form and become incrusted on the inner surface of -land-boilers and of common culinary vessels. - -The coating of the inner surface of a boiler by incrustation and -the collection of salt sediment in its lower parts, are attended -with effects highly injurious to the materials of the boiler. The -crust and sediment thus formed within the boiler are almost -non-conductors of heat, and placed, as they are, between the water -contained in the boiler and the metallic plates which form it, -they obstruct the passage of heat from the outer surface of the -plates in contact with the fire to the water. The heat, therefore, -accumulating in the boiler-plates so as to give them a much higher -temperature than the water within the boiler, has the effect of -softening them, and by the unequal temperature which will thus be -imparted to the lower plates which are incrusted, compared with -the higher parts which may not be so, an unequal expansion is -produced, by which the joints and seams of the boiler are loosened -and opened, and leaks produced. - -These injurious effects can only be prevented by either of two -methods; first, by so regulating the feed of the boiler that the -water it contains shall not be suffered to reach the point of -saturation, but shall be so limited in its degree of saltness that -no injurious incrustation or deposit shall be formed; secondly, by -the adoption of some method by which the boiler may be worked with -fresh water. This end can only be attained by condensing the steam -by a jet of fresh water, and working the boiler continually by the -same water, since a supply of fresh water sufficient for a boiler -worked in the ordinary way could never be commanded at sea. - - -(210.) The method by which the saltness of the water in the boiler -is most commonly prevented from exceeding a certain [Pg452] limit -has been to discharge from the boiler into the sea a certain -quantity of over-salted water, and to supply its place by -sea-water introduced into the condenser through the injection-cock -for the purpose of condensing the steam, this water being mixed -with the steam so condensed, and being, therefore, a weaker -solution of salt than common sea-water. To effect this, cocks -called _blow-off cocks_, are usually placed in the lower parts of -the boiler, where the over-salted, and therefore heavier, parts of -the water collect. The pressure of the steam and incumbent weight -of the water in the boiler force the lower strata of water out -through these cocks; and this process, called _blowing out_, is, -or ought to be, practised at such intervals as will prevent the -water from becoming over salted. When the salted water has been -blown out in this manner, the level of the water in the boiler is -restored by a feed of corresponding quantity. - -This process of blowing out, on the due and regular observance of -which the preservation and efficiency of the boiler mainly depend, -is too often left at the discretion of the engineer, who is, in most -cases, not even supplied with the proper means of ascertaining the -extent to which the process should be carried. It is commonly -required that the engineer should blow out a certain portion of the -water in the boiler every two hours, restoring the level by a feed -of equivalent amount; but it is evident that the sufficiency of the -process founded on such a rule must mainly depend on the supposition -that the evaporation proceeds always at the same rate, which is far -from being the case with marine boilers. An indicator, by which the -saltness of the water in the boiler would always be exhibited, ought -to be provided, and the process of blowing out should be regulated -by the indications of that instrument. To blow out more frequently -than is necessary is attended with a waste of fuel; for hot water is -thus discharged into the sea while cold water is introduced in its -place, and consequently all the heat necessary to produce the -difference of the temperatures of the water blown out and the feed -introduced is lost. If, on the other hand, the process of blowing -out be observed less frequently than is necessary, then more or less -incrustation and deposit [Pg453] may be produced, and the injurious -effects already described ensue. - -As the specific gravity of water holding salt in solution is -increased with every increase of the strength of the solution, any -form of hydrometer capable of exhibiting a visible indication of -the specific gravity of the water contained in the boiler would -serve the purpose of an indicator, to show when the process of -blowing out is necessary, and when it has been carried to a -sufficient extent. The application of such instruments, however, -would be attended with some practical difficulties in the case of -sea-boilers. - -The temperature at which a solution of salt boils under a given -pressure varies considerably with the strength of the solution; -the more concentrated the solution is, the higher will be its -boiling temperature under the same pressure. A comparison, -therefore, of a steam-gauge attached to the boiler, and a -thermometer immersed in it, showing the pressure and the -temperature, would always indicate the saltness of the water; and -it would not be difficult so to graduate these instruments as to -make them at once show the degree of saltness. - -If the application of the thermometer be considered to be attended -with practical difficulty, the difference of pressures under which -the salt water of the boiler and fresh water of the same -temperature boil, might be taken as an indication of the saltness -of the water in the boiler, and it would not be difficult to -construct upon this principle a self-registering instrument, which -would not only indicate but record from hour to hour the degree of -saltness of the water. A small vessel of distilled water being -immersed in the water of the boiler would always have the -temperature of that water, and the steam produced from it -communicating with a steam-gauge, the pressure of such steam would -be indicated by that gauge, while the pressure of the steam in the -boiler under which pressure the salted water boils might be -indicated by another gauge. The difference of the pressures -indicated by the two gauges would thus become a test by which the -saltness of the water in the boiler would be measured. The two -pressures might be made to act on opposite ends of the same column -of [Pg454] mercury contained in a siphon tube, and the difference -of the levels of the two surfaces of the mercury would thus become -a measure of the saltness of the water in the boiler. A -self-registering instrument founded on this principle formed part -of the self-registering steam-log which I proposed to introduce -into steam-vessels some time since. - - -(211.) The Messrs. Seaward of Limehouse have adopted, in some of -their recently constructed engines, a method of indicating the -saltness of the water, and of measuring the quantity of salted -water or brine discharged, by blowing out. A glass-gauge, similar -in form to that already described in land engines (156.), is -provided to indicate the position of the surface of the water in -the boiler. In this gauge two hydrometer balls are provided, the -weight of which in proportion to their magnitude is such that they -would both sink to the bottom in a solution of salt of the same -strength as common sea-water. When the quantity of salt exceeds -5/32 parts of the whole weight of the water, the lighter of the -two balls will float to the top; and when the strength is further -increased until the proportion of salt exceeds 6/32 parts of the -whole, then the heavier ball will float to the top. The actual -quantity of salt held in solution by sea-water in its ordinary -state is 1/32 part of its whole weight; and when by evaporation -the proportion of salt in solution has become 9/32 parts of the -whole, then a deposition of salt commences. With an indicator such -as that above described, the ascent of the lighter hydrometer ball -gives notice of the necessity for blowing out, and the ascent of -the heavier may be considered as indicating the approach of an -injurious state of saltness in the boiler. - -[Illustration: _Fig._ 125.] - -The ordinary method of blowing out the salted water from a boiler -is by a pipe having a cock in it leading from the boiler through -the bottom of the ship, or at a point low down at its side. -Whenever the engineer considers that the water in the boiler has -become so salted that the process of blowing out should commence, -he opens the cock communicating by this pipe with the sea, and -suffers an indefinite and uncertain quantity of water to escape. -In this way he discharges, according to the magnitude of the -boiler, from two to six tons [Pg455] of water, and repeats this -at intervals of from two to four hours, as he may consider to be -sufficient. If, by observing this process, he prevents the boiler -from getting incrusted during the voyage, he considers his duty to -be effectually discharged, forgetting that he may have blown out -many times more water than is necessary for the preservation of -the boiler, and thereby produced a corresponding and unnecessary -waste of fuel. In order to limit the quantity of water discharged, -Messrs. Seaward have adopted the following method. In _fig._ 125. -is represented a transverse section of a part of a steam-vessel; W -is the water-line of the boiler, B is the mouth of a blow-off -pipe, placed near the bottom of the boiler. This pipe rises to A, -and turning in the horizontal direction, A C is conducted to a -tank T, which contains exactly a ton of water. This pipe -communicates with the tank by a cock D, governed by a lever H. -When this lever is moved to D', the cock D is open, and when it is -moved to K, the cock D is closed. From the same tank there -proceeds another pipe E, which issues from the side of the -[Pg456] vessel into the sea governed by a cock F, which is -likewise put in connection with the lever H, so that it shall be -opened when the lever H is drawn to the position F', the cock D' -being closed in all positions of the lever between K and F'. Thus, -whenever the cock F communicating with the sea is open, the cock D -communicating with the boiler is closed, and _vice versa_, both -cocks being closed when the lever is in the intermediate position -K. By this arrangement the boiler cannot, by any neglect in -blowing off, be left in communication with the sea, nor can more -than a ton of water be discharged except by the immediate act of -the engineer. The injurious consequences are thus prevented which -sometimes ensue when the blow-off cocks are left open by any -neglect on the part of the engineer. When it is necessary to blow -off, the engineer moves the lever H, to the position D'. The -pressure of the steam in the boiler on the surface of the water W -forces the salted water or brine up the pipe B A, and through the -open cock C into the tank, and this continues until the tank is -filled: when that takes place, the lever is moved from the -position D' to the position F', by which the cock D is closed, and -the cock F opened. The water in the tank flows through the pipe E -into the sea, air being admitted through the valve V, placed at -the top of the tank, opening inwards. A second ton of brine is -discharged by moving the lever back to the position D', and -subsequently returning it to the position F'; and in this way the -brine is discharged ton by ton, until the supply of water from the -feed which replaces it has caused both the balls in the indicator -to sink to the bottom. - - -(212.) A different method of preserving the requisite freshness of -the water in the boiler has been adopted by Messrs. Maudslay and -Field, and introduced with success into the Great Western and -other steam-vessels. Pumps called _brine-pumps_ are put into -communication with the lower part of the boiler, and so -constructed as to draw the brine therefrom, and drive it into the -sea. These brine-pumps are worked by the engine, and their -operation is constant. The feed-pumps are likewise worked by the -engine, and they bear such a proportion to the brine-pumps that -the quantity of salt discharged in a given time in the brine is -equal to the quantity of salt [Pg457] introduced in solution by -the water of the feed-pumps. By this means the same actual -quantity of salt is constantly maintained in the boiler, and -consequently the strength of the solution remains invariable. If -the brine discharged by the brine-pumps contains 5/32 parts of -salt while the water introduced by the feed-pumps contains only -1/32 part, then it is evident that five cubic feet of the feeding -water will contain no more salt than is contained in one cubic -foot of brine. Under such circumstances the brine-pumps would be -so constructed as to discharge 1/5 of the water introduced by the -feed-pumps, so that 4/5 of all the water introduced into the -boiler would be evaporated, and rendered available for working the -engine. - -To save the heat of the brine, a method has been adopted in the -marine engines constructed by Messrs. Maudslay and Field similar -to one which has been long practised in steam-boilers, and in -various apparatus for the warming of buildings. The current of -heated brine is conducted from the boiler through a tube which is -contained in another, through which the feed is introduced. The -warm current of brine, therefore, as it passes out, imparts a -considerable portion of its heat to the cold feed which comes in; -and it is found that by this expedient the brine discharged into -the sea may be reduced to a temperature of about 100 deg. - -This expedient is so effectual that when the apparatus is properly -constructed, and kept in a state of efficiency, it may be regarded -as nearly a perfect preventive against the incrustation, and the -deposition of salt in the boilers, and is not attended with any -considerable waste of fuel. - - -(213.) About the year 1776, Mr. Watt invented a tubular condenser, -with a view to condense the steam drawn off from the cylinder -without the process of injection. This apparatus consisted of a -number of small tubes connecting the top and bottom of the -condenser, arranged in a manner not very different from that of -the tubes which traverse the boiler of a locomotive engine. These -tubes were continually surrounded by cold water, and the steam, as -it escaped from the cylinder passing through them, was condensed -by their cold surfaces, and collected in the form of water in a -reservoir below, from [Pg458] whence it was drawn off by a pump -in the same manner as in engines which condensed by injection. One -of the advantages proposed by this expedient was, that no -atmospheric air would be introduced into the condenser, as is -always the case when condensation by injection is practised. Cold -water, which is injected, has always combined with it more or less -common air. When this water is mixed with the condensed steam, the -elevation of its temperature disengages the air combined with it, -and this air circulating to the cylinder, vitiates the vacuum. One -of the purposes for which the air-pump in condensing steam-engines -was provided, and from which it took its name, was to draw off -this air. If, however, a tubular condenser could be made to act -with the necessary efficiency, no injection water would be -introduced for condensation, and the pump would have no other duty -except to remove the small quantity of water produced by the -condensed steam. That water being subsequently carried back to the -boiler by the feed-pumps, a constant system of circulation would -be maintained, and the boiler would never require any fresh supply -of water, except what might be necessary to make good the waste by -leakage and other causes. - -This contrivance has been of late years revived by Mr. Samuel Hall -of Basford, near Nottingham, with a view to supersede in marine -engines the necessity of using sea-water in the boilers. Mr. Hall -proposes to make marine boilers with fresh water to condense the -steam without injection, by a tubulated condenser, and to provide -by the distillation of sea-water the small quantity of fresh water -which would be necessary to make good the waste. These condensers -have been introduced into several steam-vessels: in some they have -been continued, and in others abandoned, and various opinions are -entertained of their efficacy. I have not been able to obtain the -results of any satisfactory experiments on them, and cannot -therefore form a judgment of their usefulness. Mr. Watt abandoned -these condensers from finding that the condensation of the steam -was not sufficiently sudden, and that consequently at the -commencement of the stroke the piston was subject to a resistance -which [Pg459] injuriously diminished the amount of the moving -power, whereas condensation by jet was almost instantaneous, and -the efficiency of the piston throughout the entire stroke was more -uniform. - -Mr. Watt also found that a fur collected around the tubes of the -condenser, so as to obstruct the free passage of heat from the -steam to the water of the cold cistern; and that, consequently, -the efficiency of the condenser was gradually impaired, and could -only be restored by frequent cleansing. - -It is stated by Mr. Hall that a vacuum is preserved in his -condensers as perfect as that which is maintained in the ordinary -condensers by injection. It is objected, on the other hand, that -without the injection water and the air which accompanies it being -introduced into his condensers, Mr. Hall uses as large and -powerful an air-pump as those which are used in engines of equal -power condensing by injection; that, consequently, the vacuum -which is maintained is produced, not as it ought to be altogether -by the condensation of steam, but by the air-pump drawing off the -uncondensed steam. To whatever extent this may be true, the -efficacy of the machine, as indicated by the barometer-gauge, is -only apparent; since as much power is necessary to pump away any -portion of uncondensed vapour as is obtained by the vacuum -produced by the absence of that vapour. - -A tubular condenser of the form proposed by Mr. Hall is represented -in _fig._ 126.; _a_ is the upper part of the condenser to which -steam is admitted from the slide after having worked the piston; _k_ -is the section of a thin plate, forming the top of the condenser, -perforated with small holes, in which the tubes are inserted so as -to be steam-tight and water-tight. Water is admitted to flow around -these tubes between the top _k_ and the bottom _d_ of the condenser, -so as to keep them constantly at a low temperature. The steam passes -from _a_ through the tubes to the lower chamber _f_ of the -condenser, where it is reduced to water by the cold to which it has -been exposed. A supply of cold water is constantly pumped through -the condenser, so as to keep the tubes at a low temperature. The -air-pump _g_ is of the usual construction, having valves in the -piston opening upwards, and [Pg460] similar valves in the cover of -the pump also opening upwards. The water formed by the condensed -steam in _f_ is drawn through the foot-valve, and after passing -through the piston-valves, is discharged by the up-stroke of the -piston into the hot well. Any air, or other permanent gas, which may -be admitted by leakage through the tubes of the condenser, or by any -other means, is likewise drawn out by this pump, and when drawn into -the hot well is carried from thence to the feeding apparatus of the -boiler, to which it is transferred by the feed-pump. - -[Illustration: _Fig._ 126.] - -A provision is likewise made by which the steam escaping at the -safety-valve is condensed and carried away to the feeding cistern. - - -(214.) One of the remedies proposed for the evil consequences -arising from incrustation is the substitution of copper for iron -boilers. The attraction which produces the adhesion of the -calcareous matter held in solution by salt water to the surface of -iron has no existence in copper, and all the saline and other -alkaline matter precipitated in the boiling water in [Pg461] -copper boilers is suspended in a loose form, and carried off by -the process of blowing out. - -Besides the injury arising from the deposition of salt and the -incrustation on the inner surface of boilers, an evil of a -formidable kind attends the accumulation of soot mixed with salt -in the flues, which proceeds from the leaks. In the seams of the -boiler there are numerous apertures, of dimensions so small as to -be incapable of being rendered stanch by any practicable means, -through which the water within the boiler filters, and the salt -which it carries with it mixes with the soot, forming a compound -which rapidly corrodes the boilers. This process of corrosion in -the flues takes place not less in copper than in iron boilers. In -cleansing the flues of a copper boiler, the salt and soot which -was thrown out upon the iron-plates which formed the flooring of -the engine-room, having remained there for some time, left behind -it a permanent appearance of copper on the iron flooring, arising -from the precipitation of the copper which had combined with the -soot and salt in the flues.[36] In this case the leaks from whence -the salt proceeded were found, on careful examination, so -unimportant, that the usual means to stanch them could not be -resorted to without the risk of increasing the evil. - - -(215.) In the application of the steam-engine to the propulsion of -vessels in voyages of great extent, the economy of fuel acquires -an importance greater than that which appertains to it in -land-engines, even in localities the most removed from coal-mines, -and where its expense is greatest. The practical limit to -steam-voyages being determined by the greatest quantity of coals -which a steam-vessel can carry, every expedient by which the -efficiency of the fuel can be increased becomes a means, not -merely of a saving of expense, but of an increased extension of -steam-power to navigation. Much attention has been bestowed on the -augmentation of the duty of engines in the mining districts of -Cornwall, where the question of their efficiency is merely a -question of economy, but far greater care should be given to this -subject when the practicability of maintaining intercourse by -steam between distant points of the globe will perhaps depend on -the effect produced by a given quantity [Pg462] of fuel. So long -as steam-navigation was confined to river and channel transport, -and to coasting voyages, the speed of the vessel was a paramount -consideration, at whatever expenditure of fuel it might be -obtained; but since steam-navigation has been extended to -ocean-voyages, where coals must be transported sufficient to keep -the engine in operation for a long period of time without a fresh -relay, greater attention has been bestowed upon the means of -economising it. - -Much of the efficiency of fuel must depend on the management of -the fires, and therefore on the skill and care of the stokers. -Formerly the efficiency of firemen was determined by the abundant -production of steam, and so long as the steam was evolved in -superabundance, however it might have blown off to waste, the duty -of the stoker was considered as well performed. The regulation of -the fires according to the demands of the engine were not thought -of, and whether much or little steam was wanted, the duty of the -stoker was to urge the fires to their extreme limit. - -Since the resistance opposed by the action of the paddle-wheels of -a steam-vessel varies with the state of the weather, the -consumption of steam in the cylinders must undergo a corresponding -variation; and if the production of steam in the boilers be not -proportioned to this, the engines will either work with less -efficiency than they might do under the actual circumstances of -the weather, or more steam will be produced in the boilers than -the cylinders can consume, and the surplus will be discharged to -waste through the safety-valves. The stokers of a marine engine, -therefore, to perform their duty with efficiency, and obtain from -the fuel the greatest possible effect, must discharge the -functions of a self-regulating furnace, such as has been already -described: they must regulate the force of the fires by the amount -of steam which the cylinders are capable of consuming, and they -must take care that no unconsumed fuel is allowed to be carried -away from the ash-pit. - - -(216.) Until within a few years of the present time the heat -radiated from every part of the surface of the boiler was allowed -to go to waste, and to produce injurious effects on those parts of -the vessel to which it was transmitted. This evil, [Pg463] -however, has been lately removed by coating the boilers, -steam-pipes, &c. of steam-vessels with felt, by which the escape -of heat from the surface of the boiler is very nearly, if not -altogether, prevented. This felt is attached to the boiler-surface -by a thick covering of white and red lead. This expedient was -first applied in the year 1818 to a private steam-vessel of Mr. -Watt's called the _Caledonia_, and it was subsequently adopted in -another vessel, the machinery of which was constructed at Soho, -called the _James Watt_. - -The economy of fuel depends in a considerable degree on the -arrangement of the furnaces, and the method of feeding them. In -general each boiler is worked by two or more furnaces communicating -with the same system of flues. While the furnace is fed, the door -being open, a stream of cold air rushes in, passing over the burning -fuel and lowering the temperature of the flues: this is an evil to -be avoided. But, on the other hand, if the furnaces be fed at -distant intervals, then each furnace will be unduly heaped with -fuel, a great quantity of smoke will be evolved, and the combustion -of the fuel will be proportionally imperfect. The process of coking -in front of the grate, which would insure a complete combustion of -the fuel, has been already described (147.). A frequent supply of -coals, however, laid carefully on the front part of the grate, and -gradually pushed backwards as each fresh feed is introduced, would -require the fire-door to be frequently opened, and cold air to be -admitted. It would also require greater vigilance on the part of the -stokers than can generally be obtained in the circumstances in which -they work. In steam-vessels the furnaces are therefore fed less -frequently, fuel introduced in greater quantities, and a less -perfect combustion produced. - -When several furnaces are constructed under the same boiler, -communicating with the same system of flues, the process of -feeding, and consequently opening one of them, obstructs the due -operation of the others, for the current of cold air which is thus -admitted into the flues checks the draft and diminishes the -efficiency of the furnaces in operation. It was formerly the -practice in vessels exceeding one hundred horse-power, to place -four furnaces under each boiler, communicating with the same -system of flues. Such an arrangement [Pg464] was found to be -attended with a bad draft in the furnaces, and therefore to -require a greater quantity of heating surface to produce the -necessary evaporation. This entailed upon the machinery the -occupation of more space in the vessel in proportion to its power; -it has therefore been more recently the practice to give a -separate system of flues to each pair of furnaces, or, at most, to -every three furnaces. When three furnaces communicate with a -common flue, two will always be in operation, while the third is -being cleared out; but if the same quantity of fire were divided -among two furnaces, then the clearing out of one would throw out -of operation half the entire quantity of fire, and during the -process the evaporation would be injuriously diminished. It is -found by experience, that the side plates of furnaces are liable -to more rapid destruction than their roofs, owing, probably, to a -greater liability to deposit. Furnaces, therefore, should not be -made narrower than a certain limit. Great depth from front to back -is also attended with practical inconvenience, as it renders -firing tools of considerable length, and a corresponding extent of -stoking room necessary. It is recommended, by those who have had -much practical experience in steam-vessels, that furnaces six feet -in depth from front to back should not be less than three feet in -width, to afford means of firing with as little injury to the side -plates as possible, and of keeping the fires in the condition -necessary for the production of the greatest effect. The tops of -the furnaces almost never decay, and seldom are subject to an -alteration of figure, unless the level of the water be allowed to -fall below them.[37] - - -(217.) A form of marine engine was some years since proposed and -patented by Mr. Thomas Howard, possessing much novelty and -ingenuity, and having pretensions to a very extraordinary economy -of fuel, in addition to the advantages claimed by Mr. Hall. In Mr. -Howard's engines, the steam, as in Mr. Hall's, is constantly -reproduced from the same water, so that pure or distilled water -may be used; but Mr. Howard dispenses altogether with the use of a -boiler. - -A quantity of mercury is placed in a shallow wrought-iron -vessel over a coke fire, by which it is maintained at a [Pg465] -temperature varying from 400 deg. to 500 deg. The surface exposed -to the fire was computed at three fourths of a square foot for each -horse-power. The upper surface of the mercury was covered by a -very thin plate of iron in contact with it, and so contrived as to -present about four times as much surface as that exposed beneath -the fire. Adjacent to this a vessel of water was placed, -maintained nearly at the boiling point, and communicating by a -nozzle and valve with the chamber immediately above the mercury. -At intervals corresponding to the motion of the piston a small -quantity of water was injected from this vessel, and thrown upon -the plate of iron resting upon the hot mercury. From this it -received not only the heat necessary to convert it into common -steam, but to give it the qualities of highly superheated steam. -In fact, the steam thus produced had a temperature considerably -above that which corresponded to its pressure, and was, therefore, -capable of being deprived of more or less of its heat without -being condensed. (94.) The quantity of water injected into the -steam-chamber was regulated by the power at which the engine was -intended to be worked. The fire was supplied with air by a blower -subject to exact regulation. The steam thus produced was conducted -to a chamber surrounding the working cylinder, and this chamber -itself was enclosed by another space through which the air from -the furnace passed before it reached the flue. By this contrivance -the air imparted its redundant heat to the steam, as the latter -passed to the cylinder, and raised its temperature to about 400 deg., -the pressure, however, not exceeding 25 lbs. per square inch. The -valves, governing the admission of steam to the piston, were -adapted for expansive action. - -The vacuum on the opposite side was maintained by condensation in -the following manner:--The condenser was a copper vessel placed in -a cistern of cold water, and the steam was admitted to it from the -cylinder by an eduction pipe in the usual way. A jet was -introduced from an adjacent vessel filled with distilled water, -and the condensing water and condensed steam were pumped from the -condenser as in common engines. The warm water thus pumped out of -the [Pg466] condenser was drawn through a copper worm, carried -with many coils through a cistern of cold water, so that when it -arrived at the end of this pipe it was reduced nearly to the -temperature of the atmosphere. The pipe was thus brought to the -vessel of distilled water already mentioned, and the water -supplied by it replaced. The water admitted to the condenser -through the condensing jet being purged of air, a small air-pump -was sufficient, since it had only to exhaust the condenser and -tubes at starting, and to remove the air which might be admitted -by leakage. Mr. Howard stated that the condensation took place as -rapidly and perfectly as in the best engines of the common kind. - -An engine of this construction was in the spring of 1835 placed in -the government steamer called the _Comet_. It was stated, that -though the machinery was not advantageously constructed, a part of -the engine being old, and not made expressly for a boiler of this -kind, the vessel performed a voyage from Falmouth to Lisbon, in -which the consumption of fuel did not exceed a third of her former -consumption when worked by Boulton and Watt's engines, the former -consumption of coals being about eight hundred pounds per hour, -and the consumption of Mr. Howard's engine being less than two -hundred and fifty pounds of coke per hour. - -The advantages claimed for this contrivance were the following: -_first_, the small space and weight occupied by the machinery, -arising from the absence of a boiler; _second_, the diminished -consumption of fuel; _third_, the reduced size of the flues; -_fourth_, the removal of the injurious effects arising from -deposit and incrustation; _fifth_, the absence of smoke. - - -(218.) The method by which the greatest quantity of practical effect -can be obtained from a given quantity of fuel must, however, mainly -depend on the extended application of the expansive principle. This -has been the means by which an extraordinary amount of duty has been -obtained from the Cornish engines. The difficulty of the application -of this principle in marine engines has arisen from the objections -entertained in Europe to the use of steam of high pressure under the -circumstances in which the engine must be worked at sea. To apply -the expansive principle, it is necessary that the moving power at -the commencement of the stroke shall considerably exceed the -[Pg467] resistance, its force being gradually attenuated till the -completion of the stroke, when it will at length become less than -the resistance. This condition may, however, be attained with steam -of limited pressure, if the engine be constructed with a sufficient -quantity of piston-surface. This method of rendering the expansive -principle available at sea, and compatible with low-pressure steam, -has recently been brought into operation by Messrs. Maudslay and -Field. Their improvement consists in adapting two steam-cylinders in -one engine, in such a manner that the steam shall act simultaneously -on both pistons, causing them to ascend and descend together. The -piston-rods are both attached to the same horizontal cross-head, -whereby their combined action is applied to one crank by means of a -connecting rod placed between the pistons. - -[Illustration: _Fig._ 127.] - -A section of such an engine, made by a plane passing through the -two piston-rods P P' and cylinders, is represented in _fig._ 127. -The piston-rods are attached to a cross-head C, [Pg468] which -ascends and descends with them. This cross-head drives upwards and -downwards an axle D, to which the lower end of the connecting rod -E is attached. The other end of the connecting rod drives the -crank-pin F, and imparts revolution to the paddle-shaft G. A rod H -conveys motion by means of a beam I to the rod K of the air-pump -E. - - -(219.) Connected with this, and in the same patent, another -improvement is included, consisting of the application of a hollow -wrought-iron framing carried across the vessel above the machinery, -to support the whole of the bearings of the crank-shaft. A plan of -this, including the cylinders and paddle-wheel, is represented in -_fig._ 128. The advantages proposed by these improvements are -simplicity of construction, more direct action on the crank, economy -of space and weight of material, combined with increased area of the -piston, whereby a given evaporating power of the boiler is rendered -productive, by extended application of the expansive principle, of a -greater moving power than in former arrangements. Consequently, -under like circumstances, greater power and economy of fuel is -obtained, with the further advantage at sea, that when the engine is -reduced in its speed, either by the vessel being deeply laden with -coal, as is the case at the commencement of a long sea voyage, or by -head winds, more steam may be given to the cylinders, and -consequently more speed imparted to the vessel, all the steam -produced in the boiler being usefully employed. - - -(220.) Another improvement, having the same objects, and analogous -to the preceding, has been likewise patented by Messrs. Maudslay -and Field. This consists in the adoption of a cylinder of greater -diameter, having two piston-rods P P', as represented in _fig._ -129., of considerable length, connected at the top by a cross-head -C. From this cross-head is carried downwards the connecting rod D, -which drives the crank-pin E, and thereby works the paddle-shaft -S. In this case the paddle-shaft is extended immediately above the -piston, and the double piston-rod has sufficient length to be -above the paddle-shaft when the piston is at the bottom of its -stroke. This improvement is intended to be applied more -particularly for engines for river navigation, the advantages -resulting from [Pg469] it being that a paddle-shaft placed at a -given height from the bottom of the vessel will be enabled to -receive a longer stroke of piston than by any other arrangement -now in use. A more [Pg470] compact and firm connection of the -cylinder with the crank-shaft bearings is effected by it, and a -cylinder of much greater diameter may be applied by which the -expansive action of steam may be more fully brought into play; and -a more direct action of the steam-power on the crank with a less -weight of materials and a greater economy of space may be obtained -than by any of the arrangements of marine engines hitherto used. - -[Illustration: _Fig._ 128.] - -[Illustration: _Fig._ 129.] - - -(221.) Mr. Francis Humphrys has obtained a patent for a form of -marine engine, by which some simplification of the machinery is -attained, and the same power comprised within more limited -dimensions. In this engine there is attached to the piston of the -cylinder, instead of a piston-rod, a hollow casing D D (_fig._ -130.), which moves through a stuffing-box G, constructed in a -manner similar to the stuffing-box of a piston-rod. In the figure, -this casing is presented in section, but [Pg471] its form is that -of a long narrow slit, or opening, rounded at either end as -exhibited in the plan (_fig._ 131) of the cylinder-cover. The -crank C is driven by the other end of the connecting rod H, the -crank-shaft being immediately above the centre of the piston and -the connecting rod passing through the oblong opening D, and -descending into the hollow piston-rod it is attached to an axis I -at the bottom of the piston. A box or cover K K encloses the -cross-piece or axis I with its bearings, and is [Pg472] attached -so as to be steam-tight to the bottom of the piston. A hollow -space L L is cast in the bottom of the cylinder for the reception -of the box K K, when the piston is at the bottom of the cylinder. - -[Illustration: _Fig._ 130.] - -[Illustration: _Fig._ 131.] - -By this arrangement the force by which the piston is driven in its -ascent and descent is communicated to the connecting rod, not, as -usual, through the intervention of a piston-rod, but directly from -the piston itself by the cross-pin I, and from thence to the crank -C, which it drives without the intervention of beams, cross-heads, -or any similar appendage. - -The slide-valves regulating the admission and eduction of steam -are represented at _a_; the rod of the air-pump is shown at _d_, -being worked by a crank placed on the centre of the great crank -shaft.[38] - - -(222.) To obtain from the moving power its full amount of -mechanical effect in propelling the vessel, it would be necessary -that its force should propel, by constantly acting against the -water in a horizontal direction, and with a motion contrary to the -course of the vessel. No system of mechanical propellers has, -however, yet been contrived capable of perfectly accomplishing -this. Patents have been granted for many ingenious mechanical -combinations to impart to the propelling surfaces such angles as -appeared to the respective contrivers most advantageous. In most -of these the mechanical complexity has formed a fatal objection. -No part of the machinery of a steam-vessel is so liable to become -deranged at sea as the paddle-wheels; and, therefore, that -simplicity of construction which is compatible with those repairs -which are possible on such emergencies is quite essential for safe -practical use. - -[Illustration: _Fig._ 132.] - -The ordinary paddle-wheel, as has been already stated, is a wheel -revolving upon a shaft driven by the engine, and carrying upon its -circumference a number of flat boards, called paddle-boards, which -are secured by nuts and braces in a fixed position; and that -position is such that the planes [Pg473] of the paddle-boards -diverge nearly from the centre of the shaft on which the wheel -turns. The consequence of this arrangement is that each -paddle-board can only act in that direction which is most -advantageous for the propulsion of the vessel when it arrives near -the lowest point of the wheel. In _fig._ 132. let O be the shaft -on which the common paddle-wheel revolves; the position of the -paddle-boards are represented at A, B, C, &c.; X, Y represents the -water line, the course of the vessel being supposed to be from X -to Y; the arrows represent the direction in which the paddle-wheel -revolves. The wheel is immersed to the depth of the lowest -paddle-board, since a less degree of immersion would render a -portion of the surface of each paddle-board mechanically useless. -In the position A the whole force of the paddle-board is efficient -for propelling the vessel; but as the paddle enters the water in -the position H, its action upon the water, not being horizontal, -is only partially effective for propulsion: a part of the force -which drives the paddle is expended in depressing the water, and -the remainder in driving it contrary to the course of the vessel, -and, therefore, by its re-action producing a certain propelling -effect. The tendency, however, of the paddle entering the water at -H, is to form a hollow or trough, which the water, by its ordinary -property, has a continual tendency to fill up. After passing the -lowest point A, as the paddle approaches the position B, where it -[Pg474] emerges from the water, its action again becomes oblique, -a part only having a propelling effect, and the remainder having a -tendency to raise the water, and throw up a wave and spray behind -the paddle-wheel. It is evident that the more deeply the -paddle-wheel becomes immersed, the greater will be the proportion -of the propelling power thus wasted in elevating and depressing -the water; and if the wheel were immersed to its axis, the whole -force of the paddle-boards, on entering and leaving the water, -would be lost, no part of it having a tendency to propel. If a -still deeper immersion take place, the paddle-boards above the -axis would have a tendency to retard the course of the vessel. -When the vessel is, therefore, in proper trim, the immersion -should not exceed nor fall short of the depth of the lowest -paddle; but for various reasons it is impossible in practice to -maintain this fixed immersion: the agitation of the surface of the -sea, causing the vessel to roll, will necessarily produce a great -variation in the immersion of the paddle-wheels, one becoming -frequently immersed to its axle, while the other is raised -altogether out of the water. Also the draught of water of the -vessel is liable to change, by the variation in her cargo; this -will necessarily happen in steamers which take long voyages. At -starting they are heavily laden with fuel, which as they proceed -is gradually consumed, whereby the vessel is lightened. - - -(223.) To remove this defect, and economise as much as possible -the propelling effect of the paddle-boards, it would be necessary -so to construct them that they may enter and leave the water -edgeways, or as nearly so as possible; such an arrangement would -be, in effect, equivalent to the process called feathering, as -applied to oars. Any mechanism which would perfectly accomplish -this would cause the paddles to work in almost perfect silence, -and would very nearly remove the inconvenient and injurious -vibration which is produced by the action of the common paddles. -But the construction of feathering paddles is attended with great -difficulty, under the peculiar circumstances in which such wheels -work. Any mechanism so complex that it could not be easily -repaired when deranged, with such engineering implements and -skill [Pg475] as can be obtained at sea, would be attended with -great objections; and the efficiency of its propelling action -would not compensate for the dangers which must attend upon the -helpless state of a steamer, deprived of her propelling agents. - -Feathering paddle-boards must necessarily have a motion -independently of the motion of the wheel, since any fixed position -which could be given to them, though it might be most favourable to -their action in one position would not be so in their whole course -through the water. Thus the paddle-board when at the lowest point -should be in a vertical position, or so placed that its plane, if -continued upwards, would pass through the axis of the wheel. In -other positions, however, as it passes through the water, it should -present its upper edge, not towards the axle of the wheel, but -towards a point above the highest point of the wheel. The precise -point to which the edge of the paddle-board should be directed is -capable of mathematical determination. But it will vary according to -circumstances, which depend on the motion of the vessel. The -progressive motion of the vessel, independently of the wind or -current, must obviously be slower than the motion of the -paddle-boards round the axle of the wheel; since it is by the -difference of these velocities that the re-action of the water is -produced by which the vessel is propelled. The proportion, however, -between the progressive speed of the vessel and the rotative speed -of the paddle-boards is not fixed: it will vary with the shape and -structure of the vessel, and with its depth of immersion; -nevertheless it is upon this proportion that the manner in which the -paddle-boards should shift their position must be determined. If the -progressive speed of the vessel were nearly equal to the rotative -speed of the paddle-boards, the latter should so shift their -position that their upper edges should be presented to a point very -little above the highest point of the wheel. This is a state of -things which could only take place in the case of a steamer of a -small draught of water, shallop-shaped, and so constructed as to -suffer little resistance from the fluid. On the other hand, the -greater the depth of immersion, and the less fine the lines of the -[Pg476] vessel, the greater will be the resistance in passing -through the water, and the greater will be the proportion which the -rotative speed of the paddle-boards will bear to the progressive -speed of the vessel. In this latter case the independent motion of -the paddle-boards should be such that their edges, while in the -water, shall be presented towards a point considerably above the -highest point of the paddle-wheel. - -A vast number of ingenious mechanical contrivances have been -invented and patented for accomplishing the object just explained. -Some of these have failed from the circumstance of their inventors -not clearly understanding what precise motion it was necessary to -impart to the paddle-board: others have failed from the complexity -of the mechanism by which the desired effect was produced. - - -(224.) In the year 1829 a patent was granted to Elijah Galloway -for a paddle-wheel with movable paddles, which patent was -purchased by Mr. William Morgan, who made various alterations in -the mechanism, not very materially departing from the principle of -the invention. - -[Illustration: _Fig._ 133.] - -This paddle-wheel is represented in _fig._ 133. The contrivance -may be shortly stated to consist in causing the wheel which bears -the paddles to revolve on one centre, and the radial arms which -move the paddles to revolve on another centre. Let A B C D E F G H -I K L be the polygonal circumference of the paddle-wheel, formed -of straight bars, securely connected together at the extremities -of the spokes or radii of the wheel which turns on the shaft which -is worked by the engine; the centre of this wheel being at O. So -far this wheel is similar to the common paddle-wheel; but the -paddle-boards are not, as in the common wheel, fixed at A B C, -&c., so as to be always directed to the centre O, but are so -placed that they are capable of turning on axles which are always -horizontal, so that they can take any angle with respect to the -water which may be given to them. From the centres, or the line -joining the pivots on which these paddle-boards turn, there -proceed short arms K, firmly fixed to the paddle-boards at an -angle of about 120 deg. On a motion given to this arm K, it will -therefore give a corresponding angular motion to the paddle-board, -so as to make it turn on its pivots. At [Pg477] the extremities -of the several arms marked K is a pin or pivot, to which the -extremities of the radial arms L are severally attached, so that -the angle between each radial arm L and the short paddle-arm K is -capable of being changed by any motion imparted to L; the radial -arms are connected at the other end with a centre, round which -they are capable of revolving. Now, since the points A B C, &c., -which are the pivots on which the paddle-boards turn, are moved in -the circumference of a circle, of which the centre is O, they are -always at the same distance from that point; consequently they -will continually vary their distance from the other centre P. -Thus, when a paddle-board arrives at that point of its revolution -at which the centre round which it revolves lies precisely between -it and the centre O, its distance from the former centre is less -than in any other position. As it departs from that point, its -distance from that centre gradually increases until it arrives at -the opposite point of its revolution, where the centre O is -exactly between it and the former centre; then the distance of the -paddle-board from the former centre is greatest. [Pg478] This -constant change of distance between each paddle-board and the -centre P is accommodated by the variation of the angle between the -radial arm L and the short paddle-board arm K; as the paddle-board -approaches the centre P this gradually diminishes; and as the -distance of the paddle-board increases, the angle is likewise -augmented. This change in the magnitude of the angle, which thus -accommodates the varying position of the paddle-board with respect -to the centre P, will be observed in the figure. The paddle-board -D is nearest to P; and it will be observed that the angle -contained between L and K is there very acute; at E the angle -between L and K increases, but is still acute; at G it increases -to a right angle; at H it becomes obtuse; and at K, where it is -most distant from the centre P, it becomes most obtuse. It again -diminishes at K, and becomes a right angle between A and B. Now -this continual shifting of the direction of the short arm K is -necessarily accompanied by an equivalent change of position in the -paddle-board to which it is attached; and the position of the -second centre P is, or may be, so adjusted that this paddle-board, -as it enters the water and emerges from it, shall be such as shall -be most advantageous for propelling the vessel, and therefore -attended with less of that vibration which arises chiefly from the -alternate depression and elevation of the water, owing to the -oblique action of the paddle-boards. - - -(225.) In the year 1833, Mr. Field, of the firm of Maudslay and -Field, constructed a paddle-wheel with fixed paddle-boards, but -each board being divided into several narrow slips arranged one a -little behind the other, as represented in _fig._ 134. These -divided boards he proposed to arrange in such cycloidal curves -that they must all enter the water at the same place in immediate -succession, avoiding the shock produced by the entrance of the -common board. These split paddle-boards are as efficient in -propelling when at the lowest point as the common paddle-boards, -and when they emerge the water escapes simultaneously from each -narrow board, and is not thrown up, as is the case with common -paddle-boards.[39] - -[Illustration: _Fig._ 134.] - -[Pg479] The theoretical effect of this wheel is the same as that -of the common wheel, and experience alone, the result of which has -not yet been obtained, can prove its efficiency. The number of -bars, or separate parts into which each paddle-board is divided, -has been very various. When first introduced by Mr. Galloway each -board was divided into six or seven parts: this was subsequently -reduced, and in the more recent wheels of this form constructed -for the government vessels the paddle-boards consist only of two -parts, coming as near to the common wheel as is possible, without -altogether abandoning the principle of the split paddle. - - -(226.) To obtain an approximate estimate of the extent to which -steam-power is applicable to long sea-voyages, it would be -necessary to investigate the mutual relation which, in the -existing state of this application of steam-power, exists between -the capacity or tonnage of the vessel, the magnitude, weight, and -power, of the machinery, the available stowage for fuel, and the -average speed attainable in all [Pg480] weathers, as well as the -general purposes to which the vessel is to be appropriated, -whether for the transport of goods or merchandise, or merely for -despatches and passengers, or for both of these combined. That -portion of the capacity of the vessel which is appropriated to the -moving power consists of the space occupied by the machinery and -the fuel. The distribution of it between these must mainly depend -on the length of the voyage which the vessel must make without -receiving a fresh supply of coals. If the trips be short, and -frequent relays of fuel can be obtained, then the space allotted -to the machinery may bear a greater proportion to that assigned to -the fuel; but in proportion as each uninterrupted stage of the -voyage is increased, a greater stock of coals will be necessary, -and a proportionally less space left for the machinery. Other -things being the same, therefore, steam-vessels intended for long -sea-voyages must be less powerful in proportion to their tonnage. - -It will be apparent that every improvement which takes place in -the application of the steam-engine to navigation will modify all -these data on which such an investigation must depend. Every -increased efficiency of fuel, from whatever cause it may be -derived, will either increase the useful tonnage of the vessel, or -increase the length of the voyage of which it is capable. Various -improvements have been and are still in progress, by which this -efficiency has undergone continual augmentation, and voyages may -now be accomplished with moderate economy and profit, to which a -few years since marine engines could not be applied with permanent -advantage. The average speed of steam-vessels has also undergone a -gradual increase by such improvements. During the four years -ending June, 1834, it was found that the average rate of steaming -obtained from fifty-one voyages made by the Admiralty steamers -between Falmouth and Corfu, exclusive of stoppages, was seven -miles and a quarter an hour direct distance between port and port. -The vessels which performed this voyage varied from 350 to 700 -tons measured burden, and were provided with engines varying from -100 to 200 horse-power, with stowage for coals varying from 80 to -240 tons. The proportion of the power to the [Pg481] tonnage -varied from one horse to three tons to one horse to four tons. -Thus the MESSENGER had a power of 200 horses and measured 730 -tons; the FLAMER had a power of 120 horses, and measured 500 tons; -the COLUMBIA had a power of 120 horses, and measured 360 tons. In -general it may be assumed that for the shortest class of trips, -such as those of the Channel steamers, the proportion of the power -to the tonnage should be about one horse for every two tons; but -for the longer class of voyages, the proportion of power to -tonnage should be about one horse-power to from three to four tons -measured tonnage. These data, however, must be received as very -rough approximations, subject to considerable modifications in -their application to particular vessels. We have already stated -that the nominal horse-power is itself extremely indefinite; and -if, as is now customary in the longer class of voyages, the steam -be worked expansively, then the nominal power almost ceases to -have any definite relation to the actual performance of the -vessel. It is usual to calculate the horse-power by assuming a -uniform pressure of steam upon the piston, and, consequently, by -excluding the consideration of the effect of expansion. The most -certain test of the amount of mechanical power exerted by the -machinery would be obtained from the quantity of water actually -transmitted in the form of steam from the boiler to the cylinder. -But the effect of this would also be influenced by the extent to -which the expansive principle has been brought into operation. - -From the reported performances of the larger class of steam-ships -within the last few years, it would appear that the average speed -has been increased since the estimate above mentioned, which was -obtained in 1834; and on comparing the consumption of fuel with -the actual performance, it would appear that the efficiency of -fuel has also been considerably augmented. No extensive course of -accurate experiments or observations have, however, been obtained -from which correct inferences may be drawn of the probable limits -to which steam-navigation, in its present state, is capable of -being extended. The jealousy of rival companies has obstructed the -inquiries of those who, solicitous more [Pg482] for the general -advancement of the art than for the success of individual -enterprises, have directed their attention to this question; and -it is hardly to be expected that sufficiently correct and -extensive data can be obtained for this purpose. - - -(227.) Increased facility in the extension and application of -steam-navigation is expected to arise from the substitution of iron -for wood, in the construction of vessels. Hitherto iron steamers -have been chiefly confined to river-navigation; but there appears no -sufficient reason why their use should be thus limited. For -sea-voyages they offer many advantages; they are not half the weight -of vessels of equal tonnage constructed of wood; and, consequently, -with the same tonnage they will have less draught of water, and -therefore less resistance to the propelling power; or, with the same -draught of water and the same resistance, they will carry a -proportionally heavier cargo. The nature of their material renders -them more stiff and unyielding than timber; and they do not suffer -that effect which is called _hogging_, which arises from a slight -alteration which takes place in the figure of a timber vessel in -rolling, accompanied by an alternate opening and closing of the -seams. Iron vessels have the further advantage of being more proof -against fracture upon rocks. If a timber vessel strike, a plank is -broken, and a chasm opened in her many times greater than the point -of rock which produces the concussion. If an iron vessel strike, she -will either merely receive a dinge, or be pierced by a hole equal in -size to the point of rock which she encounters. Some examples of the -strength of iron vessels were given by Mr. Macgregor Laird, in his -evidence before the Committee of the Commons on Steam Navigation, -among which the following may be mentioned:--An iron vessel, called -the ALBURKAH, in one of their experimental trials got aground, and -lay upon her anchor: in a wooden vessel the anchor would probably -have pierced her bottom; in this case, however, the bottom was only -dinged. An iron vessel, built for the Irish Inland Navigation -Company, was being towed across Lough Derg in a gale of wind, when -the towing rope broke, and she was driven upon rocks, on which she -bumped for a considerable time [Pg483] without any injury. A wooden -vessel would in this case have gone to pieces. A further advantage -of iron vessels (which in warm climates is deserving of -consideration) is their greater coolness and perfect freedom from -vermin. - -Iron steam-vessels on a very large scale are now in preparation in -the ports of Liverpool and Bristol, intended for long sea-voyages. -The largest vessel of this description which has yet been -projected is stated to be in preparation for the voyage between -Bristol and New York, by the company who have established the -steam-ship called the Great Western, plying between these places. - -Several projects for the extension of steam-navigation to voyages -of considerable length have lately been entertained both by the -public and by the legislature, and have imparted to every attempt -to improve steam-navigation increased interest. A committee of the -House of Commons collected evidence and made a report in the last -session in favour of an experiment to establish a line of -steam-communication between Great Britain and India. Two routes -have been suggested by the committee, each being a continuation of -the line of Admiralty steam-packets already established to Malta -and the Ionian Isles. One of the routes proposed is through Egypt, -the Red Sea, and across the Indian Ocean to Bombay, or some of the -other presidencies; the other across the north part of Syria to -the banks of the Euphrates, by that river to the Persian Gulf, and -from thence to Bombay. Each of these routes will be attended with -peculiar difficulties, and in both a long sea-voyage will be -encountered. - -In the route by the Red Sea it is proposed to establish steamers -between Malta and Alexandria (eight hundred and sixty miles). A -steamer of four hundred tons' burden and one hundred horse-power -would perform this voyage, upon an average of all weathers -incident to the situation, in from five to six days, consuming ten -tons of coal per day. But it is probable that it might be found -more advantageous to establish a higher ratio between the power -and the tonnage. From Alexandria the transit might be effected by -land across the isthmus to Suez--a journey of from four to five -days--by caravan and camels; or the transit might be made either -[Pg484] by land or water from Alexandria to Cairo, a distance of -one hundred and seventy-three miles; and from Cairo to Suez, -ninety-three miles, across the desert, in about five days. At Suez -would be a station for steamers, and the Red Sea would be -traversed in three runs or more. If necessary, stations for coals -might be established at Cosseir, Judda, Mocha, and finally at Aden -or at Socatra--an island immediately beyond the mouth of the Red -Sea, in the Indian Ocean; the run from Suez to Cosseir would be -three hundred miles--somewhat more than twice the distance from -Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty -miles; from Judda to Mocha, five hundred and seventeen miles; and -from Mocha to Socatra, six hundred and thirty-two miles. It is -evident that all this would, without difficulty, in the most -unfavourable weather, fall within the present powers of -steam-navigation. If the terminus of the passage be Bombay, the -run from Socatra to Bombay will be twelve hundred miles, which -would be from six to eight days' steaming. The whole passage from -Alexandria to Bombay, allowing three days for delay between Suez -and Bombay, would be twenty-six days: the time from Bombay to -Malta would therefore be about thirty-three days; and adding -fourteen days to this for the transit from Malta to England, we -should have a total of forty-seven days from London to Bombay, or -about seven weeks. - -If the terminus proposed were Calcutta, the course from Socatra -would be one thousand two hundred and fifty miles south-east to -the Maldives, where a station for coals would be established. This -distance would be equal to that from Socatra to Bombay. From the -Maldives, a run of four hundred miles would reach the southern -point of Ceylon, called the Point de Galle, which is the best -harbour (Bombay excepted) in British India: from the Point de -Galle, a run of six hundred miles will reach Madras, and from -Madras to Calcutta would be a run of about six hundred miles. The -voyage from London to Calcutta would be performed in about sixty -days. - -At a certain season of the year there exists a powerful physical -opponent to the transit from India to Suez: from [Pg485] the -middle of June until the end of September, the south-west monsoon -blows with unabated force across the Indian Ocean, and more -particularly between Socatra and Bombay. This wind is so violent -as to leave it barely possible for the most powerful steam-packet -to make head against it, and the voyage could not be accomplished -without serious wear and tear upon the vessels during these -months. - -The attention of parliament has therefore been directed to another -line of communication, not liable to this difficulty: it is -proposed to establish a line of steamers from Bombay through the -Persian Gulf to the Euphrates. - -The run from Bombay to a place called Muscat, on the southern shore -of the gulf, would be eight hundred and forty miles in a north-west -direction, and therefore not opposed to the south-west monsoon. From -Muscat to Bassidore, a point upon the northern coast of the strait -at the mouth of the Persian Gulf, would be a run of two hundred and -fifty-five miles; from Bassidore to Bushire, another point on the -eastern coast of the Persian Gulf, would be a run of three hundred -miles; and from Bushire to the mouth of the Euphrates, would be one -hundred and twenty miles. It is evident that the longest of these -runs would offer no more difficulty than the passage from Malta to -Alexandria. From Bussora, near the mouth of the Euphrates, to Bir, a -town upon its left bank near Aleppo, would be one thousand one -hundred and forty-three miles, throughout which there are no -physical obstacles to the river-navigation which may not be -overcome. Some difficulties arise from the wild and savage character -of the tribes who occupy its banks. It is, however, thought that by -proper measures, and securing the co-operation of the pacha of -Egypt, any serious obstruction from this cause may be removed. From -Bir, by Aleppo, to Scanderoon, a port upon the Mediterranean, -opposite Cyprus, is a land-journey, said to be attended with some -difficulty, but not of great length; and from Scanderoon to Malta is -about the same distance as between the latter place and Alexandria. -It is calculated that the time from London to Bombay by the -Euphrates--supposing the passage to be successfully [Pg486] -established--would be a few days shorter than by Egypt and the Red -Sea. - -Whichever of these courses may be adopted, it is clear that the -difficulties, so far as the powers of the steam engine are -concerned, lie in the one case between Socatra and Bombay, or -between Socatra and the Maldives, and in the other case between -Bombay and Muscat. This, however, has already been encountered and -overcome on four several voyages by the HUGH LINDSAY steamer from -Bombay to Suez: that vessel encountered a still longer run on -these several trips, by going, not to Socatra, but to Aden, a -point on the coast of Arabia, near the Straits of Babel Mandeb, -being a run of one thousand six hundred and forty-one miles, which -she performed in ten days and nineteen hours. The same trip has -since been repeatedly made by other steamers; and, in the present -improved state of steam navigation, no insurmountable obstacles -are opposed to their passage. - -[Illustration] - - FOOTNOTES: - - [35] This cut is taken from the plate of the engine of the Red - Rover, manufactured by Boulton and Watt, given in the last - edition of _Tredgold on the Steam Engine_. - - [36] Appendix I., _on Marine Boilers, by J. Dinnen; Tredgold_ - _on the Steam Engine_, second edition. - - [37] _Tredgold on the Steam Engine_, Appendix, I. p. 171. - - [38] Engines on a very large scale constructed upon this - principle are said to be in process of construction for an - iron steam-vessel of great tonnage, which is in preparation - for the New York passage. It is said that the cylinders of - these engines will be one hundred and twenty inches in - diameter. - - [39] A patent was subsequently taken out for these by Mr. - Galloway. Mr. Field did not persevere in its use at the time - he invented it. It has, however, been more generally adopted - since the date of Galloway's patent. - -[Pg487] - - - - -[Illustration] - -CHAP. XIV. - -AMERICAN STEAM NAVIGATION. - - STEAM NAVIGATION FIRST ESTABLISHED IN AMERICA. -- CIRCUMSTANCES - WHICH LED TO IT. -- FITCH AND RUMSEY. -- STEVENS OF HOBOKEN. -- - LIVINGSTONE AND FULTON. -- EXPERIMENTS ON THE SEINE. -- FULTON'S - FIRST BOAT. -- THE HUDSON NAVIGATED BY STEAM. -- EXTENSION AND - IMPROVEMENT OF RIVER NAVIGATION. -- SPEED OF AMERICAN STEAMERS. - -- DIFFERENCE BETWEEN THEM AND EUROPEAN STEAMERS. -- SEA-GOING - AMERICAN STEAMERS. -- AMERICAN PADDLE-WHEELS. -- LAKE STEAMERS. - -- THE MISSISIPPI AND ITS TRIBUTARIES. -- STEAMERS NAVIGATING - IT. -- THEIR STRUCTURE AND MACHINERY. -- NEW ORLEANS HARBOUR. -- - STEAM TUGS. - - -(228.) The credit of having afforded the first practical solution -of the problem to apply the steam engine to the propulsion of -ships, undoubtedly belongs to the people of the United States of -America. The geographical character of their vast country, not -less than the sanguine and enterprising spirit of the nation, -contributed to this. A coast of four thousand miles in extent, -stretching from the Gulf of St. Lawrence to the embouchures of the -Mississippi, indented and [Pg488] serrated in every part with -natural harbours and sheltered bays, and fringed with islands -forming sounds--capes, and promontories enclosing arms of the sea, -in which the waters are free from the roll of the ocean, and take -the placid character of lakes,--rivers of imposing magnitude, -navigable for vessels of the largest class, for many hundreds and -in some instances for many thousands of miles, affording access to -the innermost population of an empire, whose area vastly exceeds -the whole European continent,--chains of lakes composed of the -most extensive bodies of fresh water in the known world,--and this -extensive continent peopled by races carrying with them the habits -and feelings together with much of the skill and knowledge of the -most civilized parts of the globe, endowed also with that -inextinguishable spirit of enterprise which ever belongs to an -emigrant people,--form a combination of circumstances more than -sufficient to account for the fact of this nation snatching from -England, the parent of the steam engine, the honour of first -bringing into practical operation one of the most important--if -indeed it be not altogether the most important--of the many -applications of that machine to the uses of life. - -The circumstances which rendered these extensive tracts of inland -and coast navigation eminently suited to the application of steam -power, formed so many obstructions and difficulties to the -application of other more ordinary means of locomotion on water. -The sheltered bays and sounds which offered a smooth and -undisturbed surface to the action of the infant steamer argued the -absence of that element which gave effect to the sails and rigging -of the wind-propelled ship, and the rapid currents of the gigantic -streams formed by the drainage of this great continent, though -facilitating access to the coast, rendered the oar powerless in -the ascent. - - -(229.) The first great discovery of Watt had scarcely been -realized in practice by the construction of the single-acting -steam-engine, when the speculative and enterprising Americans -conceived the project of applying it as a moving power in their -inland navigation. So early as the year 1783 [Pg489] Fitch and -Rumsey made attempts to apply the single-acting engine to the -propulsion of vessels, and their failure is said to have arisen -more from the inherent defects of that machine in reference to -this application of it, than from any want of ingenuity or -mechanical skill on their parts. In 1791, John Stevens of Hoboken -commenced his experiments on steam navigation, which were -continued for sixteen years; during a part of this period he was -assisted by Livingstone (who was subsequently instrumental in -advancing the views of Fulton), and by Roosevelt. These projectors -had, at that time also, the assistance and advice of Brunel, since -so celebrated for the invention of the block machinery, and the -construction of the Thames Tunnel. Their proceedings were -interrupted by the appointment of Livingstone as American Minister -at Paris, under the Consular Government. - -At Paris, Livingstone met Fulton, who had been previously engaged -in similar speculations, and being struck with his mechanical -skill, and the soundness of his views, joined him in causing a -series of experiments to be made, which were accordingly carried -on at Plombieres, and subsequently on a still more extensive scale -on the Seine, near Paris. Having by this course of experiments -obtained proofs of the efficiency of Fulton's projects, sufficient -to satisfy the mind of Livingstone, he agreed to obtain for Fulton -the funds necessary to construct a steam boat on a large scale, to -be worked upon the Hudson. It was decided, in order to give the -project the best chance of success, to obtain the machinery from -Bolton and Watt. In 1803, Fulton accordingly made drawings of the -engines intended for this first steamer, which were sent to Soho, -with an order for their construction. Fulton, meanwhile, repaired -to America, to superintend the construction of the boat. The -delays incidental to these proceedings retarded the completion of -the boat and machinery until the year 1807, when all was -completed, and the first successful experiment made at New York. -The vessel was placed, for regular work, to ply between New York -and Albany, in the beginning of 1808; and, from that time to the -present, this river has been the theatre of the most [Pg490] -remarkable series of experiments on locomotion on water which has -ever been presented in the history of navigation. - - -(230.) The form and arrangement of this first marine engine was, -in many respects, similar to that which is still generally used -for marine purposes. The cold water cistern was abandoned, and an -increased condensing power obtained by enlarging the condenser. It -was usual to make the condenser half the diameter of the cylinder, -and half its length, and therefore one eighth of its capacity. The -condenser, however, was now made of the same diameter as the -cylinder, being still half its length; its capacity therefore, -instead of being only an eighth, was half of the cylinder; the -condensing jet was admitted by a pipe passing through the bottom -of the vessel. As in the present marine engines, two working beams -were provided, one at either side of the cylinder; but in order to -provide against the difficulties which might arise in the -adaptation of machinery made at Birmingham to a vessel made at New -York, beams were constructed in the form of an inverted T, the -working arms being twofold, one horizontal and the other vertical, -so that the connecting rod might be carried from the crank, either -downwards, to the end of the horizontal arm, or horizontally, to -the end of the vertical arm. In fact there was a choice, to use -either a straight beam, or a bell-crank. The latter was that which -was adopted in this instance. The paddle-shaft, driven by the -crank, passed across the vessel, and had the paddle-wheels keyed -upon it as at present; and in order to equalise the effect of the -engine spur wheels were also placed on the paddle-shaft, by which -pinions were driven, placed upon an axle, which carried a -fly-wheel. - -The speed attained by this steam boat, when it first began to ply -upon the river, did not exceed four miles an hour, but by a series -of improvements its rate of motion was soon increased to six miles -an hour. In the steam boats subsequently constructed by Fulton a -greater speed was attained; but in the latest vessels built by him -he did not exceed a speed of nine miles an hour, which he -considered to be the greatest that could be advantageously -obtained. - -While Fulton was making his plans, and engaged in the [Pg491] -construction of his first boat, Mr. Stevens of Hoboken, already -mentioned, was engaged in a like project, and completed a vessel, -to be propelled by a steam engine, within a few weeks after the -first successful voyage of Fulton. Stevens was likewise completely -successful; but the exclusive privilege of navigating the Hudson -by steam having been granted to Fulton by an act of Congress, -Stevens was compelled to select another theatre for his -operations, and he accordingly sent his steam boat by sea to -Philadelphia, to navigate the Delaware, thus securing for himself -the honour of having made the first sea voyage by steam. - -Fulton did not long retain the monopoly of the steam navigation of -the Hudson. Fortunately for the progress of steam navigation, the act -conferring upon him that privilege was declared unconstitutional; -and the navigation of that noble river was thrown open to the spirit -and enterprise of American genius. The number of passengers conveyed -upon it became enormous beyond all precedent, and inducements of the -strongest kind were accordingly held out to the improvement of its -navigation. The distance between New York and Albany, ascertained by -a late survey to be one hundred and twenty-five geographical miles by -water, had been performed by Fulton's boats occasionally in fifteen -or sixteen hours, being at the rate of about eight miles an hour, -including stoppages. It became a great object to increase the speed -of this trip, so that it might at all times of the year be performed -between sunrise and sunset. Robert L. Stevens, the son of the person -of that name already mentioned, immediately after the abolition -of Fulton's monopoly, placed on the river a vessel which had been -built for the Delaware, which easily performed the passage in twelve -hours, being at the rate of nearly ten and a half geographical miles -an hour. By this increase of speed the improved boats so entirely -monopolised the day work upon the river, that the former steamers -were either converted into steam tugs to draw barges laden with -goods, or used for night trips between New York and Albany. In the -night trips the saving of one or two hours was immaterial, it being -sufficient that the vessel which left the one port at night should -reach the other in the morning. [Pg492] - -The river Hudson rises near Lake Champlain, the easternmost of the -great chain of lakes or inland seas which extend from east to west -across the northern boundary of the United States. The river -follows nearly a straight course southwards for two hundred and -fifty miles, and empties itself into the sea at New York. The -influence of the tide is felt as far as Albany, above which the -stream begins to contract. Although this river in magnitude and -extent is by no means equal to several others which intersect the -States, it is nevertheless rendered an object of great interest by -reason of the importance and extent of its trade. The produce of -the state of New York and that of the banks of the great Lakes -Ontario and Erie are transported by it to the capital; and one of -the most extensive and populous districts of the United States is -supplied with the necessary imports by its waters. A large fleet -of vessels is constantly engaged in its navigation; nor is the -tardy but picturesque sailing vessel as yet excluded by the more -rapid steamers. The current of the Hudson is said to average -nearly three miles an hour; but as the ebb and flow of the tide -are felt as far as Albany, the passage of the steamers between -that place and New York may be regarded as equally affected by -currents in both directions, or nearly so. The passage therefore, -whether in ascending or descending the river, is made nearly in -the same time. - - -(231.) The prevalence of smooth water navigation, whether on the -surfaces of rivers or in sheltered bays and sounds, has invested -the problem of steam navigation in America with conditions so -entirely distinct and different from those under which the same -problem presents itself to the European engineer, that any -comparison of the performance of vessels, whether with regard to -speed or the absorption of power in the two cases, must be utterly -fallacious. In Europe a steamer is almost invariably a vessel -designed to encounter the agitated surface of an open sea, and is -accordingly constructed upon principles of suitable strength and -stability. It is likewise supplied with rigging and with sails, to -be used in aid of the mechanical power, and manned and commanded -by experienced seamen; in fact, it is a combination of a nautical -and mechanical structure. In America, on the other hand, [Pg493] -with the exception of the vessels which navigate the great -northern lakes, the steamers are structures exclusively -mechanical, being designed for smooth water. They require no other -strength or stability than that which is sufficient to enable them -to float and to bear a progressive motion through the water. Their -mould is conceived with an exclusive view to speed; they are -therefore slender and weak in their build, of great length in -proportion to their width, and having a very small draught of -water. In fact, they approach in their form to that of a Thames -wherry on a very large scale. - -The position and form of the machinery is likewise affected by -these conditions. Without the necessity of being protected from a -rough sea, it is placed on the deck in an elevated position. The -cylinders of large diameter and short stroke invariably used in -Europe are unknown in America, and the proportions are reversed, a -small diameter and stroke of great length being invariably -adopted. It is rarely that two engines are used. A single engine, -placed in the centre of the deck, with a cylinder from forty to -sixty inches' diameter, and from eight to ten foot stroke, drives -paddle-wheels from twenty-one to twenty-five feet in diameter, -producing from twenty-five to thirty revolutions per minute. The -great magnitude of the paddle-wheels and the velocity imparted to -them enable them to perform the office of fly-wheels, and to carry -the engine round its centres, not however without a perceptible -inequality of motion, which gives to the American steamer an -effect like that of a row boat advancing by starts with each -stroke of the piston. The length of stroke adopted in these -engines enables them to apply with great effect the expansive -principle, which is almost universally used, the steam being -generally cut off at half stroke. - -The steamers which navigate the Hudson are vessels of considerable -magnitude, splendidly fitted up for the accommodation of -passengers; they vary from one hundred and eighty to two hundred -and forty feet in length, and from twenty to thirty feet in width -of beam. In the following table is given the particulars of nine -steamers plying on this river, taken from [Pg494] the work of Mr. -Stevenson, and from the paper of Mr. Renwick, inserted in the last -edition of Tredgold:-- - - ------------------------------------------------------------- - | Length | Breadth | Draft | Drain | Length - Names. | of | of | of | of | of - | Deck. | Beam. | Water. | Wheel. | Paddles. - ------------------------------------------------------------- - | Ft. | Ft. | Ft. | Ft. | Ft. - Dewit Clinton | 230 | 28 | 5.5 | 21 | 13.7 - Champlain | 180 | 27 | 5.5 | 22 | 15 - Erie | 180 | 27 | 5.5 | 22 | 15 - North America | 200 | 30 | 5 | 21 | 13 - Independence | 148 | 26 | -- | -- | -- - Albany | 212 | 26 | -- | 24.5 | 14 - Swallow | 233 | 22.5 | 3.75 | 24 | 11 - Rochester | 200 | 25 | 3.75 | 23.5 | 10 - Utica | 200 | 21 | 3.5 | 22 | 9.5 - ------------------------------------------------------------- - - ---------------------------------------------------------------------- - | Depth | Number | Drain | Length| Number | Part of - Names. | of | of | of | of | of | Stroke - | Paddles| Engines| Cylinder| Stroke| Revs | at which - | | | | | | it is - | | | | | | cut off. - ---------------------------------------------------------------------- - | In. | | In. | Ft. | | - Dewit Clinton | 36 | 1 | 65 | 10 | 29 | 3/4 - Champlain | 34 | 2 | 44 | 10 | 27.5 | 1/2 - Erie | 34 | 2 | 44 | 10 | 27.5 | 1/2 - North America | 30 | 2 | 44.5 | 8 | 24 | 1/2 - Independence | -- | 1 | 44 | 10 | | - Albany | 30 | 1 | 65 | -- | 19 | - Swallow | 30 | 1 | 46 | -- | 27 | - Rochester | 24 | 1 | 43 | 10 | 28 | - Utica | 24 | 1 | 39 | 10 | | - ---------------------------------------------------------------------- - -None of these vessels have either masts or rigging, and -consequently never derive any propelling power except from the -engines: they are neither manned nor commanded by persons having -any knowledge of navigation: the works that are visible above -their decks are the beam and framing of the engine, and the -chimneys. - -The engines used for steamers on the Hudson, and other great -rivers and bays on the eastern coast of America, are most commonly -condensing engines, but they nevertheless work with steam of very -high pressure, being seldom less than twenty-five pounds per -square inch, and sometimes as much as fifty. By reference to the -preceding table it will be seen, that the velocity of the piston -greatly exceeds the limit generally observed in Europe. It is -customary in European marine engines to limit the speed of the -piston to about two hundred and twenty feet per minute. Even the -piston of a locomotive engine does not much exceed the rate of -three hundred feet per minute. In the American steamers, however, -the pistons commonly move at the rate of from five to six hundred -feet per minute, while the circumference of the paddle-wheels are -driven at the rate of from twenty to twenty-two miles an hour. -[Pg495] - -[Illustration: _Fig._ 135.] - -The hulls of these boats are formed with a perfectly flat bottom -and perpendicular sides, rounded at the angles, as represented in -_fig._ 135. At the bow, or cutwater, they are made very sharp, and -the deck projects to a great distance over the sides. The weight -of the machinery is distributed over an extensive surface of the -bottom of this feeble structure, by means of a frame-work of -substantial carpentry to which it is attached. - -At the height of from four to six feet above the water-line is -placed the deck, which is a platform, having the shape of a very -elongated ellipse. The extremities of its longer axis are -supported by the sternpost and the cutwater, and its sides expand -in gentle curves on either hand to a considerable distance beyond -the limits of the hull; those parts of the deck thus overhanging -the water are called the wheel guards. - -Beneath the first deck is the saloon, or dining-room, which also, -as is usual in European steamers, forms the gentlemen's -sleeping-room. It usually extends from end to end of the vessel. -The middle of the first deck is occupied by the engine, boilers, -furnaces, and chimneys, of which latter there are generally two. -Between the chimneys and the stern, above the first deck, is -constructed the ladies' cabin, which is covered by the second -deck, called the promenade deck. The great length of these boats -and the elevation of the cabins render it impossible for a -steersman at the stern to see ahead, and they are, consequently, -steered from the bow; the wheel placed there communicating with -the helm at the stern, by chains or rods carried along the sides -of the boat. Until a recent period, the wheel was connected with -the stern by ropes, but some fatal accidents, produced by fire, -[Pg496] in which these ropes were burnt, and the steersman lost -all power to guide the vessel, caused metal rods or chains to be -substituted. - - -(232.) The paddle-wheels universally used in American steam-boats -are formed, as if by the combination of two or more common -paddle-wheels, placed one outside the other, on the same axle, but -so that the paddle boards of each may have an intermediate -position between those of the adjacent one, as represented in -_fig._ 136. - -[Illustration: _Fig._ 136.] - -The spokes, which are bolted to cast-iron flanges, are of wood. -These flanges, to which they are so bolted, are keyed upon the -paddle shaft. The outer extremities of the spokes are attached to -circular bands or hoops of iron, surrounding the wheel; and the -paddle boards, which are formed of hard wood, are bolted to the -spokes. The wheels thus constructed, sometimes consist of three, -and not unfrequently four, independent circles of paddle boards, -placed one beside the other, and so adjusted in their position, -that the boards of no two divisions shall correspond. - -The great magnitude of the paddle-wheels, and the circumstance of -the navigation being carried on, for the most part, in smooth -water, have rendered unnecessary, in America, the adoption of any -of those expedients for neutralising the effects of the oblique -action of the paddles, which have been tried, but hitherto with so -little success, in Europe. - - -(233.) Sea-going steamers are not numerous in America, the chief -of them being those which ply between New York and Providence, and -between New York and Charleston. These vessels, however, do not -resemble the sea-going steamers of Europe as closely as might be -expected; and to those who are accustomed to the latter, the -sea-going [Pg497] steamers of America can hardly be regarded as -safe means of transport. - -In the following Table is given the dimensions of five of these -vessels, all plying between New York and Providence:-- - - -------------------------------------------------------------- - | Length | Breadth | Draft | Diameter | Length - Names. | of | of | | of | of - | Deck. | Beam. | | Wheel. | Paddles. - -------------------------------------------------------------- - | Ft. | Ft. | Ft. | Ft. | Ft. - Providence | 180 | 27 | 9 | -- | -- - Lexington | 207 | 21 | -- | 23 | 9 - Narragansett | 210 | 26 | 5 | 25 | 11 - Massachusetts | 200 | 29.5 | 8.5 | 22 | 10 - Rhode Island | 210 | 26 | 6.5 | 24 | 11 - -------------------------------------------------------------- - --------------------------------------------------------------------- - | Depth | Number | Diameter| Length| Number | Part of - Names. | of | of | of | of | of | Stroke - | Paddles| Engines| Cylinder| Stroke| Revs | at which - | | | | | | it is - | | | | | | cut off. - --------------------------------------------------------------------- - | In. | | In. | Ft. | | - Providence | -- | 1 | 10 | 65 | | - Lexington | 30 | 1 | 11 | 48 | 24 | - Narragansett | 30 | 1 | 60 | 12 | 2 | 1/2 - Massachusetts | 28 | 2 | 44 | 8 | 26 | - Rhode Island | 30 | 1 | 11 | 60 | 21 | - --------------------------------------------------------------------- - -The Narragansett, the finest of these vessels, is built of oak, -strengthened by diagonal straps or ties of iron, by which her -timbers are connected; she is driven by a condensing engine, and -has two boilers, exposing about three thousand square feet of -surface to the fire. The steam is maintained at a pressure of from -twenty to twenty-five lbs. per square inch: the cylinder is -horizontal. - -The cabins of these sea-boats are of great magnitude, and afford -excellent accommodation for passengers, containing generally four -hundred berths. In the Massachusetts the chief cabin is one hundred -and sixty feet long, twenty-two feet wide, and twelve feet in -height, its vast extent being uninterrupted by pillars or any other -obstruction. "I have dined," says Mr. Stevenson, "with one hundred -and seventy-five persons in this cabin, and, notwithstanding this -numerous assembly, the tables, which were arranged in two parallel -rows, extending from one end of the cabin to the other, were far -from being fully occupied, the attendance was good, and every thing -was conducted with perfect regularity and order. There are one -hundred and twelve fixed berths ranged round this cabin, and one -hundred temporary berths can be erected in the middle of the floor: -besides these there are sixty fixed berths in the ladies' cabin, and -several temporary sleeping [Pg498] places can be erected in it -also. The cabin of the Massachusetts is by no means the largest in -the United States. Some steamers have cabins upwards of one hundred -and seventy-five feet in length. Those large saloons are lighted by -Argand lamps, suspended from the ceiling, and their appearance, when -brilliantly lighted up and filled with company, is very remarkable. -The passengers generally arrange themselves in parties at the -numerous small tables into which the large tables are converted -after dinner, and engage in different amusements. The scene -resembles much more the coffee-room of some great hotel than the -cabin of a floating vessel." - - -(234.) Nothing has excited more surprise among engineers and -others interested in steam navigation in Europe, than the -statements which have been so generally and so confidently made of -the speed attained by American steamers. This astonishment is due -to several causes, the chief of which is the omission of all -notice of the great difference between the structure and operation -of the American steamers and the nature of the navigation in which -they are engaged, compared with the structure and operation of, -and the navigation in which European steamers are employed: as -well might the performance of a Thames wherry, or one of the -fly-boats on the northern canals, be compared with that of the -Great Western, or the British Queen. The statements alluded to all -have reference to steamers navigating the Hudson between New York -and Albany, the form and structure of which we have already -described; and doubtless the greatest speed ever attained on the -surface of water has been exhibited in the passages of these -vessels. - -Mr. Stevenson states, that exclusive of the time lost in -stoppages, the voyage between New York and Albany is usually made -in ten hours. Dr. Renwick, however, who has probably more -extensive opportunities of observation, states, that the average -time, exclusive of stoppages, is ten hours and a half. The -distance being 125.18 geographical miles, the average rate would -therefore be 11-9/10 miles per hour. If it be observed that the -average rate of some of the best sea-going steamers in Europe -obtained from experiments [Pg499] and observations made by -myself, more than three years ago, showed a rate of steaming -little less than ten geographical miles per hour, and that since -that time considerable improvements in steam navigation have been -made, and further, that these performances were made under -exposure to all the disadvantages of an open sea, the difference -between them and the performance of the American river steamers -will cease to create astonishment. - -Dr. Renwick states that he made, in a boat called the "New -Philadelphia," one of the most remarkable passages ever performed. -He left New York at five in the afternoon, with the first of the -flood, and landed at Catskill, distant 95.8 geographical miles -from New York, at a quarter before twelve. Passengers were landed -and taken in at seven intermediate points: the rate, including -stoppages, was therefore 14.2 miles per hour; and if half an hour -be allowed for stoppages, the actual average rate of motion would -be fifteen miles and three quarters an hour. As the current, which -in this case was with the course of the vessel, did not exceed -three miles and a half an hour, the absolute velocity through the -water would have been somewhat under twelve miles an hour. This -speed is nearly the same as the speed obtained from taking the -average time of the voyages between New York and Albany at ten -hours and a half; it would therefore appear that the great speed -attained in this trip must have been chiefly, if not altogether, -owing to the effect of the current. - - -(235.) The steamers which navigate the great northern lakes differ -so little in their construction and appearance from the European -steam-boats, that it will not be necessary here to devote any -considerable space to an account of them. These vessels were -introduced on the lakes at about the same time that steamers were -first introduced on the Clyde. These steamers are strongly built -vessels, supplied with sails and rigging, and propelled by -powerful engines. The largest in 1837, when Mr. Stevenson visited -the States, was the _James Madison_. This vessel was one hundred -and eighty-one feet in length on the deck, thirty feet in breadth -of beam, and twelve feet six inches in depth of hold: her draught -of water was ten feet, and her measured capacity seven hundred -[Pg500] tons. She plyed between Buffalo on Lake Erie and Chicago -on Lake Michigan, a distance of nine hundred and fifty miles. - -The severe storms and formidable sea encountered on the lakes -render necessary for the navigation, vessels in all respects as -strong and powerful as those which navigate the open ocean. - - -(236.) By far the most remarkable and important of all the -American rivers is the Mississippi and its tributaries. That part -of the American continent which extends from the southern shores -of the great northern lakes to the northern shores of the Gulf of -Mexico, is watered by these great streams. The main stream of the -Mississippi has its fountains in the tract of country lying north -of the Illinois and east of Lake Michigan, in latitude forty-three -degrees. At about latitude thirty-nine degrees, a little north of -St. Louis, it receives the waters of the Missouri, and further -south, at the latitude of thirty-seven degrees, the Ohio flows -into it, after traversing five degrees of longitude and four of -latitude, and winding its way from the Alleghany range through -several of the states, and forming a navigable communication with -numerous important towns of the Union, among which may be -mentioned Pittsburg, Cincinnati, Frankfort, Lexington, and -Louisville. The main stream of the Mississippi, after receiving -the waters of the Arkansas, and numerous other minor tributaries, -flows into the Gulf of Mexico by four mouths. The main stream of -the Mississippi, independently of its tributaries, forms an -unbroken course of inland navigation for a distance of nearly two -thousand three hundred miles. Its width, through a distance of one -thousand one hundred miles from its mouth, is not less than half a -mile, and its average depth a hundred feet. The Ohio, its chief -eastern tributary, flowing into it at a distance of about a -thousand miles from its mouth, traverses also about the same -extent of country, and is navigable throughout the whole of that -extent. This river also has several navigable tributaries of -considerable extent, among which may be mentioned the Muskingum, -navigable for one hundred and twenty miles; the Miami, navigable -for seventy-five miles; the Scioto, navigable for one hundred and -twenty [Pg501] miles; the Tennessee, navigable for two hundred -and fifty miles; the Cumberland, navigable for four hundred and -forty miles; the Kentucky, navigable for one hundred and thirty -miles; and the Green River, navigable for one hundred and fifty -miles. The total length of the Ohio and its tributaries is -estimated at above seven thousand miles. - - -(237.) Steam-boats were introduced on the Mississippi about the -year 1812, the period of their first introduction in Europe; and -their increase has been rapid beyond all precedent. In the year -1831 there were one hundred and ninety-eight steamers plying on -its waters; and the number in 1837 amounted to nearly four -hundred. These vessels are built chiefly on the banks of the Ohio, -at the towns of Pittsburg and Cincinnati, at distances of about -two thousand miles from the mouth of the river they are intended -to navigate. - - -(238.) These steamers, which are decidedly inferior to those which -navigate the eastern waters, are generally of a heavy build, -fitted to carry goods as well as passengers, and vary from one -hundred to seven hundred tons burthen. Their draught of water is -also greater than that of the eastern river steamers--varying from -six to eight feet. The hull, at about five feet from the water -line, is covered with a deck, under which is the hold, in which -the heavy part of the cargo is stowed. About the middle of this -deck the engines are placed, the boilers and furnaces occupying a -space nearer to the bow, near which two chimneys are placed. The -fire-doors of the furnaces are presented towards the bow, and -exposed so as to increase the draught. That part of the first deck -which extends from the machinery to the stern is the place -allotted to the crew and the deck passengers, and is described as -being filthy and inconvenient in the extreme. A second deck is -constructed, which extends from the chimneys near the bow to the -stern of the vessel. On this is formed the great cabin or saloon, -which extends from the chimneys to within about thirty feet of the -stern, where it is divided by a partition from the ladies' cabin, -which occupies the remaining space. These principal cabins are -surrounded by a gallery about three feet in width, from which, at -convenient [Pg502] places, an ascent is supplied by stairs to the -highest deck, called the hurricane or promenade deck. - - -(239.) The engines by which these boats are propelled are totally -different from the machinery already described as used in the -eastern steamers. They are invariably non-condensing engines, -worked by steam of extremely high pressure; the boilers are -therefore tubular, and the cylinders small in diameter, but -generally having a long stroke. - -The pressure of steam used in these machines is such as is never -used in European engines, even when worked on railways. A pressure -of one hundred pounds per inch is here considered extremely -moderate. The captain of one of these boats, plying between -Pittsburg and St. Louis, told Mr. Stevenson that "under ordinary -circumstances his safety valves were loaded with a pressure equal -to one hundred and thirty-eight pounds per square inch, but that -the steam was occasionally raised as high as one hundred and fifty -pounds to enable the vessel to pass parts of the river in which -there is a strong current;" and he added, by way of consolation, -that "this pressure was never exceeded except on _extraordinary -occasions_!" - -The dimensions and power of the Mississippi steamers may be -collected from those of the St. Louis, a boat which was plying on -that river in 1837. That vessel measured two hundred and fifty -feet on deck, and had twenty-eight feet breadth of beam. Her -draught of water was eight feet, and her measured capacity one -thousand tons. She was propelled by two engines with thirty-inch -cylinders, and ten feet stroke; the safety valve being loaded at -one hundred pounds per square inch. - -The paddle wheels of these vessels are attached to the paddle -shaft, in such a manner as to be thrown into and out of gear, at -discretion, by the engineer, so that the paddle shaft may revolve -without driving the wheels: by this expedient the power of the -engine is used to feed the boilers while the vessel stops at the -several stations. The vessel is therefore stopped, not, as is -usually the case, by stopping the engines, but by throwing the -wheels out of connection with the paddle shaft. The engines -continue to work, but their [Pg503] power is expended in forcing -water into the boiler. By this expedient the activity of the -engines may, within practical limits, be varied with the -resistance the vessel has to encounter. In working against a -strong current, the feed may be cut off from the boilers, and the -production of steam, and consequently the power of the engines, -thereby stimulated, while this suspension of the feed may be -compensated at the next station. - -The stoppages to take in goods and passengers, and for relays of -fuel, are frequent. "The liberty which they take with their -vessels on these occasions," says Mr. Stevenson, "is somewhat -amusing: I had a good example of this on board a large vessel, -called the Ontario. She was steered close in shore amongst stones -and stumps of trees, where she lay for some hours to take in -goods: the additional weight increased her draught of water, and -caused her to heel a good deal; and when her engines were put in -motion, she actually _crawled_ into the deep water on her paddle -wheels: the steam had been got up to an enormous pressure to -enable her to get off, and the volume of steam discharged from the -escapement pipe at every half stroke of the piston made a sharp -sound almost like the discharge of fire-arms, while every timber -in the vessel seemed to tremble, and the whole structure actually -groaned under the shocks." - -Besides the steamers used for the navigation of the Mississippi, -innumerable steam tugs are constantly employed in towing vessels -between the port of New Orleans and the open sea of the Gulf of -Mexico. Before the invention of steam navigation, this southern -capital of the United States laboured under the disadvantage of -possessing almost the only bad and inconvenient harbour in the -vast range of coast by which the country is bounded. New Orleans -lies at a distance of about one hundred miles from the Gulf of -Mexico. The force of the stream, the frequency of shoals, and the -winding course of the channel rendered it scarcely possible for a -sailing vessel to pass between the port and the sea with the same -wind. The anchorage was every where bad, and great difficulty and -risk attended the mooring of large vessels to the banks. The steam -engine has, however, overcome all [Pg504] these difficulties, and -rendered the most objectionable harbour of the Union a safe and -good seaport, perfectly easy of approach and of egress at all -times; a small steam tug will take in tow several large ships, and -carry them with safety and expedition to the offing, where it will -dismiss them on their voyage, and take back vessels which may have -arrived. - -[Illustration: GREAT WESTERN OFF NEW YORK.] - -[Pg505] - - - - -APPENDIX. - - _On the Relation between the Temperature, Pressure, and Density_ - _of Common Steam._ - - -There is a fixed relation between the temperature and pressure of -common steam, which has not yet been ascertained by theory. -Various empirical formulae have been proposed to express it, -derived from tables of temperatures and corresponding pressures -which have been founded on experiments and completed by -interpolation. - -The following formula, proposed by M. Biot, represents with great -accuracy the relation between the temperature and pressure of -common steam, throughout all that part of the thermometric scale -to which experiments have been extended. - -Let - - a = 5.96131330259 - log. a_{1} = 0.82340688193 - 1 - log. b_{1} = -.01309734295 - log. a_{2} = 0.74110951837 - log. b_{2} = -.00212510583 - -The relation between the temperature t with reference to the -centesimal thermometer, and the pressure p in millimetres of -mercury at the temperature of melting ice, will then be expressed -by the following formula:-- - - log. p = a - a_{1}b_{1}^{20 + t} - a_{2}b_{2}^{20 + t}. (1.) - -Formulae have, however, been proposed, which, though not applicable -to the whole scale of temperatures, are more manageable in their -practical application than the preceding. - -For pressures less than an atmosphere, Southern proposed the -following formula, where the pressure is intended to be expressed -[Pg506] in pounds per square inch, and the temperature in -reference to Fahrenheit's thermometer,-- - - p = 0.04948 + ((51.3 + t) / 155.7256)^{5.13} | - |. (2.) - t = 155.7256 ((p - 0.04948) - 51.3)^{1/(5.13)} | - - -The following formula was proposed by Tredgold, where p expresses -the pressure in inches of mercury:-- - - p = ((100 + t) / 177)^{6}. - -This was afterwards modified by Mellet, and represents with -sufficient accuracy experiments from 1 to 4 atmospheres. Let p -represent pounds per square inch, and t the temperature by -Fahrenheit's thermometer,-- - - p = ((103 + t) / 201.18)^{6} | - |. (3.) - t = 201.18 p^{1/6} - 103 | - -M. de Pambour has proposed the following formula, also applicable -through the same limits of the scale:-- - - p = ((98.806 + t) / 198.562)^{6} | - |. (4.) - t = 198.562 p^{1/6} - 98.806 | - -MM. Dulong and Arago have proposed the following formula for all -pressures between 4 and 50 atmospheres:-- - - p = (0.26793 + 0.0067585 t)^{5} | - |. (5.) - t = 147.961 p^{1/5} - 39.644 | - -It was about the year 1801, that Dalton, at Manchester, and -Gay-Lussac, at Paris, instituted a series of experiments on -gaseous bodies, which conducted them to the discovery of the law -mentioned in art. (96.), p. 171. These philosophers found that all -gases whatever, and all vapours raised from liquids by heat, as -well as all mixtures of gases and vapours, are subject to the -_same quantity of expansion_ between the temperatures of melting -ice and boiling water; and by experiments subsequently made by -Dulong and Petit, this uniformity of expansion has been proved to -extend to all temperatures which can come under practical -inquiries. - -Dalton found that 1000 cubic inches of air at the temperature of -melting ice dilated to 1325 cubic inches if raised to the -temperature of boiling water. According to Gay-Lussac, the -increased volume was 1375 cubic inches. The latter determination -has been subsequently found to be the more correct one.[40] - -[Pg507] It appears, therefore, that for an increase of temperature -from 32 deg. to 212 deg., amounting to 180 deg., the increase of volume -is 375 parts in 1000; and since the expansion is uniform, the increase -of volume for 1 deg. will be found by dividing this by 180, which will -give an increase of 208-1/3 parts in 100,000 for each degree of -the common thermometer. - -To reduce the expression of this important and general law to -mathematical language, let v be the volume of an elastic fluid at -the temperature of melting ice, and let nv be the increase which -that volume would receive by being raised one degree of -temperature under the same pressure. Let V be its volume at the -temperature T. Then we shall have - - V = v + nv (T - 32) = v (1 + n (T - 32)). - -If V' be its volume at any other temperature T', and under the -same pressure, we shall have, in like manner, - - V' = v (1 + n (T' - 32)). - -Hence we obtain - - V/V' = (1 + n (T - 32)) / (1 + n (T' - 32)); (6.) - -which expresses the relation between the volumes of the same gas -or vapour under the same pressure and at any two temperatures. The -co-efficient n, as explained in the text, has the same value for -the same gas or vapour throughout the whole thermometric scale. -But it is still more remarkable that this constant has the same -value for all gases and vapours. It is a number, therefore, which -must have some essential relation to the gaseous or elastic state -of fluid matter, independent of the peculiar qualities of any -particular gas or vapour. - -The value of n, according to the experiments of Gay-Lussac, is -0.002083, or 1/480. - -To reduce the law of Mariotte, explained in (97.) p. 171., to -mathematical language, let V, V' be the volumes of the same gas or -vapour under different pressures P, P', but at the same -temperature. We shall then have - - VP = V'P'. (7.) - -If it be required to determine the relation between the volumes of -the same gas or vapour, under a change of both temperature and -pressure, let V be the volume at the temperature T and under the -pressure P, and let V' be the volume at the temperature T' and -under the pressure P'. Let v be the volume at the temperature T -and under the pressure P'. - -By formula (7.) we have - - VP = vP'; - - -[Pg508] and by formula (6.) we have - - (V'/v) = (1 + n(T' - 32)) / (1 + n(T - 32)) - -Eliminating v, we shall obtain - - (V/V') = (P'/P) . (1 + n(T - 32)) / (1 + n(T' - 32)); - -or, - - (VP/V'P') = (1 + n(T - 32)) / (1 + n(T' - 32)); (8.) - -which is the general relation between the volumes, pressures, and -temperatures of the same gas or vapour in two different states. - -To apply this general formula to the case of the vapour of water, -let T' = 212 deg. It is known by experiment that the corresponding -value of P', expressed in pounds per square inch, is 14.706; and -that V', expressed in cubic inches, the water evaporated being -taken as a cubic inch, is 1700. If, then, we take 0.002083 as the -value of n, we shall have by (8.), - - VP = 1700 x 14.706 x (1 + 0.002083 (T - 32)) / (1 + 0.002083 x 180) - - = 18183(1 + 0.002083 (T - 32)). (9.) - -If, by means of this formula (9.), and any of the formulae (1.), -(2.), (3.), (4.), (5.), T were eliminated, we should obtain a -formula between V and P, which would enable us to compute the -enlargement of volume which water undergoes in passing into steam -under any proposed pressure. But such a formula would not be -suitable for practical computations. By the formulae (1.) to (5.), -a table of pressures and corresponding temperatures may be -computed; and these being known, the formula (9.) will be -sufficient for the computation of the corresponding values of V, -or the enlargement of volume which water undergoes in passing into -steam. - -In the following table, the temperatures corresponding to -pressures from 1 to 240 lbs. per square inch are given by -computation from the formulae (2.) to (5.), and the volumes of -steam produced by an unit of volume of water as computed from the -formula (9.). - -The mechanical effect is obtained by multiplying the pressure in -pounds by the expansion of a cubic inch of water in passing into -steam expressed in feet, and is therefore the number of pounds -which would be raised one foot by the evaporation of a cubic inch -of water under the given pressure. [Pg509] - - --------------------------------------------------------- - | | Volume of | Mechanical - Total pressure| | the Steam | Effect of - in Pounds | Corresponding| compared to | a Cubic Inch - per Square | Temperature. | the Volume | of Water - Inch. | | of the | evaporated - | | Water that | in Pounds - | | has | raised One - | | produced it.| Foot. - --------------------------------------------------------- - 1 | 102.9 | 20868 | 1739 - 2 | 126.1 | 10874 | 1812 - 3 | 141.0 | 7437 | 1859 - 4 | 152.3 | 5685 | 1895 - 5 | 161.4 | 4617 | 1924 - 6 | 169.2 | 3897 | 1948 - 7 | 175.9 | 3376 | 1969 - 8 | 182.0 | 2983 | 1989 - 9 | 187.4 | 2674 | 2006 - 10 | 192.4 | 2426 | 2022 - 11 | 197.0 | 2221 | 2036 - 12 | 201.3 | 2050 | 2050 - 13 | 205.3 | 1904 | 2063 - 14 | 209.1 | 1778 | 2074 - 15 | 212.8 | 1669 | 2086 - 16 | 216.3 | 1573 | 2097 - 17 | 219.6 | 1488 | 2107 - 18 | 222.7 | 1411 | 2117 - 19 | 225.6 | 1343 | 2126 - 20 | 228.5 | 1281 | 2135 - 21 | 231.2 | 1225 | 2144 - 22 | 233.8 | 1174 | 2152 - 23 | 236.3 | 1127 | 2160 - 24 | 238.7 | 1084 | 2168 - 25 | 241.0 | 1044 | 2175 - 26 | 243.3 | 1007 | 2182 - 27 | 245.5 | 973 | 2189 - 28 | 247.6 | 941 | 2196 - 29 | 249.6 | 911 | 2202 - 30 | 251.6 | 883 | 2209 - 31 | 253.6 | 857 | 2215 - 32 | 255.5 | 833 | 2221 - 33 | 257.3 | 810 | 2226 - 34 | 259.1 | 788 | 2232 - 35 | 260.9 | 767 | 2238 - 36 | 262.6 | 748 | 2243 - 37 | 264.3 | 729 | 2248 - 38 | 265.9 | 712 | 2253 - 39 | 267.5 | 695 | 2259 - 40 | 269.1 | 679 | 2264 - 41 | 270.6 | 664 | 2268 - 42 | 272.1 | 649 | 2273 - 43 | 273.6 | 635 | 2278 - 44 | 275.0 | 622 | 2282 - 45 | 276.4 | 610 | 2287 - 46 | 277.8 | 598 | 2291 - 47 | 279.2 | 586 | 2296 - 48 | 280.5 | 575 | 2300 - 49 | 281.9 | 564 | 2304 - 50 | 283.2 | 554 | 2308 - 51 | 284.4 | 544 | 2312 - 52 | 285.7 | 534 | 2316 - 53 | 286.9 | 525 | 2320 - 54 | 288.1 | 516 | 2324 - 55 | 289.3 | 508 | 2327 - 56 | 290.5 | 500 | 2331 - 57 | 291.7 | 492 | 2335 - 58 | 292.9 | 484 | 2339 - 59 | 294.2 | 477 | 2343 - 60 | 295.6 | 470 | 2347 - 61 | 296.9 | 463 | 2351 - 62 | 298.1 | 456 | 2355 - 63 | 299.2 | 449 | 2359 - 64 | 300.3 | 443 | 2362 - 65 | 301.3 | 437 | 2365 - 66 | 302.4 | 431 | 2369 - 67 | 303.4 | 425 | 2372 - 68 | 304.4 | 419 | 2375 - 69 | 305.4 | 414 | 2378 - 70 | 306.4 | 408 | 2382 - 71 | 307.4 | 403 | 2385 - 72 | 308.4 | 398 | 2388 - 73 | 309.3 | 393 | 2391 - 74 | 310.3 | 388 | 2394 - 75 | 311.2 | 383 | 2397 - 76 | 312.2 | 379 | 2400 - 77 | 313.1 | 374 | 2403 - 78 | 314.0 | 370 | 2405 - 79 | 314.9 | 366 | 2408 - 80 | 315.8 | 362 | 2411 - 81 | 316.7 | 358 | 2414 - 82 | 317.6 | 354 | 2417 - 83 | 318.4 | 350 | 2419 - 84 | 319.3 | 346 | 2422 - 85 | 320.1 | 342 | 2425 - 86 | 321.0 | 339 | 2427 - 87 | 321.8 | 335 | 2430 - 88 | 322.6 | 332 | 2432 - 89 | 323.5 | 328 | 2435 - 90 | 324.3 | 325 | 2438 - 91 | 325.1 | 322 | 2440 - 92 | 325.9 | 319 | 2443 - 93 | 326.7 | 316 | 2445 - 94 | 327.5 | 313 | 2448 - 95 | 328.2 | 310 | 2450 - 96 | 329.0 | 307 | 2453 - 97 | 329.8 | 304 | 2455 - 98 | 330.5 | 301 | 2457 - 99 | 331.3 | 298 | 2460 - 100 | 332.0 | 295 | 2462 - 110 | 339.2 | 271 | 2486 - 120 | 345.8 | 251 | 2507 - 130 | 352.1 | 233 | 2527 - 140 | 357.9 | 218 | 2545 - 150 | 363.4 | 205 | 2561 - 160 | 368.7 | 193 | 2577 - 170 | 373.6 | 183 | 2593 - 180 | 378.4 | 174 | 2608 - 190 | 382.9 | 166 | 2622 - 200 | 387.3 | 158 | 2636 - 210 | 391.5 | 151 | 2650 - 220 | 395.5 | 145 | 2663 - 230 | 399.4 | 140 | 2675 - 240 | 403.1 | 134 | 2687 - --------------------------------------------------------- - -[Pg511] In the absence of any direct method of determining the -general relation between the pressure and volume of common steam, -empirical formulae expressing it have been proposed by different -mathematicians. - -The late Professor Navier proposed the following:--Let S express -the volume of steam into which an unit of volume of water is -converted under the pressure P, this pressure being expressed in -kilogrammes per square metre. Then the relation between S and P -will be - - S = a/(b + mP), - - where a = 1000, b = 0.09, and m = 0.0000484. - -This formula, however, does not agree with experiment at pressures -less than an atmosphere. M. de Pambour, therefore, proposes the -following changes in the values of its co-efficients:--Let P -express the pressure in pounds per square foot; and let - - a = 10000 b = 0.4227 m = 0.00258, - -and the formula will be accurate for all pressures. For pressures -above two atmospheres the following values give more accuracy to -the calculation:-- - - a = 10000 b = 1.421 m = 0.0023. - -In these investigations I shall adopt the following modified -formula. The symbols S and P retaining their signification, we -shall have - - S = a/(b + P) (10.) - -where - - a = 3875969 b = 164. - -These values of a and b will be sufficiently accurate for practical -purposes for all pressures, and may be used in reference to -low-pressure engines of every form, as well as for high-pressure -engines which work expansively. - -When the pressure is not less than 30 pounds per square inch, the -following values of a and b will be more accurate:-- - - a = 4347826 b = 618. - - _On the Expansive Action of Steam._ - -The investigation of the effect of the expansion of steam which -has been given in the text, is intended to convey to those who are -not conversant with the principles and language of analysis, some -notion of the nature of that mechanical effect to which the -advantages attending the expansive principle are due. We shall -now, however, explain these effects more accurately. [Pg512] - -The dynamical effect produced by any mechanical agent is expressed -by the product of the resistance overcome and the space through -which that resistance is moved. - - Let - P = the pressure of steam expressed in pounds per square foot. - S = the number of cubic feet of steam of that pressure - produced by the evaporation of a cubic foot of water. - E = the mechanical effect produced by the evaporation of a - cubic foot of water expressed in pounds raised one foot. - -Then we shall have E = PS; and if W be a volume of water -evaporated under the pressure P, the mechanical effect produced by -it will be WPS. - -By (10.) we have - - SP = a - bS. - -Hence, for the mechanical effect of a cubic foot of water -evaporated under the pressure P we have - - E = a - bS. (11.) - -Let a cubic foot of water be evaporated under the pressure P', and -let it produce a volume of steam S' of that pressure. Let this -steam afterwards be allowed to expand to the increased volume S -and the diminished pressure P; and let it be required to determine -the mechanical effect produced during the expansion of the steam -from the volume S' to the volume S. - - Let - E' = the mechanical effect produced by the evaporation of - the water under the pressure P' without expansion. - E'' = the mechanical effect produced during the expansion - of the steam. - E = the mechanical effect which would be produced by - the evaporation under the pressure P without expansion. - _E_ = the total mechanical effect produced by the evaporation - under the pressure P' and subsequent expansion. - -Thus we have - - _E_ = E' + E''. - -Let s be any volume of the steam during the process of expansion, -p the corresponding pressure, and e'' the mechanical effect -produced by the expansion of the steam. We have then by (10.) - - p = (a/s) - b; - - [because] de'' = (ads/s) - bds. - -Hence by integrating we obtain - - e'' = a log. s - bs + C; - - -[Pg513] which, taken between the limits s = S' and s = S, becomes - - E'' = a log. S/S' - b(S - S'). (12.) - -But by (11.) we have - - E' = a - bS', - E = a - bS; - [because] E' - E = b(S - S'); - [because] E'' = a log. S/S' - E' + E; - [because] _E_ = E'' + E' = a log. S/S' + E. (13.) - -Or, - - _E_ = a (1 + log. S/S') - bS. (14.) - -Hence it appears that the mechanical effect of a cubic foot of -water evaporated under the pressure P may be increased by the -quantity a log. S/S', if it be first evaporated under the greater -pressure P', and subsequently expanded to the lesser pressure P. - -The logarithms in these formulae are hyperbolic. - -To apply these principles to the actual case of a double acting -steam engine, - - Let - L = the stroke of the piston in feet. - A = the area of the piston in square feet. - n = the number of strokes of the piston per minute. - [because] 2n AL = the number of cubic feet of space through which - the piston moves per minute. - Let cLA = the clearage, or the space between the steam valve - and the piston at each end of the stroke. - [because] The volume of steam admitted through the steam valve - at each stroke of the engine will be 2n AL(1 + c). - - Let - V = the mean speed of the piston in feet per minute, - [because] 2nL = V. - -The volume of steam admitted to the cylinder per minute will -therefore be VA (1 + c), the part of it employed in working the -piston being VA. - - Let - W = the water in cubic feet admitted per minute in the form - of steam through the steam valve. - S = the number of cubic feet of steam produced by a cubic - foot of water. - -[Pg514] Hence we shall have - - WS = VA (1 + c); - [because] S = (VA(1 + c))/W. (15.) - -Since by (10.) we have - - P = a/S - b; - [because] P = [Wa/(VA(1 + c))] - b. (16.) - -By which the pressure of steam in the cylinder will be known, when -the effective evaporation, the diameter of the cylinder, and speed -of the piston, are given. - -If it be required to express the mechanical effect produced per -minute by the action of steam on the piston, it is only necessary -to multiply the pressure on the surface of the piston by the space -per minute through which the piston moves. This will give - - VAP = W(a/(1 + c)) - VAb; (17.) - -which expresses the whole mechanical effect per minute in pounds -raised one foot. - -If the steam be worked expansively, let it be cut off after the -piston has moved through a part of the stroke expressed by e. - -The volume of steam of the undiminished pressure P' admitted per -minute through the valve would then be - - VA (e + c); - -and the ratio of this volume to that of the water producing it -being expressed by S', we should have - - S' = (VA(e + c))/W. - -The final volume into which this steam is subsequently expanded -being VA(1 + c), its ratio to that of the water will be - - S = (VA (1 + c))/W. - -The pressure P', till the steam is cut off, will be - - P' = [Wa / (VA(e + c))] - b. (18.) - -The mechanical effect E' produced per minute by the steam of full -pressure will be - - E' = P'AVe = [Wae / (e + c)] - AVbe; - -and the effect E'' per minute produced by the expansion of the -steam will by (12.) be [Pg515] - - E'' = Wa log.[(1 + c) / (e + c)] - bVA(1 - e). - -Hence the total effect per minute will be - - _E_ = Wa [(e/(e + c)) + log.([1 + c]/[e + c])] - bVA. (19.) - -If the engine work without expansion, e = 1; - - [because] _E'_ = ( Wa/(1 + c)) - bVA, (20.) - -as before; and the effect per minute gained by expansion will -therefore be - - _E_ - _E'_ = - - Wa [(e/(e + c)) - (1/(1 + c)) + log.([1 + c]/[e + c])]; (21.) - -which therefore represents the quantity of power gained by the -expansive action, with a given evaporating power. - -In these formulae the total effect of the steam is considered -without reference to the nature of the resistances which it has to -overcome. - -These resistances may be enumerated as follows:-- - - 1. The resistance produced by the load which the engine is - required to move. - - 2. The resistance produced by the vapour which remains - uncondensed if the engine be a condensing engine, or of the - atmospheric pressure if the engine do not condense the steam. - - 3. The resistance of the engine and its machinery, consisting - of the friction of the various moving parts, the resistances - of the feed pump, the cold water pump, &c. A part of these - resistances are of the same amount, whether the engine be - loaded or not, and part are increased, in some proportion - depending on the load. - -When the engine is maintained in a state of uniform motion, the -sum of all these resistances must always be equal to the whole -effect produced by the steam on the piston. The power expended on -the first alone is the _useful effect_. - -Let R = the pressure per square foot of the piston surface, which -balances the resistances produced by the load. - -mR = the pressure per square foot, which balances that part of the -friction of the engine which is proportional to the load. - -r = the pressure per square foot, which balances the sum of all -those resistances that are not proportional to the load. - -The total resistance, therefore, being R + mR + r, which, when the -mean motion of the piston is uniform, must be equal to the mean -pressure on the piston. The total mechanical effect [Pg516] must -therefore be equal to the total resistance multiplied by the space -through which that resistance is driven. Hence we shall have - - [R(1 + m) + r]VA = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] - VAb; - - [because] RVA(1 + m) = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] - VA(b + r). - -For brevity, let - - e' = a[(e/(e + c)) + log.([1 + c]/[e + c])]; - - [because] RVA(1 + m) = We' - VA(b + r). (22.) - -By solving this for VA, we obtain - - VA = We'/(R(1 + m) + b + r); - - [because] RVA = We'R/(R(1 + m) + b + r). (23.) - -This quantity RVA, being the product of the resistance RA, of the -load reduced to the surface of the piston, multiplied by the space -through which the piston is moved, will be equal to the load -itself multiplied by the space through which it is moved. This -being, in fact, the useful effect of the engine, let it be -expressed by U, and we shall have - - U = We'R/(R(1 + m) + b + r). (24.) - -Or by (22.), - - U(1 + m) = We' - VA(b + r). (25.) - -The value of the useful effect obtained from these formulae will be -expressed in pounds, raised one foot per minute, W being the -effective evaporation in cubic feet per minute, A the area of the -piston in square feet, and V the space per minute through which it -is moved, in feet. - -Since a resistance amounting to 33,000 pounds moved through one -foot per minute is called one-horse power, it is evident that the -horse power H of the engine is nothing more than the useful effect -per minute referred to a larger unit of weight or resistance; that -is to 33,000 pounds instead of one pound. Hence we shall have - - H = U/33000. (26.) - -Since the useful effect expressed in (24.) and (25.) is that due -to a number of cubic feet of water, expressed by W, we shall -obtain the effect due to one cubic foot of water, by dividing U by -W. If, therefore, U' be the effect produced by the effective -evaporation of a cubic foot of water, we shall have [Pg517] - - U' = U/W. (27.) - -If the quantity of fuel consumed per minute be expressed by F, the -effect produced by the unit of fuel, called the DUTY of the -engine, will, for like reason, be - - D = U/F. (28.) - -If the fuel be expressed in hundredweights of coal, then D will -express the number of pounds' weight raised one foot by a -hundredweight of coal. - -By solving (24.) and (25.) for W, we obtain - - W = [U(R(1 + m) + b + r)]/Re', (29.) - - W = (1/e')[U(1 + m) + VA(b + r)]. (30.) - -By eliminating U, by (26.), we shall have - - W = [33000 H(R(1 + m) + b + r)]/Re', (31.) - - W = (1/e')[33000 H(1 + m) + VA(b + r)]. (32.) - -The evaporation necessary per horse power per minute will be found -by putting H = 1 in these formulae.[41] - -It will be observed that the quantities A and V, the area of the -cylinder and the speed of the piston, enter all these formulae as -factors of the same product. Other things, therefore, being the -same, the speed of the piston will be always inversely as the area -of the cylinder. In fact, VA is the volume of steam per minute -employed in working the piston, and if the piston be increased or -diminished in magnitude, its speed must be inversely [Pg518] -varied by the necessity of being still moved through the same -number of cubic feet by the same volume of steam. - -It has been already stated in the text, that no satisfactory -experiments have yet been made, by which the numerical value of -the quantity r can be exactly known. In engines of different -magnitudes and powers, this resistance bears very different -proportions to the whole power of the machine. In general, -however, the larger and more powerful the engine, the less that -proportion will be. - -That part of this resistance which arises from the reaction of the -uncondensed vapour on the piston is very variable, owing to the -more or less perfect action of the condensing apparatus, the -velocity of the piston, and the magnitude and form of the steam -passages. M. de Pambour states, that, by experiments made with -indicators, the mean amount of this resistance in the cylinder is -2-1/2 lbs. per square inch more than in the condenser, and that -the pressure in the latter being usually 1-1/2 lb. per square -inch, the mean amount of the pressure of the condensed vapour in -the cylinder is about 4 lbs. per square inch. Engineers, however, -generally consider this estimate to be above the truth in -well-constructed engines, when in good working order. - -In condensing low pressure engines of forty horse power and -upwards, working with an average load, it is generally considered -that the resistance produced by the friction of the machine and -the force necessary to work the pumps may be taken at about 2 lbs. -per square inch of piston surface. - -Thus the whole resistance represented by r in the preceding -formulae, as applied to the larger class of low pressure engines, -may be considered as being under 6 lbs. per square inch, or 864 -lbs. per square foot, of the piston. It is necessary, however, to -repeat, that this estimate must be regarded as a very rough -approximation; and as representing the mean value of a quantity -subject to great variation, not only in one engine compared with -another, but even in the same engine compared with itself at -different times and in different states. - -In the same class of engines, the magnitude of the clearage is -generally about a twentieth part of the capacity of the cylinder, -so that c = 0.05. - -That part of the resistance which is proportional to the load, and -on which the value of m in the preceding formulae depends, is still -more variable, and depends so much on the form, magnitude, and the -arrangement of its parts, that no general rule can be given for -its value. It must, in fact, be determined in every particular -case. - -In the practical application of the preceding formulae in -condensing engines we shall have [Pg519] - - a = 3875969 b = 164 c= 0.05; - - e' = 3875969([e/(e + 0.05)] + log.[1.05/(e + 0.05)]). - -In engines which work without condensation, and therefore with -high pressure steam, we shall have - - a = 4347826 b = 618 c = 0.05 - - e' = 4347826([e/(e + 0.05)] + log.[1.05/(e + 0.05)]) - -To facilitate computation, the values of e' corresponding to all -values of e, from e = .10 to e = .90, are given in the following -table:-- - - -------------------------------------------------------------- - |Condensing|Non-condensing|| |Condensing|Non-condensing - | Engines | Engines || | Engines | Engines - e | e'. | e'. || e | e'. | e'. - -------------------------------------------------------------- - .10 | 10126265 | 11359029 ||.51 | 5966367 | 6692708 - .11 | 9956867 | 11169008 ||.52 | 5903837 | 6622565 - .12 | 9793136 | 10985344 ||.53 | 5842288 | 6553525 - .13 | 9634926 | 10807875 ||.54 | 5781693 | 6485552 - .14 | 9482029 | 10636364 ||.55 | 5722024 | 6418619 - .15 | 9334219 | 10470560 ||.56 | 5663251 | 6352693 - .16 | 9191251 | 10310186 ||.57 | 5605353 | 6287745 - .17 | 9052888 | 10154978 ||.58 | 5548297 | 6223742 - .18 | 8918896 | 10004675 ||.59 | 5492064 | 6160662 - .19 | 8789043 | 9859014 ||.60 | 5436628 | 6098478 - .20 | 8663120 | 9717760 ||.61 | 5381969 | 6037166 - .21 | 8540918 | 9580682 ||.62 | 5328065 | 5976699 - .22 | 8422242 | 9447559 ||.63 | 5274896 | 5917057 - .23 | 8306916 | 9318193 ||.64 | 5222444 | 5858219 - .24 | 8194770 | 9192396 ||.65 | 5170684 | 5800159 - .25 | 8085644 | 9069984 ||.66 | 5119605 | 5742860 - .26 | 7979392 | 8950796 ||.67 | 5069186 | 5686304 - .27 | 7875870 | 8834674 ||.68 | 5019410 | 5630469 - .28 | 7774952 | 8721468 ||.69 | 4970263 | 5575340 - .29 | 7676514 | 8611048 ||.70 | 4921727 | 5520894 - .30 | 7580447 | 8503284 ||.71 | 4873790 | 5467121 - .31 | 7486640 | 8398056 ||.72 | 4826434 | 5414000 - .32 | 7394990 | 8295250 ||.73 | 4779648 | 5361519 - .33 | 7305407 | 8194760 ||.74 | 4733417 | 5309659 - .34 | 7217807 | 8096496 ||.75 | 4687728 | 5258408 - .35 | 7132097 | 8000352 ||.76 | 4642569 | 5207751 - .36 | 7048206 | 7906249 ||.77 | 4597928 | 5157676 - .37 | 6966058 | 7814100 ||.78 | 4553794 | 5108170 - .38 | 6885585 | 7723832 ||.79 | 4510155 | 5059218 - .39 | 6806720 | 7635365 ||.80 | 4466999 | 5010808 - .40 | 6729408 | 7548642 ||.81 | 4424317 | 4962931 - .41 | 6653578 | 7463580 ||.82 | 4382096 | 4915569 - .42 | 6579187 | 7380132 ||.83 | 4340332 | 4868720 - .43 | 6506174 | 7298230 ||.84 | 4299010 | 4822368 - .44 | 6434491 | 7217822 ||.85 | 4258120 | 4776500 - .45 | 6364099 | 7138858 ||.86 | 4217658 | 4731113 - .46 | 6294944 | 7061285 ||.87 | 4177613 | 4686192 - .47 | 6226989 | 6985058 ||.88 | 4137974 | 4641728 - .48 | 6160190 | 6910126 ||.89 | 4098737 | 4597713 - .49 | 6094510 | 6836450 ||.90 | 4059893 | 4554140 - .50 | 6029916 | 6763992 || | | - -------------------------------------------------------------- - - -[Pg520] In engines which work without expansion we have - - e' = a/(1 + c). - -For condensing engines without expansion, we shall then have - - e' = 3875969/1.05 = 3691399; (33.) - -and for non-condensing engines, - - e' = 4347826/1.05 = 4140787. (34.) - -As the diameters of the cylinders of engines are generally -expressed in inches, the corresponding areas of the pistons -expressed in square feet are given in the following table, so that -the values of A may be readily found:-- - - ----------------------------------------------------------------------- - Diam. | Area. | Diam. | Area. | Diam. | Area. | Diam. | Area. - ----------------------------------------------------------------------- - Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet. - 10 | 0.545 | 48 | 12.566 | 86 | 40.339 | 124 | 83.863 - 11 | 0.660 | 49 | 13.095 | 87 | 41.283 | 125 | 85.221 - 12 | 0.785 | 50 | 13.635 | 88 | 42.237 | 126 | 86.590 - 13 | 0.922 | 51 | 14.186 | 89 | 43.202 | 127 | 87.970 - 14 | 1.069 | 52 | 14.748 | 90 | 44.179 | 128 | 89.361 - 15 | 1.227 | 53 | 15.321 | 91 | 45.166 | 129 | 90.763 - 16 | 1.396 | 54 | 15.904 | 92 | 46.164 | 130 | 92.175 - 17 | 1.576 | 55 | 16.499 | 93 | 47.173 | 131 | 93.599 - 18 | 1.767 | 56 | 17.104 | 94 | 48.193 | 132 | 95.033 - 19 | 1.969 | 57 | 17.721 | 95 | 49.224 | 133 | 96.479 - 20 | 2.182 | 58 | 18.348 | 96 | 50.265 | 134 | 97.935 - 21 | 2.405 | 59 | 18.986 | 97 | 51.318 | 135 | 99.402 - 22 | 2.640 | 60 | 19.635 | 98 | 52.382 | 136 | 100.880 - 23 | 2.885 | 61 | 20.295 | 99 | 53.456 | 137 | 102.369 - 24 | 3.142 | 62 | 20.966 | 100 | 54.542 | 138 | 103.869 - 25 | 3.409 | 63 | 21.648 | 101 | 55.638 | 139 | 105.380 - 26 | 3.687 | 64 | 22.340 | 102 | 56.745 | 140 | 106.901 - 27 | 3.976 | 65 | 23.044 | 103 | 57.863 | 141 | 108.434 - 28 | 4.276 | 66 | 23.758 | 104 | 58.992 | 142 | 109.977 - 29 | 4.587 | 67 | 24.484 | 105 | 60.132 | 143 | 111.532 - 30 | 4.909 | 68 | 25.220 | 106 | 61.283 | 144 | 113.097 - 31 | 5.241 | 69 | 25.967 | 107 | 62.445 | 145 | 114.674 - 32 | 5.585 | 70 | 26.725 | 108 | 63.617 | 146 | 116.261 - 33 | 5.940 | 71 | 27.494 | 109 | 64.801 | 147 | 117.859 - 34 | 6.305 | 72 | 28.274 | 110 | 65.995 | 148 | 119.468 - 35 | 6.681 | 73 | 29.065 | 111 | 67.201 | 149 | 121.088 - 36 | 7.069 | 74 | 29.867 | 112 | 68.417 | 150 | 122.719 - 37 | 7.467 | 75 | 30.680 | 113 | 69.644 | 151 | 124.361 - 38 | 7.876 | 76 | 31.503 | 114 | 70.882 | 152 | 126.013 - 39 | 8.296 | 77 | 32.338 | 115 | 72.131 | 153 | 127.676 - 40 | 8.727 | 78 | 33.183 | 116 | 73.391 | 154 | 129.351 - 41 | 9.168 | 79 | 34.039 | 117 | 74.662 | 155 | 131.036 - 42 | 9.621 | 80 | 34.907 | 118 | 75.944 | 156 | 132.732 - 43 | 10.085 | 81 | 35.785 | 119 | 77.236 | 157 | 134.439 - 44 | 10.559 | 82 | 36.674 | 120 | 78.540 | 158 | 136.157 - 45 | 11.045 | 83 | 37.574 | 121 | 79.854 | 159 | 137.886 - 46 | 11.541 | 84 | 38.485 | 122 | 81.180 | 160 | 139.626 - 47 | 12.048 | 85 | 39.406 | 123 | 82.516 | 161 | 141.377 - --------------------------------------------------------------------- - - -[Pg521] The practical application of the preceding formulae will be -shown by the following examples. - - -EXAMPLES. - -1. _A 36-inch cylinder with 5-1/2 feet stroke is supplied by a -boiler evaporating effectively 60 cubic feet of water per hour, -and the piston makes 20 strokes per minute without expansion;-- -what is the power of the engine and the pressure of steam -in the cylinder?_ - -Let it be assumed that r = 6 x 144 = 864 and m = 0.1. Since the -engine is a condensing engine, we have b = 164 and e' = 3691399. -By the formulae (25.) and (26.) we have - - H = [We' - VA(b + r)]/[33000(1 + m)]; - -and since by the data we have - - W = 1 A = 7.069 V = 2nL = 40 x 5.5 = 220, - -the formula, by these substitutions, becomes - - H = (3691399 - 220 x 1028 x 7.069) / (33000 x 1.1); - [because] H = 57.6. - -Since e = 1, the pressure P of steam in the cylinder, by (18.), is - - P = (We'/VA) - b. - -Therefore - - P = (3691399/1555.18) - 164 = 2210; - -which being the pressure in pounds per square foot, the pressure -per square inch will be 15-1/3 lbs. - -2. _To find the effective evaporation necessary to produce a power -of 80 horses with the same engine. Also, find the pressure of -steam in the cylinder, the speed of the piston being the same._ - -By the formula (32.), with the above substitutions, we have - - W = (33000 x 80 x 1.1 + 220 x 7069 x 1028)/3691399 = 1.22. - -The evaporating power would therefore be only increased 22 per -cent., while the working power of the engine would be increased -nearly 40 per cent. - -The pressure P in the cylinder will be given, by (18.), as before. - - P = [(1.22 x 3691399)/1555.18] - 164 = 2732; - -which is equivalent to 19 lbs. per square inch. [Pg522] - -3. _What must be the diameter of a cylinder to work with a power -of a hundred horses, supplied by a boiler evaporating effectively -70 cubic feet of water per hour, the mean speed of the piston -being 240 feet per minute, and the steam being cut off at half -stroke? Also, what will be the full pressure of steam on the -piston?_ - -Taking, as in the former examples, m = 0.1, b = 164, and r = 864, -we shall have - - H = 100 W = 7/6 V = 240, - -and by the column for condensing engines, in table, p. 519, we -have e' = 6029916, where e = 0.50. Making these substitutions in - - We' = 33000 H (1 + m) + VA (b + r), - -we shall have - - (7/6) x 6029916 = 3300000 x 1.1 + 240 x 1028 x A. - -Whence we find - - A = 13.8; - -and by the table, p. 520, the corresponding diameter of the -cylinder will be 50-1/3 inches. - -If P' be the full pressure of the steam, we shall have, by (18.), - - P' = (Wa/VA(e + c)) - b. - -Making in this the proper substitutions, we have - - P' = ((7/6) x 3875969) / (240 x 13.8 x 0.55) - 164 = 2318; - -which being in pounds per square foot, the pressure per square -inch will be 16-1/10 lbs. - - FOOTNOTES: - - [40] M. de Pambour states that the increased volume is 1364 - cubic inches. - - [41] Formulae equivalent to some of the preceding are given, - with numerous others, by M. de Pambour, in his Theory of the - Steam Engine. These mathematical details contain nothing new - in principle, being merely the application of the known - principles of general mechanics to this particular machine. M. - de Pambour objects against the methods of calculating the - practical effects of steam engines generally adopted by - engineers in this country. Their estimates of the loss of - power by friction, imperfect condensation, and other causes, - are, as I have stated in this volume, vague, and can be - regarded at best as very rough approximations; but, subject to - the restrictions under which their methods of calculation are - always applied, they are by no means so defective as M. de - Pambour supposes. He proves what he considers to be their - inaccuracy, by applying them in cases in which they are never - intended to be applied by English engineers. Those who desire - to reduce to general algebraical formulae the effects of the - different kinds of steam engines will, however, find the - volume of M. de Pambour of considerable use. - -[Pg523] - - - - -INDEX. - - - Air, elasticity of, 28; - May be partially expelled from a vessel by the application of - heat, 44. - - America, steam navigation first established in, 487; - Circumstances which led to it, 488; - Fitch and Rumsey, their attempts to apply the single-acting - engine to the propulsion of vessels, 489; - Stevens of Hoboken commences experiments on steam navigation, - 489; - Experiments of Livingstone and Fulton, 489; - Fulton's first boat, 490; - The Hudson navigated by steam, 491; - Extension and improvement of river navigation, 492; - American steamers, 494; - Difference between them and European steamers, 494; - Steamers on the Hudson, 494; - American paddle-wheels, 495; - Sea-going American steamers, 496; - Speed attained by American steamers, 497; - Lake steamers, 499; - The Mississippi and its tributaries, 499; - Steam-boats navigating it, 500; - Their structure and machinery, 500; - New Orleans Harbour, 503; - Steam tugs, 503. - - Atmosphere, 38; - Weight of, 39. - - Atmospheric air, mechanical properties of, 38; - Composition of, 253. - - Atmospheric engine, Thomas Newcomen the reputed inventor of, 62; - Description of, as first constructed by Newcomen, 67; - The operation of considered, 69; - Not unfrequently used in preference to the modern steam - engine, 72; - Advantages which it possessed over Savery's, 73; - Considerably improved by Beighton, 75; - John Smeaton investigates this machine, 76; - Brindley obtains a patent for improvements in, 76; - Applied by Champion of Bristol to raise water, 181; - Possessed but limited power of adaptation to a varying load, - 151; - Expedient to remedy this, 151; - Working-beam, cylinder, and piston applied to by Newcomen, - 322. - - Atmospheric pressure rendered available as a mechanic agent by - Denis Papin, 38; - Means of measuring the force of, 39; - The idea of using against a vacuum or partial vacuum to work - a piston in a cylinder, suggested by Otto Guericke, 73. - - - Barometer gauge, 272. - - Barton's piston, 248. - - Beighton, his improvement of the atmospheric engine, 75. - - Black, Dr., his doctrine of latent heat, 93. - - Blasco de Garay, his contrivance to propel vessels, 16; - The contrivance of, probably identical with that of Hero, 17. - - Blinkensop, his locomotive engine, 337. - - Blowing-box, 429. - - Blowing out, Seaward's method of, 454. - - Blow-off cocks, 452. - - Boiler, forms of, most convenient, 255; - The waggon boiler adopted by Watt, 255; - Furnace, 256; - Method of feeding, 257; - Combustion of gas in flues, 260; - Mr. Williams's method of consuming the unburned gases which - escape from the grate, and are carried through the flues, - 260; - Construction of grate and ash-pit, 261; - Magnitude of heating surface of boiler, 262; - Capacity of, must be proportioned to the quantity of water to - be evaporated, 263; - Water-space and steam-space in boiler, 263; - Proportion of water-space in the boiler, how to be regulated, - 264; - Position of flues, 264; - Method of feeding, 265; - The magnitude of the feed should be equal to the quantity of - water evaporated, 265; - Different methods for indicating the level of the water in - the boiler, 266; - Level guages, 266; - Self-regulating feeder, 267; - Another method of arranging, 269; - Steam gauge, 270; - Thermometer gauge, 271; - Barometer gauge, 272; - The indicator to measure the mean efficient force of the - piston invented by Watt, 274; - The counter contrived by Watt, 278; - Safety valve, 279; - Fusible plugs used in high pressure boilers, 280; - Self-regulating damper, 281; - Self-regulating furnace invented by Brunton, 283; - Duty of a boiler, 294; - Boilers of locomotive engines, 351; - Construction of the boiler of Gurney's steam carriage, 423; - All boilers require occasional cleansing, 427; - Gurney's method of removing crust of deposited matter in - boilers, 427; - The boiler of Dr. Church's engine formed of copper, 439; - Boilers in marine engines, 449; - Effects of sea-water in, 450; - Remedies for them, 451; - Substitution of copper for iron, 460; - Expedient of coating boilers with felt, applied by Watt, 463. - - Booth, Mr., his report on locomotive engines, 361. - - Boulton and Watt's experiments on the horse power of engines, - 288. - - Branca, Giovanni, his machine for propelling a wheel by a blast - of steam, 22. - - Brindley (James) obtains a patent for improvements in - atmospheric engine, 76; - Undertook to erect an engine at Newcastle-under-Lyne, 76; - Discouraged by the obstacles thrown in his way, 76. - - Brougham, Lord, his sketch of Watt's character, 313; - Inscription from the pen of, on Watt's monument in Westminster - Abbey, 320. - - Buffers, 404. - - Cartwright's engine to use the vapour of alcohol to work the - piston, 245; - His piston, 247. - - Cawley and Newcomen obtain a patent for the atmospheric engine, - 64. - - Champion applies atmospheric engine to raise water, 181. - - Chapman, Messrs., their locomotive engine, 337. - - Chlorine introduced in bleaching by Watt, 310. - - Church, Dr., his steam engine, 439; - The boiler formed of copper, 439. - - Coals, the virtues and powers which steam has conferred upon, - 6; - The amount of labour a bushel of performs by means of the - steam engine, compared with horse power, 7; - Constituents of, 252; - Process of combustion, 252. - - Coal mines, apprehensions as to the possibility of the - exhaustion of groundless, 8. - - Cocks, friction on, 240. - - Cocks and valves, 227. - - Combustion of gas in flues, 260. - - Condensation by injection, accidental discovery of, 69. - - Condensation in the cylinder incompatible with a due economy - of fuel, 120. - - Condensing principle, circumstance which led to Savery's - discovery of, 47. - - Condensing pipe in Savery's engine, 52. - - Condensing out of the cylinder, 120. - - Condensing jet, 191. - - Conical steam valves, 228. - - Conversion of ice into water, 103; - Of water into steam, 105. - - Copying press invented by Watt, 302. - - Cornish system of inspection, 297. - - Cornish engines, improvement of, 298; - Historical detail of the duty of, 299. - - Cylinders, Wilkinson's machine for accurately boring the - insides of, 149. - - - D valve, 230. - - Dalton and Gay-Lussac, law of, relating to the pressure of - elastic bodies, 171. - - Dixon, Mr. The substitution of brass for copper tubes in - locomotive engines ascribed to him, 370. - - Double clack-valve, 228. - - - Eccentric, 225; - Two expedients to reverse the position of, 379. - - Effect of an engine, 285. - - Elastic fluids. The law according to which the pressure of, - increases with their temperature, discovered by Dalton and - Gay-Lussac, 171. - - Evaporation of water and other liquids, physical and - mechanical principles connected with, 97. - - Expansion of common steam, effects of, 173. - - Expansive action of steam, 159; - Stated by Watt in a letter to Dr. Small, 157; - Its principle explained, 158; - Mechanical effect resulting from it, 161; - Computed effect of cutting off steam at different portions - of the stroke, 162; - Involves the condition of a variation in the intensity of - the moving power, 163; - Expedients for equalising the power, 164; - The expansive principle in the engines constructed by - Boulton and Watt, limited, 165; - Its more extensive application in the Cornish engines, 165; - Methods of equalising, 174; - Description of Hornblower's engine for this purpose, 174. - - Expansive principle, application of in marine engines, 466. - - - Farey on the steam engine, quotation from, relative to - Savery's engine, 58; - His evidence before the House of Commons, 435. - - Field, construction of his split paddle, 478. - - Fitch and Rumsey, their attempts to apply the single-acting - engine to the propulsion of vessels, 489. - - Flues, position of, 264. - - Fluids, of two kinds, 25; - Mechanical properties of, 25; - Elastic, 27; - Experimental proof that they press equally in all - directions, 41. - - Fly-wheel, 205. - - Four-way cock, 239; - Disadvantages of, 240. - - Fuel, means of economising, in marine furnaces, 463. - - Fulton and Livingstone, their experiments in steam navigation, - 489. - - Fulton's first boat, 490. - - Furnace, self-regulating, invented by Brunton, 283. - - Fusible plugs used in high-pressure boilers, 280. - - - Galloway, his paddle-wheel described, 476. - - Gas, elasticity of, 28. - - Gay-Lussac and Dalton, law of, relating to the pressure of - elastic bodies, 171. - - Governor, adaptation of, 209. - - Gradients, restrictions on, 411; - Disposition of, should be uniform, 415. - - Great Western Railway, Dr. Lardner's experiments on, 408. - - Griff, proposals to drain a colliery at, mentioned by - Desaguliers, 64. - - Gurney's steam carriage, 423; - Construction of the boiler of, 423; - His method of removing crust of deposited matter in boilers, - 427; - His experiments on common roads, 432. - - - Hall, his condensers described, 458. - - Hancock, his steam carriage, 436; - In what manner it differs from that of Gurney, 437. - - Harris, Dr., mentions Savery's engine in his "Lexicon - Technicum," 56. - - Heat, effects of upon water, 29; - Waste of in atmospheric engine, 89; - An examination of the analogous effects produced by the - continued application of, to water in the liquid state, - 102; - Radiation of, 254. - - Heating by steam brought forward by Watt, 303. - - "Hecla," experiments with the, 412. - - Hero of Alexandria, description of his machine, 12. - - High pressure engines described, 321; - One of the earliest forms of the steam engine, 322; - Obscurely described in the "Century of Inventions," 322; - Construction of the first, by Messrs. Trevethick and Vivian, - 324. - - Hooke exposes the fallacy of Papin's project, 64. - - Horse carriages compared with steam, 435. - - Horse power of steam engines, 288; - Smeaton's estimation of, 288; - Boulton and Watt's experiments on, 288. - - Howard's description of his marine engine, 464. - - Hudson, the, navigated by steam, 491. - - Hull, Jonathan, his application of the steam engine to water - wheels, 180. - - Humphrey. His marine engine described, 470. - - Huskisson, Mr., death of, 329. - - Hydrogen, 253. - - - India, steam navigation to, 483. - - Indicator invented by Watt, 274. - - - Jeffrey, Lord; his sketch of the character of Watt, 315. - - - Kinneal, description of Watt's experimental engine at, 131. - - - Lake steamers, 499. - - Lardner's, Dr., experiments on the Manchester Railway in 1832, - 357; - His experiments in 1838, 406; - Experiments on the Great Western Railway, 408. - - Leupold's engine, description of, 323. - - Level gauges, 266. - - Linen, machine for drying by steam, invented by Watt, 303. - - Liverpool and Manchester railroad, effects of the introduction - of steam transport on, 329; - Want of experience in the construction of the engines, 329; - Death of Mr. Huskisson, 329; - Proceedings of the directors, 342; - Premium offered by them for the best engine, 344; - Experimental trial, 344. - - Livingstone and Fulton, experiments of in steam navigation, - 489. - - Locomotive engine, history of, 328; - Blinkensop's engine, 337; - Chapman's engine, 337; - Walking engine, 337; - Mr. Stephenson's engine at Killingworth, 339; - Defect of, 341; - Description of the "Rocket," 345; - The "Sanspareil," 347; - The "Novelty," 349; - Superiority of the "Rocket," 350; - Subsequent improvements in the locomotive engine, 352; - Table, showing the economy of fuel gained by subdividing the - flue into tubes, 354; - Engines constructed in the form of the "Rocket" subject to - two principal defects, 354; - These defects remedied, 355; - Improved by the adoption of a more contracted blast pipe, - 356; - Dr. Lardner's experiments in 1832, 357; - Adoption of brass tubes, 361; - Mr. Booth's report, 361; - Detailed description of the most improved locomotive - engines, 364; - Substitution of brass for copper tubes ascribed to Mr. - Dixon, 370; - Mr. Stephenson constructed the driving wheels without - flanges, 383; - Pressure of steam in the boiler limited by two safety-valves, 402; - Buffers, 404; - Steam whistle, 404; - Water tank, 404; - Power of locomotive engines, 405; - Evaporation of boilers, 406; - Dr. Lardner's experiments in 1838, 406; - Resistance to railway trains, 407; - Dr. Lardner's experiments on the Great Western Railway, 408; - Restriction on gradients, 411; - Experiment with the "Hecla," 412; - Disposition of gradients should be uniform, 415; - Method of surmounting steep inclinations, 415; - Steam carriages on common roads, 419; - Difference between steam engines on railways and those used - to propel carriages on turnpike roads, 422; - Gurney's steam carriage, 423; - Construction of the boiler of, 423; - Escape of steam from the engines on the Liverpool road, 428; - Blowing-box, 429; - Separator, 430; - Difficulties in the practical working of steam carriages - upon common roads, 432; - Gurney's experiments on common roads, 432; - Prejudice against locomotive engines on common roads, 432; - Not more destructive to roads than carriages drawn by - horses, 433; - Report of the committee of the House of Commons, 433; - Weight of steam carriages, 433; - Two methods of applying locomotives upon common roads, 434; - Horse carriages compared with, 435; - Farey's evidence before the House of Commons, 435; - Risk of accident from explosion extremely slight, 435; - Hancock's steam carriage, 436; - In what manner it differs from that of Gurney, 437; - Ogle's steam carriage, 438; - His evidence before the House of Commons, 439; - Dr. Church's steam engine, 439; - The boiler of formed of copper, 439. - - Lunar Society, Boulton and Watt leading members in, 302. - - - Marine engines, form and arrangement of, 441; - Difference between marine and land engines, 443; - Engine-room, arrangement of, 446; - Boilers in, 449; - Effects of sea-water on boilers, 450; - Remedies for them, 451; - Blow-off cocks, 452; - Indicators of saltness, 452; - Seaward's indicator, 454; - His method of blowing out, 454; - Method of Maudslay and Field to preserve freshness of water - in the boiler, 456; - Brine pumps, 457; - Tubular condensers applied by Mr. Watt, 457; - Hall's condensers, 458; - Substitution of copper for iron boilers, 461; - Process of stoking, 462; - Marine furnaces, 463; - Expedient of coating boilers with felt applied by Watt, 463; - Means of economising fuel, 463; - Description of Howard's engine, 464; - Application of the expansive principle in marine engines, - 466; - Recent improvements of Messrs. Maudslay and Field, 467; - Humphrey's engine, 470; - Common paddle-wheel, 472; - Defect of, 474; - Feathering paddles, 474; - Galloway's paddle-wheel, 476; - Field's split paddle, 478; - Proportion of power to tonnage, 480; - Iron steam vessels, 482. - - Mariotte's law relating to pressure, 171. - - Maudslay and Field, their method to preserve the requisite - freshness of water in the boiler, 456; - Brine pumps, 457; - Recent improvements of in marine engines, 466. - - Metallic pistons, 244; - Cartwright's engine, 245; - An improved form given to by Barton, 248. - - Mill work, Stewart's application of the steam engine to, 182. - - Mines, the drainage of, Watt endeavours to bring to perfection - the application of the steam engine to, 178. - - Mississippi and its tributaries, 499; - Steam-boats on, 500; - Their structure and machinery, 500. - - Morland, Sir Samuel, his application of steam to raise water, - 34; - The reputed inventor of several ingenious contrivances, 34; - His work in French upon the raising of water, 35; - Extract from it, 35; - Evelyn's account of his visit to, 36. - - Murray's slide-valve, 229. - - - Newcomen, Thomas, the reputed inventor of the atmospheric - engine, 62; - His acquaintance with Dr. Hooke, 62; - Acquainted with Papin's writings, 64; - The merits of his engine ascribed principally to its - mechanism and combinations, 73; - Obtains with Cawley a patent for the atmospheric engine, 64; - Resumes the old method of raising water from mines by - ordinary pumps, 65; - The means proposed to effect this, 66; - First conception of the atmospheric engine, 66; - Description of his construction of atmospheric engine, 67; - Suggestion of a better method of condensation than the - application of cold water on the external surfaces of the - cylinder, 69; - He abandons the external cylinder, 69; - Applied the working-beam, cylinder, and piston to the - atmospheric engine, 322. - - New Orleans Harbour, 503. - - "Novelty," description of the, a locomotive engine, 349. - - - Ogle, his steam carriage, 438; - His evidence before the House of Commons, 439. - - Otto Guericke, his suggestion relative to atmospheric - pressure, 73. - - Oxley made the first attempt to drive water-wheels by the - steam engine, 182. - - - Paddle-wheel described, 472; - Defect of, 474; - Feathering paddles, 474; - Galloway's paddle-wheel, 476; - Field's split paddle, 478. - - Paddle-wheels of American steamers, 495. - - Papin, Denis, conceived the idea of rendering atmospheric - pressure available as a mechanical agent, 37; - Description of his contrivance, 37; - His discovery of condensation of steam, 45; - Quotation from his work relative to this discovery, 45; - Explanation of this important discovery, 46; - Discovers the method of producing a vacuum by the - condensation of steam, 178; - His projected applications of the steam engine, 178; - His proposition for the construction of an engine working by - atmospheric pressure, 62; - Abandons the project when informed of the principle and - structure of Savery's engine, 62; - His engine described, 62; - This project nothing more than a reproduction of the Marquis - of Worcester's engine, 63; - The fallacy of his project exposed by Hooke, 64; - His project for producing a vacuum under a piston by - condensing the steam, published in the "Actae Eruditorum," - 64. - - Parallel motion, 195. - - Physical science, the rapid progress of, 8. - - Pistons, 242; - The common hemp-packed, 242; - Woolf's method of tightening the packing of, without - removing the lid of the cylinder, 244; - This method further simplified, 244; - Metallic, 244; - Cartwright's engine, 245; - Cartwright's piston, 247; - Invention of the indicator by Watt to measure the mean - efficient force of, 274. - - Piston rod and beam, methods of connecting in the double-acting - engine, 193. - - Pneumatic institution at Clifton, Watt one of the founders of, - 310. - - Potter, Humphrey, his contrivance for working the valves, 71; - Improved by the substitution of a plug-frame, 72. - - Power, proportion of, to tonnage in marine engines, 480. - - Power and duty of steam engines, 287. - - Priestley, Watt's letter to, relative to the composition of - water, 307. - - Pump, an illustration of force attained by a vacuum, 43. - - Puppet clacks, or button valves, 144. - - - Rack and Sector, 194. - - Railways, speed of coaches on, compared with that of stage-coaches - on a common road, 7. - - Railway transport, effects of, 328. 330. - - Railways and stone roads compared, 420. - - River navigation, extension and improvement of, 492. - - "Rocket," description of the, a locomotive engine, 345; - Engines constructed in the form of, subject to two principal - defects, 354; - These defects remedied, 355; - Improved by the adoption of a more contracted blast-pipe, 356. - - Roebuck, Dr., Watt's partnership with, 130. - - Rotatory motion, method of producing by sun and planet wheels, - 187. - - - Safety-valve not adopted by Savery, 57; - Invented by Papin, 57; - Description of, 57; - First applied to Savery's engine by Desaguliers, 58. - - "Sanspareil," description of the, a locomotive engine, 347. - - Savery, Thomas, obtains a patent for an engine to raise water, - 47; - Circumstance which led to his discovery of the condensing - principle, 47; - An account of his engine, 49; - Description of the working apparatus in which the steam is - used as a moving power, 51; - His engine described in a work entitled "The Miner's Friend," - 56; - Mentioned by Dr. Harrison in his "Lexicon Technicum," 56; - Quotation from his address to the Royal Society, 56; - Quotation from his address to the Miners of England, 57; - Mentioned by Bradley in his "Improvements of Planting and - Gardening," 57; - The safety-valve not adopted by him, 57; - The safety-valve first applied to his engine by Desaguliers, - 58; - Farey on the steam engine quoted, 58; - Further Improvements made by Desaguliers, 58; - Defects of his engine, 59; - His engine applied to the drainage of mines, 59; - Further defects of, 60; - The first to suggest the method of expressing the power of - an engine with reference to that of horses, 61; - Failure of his engine in the work of drainage, 61; - The tendency of high pressure to weaken and gradually destroy - the vessels, 72; - The power of his engines restricted, 73; - The atmospheric engine superior to, 73; - The boiler, guage-pipes, and regulator borrowed from his - engine, 73; - Proposes to apply his engine as a prime mover for all sorts - of machinery, 180. - - Scott, Sir Walter, his sketch of the character of Watt, 314. - - Sculpture, Watt's invention of machine for copying, 318. - - Sea-going American steamers, 496. - - Sea-water, effects of upon boilers, 450. - - Seaward's slides, 235; - Indicator of saltness, 454; - His method of blowing out, 454. - - Self-regulating damper, 281; - Furnace, 283. - - Separator, 430. - - Single-acting engine, description of Watt's, 133. 144. - - Single clack-valve, 227. - - Single cock, 238. - - Slide-valves, 229; - That contrived by Mr. Murray, 229. - - Smeaton, John, investigates the atmospheric engine, 76; - Applies himself to the improvement of wind and water mills, - 181; - His estimate of the horse power of engines, 288. - - Solomon De Caus, description of the apparatus of, 17; - M. Arago claims for him a share of the honour of the - invention of the steam engine, 21; - Republished, with additions, the work of Isaac De Caus, 22. - - Somerset, Edward, Marquis of Worcester. Invention of the steam - engine ascribed to him, 23; - Description of his contrivance, 23; - His "Century of Inventions," 24; - Brief account of his engine described in this work, 31; - His contrivance compared with that of De Caus, 33; - Many of his inventions have been reproduced and brought into - general use, 34. - - Steam cannot be applied _immediately_ to any useful purpose, - but requires the interposition of mechanism, 11; - Elastic force of, recognised by the ancients only in vague - and general terms, 14; - The power of, formerly made to minister to the objects of - superstition, mentioned by Arago, 15; - Anecdote showing the knowledge which the ancients had of the - mechanical force of, 15; - The discovery of the condensation of, by Papin, 45; - Mechanical power obtained from the direct pressure of the - elastic force of, suggested by De Caus and Lord Worcester, - 73; - Latent heat of, 107; - The mechanical force of considered, 115; - Watt's early experiments on, 87; - Discovery of the expansive action of, 157; - Expansive action of stated by Watt in a letter to Dr. Small, - 157; - Its principle explained, 158; - Mechanical effect resulting from it, 161; - Properties of, 168; - Common and super-heated steam, 168; - Pressure and temperature of, 171; - Relation between the temperatures of common steam and its - pressure and density, 172; - Effects of the expansion of common steam, 173; - Mechanical effects of, 173; - Methods of equalising the varying force of expanding steam, - 174; - Method of producing a vacuum by the condensation of, - discovered by Papin, 178; - Applied to move machinery, 179; - Steam guage, 270; - Heating by steam brought forward by Watt, 303; - A machine for drying linen by, invented by Watt, 303; - Mode of escape of, from the engines on the Liverpool road, - 429. - - Steam case or jacket, invented by Watt, 124. - - Steam engine, a subject of popular interest, 3; - The effects which it has produced upon the well-being of the - human race considered, 4; - Presents peculiar claims upon the attention of the people of - Great Britain, 5; - The exclusive offspring of British genius, 5; - The virtues and powers which it has conferred upon coals, 6; - Water the means of calling these powers into activity, 6; - Used in the drainage of Cornish mines, 7; - Comparison of its power with human labour, 8; - Investigation of the origin of, 10; - A combination of a great variety of contrivances and the - production of several inventions, 12; - Before the discoveries of James Watt was of extremely - limited power, 12; - Invention of, ascribed to the Marquis of Worcester, 23; - Account of Savery's, 49; - Farey quoted, 58; - Improvements made by Desaguliers, 58; - Applied to the drainage of mines, 59; - Humphrey Potter's contrivance, 72; - Advantages of the atmospheric engine over that of Savery, 73; - Progress of the atmospheric engine, 75; - Description of Papin's engine, 62; - Smeaton's improvements, 76; - First experiments of Watt and subsequent improvements, 83; - Watt's experiments on the force of steam at high pressure, - 83; - Watt discovers the great defects of the atmospheric engine, - 85; - Waste of heat in atmospheric engine, 89; - Dr. Black's theory of latent heat, 93; - Description of Watt's experimental engine at Kinneal, 131; - Description of his single-acting engine, 133; - Disadvantages of the atmospheric compared with the old - engine, 150; - Expedients to force the atmospheric engines into use, 152; - Watt's exertions to improve the manufacture of, at Soho, 155; - Efficiency of fuel in the new engines, 156; - Hornblower's engine, 175; - Woolf's engine, 176; - Watt endeavours to bring to perfection the application of, - to the drainage of mines, 178; - Papin's projected application of, 178; - Savery proposed to apply his steam engine as a prime mover - for all sorts of machinery, 180; - Jonathan Hull's application of, to water-wheels, 180; - Steam engine used for driving water wheels, 182; - First attempt of this kind made by Oxley, 182; - Stewart's application of, to mill work, 182; - Wasbrough's application of the fly-wheel and crank, 183; - Reasons why Watt's single-acting engine was not adapted to - produce continuous uniform motion of rotation, 184; - Watt's second patent, 186; - Valves of double-acting engine, 189; - Condensing jet, 191; - Methods of connecting the piston-rod and beam in the - double-acting engine, 193; - Rack and sector, 194; - Parallel motion, 195; - Connecting rod and crank, 202; - Fly-wheel, 205; - Throttle-valve, 207; - Adaptation of the governor, 209; - Double-acting engine considered as a whole, 216; - Process of its operation investigated, 217; - The eccentric, 225; - Cocks and valves, 227; - Single clack-valve, 227; - Double clack-valve, 228; - Conical steam-valves, 228; - Slide-valves, 229; - Murray's slide-valve, 229; - D valve, 230; - Seaward's slides, 235; - Single cock, 238; - Four-way cock, 239; - Pistons, 242; - Gross effect and useful effect of engines, 285; - Power and duty of, 287; - Horse power of, 288; - The means whereby mechanical power is expended in working - the engines enumerated, 290; - Common rules followed by engine makers, 292; - Duty of engines, 294; - Duty distinguished from power, 295; - Proportion of stroke to diameter of cylinder, 295; - Cornish system of inspection, 297; - Improvement of the Cornish engines, 298; - Historical detail of the duty of Cornish engines, 299; - High-pressure engines, 321; - Leupold's engine described, 323; - Construction of the first high-pressure engine by Messrs. - Trevethick and Vivian, 324; - First application of the steam engine to propel carriages - on railroads, 328; - Computation of how much corn could be saved by the - substitution of steam engines for horse power, 332; - Marine engines, form and arrangement of, 441; - Difference between marine and land engines, 443; - Mr. Howard's patent engine described, 464; - Humphrey's engine described, 470. - - Steam navigation to India, 483; - First established in America, 487; - Circumstances which led to it, 488; - Attempts of Fitch and Rumsey to apply the single-acting - engine to the propulsion of vessels, 489; - Stevens of Hoboken commences experiments in, 489; - Experiments of Livingstone and Fulton, 489; - Fulton's first boat, 490; - The Hudson navigated by steam, 491; - Extension and improvement of river navigation, 492; - American steamers, 494; - Difference between them and European steamers, 494; - Steamers on the Hudson, 494; - Sea-going American steamers, 496; - Speed attained by American steamers, 497; - Lake steamers, 499; - Steam-boats on the Mississippi, 500. - - Steam tugs, 503. - - Steep inclinations, method of surmounting, 415. - - Stephenson, his locomotive engine at Killingworth, 339; - Defect of, 341; - Constructed the driving wheels without flanges, 383. - - Stevens, of Hoboken, commences experiments on steam navigation, - 489. - - Stewart, his application of the steam engine to mill work, 182. - - Stoking, process of, 462. - - Stuffing-box, contrivance of, 147. - - Sun and planet wheels, method of producing rotatory motion, - 187. - - - Thermometers, the process of filling described, 44; - Explanation of the principle of, 98; - Construction of mercurial thermometer, 98; - Method of graduating, 99. - - Thermometer gauge, 270. - - Throttle-valve, description of, 207. - - Tredgold, his remark relative to Newcomen's engine, 73. - - Trevethick and Vivian's engine described, 325. - - - Vacuum, force obtained by a, 43; - The pump an illustration of this, 43. - - Valves of double-acting engine, 189. - - - Wasbrough, his application of the fly-wheel and crank, 183. - - Water, a pint of, the mechanical force produced by its - evaporation, 6; - The alternate decomposition and recomposition of, by - magnetism and electricity, analogous to vaporisation and - condensation, 8; - The fixed temperature which it assumes in boiling subject to - variation, 108; - Experiments to illustrate this, 109; - Table to show the temperature at which it will boil under - different pressures of the atmosphere, 113; - Mechanical force of a cubic inch of, converted into steam, - 118; - Discovery of the composition of, 303; - The merit of this discovery shared between Cavendish, - Lavoisier, and Watt, 305; - Latent heat of, 101; - Conversion of ice into, 103. - - Water tank, 404. - - Water-wheels, steam engine used for turning, 182. - - Watt (James), birth of, 77; - His infancy, 78; - Anecdotes respecting, 78; - His boyhood, 79; - Goes to London, 80; - Returns to Glasgow, 80; - Appointed mathematical instrument-maker to the university, - 81; - Adam Smith one of his earliest friends and patrons, 81; - Also Black and Robert Simson, 81; - Extract from an unpublished manuscript of Robison respecting - the character of, 82; - His first experiments on steam, 83; - Observes defects of atmospheric engine, 84; - His first attempt to improve it, by using a wooden instead - of an iron cylinder, 85; - His method to ascertain the temperatures at which water would - boil under pressures less than that of the atmosphere, 86; - His early experiments on steam, 87; - His notice of the waste of heat in atmospheric engines, 89; - His experiments to determine the extent to which water - enlarged its volume when it passed into steam, 90; - Discovers the latent heat of steam, 91; - Learns the theory of latent heat, 93; - His letter to Dr. Brewster, explaining the circumstances - which led to the error that a large share of the merit of - his discoveries were due to Black, 93; - Finds that condensation in the cylinder is incompatible with - a due economy of fuel, 120; - Conceives the notion of condensing out of the cylinder, 120; - Discovers separate condensation, 121; - Invents the air-pump, 122; - Substitutes steam pressure for atmospheric pressure, 123; - Invents the steam case or jacket, 124; - His first experiments to realise these inventions, 125; - His experimental apparatus, 126; - Difficulties of bringing the improved engines into use, 128; - Practises as a civil engineer, 129; - Makes a survey of the river Clyde, 129; - His partnership with Dr. Roebuck, 130; - His first patent, 130; - Description of his experimental engine at Kinneal, 131; - Removes to Soho, 131; - Abstract of the act of parliament for the extension of his - patent, 132; - Description of his single-acting engine, 133-144; - His condenser worked by an injection, 146; - Objections attending condensation by surface, 146; - Improvements in construction of piston, 147; - Effected by a contrivance called a stuffing-box, 147; - Method of packing, 148; - Improved methods of boring the cylinder, 149; - His letter to Smeaton on this subject, 149; - Used black-lead dust for the purpose of lubrication, 149; - This found to wear the cylinder, 149; - Disadvantages of the atmospheric compared with the old - engines, 150; - Greatly increased economy of fuel, 151; - Expedients to force the atmospheric engines into use, 152; - His correspondence with Boulton, 153; - His correspondence with Smeaton, 154; - Exertions to improve the manufacture of engines at Soho, 155; - Efficiency of fuel in the new engines, 156; - Endeavours to bring to perfection the application of the - steam engine to the drainage of mines, 178; - The reasons why his single-acting engine was not adapted to - produce continuous uniform motion of rotation, 184; - His notes upon Dr. Robison's article on the steam engine, - 184; - His second patent, 186; - His third patent, 189; - His application of the fly-wheel, 205; - His application of the throttle-valve, 207; - His adaptation of the governor, 209; - His double-acting engine considered as a whole, 216; - Investigation of the process of its operation, 217; - Eccentric, 225; - Cocks and valves, 227; - Single clack-valve, 227; - Double clack-valve, 228; - Conical steam-valve, 228; - Slide-valves, 229; - The waggon boiler adopted by him, 225; - Invents the indicator, 274; - The counter contrived by him, 278; - The Lunar Society in which Watt and Boulton were leading - members, 302; - Invents the copying press, 302; - His friends and associates at Birmingham, 302; - Method of heating by steam brought forward by him, 303; - His invention of a machine for drying linen by steam, 303; - His share in the discovery of the composition of water, 303; - His letter to Priestley on this subject, 307; - Anecdote of his inventive genius, 309; - Introduces the use of chlorine in bleaching, 310; - One of the founders of the Pneumatic institution at Clifton, - 310; - His first marriage, 310; - Private life of, 311; - Death of his first wife, 311; - His second marriage, 311; - He retires from business, 311; - Death of his younger son, 311; - Extracts from his letters, 312; - His death, 313; - Character of, by Lord Brougham, 313; - By Sir Walter Scott, 314; - By Lord Jeffrey, 315; - Occupation of his old age, 318; - Invention of machine for copying sculpture, 318; - His last days, 318; - Monuments, 319; - Inscription on the monument in Westminster Abbey from the - pen of Lord Brougham, 319; - His application of tubular condensers, 457; - His expedient for coating boilers with felt, 463. - - Wilkinson, his machine for accurately boring the insides of - cylinders, 149. - - Williams's method of consuming the unburned gases which escape - from the grate, and are carried through the flues, 260. - - Woolf's engine, 176; - Woolf's piston, 243. - - [Illustration: RICHMOND BRIDGE.] - - -LONDON: - -Printed by A. SPOTTISWOODE, -New-Street-Square. - - - - - * * * * * - - - - -Transcriber's endnote: - - Original spelling and grammar has mostly been retained. For - example, the forms "Cyclopoedia", "cyclopaedia", "Encyclopaedia", - "Encyclopoedia", "guage", and "gauge" are all retained. Figures - were moved from within paragraphs to between paragraphs. - Footnotes were re-indexed and moved to the ends of chapters. - - An entry for the INDEX was inserted into the Table of Contents. - - In the Table of Contents, changed "MM. Dulong and Arrago" to "MM - Dulong and Arago". Also "Blinkinsop" to "Blinkensop". Also - "Wasborough's" to "Wasbrough's". - - Figs. 4, 5 and 6 are all in one image. Two tubes in Fig. 4 - were incorrectly labeled T'; one of these has been crossed out - and changed to T. Both tubes in Fig. 6 were incorrectly - labeled G. One of these was crossed out and replaced by G'. - Note also that Figs. 4, 5, 6 are repeated in the text on - different pages; this feature has been retained. - - Page 10: "it s already" to "it is already". - - Page 43: "Thu if heat" changed to "Thus if heat". - - Page 45: "had a diameter of only one square foot" changed to - "had a diameter of only one foot". - - Page 47: "immedate" to "immediate". - - Page 51: "a a level" to "a level". Also, comma removed from "A - gauge, pipe is inserted". - - Page 53: "proportionably" to "proportionally". - - Page 79: A paragraph beginning "He was not fourteen" contains three - double quotation marks; this is presumably an error. Possibly there - should be two double quotation marks and two single quotation - marks. - - Page 80: "S'. Gravesande" is retained, although this probably - refers to a person known as "'s Gravesande". - - Page 103: "gases n general" to "gases in general". - - Page 122: comma removed from "process may, be continued". - - Page 123: "two thin pipes F G of tin" to "two thin pipes F, G - of tin". - - Page 172: "empyrical" to "empirical". - - Page 187, Fig. 32.: The text refers to "end I of the - connecting rod", but this was labeled L on the Figure. This L - has been crossed out and replaced by I. - - Page 285: In "surrounding the boiler with iron-conducting - substances", changed "iron-" to "non-". - - Page 308: "exeitement" to "excitement". - - Page 362: "acomplish" to "accomplish". - - There were several extended quotations, for example beginning on - page 312, in which each line began with a quotation mark, with - ending quotation marks at the end of each paragraph. In this - edition, these passages have been marked by indentation, and all - but the first and last quotation marks from each paragraph were - removed. - - Page 366: Figs. 97-104 appeared originally between pages 385 - and 399, as full-page prints. Numerically, however, they - belong between Figs. 96 and 105--therefore between pages - 366 and 369. Therefore, they have been moved to a location - between two paragraphs on page 367. - - Page 368: "rivetted" to "riveted". - - Page 419: "TREVITHECK'S INVENTION" changed to "TREVETHICK'S - INVENTION", in the chapter heading. However, the references to - Trevethick occur in a previous chapter, around page 324. - - Page 468: Period added to end sentence "[...] piston is at the - bottom of its stroke". - - Page 490: Period added to end sentence "[...] therefore one - eighth of its capacity". - - Pages 494, 497: large data tables were split into two pieces - each. - - Page 505: The logarithm originally given as log x = - "[=1].82340688193", where "[=1]" represents a numeral one with - a horizontal line over it, is herein changed to log x = - "0.82340688193 - 1", as that is the meaning of this convention. - - Page 513 "formulae are hyberbolic" to "formulae are hyperbolic". - - In the Appendix, pp 505-522, mathematical variables such as - "a", "p", "t", etc. were originally italicized. In these text - file versions, italicized variables have been removed from this - section of the book. This rule has two unfortunate exceptions: - E, _E_, E', and _E'_ on pp 512-515 are different variables, and - have been retained. Italics have been removed from tables - throughout the work. - - The tables on page 494 and 497 were divided into two parts, - better to fit the width constraints of this format. 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