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diff --git a/old/55051-0.txt b/old/55051-0.txt deleted file mode 100644 index c1d15cf..0000000 --- a/old/55051-0.txt +++ /dev/null @@ -1,5380 +0,0 @@ -The Project Gutenberg EBook of Rock Blasting, by Geo. G. André - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world 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. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - -Title: Rock Blasting - A Practical Treatise on the Means Employed in Blasting - Rocks for Industrial Purposes - -Author: Geo. G. André - -Release Date: July 5, 2017 [EBook #55051] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK ROCK BLASTING *** - - - - -Produced by Chris Curnow, Harry Lamé and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - Transcriber’s Notes - - Texts printed in italics in the source document have been transcribed - _between underscores_; small capitals have been replaced by ALL - CAPITALS. ^{T} represents a superscript T, ~T~ a T-shape part rather - than the letter T. - - More Transcriber’s Notes will be found at the end of this text. - - - - -ROCK BLASTING. - - - - - ROCK BLASTING. - - A - PRACTICAL TREATISE - ON THE - MEANS EMPLOYED IN BLASTING ROCKS - FOR INDUSTRIAL PURPOSES. - - BY - GEO. G. ANDRÉ, F.G.S., ASSOC. INST. C.E., - MINING CIVIL ENGINEER; MEMBER OF THE SOCIETY OF ENGINEERS. - - [Illustration] - - LONDON: - E. & F. N. SPON, 46, CHARING CROSS. - NEW YORK: - 446, BROOME STREET. - - 1878. - - - - -PREFACE. - - -During the past decade, numerous and great changes have taken place in -the system followed and the methods adopted for blasting rocks in -industrial operations. The introduction of the machine drill led -naturally to these important changes. The system which was suitable to -the operations carried on by hand was inefficient under the requirements -of machine labour, and the methods which had been adopted as the most -appropriate in the former case were found to be more or less unsuitable -in the latter. Moreover, the conditions involved in machine boring are -such as render necessary stronger explosive agents than the common -gunpowder hitherto in use, and a more expeditious and effective means of -firing them than that afforded by the ordinary fuse. These stronger -agents have been found in the nitro-cotton and the nitro-glycerine -compounds, and in the ordinary black powder improved in constitution -and fired by detonation; and this more expeditious and effective means -of firing has been discovered in the convenient application of -electricity. Hence it is that the changes mentioned have been brought -about, and hence, also, has arisen a need for a work like the present, -in which the subjects are treated of in detail under the new aspects due -to the altered conditions. - - GEO. G. ANDRÉ. - - LONDON, 17, KING WILLIAM STREET, STRAND, - - _January 1st, 1878._ - - - - -CONTENTS. - - - CHAPTER I. - THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN ROCK BLASTING. - - PAGE - Section I. _Hand-boring Tools._--Drills. Hammers. Auxiliary - Tools. Sets of Blasting Gear 1 - - Section II. _Machine-boring Tools._--Machine Rock-drills. Borer- - bits. Drill Carriages 23 - - Section III. _Appliances for firing Blasting Charges._--Squibs. - Safety Fuse. Electric Fuses. Cables. Detonators. Electric Firing- - Machines 42 - - - CHAPTER II. - EXPLOSIVE AGENTS USED IN ROCK BLASTING. - - Section I. _Phenomena accompanying an Explosion._--Nature of an - Explosion. Heat liberated by an Explosion. Gases generated by an - Explosion. Force developed by an Explosion 64 - - Section II. _Nature of Explosive Agents._--Mechanical Mixtures. - Chemical Compounds 76 - - Section III. _Relative Strength of the common Explosive Agents._-- - Force developed by Gunpowder. Relative Force developed by - Gunpowder, Gun-cotton, and Nitro-Glycerine 88 - - Section IV. _Means of firing the common Explosive Agents._--Action - of Heat. Detonation 92 - - Section V. _Some Properties of the common Explosive Agents._-- - Gunpowder, Gun-cotton, Dynamite. Firing Temperatures 97 - - Section VI. _Some Varieties of the Nitro-Cellulose and the Nitro- - Glycerine Compounds._--Nitrated Gun-cotton. Tonite, or Cotton- - Powder. Schultze’s Powder. Lithofracteur. Brain’s Powder. - Cellulose-Dynamite 103 - - - CHAPTER III. - THE PRINCIPLES OF ROCK BLASTING. - - Line of least Resistance. Force required to cause Disruption. - Conditions of Disruption. Example of a Heading. Economical - Considerations. Tamping 106 - - - CHAPTER IV. - THE OPERATIONS OF ROCK BLASTING. - - _Hand Boring._--Boring the Shot-holes. Charging the Shot-holes. - Firing the Charges 128 - - _Machine Boring._--Boring the Shot-holes. Charging and Firing. - Removing the dislodged Rock. Division of Labour 142 - - _Examples of Drivings._--The St. Gothard Tunnel. The Hoosac - Tunnel. The Musconetcong Tunnel. Headings at Marihaye, Anzin, and - Ronchamp 157 - - - CHAPTER V. - SUBAQUEOUS BLASTING. - - Preparation of the Charge. Boring under Water. Submarine Rocks. - Obstructions in Water-courses 164 - - - - -ROCK BLASTING. - - - - -CHAPTER I. - -THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS. - - -SECTION I.--HAND BORING. - - -_Drills._--The operations of blasting consist in boring suitable holes -in the rock to be dislodged, in inserting a charge of some explosive -compound into the lower portion of these holes, in filling up, -sometimes, the remaining portion of the holes with suitable material, -and in exploding the charge. The subjects which naturally first present -themselves for consideration are: the nature, form, and construction of -the tools, machines, and other appliances used. Of these tools, the -“drill” or “borer” constitutes the chief. To understand clearly the -action of the rock drill, we must consider the nature of the substance -which has to be perforated. He who has examined the mineral constitution -of rocks will have recognised the impossibility of _cutting_ them, using -that term in its ordinary acceptation, inasmuch as the rock constituents -are frequently harder than the material of the tools employed to -penetrate them. As a rock cannot be cut, the only way of removing -portions of it is to fracture or to disintegrate it by a blow delivered -through the medium of a suitable instrument. Each blow so delivered may -be made to chip off a small fragment, and by this means the rock may be -gradually worn away. To effect this chipping, however, the instrument -used must present only a small surface to the rock, in order to -concentrate the force, and that surface must be bounded by inclined -planes or wedge surfaces, to cause a lateral pressure upon the particles -of rock in contact with them. In other words, the instrument must be -provided with an edge similar to that possessed by an ordinary _cutting_ -tool. - -The conditions under which the instrument is worked are obviously such -that this edge will be rapidly worn down by attrition from the hard rock -material, and by fracture. To withstand these destructive actions, two -qualities are requisite in the material of which the instrument is -composed, namely, hardness and toughness. Thus there are three important -conditions concurring to determine the nature and the form of a cutting -tool to be used in rock boring--1, a necessity for a cutting edge; 2, a -necessity for a frequent renewal of that edge; and 3, a necessity for -the qualities of hardness and toughness in the material of the tool. - -In very hard rock, a few minutes of work suffice to destroy the cutting -edge, and then the tool has to be returned to the smithy to be -re-sharpened. Hence it is manifest that the form of the edge should not -be one that is difficult to produce, since, were it so, much time would -be consumed in the labour of re-sharpening. Experience has shown that -the foregoing conditions are most fully satisfied in the steel rod -terminating in a simple chisel edge, now universally adopted. - -This form of drill is exhibited in Fig. 1, which represents a common -“jumper” borer. It consists of a rod terminating at each end in a chisel -edge, and having a swell, technically described as the “bead,” between -the extremities to give it weight. The bead divides the jumper into two -unequal portions, each of which constitutes a chisel bit, with its shank -or “stock.” The shorter stock is used while the hole is shallow, and the -longer one to continue it to a greater depth. - -[Illustration: FIG. 1.] - -[Illustration: FIG. 2.] - -[Illustration: FIG. 3.] - -With the jumper, the blow is obtained from the direct impact of the -falling tool. The mode of using the instrument is to lift it with both -hands to a height of about a foot, and then to let it drop. In lifting -the jumper, care is taken to turn it partially round, that the edge may -not fall twice in the same place. By this means, the edge is made to act -most favourably in chipping away the rock, and the hole is kept fairly -circular. So long as the holes are required to be bored vertically -downwards, the jumper is a convenient and very efficient tool, and hence -in open quarrying operations, it is very commonly employed. But in -mining, the shot-holes are more often required to be bored in some other -direction, or, as it is termed, “at an angle;” that is, at an angle with -the vertical. Or it may be that a shot-hole is required to be bored -vertically upward. It is obvious that in any one of these directions the -jumper is useless. To meet the requirements of such cases, recourse is -had to the hammer wherewith to deliver the blow, and the drill is -constructed to be used with the hammer. We have a suitable form of tool -for application in this wise when we cut out the bead of the jumper and -leave the ends flat for a striking face, as shown in Figs. 2 and 3. The -form of the two chisels thus obtained is that adopted for the ordinary -rock drill. - -It will be understood from these descriptions that a rock drill consists -of the chisel edge or _bit_, the _stock_, and the _striking face_. -Formerly drills were made of wrought iron, and steeled at each end to -form the bit and the striking face. Now they are commonly made of cast -steel, which is supplied for that purpose in octagonal bars of the -requisite diameter. The advantages offered by steel stocks are numerous. -The superior solidity of texture of that material renders it capable of -transmitting the force of a blow more effectively than iron. Being -stronger than the latter material, a smaller diameter of stock, and, -consequently, a less weight, are sufficient. This circumstance also -tends to increase the effect of the blow by diminishing the mass through -which it is transmitted. On the other hand, a steel stock is more easily -broken than one of iron. - -The cutting edge of a drill demands careful consideration. To enable the -tool to free itself readily in the bore-hole, and also to avoid -introducing unnecessary weight into the stock, the bit is made wider -than the latter; the difference in width may be as much as 1 inch. It is -evident that in hard rock, the liability of the edge to fracture -increases as the difference of width. The edge of the drill may be -straight or slightly curved. The straight edge cuts its way somewhat -more freely than the curved, but it is weaker at the corners than the -latter, a circumstance that renders it less suitable for very hard rock. -It is also slightly more difficult to forge. The width of the bit -varies, according to the size of the hole required, from 1 inch to 2½ -inches. Figs. 4, 5, and 6 show the straight and the curved bits, and the -angles of the cutting edges for use in rock. - -[Illustration: FIG. 4.] - -[Illustration: FIG. 5.] - -[Illustration: FIG. 6.] - -The stock is octagonal in section; it is made in lengths varying from -20 inches to 42 inches. The shorter the stock the more effectively does -it transmit the force of the blow, and therefore it is made as short as -possible. For this reason, several lengths are employed in boring a -shot-hole, the shortest being used at the commencement of the hole, a -longer one to continue the depth, and a still longer one, sometimes, to -complete it. To ensure the longer drills working freely in the hole, the -width of the bit should be very slightly reduced in each length. It has -already been remarked that the diameter of the stock is less than the -width of the bit; this difference may be greater in coal drills than in -rock or “stone” drills; a common difference in the latter is ⅜ of an -inch for the longer. The following proportions may be taken as the -average adopted:-- - - +---------+---------+ - | Width of| Diameter| - | the Bit.| of the | - | | Stock. | - +---------+---------+ - |1 inch | ⅝ inch | - |1⅛ „ | ¾ „ | - |1¼ „ | ⅞ „ | - |1½ „ |1 „ | - |1¾ „ |1⅛ „ | - |2 inches|1⅜ „ | - |2¼ „ |1½ „ | - |2½ „ |1⅝ „ | - +---------+---------+ - -The striking face of the drill should be flat. The diameter of the face -is less than that of the stock in all but the smallest sizes, the -difference being made by drawing in the striking end. The amount of -reduction is greater for the largest diameters; that of the striking -face being rarely more than one-eighth of an inch. - -The making and re-sharpening of rock drills constitute an extremely -important part of the labour of the mine smith. The frequent use of the -drill, and its rapid wear, necessitate a daily amount of work of no -trifling proportions, and the judgment and skill required in proper -tempering render some degree of intelligence in the workman -indispensable; indeed, so much depends upon the smith whose duty it is -to repair the miners’ tools, that no pains should be spared to obtain a -man capable of fulfilling that duty in the most efficient manner -possible. - -When the borer-steel bars are supplied to the smith, he cuts them up, as -required, into the desired lengths. To form the bit, the end of the bar -is heated and flattened out by hammering to a width a little greater -than the diameter of the hole to be bored. The cutting edge is then -hammered up with a light hammer to the requisite angle, and the corners -beaten in to give the exact diameter of the bore-hole intended. As the -drills are made in sets, the longer stocks will have a bit slightly -narrower than the shorter ones, for reasons already given. The edge is -subsequently touched up with a file. In performing these operations, -heavy hammering should be avoided, as well as high heats, and care -should be taken in making the heat that the steel should be well covered -with coal, and far enough removed from the tuyere to be protected from -the “raw” air. Overheated or “burned” steel is liable to fly, and drills -so injured are useless until the burned portion has been cut away. - -[Illustration: FIG. 7.] - -[Illustration: FIG. 8.] - -[Illustration: FIG. 9.] - -Both in making and in re-sharpening drills, great care is required to -form the cutting edge evenly, and of the full form and dimensions. If -the corners get hammered in, as shown in Fig. 7, they are said to be -“nipped,” and the tool will not free itself in cutting. When a -depression of the straight, or the curved, line forming the edge occurs, -as shown in Fig. 8, the bit is said to be “backward,” and when one of -the corners is too far back, as in Fig. 9, it is spoken of as -“odd-cornered.” When either of these defects exist--and they are -unfortunately common--not only does the bit work less effectively on the -rock, but the force of the blow is thrown upon a portion only of the -edge, which, being thereby overstrained, is liable to fracture. - -The hardening and tempering of steel is a matter requiring careful study -and observation. It is a well-known fact that a sudden and great -reduction of temperature causes a notable increase of hardness in the -metal. The reason of this phenomenon is not understood, but it is -certain that it is in some way dependent upon the presence of carbon. -The degree of hardness imparted to steel by this means depends upon the -amount of the reduction of the temperature, and the proportion of carbon -present in the metal, highly carburetted steel being capable of -hardening to a higher degree, under the same conditions, than steel -containing less carbon. Thus, for steel of the same quality, the wider -the range of temperature the higher is the degree of hardness. But here -we encounter another condition, which limits the degree of hardness -practically attainable. - -The change which takes place among the molecules of the metal in -consequence of the change of temperature causes internal strains, and -thereby puts portions in a state of unequal tension. This state renders -the strained parts liable to yield when an additional strain is thrown -upon them while the tool is in use; in other words, the brittleness of -the steel increases with its hardness. Here again the proportion of -carbon present comes into play, and it must be borne in mind that for -equal degrees of hardness the steel which contains the least carbon -will be the most brittle. In hardening borer-steel, which has to combine -as far as possible the qualities of hardness and toughness, this matter -is one deserving careful attention. It is a remarkable fact, and one of -considerable practical value, that when oil is employed as the cooling -medium instead of water, the toughness of steel is enormously increased. - -The tempering of steel, which is a phenomenon of a similar character to -that of hardening, also claims careful consideration. When a bright -surface of steel is subjected to heat, a series of colours is produced, -which follow each other in a regular order as the temperature increases. -This order is as follows: pale yellow, straw yellow, golden yellow, -brown, brown and purple mingled, purple, light blue, full clear blue, -and dark blue. Experience has shown that some one of these colours is -more suitable than the rest for certain kinds of tools and certain -conditions of working. - -The selection of the proper colour constitutes a subject for the -exercise of judgment and skill on the part of the smith. For rock -drills, straw colour is generally the most suitable when the work is in -very hard rock, and light blue when the rock is only of moderate -hardness. - -The processes of hardening and tempering drills are as follows: When the -edge of the bit has been formed in the manner already described, from 3 -to 4 inches of the end is heated to cherry redness, and dipped in cold -water to a depth of about an inch to harden it. While in the water, the -bit should be moved slightly up and down, for, were this neglected, the -hardness would terminate abruptly, and the bit would be very liable to -fracture along the line corresponding with the surface of the water. In -cold weather, the water should be slightly warmed, by immersing a piece -of hot iron in it, before dipping the steel. When a sufficient degree of -hardness has been attained, the remainder of the hot portion is immersed -until the heat is reduced sufficiently for tempering. At this stage it -is withdrawn, and the colours carefully watched for. The heat which is -left in the stock will pass down to the edge of the bit, and as the -temperature increases in that part the colours will appear in regular -succession upon the filed surface of the edge. When the proper hue -appears, the whole drill is plunged into the water and left there till -cold, when the tempering is complete. When the edge is curved or -“bowed,” the colours will reach the corners sooner than the middle of -the bit. This tendency must be checked by dipping the corners in the -water, for otherwise the edge will not be of equal hardness throughout. -As the colour can be best observed in the dark, it is a good plan to -darken that portion of the smithy in which tempering is being carried -on. - -The degree of temper required depends upon the quality of the steel and -the nature of the work to be performed. The larger the proportion of -carbon present in the metal, the lower must be the temper. Also the -state of the blunted edges, whether battered or fractured, will show -what degree of hardness it is desirable to produce. From inattention to -these matters, good steel is not unfrequently condemned as unsuitable. - -To form the striking face, the end of the stock is heated to a dull red, -and drawn out by a hammer to form a conical head. The extremity is then -flattened to form a face from ½ inch to 1 inch in diameter. This head is -then annealed to a degree that will combine considerable toughness with -hardness. The constant blows to which the head is subjected tend to wear -it down very rapidly. There is great difference in the lasting qualities -of steel in this respect; some drills will wear away more quickly at the -striking than at the bit end. - -A smith will, with the assistance of a striker, sharpen and temper about -thirty single-hand drills of medium size in an hour, or twenty -double-hand drills of medium size in the same time. Of course, much will -depend on the degree of bluntness in the cutting edge; but assuming the -drills to be sent up only moderately blunted, this may be taken as a -fair average of the work of two men. - -It will be evident from the foregoing remarks, that to enable a drill to -stand properly it must be made of good material, be skilfully tempered -in the smithy, and provided with a cutting edge having an angle and a -shape suited to the character of the rock in which it is used. To these -conditions, may be added another, namely, proper handling; for if the -drill be carelessly turned in the hole so as to bring all the work upon -a portion only of the cutting edge, or unskilfully struck by the sledge, -fracture or blunting will speedily result. Improper handling often -destroys the edge in the first five minutes of using. - -Drills, as before remarked, are used in sets of different lengths. The -sets may be intended for use by one man or by two. In the former case, -the sets are described as “single-hand” sets, and they contain a hammer -for striking the drills; in the latter case, the sets are spoken of as -“double-handed,” and they contain a sledge instead of a hammer for -striking. It may appear at first sight that there is a waste of power in -employing two men, or, as it is termed, the double set, for that two men -cannot bore twice as fast as one. This rate of speed can, however, be -obtained, and is due less to the greater effectiveness of the stroke -than to the fact that two men can, by repeatedly changing places with -each other, keep up almost without intermission a succession of blows -for an indefinite length of time; whereas, with the single set, the man -is continually obliged to cease for rest. - - -_Hammers._--To deliver the blow upon a rock drill, hammers and sledges -are used. The distinction between a hammer and a sledge is founded on -dimensions only: the hammer being intended for use in _one_ hand, is -made comparatively light and is furnished with a short handle, while the -sledge, being intended for use in _both_ hands, is furnished with a much -longer handle and is made heavier. The striking face of the blasting -sledge should be flat, to enable the striker to deliver a direct blow -with certainty upon the head of the drill; and to facilitate the -directing of the blow, as well as to increase its effect, the mass of -metal composing the head should be concentrated within a short length. -To cause the sledge to fly off from the head of the drill in the case of -a false blow being struck, and thereby to prevent it from striking the -hand of the man who holds the drill, the edges of the striking face -should be chamfered or bevelled down till the diameter is reduced by -nearly one-half. This requirement is, however, but seldom provided for. - -[Illustration: FIG. 10.] - -[Illustration: FIG. 11.] - -[Illustration: FIG. 12.] - -[Illustration: FIG. 13.] - -The head of a sledge is of iron; it consists of a pierced central -portion called the “eye,” and two shanks or “stumps,” the steeled ends -of which form the striking faces or “panes.” The form of the head varies -in different localities, but whatever the variations may be, the form -may be classed under one of four types or “patterns.” A very common -form is that shown in Fig. 10 and known as the “bully” pattern. By -varying the width, as shown in Fig. 11, we obtain the “broad bully,” the -former being called for the sake of distinction the “narrow” bully. -Another common form is the “pointing” pattern, represented in Fig. 12. -The form shown in Fig. 13 is designated as the “bloat” pattern; and that -given in Fig. 14 the “plug” pattern. Each of these forms possesses -peculiar merits which renders it more suitable for certain uses than -the others. The same forms are used for hammers. The eye is generally -made oval in shape, but sometimes, especially with the bloat pattern, it -is made circular, as shown in Fig. 13. The weight of a sledge head may -vary from 5 lb. to 10 lb., but a common and convenient weight is 7 lb. -The length of the helve varies from 20 inches to 30 inches; a common -length for blasting sledges is 24 inches. The average weight of hammer -heads is about 3 lb., and the average length of the helve 10 inches. - -[Illustration: FIG. 14.] - -[Illustration: FIG. 15.] - -Fig. 15 represents a blasting sledge used in South Wales. The stumps are -octagonal in section, and spring from a square block in the centre. The -panes or striking faces, however, are circular and flat. The length of -the head is 8¾ inches, and that of the helve 27 inches, and the weight -of the tool complete 7 lb. - -[Illustration: FIG. 16.] - -Fig. 16 represents a blasting sledge used in North Wales. The central -block is an irregular octagon in section, formed by slightly chamfering -the angles of a square section, and the stumps are chamfered down to -form a regular octagon at the panes, which are flat. The length of the -head is 7¾ inches, and that of the helve 22 inches, and the weight of -the tool complete 6 lb. 7 oz. - -[Illustration: FIG. 17.] - -The sledges used in the north of England have shorter heads, and are -lighter than the foregoing. Fig. 17 represents one of these blasting -sledges. The head is nearly square in section at the centre, and the -panes are flat. The length of the head is 5 inches, and that of the -helve 24½ inches, and the weight of the sledge complete 4 lb. 14 oz. - - -_Auxiliary Tools._--Besides the drill and the hammer, other tools are -needed in preparing the hole for the blasting charge. If the bore-hole -is inclined downwards, the débris or “bore-meal” made by the drill -remains on the bottom of the hole, where it is converted into mud or -“sludge” by the water there present. This sludge has to be removed as -the work progresses, to keep the rock exposed to the action of the -drill. The removal of the sludge is effected by a simple tool called a -“scraper.” It consists of a rod of iron from ¼ inch to ½ inch in -diameter, and of sufficient length to reach the bottom of the bore-hole. -One end of the rod is flattened out on the anvil and made circular in -form, and then turned up at right angles to the stem. The disc thus -formed must be less in diameter than the bore-hole, to allow it to pass -readily down. When inserted in the hole, the scraper is turned round -while it is being pressed to the bottom; on withdrawing the instrument, -the sludge is brought up upon the disc. The operation, two or three -times repeated, is sufficient to clear the bore-hole. The other end of -the scraper is sometimes made to terminate in a ring for convenience in -handling, as shown in Fig. 18. Instead of the ring, however, at one end, -a disc may be made at each end, as shown in Fig. 19, the discs in this -case being of different diameter, to render the scraper suitable for -different size bore-holes. Sometimes the scraper is made to terminate -in a spiral hook or “drag-twist,” as represented in Fig. 20. The use of -the drag is to thoroughly cleanse the hole before inserting the charge. -A wisp of hay is pushed down the hole, and the drag end of the scraper -introduced after it, and turned round till it has become firmly -entangled. The withdrawal of the hay by the drag wipes the bore-hole -clean. Instead of the twist drag, the “loop” drag is frequently -employed. This consists of a loop or eye, through which a piece of rag -or tow is passed. The rag or tow is used for the same purpose as the -hay, namely, to thoroughly cleanse and dry the bore-hole previous to the -introduction of the charge. Very frequently the “swab-stick” is used -instead of the scraper to clear out the bore-hole. This is simply a deal -rod bruised at one end by blows with a hammer until the fibres separate -to form a kind of stumpy brush or “swab.” When this is pushed down the -hole, the sludge passes up around and between the fibres, which are then -spread out by being pressed against the bottom of the hole. On -withdrawing the swab, the sludge is brought out with it. - -[Illustration: FIG. 18.] - -[Illustration: FIG. 19.] - -[Illustration: FIG. 20.] - -When the charge has been placed in the bore-hole, and the fuse laid to -it, the hole needs to be tamped, that is, the portion above the charge -has to be filled up with some suitable substance. For this purpose, a -“rammer,” “stemmer,” or “tamping iron,” as the instrument is variously -called, is required. This instrument is illustrated in Fig. 21. It -consists of a metal bar, the tamping end of which is grooved to receive -the fuse lying against the side of the bore-hole. The other end is flat, -to afford a pressing surface for the hand, or a striking face for the -hammer when the latter is needed. To prevent the danger of accidental -ignition from sparks caused by the friction of the metal against -silicious substances, the employment of iron stemmers has been -prohibited by law. They are usually made of copper or phosphor-bronze, -the latter substance being more resisting than the former. - -[Illustration: FIG. 21.] - -[Illustration: FIG. 22.] - -[Illustration: FIG. 23.] - -Sometimes in wet ground it becomes necessary to shut back the water from -the bore-hole before introducing the charge of gunpowder. This happens -very frequently in shaft sinking. The method employed in such cases is -to force clay into the interstices through which the water enters. The -instrument used for this purpose is the “claying-iron” or “bull,” -represented in Fig. 22. It consists of a round bar of iron, called the -stock or shaft, a little smaller in diameter than the bore-hole, and a -thicker portion, called the head or poll, terminating in a striking -face. The lower end of the shaft is pointed, to enable it to penetrate -the clay, and the head is pierced by a hole about an inch in diameter to -receive a lever. Clay in a plastic state having been put into the -bore-hole, the bull is inserted and driven down by blows with the -sledge. As the shaft forces its way down, the clay is driven into the -joints and crevices of the rock on all sides. To withdraw the bull, a -bar of iron is placed in the eye and used as a lever to turn it round to -loosen it; the rod is then taken by both hands and the bull lifted out. -To allow the bull to be withdrawn more readily, the shaft should be made -with a slight taper and kept perfectly smooth. As the bull is subjected -to a good deal of heavy hammering on the head, the latter part should be -made stout. This tool, which should be considered as an extra instrument -rather than as an essential part of a blasting set, is a very -serviceable one, and should always be at hand in wet ground when loose -gunpowder is employed. - -Another instrument of this auxiliary character is the beche, Fig. 23, -used for extracting a broken drill. It consists of an iron rod of nearly -the diameter of the bore-hole, and hollow at the lower end. The form of -the aperture is slightly conical, so that the lower end may easily pass -over the broken stock of the drill, and, on being pressed down with -some force, may grasp the stock in the higher portion of the aperture -with sufficient firmness to allow of the two being raised together. When -only a portion of the bit remains in the hole, it may often be extracted -by means of the drag-twist end of the scraper, or the swab-stick may be -driven down upon the broken portion, and latter withdrawn with the swab. - - -_Sets of Blasting Gear._--On Plates I., II., and III., will be found -three sets of blasting gear; a set of coal-blasting gear; a set of -single-hand stone-blasting gear; and a set of double-hand stone-blasting -gear. In the first set, the drill, shown in Fig. 1, is 22 inches in -length; the cutting edge is straight and 1½ inch wide, and the weight is -2½ lb. The other drill, Fig. 2, is 42 inches in length; it has a -straight cutting edge 1⁷/₁₆ inch wide, and weighs 4 lb. 10 oz. The -hammer used in this set and shown in Fig. 3 weighs 2 lb. 14 oz.; the -length of the head is 4½ inches, and that of the handle 7¾ inches. In -the second or single-hand stone set, the shorter drill, Fig. 6, Plate -II., is 22 inches in length; the cutting edge is strongly curved, and is -1½ inch in width, and the weight is 3 lb. 10 oz. The longer drill, Fig. -7, is 36 inches in length; the width of the cutting edge, which is -curved as in the shorter drill, is 1⁷/₁₆ inch, and the weight is 6 lb. 5 -oz. The hammer used with this set, and represented in Fig. 8, weighs 3 -lb. 6 oz.; the length of the head is 5 inches, and that of the handle -10 inches. In the third or double-hand stone set, Plate III., the first -or shortest drill, Fig. 12, is 18 inches in length, 1¾ inch wide on the -cutting edge, and weighs 4¼ lb. The second drill, Fig. 13, is 27 inches -in length, 1¹¹/₁₆ wide on the cutting edge, and weighs 6 lb. The third -or longest drill, Fig. 14, is 40 inches in length, 1⅝ inch wide on the -cutting edge, and weighs 9¼ lb. The cutting edges of all these drills -are strongly curved as in the preceding set. The sledge used with this -set, and represented in Fig. 15, weighs about 5 lb. - - -SECTION II.--MACHINE BORING. - - -_Machine Rock-Drills._--The most remarkable advance, which in recent, or -perhaps in any, times has been made in the practice of mining consists -in the substitution of machine for hand labour in rock boring. The -importance of this change is obvious, and very great. Not only is the -miner relieved thereby of the labour of boring, but the speed with which -the shot-holes may be bored is increased a hundredfold. This gain of -speed offers many practical advantages. The ability to sink a shaft or -to drive a heading rapidly may ensure the success of an undertaking, and -save indirectly the expenditure of large sums of money; and, in all -cases, it allows the time spent in preparatory work to be materially -shortened. Indeed, it would be difficult to over-estimate the magnitude -of the advantage accruing from the increased rate of progress due to -the substitution of machine power for hand labour, and in the future we -may expect to see its application greatly extended. In making this -substitution, numerous difficulties have had to be overcome, and in -encountering these many failures have had to be recorded. But it must -now be conceded by the most prejudiced that rock-boring machines have -successfully passed through what may be described as the tentative stage -of their existence, and have taken a foremost place among the mechanical -appliances which experience has shown to be capable of effectually -performing the work required of them. In the author’s work on ‘Mining -Engineering,’ the requirements of a rock drill will be found fully -discussed, and the principles and the construction of the most important -machines now in use carefully explained and described. In the present -work, only one example can be given. - -Machine drills penetrate rock in the same way as the ordinary hand -drills already described, namely, by means of a percussive action. The -cutting tool is in most cases attached directly to the piston rod, with -which it consequently reciprocates. Thus the piston with its rod is made -to constitute a portion of the cutting tool, and the blow is then given -by the direct action of the steam, or the compressed air, upon the tool. -As no work is done upon the rock by the back stroke of the piston, the -area of the forward side is reduced to the dimensions necessary only to -lift the piston, and to overcome the resistance due to the friction of -the tool in the bore-hole. The piston is made to admit steam or air into -the cylinder, and to cut off the supply, and to open the exhaust, as -required, by means of tappet valves, or other suitable devices; and -provision is made to allow, within certain limits, a variation in the -length of the stroke. During a portion of the stroke, means are brought -into action to cause the piston to rotate to some extent, for the -purposes that have been already explained. To keep the cutting edge of -the tool up to its work, the whole machine is moved forward as the rock -is cut away. This forward or “feed” motion is usually given by hand, but -in some cases it is communicated automatically. The machine is supported -upon a stand or framing which varies in form according to the situation -in which it is to be used. This support is in all cases constructed to -allow of the feed motion taking place, and also of the cutting tool -being directed at any angle. The support for a rock drill constitutes an -indispensable and a very important adjunct to the machine, for upon the -suitability of its form, material, and construction, the efficiency of -the machine will largely depend. - -The foregoing is a general description of the construction and mode of -action of percussive rock-drills. The numerous varieties now in use -differ from each other rather in the details of their construction than -in the principles of their action, and the importance of the difference -is, of course, dependent upon that of the details. It is but just to -remark here that the first really practical solution of the -rock-drilling problem is due to M. Sommeiller, whose machine was -employed in excavating the Mont Cenis tunnel. - - -_The Darlington Drill._--The machine which, in England, has stood the -test of experience most satisfactorily, and which, consequently, is -surely working itself into general favour in this country, and also in -some of the important mining districts of the Continent, is the -invention of John Darlington, and is known as the “Darlington drill.” -This drill is remarkable as the attainment of the highest degree of -simplicity of parts possible in a machine. The valve gear of a machine -drill is especially liable to derangement. It must necessarily consist -of several parts, and these parts must as necessarily be of a somewhat -fragile character. Besides this, when actuated by the piston through the -intervention of tappets, the violence of the blow delivered at each -stroke is such as to rapidly destroy the parts. In some machines, the -force of these blows and their destructive tendency have been reduced to -a minimum; but when every means of remedying the evil has been employed, -there remains a large amount of inevitable wear and tear, and a -liability to failure from fracture or displacement exists in a greater -or less degree. Moreover, as these effects are greatly intensified by -increasing the velocity of the piston, it becomes at least undesirable -to use a high piston speed. To remedy these defects, which are inherent -in the system, Darlington proposed to remove altogether the necessity -for a valve gear by radically changing the mode of admitting the motor -fluid to the cylinder. This proposal he has realized in the machine -which is illustrated on Plate IV. - -The Darlington rock-drill consists essentially of only two parts: the -cylinder A, Figs. 20 and 21, with its cover; and the piston B, with its -rod. The cover, when bolted on, forms a part of the cylinder; the piston -rod is cast solid with the piston, and is made sufficiently large at its -outer end to receive the tool. These two parts constitute an engine, and -with less than one fixed and one moving part it is obviously impossible -to develop power in a machine by the action of an elastic fluid. The -piston itself is made to do the work of a valve in the following manner: -The annular space affording the area for pressure on the fore part of -the piston gives a much smaller extent of surface than that afforded by -the diameter of the cylinder, as shown in the drawing; and it is obvious -that by increasing or diminishing the diameter of the piston rod, the -area for pressure on the one side of the piston may be made to bear any -desired proportion to that on the other side. The inlet aperture, or -port C, being in constant communication with the interior of the -cylinder, the pressure of the fluid is always acting upon the front of -the piston, consequently when there is no pressure upon the other side, -the piston will be forced backward in the cylinder. During this backward -motion, the piston first covers the exhaust port D, and then uncovers -the equilibrium port E, by means of which communication is established -between the front and back ends of the cylinder, and, consequently, the -fluid is made to act upon both sides of the piston. The area of the back -face of the piston being greater than that of the front face by the -extent occupied by the piston rod, the pressure upon the former first -acts to arrest the backward motion of the piston, which, by its -considerable weight and high velocity, has acquired a large momentum, -and then to produce a forward motion, the propelling force being -dependent for its amount upon the difference of area on the two sides of -the piston. As the piston passes down, it cuts off the steam from the -back part of the cylinder and opens the exhaust. The length or thickness -of the piston is such that the exhaust port D is never open to its front -side, but, in the forward stroke, it is opened almost immediately after -the equilibrium port is closed, and nearly at the time of striking the -blow. It will be observed that the quantity of fluid expended is only -that which passes over to the back face of the piston, since that which -is used to effect the return stroke is not discharged. - -The means employed to give a rotary motion to the tool are deserving of -special attention, as being simple in design, effective in action, and -well situate within the cylinder. These means consist of a spiral or -rifled bar H, having three grooves, and being fitted at its head with a -ratchet wheel G, recessed into the cover of the cylinder. Two detents J, -J, Fig. 22, also recessed into the cover, are made to fall into the -teeth of the ratchet wheel by spiral springs. These springs may, in case -of breakage, be immediately renewed without removing the cover. It will -be observed that this arrangement of the wheel and the detents allow the -spiral bar H to turn freely in one direction, while it prevents it from -turning in the contrary direction. The spiral bar drops into a long -recess in the piston, which is fitted with a steel nut made to -accurately fit the grooves of the spiral. Hence the piston, during its -instroke, is forced to turn upon the bar; but, during its outstroke, it -turns the bar, the latter being free to move in the direction in which -the straight outstroke of the piston tends to rotate it. Thus the -piston, and with it the tool, assumes a new position after each stroke. - -The mode of fixing the cutting tool to the piston rod is a matter -deserving some attention. As the tool has to be changed more than once -during the progress of a bore-hole, it is important that the change -should be accomplished in as short a time as possible; and as the -vibration of the machine and the strain upon the tool are necessarily -great, it is equally important that the tool be firmly held. It is also -desirable that the mode of fixing the tool shall not require a shoulder -upon the latter, a slot in it, or any peculiarity of form difficult to -be made in the smithy. The Darlington machine fulfils the requirements -of expedition in fixing, firmness of retention, and simplicity of form -most satisfactorily. The means and the method are the following: The -outer end of the rod or holder is first flattened to afford a seat for -the nut, as shown in Figs. 21 and 25. The slot is then cut and fitted -tightly with a piece of steel K forged of the required shape for the -clamp, and the holder is afterwards bored to receive the tool while the -clamp is in place. This clamp K is then taken out, its fittings eased a -little, and its end screwed and fitted with a nut. When returned to its -place in the holder, the clamp, in consequence of the easing, can be -easily drawn tight against the tool, by which means it is firmly held in -position. The shank of the tool is turned to fit the hole easily, and -the end of it is made hemispherical to fit the bottom of the hole, upon -which the force of the reaction of the blow is received. - -It would seem impossible to attain a higher degree of simplicity of -form, or to construct a machine with fewer parts. The absence of a valve -or striking gear of any kind ensures the utmost attainable degree of -durability, and allows a high piston speed to be adopted without risk or -injury. As the piston controls its own motion, there is no liability to -strike against the cylinder cover. The stroke may be varied in length -from half an inch to four inches, and as the machine will work -effectively with a pressure of 10 lb. to the inch, holes may be started -with the greatest ease. With a pressure of 40 lb., the machine makes -1000 blows a minute, a speed that may be attained without causing undue -strains or vibration. This alone constitutes a very great advantage. It -must indeed be conceded that an unprejudiced consideration of the merits -of this drill shows it to be admirably adapted to the work required of -it. - - -_Borer-Bits._--The form and the dimensions of the cutting tools, -variously described as “drills,” “borers,” and “bits,” used with machine -rock-perforators are matters of great practical importance. The -dimensions are determined mainly by two conditions, namely, the -necessity for sufficient strength in the shank of the tool, and the -necessity for sufficient space between the shank and the sides of the -hole to allow the débris to escape. Experience has shown that the latter -condition is best fulfilled when the distance between the sides of the -hole and the shank of the tool is from ³/₁₆ inch to ¼ inch, regard being -had to the former condition. - -The form of the cutting edge is determined by several conditions, some -of which have been already discussed in relation to hand drills. The -form first adopted was naturally that possessed by the hand drill, -namely, the chisel edge. To increase the useful effect of the blow, the -cutting edge was subsequently doubled, the bit being formed of two -chisel edges crossing each other at right angles. This bit, which from -its form was called the “cross” bit, was found to penetrate the rock -more rapidly than the straight or chisel bit. The gain in speed was very -marked at the commencement of the hole; but it diminished gradually as -the hole progressed in depth, owing to the difficulty with which the -débris escaped. To remedy this defect, the cutting edges were next made -to cross each other obliquely, so as to form the letter X. In this way, -the two chisel edges were retained, while the breadth of the bit was -considerably reduced. This form, described as the X bit, cleared the -hole much more effectively than the cross, but not in a manner that was -altogether satisfactory. Another modification of the form was, -therefore, made, and this time that of the Z was adopted, the upper and -the lower portions of which were arcs of circles struck from the centre -of the bit in the direction contrary to that of the rotation. - -This form of tool, which is known as the Z bit, readily cleared itself -of the débris. But besides this advantage, it was found to possess -others of an important character. With the chisel-edge forms, the -corners of the bit were rapidly worn off by friction against the sides -of the hole. With the Z form, this wearing no longer occurred, by -reason of the large surface exposed to friction. Another advantage of -the Z form of bit lies in its tendency to bore the hole truly circular. -Generally then, it may be stated that this form satisfies most fully the -determining conditions. The form of bit, however, that is most suitable -in a given case will, in some degree, be determined by particular -circumstances. Of these, the nature and the character of the rock will -operate most strongly to influence the choice. Thus the cross bit will -generally be found the most suitable in fissured rock, while the single -chisel edge may be used with advantage in rock of a very solid and hard -character. Indeed, on the judicious selection of the most suitable form -of cutting edge, the success of machine boring largely depends. The -chisel bit, the cross bit, the X bit, and the Z bit, are shown in Figs. -24 to 27. - -[Illustration: FIG. 24.] - -[Illustration: FIG. 25.] - -[Illustration: FIG. 26.] - -[Illustration: FIG. 27.] - -The sharpening of bits of a form other than that of the chisel is done -by means of “swages.” The tempering is effected in the way already -described in reference to hand drills. As in the latter case, the -degree of temper must be suited to the hardness of the rocks to be -penetrated. Generally the straw colour will be found to be the best -degree. It is a remarkable fact that the wear of the cutting edge of a -machine drill is, for a given length of boring, five or six times less -than that of a hand drill. Steel of the best quality should always be -used. - -As in the case of hand boring, each successive length of drill must -diminish slightly in the width of its cutting edge; a diminution of -about ¹/₃₂ inch may be considered sufficient. Care should, however, be -taken to ensure the proper dimensions being given to the edge, and it -will be found advantageous to have at hand an accurate gauge through -which the tool may be passed previously to its being fixed to the -machine. It is important that the tool be truly “centred,” that is, the -centres of the edge of the bit, of the shank, and of the piston rod, -should be perfectly coincident. - - -_Rock-Drill Supports._--A machine rock-drill may satisfy every -requirement, and yet, by reason of the defective character of the -support to which it is attached, it may be unsuitable to the work -required of it. Hence it becomes desirable to carefully study the design -and construction of a drill support, and to consider the requirements -which it is needful to fulfil. Assuming the necessity for a high degree -of strength and rigidity in the support, a primary condition is that it -shall allow the machine to be readily adjusted to any angle, so that the -holes may be bored in the direction and with the inclination required. -When this requirement is not fulfilled, the machine is placed, in this -respect, at a great disadvantage with hand labour. If a machine drill -were not capable of boring in any position and in any direction, hand -labour would have to be employed in conjunction with it, and such -incompleteness in the work of a machine would constitute a serious -objection to its adoption. - -Besides allowing of the desired adjustment of the machine, the support -must be itself adjustable to uneven ground. The bottom of a shaft which -is being sunk, or the sides, roof, and floor of a heading which is being -driven, present great irregularities of surface, and, as the support -must of necessity in most cases be fixed to these, it is obvious that -its design and construction must be such as will allow of its ready -adjustment to these irregularities. The means by which the adjustment is -effected should be few and simple, for simplicity of parts is important -in the support as well as in the machine, and for the same reasons. A -large proportion of the time during which a machine drill is in use is -occupied in shifting it from one position or one situation to another; -this time reduces, in a proportionate degree, the superiority of machine -over hand labour, in respect of rapidity of execution, and it is -evidently desirable that it should be shortened as far as possible. -Hence the necessity for the employment of means of adjustment which -shall be few in number, rapid in action, and of easy management. - -For reasons similar to the foregoing, the drill support must be of small -dimensions, and sufficiently light to allow of its being easily -portable. The limited space in which rock drills are used renders this -condition, as in the case of the machine itself, a very important one. -It must be borne in mind that, after every blast, the dislodged rock has -to be removed, and rapidity of execution requires that the operations of -removal should be carried on without hindrance. A drill support that -occupies a large proportion of the free space in a shaft or a heading is -thus a cause of inconvenience and a source of serious delay. Moreover, -as it has to be continually removed from one situation to another, it -should be of sufficiently light weight to allow of its being lifted or -run along without difficulty. In underground workings, manual power is -generally the only power available, and therefore it is desirable that -both the machine and its support should be of such weight that each may -be lifted by one man. Of course, when any endeavour is made to reduce -the weight of the support, the necessity for great strength and rigidity -must be kept in view. - -In spacious headings, such as are driven in railway tunnel work, -supports of a special kind may be used. In these situations, the -conditions of work are different from those which exist in mines. The -space is less limited, the heading is commenced at surface, and the -floor is laid with a tramway and sidings. In such a case, the support -may consist of a more massive structure mounted upon wheels to run upon -the rails. This support will carry several machines, and to remove it -out of the way when occasion requires, it will be run back on to a -siding; but for ordinary mining purposes, such a support is suitable. - - -_The Stretcher Bar._--The simplest kind of support is the “stretcher -bar.” This consists essentially of a bar so constructed that it may be -lengthened or shortened at pleasure, by means of a screw. It is fixed in -position by screwing the ends into firm contact with the sides, or with -the roof and the floor, of a heading. The machine is fixed to this bar -by means of a clamp, which, when loosened, slides along the bar, and -allows the drill to be placed in the required position, and to be -directed at the required angle. The bar illustrated in Fig. 26, Plate -V., is that which is used with the Darlington drill; in it, lightness -and rigidity are combined in the highest possible degree by the adoption -of the hollow section. The mode of setting the bar in a heading is shown -in the drawing; the end claws are set against pieces of wood on the -floor and the roof, and are tightened by turning the screw with a common -bar. - -The simple stretcher bar is frequently used in narrow drivings and in -shafts of small diameter. But a more satisfactory support in drivings is -afforded by a bar suitably mounted upon a carriage designed to run upon -rails. The carriage consists simply of a trolly, to the fore part of -which the bar is fixed usually by some kind of hinge-joint. It is -obvious that the details of the construction of this support may be -varied greatly, and numerous designs have been introduced and adopted. -In Figs. 27 and 28, Plate VI., is shown a support of this character -designed by J. Darlington. A single vertical bar is carried on the fore -part of the trolly, and fixed, by the usual means, against the centre of -the roof. This vertical bar carries an arm, which is capable of turning -upon it, as upon a centre, and of sliding up and down it. This arm -carries the drill. The central bar having been fixed in position, the -arm is slid up to the highest position required, and fixed against the -side of the heading. A row of holes are then bored from this arm. When -these are completed, the arm is lowered the requisite distance, and -another row of holes are bored. This is continued until all the holes -are bored over one-half the face. The arm is then swung round, and fixed -against the other side of the heading, and the holes are bored over that -half the face in like manner. In this way, one-half the heading is kept -clear to allow the operations of removing the dislodged rock to be -carried on at the same time. If desired, two arms may be used. This -arrangement gives undoubtedly great facilities for working the drill, -and leaves the heading comparatively unencumbered. - -In shaft sinking, the same support, slightly modified, is used without -the trolly. The arrangement adopted in this case is shown in Fig. 29, -Plate VII. The central bar is held firmly in its position by a cross -stretcher bar set against the sides of the shaft. The arms are made to -revolve upon this bar to allow the holes to be bored in the positions -required. When all the holes have been bored, the support, with the -machines, is hauled up, by means of a chain attached to the central bar, -out of the way of the blast. With this support, the time of fixing, -raising, and lowering is reduced to a minimum; while the facility with -which the machines may be slid along and fixed to the arm, and the -positions of the latter changed, allows the boring to be carried on -rapidly. - -For open work, as in quarrying, where the stretcher bar cannot be used, -the tripod stand is adopted. - - -_The Dubois-François Carriage._--The support commonly used in France and -in Belgium consists of a kind of carriage carrying bars upon which the -drills are set. This carriage is used in drivings of all kinds; but it -is particularly suitable for tunnelling. It has been adopted, with but -slight modification, in the St. Gothard tunnel, and in several other -important works of the like character. - -A modification of the carriage is shown in Figs. 30 and 31. Being -designed for ordinary mining operations, it carries but two machines; -but it will be readily perceived that, by increasing the number of -vertical screws, the same support may be made to carry a larger number. -It consists essentially of a vertical frame of flat bar iron _a b c d_, -8 feet in length, and 4 feet 9 inches in height above the rails, the -hinder portion of which rests upon a cast-iron plate _e f g h_, carried -upon two wheels; on this are fixed the two uprights _l_, _l′_, which, -being bound to the upper part by a transverse bar _m m′_, form a framing -to serve as a support to the two vertical screws _p′_, _q′_. The front -framing is formed of two longitudinals _b c_ and _b′ c′_ and the -uprights _a_, _a′_, and the vertical screws _p_, _q_, which are -connected to the upper part by the single piece _a d_. This framing is -supported below upon a small trolly with four wheels, connected to the -two longitudinals of the framing by a pivot bolt _n_ of ~T~ form, the -bar of the ~T~ being inserted into the elongated openings _o_ cut -through the middle of the curved portion of the longitudinals. The -cast-iron plate behind, the use of which is only to give stability to -the carriage, carries above it, by means of the two curved pieces _h_, -_h′_, a wrought-iron plate V, upon which the small tools needed for -repairs are kept. Two screws, _s_, _s′_, carried by lugs cast upon the -back of the plate, serve, by turning them down upon the rails, to fix -the carriage, the latter being slightly lifted by the screws. - -Each machine is supported at two points. Behind, the point of support is -given by a cast-iron bracket _t_, having a projecting eye which enters -between the two cheeks formed at the back end of the machine by the -continuations of the two longitudinals of the framing. A pin bolt, -carried by the machine, allows the latter to be fixed to the bracket, -while leaving sufficient freedom of motion to allow of its being -directed at the required angle. This bracket, shown in plan in Fig. 33, -is supported by a kind of nut, Fig. 32, having two handles whereby it -may be easily turned. By raising or lowering this, the hinder support of -the drill may be brought to the requisite height. To prevent it turning -upon the screw, a pin is passed through the hole _o_, which pin forms a -stop for the handles aforementioned. The rotation of the bracket itself -is rendered impossible by the form of the vertical screw upon which it -is set, as shown in Fig. 33. In front, the support is a fork, the shank -of which slides along in the piece U, Figs. 30 and 31. This support, -which is not screwed on the inside, rests upon a nut of the same form as -that already described, and the same means are employed to prevent -rotation as in the case of the hinder supports. - - -SECTION III.--APPLIANCES FOR FIRING BLASTING CHARGES. - -In the foregoing sections, the machines and tools used in rock boring -have been treated of. It now remains to describe those which are -employed in firing the charges after they have been placed in the -bore-holes. In this direction, too, great progress has been made in -recent times. With the introduction of new explosive agents, arose the -necessity for improved means of firing them. Attention being thus -directed to the subject, its requirements were investigated and its -conditions observed, the outcome being some important modifications of -the old appliances and the introduction of others altogether new. Some -of the improvements effected are scarcely less remarkable than the -substitution of machine for hand boring. - -[Illustration: FIG. 28.] - -[Illustration: FIG. 29.] - -The means by which the charge of explosive matter placed in the -bore-hole is fired constitute a very important part of the set of -appliances used in blasting. The conditions which any such means must -fulfil are: (1) that it shall fire the charge with certainty; (2) that -it shall allow the person whose duty it is to explode the charge to be -at a safe distance away when the explosion takes place; (3) that it -shall be practically suitable, and applicable to all situations; and (4) -that it shall be obtainable at a low cost. To fulfil the second and most -essential of these conditions, the means must be either slow in -operation, or capable of being acted upon at a distance. The only known -means possessing the latter quality is electricity. The application of -electricity to this purpose is of recent date, and in consequence of the -great advantages which it offers, its use is rapidly extending. The -other means in common use are those which are slow in operation, and -which allow thereby sufficient time to elapse between their ignition and -the explosion of the charge for a person to retire to a safe distance. -These means consist generally of a train of gunpowder so placed that the -ignition of the particles must necessarily be gradual and slow. The old, -and in some parts still employed, mode of constructing this train was as -follows: An iron rod of small diameter and terminating in a point, -called a “pricker,” was inserted into the charge and left in the -bore-hole while the tamping was being rammed down. When this operation -was completed, the pricker was withdrawn, leaving a hole through the -tamping down to the charge. Into this hole, a straw, rush, quill, or -some other like hollow substance filled with gunpowder, was inserted. A -piece of slow-match was then attached to the upper end of this train, -and lighted. - -The combustion of the powder confined in the straw fired the charge, the -time allowed by the slow burning of the match being sufficient to -enable the man who ignited it to retire to a place of safety. This -method of forming the train does not, however, satisfy all the -conditions mentioned above. It is not readily applicable to all -situations. Moreover, the use of the iron pricker may be a source of -danger; the friction of this instrument against silicious substances in -the sides of the bore-hole or in the tamping has in some instances -occasioned accidental explosions. This danger is, however, very greatly -lessened by the employment of copper or phosphor-bronze instead of iron -for the prickers. But the method is defective in some other respects. -With many kinds of tamping, there is a difficulty in keeping the hole -open after the pricker is withdrawn till the straw can be inserted. When -the holes are inclined upwards, besides this difficulty, another is -occasioned by the liability of the powder constituting the train to run -out on being ignited. And in wet situations, special provision has to be -made to protect the trains. Moreover, the manufacture of these trains by -the workmen is always a source of danger. Many of these defects in the -system may, however, be removed by the employment of properly -constructed trains. One of these trains or “squibs” is shown full size -in Fig. 28. - - -_Safety Fuse._--Many of the defects pertaining to the system were -removed by the introduction of the fuse invented by W. Bickford, and -known as “safety fuse.” The merits of this fuse, which is shown full -size in Fig. 29, are such as to render it one of the most perfect of the -slow-action means that have yet been devised. The train of gunpowder is -retained in this fuse, but the details of its arrangement are changed so -as to fairly satisfy the conditions previously laid down as necessary. -It consists of a flexible cord composed of a central core of fine -gunpowder, surrounded by hempen yarns twisted up into a tube, and called -the countering. An outer casing is made of different materials, -according to the circumstances under which it is intended to be used. A -central touch thread, or in some cases two threads, passes through the -core of gunpowder. This fuse, which in external appearance resembles a -piece of plain cord, is tolerably certain in its action; it may be used -with equal facility in holes bored in any direction; it is capable of -resisting considerable pressure without injury; it may be used without -special means of protection in wet ground; and it may be transported -from place to place without risk of damage. - -In the safety fuse, the conditions of slow burning are fully satisfied, -and certainty is in some measure provided for by the touch thread -through the centre of the core. As the combustion of the core leaves, in -the small space occupied by it, a carbonaceous residue, there is little -or no passage left through the tamping by which the gases of the -exploding charge may escape, as in the case of the squibs. Hence results -an economy of force. Another advantage offered by the safety fuse is, -that it may be made to carry the fire into the centre of the bursting -charge if it be desired to produce rapid ignition. This fuse can be also -very conveniently used for firing charges of compounds other than -gunpowder, by fixing a detonating charge at the end of it, and dropping -the latter into the charge of the compound. This means is usually -adopted in firing the nitro-glycerine compounds, the detonating charge -in such cases being generally contained within a metallic cap. In using -this fuse, a sufficient length is cut off to reach from the charge to a -distance of about an inch, or farther if necessary, beyond the mouth of -the hole. One end is then untwisted to a height of about a quarter of an -inch, and placed to that depth in the charge. The fuse being placed -against the side of the bore-hole with the other end projecting beyond -it, the tamping is put in, and the projecting end of the fuse slightly -untwisted. The match may then be applied directly to this part. The -rate of burning is about two and a half feet a minute. - -Safety fuse is sold in coils of 24 feet in length. The price varies -according to the quality, and the degree of protection afforded to the -train. - - -_Electric Fuses._--The employment of electricity to fire the charge in -blasting rock offers numerous and great advantages. The most important, -perhaps, is the greatly increased effect of the explosions when the -charges are fired simultaneously. But another advantage, of no small -moment, lies in the security from accident which this means of firing -gives. When electricity is used, not only may the charge be fired at the -moment desired, after the workmen have retired to a place of safety, but -the danger due to a misfire is altogether avoided. Further, the facility -afforded by electricity for firing charges under water is a feature in -this agent of very great practical importance. It would therefore seem, -when all these advantages are taken into account, that electricity is -destined to become of general application to blasting purposes in this -country, as it is already in Germany and in America. - -An electric fuse consists of a charge of an explosive compound suitably -placed in the circuit of an electric current, which compound is of a -character to be acted upon by the current in a manner and in a degree -sufficient to produce explosion. The mode in which the current is made -to act depends upon the nature of the source of the electricity. That -which is generated by a machine is of high tension, but small in -quantity; while that which is generated by a battery is, on the -contrary, of low tension, but is large in quantity. Electricity of high -tension is capable of leaping across a narrow break in the circuit, and -advantage is taken of this property to place in the break an explosive -compound sufficiently sensitive to be decomposed by the passage of the -current. The electricity generated in a battery, though incapable of -leaping across a break in the circuit, is in sufficient quantity to -develop a high degree of heat. Advantage is taken of this property to -fire an explosive compound by reducing the sectional area of the wire -composing a portion of the circuit at a certain point, and surrounding -this wire with the compound. It is obvious that any explosive compound -may be fired in this way; but for the purpose of increasing the -efficiency of the battery, preference is given to those compounds which -ignite at a low temperature. Hence it will be observed that there are -two kinds of electric fuses, namely, those which may be fired by means -of a machine, and which are called “tension” fuses, and those which -require a battery, and which are known as “quantity” fuses. - -In the tension, or machine fuses, the circuit is interrupted within the -fuse case, and the priming, as before remarked, is interposed in the -break; the current, in leaping across the interval, passes through the -priming. In the quantity or battery fuses, the reduction of the -sectional area is effected by severing the conducting wire within the -fuse case, and again joining the severed ends of the wire by soldering -to them a short piece of very fine wire. Platinum wire, on account of -its high resistance and low specific heat, is usually employed for this -purpose. The priming composition is placed around this fine wire, which -is heated to redness by the current as soon as the circuit is closed. - -The advantages of high tension lie chiefly in the convenient form and -ready action of the machines employed to excite the electricity. Being -of small dimensions and weight, simple in construction, and not liable -to get quickly out of order, these sources of electricity are -particularly suitable for use in mining operations, especially when -these operations are entrusted, as they usually are, to men of no -scientific knowledge. - -Another advantage of high tension is the small effect of line resistance -upon the current, a consequence of which is that mines may be fired at -long distances from the machine, and through iron wire of very small -section. A disadvantage of high tension is the necessity for a perfect -insulation of the wires. - -When electricity of low tension is employed, the insulation of the -wires needs not to be perfect, so that leakages arising from injury to -the coating of the wire are not of great importance. In many cases, bare -wires may be used. Other advantages of low tension are the ability to -test the fuse at any moment by means of a weak current, and an almost -absolute certainty of action. For this reason, it is usually preferred -for torpedoes and important submarine work. On the other hand, the -copper wires used must be of comparatively large section, and the -influence of line resistance is so considerable that only a small number -of shots can be fired simultaneously when the distance is great. - -[Illustration: FIG. 30.] - -[Illustration: FIG. 31.] - -[Illustration: FIG. 32.] - -In Fig. 30 is shown an external view of an electric tension fuse. It -consists of a metal cap containing a detonating composition, upon the -top of which is placed the priming to be ignited by the electric spark. -The ends of two insulated wires project into this priming, which is -fired by the passage of the spark from one of these wires to the other. -The insulated wires are sufficiently long to reach a few inches beyond -the bore-hole. - -Sometimes the fuse is attached to the end of a stick, and the wires are -affixed to the latter in the manner shown in Fig. 31. The rigidity of -the stick allows the fuse to be readily pushed into the bore-hole. When -the ground is not very wet, bare wires are, for cheapness, used, and the -stick is in that case covered with oiled paper, or some other substance -capable of resisting moisture. These “blasting sticks,” as they are -called, are extensively used in Germany. When heavy tamping is employed, -the stick is not suitable, by reason of the space which it occupies in -the bore-hole. - -A mode of insulating the wires, less expensive than the guttapercha -shown in Fig. 30, is illustrated in Fig. 32. In this case, the wires are -cemented between strips of paper, and the whole is dipped into some -resinous substance to protect it from water. These “ribbon” wires may be -used in ground that is not very wet. They occupy little or no space in -the bore-hole, and therefore are suitable for use with tamping. - -To connect the fuses with the machine or the battery, two sets of wires -are required when a single shot is fired, and three sets may be needed -when two or more shots are fired simultaneously. Of these several sets -of wires, the first consists of those which are attached to the fuses, -and which, by reason of their being placed in the shot-hole, are called -the “shot-hole wires.” Two shot-hole wires must be attached to each -fuse, and they must be of such a length that, when the fuse has been -placed in its proper position in the charge, the ends may project a few -inches from the hole. These wires must also be “insulated,” that is, -covered with a substance capable of preventing the escape of -electricity. - -The second set of wires consists of those which are employed to connect -the charges one with another, and which, for this reason, are called -“connecting wires.” In connecting the charges in single circuit, the end -of one of the shot-hole wires of the first charge is left free, and the -other wire is connected, by means of a piece of this connecting wire, to -one of the shot-hole wires in the second hole; the other wire in this -second hole is then connected, in the same manner, to one of the wires -in the third hole; and so on till the last hole is reached, one -shot-hole wire of which is left free, as in the first. Whenever the -connecting wires can be kept from touching the rock, and also from -coming into contact one with another--and in most cases this may be -done--bare wire may be used, the cost of which is very little. But when -this condition cannot be complied with, and, of course, when blasting in -water, the connecting wires, like the shot-hole wires, must be -insulated. When guttapercha shot-hole wires are used, it is best to have -them sufficiently long to allow the ends projecting from one hole to -reach those projecting from the next hole. This renders connecting wire -unnecessary, and moreover saves one joint for each shot. - -[Illustration: FIG. 33.] - -[Illustration: FIG. 34.] - - -_Cables._--The third set of wires required consists of those used to -connect the charges with the machine or the battery. These wires, which -are called the “cables,” consist each of three or more strands of copper -wire well insulated with guttapercha, or better, indiarubber, the -coating of these materials being protected from injury by a sheathing of -tape or of galvanized iron wire underlaid with hemp. Two cables are -needed to complete the circuit; the one which is attached to the -positive pole of the machine, that is, the pole through which the -electric current passes out, is distinguished as the “leading cable,” -and the other, which is attached to the negative pole, that is, the pole -through which the current returns to the machine, is described as the -return cable. Sometimes both the leading and the return cables are -contained within one covering. When a cable having a metallic sheathing -is used, the sheathing may be made to serve as a return cable, care -being taken to make good metallic contact with the wires that connect -the sheathing to the fuses and to the terminal of the machine. The best -kind of unprotected cable consists of a three-strand tinned copper wire, -each 0·035 inch in diameter, insulated with three layers of indiarubber -to 0·22 inch diameter, and taped with indiarubber-saturated cotton to -0·24 inch diameter, as shown in Fig. 33. The best protected cable -consists of a similar strand of copper wire, covered with guttapercha -and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08 -inch diameter each, to a total diameter of 0·48 inch, as shown in Fig. -34. - - -_Detonators._--The new explosives of the nitro-cotton and -nitro-glycerine class cannot be effectively fired by means of safety or -other fuse alone. To bring about their instantaneous decomposition, it -is necessary to produce in their midst the explosion of some other -substance. The force of this initial explosion causes the charge of -gun-cotton, or dynamite, as the case may be, to detonate. It has been -found that the explosion of the fulminate of mercury brings about this -result most effectively and with the greatest certainty; and this -substance is therefore generally used for the purpose. The charge of -fulminate is contained in a copper capsule about a quarter of an inch in -diameter, and from 1 inch to 1¼ inch in length. These caps, with their -charge of fulminate, which are now well known to users of the -nitro-compounds, are called “detonators.” It is of the highest -importance that these detonators should contain a sufficiently strong -charge to produce detonation, for if too weak, not only is the whole -force of the explosive not developed, but a large quantity of noxious -gas is generated. Gun-cotton requires a much stronger charge of -fulminate than dynamite. - -[Illustration: FIG. 35.] - -In the electric fuses illustrated, the metal case shown is the -detonator, the fuse being placed inside above the fulminate. When safety -fuse is used, the end is cut off clean and inserted into the cap, which -is then pressed tightly upon the fuse by means of a pair of nippers, as -shown in Fig. 35. When water tamping is used, and when, with ordinary -tamping, the hole is very wet, a little white-lead or grease must be put -round the edge of the cap as a protection. The electric fuses are always -made waterproof; consequently, they are ready for use under all -circumstances. When the safety fuse burns down into the cap, or when, in -the other case, the priming of the electric fuse is fired, the fulminate -explodes and causes the detonation of the charge in which it is placed. - - -_Firing Machines and Batteries._--The electrical machines used for -firing tension fuses are of two kinds. In one kind, the electricity is -excited by friction, and stored in a condenser to be afterwards -discharged by suitable means provided for the purpose. In the other -kind, the electricity is excited by the motion of an armature before the -poles of a magnet. The former kind are called “frictional electric” -exploders; the latter kind are known as “magneto-electric” exploders. -When a magneto-electric machine contains an electro-magnet instead of a -permanent magnet, it is described as a “dynamo-electric exploder.” - -Frictional machines act very well as exploders so long as they are kept -in a proper state. But as they are injuriously affected by a moist -atmosphere, and weaken rapidly with use by reason of the wearing away of -the rubbers, it is necessary to take care that they be in good -electrical condition before using them for firing. Unless this care be -taken, the quantity of electricity excited by a given number of -revolutions of the plate will be very variable, and vexatious failures -will ensue. If, however, the proper precautions be observed, very -certain and satisfactory results may be obtained. In Germany and in -America, frictional exploders are generally used. - -Magneto-electric machines possess the very valuable quality of -constancy. They are unaffected, in any appreciable degree, by -atmospheric changes, and they are not subject to wear. These qualities -are of inestimable worth in an exploder used for ordinary blasting -operations. Moreover, as they give electricity of a lower tension than -the frictional machines, defects of insulation are less important. Of -these machines, only the dynamo variety are suitable for industrial -blasting. It is of primary importance that an exploder should possess -great power. The mistake of using weak machines has done more than -anything else to hinder the adoption of electrical firing in this -country. - -[Illustration: FIG. 36.] - -The machine most used in Germany is Bornhardt’s frictional exploder, -shown in Fig. 36. This machine is contained in a wooden case 20 inches -in length, 7 inches in breadth, and 14 inches in depth, outside -measurement. The weight is about 20 lb. - -To fire the charges by means of this exploder, the leading wire is -attached to the upper terminal B, and the return to the lower terminal -C, the other ends of these wires being connected to the fuses. The -handle is then turned briskly from fifteen to thirty times, according to -the number of the fuses and the state of the machine, to excite the -electricity. The knob A is then pressed suddenly in, and the discharge -takes place. To ascertain the condition of the machine, a scale of -fifteen brass-headed nails is provided on the outside, which scale may -be put in communication with the poles B and C by means of brass chains, -as shown in the drawing. If after twelve or fourteen turns, the spark -leaps the scale when the knob is pushed in, the machine is in a -sufficiently good working condition. To give security to the men -engaged, the handle is designed to be taken off when the machine is not -in actual use; and the end of the machine into which the cable wires are -led is made to close with a lid and lock, the key of which should be -always in the possession of the man in charge of the firing operations. - -[Illustration: FIG. 37.] - -[Illustration: FIG. 38.] - -In America, there are two frictional exploders in common use. One, shown -in Fig. 37, is the invention of H. Julian Smith. The apparatus is -enclosed in a wooden case about 1 foot square and 6 inches in depth. -The handle is on the top of the case, and is turned horizontally. This -handle is removable, as in Bornhardt’s machine. The cable wires having -been attached to the terminals, the handle is turned forward a certain -number of times to excite the electricity, and then turned a quarter of -a revolution backward to discharge the condenser and to fire the blast. -By this device, the necessity for a second aperture of communication -with the inside is avoided, an important point in frictional machines, -which are so readily affected by moisture. The aperture through which -the axis of the plate passes, upon which axis the handle is fixed, is -tightly closed by a stuffing-box. A leathern strap on one end of the -case allows the machine to be easily carried. The weight of this -exploder is under 10 lb. - -The other exploder used is that designed by G. Mowbray. This machine, -which is shown in Fig. 38, is contained in a wooden barrel-shaped case, -and is known as the “powder-keg” exploder, the form and dimensions of -the case being those of a powder-keg. The action is similar to that of -the machine last described. The cable wires having been attached to the -terminals at one end of the keg, the handle at the other end is turned -forward to excite the electricity, and the condenser is discharged by -making a quarter turn backward, as in Smith’s machine. The handle is in -this case also removable. The weight of the powder-keg exploder is about -26 lb. - -Both of these machines are very extensively used, and good results are -obtained from them. They stand well in a damp atmosphere, and do not -quickly get out of order from the wearing of the rubbers. They are also, -especially the former, very easily portable. - -[Illustration: FIG. 39.] - -The machine commonly used in England is the dynamo-electric exploder of -the Messrs. Siemens. This machine, which is the best of its kind yet -introduced for blasting purposes, is not more than half the size of -Bornhardt’s frictional exploder; but it greatly exceeds the latter in -weight, that of Siemens’ being about 55 lb. The apparatus, which is -contained within the casing shown in Fig. 39, consists of an ordinary -Siemens’ armature, which is made, by turning the handle, to revolve -between the poles of an electro-magnet. The coils of the electro-magnet -are in circuit with the wire of the armature; the residual magnetism of -the electro-magnet cores excites, at first, weak currents; these pass -into the coils, thereby increasing the magnetism of the cores, and -inducing still stronger currents in the armature wire, to the limit of -magnetic saturation of the iron cores of the electro-magnets. By the -automatic action of the machine, this powerful current is, at every -second turn of the handle, sent into the cables leading to the fuses. - -To fire this machine, the handle is turned gently till a click is heard -from the inside, indicating that the handle is in the right position to -start from. The cable wires are then attached to the terminals, and the -handle is turned quickly, but steadily. At the completion of the second -revolution, the current is sent off into line, as it is termed, that is, -the current passes out through the cables and the fuses. As in the case -of the frictional machines, the handle is, for safety, made removable. -This exploder is practically unaffected by moisture, and it is not -liable to get out of order from wear. - -Induction coils have been used to fire tension fuses; but it is -surprising that they have not been more extensively applied to that -purpose. A coil designed for the work required of it is a very effective -instrument. If constructed to give a spark not exceeding three inches in -length, with comparatively thick wire for quantity, it makes a very -powerful exploder. An objection to its use is the necessity for a -battery. But a few bichromate of potash cells, provided with spiral -springs to hold the zincs out of the liquid, and designed to be set in -action by simply pressing down the zincs, give but little trouble, so -that the objection is not a serious one. The writer has used an -induction exploder in ordinary mining operations without experiencing -any difficulty or inconvenience. It is cheap, easily portable, and -constant in its action. - -Batteries are used to fire what are known as “quantity” or “low tension” -fuses. Any cells may be applied to this purpose; but they are not all -equally suitable. A firing battery should require but little attention, -and should remain in working order for a long time. These conditions are -satisfactorily fulfilled by only two cells, namely, the Léclanché and -the Bichromate of Potash. The latter is the more powerful, and generally -the more suitable. The Léclanché is much used in this country for firing -purposes, under the form known as the “Silvertown Firing Battery.” This -battery consists of a rectangular teak box, containing ten cells. Two, -or more, of these may be joined up together when great power is -required. In France, the battery used generally for firing is the -Bichromate. This battery is much more powerful than the Léclanché, and -as no action goes on when the zincs are lifted out of the liquid, it is -equally durable. It is moreover much cheaper. At the suggestion of the -writer, Mr. Apps, of the Strand, London, has constructed a bichromate -firing battery of very great power. It is contained in a box of smaller -dimension than the 10-cell Silvertown. The firing is effected by simply -lowering the zincs, which rise again automatically out of the liquid, so -that there is no danger of the battery exhausting itself by continuous -action in case of neglect. Externally, this battery, like the -Silvertown, appears a simple rectangular box, so that no illustration is -needed. With either of these, the usual objections urged against the -employment of batteries, on the ground of the trouble involved in -keeping them in order, and their liability to be injured by ignorant or -careless handling, do not apply, or at least apply in only a very -unimportant degree. - -To guard against misfires, the machine or the battery used should be -constructed to give a very powerful current. If this precaution be -observed, and the number of fuses in circuit be limited to one-half that -which the machine is capable of firing with a fair degree of certainty, -perfectly satisfactory results may be obtained. The employment of weak -machines and batteries leads inevitably to failure. In the minds of -those who have hitherto tried electrical blasting in this country, there -seems to be no notion of any relation existing between the work to be -done and the force employed to do it. The electrical exploder is -regarded as a sort of magic box that needs only to be set in action to -produce any required result. Whenever failure ensues, the cause is -unhesitatingly attributed to the fuses. - - - - -CHAPTER II. - -EXPLOSIVE AGENTS USED IN BLASTING ROCKS. - - -SECTION I.--PHENOMENA ACCOMPANYING AN EXPLOSION. - - -_Nature of an Explosion._--The combination of oxygen with other -substances for which it has affinity is called generally “oxidation.” -The result of this combination is a new substance, and the process of -change is accompanied by the liberation of heat. The quantity of heat -set free when two substances combine chemically is constant, that is, it -is the same under all conditions. If the change takes place within a -short space of time, the heat becomes sensible; but if the change -proceeds very slowly, the heat cannot be felt. The same _quantity_, -however, is liberated in both cases. Thus, though the quantity of heat -set free by a chemical combination is under all conditions the same, the -degree or _intensity_ of the heat is determined by the rapidity with -which the change is effected. - -When oxidation is sufficiently rapid to cause a sensible degree of heat, -the process is described as “combustion.” The oxidation of a lump of -coke in the furnace, for example, is effected within a short space of -time, and, as the quantity of heat liberated by the oxidation of that -weight of carbon is great, a high degree results. And it is well known -and obvious that as combustion is quickened, or, in other words, as the -time of change is shortened, the intensity of the heat is proportionally -increased. So in the case of common illuminating gas, the oxidation of -the hydrogen is rapidly effected, and, consequently, a high degree of -heat ensues. - -When oxidation takes place within a space of time so short as to be -inappreciable to the senses, the process is described as “explosion.” -The combustion of a charge of gunpowder, for example, proceeds with such -rapidity that no interval can be perceived to intervene between the -commencement and the termination of the process. Oxidation is in this -case, therefore, correctly described as an explosion; but the combustion -of a train of gunpowder, or of a piece of quick-match, though -exceedingly rapid, yet, as it extends over an appreciable space of time, -is not to be so described. By analogy, the sudden change of state which -takes place when water is “flashed” into steam, is called an explosion. -It may be remarked here that the application of this expression to the -bursting of a steam boiler is an abuse of language; as well may we speak -of an “explosion” of rock. - -From a consideration of the facts stated in the foregoing paragraphs, -it will be observed that oxidation by explosion gives the maximum -intensity of heat. - - -_Measure of Heat, and specific Heat._--It is known that if a certain -quantity of heat will raise the temperature of a body one degree, twice -that quantity will raise its temperature two degrees, three times the -quantity, three degrees, and so on. Thus we may obtain a measure of heat -by which to determine, either the temperature to which a given quantity -of heat is capable of raising a given body, or the quantity of heat -which is contained in a given body at a given temperature. The quantity -of heat requisite to produce a change of one degree in temperature is -different for different bodies, but is practically constant for the same -body, and this quantity is called the “specific heat” of the body. The -standard which has been adopted whereby to measure the specific heat of -bodies is that of water, the unit being the quantity of heat required to -raise the temperature of 1 lb. of water through 1° Fahr., say from 32° -to 33°. The quantity of heat required to produce this change of -temperature in 1 lb. of water is called the “unit of heat,” or the -“thermal unit.” Having determined the specific heat of water, that of -air may in like manner be ascertained, and expressed in terms of the -former. It has been proved by experiments that if air be heated at -constant pressure through 1° Fahr., the quantity of heat absorbed is -0·2375 thermal units, whatever the pressure or the temperature of the -air may be. Similarly it has been shown that the specific heat of air at -constant volume is, in thermal units, 0·1687; that is, if the air be -confined so that no expansion can take place, 0·1687 of a thermal unit -will be required to increase its temperature one degree. - - -_Heat liberated by an Explosion._--In the oxidation of carbon, one atom -of oxygen may enter into combination with one atom of that substance; -the resulting body is a gas known as “carbonic oxide.” As the weight of -carbon is to that of oxygen as 12 is to 16, 1 lb. of the former -substance will require for its oxidation 1⅓ lb. of the latter; and since -the two enter into combination, the product, carbonic oxide, will weigh -1 + 1⅓ = 2⅓ lb. The combining of one atom of oxygen with one of carbon -throughout this quantity, that is, 1⅓ lb. of oxygen, with 1 lb. of -carbon, generates 10,100 units of heat. Of this quantity, 5700 units are -absorbed in changing the carbon from the solid into the gaseous state, -and 4400 are set free. The quantity of heat liberated, namely, the 4400 -units, will be expended in raising the temperature of the gas from 32° -Fahr., which we will assume to be that of the carbon and the oxygen -previous to combustion, to a much higher degree, the value of which may -be easily determined. The 4400 units would raise 1 lb. of water from 32° -to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is -0·17 when there is no increase of volume, the same quantity of heat will -raise 1 lb. of that gas from 32° to - - 4400 - 32 + ---- = 25,914°. - 0·17 - -But in the case under consideration, we have 2⅓ lb. of the gas, the -resulting temperature of which will be - - 25,914 - ------ = 9718°. - 2⅓ - -In the oxidation of carbonic oxide, one atom of oxygen combines with one -atom of the gaseous carbon; the resulting body is a gas known as -“carbonic acid.” Since 2⅓ lb. of carbonic oxide contains 1 lb. of -carbon, that quantity of the oxide will require 1⅓ lb. of oxygen to -convert it into the acid, that is, to completely oxidize the original -pound of solid carbon. By this combination, 10,100 units of heat are -generated, as already stated, and since the carbon is now in the gaseous -state, the whole of that quantity will be set free. Hence the -temperature of the resulting 3⅔ lb. of carbonic acid will be - - 4400 + 10,100 - 32 + ------------- = 23,516°. - 0·17 × 3·667 - -It will be seen from the foregoing considerations that if 1 lb. of pure -carbon be burned in 2⅔ lb. of pure oxygen, 3⅔ lb. of carbonic acid is -produced, and 14,500 units of heat are liberated; and further, that if -the gas be confined within the space occupied by the carbon and the -oxygen previously to their combination, the temperature of the product -may reach 23,516° Fahr. - -In the oxidation of hydrogen, one atom of oxygen combines with two atoms -of the former substance; the resulting body is water. As the weight of -hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas -will require for its oxidation 8 lb. of the latter; and since the two -substances enter into combination, the product, water, will weigh 1 + 8 -= 9 lb. By this union, 62,032 units of heat are generated. Of this -quantity, 8694 are absorbed in converting the water into steam, and -53,338 are set free. The specific heat of steam at constant volume being -0·37, the temperature of the product of combustion, estimated as before, -will be - - 53,338 - 32 + -------- = 16,049°. - 0·37 × 9 - -Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb. -of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat -will be liberated; and further, that the temperature of the product may -reach 16,049°. - - -_Gases generated by an Explosion._--It was shown in the preceding -paragraph that in the combustion of carbon, one atom of oxygen may unite -with one atom of carbon to form carbonic oxide, or two atoms of oxygen -may unite with one atom of carbon to form carbonic acid. When the -combination takes place according to the former proportions, the -reaction is described as “imperfect combustion,” because the carbon is -not fully oxidized; but when the combination is effected in the latter -proportions, the combustion is said to be “perfect,” because no more -oxygen can be taken up. The products of combustion are in both cases -gaseous. Carbonic oxide, the product of imperfect combustion, is an -extremely poisonous gas; it is this gas which is so noisome in close -headings, and in all ill-ventilated places, after a blast has been -fired. A cubic foot of carbonic oxide, the specific gravity of which is -0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1 -lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon -imperfectly oxidized will give 2⅓ lb. of carbonic oxide, which, at the -mean atmospheric pressure of 30 inches and the mean temperature of 62° -Fahr., will occupy a space of 13·5 × 2⅓ = 31·5 cubic feet. The product -of perfect combustion, carbonic acid, is a far less noxious gas than the -oxide, and it is much more easily expelled from confined places, because -water possesses the property of absorbing large quantities of it. In an -ill-ventilated but wet heading, the gas from a blast is soon taken up. -Carbonic acid is a comparatively heavy gas, its specific gravity -relatively to that of common air being 1·524. Hence a cubic foot at the -ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the -gas under the same conditions will occupy a space of 8·6 cubic feet. -Thus if 1 lb. of carbon be completely oxidized, there will result 3⅔ lb. -of carbonic acid, which will fill a space of 8·6 × 3⅔ = 31·5 cubic feet. -It will be observed that, though an additional pound of oxygen has been -taken up during this reaction, the product occupies the same volume as -the oxide. In complete combustion, therefore, a contraction takes place. - -In the oxidation of hydrogen, as already pointed out, one atom of oxygen -combines with two atoms of the former substance to form water. In this -case, the product is liquid. But the heat generated by the combustion -converts the water into steam, so that we have to deal with this product -also in the gaseous state, in all considerations relating to the effects -of an explosion. A cubic foot of steam, at atmospheric pressure and a -temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these -conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the -combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which, -under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26 -cubic feet. - -Usually in an explosion a large quantity of nitrogen gas is liberated. -This gas, which is not in itself noxious, has a specific gravity of -0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb. -will occupy a space of 13·5 cubic feet, which are the weight and the -volume of carbonic oxide. Other gases are often formed as products of -combustion; but the foregoing are the chief, viewed as the results of an -explosion, since upon these the force developed almost wholly depends. - - -_Force developed by an Explosion._--A consideration of the facts -enunciated in the foregoing paragraphs will show to what the tremendous -energy developed by an explosion is due. It was pointed out that the -combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If -this volume of gas be compressed within the space of 1 cubic foot it -will obviously have a tension of 31·5 atmospheres; that is, it will -exert upon the walls of the containing vessel a pressure of 472 lb. to -the square inch. If the same volume be compressed into a space -one-eighth of a cubic foot in extent, say a vessel of cubical form and 6 -inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the -pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen -to exist in the solid state, and the two bodies carbon and oxygen to -occupy together a space of one-eighth of a cubic foot, the combustion of -the carbon will develop upon the walls of an unyielding containing -vessel of that capacity a pressure of 252 atmospheres. Also the -combustion of 1 lb. of hydrogen gives rise, as already remarked, to -190·26 cubic feet of steam; and if combustion take place under similar -conditions with respect to space, the pressure exerted upon the -containing vessel will be 22,830 lb., or nearly 10·5 tons, to the -square inch, the tension being 190·26 × 8 = 1522 atmospheres. - -The force thus developed is due wholly to the volume of the gas -generated, and by no means represents the total amount developed by the -explosion. The volume of the gases evolved by an explosion is estimated -for a temperature of 62°; but it was shown in a former paragraph that -the temperature of the products of combustion at the moment of their -generation is far above this. Now it is a well-known law of -thermo-dynamics that, the volume remaining the same, the pressure of a -gas will vary directly as the temperature; that is, when the temperature -is doubled, the pressure is also doubled. By temperature is understood -the number of degrees measured by Fahrenheit’s scale on a perfect gas -thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale, -that is, 493°·2 below the freezing point of water. Thus the temperature -of 62° for which the volume has been estimated is equal to 461·2 + 62 = -523°·2 absolute. - -It was shown that the temperature of the product of combustion when -carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent -to 10179°·2 absolute. Hence it will be observed that the temperature has -been increased - - 10179°·2 - -------- = 19·45 times. - 523°·2 - -According to the law above enunciated, therefore, the pressure will be -increased in a like ratio, that is, it will be, for the volume and the -space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square -inch. - -When carbon is burned to carbonic acid, the temperature of the product -was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute. -In this case, it will be observed that the temperature has been -increased - - 23977·2 - ------- = 45·83 times. - 523·2 - -Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3 -tons to the square inch. It will be seen from these pressures that when -combustion is complete, the force developed is 2·36 greater than when -combustion is incomplete; and also that the increase of force is due to -the larger quantity of heat liberated, since the volume of the gases is -the same in both cases. If we suppose the carbon burned to carbonic -oxide in the presence of a sufficient quantity of oxygen to make -carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic -feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If -this volume be compressed within the space of one-eighth of a cubic -foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will -exert upon the walls of the containing vessel a pressure of 5124 lb. to -the square inch. The temperature of the gases will be - - 4400 - 32 + ------------- = 6347° Fahr. = 6808°·2 absolute, - 0·190 × 3·667 - -the mean specific heat of the gases being 0·190; whence it will be seen -that the temperature has been increased - - 6808°·2 - ------- = 13·01 times. - 523·2 - -According to the law of thermo-dynamics, therefore, the pressure under -the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons -to square inch. So that, under the conditions assumed in this case, the -pressures developed by incomplete and by complete combustion are as 29·8 -to 77·3, or as 1 to 2·59. - -Similarly, when hydrogen is burned to water, the temperature of the -product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2 -absolute; and the pressure will be - - 16510·2 - 22,830 × ------- = 720,286 lb. = 321·1 tons to the square inch. - 523·2 - -It will be observed, from a consideration of the foregoing facts, that a -very large proportion of the force developed by an explosion is due to -the heat liberated by the chemical reactions which take place. And hence -it will plainly appear that, in the practical application of explosive -agents to rock blasting, care should be taken to avoid a loss of the -heat upon which the effects of the explosion manifestly so largely -depend. - - -SECTION II.--NATURE OF EXPLOSIVE AGENTS. - - -_Mechanical Mixtures._--In the preceding section, it was shown that an -explosion is simply the rapid oxidation of carbon and hydrogen. To form -an explosive agent, the problem is, how to bring together in a -convenient form the combustible, carbon or hydrogen, and the oxygen -required to oxidize it. Carbon may be obtained pure, or nearly pure, in -the solid form. As wood charcoal, for example, that substance may be -readily procured in any needful abundance; but pure oxygen does not -exist in that state, and it is hardly necessary to point out that only -the solid form is available in the composition of an explosive agent. In -nature, however, oxygen exists in the solid state in very great -abundance in combination with other substances. Silica, for example, -which is the chief rock constituent, is a compound of silicon and -oxygen, and the common ores of iron are made up chiefly of that metal -and oxygen. The elementary constituents of cellulose, or wood fibre, are -carbon, hydrogen, and oxygen; and the body known as saltpetre, or -nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But -though oxygen is thus found in combination with many different -substances, it has not the same affinity for all. When it is combined -with a substance for which its affinity is strong, as in the silica and -the iron oxide, it cannot be separated from that substance without -difficulty; but if the affinity be weak, dissociation may be more easily -effected. The former combination is said to be “stable,” and the latter -is, in contradistinction, described as “unstable.” It will be evident on -reflection that only those compounds in which the oxygen exists in -unstable combination can be made use of as a constituent part of an -explosive agent, since it is necessary that, when required, the oxygen -shall be readily given up. Moreover, it will also appear that when one -of these unstable oxygen compounds and carbon are brought together the -mixture will constitute an explosive agent, since the oxygen which is -liberated by the dissociation of the unstable compound will be taken up -by the carbon for which it has a stronger affinity. Saltpetre is one of -those compounds, and a mixture of this body with charcoal constitutes -gunpowder. The means employed to dissociate the elements of saltpetre is -heat. It is obvious that other compounds of oxygen might be substituted -for the saltpetre, but this body being easily procurable is always -employed. The chlorate of potash, for example, is less stable than the -nitrate, and therefore an explosive mixture containing the former -substance will be more violent than another containing the latter. For -the violence of an explosion is in a great measure determined by the -readiness with which the oxygen is given up to the combustible. But the -chlorate is much more costly than the nitrate. As, however, the force -developed is greater, the extra cost would perhaps be compensated by the -increased effect of the explosion. But the instability of the chlorate -is such that friction or a moderately light blow will produce explosion -in a mixture containing that substance, a circumstance that renders it -unfit to be the oxidizer in an explosive agent in common use. The -nitrate is therefore preferred on the ground of safety. Saltpetre, or -nitrate of potash, consists, as already pointed out, of the metal -potassium in combination with the substances nitrogen and oxygen. Of -these, the last only is directly concerned in the explosion; but the two -former, and especially the nitrogen, act indirectly to intensify its -effects in a manner that will be explained hereafter. - -The chemical formula for nitrate of potash is KNO₃, which signifies that -three atoms of oxygen exist in this body in combination with one atom of -nitrogen and one atom of kalium or potassium. As the atomic weights of -these substances are 16, 14, and 39 respectively, the weight of the -molecule is 101, that is, in 101 lb. of nitrate of potash there are 39 -lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen. -Hence the proportion of oxygen in nitrate of potash is by weight 47·5 -per cent. It will be seen from this proportion that to obtain 1 lb. of -oxygen, 2·1 lb. of the nitrate must be decomposed. - -The carbon of gunpowder is obtained from wood charcoal, the light woods, -such as alder, being preferred for that purpose. The composition of the -charcoal varies somewhat according to the degree to which the burning -has been carried, the effect of the burning being to drive out the -hydrogen and the oxygen. But, generally, the composition of gunpowder -charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per -cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the -charcoal, it is easy to calculate the proportion of saltpetre required -in the explosive mixture. - -Thus far we have considered gunpowder as composed of charcoal and -saltpetre only. But in this compound, combustion proceeds too slowly to -give explosive effects. Were the chlorate of potash used instead of the -nitrate, the binary compound would be sufficient. The slowness of -combustion in the nitrate mixture is due to the comparatively stable -character of that body. To accelerate the breaking up of the nitrate, a -quantity of sulphur is mixed up with it in the compound. This substance -possesses the property of burning at a low temperature. The proportion -of sulphur added varies from 10 per cent. in powder used in fire-arms, -to 20 per cent. in that employed for blasting purposes. The larger the -proportion of sulphur, the more rapid, within certain limits, is the -combustion. Thus ordinary gunpowder is a ternary compound, consisting of -charcoal, saltpetre, and sulphur. - -As the composition of charcoal varies, it is not practicable to -determine with rigorous accuracy the proportion of saltpetre required in -every case; a mean value is therefore assumed, the proportions adopted -being about-- - - Charcoal 15 - Saltpetre 75 - Sulphur 10 - --- - 100 - --- - -With these proportions, the carbon should be burned to carbonic acid, -and the sulphur should be all taken up by the potassium. Powder of this -composition is used for fire-arms. For blasting purposes, as before -remarked, the proportion of sulphur is increased at the expense of the -saltpetre, in order to quicken combustion and to lessen the cost, to 20 -per cent. as a maximum. With such proportions, some of the carbon is -burned to carbonic oxide only, and some of the sulphur goes to form -sulphurous acid, gases that are particularly noisome to the miner. - -It is essential to the regular burning of the mixture that the -ingredients be finely pulverized and intimately mixed. The manufacture -of gunpowder consists of operations for bringing about these results. -The several substances are broken up by mechanical means, and reduced to -an impalpable powder. These are then mixed in a revolving drum, and -afterwards kneaded into a paste by the addition of a small quantity of -water. This paste is subjected to pressure, dried, broken up, and -granulated; thus, the mixing being effected by mechanical means, the -compound is called a mechanical mixture. It will be observed that in a -mechanical mixture the several ingredients are merely in contact, and -are not chemically united. They may therefore be separated if need be, -or the proportions may be altered in any degree. Mechanical mixtures, -provided the bodies in contact have no chemical action one upon another, -are stable, that is, they are not liable, being made up of simple -bodies, to decompose spontaneously. - - -_Chemical Compounds._--In a mechanical mixture, as we have seen, the -elements which are to react one upon another are brought together in -separate bodies. In gunpowder, for example, the carbon is contained in -the charcoal, and the oxygen in the saltpetre. But in a chemical -compound, these elements are brought together in the same body. In a -mechanical mixture, we may put what proportion of oxygen we please. But -elements combine chemically only in certain definite proportions, so -that in the chemical compound we can introduce only a certain definite -proportion of oxygen. The oxygen in saltpetre is in chemical combination -with the potassium and the nitrogen, and, as we have already seen, these -three substances hold certain definite proportions one to another. That -is, to every atom of potassium, there are one atom of nitrogen and -three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of -saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and -0·475 lb. of oxygen. Moreover, these elements occupy definite relative -positions in the molecule of saltpetre. But in the mechanical mixture, -the molecules of which it is made up have no definite relative -positions. Even if the three substances--charcoal, saltpetre, and -sulphur--of which gunpowder is composed, could be so finely divided as -to be reduced to their constituent molecules, the relative position of -these would be determined by the mixing, and it would be impossible so -to distribute them that each should find itself in immediate proximity -to those with which it was to combine. But so far are we from being able -to divide substances into their constituent molecules, that when we have -reduced them to an impalpable powder, each particle of that powder -contains a large number of molecules. Thus, in a mechanical mixture, we -have groups of molecules of one substance mingled irregularly with -groups of molecules of another substance, so that the atoms which are to -combine are not in close proximity one to another, but, on the contrary, -are, many of them, separated by wide intervals. In the chemical -compound, however, the atoms are regularly distributed throughout the -whole mass of the substance, and are, relatively to one another, in the -most favourable position for combining. Viewed from this point, the -chemical compound may be regarded as a perfect mixture, the mechanical -mixture being a very imperfect one. This difference has an important -influence on the effect of an explosion. All the atoms in a chemical -compound enter at once into their proper combinations, and these -combinations take place in an inconceivably short space of time, while, -in a mechanical mixture, the combinations are less direct, and are much -less rapidly effected. This is the reason why the former is more violent -in its action than the latter. The one is crushing and shattering in its -effects, the other rending and projecting. The compound gives a sudden -blow; the mixture applies a gradually increasing pressure. It is this -sudden action of the compound that allows it to be used effectively -without tamping. The air, which rests upon the charge, and which offers -an enormous resistance to motion at such inconceivably high velocities, -serves as a sufficient tamping. - -Gun-cotton may be taken as an example of a chemical compound. The woody -or fibrous part of plants is called “cellulose.” Its chemical formula is -C₆H₁₀O₅, that is, the molecule of cellulose consists of six atoms of -carbon in combination with ten atoms of hydrogen and five atoms of -oxygen. If this substance be dipped into concentrated nitric acid, some -of the hydrogen is displaced and peroxide of nitrogen is substituted for -it. The product is nitro-cellulose, the formula of which is -C₆H₇(NO₂)₃O₅. If this formula be compared with the last, it will be -seen that three atoms of hydrogen have been eliminated and their place -taken by three molecules of the peroxide of nitrogen NO₂; so that we now -have a compound molecule, which is naturally unstable. The molecules of -the peroxide of nitrogen are introduced into the molecule of cellulose -for the purpose of supplying the oxygen needed for the combustion of the -carbon and the hydrogen, just as the groups of molecules of saltpetre -were introduced into the charcoal of the gunpowder for the combustion of -the carbon and the hydrogen of that substance. Only, in the former case, -the molecules of the peroxide are in chemical combination, not merely -mixed by mechanical means as in the latter. The compound molecule of -nitro-cellulose may be written C₆H₇N₃O₁₁, that is, in 297 lb. of the -substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of -hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen; -or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent. -nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by -the action of heat, the oxygen combines with the carbon and the -hydrogen, and sets the nitrogen free. But it will be observed that the -quantity of oxygen present is insufficient to completely oxidize the -carbon and the hydrogen. This defect, though it does not much affect the -volume of gas generated, renders the heat developed, as shown in a -former section, considerably less than it would be were the combustion -complete, and gives rise to the noxious gas carbonic oxide. - -Cotton is one of the purest forms of cellulose, and, as it may be -obtained at a cheap rate, it has been adopted for the manufacture of -explosives. This variety of nitro-cellulose is known as “gun-cotton.” -The raw cotton made use of is waste from the cotton mills, which waste, -after being used for cleaning the machinery, is swept from the floors -and sent to the bleachers to be cleaned. This is done by boiling in -strong alkali and lime. After being picked over by hand to remove all -foreign substances, it is torn to pieces in a “teasing” machine, cut up -into short lengths, and dried in an atmosphere of 190° F. It is then -dipped into a mixture of one part of strong nitric acid and three parts -of strong sulphuric acid. The use of the sulphuric acid is, first, to -abstract water from the nitric acid, and so to make it stronger; and, -second, to take up the water which is formed during the reaction. After -the dipping, it is placed in earthenware pots to digest for twenty-four -hours, in order to ensure the conversion of the whole of the cotton into -gun-cotton. To remove the acid, the gun-cotton is passed through a -centrifugal machine, and subsequently washed and boiled. It is then -pulped, and again washed with water containing ammonia to neutralize -any remaining trace of acid. When rendered perfectly pure, it is -compressed into discs and slabs of convenient dimensions for use. - -Another important chemical compound is nitro-glycerine. Glycerine is a -well-known, sweet, viscous liquid that is separated from oils and fats -in the processes of candle-making. Its chemical formula is: C₃H₈O₃; that -is, the molecule is composed of three atoms of carbon, in combination -with eight atoms of hydrogen, and three atoms of oxygen. In other words, -glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and -oxygen 52·2 per cent. When this substance is treated, like cellulose, -with strong nitric acid, a portion of the hydrogen is displaced, and -peroxide of nitrogen is substituted for it; thus the product is: -C₃H₅(NO₂)₃O₃, similar, it will be observed, to nitro-cellulose. This -product is known as nitro-glycerine. The formula may be written -C₃H₅N₃O₉. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36 -lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen; -and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per -cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When -the molecule is broken up by the action of heat, the oxygen combines -with the carbon and the hydrogen, and sets the nitrogen free. And it -will be seen that the quantity of oxygen present is more than sufficient -to completely oxidize the carbon and the hydrogen. In this, the -nitro-glycerine is superior to the nitro-cotton. In both of these -compounds, the products of combustion are wholly gaseous, that is, they -give off no smoke, and leave no solid residue. - -In the manufacture of nitro-glycerine, the acids, consisting of one part -of strong nitric acid and two parts of strong sulphuric acid, are mixed -together in an earthenware vessel. When quite cold, the glycerine is run -slowly into this mixture, which, during the process, is kept in a state -of agitation, as heat is developed in the process; and, as the -temperature must not rise above 48° F., the vessels are surrounded with -iced water, which is kept in circulation. When a sufficient quantity of -glycerine has been run into the mixture, the latter is poured into a tub -of water. The nitro-glycerine being much heavier than the dilute acid -mixture, sinks to the bottom; the acid liquid is then poured off, and -more water added, this process being repeated until the nitro-glycerine -is quite free from acid. - -Nitro-glycerine is, at ordinary temperatures, a clear, nearly -colourless, oily liquid, having a specific gravity of about 1·6. It has -a sweet, pungent taste, and if placed upon the tongue, or even if -allowed to touch the skin in any part, it causes a violent headache. -Below 40° F. it solidifies in crystals. - -Dynamite is nitro-glycerine absorbed in a silicious earth called -kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine -and 25 per cent. kieselguhr. The use of the absorbent is to remove the -difficulties and dangers attending the handling of a liquid. Dynamite is -a pasty substance of the consistence of putty, and is, for that reason, -very safe to handle. It is made up into cartridges, and supplied for use -always in that form. - - -SECTION III.--RELATIVE STRENGTH OF THE COMMON EXPLOSIVE AGENTS. - - -_Force developed by Gunpowder._--In the combustion of gunpowder, the -elements of which it is composed, which elements, as we have seen, are -carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to -form, as gaseous products, carbonic acid, carbonic oxide, nitrogen, -sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as -solid products, sulphate, hyposulphite, sulphide, and carbonate of -potassium. Theoretically, some of these compounds should not be -produced; but experiment has shown that they are. It has also been -ascertained that the greater the pressure, the higher is the proportion -of carbonic acid produced, so that the more work the powder has to do, -the more perfect will be the combustion, and, consequently, the greater -will be the force developed. This fact shows that overcharging is not -only very wasteful of the explosive, but that the atmosphere is more -noxiously fouled thereby. The same remark applies even more strongly to -gun-cotton and the nitro-glycerine compounds. - -The careful experiments of Messrs. Noble and Abel have shown that the -explosion of gunpowder produces about 57 per cent. by weight of solid -matters, and 43 per cent. of permanent gases. The solid matters are, at -the moment of explosion, in a fluid state. When in this state, they -occupy 0·6 of the space originally filled by the gunpowder, consequently -the gases occupy only 0·4 of that space. These gases would, at -atmospheric pressure and 32° F. temperature, occupy a space 280 times -that filled by the powder. Hence, as they are compressed into 0·4 of -that space, they would give a pressure of - - 280 - --- × 15 = 10,500 lb., - 0·4 - -or about 4·68 tons to the square inch. But a great quantity of heat is -liberated in the reaction, and, as it was shown in a former section, -this heat will enormously increase the tension of the gases. The -experiments of Noble and Abel showed that the temperature of the gases -at the instant of explosion is about 4000° F. Thus the temperature of -32° + 461°·2 = 493°·2 absolute, has been raised - - 4000 - ------ = 8·11 times, - 493°·2 - -so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons -to the square inch. And this pressure was, in the experiments referred -to, indicated by the crusher-gauge. When, therefore, gunpowder is -exploded in a space which it completely fills, the force developed may -be estimated as giving a pressure of about 42 tons to the square inch. - - -_Relative Force developed by Gunpowder, Gun-cotton, and -Nitro-glycerine._--Unfortunately no complete experiments have hitherto -been made to determine the absolute force developed by gun-cotton and -nitro-glycerine. We are, therefore, unable to estimate the pressure -produced by the explosion of those substances, or to make an accurate -evaluation of their strength relatively to that of gunpowder. It should, -however, be borne in mind that a correct estimate of the pressure -produced to the square inch would not enable us to make a full -comparison of the _effects_ they were capable of causing. For though, by -ascertaining that one explosive gives twice the pressure of another, we -learn that one will produce twice the effect of another; yet it by no -means follows from that fact that the stronger will produce no more than -twice the effect of the weaker. The rending effect of an explosive -depends, in a great measure, on the rapidity with which combustion takes -place. The force suddenly developed by the decomposition of the chemical -compounds acts like a blow, and it is a well-known fact that the same -force, when applied in this way, will produce a greater effect than when -it is applied as a gradually increasing pressure. But some calculations -have been made, and some experiments carried out, which enable us to -form an approximate estimate of the relative strength of these explosive -substances. - -Messrs. Roux and Sarrau give the following as the result of their -investigations, derived from a consideration of the weight of the gases -generated and of the heat liberated. The substances are simply exploded, -and the strength of gunpowder is taken as unity. - - ---------------+---------+-------------+--------- - |Relative |Heat in Units| - Substance. | Weight | liberated |Relative - |of Gases.| from 1 lb. |Strength. - ---------------+---------+-------------+--------- - Gunpowder | 0·414 | 1316 | 1·00 - Gun-cotton | 0·850 | 1902 | 3·00 - Nitro-glycerine| 0·800 | 3097 | 4·80 - ---------------+---------+-------------+--------- - -The relative strength is that due to the volume of the gases and the -heat, no account being taken of the increased effect due to the rapidity -of the explosion. - -Alfred Noble has essayed to appreciate the effects of these different -explosives by means of a mortar loaded with a 32-lb. shot and set at an -angle of 10°, the distances traversed by the shot being taken as the -results to be compared. Considered, weight for weight, he estimates as -follows the relative strengths of the substances compared, gunpowder -being again taken as unity:-- - - Gunpowder 1·00 - Gun-cotton 2·84 - Dynamite 2·89 - Nitro-glycerine 4·00 - -The relative strength, bulk for bulk, is, however, of greater importance -in rock blasting. This is easily computed from the foregoing table and -the specific gravity of the substances, which is 1·00 for gunpowder and -compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite. -Compared in this way, bulk for bulk, these explosives range as -follows:-- - - Gunpowder 1·00 - Gun-cotton 2·57 - Dynamite 4·23 - Nitro-glycerine 5·71 - -Hence, for a given height of charge in a bore-hole, gun-cotton exerts -about 2½ times the force of gunpowder, and dynamite about 4¼ times that -force. - - -SECTION IV.--MEANS OF FIRING THE COMMON EXPLOSIVE AGENTS. - - -_Action of Heat._--We have seen that the oxygen required for the -combustion of the carbon in gunpowder is stored up in the saltpetre. So -long as the saltpetre remains below a certain temperature, it will -retain its oxygen; but when that temperature is reached, it will part -with that element. To fire gunpowder, heat is therefore made use of to -liberate the oxygen, which at once seizes upon the carbon with which it -is in presence. The means employed to convey heat to an explosive have -been described in the preceding chapter. It is necessary to apply heat -to one point only of the explosive; it is sufficient if it be applied to -only one grain. That portion of the grain which is thus raised in -temperature begins to “burn,” as it is commonly expressed, that is, this -portion enters at once into a state of combustion, the saltpetre giving -up its oxygen, and the liberated oxygen entering into combination with -the carbon. The setting up of this action is called “ignition.” The hot -gases generated by the combustion set up ignite other grains surrounding -the one first ignited; the gases resulting from the combustion of these -ignite other grains; and, in this way, ignition is conveyed throughout -the mass. Thus the progress of ignition is gradual. But though it takes -place, in every case, gradually, if the gases are confined within the -space occupied by the powder, it may be extremely rapid. It is easy to -see that the gases evolved from a very small number of grains are -sufficient to fill all the interstices, and to surround every individual -grain of which the charge is composed. But besides this ignition from -grain to grain, the same thing goes on from the outside to the inside of -each individual grain, the grain burning gradually from the outside to -the inside in concentric layers. The successive ignitions in this -direction, however, of layer after layer, is usually described as the -progress of combustion. Thus the time of an explosion is made up of that -necessary for the ignition of all the grains, and of that required for -their complete combustion. - -The time of ignition is determined in a great measure by the proportion -which the interstices, or empty spaces between the grains, bear to the -whole space occupied by the powder. If the latter be in the form of an -impalpable dust, ignition cannot extend throughout the mass in the -manner we have described; but we shall have merely combustion proceeding -from grain to grain. If, on the contrary, the powder be in large -spherical grains or pellets, the interstices will be large, and the -first gases formed will flash through these, and ignite all the grains -one after another with such rapidity that ignition may be regarded as -simultaneous. Thus the time of ignition is shortened by increasing the -size of the grains and approximating the latter to the spherical form. - -But the time of combustion is determined by conditions contrary to -these. As combustion proceeds gradually from the outside to the inside -of a grain, it is obvious that the larger the grain is, the longer will -be the time required to burn it in. Also it is evident that if the grain -be in the form of a thin flake, it will be burned in a much shorter time -than if it be in the spherical form. Thus the conditions of rapid -ignition and rapid combustion are antagonistic. The minimum time of -explosion is obtained when the grains are irregular in shape and only -sufficiently large to allow a fairly free passage to the hot gases. -There are other conditions which influence the time of combustion; among -them is the _density_ of the grain. This is obvious, since the denser -the grain, the greater is the quantity of material to be consumed. But -besides this, combustion proceeds more slowly through a dense grain than -through an open one. The presence of moisture also tends to retard -combustion. - -The progress both of ignition and of combustion is accelerated, not -uniform. In proportion as the grains are ignited, the gases evolved -increase in volume, and as the progress of combustion continues to -generate gases, the tension of these increases, until, as we have seen, -the pressure rises as high as 42 tons to the square inch. As the -pressure increases, the hot gases are forced more and more deeply into -the grains, and combustion, consequently, proceeds more and more -rapidly. - - -_Detonation._--By detonation is meant the simultaneous breaking up of -all the molecules of which the explosive substance is composed. Properly -the term is applicable to the chemical compounds only. But it is applied -to gunpowder to denote the simultaneous ignition of all the grains. The -mode of firing by detonation is obviously very favourable to the rending -effect required of blasting powder, since it reduces to a minimum the -time of explosion. It is brought about, in all cases, by means of an -initial explosion. The detonator, which produces this initial -explosion, consists of an explosive compound, preferably one that is -quick in its action, contained within a case sufficiently strong to -retain the gases until they have acquired a considerable tension. When -the case bursts, this tension forces them instantaneously through the -interstices of the powder, and so produces simultaneous ignition. A -pellet of gun-cotton, or a cartridge of dynamite, the latter especially, -makes a good detonator for gunpowder. Fired in this way, very much -better effects may be obtained from gunpowder than when fired in the -usual manner. Indeed, in many kinds of rock, more work may be done with -it than with gun-cotton or with dynamite. - -The action of a detonator upon a chemical compound is different. In this -case, the explosion seems to be due more to the vibration caused by the -blow than by the heat of the gases from the detonator. Probably both of -these causes operate in producing the effect. However this may be, the -fact is certain that under the influence of the explosion of the -detonator, the molecules of a chemical compound, like nitro-glycerine, -are broken up simultaneously, or at least, so nearly simultaneously, -that no tamping is needed to obtain the full effect of the explosion. -Dynamite is always, and gun-cotton is usually, fired by means of a -detonator. A much larger quantity of explosive is needed to detonate -gunpowder than is required for dynamite, or gun-cotton, since, for the -former explosive, a large volume of gases is requisite. Dynamite -detonators usually consist of from six to nine grains of fulminate of -mercury contained in a copper cap, as described in the preceding -chapter. Gun-cotton detonators are similar, but have a charge of from -ten to fifteen grains of the fulminate. An insufficient charge will only -scatter the explosive instead of firing it, if it be unconfined, and -only explode it without detonation, if it be in a confined space. - - -SECTION V.--SOME PROPERTIES OF THE COMMON EXPLOSIVE AGENTS. - - -_Gunpowder._--The combustion of gunpowder, as we have seen, is gradual -and comparatively slow. Hence its action is rending and projecting -rather than shattering. This constitutes one of its chief merits for -certain purposes. In many quarrying operations, for instance, the -shattering action of the chemical compounds would be very destructive to -the produce. In freeing blocks of slate, or of building stone, a -comparatively gentle lifting action is required, and such an action is -exerted by gunpowder. Moreover, this action may be modified by using -light tamping, or by using no tamping, a mode of employing gunpowder -often adopted in slate quarries. The effect of the violent explosives -cannot be modified in this way. - -Gunpowder is injured by moisture. A high degree of moisture will destroy -its explosive properties altogether, so that it cannot be used in water -without some protective covering. Even a slight degree of moisture, as -little as one per cent. of its weight, materially diminishes its -strength. For this reason, it should be used, in damp ground, only in -cartridges. This is, indeed, the most convenient and the most economical -way of using gunpowder in all circumstances. It is true that there is a -slight loss of force occasioned by the empty space around the cartridge, -in holes that are far from circular in shape. But at least as much will -be lost without the cartridge from the moisture derived from the rock, -even if the hole be not wet. But in all downward holes, the empty spaces -may be more or less completely filled up with dry loose sand. - -The products of the explosion of gunpowder are partly gaseous, partly -solid. Of the former, the most important are carbonic acid, carbonic -oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are -formed in only small quantities. The carbonic oxide is a very noxious -gas; but it is not formed in any considerable quantity, except in cases -of overcharging. The solid products are compounds of potassium and -sulphur, and potassium and carbon. These constitute the smoke, the dense -volumes of which characterize the explosion of gunpowder. This smoke -prevents the immediate return of the miner to the working face after -the blast has taken place. - - -_Gun-cotton._--The combustion of gun-cotton takes place with extreme -rapidity, in consequence of which its action is very violent. Its effect -is rather to shatter the rock than to lift it out in large blocks. This -quality renders it unsuitable to many quarrying operations. In certain -kinds of weak rock, its disruptive effects are inferior to those -produced by gunpowder. But in ordinary mining operations, where strong -tough rock has to be dealt with, its superior strength and quickness of -action, particularly the latter quality, produce much greater disruptive -effect than can be obtained from gunpowder. Moreover, its shattering -action tends to break up into small pieces the rock dislodged, whereby -its removal is greatly facilitated. - -Gun-cotton may be detonated when in a wet state by means of a small -quantity of the dry material. This is a very important quality, inasmuch -as it allows the substance to be used in a wet hole without protection, -and conduces greatly to the security of those who handle it. When in the -wet state, it is uninflammable, and cannot be exploded by the heaviest -blows. Only a powerful detonation will bring about an explosion in it -when in the wet state. It is, therefore, for safety, kept and used in -that state. Since it is insensible to blows, it may be rammed tightly -into the bore-hole, so as to fill up all empty spaces. The primer of -dry gun-cotton, however, which is to detonate it, must be kept perfectly -dry, and handled with caution, as it readily detonates from a blow. -Gun-cotton, when ignited in small quantities in an unconfined space, -burns fiercely, but does not explode. - -The products of the combustion of gun-cotton are:--carbonic acid, -carbonic oxide, water, and a little carburetted hydrogen or marsh-gas. -On account of the insufficiency of oxygen, already pointed out, a -considerable proportion of carbonic oxide is formed, which vitiates the -atmosphere into which it is discharged. Overcharging, as in the case of -gunpowder, causes an abnormal quantity of the oxide to be formed. - - -_Dynamite._--As combustion takes place more rapidly in nitro-glycerine -than in gun-cotton, the effects of dynamite are more shattering than -those of the latter substance. Gun-cotton holds, indeed, a mean position -in this respect between dynamite, on the one hand, and gunpowder on the -other. Dynamite is, therefore, even less suitable than gun-cotton for -those uses which are required to give the produce in large blocks. But -in very hard and tough rock, it is considerably more effective than -gun-cotton, and, under some conditions, it will bring out rock which -gun-cotton fails to loosen. - -Dynamite is unaffected by water, so that it may be used in wet holes; -indeed, water is commonly used as tamping, with this explosive. In -upward holes, where water cannot, of course, be used, dynamite is -generally fired without tamping, its quick action rendering tamping -unnecessary. - -The pasty form of dynamite constitutes a great practical advantage, -inasmuch as it allows the explosive to be rammed tightly into the -bore-hole so as to fill up all empty spaces and crevices. This is -important, for it is obvious that the more compactly the charge is -placed in the hole, the greater will be the effect of the explosion. -Moreover, this plastic character renders it very safe to handle, as -blows can hardly produce sufficient heat in it to cause explosion. If a -small quantity of dynamite be placed upon an anvil and struck with a -hammer, it explodes readily; but a larger quantity so struck does not -explode, because the blow is cushioned by the kieselguhr. If ignited in -small quantities in an unconfined space, it burns quietly without -explosion. - -If dynamite be much handled out of the cartridges, it causes violent -headaches; and the same effect is produced by being in a close room in -which there is dynamite in the unfrozen state. - -Dynamite possesses one quality which places it at a disadvantage with -respect to other explosives, namely, that of freezing at a comparatively -high temperature. At about 40° F. the nitro-glycerine solidifies, and -the dynamite becomes chalky in appearance. In this state, it is exploded -with difficulty, and, consequently, it has to be thawed before being -used. This may be safely done with hot water; performed in any other way -the operation is dangerous. - -The products of the combustion of dynamite are carbonic acid, carbonic -oxide, water, and nitrogen. As, however, there is more than a -sufficiency of oxygen in the compound, but little of the oxide is formed -when the charge is not excessive. If, therefore, dynamite be properly -detonated, and overcharging be avoided, its explosion will not greatly -vitiate the atmosphere. But if it be only partially detonated -hypo-nitric fumes are given off, which have a very deleterious effect -upon the health. It is, thus, of the highest importance that complete -detonation should be effected, not merely to obtain the full effect of -the explosive, but to avoid the formation of this noxious gas. This may -be done by using a detonator of sufficient strength, and placing it well -into the primer. - - -_Firing Points of the Common Explosive Compounds._--The following table -shows the temperatures at which the commonly used compounds explode:-- - - -------------------+-----------+----------------- - |When slowly| When suddenly - | Heated. | Heated. - +-----------+----------------- - Gunpowder | .. |from 500° to 540° - Gun-cotton | 360° | 482° - Kieselguhr dynamite| 356° | 446° - Cellulose dynamite | 342° | 446° - -------------------+-----------+----------------- - -Cotton powder explodes at the same temperatures as gun-cotton, and -lithofracteur at the same temperature as kieselguhr dynamite. - - -SECTION VI.--SOME VARIETIES OF THE NITRO-CELLULOSE AND THE -NITRO-GLYCERINE COMPOUNDS. - - -_Nitrated Gun-cotton._--It has been shown that gun-cotton contains an -insufficient quantity of oxygen for its complete combustion. To furnish -that which is wanting, gun-cotton has sometimes incorporated with it a -certain proportion of nitrate of potash, or of nitrate of baryta. This -compound, which, it will be observed, is at once a chemical compound and -a mechanical mixture, is known as “nitrated gun-cotton.” - - -_Cotton Powder, or Tonite._--The explosive which is now well known as -“tonite” or “cotton powder,” is essentially nitrated gun-cotton. It is -produced in a granulated form, and is compressed into cartridges of -various dimensions to suit the requirements of practice. The convenient -form in which tonite is made up, ready to the miner’s hand, has greatly -contributed towards bringing it into favour. But irrespective of this, -the fact of its being so highly compressed as to give it a density -equal, or nearly equal, to dynamite gives it a decided advantage over -the other nitro-cotton compounds as they are at present used. - - -_Schultze’s Powder._--In Schultze’s powder, the cellulose is obtained -from wood. The wood is first sawn into sheets, about ¹/₁₆ inch thick, -and then passed through a machine, which punches it up into grains of a -uniform size. These are deprived of their resinous matters by a process -of boiling in carbonate of soda, and are further cleansed by washing in -water, steaming, and bleaching by chloride of lime. The grains, which -are then pure cellulose, are converted into nitro-cellulose in the same -way as cotton, namely, by being treated with a mixture of nitric and -sulphuric acids. The nitro-cellulose thus produced is subsequently -steeped in a solution of nitrate of potash. Thus the finished compound -is similar in character to nitrated gun-cotton. - - -_Lithofracteur._--Lithofracteur is a nitro-glycerine compound in which a -portion of the base is made explosive. In dynamite, the base, or -absorbent material, is, as we have said, a silicious earth, called -“kieselguhr.” In lithofracteur, the same substance is used; but in -addition, a mixture of nitrate of baryta and charcoal, a kind of -gunpowder, is introduced. The object of employing this explosive mixture -is to increase the force of the explosion, the kieselguhr being an inert -substance. Obviously this object would be attained if the explosive -mixture possessed the same absorbent power as the kieselguhr. But -unfortunately it does not, and, as a consequence, less nitro-glycerine -is used. Thus what is gained in the absorbent is lost in the substance -absorbed. The composition of lithofracteur varies somewhat; but its -average proportion of ingredients are the following:-- - - Nitro-glycerine 52·50 - Nitrate of baryta 16·40 - Charcoal 2·85 - Sulphur 25·75 - Kieselguhr 22·50 - ------ - 100·00 - ------ - - -_Brain’s Powder._--Brain’s powder is a nitro-glycerine compound, similar -in character to lithofracteur. The exact composition of the base has -never been published, so far as relates to the proportions of the -ingredients. But it is composed of chlorate of potash, charcoal, and -nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per -cent. Horseley’s powder contains about the same proportion of -nitro-glycerine in a base of chlorate of potash and nut-galls. - - -_Cellulose Dynamite._--In Germany, gun-cotton is used as an absorbent -for nitro-glycerine, the compound being known as “Cellulose dynamite.” -It is chiefly used for primers to explode frozen dynamite. It is more -sensitive to blows than the kieselguhr dynamite. - - - - -CHAPTER III. - -THE PRINCIPLES OF ROCK BLASTING. - - -_Line of Least Resistance._--The pressure of a fluid is exerted equally -in all directions; consequently the surrounding mass subjected to the -force will yield, if it yield at all, in its weakest part, that is, the -part which offers least resistance. The line along which the mass -yields, or line of rupture, is called the “line of least resistance.” If -the surrounding mass were perfectly homogeneous, it would always be a -straight line, and it would be the shortest distance from the centre of -the charge to the surface. Such, however, is never the case, and the -line of rupture is, therefore, always a more or less irregular line, and -often much longer than that from the centre direct to the surface. It -will be obvious, on reflection, that the line of least resistance will -be greatly dependent upon (1) the texture of the rock, which may vary -from one point to another; (2) its structure, which renders it more -easily cleavable in one direction than in another; (3) the position, -direction, and number of the joints, which separate the rock into more -or less detached portions; and (4) the number and relative position of -the unsupported faces of the rock. All these circumstances must be -ascertained, and the position and the direction of the bore-hole -determined in accordance with them, in order to obtain the maximum -effect from a given quantity of explosive. It must not be supposed, -however, that this is a labour involving minute examination and long -consideration. On the contrary, a glance is generally sufficient to -enable the trained eye to estimate the value of those circumstances, and -to determine accordingly the most effective position for the shot. In -practice, the line of least resistance is taken as the shortest distance -from the centre of the charge to the surface of the rock, unless the -existence of joint planes, a difference of texture, or some other -circumstance, shows it to lie in some other direction. - - -_Force required to cause Disruption._--When the line of least resistance -is known, it remains to determine the quantity of the explosive compound -required to overcome the resistance along that line. This matter is one -of great importance, for not only is all excess waste, but this waste -will be expended in doing mischief. In mining operations, the dislodged -rock is violently projected, and the air is vitiated in an unnecessary -degree; and in quarrying, stones are shattered which it is desirable to -extract in a sound state. The evil effects of overcharging, in -occasioning the formation of noxious gases, was pointed out in the last -chapter. Of course it is not possible so to proportion a charge to the -resistance that the rock shall be just lifted out, and no more; because -neither the force developed by the charge, nor the value of the -resistance can be known with precision. But a sufficient approximation -may be easily arrived at to enable us to avoid the loud report that is -indicative of wasted force. - -Charges of an explosive compound of uniform strength produce effects -that vary as the weight of those charges, that is, a double charge will -move a double mass. And, as homogeneous masses vary as the cube of any -similar line within them, the general rule is established that charges -of powder capable of producing the same effects are to each other as the -cubes of the lines of least resistance. Generally, the quantity of black -blasting powder requisite to overcome the resistance will vary from ¹/₂₀ -to ¹/₃₀ of the cube of the line of least resistance, the latter being -measured in feet and the former in pounds. Thus, if the rock to be -blasted be moderately strong limestone, for example, and the shortest -distance from the centre of the charge to the surface of the rock be 3 -feet, we shall have 3 × 3 × 3 = 27, the cube of the line, and ²⁷/₂₅ lb. -= 1²/₂₅ lb., or about 1 lb. 1 oz., as the weight of the powder required. -If dynamite be used, and we assume it to be four times as strong as -common black powder, of course, only one-fourth of this quantity will be -required. Also if gun-cotton, or cotton-powder, be used, and we assume -its strength to be three times that of black powder, one-third only -will be needed. Again, if Curtis’s and Harvey’s new extra-strong mining -powder fired by a detonator be employed, we may assume it to be twice as -strong as common black powder fired by the ordinary means, and -consequently we shall need only one-half the quantity indicated by the -formula. - -It is neither practicable nor desirable that such calculations and -measurements as these should be made for every blast; their practical -value lies in this, namely, that if the principles involved in them be -clearly understood, the blaster is enabled to proportion his charges _by -sight_ to the resistance to be overcome, with a sufficient degree of -precision. A few experiments in various kinds of rock, followed by some -practice, will enable a man to acquire this power. - -As it is a common and a convenient practice to make use of the bore-hole -as a measure of the quantity of explosive to be employed, we have -calculated the following table:-- - - ---------+------------+----------+------------ - Diameter | | |Dynamite (or - of |Black Powder|Gun-cotton| Tonite) - the Hole.| in 1 inch. |in 1 inch.| in 1 inch. - ---------+------------+----------+------------ - ins. | ozs. | ozs. | ozs. - 1 | 0·419 | 0·419 | 0·670 - 1¼ | 0·654 | 0·654 | 1·046 - 1½ | 0·942 | 0·942 | 1·507 - 1¾ | 1·283 | 1·283 | 2·053 - 2 | 1·675 | 1·675 | 2·680 - 2¼ | 2·120 | 2·120 | 3·392 - 2½ | 2·618 | 2·618 | 4·189 - 2¾ | 3·166 | 3·166 | 5·066 - 3 | 3·769 | 3·769 | 6·030 - ---------+------------+----------+------------ - -[Illustration: FIG. 40.] - -[Illustration: FIG. 41.] - -[Illustration: FIG. 42.] - -[Illustration: FIG. 43.] - - -_Conditions of Disruption._--Having explained the law according to which -the elastic gases evolved by an explosion act upon the surrounding rock, -and shown how the force required to cause disruption may be calculated, -it now remains to consider the conditions under which disruption may -take place. Suppose a block of unfissured rock detached on all sides, as -shown in plan, in Fig. 40, and a bore-hole placed in the centre of this -block. If a charge be fired in this position, the lines of rupture will -radiate from the centre towards any two, or towards all four of the -unsupported faces of the block, because the forces developed will act -equally in all directions, and the lines of rupture will be those of -least resistance. Evidently this is the most favourable condition -possible for the charge, since the rock offers an unsupported face on -every side; and it is evident that the line of rupture must reach an -unsupported face to allow of dislodgement taking place. Suppose, again, -as shown in Fig. 41, the block to be unsupported on three sides only, -and the charge placed at _h_. In this case, the lines of rupture may -run to any two, or to all three, of the unsupported faces; and hence -this will be the next most favourable condition for the action of the -charge. The greatest useful effect, however, will be obtained in this -case by placing the charge farther back at _h′_, when the lines of -rupture must necessarily run to the opposite faces _b c_, and, -consequently, the whole of the block will be dislodged. Assume another -case, in which the rock is unsupported upon only two sides, as shown in -Fig. 42, and the charge placed at _h_. In this case, the lines of -rupture must run to each of the unsupported faces _a b_. Thus, it is -evident that this condition, though still a favourable one for the good -effect of the charge, is inferior to the preceding. As rock is never -homogeneous in composition nor uniform in texture, the lines of rupture, -which, as before remarked, will be those of least resistance, may reach -the faces at any point, as at _m n_, _m′ n′_, or any point intermediate -between these. But it will be seen that the useful effect will be -greatest when these lines, radiating from the charge, make an angle of -180°, or, in other words, run in directly contrary directions, and that -the useful effect diminishes with the angle made by these lines of -rupture. Suppose, again, the rock to be unsupported upon one side only, -as shown in Fig. 43, and the charge placed at _h_. In this case, the -lines of rupture must run to the face _a_, and the condition must -therefore be considered as less favourable than the preceding. As in -those cases, the useful effect will depend upon the angle made by the -lines of rupture _h m_ and _h n_, which angle may be very small, and -which must necessarily be much less than 180°. A greater effect may be -obtained, under this condition, by firing several charges -simultaneously. If, for example, we have two charges placed, one at _h_, -and the other at _h′_, and fired successively, the lines of rupture will -run in or near the directions _h m_, _h n_, _h′ m′_, _h′ n′_, and the -portion of rock dislodged will be _m h n h′ n′_. But if these two -charges be fired simultaneously, the lines of rupture will be _h m_, _h -o_, _h′ o_, _h′ n′_, and the mass of rock dislodged will be _m h h′ n′_. -Simultaneous firing is in this way productive of a greatly increased -useful effect in numerous cases, and the mining engineer, and the -quarryman especially, will do well to direct their attention to this -source of economy. There is yet another case to be considered, in which -the conditions are still less favourable. Suppose two unsupported faces -at right angles to each other, and the charge placed at _h_, as shown in -Fig. 44. In this case, the lines of rupture will run to each of the two -unsupported faces; but as these lines must necessarily make a very small -angle with each other--for the length of the lines increases rapidly -with the angle--the useful effect will be less than in the last case. It -follows, therefore, that this is the most unfavourable condition -possible, and as such it should be avoided in practice. - -[Illustration: FIG. 44.] - -[Illustration: FIG. 45.] - -[Illustration: FIG. 46.] - -In the foregoing considerations, the holes have been assumed to be -vertical, and for this reason the unsupported face which is -perpendicular to the hole, that is, the face into which the hole is -bored, has been neglected. For it is evident that, under the conditions -assumed, the lines of rupture cannot reach this face, which, therefore, -has practically no existence. Suppose, for example, a bore-hole placed -at _h_, in Fig. 45, and the rock to be supported upon every side except -that at right angles to the hole. The forces acting perpendicularly to -the direction of the bore-hole are opposed on all sides by an infinite -resistance. Hence, in this case, either the tamping will be blown out, -or, if the forces developed are unequal to the work, no effect will be -produced beyond a slight enlargement of the hole at the base. This, -however, is a case of frequent occurrence in practice, and it becomes -necessary to adopt measures for making this unsupported face available. -Evidently this object can be attained only by so directing the bore-hole -that a line perpendicular to it may reach the face; that is, the line of -the bore-hole must make with the unsupported face an angle less than -90°. This direction of the bore-hole is shown in Fig. 46, which may be -regarded as a sectional elevation of Fig. 45. In this case, the lines of -rupture, which will run similarly to those produced in the case shown in -Fig. 43, will reach the unsupported face at _b_, and the length of these -lines, and consequently the depth of the excavation, for a given length -of bore-hole, will depend upon the angle which the latter makes with -the face. This mode of rendering a single exposed surface available is -called “angling the holes,” and it is generally resorted to in shaft -sinking and in driving headings. The conditions involved in “angling” -are favourable to the action of strong explosives. - - -_Example of a Heading._--To show how these principles are applied in -practice, we will take a typical case of a heading, 7 feet by 9 feet, as -shown in Fig. 47. In this case, we have at starting only one exposed -face, which is perpendicular to the direction of the driving. Hence it -is evident that we shall have to proceed by angling the holes. We might -begin in any part of the exposed face; but, as it will hereafter appear, -the most favourable position is the centre. We therefore begin at this -point by boring a series of holes, numbered 1 on the drawing. These -holes are angled towards each other; that is, the two sets of three -holes vertically above each other converge in the direction of their -lower ends, as shown in the sectional plan, Fig. 48. In this instance, -we have assumed six holes as necessary and sufficient. But it is obvious -that the number of holes, as well as their distance apart horizontally, -will be determined by their depth, the tenacity of the rock, and the -strength of the explosive used. When these holes are fired, a -wedge-shaped portion of the rock will be forced out, and this result -will be more effectually and certainly obtained if the charges be -fired simultaneously. The removal of this portion of the rock is called -“taking out the key.” The effect of removing this key is to leave the -surrounding rock unsupported on the side towards the centre; that is, -another face is formed perpendicular to the first. - -[Illustration: FIG. 47.] - -[Illustration: FIG. 48.] - -[Illustration: FIG. 49.] - -Having thus unkeyed the rock by the removal of this portion from the -centre, it will evidently be unnecessary, except for convenience or -increased effect, to angle any more of the shot-holes. The second series -therefore, numbered 2 in the drawing, may be bored perpendicularly to -the face of the heading. When this series is fired, the lines of rupture -will all run to the unsupported face in the centre--and from hole to -hole, if the shots be fired simultaneously--and the annular portion of -rock included between the dotted lines 1 and 2 will be removed. If the -shots be fired successively, the first will act under the condition of -one unsupported face, as illustrated in Fig. 43; but as another -unsupported face will be formed by the removal of the rock in front of -this charge, the succeeding shots will be subject to the more favourable -condition represented in Fig. 42. The firing of this second series of -shots still leaves the surrounding rock unsupported towards the centre, -and consequently the same conditions will exist for the third series, -numbered 3 on the drawing, the firing of which series will complete the -excavation. Fig. 49 shows the appearance of Fig. 48 after the firing of -the central holes. - -It may be remarked here that, owing to the want of homogeneity in the -rock, and to the existence of joints and fissures, the outer line of -rupture will not, in practice, run so regularly as indicated, in this -assumed case, by the dotted lines. This circumstance will influence the -position of the holes, or the quantity of explosive, in the next series, -and furnish an opportunity for the exercise of judgment on the part of -the blaster. - -There exist also other circumstances which will influence the position -and the number of the holes in a very important degree, and which -therefore must be taken fully into account at every advance. One of -these is the irregularity of the face of the excavation. Instead of -forming an unbroken plane at right angles to the direction of the -heading, or of the shaft, this face is broken up by projecting bosses -and more or less deep depressions. Obviously these protuberances and -cavities will influence, in no inconsiderable degree, the lines of least -resistance; the latter being lengthened or shortened, or changed in -direction, by the presence of the former, which give existence to -unsupported faces to which the lines may radiate. These conditions must, -in every case, be taken into account when determining the best position -for the bore-hole. Of yet greater importance, is the existence of joint -planes and bedding planes. A bed of rock may be, and frequently is, cut -up by these planes into detached blocks of greater or less dimensions, -according to the more or less perfect development of the different sets. -Hence it becomes necessary, in determining a suitable position for -blasting the charge, to consider such planes as unsupported faces, and -to ascertain the direction and length of the lines of resistance under -such conditions. If a charge be placed in close proximity to one of -these planes, not only may the lines of rupture run in unforeseen -directions, but the greater part of the force of the explosion will be -lost by the escape of the gases along the plane. The same loss of force -may be occasioned by the presence of a cavity, such as are of frequent -occurrence in cellular or vughy rock. When the joint planes are fully -developed, their existence can be ascertained by inspection; but when -their development is imperfect, there may be considerable difficulty in -discovering them. In such cases, the rock should be carefully inspected, -and sounded with a hammer or pick. When a cavity is bored into, it may -be rammed full of clay, and the boring continued through the clay; or if -sufficient depth has been obtained, the charge may be placed upon the -clay, which will prevent the wasteful dissipation of the gases. As none -of the aforementioned circumstances occur under precisely similar -conditions, no general rule of much service can be laid down; they are -matters upon which the blaster must be left to use his own judgment, and -to do this effectively, it is necessary that he possess some knowledge -of the materials with which he deals. - - -_Economical Considerations._--Besides the important economical -considerations involved in the foregoing, there are others which claim -attention. Foremost among these is the question whether, for a given -effect, it be better to augment or to diminish the individual importance -of the shots; that is, whether it be better to diminish the number of -the holes and to increase their diameter, or to diminish their diameter -and increase their number; or, again, to diminish their diameter and to -increase their depth, or to increase their diameter and to diminish -their number and their depth. It may be readily shown mathematically, -and the results are confirmed by experience, that there is an important -gain in reducing the diameter of the shot-holes to the lowest limit -allowed by the strength and the gravimetric density of the explosive, -and increasing their depth. The gain is mainly in the direction of a -saving of labour, and it is especially remarkable in the case of machine -boring. Here again we perceive the advantage of strength in the -explosive agent employed. - -The simultaneous firing of the shots offers several important -advantages. It has already been shown how one charge aids another, under -such a condition, and in what way the line of rupture is affected by it. -When the shots are fired successively, each one has to _tear out_ the -portion of rock allotted to it; but when they are fired simultaneously, -their collective force is brought to bear upon the whole mass to be -dislodged. This is seen in the diagram, Fig. 43. When deep holes are -used, the greater useful effect caused by simultaneous firing becomes -very marked. Hence electricity associates itself naturally with machine -drills and strong explosives. - - -_Tamping._--To “tamp” a shot-hole is to fill it up above the charge of -explosive with some material, which, when so applied, is called the -“tamping.” The object of tamping is to oppose a resistance to the escape -of the gases in the direction of the bore-hole. Hence a primary -condition is that the materials used shall be of a strongly resisting -character. A second determining condition is that these materials shall -be of easy application. This condition precludes the use of all such -devices as plugs, wedges, and forms of a similar character, which have -been from time to time proposed. - -The only material that, in practice, has been found to satisfactorily -fulfil the requirements, is rock in a broken, pulverulent, or plastic -state. As, however, all rock is not equally suitable, either from the -point of view of its resisting character, or from that of convenience of -handling, it becomes necessary to consider which satisfies the two -conditions in the most complete manner. - -Though it is not easy to assign a perfectly satisfactory reason why one -kind of rock substance opposes a greater resistance to motion in a -bore-hole than another, yet it is certain that this resistance is mainly -due to the friction among the particles of that substance. If a column -of solid, hard rock, of the same diameter as the bore-hole, be driven -down upon the charge, the resistance opposed by the column to the -imprisoned gases will be, neglecting the weight of the former, that of -the friction between the sides of the column and those of the hole. But -if disintegrated rock be used, not only is an absolute motion imparted -to the particles, but, on account of the varying resistances, a relative -motion also. Consequently, friction occurs amongst the particles, and as -the number of these is immense, the sum of the slight friction of one -particle against another, and of the great friction of the outside -particles against the sides of the hole, amounts to a much greater value -than that of the outside particles of the solid column against the sides -of the bore-hole. If this view of the facts alone be taken, it follows -that dry sand is the most resistant material, and that the finer the -grains, the greater will be the resistance which it offers. In practice, -however, it has been found that though the resistance offered by sand -tamping is very great, and though also the foregoing inference is true -when the tamping is lifted by the pressure of a solid against it from -below, this substance is notably inferior to some others when acted upon -by an explosion of gases. The explanation of this apparent anomaly is -that the gases, under the enormous tension to which they are subjected -in the bore-hole, insinuate themselves between the particles, and so -prevent the friction which would otherwise take place. When the -readiness with which water, through the influence of gravity alone, -permeates even closely compacted sand, is borne in mind, there will be -no difficulty in conceiving a similar action on the part of more subtile -gases in a state of extreme tension. Under such conditions as these, -there is no resistance whatever due to friction, and the only resistance -opposed to the escape of the gases is that proceeding from the inertia -of the mass. How this resistance may be very great, we have shown in the -case of air tamping. Hence, it becomes necessary to have recourse to -some other material of a composition less liable to be thus acted upon, -or to seek means of remedying the defect which renders such action -possible. - -Clay, dried either in the sun, or, preferably, by a fire, appears to -fulfil the requirements of a tamping material in the fullest degree. -This substance is composed of exceedingly minute grains of silicious -matters, bound together by an aluminous and calcareous or ferruginous -cement. Thus constituted, there are no voids between the particles, as -in porous substances, and, consequently, there is no passage for the -gases, the substance being impervious alike to water and gas. Hence, -when this material is employed as tamping, the forces act only upon the -lower surface, friction takes place among the particles, and the -requisite degree of resistance is produced. By reason of its possession -of this property, clay is generally used as the tamping material. - -In rock blasting, it is usual to prepare the clay beforehand, and this -practice is conducive both to effective results and to rapidity of -tamping. The latter consideration is an important one, inasmuch as the -operation, as commonly performed, requires a good deal of time. To -prepare the pellets of clay, a lump is taken and rolled between the -palms of the hands until it has assumed the form of a sausage, from -three to four inches in length, and of the diameter of the bore-hole. -These pellets are then baked until they are thoroughly dry, when they -are ready for use. In making them up to the requisite diameter, a little -excess should be allowed for shrinkage, since it is essential that they -fit tightly into the hole. When the charge has been put in, and covered -with a wad of hay, or a handful of sand or rubbish, one of these pellets -is inserted and pushed home with a wooden rammer. Considerable pressure -should be applied to make the clay fill the hole completely, but blows -should be avoided. A second pellet is then pushed down in the same way, -and the operations are repeated until the whole of the hole is tamped. -To consolidate the whole, light blows may be applied to the outer -pellet. It will be found advantageous to place an undried pellet -immediately above the charge, because the plasticity of such a pellet -enables it to fill all the irregularities of the sides of the hole, and -to securely seal the passage between the sides and the tamping, along -which the gases might otherwise force their way. In coal blasting, soft -shale is always used for tamping, because it is ready at hand, and heavy -shots are not required. - -Broken brick constitutes a fairly good tamping material, especially when -tempered with a little moisture; but as it is not readily procurable, -its application is necessarily limited. The dust and chippings of the -excavated rock are largely employed as tamping in quarries. This -material, however, has but little to recommend it for the purpose beyond -its readiness to hand. - -It now remains to consider what means are available for remedying the -defect inherent in sand as a tamping material. This constitutes a very -important practical question, because if the defect can be removed, sand -will constitute by far the most suitable material whenever the bore-hole -has a downward direction. It can be everywhere obtained at a low cost; -it may be poured into the hole as readily as water; and its application -gives rise to no danger. Obviously the difficulty will be overcome if we -can find suitable means for preventing the gases from penetrating the -sand. - -The end proposed may be successfully attained by means of the plastic -clay pellet applied in the following manner. Immediately above the -charge, place a handful of perfectly dry and very fine sand. This may be -obtained by sifting, if not otherwise procurable. Upon this sand, force -firmly down with a wooden rammer, so as to fill every irregularity, a -plastic clay pellet, about four inches in length, and of the same -diameter as the bore-hole, prepared by rolling between the hands in the -manner already described. Above this pellet, fill the hole with dry -sand. The impervious nature of the clay prevents the gases from reaching -the sand, except along the line of junction of the clay with the sides -of the hole. Tamped in this way, a resistance is obtained scarcely, if -at all, inferior to that opposed by the most carefully placed dried -clay. - -By the employment of a detonator, the defect due to the porous character -of sand is not removed, but its influence is greatly diminished. When -detonation is produced in an explosive compound, the full force of the -elastic gases is developed instantaneously; and it has already been -shown that, under such conditions, the resistance occasioned by the -presence of any substance in the bore-hole, even the air alone, in the -case of nitro-glycerine, is sufficient to throw the chief portion of the -force upon the sides of the hole. Loose sand, therefore, may be -successfully employed as tamping under these conditions, since its -inertia will oppose a sufficient resistance to the escape of the gases. -But though the rock may be dislodged when light tampings are used with -detonation, there can be no doubt that a considerable proportion of the -force of the explosion is lost; and hence it will always be advantageous -to tamp securely by means of the clay pellet, as already described. The -highest degree of economy is to be obtained by detonating the charge, -and tamping in this manner. - - - - -CHAPTER IV. - -THE OPERATIONS OF ROCK BLASTING. - - -_Hand Boring._--When the positions and the directions of the shot-holes -have been determined, the operations of blasting are begun by striking a -few blows with the hammer upon the spot from which the hole is to start, -for the purpose of preparing the surface to receive the drill. In some -cases, this preliminary operation will not be needed; but generally some -preparation is desirable, especially if the surface be smooth, and the -hole be to be bored at an angle with it. For the purpose of -illustration, we will take the case of a hole bored vertically -downwards, and will suppose the boring to be carried on by double-hand. - - -_Boring the Shot-holes._--The surface of the rock having been prepared -to receive the drill, one man sits down, and placing the shortest drill -between his knees, holds it vertically, with both hands. The other man, -who stands opposite, if possible, then strikes the drill upon the head -with the sledge, lightly at first, but more heavily when the tool has -fairly entered the rock. The man who holds the drill raises it a little -after each blow, and turns it partly round, the degree of turn usually -given being about one-eighth of a revolution. By this means, the hole is -kept circular, and the cutting edge of the drill is prevented from -falling twice in the same place. To keep the tool cool, and to convert -the dust and chippings into sludge, the hole is kept partially filled -with water, whenever it is inclined downwards. For this reason, downward -holes are sometimes described as “wet” holes, and upward holes as “dry” -holes. The presence of water greatly facilitates the work of boring. It -has been found by experience that the rate of boring in a dry and in a -wet hole varies as 1 : 1·5; that is, it takes one and a half times as -long to bore a dry hole as to bore a wet hole. Thus, by using water, the -time may be reduced by one-third. To prevent the water from spurting out -at each stroke and splashing the man who holds the drill, a kind of -leathern washer is placed upon the drill immediately above the hole, or -a band of straw is tied round it. When the hole has become too deep for -the short drill, the next length is substituted for it, which is in its -turn replaced by the third or longest drill as the depth becomes -greater. Each drill, on the completion of the length of hole for which -it is intended, is sent away to the smithy to be re-sharpened. In very -hard rock, the drills may have to be frequently changed, a circumstance -that renders it necessary to have several of the same length at hand. -The depth of shot-holes varies from 1 foot to 10 feet, according to the -nature of the rock, the character of the excavation, and the strength of -the explosive to be used. In shafts and in headings, the depth varies -generally between 2 feet 6 inches and 4 feet, a common depth being 3 -feet. - -The débris which accumulates at the bottom of the hole must be removed -from time to time to keep the rock exposed to the edge of the drill. The -removal of this sludge is effected by means of the tool called a -“scraper.” If the sludge is in too liquid a state to allow of its ready -removal by this means, a few handfuls of dust are thrown in to render -the mass more viscous. The importance of keeping the bore-hole clear of -sludge, and of shortening the time expended in using the scraper, has -led, in some localities, to the adoption of means for rendering the -sludge sufficiently viscous to adhere to the drill. When in this state, -the sludge accumulates around the tool rather than beneath it, the fresh -portion formed pushing the mass upward till it forms a thick coating -upon the drill throughout a length of several inches. When the tool is -withdrawn from the hole, this mass of débris is withdrawn with it; in -this way, the employment of a scraper is rendered unnecessary. This mode -of clearing the bore-hole is commonly adopted by the Hartz miners, who -use slaked lime for the purpose. This lime they reduce to the -consistency of thick paste by the addition of water, and they store it, -covered with water, in a small tin box, which they carry with them to -their work. To use this paste, they take a piece about the size of a -walnut, dilute it with water, and pour it into the bore-hole. This lime -paste is, for the purpose intended, very effective in friable rock, -especially if it be of a granular structure, as sandstone. As the grains -of sand resulting from the trituration of such rocks have no more -tendency to adhere to each other than to the drill, each of them becomes -covered with a coating of lime, which causes them to agglutinate into a -viscous mass possessing sufficient adhesiveness to enable it to cling to -the tool in the manner described. - -When the hole has been bored to the required depth, it is prepared for -the reception of the charge. The sludge is all carefully scraped out to -clear the hole, and to render it as dry as possible. This is necessary -in all cases; but the subsequent operations will be determined by the -nature of the explosive, and the manner in which it is to be used. If -black powder be employed in a loose state, the hole must be dried. This -is done by passing a piece of rag, tow, or a wisp of hay, through the -eye of the scraper and forcing it slowly up and down the hole, to absorb -the moisture. If water is likely to flow into the hole from the top, a -little dam of clay is made round the hole to keep it back. When water -finds its way into the hole through crevices, claying by means of the -“bull” must be resorted to. In such cases, however, it is far more -economical of time and powder to employ the latter in waterproof -cartridges. Indeed, excepting a few cases that occur in quarrying, -gunpowder should always be applied in this way. For not only is a -notable saving of time effected by avoiding the operations of drying the -hole, but the weakening of the charge occasioned by a large proportion -of the grains being in contact with moist rock is prevented. But besides -these advantages, the cartridge offers security from accident, prevents -waste, and affords a convenient means of handling the explosive. It may -be inserted as easily into upward as into downward holes, and it allows -none of the powder to be lost against the sides of the hole, or by -spilling outside. These numerous and great advantages are leading to the -general adoption of the cartridge. - - -_Charging the Shot-holes._--When the hole is ready to receive the -explosive, the operations of charging are commenced. If the powder be -used loose, the required quantity is poured down the hole, care being -taken to prevent the grains from touching and sticking to the sides of -the hole. This precaution is important, since not only is the force of -the grains so lodged lost, but they might be the cause of a premature -explosion. As it is difficult to prevent contact with the sides when the -hole is vertical, and impossible when it is inclined, recourse is had to -a tin or a copper tube. This tube is rested upon the bottom of the -hole, and the powder is poured in at the upper end; when the tube is -raised, the powder is left at the bottom of the hole. In horizontal -holes, the powder is put in by means of a kind of spoon. In holes that -are inclined upwards, loose powder cannot be used. When the powder is -used in cartridges, the cartridge is inserted into the hole and pushed -to the bottom with a wooden rammer. - -If the charge is to be fired by means of a squib, a pointed metal rod, -preferably of bronze, of small diameter, called a “pricker,” is placed -against the side of the bore-hole, with its lower pointed end in the -charge. The tamping is then put in, in small portions at a time, and -firmly pressed down with the tamping iron, the latter being so held that -the pricker lies in the groove. The nature of tamping has been already -fully described. When the tamping is completed, the pricker is -withdrawn, leaving a small circular passage through the tamping down to -the charge. Care must be taken in withdrawing the pricker not to loosen -the tamping, so as to close up this passage. A squib is then placed in -the hole thus left, and the charge is ready for firing. - -If the charge is to be fired by means of safety fuse, a piece -sufficiently long to project a few inches from the hole is cut off and -placed in the hole in the same position as the pricker. When the powder -is in cartridges, the end of the fuse is inserted into the cartridge -before the latter is pushed into the bore-hole. The fuse is held in its -position during the operation of tamping by a lump of clay placed upon -the end which projects from the hole, this end being turned over upon -the rock. The tamping is effected in precisely the same manner as when -the pricker is used. - -If the charge is to be fired by electricity, the fuse is inserted into -the charge, and the wires are treated in the same way as the safety -fuse. When the tamping is completed, the wires are connected for firing -in the manner described in a former chapter. - -In all cases, before tamping a gunpowder charge placed loose in the -hole, a wad of tow, hay, turf, or paper is placed over the powder -previously to putting in the tamping. If the powder is in cartridges, a -pellet of plastic clay is gently forced down upon the charge. Heavy -blows of the tamping iron are to be avoided until five or six inches of -tamping have been put in. - -When gun-cotton is the explosive agent employed, the wet material which -constitutes the charge is put into the shot-hole in cartridges, one -after another, until a sufficient quantity has been introduced. Each -cartridge must be rammed down tightly with a wooden rammer to rupture -the case and to make the cotton fill the hole completely. A length of -safety fuse is then cut off, and one end of it is inserted into a -detonator cap. This cap is fixed to the fuse by pressing the open end -into firm contact with the latter by means of a pair of nippers -constructed for the purpose. The cap, with the fuse attached, is then -placed into the central hole of a dry “primer,” which should be well -protected from moisture. When an electric fuse is used, the cap of the -fuse is inserted in the same way into the primer. The primer is put into -the shot-hole and pushed gently down upon the charge. As both the dry -gun-cotton and the detonator may be exploded by a blow, this operation -must be performed with caution. - -Cotton-powder or tonite requires a somewhat different mode of handling. -It is made up in a highly compressed state into cartridges, having a -small central hole for the reception of the detonator cap. This cap, -with the safety fuse attached in the way described, or the cap of the -electric fuse, is inserted into the hole, and fixed there by tying up -the neck of the cartridge with a piece of copper wire placed round the -neck for that purpose. The cartridge is then pushed gently down the -shot-hole, or, if a heavier charge is required, a cartridge without a -detonator is first pushed down, and the “primed” cartridge put in upon -it. No ramming may be resorted to, as the substance is in the dry state. - -When dynamite is the explosive agent used, a sufficient number of -cartridges is inserted into the shot-hole to make up the charge -required. Each cartridge should be rammed home with a moderate degree -of force to make it fill the hole completely. Provided a wooden rammer -be employed, there is no danger to be feared from explosion. A detonator -cap is fixed to the end of a piece of safety fuse, and, if water tamping -is to be used, grease, or white-lead, is applied to the junction of the -cap with the fuse. A “primer,” that is, a small cartridge designed to -explode the charge, is then opened at one end, and the detonator cap, or -the cap of the electric fuse, is pushed into the dynamite to a depth -equal to about two-thirds of its length, and the paper covering of the -primer is firmly tied to the cap with a string. If the cap be pushed too -far into the dynamite, the latter may be fired by the safety fuse, in -which case the substance is only burned, not detonated. With an electric -fuse this cannot occur. The same result ensues if the cap be not in -contact with the dynamite. The object of tying in the cap is to prevent -its being pulled out. The primer thus attached to the fuse is then -pushed gently down upon the charge in the shot-hole. It should be -constantly borne in mind that no ramming may take place after the -detonator is inserted. - -Gun-cotton and tonite require a light tamping. This should consist of -plastic clay; or sand may be used in downward holes. The tamping should -be merely pushed in, blows being dangerous. A better effect is obtained -from dynamite when tamped in this way than when no tamping is used. In -downward holes, water is commonly employed as tamping for a dynamite -charge, especially in shaft sinking, when the holes usually tamp -themselves. But in other cases, it is a common practice to omit the -tamping altogether to save time. - - -_Firing the Charges._--When all the holes bored have been charged, or as -many of them as it is desirable to fire at one time, preparation is made -for firing them. The charge-men retire, taking with them the tools they -have used, and leaving only him of their number who is to fire the -shots, in the case of squibs or safety fuse being employed. When this -man has clearly ascertained that all are under shelter, he assures -himself that his own way of retreat is open. If, for example, he is at -the bottom of a shaft, he calls to those above, in order to learn -whether they be ready to raise him, and waits till he receives a reply. -When this reply has been given, he lights the matches of the squibs or -the ends of the safety fuse, and shouts to be hauled up; or if in any -other situation than a shaft, he retires to a place of safety. Here he -awaits the explosion, and carefully counts the reports as they occur. -After all the shots have exploded, a short time is allowed for the fumes -and the smoke to clear away, and then the workmen return to remove the -dislodged rock. If one of the shots has failed to explode, fifteen or -twenty minutes must be allowed to elapse before returning to the place. -Nine out of ten of the accidents that occur are due to these delayed -shots. Some defect in the fuse, or some injury done to it, may cause it -to smoulder for a long time, and the blaster, thinking the shot has -missed, approaches the fuse to see the effects produced by the shots -that have fired. The defective portion of the fuse having burned -through, the train again starts, and the explosion takes place, probably -with fatal consequences. Thus missed shots are not only a cause of long -delays, but are sources of great danger. Accidents may occur also from -premature explosion. In this case, the fuse is said to “run,” that is, -burn so rapidly that there is not sufficient time for retreat. - -[Illustration: FIG. 50.] - -When the firing is to take place by means of electricity, the man to -whom the duty is entrusted connects the wires of the fuses in the manner -described in a former chapter, and as shown in Fig. 50. He then connects -the two outer wires to the cables, and retires from the place. Premature -explosion is, in this case, impossible. When he has ascertained that all -are under shelter, he goes to the firing machine, and, having attached -the cables to the terminals, excites and sends off the electric current. -The shots explode simultaneously, so that only one report is heard. But -there is no danger to be feared from a misfire, since there can be no -smouldering in an electric fuse. The face may, therefore, be approached -immediately, so that no delay occurs, and there is no risk of accident. -Moreover, as all the holes can be fired at the moment when all is in -readiness, a considerable saving of time is effected. It is essential to -the success of a blast fired by this means that a sufficient charge of -electricity be generated to allow for a considerable loss by leakage. If -Siemens’ large dynamo-machine be used, the handle should be turned -slowly till a click is heard inside, and then, not before, the cable -wires should be attached to the terminals. To fire, the handle must be -turned as rapidly as possible, a jerky motion being avoided. As -considerable force is required, the machine must be firmly fixed. If a -frictional machine be used, care must be had to give a sufficient number -of turns. As this kind of machine varies greatly, according to the state -of the rubbing surfaces and the degree of moisture in the atmosphere, it -should always be tested for a spark before firing a blast. In this way -only, can the number of turns required be ascertained. It is important -that the discharging knob should be pushed in, or, as the case may be, -the handle turned backward, suddenly. A slow motion may be fatal to the -success of a blast. In testing Bornhardt’s machine, the handle should -always be turned forwards; but in firing, half the number of turns -should be given in one direction and half in the other. The following -table shows the number of turns required for a given number of André’s -fuses with Bornhardt’s machine. The first column, containing the least -number of turns, may be taken also for Julian Smith’s machine as -manufactured by the Silvertown Company with the modifications suggested -by W. B. Brain. - -FIRING TABLE FOR FRICTIONAL MACHINE. - - -----------------+----------------+----------------+---------------- - | When the | When the | When the - | Machine sparks | Machine sparks | Machine sparks - | with 10 Turns. | with 12 Turns. | with 14 Turns. - -----------------+----------------+----------------+---------------- - Fuses in Circuit.|Number of Turns.|Number of Turns.|Number of Turns. - -----------------+----------------+----------------+---------------- - 4 | 12 | 15 | 17 - 5 | 12 | 15 | 17 - 6 | 14 | 17 | 20 - 7 | 16 | 19 | 22 - 8 | 18 | 22 | 25 - 9 | 20 | 24 | 28 - 10 | 22 | 26 | 31 - 11 | 24 | 28 | 34 - 12 | 25 | 30 | 35 - 13 | 26 | 31 | 36 - 14 | 27 | 33 | 38 - 15 | 28 | 34 | 39 - -----------------+----------------+----------------+---------------- - - NOTE.--If the machine does not spark with 14 turns, the rubber should - be taken out and brushed. - -Places of refuge, called man-holes, are often provided in headings for -the blaster to retire into; these man-holes are small excavations made -in the sides of the heading. Sometimes it is necessary to erect a shield -of timbers in the heading for the protection of the men; such a shield -is frequently needed to protect machine drills from the effects of a -blast. In Belgium, it is a common practice to provide man-holes in the -sides of a shaft as places of retreat for the men; these holes are -called _caponnières_. Instead of caponnières, a hollow iron cylinder is -sometimes used as a protection to the men. This cylinder is suspended in -the shaft at a height of a few yards from the bottom, and is lowered as -the sinking progresses. The men climb into this cylinder to await the -explosion of the shots beneath them. - -The workmen, on returning to the working face, remove the dislodged -rock, and break down every block that has been sufficiently loosened. -For this purpose, they use wedges and sledges, picks, and crowbars. And -not until every such block has been removed, do they resume the boring -for the second blast. Sometimes, to facilitate the removal of the rock -dislodged by the shots, iron plates are laid in front of the face in a -heading. The rock falling upon these plates is removed as quickly as -possible, to allow the boring for the succeeding blast to commence. It -is important, in the organization of work of this character, that one -gang of men be not kept waiting for the completion of the labour of -another. - -MACHINE BORING.--In machine drilling, the operations necessarily differ -somewhat in their details from those of hand boring, and, in some cases, -other methods of procedure will be adopted more suitable to the -requirements of machine labour. It may even be, and in most cases indeed -is, inexpedient to follow closely the principles which lead to economy -of the explosive substance employed, since the more restricted -conditions under which machine power may be applied may point to more -important gains in other directions. Thus it may be found more conducive -to rapidity of execution to determine the position and the direction of -the shot-holes rather to satisfy the requirements of the machine than -those of the lines of least resistance; or, at least, these requirements -must be allowed to have a modifying influence in determining those -positions and directions. For it is obvious that holes cannot be angled -with the same ease when a machine drill is used, as they can when the -boring is executed by hand. - - -_Boring the Shot-holes._--It has already been remarked that the -exigencies of machine labour render it impracticable to follow closely -the principles which lead to economy of labour and material in blasting. -In hand boring, economy is gained by reducing to a minimum the number of -holes and the quantity of explosive substance required. But in machine -boring, economy is to be sought mainly in the reduction of the time -needed to accomplish the driving. - -Attempts have been made to assimilate the methods of machine boring to -those adopted for hand labour, but the results have not been -satisfactory. On the contrary, the conditions determining the position -and the direction of the holes relatively to the production of the -greatest useful effect have been wholly ignored in favour of those which -determine the most rapid boring. This system has been attended with more -satisfactory results. Another system, partaking of both the preceding, -is widely adopted, and hitherto the best results have been obtained from -this, which may be regarded as a compromise between conflicting -conditions. Thus we have three systems of executing machine boring: one -in which a single machine is used upon a support capable of holding it -in any position, so as to be able to bore at any angle, and in which the -holes are placed according to the lines of least resistance, as in hand -boring. A second, in which several machines are fixed upon a heavy -support, allowing but little lateral or angular motion, and in which the -holes are placed at regular intervals apart, and bored parallel, or -nearly parallel, with the axis of the excavation, irrespective of the -varying nature of the rock, and the lines of least resistance. And a -third, in which it is sought, by the employment of one, two, or at the -most three machines, upon a simple and light support allowing the -position and direction of the machine to be readily changed, to satisfy -in some degree the two sets of conditions determining the two former -systems, by placing the shot-holes as far in accordance with the lines -of resistance as the exigencies of a fairly rapid handling of the -machine will allow. - -In the first of these systems, the necessity for extreme lightness in -the machine is unfavourable to its efficient action, and the great -length of time consumed in changing the position of the machine, so as -to comply with the conditions of resistance in the rock, render it -impossible to attain a much higher rate of progress than is reached by a -well-regulated system of hand boring. With such a result, there is -nothing to compensate the first cost of the machinery, or in any way to -justify its adoption. In the second system, the time consumed in -removing and fixing the machines is reduced to a minimum, and the chief -portion of the time during which the machines are at the working face -is, consequently, occupied in actual boring, a circumstance that is -highly favourable to machine labour. Hence the rate of progress attained -by this system is greatly in excess of that accomplished by hand labour; -and this superiority has led to the adoption of the system in several -important cases, and has also led many to regard it as the most -favourable to the exigencies of machine drilling. But as the holes are -bored to suit the requirements of the machine, quite irrespectively of -the resistance of the rock, their positions and directions are very -unfavourable to the action of the explosive. This circumstance -necessitates a much greater number of holes to ensure the fracture of -the rock around each charge, and hence the time saved in shifting the -machines is in part lost in extra boring; besides which, the consumption -of powder is enormously increased. It would, therefore, appear that the -full advantages of machine boring are to be obtained from the -intermediate system, if carried out in accordance with all the -conditions of the case. - -Assuming that we have a machine and a support of such dimensions, -weight, and construction as to be capable of being readily placed in -position, it is evident that we shall still require a much larger number -of holes than would be needed if the boring were performed by hand, -because they are not placed wholly in accordance with the lines of least -resistance. In some parts of the heading, indeed, these lines will have -to be wholly neglected, in order to avoid the loss of time involved in -shifting the supports; for the principle upon which an intermediate -system is based is to fulfil the requirements of the lines of least -resistance, when that can be conveniently done, and to neglect them, -when such fulfilment would involve a considerable expenditure of labour -and time. - -In this way, the time both for fixing and removing the machines and of -boring is reduced to a minimum, and thus two conditions favourable to -rapid and economical progress is ensured. It is evident that when this -system is followed, the face will not require the same number of holes -at each blast. Another circumstance operating to increase the number of -shot-holes is the desirability of bringing down the face in fragments -small enough to be lifted without great difficulty. When the rock is -completely broken up, the labour, and, consequently, the time of -removing it after each blast, are lessened in an important degree. Hence -there will be an advantage in placing the shot-holes sufficiently close -together to ensure the fracture of the mass between each. These -circumstances render it necessary to bore a large number of holes when -the work is done by mechanical means. The boring of the additional holes -reduces the superiority of machine over hand labour, and the additional -quantity of the explosive required augments the cost of the work. To -counterbalance these disadvantages, the shot-holes should be bored deep. -It has already been pointed out that when a hole is once started with a -machine, the rate of progress is immensely superior to that attained in -hand boring, and to profit by this advantage, the hole should be -continued to as great a depth as practicable. This is sufficiently -obvious, since it amounts to increasing the proportion of the whole time -consumed that is occupied in actual boring; for as it is in the -rapidity of the operation of boring alone that the superiority of -machine labour exists, it is plain that the longer the proportion of the -time so occupied, the more marked that superiority will be. Thus, by -increasing the depth of the holes to the farthest practicable limit, we -approximate as much as possible to the condition most favourable to -machine boring. The intermediate system, therefore, which takes full -advantage of this means, will lead to the best results. To recapitulate -the main points of such a system; it should follow the lines of least -resistance when that can be conveniently done, and neglect them when the -fulfilment of their requirements would occasion a considerable -expenditure of time; and to counterbalance the disadvantages of machine -boring, it should employ shot-holes of as great a depth as is -practicable. - -Supposing such a system in use, it now remains to consider the -operations of boring, and the subsequent operations of charging, firing, -and removing the rock dislodged by the blast. Of the method of executing -the boring, little remains to be said. It may, however, be well to -direct attention to the necessity of keeping the holes clear of the -débris. To ensure this, the bits should be chosen of a form suitable to -the nature and the structure of the rock, and the hole kept well -supplied with water. When the hole becomes deep, it should be cleared -out with a scraper during the time of changing the bit, and in very -argillaceous rock it may become necessary sometimes to withdraw the -tool, and to remove the accumulation with the scraper. When the débris -does not work out freely, its escape may be facilitated by giving a slow -motion to the tool, and then suddenly changing to a rapid motion. When -several machines are employed, the maximum number that can be applied -with advantage is one to the square yard of working face. The absolute -number of holes required in any case, will, of course, depend upon the -tenacity of the rock and the development of the joint planes, and also, -in some degree, by the lines of fracture due to the preceding blast. The -same circumstance will determine the distribution of the holes. Leaving -minor variations out of account, however, the same distribution will be -adhered to throughout the driving. - -The manner of distributing the holes over the face of the heading may be -varied according to the judgment of the engineer in charge; that is, the -general features of the distribution to be adopted may be chosen to suit -the requirements of the machines and their supports. Also, it should be -noted that one method of distributing the shot-holes will require a less -number of them than another. Some examples will be found on Plate IX., -where there are represented the Göschenen end of the St. Gothard tunnel; -the Airolo end of the same tunnel; the face of a stone drift driven at -Marihaye; that of a similar drift at Anzin; and that of a drift of the -same character at Ronchamp; the latter three examples being typical of -the distribution adopted in the French collieries. - -The same mode of unkeying the face is adopted with machine as with hand -boring. Generally, two parallel rows of holes, from two to five in a -row, are bored in the middle of the face or fore-breast, the rows being -from 18 inches to 30 inches, according to the strength of the rock, -apart on the surface, and angled so as to be from 9 inches to 15 inches -apart at the bottom. These shots unkey the fore-breast; and it is -greatly conducive to a successful accomplishment of the operation, to -fire these shots simultaneously. Sometimes, when dynamite is used, -another method is adopted. A hole is bored horizontally in the centre; -at about three inches distant, are bored three other holes at an equal -distance apart. These latter are heavily charged with dynamite, the -centre hole being left empty. When these charges are fired, the rock -between them is crushed, and a large hole made. The lines of fracture of -the subsequent shots run into this hole. In this case, it is even more -desirable than in the preceding to fire the central shots -simultaneously. - -In shaft sinking, if the strata are horizontal or nearly so, it is usual -to unkey from the centre, as in the heading. But if they be highly -inclined, it will be better to unkey from one side of the excavation. -The water which flows into the workings must be collected into one -place, both for convenience in raising it, and for the purpose of -keeping the surface of the rock clear for the sinkers. The depression -caused by the removal of the key serves to collect the water, and, on -that account, it is called “the sump.” Into this sump, the tub dips, or, -when pumps are used, the suction pipe drops. When the strata are highly -inclined, the water gravitates towards the dip side of the excavation, -and it becomes, therefore, necessary to place the sump in that -situation. The unkeying of the rock from this direction is, moreover, -favourable to the effect of the shots. In putting in the shot-holes, it -is well to avoid, as far as possible, terminating them in, or nearly in, -a bedding plane, because when so terminated, the force of the charge -expends itself along this plane. The position and the direction of the -holes will, however, be determined in some degree by the character of -the support used for the drills, and by other conditions of convenience. - - -_Charging and Firing._--The operations of charging the holes and firing -the shots demand particular attention when machine labour is employed. -It has been pointed out in the foregoing paragraphs that holes bored by -machine drills cannot be placed or directed strictly in accordance with -the requirements of the lines of least resistance; but that, on the -contrary, these requirements can only be approximately complied with, -and in some cases must be wholly neglected. To compensate in some degree -this defect of machine labour, the strength of the charges should be -varied according to the resistance which they will be required to -overcome. That is, the principles of blasting described in a former -chapter, which cannot be complied with by the borer, should be strictly -followed by the blaster in apportioning his charges. By this means, a -great saving of the explosive compound may be effected, and that without -difficulty or loss of time, if the blaster be intelligent and understand -his work. A glance will be sufficient to show what charge a given hole -of a known depth will require, and as cartridges of different sizes are -ready at hand, no delay is occasioned in making up the charge. The holes -in the centre, which are intended to unkey the face, require, of course, -the heaviest charge, since the conditions are there most unfavourable to -the effects of the explosion. And the more complete is the unkeying -resulting from this first explosion, and the more fractured and jointed -is the rock surrounding the cavity thus formed, the more may the charges -placed behind these unsupported faces be reduced. - -As economy of time is, in machine boring, the chief end to be attained, -the tamping should be done with dried clay pellets previously prepared. -This material gives the greatest resistance, and thereby ensures the -maximum of useful effect; and if prepared beforehand, in the manner -described in the preceding chapter, the time consumed in tamping will be -reduced to a minimum. An abundant supply of such pellets should always -be ready at hand. In downward holes, such as are used in shaft sinking, -the plastic clay pellet and sand may be employed. This tamping may be -put in very rapidly, and, in all but very shallow holes, it is very -effective. When it is desired to use sand tamping in horizontal holes, -and holes bored in an ascending direction, the sand should be made up in -paper cartridges. The tamping employed in the St. Gothard tunnel -consisted of sand prepared in this manner. At the Mont Cenis tunnel, an -argillaceous earth was similarly prepared in paper cartridges for -tamping. - -Firing the charges also affords an occasion for the exercise of -knowledge and judgment. A skilful determination of the order in which -the charges are to be fired will in a great measure compensate the ill -effects of badly-placed holes. The firing of a shot leaves the -surrounding rock more or less unsupported on certain sides; and it is -evident that to profit fully by the existence of these unsupported -faces, the succession of explosions must be regulated so that each shall -have the advantage of those formed by the preceding shots. This -condition can be wholly fulfilled only by simultaneous firing; but when -the firing is to take place successively, the condition may be -approximated to by regulating the succession according to the -indications observed on a careful inspection of the rock. Before firing -the charges, the blaster should consider the relative positions of the -holes, the stratification and jointing of the rock, the fissures caused -by the preceding blast, and any other circumstances that may influence -the results. The charges intended to unkey the face will be fired first, -and those in the concentric series will be then fired, in the order -determined upon, by means of different lengths of fuse. The series will -follow each other from the centre outwards. When a large number of shots -regularly placed in series have to be fired, a convenient practical -means of ensuring the successive explosion of the series, in the case of -the whole being lighted simultaneously, consists in bringing the fuses -from all the shot-holes together to one point at the centre. This method -of regulating the length of the fuses was adopted at the St. Gothard -tunnel. - -It is obvious that the acceleration of the labour of excavation, which -has been effected in so remarkable a degree by the introduction of -machine drills and strong explosives, may be still further promoted by -the adoption of electricity as the firing agent. The advantages of -firing a number of shots simultaneously, some of which have already been -pointed out, are great and manifest. In the case of a driving, for -example, when all the holes have been bored and charged, and the -machines withdrawn, it is clearly desirable to blast down the face as -quickly and as effectively as possible. If the whole of the shots can -be fired at once, the time is reduced to a minimum, and, consequently, -the maximum of progress in a given time is ensured. Electricity affords, -indeed, the most convenient, the most effective, and the most safe means -of firing blasts. Hofrath Ritter von Pischof, the Austrian Chief -Inspector of Railways, in one of his reports, says:--“A greatly -increased amount of work and a notable saving of cost are effected when -the shots can be so disposed and fired as to mutually aid one another. -These results are obtained by employing electricity as the firing agent. -The experience which has been gained at the Büchenberg cutting, where -electrical firing has been extensively adopted, has shown that, when -properly employed, this means allows, in comparison with the ordinary -methods, twice the amount of work to be performed in a given time. It is -therefore highly desirable to adopt electrical blasting whenever it is a -question of economy of time and money.” - - -_Removing the dislodged Rock._--As the removal of the rock brought down -by the blast consumes a large proportion of the time saved by machine -boring, it becomes necessary to provide means for reducing this loss to -a minimum. The most important of these means is a suitable provision for -the rapid removal of the machine to a place of safety, and a -conveniently designed and well-laid tramway, upon which the rock may be -quickly run out without confusion and its consequent delay. The number -of wagons required to remove a given cube of rock may be readily -ascertained, and sufficient provision should be made for the transport -of these to “day” in the most rapid succession. The wagons should be of -such dimensions as to be capable of being handled without great -difficulty; the importance of this condition will be understood when the -frequency of derailments is borne in mind. The shovelling up of the -rubbish is greatly facilitated by laying iron plates in front of the -face to be brought down previously to the firing of the blast. This -expedient is often adopted in important drivings. It has also been -remarked that the dislodged rock can be more rapidly removed when it -exists in small blocks. Thus there will be an advantage in placing the -charges and in regulating their strength so as to completely break up -the rock. Another matter of importance in the arrangements for the rapid -removal of the rock brought down by the blast, is the proportioning of -the number of hands employed to the requirements of the case. This -number will increase with the size of the blocks to be lifted, the -distance to be run over, and the want of suitability in the _matériel_ -employed. - - -_Division of Labour._--A proper division of labour is greatly conducive -to rapid and economical progress. The operations may be divided into -three series, namely: boring the shot-holes, charging and firing, and -removing the rock dislodged. Each of these series of operations may be -performed by different sets of men, and in several instances this -division of labour has been adopted. But it does not appear that such a -division leads to the most satisfactory results. The work of boring -occupies a much longer time than either of the other two series of -operations, and hence the distribution of the time is unequal. It has -been found that, generally, where all the arrangements have been well -considered, the labour of charging the shot-holes, firing the blast, and -removing the rock brought down, can be performed in about the same time -as that of boring. Thus it would seem to be more conducive to economy of -time to divide the men employed into only two sets: one set to bore the -holes, the other to perform all the subsequent operations. This division -has been adopted in numerous instances with favourable results. -Sometimes the whole of the operations have been performed by the same -set; but such an arrangement is not to be recommended. The labour of -directing the machines is of too distinct and skilled a character to be -confounded with that of removing the débris, without a strong reason for -such a proceeding, which does not appear to exist. Besides reserving a -set of men specially for this portion of the work, it is desirable to -keep the same men to the same machine, for in such a case each man gets -accustomed to the peculiarities of the machine entrusted to him, and -besides conceives a kind of affection for it that leads to careful -handling and watchful attention. In addition to the men required for the -operations referred to above, smiths will be needed to re-sharpen the -bits and to repair the machines. The amount of this labour will -obviously depend upon the number of machines employed, and the hardness -of the rock to be passed through. - - -EXAMPLES OF DRIVINGS. - - -_The St. Gothard Tunnel._--The St. Gothard tunnel is driven in five -sections. First, the “heading” is driven at the roof level 6 feet 6 -inches wide, and 7 feet high. The position of the holes is shown in the -drawings on Plate IX. The number of holes at the Göschenen end is 28, -and the depth about 40 inches. The shots are fired by means of safety -fuse, the ends of the fuse being brought together at the centre. This -arrangement causes the shots to explode in the proper order of -succession. At a certain distance back from the face, is the “right -enlargement;” this is a widening of the heading to the limits of the -tunnel in that direction. Farther back is the “left enlargement,” by -which the heading is widened to the full width of the tunnel. Still -farther back is the first “bench cut,” in which one half of the floor is -blasted out to the full depth of the tunnel, and behind this again is -the second bench cut, in which the remaining half is removed. The -boring machines employed are the Dubois-François, the McKean, and the -Ferroux. The explosive agent used is dynamite. The rock is a tough -granite. - - -_The Hoosac Tunnel._--At the west end of the Hoosac tunnel, the system -adopted was the following. First, a centre cut was made by drilling two -rows of five or six holes each, about 9 feet apart on the face, and -converging to about 3 feet at their lower ends. The depth of these holes -was from 9 to 12 feet, according to the hardness of the rock. These -holes are numbered from 1 to 11 on Plate X. They were charged with -nitro-glycerine, and fired by electricity, Mowbray’s frictional machine -being used. As soon as the rock had been removed, the next series of -fourteen holes, numbered from 12 to 25, were drilled. These holes were -then charged and fired simultaneously like those of the first series. -When the rock dislodged had been removed, the third series of holes, -numbered from 26 to 41, were bored. This series, like the other two, -were charged, and fired by electricity. The effect of these three -blasts, which were fired within twenty-four hours, was to advance the -heading, 9 feet in height by the full width of 24 feet, to the extent of -7 feet 6 inches. The drawings on Plate XI. are: an elevation of the -fore-breast, which shows the positions of the shot-holes; a sectional -plan, which shows the directions of the first series of holes; a similar -plan, showing the directions of the second series of holes, and the -centre cut removed; and a sectional plan of the heading after the second -series have been fired, showing the direction of the third series of -holes. - -The operations of taking out the “bench” were carried on at a distance -of about 170 yards back from the fore-breast. This was effected by first -drilling six holes 7 feet deep; two of these were each about 4 feet from -the face of the bench and close to the side of the tunnel, whilst two -others were each 4 feet behind these first holes, and the remaining two -holes were 8 feet from the face, 8 feet from the sides of the tunnel, -and 8 feet from each other. These were fired simultaneously, the result -being to lower the bench about 7 feet throughout the full width of the -tunnel. At a safe distance beyond this first bench cut, the same -operations were carried on by another gang of men, whereby the bench was -lowered to the floor of the tunnel, the full area of 24 feet in width by -22 in height being thus completed. The rock was a moderately tough -granite. - - -_The Musconetcong Tunnel._--The heading of the tunnel, shown on Plate -XII., like that of the Hoosac, was driven to the full width of the -tunnel. It is clear from theoretical considerations, and experience has -confirmed the conclusions, that the method of taking, with machine -drills, the whole width of the excavation at once conduces to rapidity -of advance, and to economy of explosive. In the example under -consideration, three tram lines were laid up to the face. The carriages -carrying the drills were run upon the two outside lines. These carriages -were simply stout frameworks of oak, each having in front three -horizontal iron bars, on which the drills were clamped in a way that -ensured easy lateral and vertical motion. After the firing of a blast, -all hands were set to shovel the dislodged rock into the middle between -the machine lines for the purpose of clearing the latter as soon as -possible to make way for the machines to be brought up for the next -boring. The lines being thus cleared, drilling was recommenced, and the -broken rock removed in wagons upon the centre line of rails. The heading -being 26 feet wide, there was ample room, and, a convenient system of -switching having been adopted, no delay was occasioned by a want of -wagons. - -The system followed was that of centre cuts, and subsequent squaring up. -It consists in first blasting out an entering wedge or “key,” about 10 -feet deep in this case, in the centre, and afterwards squaring up the -sides by several blasts. In the Musconetcong heading, twelve holes were -first drilled, as shown in the drawing, and marked C, A being the floor -of the heading. These holes were drilled with from 1½-inch to 2¾-inch -“bits,” in two rows of six, 9 feet apart on the face, and angled to meet -at the bottom. They were charged with 25 lb. of No. 1 and 50 lb. of No. -2 dynamite, and fired simultaneously by electricity. The No. 1 dynamite -was used in the bottom of these centre holes; in all the subsequent -blasts in squaring up, No. 2 only was used. - -As soon as the cut was out, a second round of holes was started for the -first squaring up, as shown in the drawings, where they are numbered 1, -1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2, -&c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course, -not so great as in the cut. In the first and the second squaring-up -rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third -round, this quantity was increased to 80 lb. or 90 lb., the resistance -becoming greater as the roof arch falls at the sides. In this third -round, there were generally one or two additional roof holes; these are -not shown in the drawing, as their position varied, according to the lay -of the rock. The top holes in the first round are also intended to bring -down any roof not shaken by the cut, and these are therefore angled -sharply towards the centre, and bored from 12 feet to 14 feet deep. In -the plan, Plate XII., the number 3 indicates the cut holes, and 4, 5, -and 6, the squaring-up rounds. The holes of the first squaring round -were always drilled about a foot deeper than the cut holes; when -blasted, these generally brought out an additional foot of shaken rock -at the apex of the cut. The following table shows approximately the -number and the depth of the holes required, and the quantity of dynamite -used for a linear advance of 10 feet. - - ---------------------+------+---------+------+------+------ - | | |Total | | - |No. of| Depth |Depth | | - |Holes.|of Holes.| of |No. 1.|No. 2. - | | |Holes.| | - +------+---------+------+------+------ - | | ft. in. | ft. | lb. | lb. - Cut | 12 | 10 6 | 126 | 25 | 50 - 1st square up | 8 | 12 0 | 96 | .. | 55 - 2nd „ | 8 | 12 0 | 96 | .. | 55 - 3rd „ | 6 | 12 0 | 72 | .. | 85 - Additional roof holes| 2 | {10 0} | 18 | .. | .. - | | { 8 0} | | | - +------+---------+------+------+------ - | 36 | .. | 408 | 25 | 245 - ---------------------+------+---------+------+------+------ - -The cut holes being 10 feet 6 inches deep, the blast usually brought out -about 9 feet full, which, as explained above, was increased to 10 feet -in the subsequent rounds. The cross section being about 175 square feet, -in an advance of 10 linear feet, there are about 65 cubic yards of rock -to be broken; this gives on an average 0·4 lb. of No. 1 and 4 lb. of No. -2 dynamite, and a little over 6 feet of drilling per cubic yard. - -The “bench” was kept from 150 yards to 200 yards back from the face of -the heading, to avoid interruptions from the heading blasts, and to -allow plenty of room for handling the wagons, and for running back the -machines to a safe distance, previously to firing. The system adopted in -removing the bench is shown on Plate XII. First, six top holes, from 12 -feet to 13 feet deep, were drilled and blasted; their relative positions -are shown in the drawings, A being the centre line, B, the sides in the -enlargement, B′, the sides of the heading, C, the face of the bench, and -1, 2, 3, 4, 5, 6, the holes. These six holes lifted the greater portion -of the rock; what was left was broken by several horizontal holes. These -two sets of holes, at the top and at the bottom, gave an average advance -of about 9 feet. The following table shows, for that advance, the number -of feet drilled, and the quantity of dynamite burned. - - ------------+------+------+---------+--------- - | |Depth | Total | - |No. of| of | Depth | No. 2 - |Holes.|Holes.|of Holes.|Dynamite. - +------+------+---------+--------- - | | ft. | ft. | lb. - Top holes | 6 | 12 | 72 | 62 - Bottom holes| 4 | 10 | 40 | 45 - +------+------+---------+--------- - Totals | 10 | 22 | 112 | 107 - ------------+------+------+---------+--------- - -The sectional area of the bench being about 306 square feet, an advance -of 9 linear feet gives about 102 cubic yards of rock to be removed. The -quantity of dynamite used was therefore 1·05 lb., and the depth of -boring 1·1 foot, per cubic yard of rock broken. - -Three machines were used at this bench, two on the top and one below. -The holes were commenced with 2¾-inch bits, and terminated by 1½-inch -bits. The rock was a tough syenite. - - - - -CHAPTER VI. - -SUBAQUEOUS BLASTING. - - -_Preparation of the Charge._--It is essential to the success of -subaqueous blasting operations, that the explosive substance used should -be suitable to the conditions under which it is to be applied. This is -true of all blasting, but the requirement is frequently overlooked in -some of the operations that have to be performed under water. In -clearing a wreck for salvage purposes, gunpowder will in most cases act -more effectually than either gun-cotton or dynamite. Also, in many -cases, this compound will prove more suitable than the stronger -substances in removing obstructions in water-courses. Examples of this -will be given hereafter. But when a wreck has to be broken up, when -piles, or objects of a similar character, have to be removed, or when -rocks have to be blasted, the more violent compounds will be found to -accomplish the purpose much more effectively. Generally, it may be -stated that when it is required merely to _remove_ objects, gunpowder is -the most suitable explosive agent to employ; and that when it is -required to _break_ objects, the nitro-cotton and the nitro-glycerine -compounds are the agents whose application is likely to be attended -with the greatest degree of success. - -When gunpowder is used, means must be adopted to protect it from the -water, since a small proportion of moisture is sufficient to lessen, in -a very important degree, the force developed, while a large proportion -of moisture will destroy altogether its explosive properties. It is no -easy matter, under the most favourable circumstances, to keep the water -from the charge; but when the depth of water is considerable, it becomes -very difficult to attain that object. The pressure of a considerable -“head” will force the water through substances that, without a pressure, -are sufficiently impervious. At ordinary depths, metal canisters are -usually employed to contain gunpowder. Old oil-cans are as good as -anything for this purpose. The fuse, whether safety or electric, is -passed through the cork, and the latter is luted with some waterproofing -composition. The best consists of: - - Tallow 1 part. - Rosin 3 parts. - Guttapercha 4 parts. - Swedish pitch 12 parts. - -Instead of metal canisters, indiarubber bags are sometimes used. These -are, however, more expensive than the oil-cans, and, in many cases, they -are scarcely more efficient or suitable. Small charges of gunpowder may -be put into short lengths of indiarubber tubing, so as to form a kind -of cartridge. But care must be taken to close the ends securely. The -best way is to insert a cork, or if that cannot be obtained, a -cylindrical piece of wood, and to tie the tubing to this very tightly -with twine. The ends should then be dipped into the luting composition -described above. Tubing suitable for this purpose is sold under the -designation of “blasting tubes.” For large blasts, wooden casks are the -most suitable receptacle for the charge. The casks should be well -tarred, or, if the depth of water be great, laid over with pitch applied -very hot. Great care must be taken to protect the aperture through which -the safety fuse, or the wire of the electric fuse, passes. - -In blasting under water with gunpowder, only the best and strongest -qualities of that compound should be used. The extra strong mining -powder of the Messrs. Curtis’s and Harvey’s, commercially known as the -E.S.M. powder, is, of all, the most suitable. It is also highly -conducive to success to detonate the charge. If the charge be not -detonated, the enclosing vessel is ruptured when only a small proportion -of the number of grains have been ignited, and, consequently, a large -proportion of the charge is blown away into the water unburned. Were -gunpowder in blasting charges always fired by a detonation, it would -compare in its effects far more favourably with the nitro-cotton and the -nitro-glycerine compounds than it does under the circumstances -attending the common method of firing it. - -When gun-cotton is used, the difficulty of waterproofing is much -lessened, but not wholly removed. Inasmuch as this compound may be -detonated in the wet state, it is not required to take those precautions -which are necessary in the case of gunpowder. But, as we have pointed -out in a former chapter, the detonation of wet gun-cotton is effected by -means of that of a small quantity of the dry substance. This quantity, -which is generally employed in the form of a cylinder, and is called the -“priming,” must be thoroughly protected from the water. For this -purpose, indiarubber tubing may be used, or, if the primer be large, -indiarubber bags. When the pressure of the water is not great, a very -efficient protective covering is obtained by dipping the primer into -melted paraffine. Care should be taken to avoid raising the temperature -of the paraffine above the degree required to melt it completely. The -primer should be placed in contact with the charge, and it is desirable -that the latter, when it can be conveniently made to do so, should -surround the former. - -Charges of gun-cotton for subaqueous blasts are usually made up of discs -of a large diameter, or of slabs of a rectangular form. When, however, -the charge has to be put into a bore-hole in rock, the common cartridge -is employed. - -Tonite, or cotton powder, is largely used in subaqueous blasting -operations. This substance is always applied in a dry state, and -requires, therefore, to be protected from the water. This protection it -is however, not difficult to give. Being prepared for use in a very -highly compressed state, it does not readily absorb moisture. In this -state, it is enclosed in cartridges, which are subsequently dipped into -melted paraffine. This is the form and preparation adopted for ordinary -use. For application under water, especially when the depth is -considerable, additional protection is given. For wreckage purposes, -tonite may be obtained in convenient charges, made up in suitable forms, -and sufficiently protected. - -When dynamite is used, the conditions are similar to those prevailing in -the case of gun-cotton. Since nitro-glycerine is unaffected by water, no -necessity exists for protecting it from moisture. But when a charge of -dynamite is immersed in water, and not contained in a bore-hole, the -nitro-glycerine rapidly exudes. The writer once made several ineffectual -attempts to explode a charge of dynamite at a depth of 70 fathoms -beneath the surface. The cause of failure was found to be this -exudation; for subsequent experiments showed that, though the dynamite -was in the form of the ordinary parchment paper cartridges, and was -contained in a stout canvas bag, the kieselguhr retained hardly a trace -of nitro-glycerine when the charge reached the surface from that depth, -after being rapidly lowered and raised. Hence it becomes necessary to -enclose dynamite within some fairly impervious substance, to prevent the -exudation of the nitro-glycerine. Waxed linen, or fine canvas overlaid -with the composition already described, may be used as a protective -covering; for blasts in deep water, indiarubber bags and tubing are -employed. When the charge is contained in a bore-hole in rock, exudation -can hardly occur, and therefore in such cases waterproofing is -unnecessary. - -For firing subaqueous blasts with safety fuse, only the guttapercha -covered kinds are suitable. Great care must be taken to render the -junction of the fuse and the detonator water-tight. A stronger detonator -is required under water than in dry ground. Electric fuses offer not -only a cheaper, but a far more certain and suitable means of firing in -water. This means is now very generally employed. When tension currents -are used, the insulation must be very good. In all cases, ample power -should be possessed by the firing machine or battery. - -The shattering class of explosives are very suitable for subaqueous-rock -blasting. In many cases, their employment renders the boring of -shot-holes unnecessary, an advantage of obviously great importance. When -detached or projecting masses of rock have to be broken up, it is -sufficient to place the charges upon them. Of course, when so applied, -larger quantities of the explosive are required; but though the method -is wasteful of explosive, it is very economical of labour and time. -Even when large undetached masses of rock have to be removed, the same -method may often be successfully followed. Suppose a level surface of -rock, for example. A few heavy charges judiciously distributed over this -surface will blow out craters of a considerable radius, and more or less -fracture the rock in their immediate neighbourhood. A few other blasts -then fired between these shattered points will break up the intervening -solid portions. Sometimes the rock will be disintegrated to a -considerable depth, and so broken up generally that it may be removed by -dredging. By proceeding in this way, the whole of the rock may often be -removed without any labour of boring. - -But when the rock is too tough to be removed in this way, recourse must -be had to boring, though even when boring is necessary, an occasional -“loose” shot may be found to be very efficacious. - - -_Boring under Water._--The percussive drills, one of which, the -Darlington, was described in a former chapter, may be used effectively -under water. Compressed air is used as the motor fluid. The tripod -stand, having its legs weighted to give it stability, is generally the -most suitable support. These drills need the immediate attention of a -diver. Sometimes the boring is carried on by hand from the deck of a -vessel or from a raft provided for the purpose. The following -description will give a general notion of the operations involved in -subaqueous boring:-- - -The working vessel having been moored over the rock by means of -mooring-lines attached to buoys placed about 50 yards from each quarter -of the vessel, the diver descends and selects the most suitable position -for the blast; he then signals, by a certain number of pulls upon his -signal line, to have the drill and stand lowered to him. This being -quickly done by means of a steam derrick, he guides the drill-stand to -its place, and finally fixes it in position by means of its adjustable -legs. This being done, he signals for air to commence drilling. - -It has been found that the drill can be worked in a rapid current as -well as in slack water. This allows the operations of drilling and -blasting, by a proper division of time and labour, to be conducted in an -extremely rapid tidal current, so that the principal work of the diver, -in inserting charges for blasting and slinging stone, may be done near -the periods of slack water, while the drilling may be advantageously -continued during the period of rapid flow. In a rapid current, the -stoppage of the drill for the purpose of “spooning out” the hole becomes -unnecessary, as the motion of the drill works up the débris to the mouth -of the hole, whence it is sucked out and carried off by the current in a -dark stream, like the smoke from the funnel of a locomotive. In a -sluggish current, or during slack water, the hose of the air-pump is -sometimes introduced, and air forced into the bore-hole to create a -current of water, by which means the hole is cleared more thoroughly -than by the most careful “spooning out.” - -As soon as the hole is drilled to the required depth, the drill is -stopped; the diver then fastens the derrick chain, which is lowered to -him for the purpose, to the drill-stand, and signals to hoist away, -whereupon the machine is quickly hoisted on deck. - -After having examined the hole and cleared away any débris remaining at -the bottom, the diver comes to the surface, and taking in his hand the -charge contained in a water-tight cartridge, and provided with its -electric fuse to which a sufficient length of insulated wire is -attached, returns with it, and inserts it into the drill hole, carefully -pressing it to the bottom with a rod. The tamping, if any is used, is -then inserted above the cartridge, and the diver comes up. - -The working vessel having been quickly hauled by the mooring-lines to a -safe distance by means of capstans worked, whenever practicable, by the -steam-engine, the wires are attached to the machine, and at the signal -“all ready” the charge is fired. - -The working vessel is then hauled back to her position, and as soon as -the water becomes sufficiently cleared of the dark muddy matter stirred -up by the blast, to enable the diver to see in it, he descends and -examines the result. - -If the blast has been effective, he signals for the stone chains to be -lowered to him; which being done, he proceeds to sling the large pieces -of broken rock, one after another, as they are hoisted up and deposited -on deck. All the pieces large enough to sling having been thus removed, -he signals for the tub and shovel, and upon their being lowered to him, -proceeds to shovel into the tub the small fragments, and to have them -hoisted up and piled on deck, until the surface of the rock is -sufficiently cleared to place the drill for a new blast. - - -_Submarine Rocks._--The following brief account of the removal of the -“Tower” and the “Corwin” Rocks from the Narrows, at the entrance of -Boston Harbour, U.S., from the pen of J. G. Foster, is instructive as -illustrating the method of procedure in submarine blasting, and as -showing the unfitness, for work of that character, of the slow-burning -explosives:-- - -“Tower Rock,” being the smaller of the two, was selected as the one to -be first removed. Its horizontal dimensions being only 50 by 26 feet, it -was estimated that one large central charge surrounded by five or six -others, all in large and deep drill-holes, would be able to rend the -rock into pieces. - -The working vessel, the sloop “Hamilton,” of 70 tons, was moored over -this rock on the 30th of July, 1867, and the new submarine drilling -machine, designed for this work, by Mr. Townsend, the contractor, was -placed in position and tried. - -Several imperfections were found at the first trial, which prevented its -efficient working. While these were being remedied, a trial was made of -surface blasts, placed in and around the rock in the positions most -favourable to their action. These proved to be entirely without effect. -No seams or breaks were made by them in the smooth surface of the rock. - -As soon as the submarine drilling machine was perfected, it was put in -operation, and successfully worked. The central and the surrounding -holes were drilled to depths varying from 2 to 8 feet, each hole being -3½ inches in diameter. These were well charged with black blasting -powder, and tamped with sand. In some holes, the charges produced no -visible effect, the tamping being blown out like the charge from a -cannon. In others, a crater was formed, but with a radius only about -one-half the line of least resistance. The holes that were intact were -then deepened, and new ones drilled; these were charged with Dupont’s -sporting powder. The result was much better, but not what was desired. -The pressure of the water, from 23 to 33 feet in depth, seemed to -diminish largely the ordinary explosive effect of gunpowder upon rock, -as seen in blasts in the open air. - -Trial was then made of the patent safety blasting powder, manufactured -by the Oriental Company of Boston, the proportions of the ingredients -having been modified to increase its strength for this especial use. -This produced the desired effect. The rock was rent in pieces; and by -drilling additional holes and continuing large charges of the powder, -the rock was finally reduced to the required depth. - -To smooth off its upper surface and break down the sharp projecting -points, large surface charges of sporting powder were employed. These -accomplished the result to a limited extent, but not completely. A large -15-inch shell was then placed in a crevice near the centre of the rock -and fired. Its explosion swept the rock completely, breaking down and -levelling the projecting points. - -The work upon this rock occupied eight weeks. In that time, 80 tons of -stone had been blasted out, hoisted up, and deposited on shore, -attaining the required depth of 23 feet at mean low water. About 70 tons -of small fragments were suffered to remain on the bottom around the -rock, where they had been thrown by the blasts, and where they could do -no harm. - -The cost per ton of the quantity hoisted up and deposited on shore was -64·93 dollars, no account being taken of the quantity blown, in small -fragments, into deep water. - -“Tower Rock” having been entirely removed to the required depth, the -moorings of the working vessel were at once removed to “Corwin Rock,” -and work commenced upon it on the 1st of October, 1867. This rock was -found to be much more difficult to blast, on account of its extremely -tortuous lamination, its great toughness, and the presence of a great -number of iron pyrites. - -Surface blasts were also tried upon this rock at the outset, in hopes -that, by being placed in the most favourable positions between the sharp -ridges of the rock, they might break them down. These, however, like -those upon Tower Rock, entirely failed to produce any noticeable effect, -even when they contained four and five hundred pounds of the best -sporting powder. The drilling machine was therefore called into -requisition as before, and used continuously till the completion of the -work. - -On account of the extent of this rock, a different plan of operations -for its removal was adopted. One side of the rock most favourable for -blasting was selected, and a row of holes drilled parallel to the edge, -and at a distance from it equal to the depth of the holes, which was -taken to extend 1 foot below the required level, 23 feet at mean low -tide. After blasting out these holes, a new line of holes was drilled -parallel to the former line, or to the “face” left by the blasts, and -these also were blasted out; then a third line, and so on, progressing -regularly across the rock, continually blasting it off in parallel -blocks, extending downward a little below the depth required. - -The advantages of this mode of operation were that it enabled the blasts -to act laterally, in which direction they were the most powerful; and -the rock was left, after each series of blasts, with a nearly vertical -side, or “face,” in which the presence of seams could be more readily -detected, and the character of the strata observed, so that the most -favourable positions could be selected for the next blasts. - -Sometimes the craters, following the strata, ran under, or left an -overhanging “face,” in which case a large charge placed under its -projecting edge, usually had the effect of throwing off the overhanging -portion, and sometimes of dislodging large masses. - -After the rock had been in this way blasted entirely across, and to the -general depth required, a careful survey was made, the soundings being -taken in lines from 5 to 10 feet apart, and at right angles to each -other, the lower end of the sounding pole being placed by the diver -alternately upon the highest and the lowest points. - -This survey showed that although more than the required depth had been -generally attained, yet many points projected above this level by -distances varying from 2 to 14 inches. - -To remove these, large surface charges were again tried, but with the -same ineffective result. Their only effect was to pile up the sand and -small fragments of stone into irregular windrows on the surface of the -rock. Small holes had, therefore, to be drilled at each of these points -to blast them off. This occupied much more time than could reasonably -have been expected; so that it was not until two months’ labour had been -expended, that all the points were finally reduced to the required -level. - - -_Obstructions in Water-courses._--The removal of obstructions from -water-courses often leads to much subaqueous blasting. Trees that have -fallen into the stream are most effectively broken up by charges of -gunpowder fired by a detonation. The success of the operation will, -however, be greatly dependent upon the judicious placing of the charges. -Brickwork may also be very effectively dealt with by charges of -gunpowder. But stone masonry and blocks of rock may be more effectively -broken up by gun-cotton, tonite, or dynamite. For work of this -character, electrical firing offers great advantages, for, besides its -convenience, it allows of several charges being exploded simultaneously, -a condition that is always favourable, and in many cases essential, to -success. - -The following highly interesting and instructive account of the removal -by blasting of some obstructions in certain rivers in India is given by -Lieut. A. O. Green, R.E. - -[Illustration: FIG. 51.] - -[Illustration: FIG. 52.] - -He, in company with some assistants, left Calcutta for Maldah on the 8th -of April, 1874, where they commenced work on the following day upon the -wreck of a large county boat, which lay on the top of a tree in -mid-stream, as shown in Fig. 51. Soundings were taken over and around -this tree, which was found to be about 3 feet 6 inches in diameter at -its base. The gunpowder intended to be used in these operations not -having arrived, three 5-lb. charges of gun-cotton were made up; it was -thought that under the 15-feet head of water, these would have been -sufficient to break the tree in half. The gun-cotton was in the form of -compressed discs, 2½ inches in diameter, and 2 inches thick, each disc -weighing about 5 ounces. These discs were filled into a tin cylinder to -within about 4 inches of the top. An electric fuse, with wires attached, -having been securely pushed into the hole in the centre of the top disc, -the empty space above was filled up, with first a layer of sawdust, and -then a layer of plastic clay, well rammed. The whole was then painted -over, and the upper end tied up in a covering of waxed cloth, the holes -through which the fuse wires passed being carefully luted. The charge, -thus made up, is shown in section in Fig. 52. It was fired by means of -a dynamo-electric machine. - -The first charge produced but little effect; a second failed from the -case not being water-tight; a third charge was more effective, as it -lifted the tree and the boat partially out of the water. The positions -of these gun-cotton charges are indicated by circles on the figure. The -next day, two charges of gunpowder, of about 70 lb. each, were placed -under the boat, these charges being lashed on to the snag by the divers. -These charges consisted simply of common oil-tins, carefully cleaned and -painted over with red-lead paint. The bunghole was closed by a wooden -plug, bored through to allow the fuse wires to pass. This plug, after -being inserted, was coated over with a waterproofing compound. The -effect of the two charges was to completely demolish the boat. Another -charge of 50 lb. removed the tree underneath. The positions of these -gunpowder charges are indicated by squares in Fig. 51. - -The next obstruction met with was a sand bank caused by a boat which had -broken in half and then sunk. The sand nearly covered the boat, so that -there was little else to operate upon. A charge of 80 lb. (one large -charge being considered preferable to two or three small ones in getting -rid of the sand), placed close to the part of the boat that was visible, -made a considerable crater, and a second charge of 80 lb. was placed in -a much more favourable position, as nearly all the boat was removed -except portions of the bow and stern, which required two separate -charges of 50 lb. each before they disappeared. In half an hour, the -whole of the sand bank had been washed away by the stream, and there was -from 3 to 4 feet of water over the spot where before the sand was high -and dry out of the water. The removal of this obstruction was dangerous, -owing to the nearness of the boat to the surface, the consequent small -resistance offered to the projection of its pieces through the air, and -the largeness of the charges used. Had, however, small charges been -used, it is more than probable that the small craters made by them would -have become too quickly filled up again to have been of any good in -facilitating the placing in position of subsequent ones. - -The following day, a large mango tree, about 4 feet 6 inches in -diameter, was destroyed by two 50-lb. charges, which broke it up into -three pieces, easily removable ashore. - -A few days later, a large trunk of a tree, about 3 feet in diameter, was -removed with two 50-lb. charges; but the depth of water over it was so -small that a large portion of the trunk was thrown a considerable -distance on shore. The next day a large tree which had formed a sand -bank was very successfully removed by a charge of 50 lb. placed among -the roots, it being considered that a smaller charge than 50 lb. would -not have effected the purpose. Opposite to Kásimpore, a boat was removed -with a charge of 50 lb. placed in the centre up-stream, which entirely -demolished it, the pieces being all dragged ashore. At Mootyá, a large -cotton tree, the wood of which is extremely tough, was found with many -large branches projecting out of the water. A charge of 70 lb. tied -under the tree at the springing of the branches effectually broke it up, -and the pieces were all hauled to land. Three miles farther down the -river, an attempt was made to destroy another large cotton tree with a -similar charge, but it only broke it into three pieces, and two more -charges of 50 lb. each were necessary to clear it away effectually. This -tree was, if anything, slightly larger than the last, i. e. from 3 to 4 -feet in diameter, and there was less water over it. - -Farther on, the party came across a collection of three or four trees, -with their branches interlaced, lying on a sand bank near Alumpore -Dáldah; these were sufficiently broken up by a 70-lb. charge to make -them easy of removal by coolie labour. Opposite to the village, another -awkward snag, in the shape of a large tree sticking up in 30 feet of -water, was destroyed by tying a 70-lb. charge at its base. A charge of -50 lb. of powder under this head of water, or even a smaller amount, -might have been sufficient, but as the work had to be done quickly, not -much account was taken of a few pounds of powder more or less, provided -the object was attained. At Gomashtapore, a large tree, branches and -all, was found in 25 feet of water, lying in the channel under the bank. -The current here was considerable, and some difficulty was experienced -in placing the charges. One charge of 70 lb. broke the tree in half; -another of 50 lb. at the springing of the branches broke them up; and -another of the same weight got rid of the roots. Below Gomashtapore, a -large mango tree was demolished with 60 lb. of powder. A short distance -farther on, a bad bamboo snag was met with. These bamboo snags, which -were merely the roots of the bamboos with perhaps a dozen or so whole -ones left, gave much trouble. Fig. 53 gives an idea of what these snags -are like. It was found impossible to place a charge underneath this one, -so an opening was prized between the bamboos, and a charge of 70 lb. -rammed down pretty well into the middle. This cleared the whole of it -away and opened the channel. - -[Illustration: FIG. 53.] - -At Chandpore, at a re-entering angle of the river and in a place -peculiarly dangerous to navigation during the rains, was an enormous -banyan tree (_Ficus Indica_), the main trunk of which, to judge from the -branches, must have been at least from 12 to 15 feet in diameter. An -approximate measurement was made with a pole, but any such measurement -can only have been a very rough one. - -[Illustration: FIG. 54.] - -[Illustration: FIG. 55.] - -The trunk was lying in deep water, but the branches, more like an -accumulation of large trees, were lying stretched out for a considerable -distance over the bank, covering an area of more than 80 square feet. A -charge of 200 lb. of powder was made up in an indiarubber bag, and -placed by the divers in about 28 feet of water, well under the trunk of -the tree. The effect of this was to split the trunk up into several -pieces, each of which subsequently required separate removal. A 70-lb. -charge was next fired under two of the largest pieces in 18 feet of -water, and this broke them up completely. Having now run out of all the -cases for powder, three charges of gun-cotton, similar to the first, -were made up, and fired separately, each placed under a good thick -branch, about 8 feet in girth. The effect of all three was prodigious; -seemingly greater than that of the 70 lb. of gunpowder. As there were no -more cases left, and time was precious, some common earthenware ghurrahs -were obtained from the village as a makeshift. These held about 20 lb. -of powder; the fuse was placed in the centre in a disc of gun-cotton, -and the neck was closed up with damp earth, white-lead paint, &c., just -in the same way as the gun-cotton charges had been. A rope for lashing -them to the obstacle was securely fastened round the neck, and the fuse -wires were tied under, this lashing, leaving a small loop towards the -fuse free, so as to avoid any chance of a strain being brought on the -fuse in lowering the charge. Figs. 54 and 55 show the arrangement of -this charge. The first one tried had but little effect when placed -under a branch of the tree in deep water, and it was accordingly -determined to wait for cases from Calcutta; but after waiting five days -without their appearing, three more of these charges were tried, and -this time with very excellent results. They were indeed so satisfactory, -that the same evening four more were made up and fired. The first under -a mango tree a little farther down the river. This broke it in half, -throwing one part high and dry on shore, and the other into deep water. -The other three were fired under the remaining branches of the banyan -tree with very good effect, cutting them away. - -It is more than probable, observes Lieut. Green, that the good results -obtained with all these ghurrah charges were entirely due to the -gun-cotton disc inside causing the gunpowder itself to detonate, so that -the thinness of the envelope was of little moment in determining the -force of the explosion. - -The tin cases having arrived, the rest of the powder was made up into -five charges of 48 lb. and three ghurrah charges of 20 lb each. About -four miles farther down the river, there was an old peepul tree lying in -mid-channel, with several of the branches above water. Two tins, one -placed under the springing of the branches and the other under the -roots, blew away the lower branches on which the tree was resting, and -it sank slightly in the water. A ghurrah was next fired under the trunk -with splendid results, the tree disappearing entirely except one branch, -which required another small charge to remove it. The trunk of this tree -was nearly 8 feet in diameter, but of soft stringy wood. - -On returning to camp, a small charge of 2 lb. of gun-cotton was made up -in a section of bamboo, and used against the banyan tree with very good -effect, and a ghurrah charge demolished the last branch but one. The -next day 1½ lb. of gun-cotton in a piece of bamboo finished the last of -this enormous tree. - -After clearing away several more trees, the foundations of an old -factory, which had slipped into the stream, were removed by introducing -two charges of 1½ lb. of gun-cotton, in the ends of two bamboos, well -into the crevices of the masonry under the water. - -Another obstruction consisted of a row of old piles, about 15 inches -square, stretching across the river below the surface of the water. Six -of the most dangerous of these were removed from the dry season channel -with ghurrah charges tied to the foot of the piles. - -An old well that had fallen bodily into the water was afterwards met -with. The position of this well is shown in Fig. 56. A charge of 4 lb. -of gun-cotton completely destroyed it. - -Near Azimgunge, the trunk of a very large peepul tree was found sunk in -deep water. It was so large that it was thought necessary to place a -100-lb. charge underneath it; this charge broke it up completely, but -two small charges of 20 lb. each were subsequently required to remove -the pieces. - -[Illustration: FIG. 56.] - -Later on, a well, similar to the one previously destroyed, was met with. -The brickwork was remarkably good and about 3 feet thick, and the mortar -was excellent. One charge of 4 lb. of gun-cotton broke it up into large -pieces; but it took another similar charge, and two charges of 20 lb. of -gunpowder to destroy it completely. On the same day, two trees were -removed with ghurrah charges, which had been used throughout, for small -charges, with unvarying success. - -At a place called Farrashdangah, there was a very bad obstruction in the -river, caused by the remains of an old bathing ghat and bridge having -been cut out from the bank by the water getting underneath the masonry. -Both were projecting about 3 feet above the water, and in the rainy -season they formed the centre of a very nasty and dangerous whirlpool, -in which many boats had, according to the Executive Engineer of the -Nuddea Rivers Division, been lost. There was an immense mass of masonry, -but no means of getting a charge placed underneath it; so a charge of -100 lb. of powder was placed close alongside it in about 15 feet of -water. This shunted the mass bodily over and underneath the water. Two -50-lb. charges were next placed underneath the mass, and these shattered -it all up, except one piece, which was got rid of with a fourth charge -of 20 lb. placed well underneath it. The Executive Engineer wishing that -the wing-wall of the bridge, which was on dry land during the dry -season, might be removed as well, a small hole was made at the foot of -the visible portion of the brickwork, and a charge of 2 lb. of -gun-cotton was introduced into this, and fired with only a tolerable -effect, the brickwork being cracked for a distance of 3 or 4 feet from -the centre of the charge. A hole was next dug down about 5 feet at one -side of the wing-wall, and a charge of 4 lb. of gun-cotton well tamped -was fired. The tamping was blown out, and the wall foundations cracked -a good deal. The excavation was now deepened to 6 feet, and a hole made -under the brickwork big enough to contain a 100-lb. charge. It was then -well tamped up and fired. Its effect was excellent. All the brickwork of -the wing-wall was got rid of, and a crater about 30 feet in width at the -top blown out in the point of the bank that was required to be removed, -and which was one of the chief causes of the whirlpool, so that the next -rise of the river was sure to carry it all away. The following day an -old pucka ghat opposite to Berhampore was entirely broken up with three -20-lb. charges, and an enormous quantity of old bricks were thrown into -the river. - -The last operation undertaken consisted in the blowing up of a very -large ghat opposite to the Nawáb of Moorshedabád’s palaces. The river -during successive rains had cut into and underneath the steps of the -ghat, bringing down large masses of it into the river, where they formed -most dangerous obstacles to navigation. The work was necessarily carried -out in a very rough way, for want of the proper tools. Deep excavations -were made under the three largest masses of masonry, at about 25 feet -apart, and into these were introduced three 50-lb. and one 20-lb. -charges of powder. These charges were well tamped, connected up in -divided circuit, and fired simultaneously. All the masonry was broken up -completely, so as to be easily removable afterwards by coolie labour, -which was all that was required. - -The conclusions to be drawn from the foregoing notes are, that large -trees lying in shallow water require charges of 50 lb. of gunpowder and -upwards for their effectual removal; but that where there is plenty of -water, and the trees are not very large, 20 lb. is sufficient. - -For these small charges, it has been seen that the common earthenware -ghurrah answers admirably, and under similar circumstances it would -undoubtedly be advantageous to use them, as they are inexpensive, and -obtainable in nearly every Indian village. - -The charges used might, in many cases, at first have been no doubt made -smaller with advantage, both for safety and economy; but as speed was -the great object, these were not so much thought of. - -For the removal of masonry under water, it is not necessary to place the -charge underneath the mass, which is often impossible; a large charge -alongside it being generally quite sufficient to break it up pretty -effectually where there is sufficient head of water. Smaller charges can -of course be easily used afterwards, whenever required, and for these -small charges, gun-cotton is very effective, as it can be easily -introduced, in the end of a bamboo, into holes and crevices where it -would be impossible to get any but the smallest charges of gunpowder. - - - - -INDEX. - - - Appliances for firing blasting charges, 42 - Auxiliary tools, 17 - - Batteries, firing, 62 - Beche, 21 - Bichromate firing battery, 62 - Bits, borer, 31 - Blasting gear, sets of, 22 - ---- sticks, 51 - ---- subaqueous, 164 - Borer-bits, 31 - Boring under water, 170 - ---- the shot-holes, 128, 142 - Bornhardt’s firing machine, 57 - Brain’s powder, 105 - Bull, 20 - - Cables, 53 - Cellulose dynamite, 105 - Charging and firing, 150 - ---- the shot-holes, 132 - Chemical compounds, 81 - Claying iron, 20 - Conditions of disruption, 110 - Connecting wires, 52 - Cotton powder, 103 - - Darlington drill, 26 - Detonation, 95 - Detonators, 54 - Dislodged rock, removal of, 154 - Disruption, conditions of, 110 - ---- force required to cause, 107 - Division of labour, 155 - Drills, dimension of, 6 - ---- form of, 4 - ---- hand, 1 - ---- hardening and tempering, 10 - ---- making and sharpening, 7 - ---- sets of, 13 - Drivings, examples of, 157 - Dubois-François carriage, 39 - Dynamite, 100 - ---- composition of, 87 - - Electrical firing, advantage of, 153 - Electric fuses, 47 - ---- tension fuse, 50 - Example of a heading, 115 - Examples of drivings, 157 - Explosion, force developed by, 72 - ---- heat liberated by, 67 - ---- gases generated by, 69 - ---- nature of, 64 - Explosive agents, nature of, 76 - - Firing batteries, 62 - ---- blasting charges, appliances for, 42 - ---- by electricity, 138 - ---- machines, 55 - ---- machine, Bornhardt’s, 57 - ---- ---- induction coil, 61 - ---- ---- Mowbray’s, 59 - ---- ---- Siemens’, 60 - ---- ---- Smith’s, 58 - ---- points of the common explosive agents, 102 - ---- table for frictional electric machine, 140 - ---- the charges, 137 - Force developed by an explosion, 72 - ---- developed by gunpowder, 88 - ---- required to cause disruption, 107 - Fuse, safety, 45 - Fuses, electric, 47 - - Gases generated by an explosion, 69 - Gun-cotton, 99 - ---- constitution of, 81 - Gunpowder, 97 - ---- composition of, 80 - ---- force developed by, 88 - - Hammers, 14 - ---- patterns of, 15 - Hand boring, 128 - Heading, example of, 115 - Heat, action of, in firing, 92 - ---- liberated by an explosion, 67 - ---- measure of, 66 - ---- specific, 66 - Hoosac Tunnel, 158 - - Induction firing coils, 61 - - Joint and bedding planes, 118 - Jumper, 3 - - Labour, division of, 155 - Line of least resistance, 106 - Lithofracteur, 104 - - Machine boring, 142 - ---- rock-drills, 23 - Machines, firing, 55 - Means of firing the common explosive agents, 92 - Measure of heat, 66 - Mechanical mixture, 76 - Mowbray’s firing machine, 59 - Musconetcong Tunnel, 159 - - Nature of an explosion, 64 - Nitrated gun-cotton, 103 - Nitro-glycerine, constitution of, 86 - - Obstructions in water-courses, 178 - Operations of rock blasting, 128 - - Preparation of subaqueous charges, 164 - Principles of blasting, 106 - - Rammer, 20 - Relative strength of gunpowder, gun-cotton, and dynamite, 91 - ---- ---- of the common explosive agents, 88 - Removing dislodged rock, 154 - Rock-drill supports, 34 - - Safety Fuse, 45 - St. Gothard Tunnel, 157 - Schultze’s powder, 104 - Scraper, 18 - Shot-holes, boring, 128, 142 - ---- charging, 132 - Siemens’ firing machine, 60 - Silvertown firing battery, 62 - Sledges, 14 - ---- North of England, 17 - ---- North Wales, 17 - ---- South Wales, 16 - Smith’s firing machine, 58 - Some properties of the common explosive agents, 97 - Some varieties of the nitro-cellulose and the nitro-glycerine - compounds, 103 - Squibs, 44 - Steel, hardening and tempering, 9 - Stemmer, 20 - Sticks, blasting, 51 - Stretcher bar, 37 - Subaqueous blasting, 164 - ---- charges, preparation of, 164 - Submarine rocks, removal of, 173 - Swab-stick, 19 - - Tamping, 121 - Tonite, 103 - - Water, boring under, 170 - Water-courses, obstructions in, 178 - Waterproofing composition, 165 - Weight of explosive in bore-hole, table of, 109 - Wires, connecting, 52 - - -LONDON: PRINTED BY WM. CLOWES AND SONS, STAMFORD STREET AND CHARING -CROSS. - - - - -[Illustration: Plate I. - -SET OF COAL BLASTING GEAR. - -_Fig. 1._ - -_Fig. 2._ - -_Fig. 4._ - -_Fig. 5._ - -_Fig. 3._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate II. - -SET OF SINGLE HAND STONE BLASTING GEAR. - -_Fig. 6._ - -_Fig. 7._ - -_Fig. 8._ - -_Fig. 9._ - -_Fig. 10._ - -_Fig. 11._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate III. - -SET OF DOUBLE HAND STONE BREAKING GEAR. - -_Fig. 12._ - -_Fig. 15._ - -_Fig. 13._ - -_Fig. 14._ - -_Fig. 16._ - -_Fig. 17._ - -_Fig. 18._ - -_Fig. 19._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate IV. - -THE DARLINGTON MACHINE DRILL. - -_Fig. 20._ - -_Fig. 22._ - -_Fig. 21._ - -_Fig. 23._ - -_Fig. 24._ - -_Fig. 25._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate V. - -STRETCHER BAR. - -_Fig. 26._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate VI. - -MACHINE DRILL SUPPORTS. - -_Fig. 27._ - -_Fig. 28._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate VII. - -MACHINE DRILL SUPPORT. - -_Fig. 29._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate VIII. - -DUBOIS-FRANÇOIS MACHINE DRILL CARRIAGE. - -_Fig. 30. Elevation._ - -_Fig. 31. Plan._ - -_Fig. 32._ - -_Raising and lowering Screw._ - -_Fig. 33._ - -_Back support for the Drill._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate IX. - -S^{T}. GOTHARD TUNNEL. - -_Göschenen End._ - -_Airolo End._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate X. - -EXAMPLES OF HEADINGS. - -_Heading, Marihaye._ - -_Heading, Anzin._ - -_Heading, Ronchamp._ - -HOOSAC TUNNEL--_West End._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate XI. - -HOOSAC TUNNEL. - -_Centre Cut._ - -_Side Cuts._ - -_Squaring the Heading._ - -E & F. N. Spon. London & New York.] - -[Illustration: Plate XII. - -MUSCONETCONG TUNNEL. - -_The Heading; Plan._ - -_The Bench._ - -_Sectional Elevation._ - -_The Bench, Plan._ - -_The Heading,_ - -_Elevation._ - -E & F. N. Spon. London & New York.] - - - - - Transcriber’ Notes - - - Inconsistent and unusual spelling, hyphenation, use of accents and - lay-out have been retained, except as mentioned below. - - There are two sets of numbered illustrations: numbered illustrations - in the text, and numbered illustrations in the plates in the back of - the book. - - Calculations and tables have been copied verbatim although some - contain calculation errors. - - Some reference letters in the text are missing from the illustrations. - - - Changes made: - - Tables and illustrations have been moved to between paragraphs. - - Some minor typographic errors have been corrected and some missing - punctuation has been added silently. - - Page 40: plate ~V~ changed to plate V. - - Page 109: Curtiss’ and Harvey’s changed to Curtis’s and Harvey’s. - - Page 166: Curtis’s and Harvey changed to Curtis’s and Harvey’s. - - - - - -End of the Project Gutenberg EBook of Rock Blasting, by Geo. G. 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