of bore-hole admits of a large mass of rock being easily dislodged by means of high-grade explosives and electric blasting. The feed or forward movement of this class of drill is 24 inches, consequently with each succeeding length of steel there is an advance of two feet until the desired depth is reached. The condition which the cutting edge of the drill-bits will present after boring 2 feet will depend on the hardness of the rock, and on the care exercised in hardening and tempering them. In average whinstone, and even in granite, a drill-bit if properly dressed and tempered may be used a second time, and in many instances one drill-bit has been used in boring 6 feet,--that is, made use of three times before being returned to the smithy to be re-sharpened. It is a remarkable fact that the wear of the cutting edge of a machine drill-bit working in hard rock will, for a given amount of boring, be only or that of a hand-boring bit. Keeping the bore-hole clear of the chippings is a matter requiring some attention, the actual amount of boring being increased at least 30 per cent. by using water in the bore-hole; this also serves the purpose of converting the dust into sludge, which can be readily removed by the 'scraper.' A much better and at the same time quicker appliance for removing the sludge, is the small pump shown in fig. 72, having a ball or catch valve working at the bottom end of the pump, which allows the liquid to be forced up the tube FIG. 72.-Bore-hole pump. to a certain point, on the return of which the valve falls and retains the contents, which are then removed from the bore-hole. A piece of -inch iron rod is attached to the pump and forms the handle; it should be of sufficient length to easily reach the bottom of the deepest bore-hole. The pumping is performed at each change of bit; it is necessary at times, however, to remove the steel and clean out the bore-hole before the drill-bit has penetrated the rock to its full length; but as this involves an unnecessary loss of time, such delay should be avoided by keeping up a good supply of water. When the débris formed by drilling is of a viscous nature and adheres to the steels, it may be advisable to direct a jet of steam through a small piece of piping inserted into the bore-hole and attached to the steam hose; this will effectually clean out the sludge when other means fail. When the steel sticks in boring through seamy rock it may be relieved by striking it with a light hammer; but care must be exercised to deliver the blow on the stock of the drilling-bit, and not on the piston or chuck. If the rotating mechanism fails to operate after the drill-bit strikes a blow, the cause will invariably be a want of pressure on the washer at the back head of the machine. This can be quickly remedied by tightening the nuts of the side bolts, which will cause the back head springs to press firmly on the cap. 188. Work done by a Rock Drill.—The amount of work which can be accomplished by a rock drill in one day or in any given time, depends on the capacity of the machine itself, the nature of the rock to be bored, the manner in which the drill-bits are formed and tempered, and the size or diameter of the hole drilled. In hard solid whinstone, two feet may be penetrated in five minutes with a drill-bit having a cutting edge 2 inches across. As each drill-bit has, however, to be replaced by a longer one for every two feet drilled, and the bore-hole also requires to be cleaned by pumping, some time must be allowed for this, say from three to four minutes for the combined operation. Starting with a drill-bit 2 inches in diameter, a depth of 12 feet can usually be accomplished in an hour, including the time required for changing the bits and pumping out the bore-hole. As the machine has to be changed from one point in the quarry to another as the boring proceeds, the actual time occupied in drilling is even less than this, especially when shallow bore-holes only are resorted to. The actual work done in a day of nine hours will depend on the number of times the machine is changed from one point to another, and on whether the nature of the rock is solid or seamy. The time occupied in changing the machine and starting another bore-hole will depend to a certain extent on the distance to which the drill may have to be taken, and on the irregular nature of the rock surface. It is also contingent on the aptitude of the drill operator. The average time required is from 10 to 15 minutes. It is obvious that when the machine has to be removed often during a day's work, the time so occupied will bear a considerable proportion to the time actually expended in drilling. Taking all these circumstances into consideration, it will be evident that no criterion as to the capacity of a rock drill can be formed by simply putting it to work on a block of stone for a few minutes. A complicated machine might do good work in a short space of time, and yet be entirely useless for practical work. All those who are interested in quarrying operations should consider this matter carefully, and adopt those machines only that are made on proper principles, and are capable of withstanding the usage of actual work. The actual amount of boring which can be performed in a given time either for comparison, or in order to determine the cost per lineal foot of hole drilled, can be arrived at only by averaging the day's work over a considerable period of time. In solid rock the author has, with an 'Ingersoll-Sergeant' drill, bored 70 feet of vertical holes in one day of ten hours; this is, however, too great an amount of work for every-day practice, as in boring through fissured rock, the actual performance may only amount to 35 feet, and even less in extreme cases. The experience gained in working this particular type of rock drill for a number of years, taking all classes of rock into consideration, and working under varying conditions, shows that the amount of boring which can be accomplished averages 42 feet per day of nine hours, each hole being of an average depth of from 10 to 14 feet, the cutting edge of bit used being 2 inches across the mouth, and finishing at the bottom of a 14-feet borehole at 12 inches, or an average diameter of 2 inches. In calculating the amount of work done, either by hand or by machine drills, it is advisable to follow some standard in determining the amount of drilling performed, so that a comparison may be made of the relative merits and drilling capacity of manual labour, and of the different types of rock-drilling machines in use. The cylindrical inch of hole bored, previously mentioned, affords a ready means to this end, and when applying this method in calculating the results of an average day's drilling, namely, 42 feet, it will be found that this amounts to 1071 cylindrical inches of hole bored, or 119 per hour. These figures include the time occupied in removing the machine from one point in the quarry to another, changing the steels, and pumping out the bore-holes. As already stated, a 12-feet hole may be drilled in one hour, including, of course, the necessary time for changing bits, etc.; this amounts to 315 cylindrical inches of hole bored in that time. From these results it is evident how essential it is to drill deep bore-holes, in order to avoid the delay occasioned by the repeated changing of the machine, during which time the drill is not working. In the event of bore-holes of shallow depth being adopted, as is sometimes done, it is very evident that changing the position of the machine repeatedly will occupy nearly as much time as the actual drilling, in which case the least possible advantages are attained by employing rock drilling machines. The means by which rock drills are actuated, as already mentioned, is compressed air or steam. In large quarries where boring operations are carried on during the greater part of the year, and probably employing a large number of drills, the most suitable and economical motive power is compressed air, which, besides these important considerations, serves as a means of preserving a clear atmosphere at the working face, and adds considerably to the best possible conditions for carrying on the operations. In establishing a compressed air plant it is necessary to supply a compressor capable of providing a sufficient pressure for the number of rock drills which may be at work, the air being conveyed in pipes of relatively large diameter. By using compressed air as the motive power for rock drills, it is immaterial at what distance the compressor is situated from the point where the drilling operations are being carried on, as, provided there is no leakage, the pressure of the fluid working the drills is practically the same as at the compressor. In other words there is little or no loss of pressure between the point of compressing the air and that of using the compressed air. In quarries where the work is carried on in an intermittent mauner, such an expensive plant as has been described, while possessing many practical advantages, could not be economically employed. It is necessary, therefore, to make use of a boiler to generate steam for actuating the drill or drills. The most serious objection to the use of steam for this purpose is that at certain seasons, owing to the humidity of the atmosphere, the steam hangs about the working face, and greatly impedes the movements of the workmen. A boiler generating steam, to be of any practicable use for such work, inust be placed as near as possible to the point at which the drill is working, so that the steam may be had as dry as possible. If the distance between these two points be great, the steam in going through the long line of pipes becomes condensed, which, combined with the less effective pressure, places the working of the machine at a great disadvantage. This points to the necessity of the boiler being placed on wheels, so that it can be readily run from one point to another along the quarry face. The pipes conveying steam as a motive power should be of as small diameter as is consistent with the work to be done, as the condensation on a large surface would be prejudicial to effective and economical working. In quarrying rock for road purposes the usual, and, in fact, the only suitable way of providing the motive power is by means of a road or traction-engine, which, besides supplying the requisite steam for operating the drill, can be utilized for removing the drilling plant, and sleeping van for the use of the workmen, from quarry to quarry as occasion requires. The connection with the boiler should be so placed that the steam for working the drill may be had as dry as possible. It is advisable when the drilling plant is once started in a quarry, although only a small quantity of material may be required, to drill a sufficient number of bore-holes which can be blasted from time to time as necessity arises. The reason for this is, that to change the plant from one quarry to another every day or second day increases the cost of drilling operations considerably, by reason of the time occupied in travelling between the different quarries. It will be apparent that the cost of drilling may be increased to twice what it should be by unnecessary changing from quarry to quarry, involving in some instances a whole day, before a sufficient amount of actual work can be performed in proportion to the time occupied. In other words, the time occupied in travelling to a quarry for one day's drilling, which may be sufficient in order to produce the requisite quantity of material for immediate purposes, will involve an increase of cost compared with the same time and expenditure spread over a working period of from one to three weeks' duration. 189. Blasting. In the foregoing pages the tools and machines used in rock boring, and the conditions necessary to be observed in placing the boreholes, a matter of the highest importance, have been considered; it now remains to describe the methods adopted in charging and firing them, and the different kinds of explosives used for this particular class of work. When it is advisable to simply rend the mass of rock so that blocks may be obtained as large as possible, it is necessary to make use of an explosive having a comparatively slow action. In the case of procuring material for road metal quite an opposite effect is desirable, pointing to the necessity of employing an explosive having an extremely rapid and violent action, which will shatter the mass of rock sufficiently, and obviate, as much as possible, the necessity of after manipulation. In blasting rock from which dimension stones are to be produced, gunpowder is sometimes used; but special qualities of blasting powder are manufactured to suit the nature of the rock and the class of work for which it is intended. 190. Gunpowder as a Blasting Agent.-Gunpowder, the grains of which are small, is more rapid in action than coarse-grained powder; the explosion of the latter is adopted to simply rend and, by its slow moving and accumulative force, dislodge the mass of rock. Gunpowder is termed a direct explosive agent, that is, its force can be developed by direct means, such as ignition; it can also, however, be detonated by indirect means, which materially increases its explosive effect. The composition of the blasting powder generally used in Great Britain is 75 per cent. saltpetre, 10 per cent. sulphur, and 15 per cent. charcoal. Dynamite of a low grade, or containing a small percentage of nitro-glycerine, may be adopted with advantage at times for blasting rock under similar conditions. 191. High-grade Explosives.--The high explosive effect necessary to shatter the class of rocks used for macadamizing purposes, such as whinstone, is best obtained with high-grade nitro-glycerine compounds, fired by electricity. These high-grade explosives are termed indirect explosive agents, as they require an intermediary, such as a fulminate detonator, to cause them to explode properly. 192. The nitro-compound explosives of high grade are: Blasting gelatine No. 1, which contains 94 per cent. nitro-glycerine. Gelignite, 74 99 621 Dynamite, No. 1, Blasting gelatine is composed of collodion cotton, which is transformed into a jelly-like mass by being acted upon and dissolved by the nitroglycerine. Dynamite is composed of nitro-glycerine and a diatomaceous earth called |