Several of these have been very largely adopted, and are now manufactured on a large scale. A great number of experiments have been made, showing that they could be discharged into an explosive mixture of fire-damp or coal-dust, or both combined, without inflaming them, and that when fired in a pit no flame could be observed. By others it is, however, alleged that flame has been observed. However this may be, there is no doubt that the amount of flame is very much less than in the case of the admittedly flame-producing explosives, and it is considered safe to use these explosives where gunpowder would be dangerous. Among the flameless explosives which have been made are: ammonite, amvis, bellite, dahmenite, electronite, Faversham powder, pembrite, roburite, westfalite. In addition to the above, there are various "permitted" explosives made with nitroglycerine, and others composed chiefly of the ingredients of ordinary gunpowder, which, on being tested, give much less flame than the old-fashioned blasting powder. The balance of the evidence obtainable is in favour of the explosives made with nitrate of ammonium, in regard to safety in gassy and dusty coal-mines. The Secretary of State from time to time publishes a list of permitted explosives. The orders dated July 24, 1899, are given in the Appendix in full, and show the composition, and rules regulating the use, of all the "permitted explosives." Some of these explosives require careful handling, because some of the constituents are poisonous and may be absorbed through the skin, and, as in the case of ordinary blasting powder, the fumes are injurious, and the workmen should not return to the place after a shot until the ventilation has had time to clear them away.

The rules to be observed in blasting, in order to keep within the provisions of the Mines Regulation Act, 1887, must be taken from the general rules, but the following is a summary of some of the principal provisions : 1

The scraper and rammer must be of brass or copper, or must at least have a brass or copper end.

The pricker (if any) must also be of brass or copper.

No coal-dust must be used in ramming the shot, but such materials as clay and shale.

The explosive must not be forcibly pressed into a hole of insufficient size; and when the hole has been charged, the explosive must not be unrammed; and no hole should be bored for a charge at a distance of less than 6 inches from any hole where the charge has missed fire.

An explosive should not be stored in the mine, and it must not be taken into the mine except in cartridges in a secure case or See Appendix, Explosives in Coal Mines Orders.

canister, and containing not more than 5 lbs., and a workman must not have more than one of such cases or canisters in one place at the same time.

In any place where the use of locked safety-lamps is for the time being required, or which is dry and dusty, no shot should be fired except by a competent person specially appointed for the purpose, who must examine the place and all contiguous accessible places within a radius of 20 yards before firing the shot.

In cases where, at either of the four inspections last recorded, inflammable gas has been found in the same ventilating district, then the shot must not be fired unless the place where the gas was found has been cleared of gas, and there is not sufficient to be a source of danger; or unless the explosive employed in firing the shot is so used with water, or other contrivance, as to prevent it from inflaming gas, or is of such a nature that it cannot inflame gas.

If the place where the shot is to be fired is dry and dusty, then the shot should not be fired, unless the place of firing and all contiguous accessible places within a radius of 20 yards from it are, at the time of firing, in a wet state, or have had a thorough watering in all parts where dust is lodging, whether roof, floor, or sides; or, in places where watering would injure the roof or floor, unless the explosive is so used with water or other contrivance as to prevent it from inflaming the gas or dust, or is of such a nature that it cannot inflame gas or dust; but if the dry and dusty place is part of a main haulage road, or is contiguous thereto, and showing dust adhering to the roof and sides, unless the place shall have been not only watered, but a flameless explosive used; or where either the water has been used, or the water-cartridge and flameless explosive only, and all the workmen have been removed from the seam in which the shot is to be fired, and from all seams communicating with the shaft on the same level except the men engaged in firing the shot, and such other persons, not exceeding ten, as are necessarily employed in attending to the ventilating furnaces, engines, machinery, winding apparatus, signals, or horses, or in inspecting the mine.

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CHAPTER XIII.

POWER DRILLS AND HEADING MACHINERY.

ALL underground works, whether sinking, driving levels, tunnels, or excavating masses of rock or ore, where the rock or mineral is very hard, require the use of power drills for economical work. The necessity for such drills was forced upon the engineers of the Mont Cenis tunnel five and thirty years ago. Before these were adopted, the progress of the tunnel was very slow; with the use of these machines, the speed of from 8 feet to 13 feet in twenty-four hours was attained. For this work Sommeiller's drill was used; this was a large and heavy machine, not often seen nowadays. Since that time a great number of excellent machines have been made, of which one or two specimens are here described; but at the same time, it must not be supposed that the prominence given to these machines indicates that they are any better than the others not mentioned. One of these, called the Climax drill (see Figs. 172, 173), has a cylinder say from 3 inches to 4 inches in diameter, stroke of the piston about 4 inches; a tappet on the piston-rod gives movement to an oscillating slide-valve; there is no cushion of air to prevent the drill from striking with full force. Fig. 396

FIG. 396.-Rio Tinto power drill.

shows the Rio Tinto drill; a tappet on the piston rod gives movement to a piston slide-valve by means of an oscillating forked lever. The Darlington drill (see Fig. 397) has no valves; the piston is very heavy, and as it moves, opens and covers the ports. The Minera drill also has no valves, and has a double piston. These valveless drills work very quickly and safely; the piston is cushioned at each end of the stroke, and therefore there is a tendency to

X

strike a gentler blow than in the case of drills which have valves and not so much cushioning for the piston. In all these machines the feed is by hand; that is to say, the miner turns the screw which causes the machine to advance along the slide, which is carried

FIG. 397.-Darlington drill: section of cylinder.

on a tripod stand or fixed bar. The number of blows struck is say from 400 to 600 a minute; the dirt is cleared from the holes by a jet of water and the movement of the drill-rod; holes may be bored to a great depth by changing the drill-bar. The machines above named are not intended to bore holes much more than 6 feet in depth, and from 1 inch to 2 inches in diameter. When the machine is fixed, the boring may be done at a speed of from 2 inches a minute up to 9 or 10 inches a minute in granite. To bore a hole 3 feet deep in fifteen minutes, including changing the drill-bar, would be good work for ordinary practice; it would very likely take two hours for three men to bore the same hole by hand.

The drills are driven by compressed air at a pressure varying from 30 to 60 lbs. per square inch. As a rule, the machine is placed on a tripod stand as shown in Fig. 172, and where the rock is hard the stand does not move; if, on the other hand, the ground is soft, the feet of the stand may move, and then the drill is apt to get jammed in the hole. For tunnel work the machines are sometimes fixed on a cross-bar; a similar arrangement is sometimes used for sinking. For rapid work in driving levels, the machines are fixed on an iron carriage (see Fig. 398). Two or four machines may be carried on this carriage, according to the size of the tunnel. When the requisite number of holes have been drilled, the machine is rolled back along the rails to a safe distance until after the blasts have been discharged and the dirt cleared away. The compressed air that drives these machines is useful for clearing out the smoke, and, if the mine is hot, for cooling the heading. Where every arrangement is made for speed, from 30 to 50 yards of heading may be driven in one week, where by hand it would have been difficult, with every exertion, to drive 6 yards a week. In ordinary mining work, however, great speed is not usually obtained, because of the capital required to provide the requisite machinery and the extra

expense in labour, and a speed of from two to three times that of hand-labour is usually considered sufficient.

1

PLAN

FIG. 398.-Drills on carriage.

Rotary Drills.—A carriage carrying two rock-drills is shown in Fig. 399. These drills are rotary, as in hand-boring machines, with a twisted steel auger; revolving motion may be given from

FIG. 399.-Walker's drilling-machine.

any source of power, such as a compressed air-motor, hydraulic motor, or electrical motor. A novel way of driving these drills is by the oil-engine, which is placed on the carriage, and a belt from the fly-wheel gives the required rotary movement to the drill. The writer has seen one of these machines drill a hole in Cleveland ironstone at the rate of one yard in one minute. It was driven by water-pressure.

Tunneling Machines. Machines have been made for driving levels without the aid of explosives; one of these, called Brunton's, was set to work in a slate-quarry in North Wales. The power was conveyed to the machine by a wire rope, and this gave movement through gearing to a revolving head of iron; on it were

1 The long horizontal arms carrying the drills and balance-weights are shown broken to reduce the length of the drawing.