Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/464

Rh 446 MINING [BLASTING. places, consists in inserting the needle into the charge and then tamping up the hole. Care is taken to draw out the needle a little as the tamping proceeds, so as to prevent too much force being re quired for its final withdrawal. The small hole left in this way serves for the insertion of a straw, rush, or series of small quills, filled with fine powder, which like the fuse reaches from the charge to the outside. A short squib which shoots a stream of sparks down the needle hole is also used occasionally. The straw or squib is lighted by some kind of slow match, made either by dipping a cotton strand in melted sulphur or soaking a piece of paper or a lucifer in the tallow of a candle ; touch-paper also is used. Dynamite, blasting gelatin, gun-cotton, and cotton-powder are fired by the detonation of a fulminating cap. A long copper cap containing fulminate of mercury is fastened into the safety-fuse by squeezing with a pair of nippers, and is then inserted into a small cartridge of the explosive (primer], and placed above the rest of the charge. Fig. 24 shows a hole charged with two dynamite cartridges, a primer with cap, and filled up with water as tamping. Sometimes gun-cotton is fired by a small charge of powder above it. Several substitutes for explosives have been tried with the object of getting rid of the flame, which is dangerous in collieries giving oif fire damp. Among these may be men tioned plugs of dry wood which swell when wetted, wedges worked by hydraulic pressure, cartridges con taining compressed air at extremely high pressures, and lastly cartridges of compressed lime which ex pands when water is brought into it. For the purpose of firing several holes simultaneously, Messrs Bickford, Smith, &, Co., the original inventors and makers of the safety-fuse, have brought out a new fuse (fig. 25), the action of which will be easily understood from the figure. An ordi nary fuse is fixed into a metal case called the igniter, from which a number of instantaneous fuses convey fire to as many separate holes. It is found in practice that this fuse answers very well. Blasting Charges may be readily fired singly or simultaneously by elec- w ith the aid of electricity, either of high tension obtained tricity. f rom a factional, magneto-electric, or dynamo-electric machine, or of low tension from a galvanic battery. The former is preferred. Fig. 26 shows a section of one of Brain s high-tension fuses. A is a cylindrical wooden case containing a paper cartridge B, with an electric igniting composition C at the bottom. Two copper wires D, D enclosed in gutta-percha E, E reach down to the composition, where they are about -fa inch apart. A copper cap or detonator G is fixed on to the small end of the wooden case. The insulated wires D, D are long enough to reach beyond the bore hole. The ends of the wires are scraped bare, and one wire of the first hole is twisted together with a wire of the next hole, and so on, and finally the two odd wires of the first and last hole are connected to the two wires of a single cable, or to two separate cables, extending to some place of safety to which the men can retreat. Here the two cable wires are connected by binding screws to a frictional electrical machine or dynamo exploder. A few turns of the handle charge a condenser, and by pressing a knob or by some other device the circuit is completed and the discharge effected. The electricity passes through the fuse wires making a spark at each break, and so firing the electric igniting composition. The flame flashes through the hole H, and ignites the fulminating mercury I, the detonation of which causes the explosion of the dynamite, blasting gelatin, or tonite surrounding the cap. One great advantage of electric firing is that the miner can retire to a perfectly safe place before attempting to explode the charge. This is important in sinking shafts, where the means of escape are less easy than in levels. A second advantage is that there is no danger of a &quot; hang fire,&quot; an occasional source of accidents with the ordinary safety-fuse. One of the greatest improvements in the art of mining Machine during the last few years has been the introduction of drills, machinery for boring holes for blasting ; most of the machines imitate percussive boring by hand, but a few rotary machines are also in use. A percussive drill or perforator consists of a cylinder with a piston to which the drill is fastened. Compressed air is made to act alternately on each side of the piston, and in this manner the drill receives its reciprocating motion. Various arrangements have been adopted for securing the automatic rotation of the drill. In some cases also the advance forward of the machine, as the hole is deepened, is also effected automatically ; but in many of the best drills this work is left to the man in charge. It is impossible within the limits of this article to describe the various drills now in use, or even to make a complete enumeration of them. The following, in alphabetical order, are the names of some of the best-known drills: Barrow, Beaumont, Burleigh, Champion, Cornish, Cranston, Darlington, Desideratum, Dcering, Dubois and Fran9ois, Dynamic, Eclipse, Excelsior, Ferroux, Frb hhch, Inger- soll, Laxey, Mack can, Osterkampf, Hand, Roanhead, Sandycroft, Schram. An account of two of the simplest, the Barrow and the Darlington drills, will be sufficient to give a general idea of the construction of these machines. &quot;The Barrow drill (fig. 27) consists essentially of a gun-metal Barrow D&quot; drill. Fig. 27. cylinder C about 2 feet in length and 4 inches in diameter, in which works a cast-steel piston-rod D, fitted with two pistons G, about 12 inches apart, mid- way between which is the tappet, or boss, G. In a valve-box on the top of the cylinder is placed the oscillating slide-valve H (shown separately), hinged at M, which is worked by the reciprocation of the tappet G coming in con tact with its lower edges, which for this purpose are formed with two slopes at each end, as shown. It has ports corresponding with openings in the slide-valve face for admitting the fresh steam or compressed air from the inlet pipe I (fig. 28) to the ports j at each end of the cylinder, and for letting the spent or exhaust air or steam escape by the exhaust pipe J. This simple arrangement constitutes the whole valve gear of the machine. &quot;The borer is inserted into a hole formed in the fore end of the piston rod, and is fixed therein by means of a screw. Its rotation is effected by hand, by means of the handle D&quot;, turning a spindle D, which is so fitted by means of the cotter d, made fast in the piston DG, and fitting in a slot in the spindle D, that the latter can slide in the piston DG, but when turned by the handle causes the piston to turn with it. The spindle D has a pinion E gearing into the pinion F, on the adjusting and feeding screw C, so that when the piston D is turned by means of the handle D&quot; the cylinder C is simultaneously pushed along the bed-plate A. These pinions can be easily disconnected by loosening the nut/, and thus the piston and the adjusting screw can be turned independently of one another when required. &quot; The borers used are respectively 1 finches, 1 inches, and 1 inch in diameter, the length of the stroke 4 inches, and the maximum number of blows about three hundred per minute. The air is