Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/463

Rh EXPLOSIVES.] MINING 445 largely employed in quarries; occasionally it is ttsed in boring holes horizontally, as for instance in the salt mines of Cheshire. The jumper is held in the desired direc tion, lifted up, and thrust down ; it is turned a little after each stroke. However, the miner s tool is generally the borer proper, or drill (fig. 22), which is a bar of round or octagonal steel, usually from ^ inch to 1 1 inches in diameter, with one end forged into a chisel-shaped edge, the exact shape and degree of sharpness varying according to the hardness of the rock. The hole is bored by striking the drill with a hammer or sledge and turning it after each blow. Boring is said to be single-handed if the miner lg holds the drill in one hand and strikes with the hammer in the other, whilst it is called double-handed when one man turns and an- IF &quot; other strikes. The f (n hammers for single- jf handed boring usu ally vary in weight from 2 to 6 or 7 Hi. FlG - 23. Sledge-Hammer. The double-handed boring hammer, or sledge (fig. 23), weighs from 6 to 10 lb or more. If a hole is directed downwards, the miner pours in a little water and bores the hole wet. From time to time he draws out the sludge with the scraper, a little disk at the end of a metal rod, and he takes a fresh borer when the tool he is using has become blunt. The depth bored varies with the rock and the nature of the excavation ; but in driving levels in the ordinary way the depth is commonly from 18 inches to 3 feet. Holes for blasting are sometimes bored by tools like carpenters augers. One of the simplest, which is used in some French slate-mines, is very like a brace and bit, and the tool is kept pressed against the rock by means of a screw fixed in a frame resting on the ground. The pricker, or needle, is a slender tapering rod of copper or bronze, with a ring at the large end. It is used for maintaining a hole in the tamping through which the charge can be fired. The use of needles made of iron is prohibited in many countries, on account of the danger of their striking sparks which might fire the charge. The tamping bar, or stemmer, is a rod of iron, copper, or bronze, or iron shod with copper, and it is used for ramming in dried clay, slate pounded up, or other fine material, upon the powder, and so creating a resistance sufficient to make the gases generated by the explosion of the charge rend the rock in the manner required. The claying bar is used for lining wet holes with clay, and so rendering them temporarily watertight. Shovels vary much in different districts. In the south west of England the long-handled shovel is preferred to the common one with a short handle ; in Germany the ore or rubbish is frequently scraped into a tray with a sort of hoe. In addition to these tools the miner requires an ex plosive, and a means of firing the charge at the bottom of the hole which will give him time to escape. Twenty years ago gunpowder was the only explosive in common use in mines, but at the present day its place has been taken to a very large extent by mixtures containing nitro-glycerin or gun-cotton. The powder used for blasting in mines usually contains less saltpetre than that which is employed for sporting or military purposes. The following is an analysis of mining powder by Captain Noble and Sir F. Abel: 1 Saltpetre 61-66 Potassium sulphate 12 ,, chloride 014 Sulphur 15-06 Carbon 17 93 Hydrogen 0-66 Oxygen 2 23 Ash 59 Water... , 1-61 100-00 &quot;On Fired Gunpowder,&quot; Phil. Trans., 1880, p. 225. Gunpowder compressed into cylinders of diameters suitable for bore-holes, and provided with a central hole for the insertion of the fuse, has lately been brought forward with some success. Nitro-glycerin or glyceryl nitrate is a light-yellow oily liquid which is very sensitive to shocks ; under the action of a fulminating cap it explodes with great violence. Its chemical composition is expressed by the formula C 3 H (NO 2 ) 3 O 3 or (C 3 H 5 )3N0 3 ; its specific gravity is 1 6. It has been found so dangerous that its use by itself has been given up ; but on the other hand the mix ture of nitro-glycerin and infusorial earth (Kieselguhr} called dynamite or giant powder is now one of the commonest explosives met with. It has the advantage over powder that it is far more powerful, that it may be used in wet holes or under water, that it is very effective even in ground full of &quot; vughs&quot; or cavities, and that it requires no hard tamping, which is always a source of danger. Its plasticity too enables it to fill the space at the bottom of a bore-hole, which is rarely a true cylinder, more completely than any solid cartridge can do. One disadvantage is that it has to be thawed in cold weather, and there is also the fact that occasionally the whole of a charge of dyna mite fails to go off, and unnoticed remnants have exploded and caused serious and even fatal accidents when struck with the pick or borer. The danger is enhanced when the remnants have been left in contact with water, which causes a separation of the sensitive nitro-glycerin, so that even a blow upon the adjacent rock may lead to an accident if any of the explosive oil has leaked into cracks. The strongest dynamite contains about 75 per cent, of nitro- glycerin, the rest being kieselguhr. A newer explosive is blasting gelatin; it is made by mixing nitro-cotton with nitro-glycerin, until enough nitro-cotton has been dissolved to convert the nitro-glycerin into a jelly-like mass. The blasting gelatin in ordinary use contains no less than 93 per cent, of nitro-glycerin, with 7 per cent, of nitro-cotton, and its strength is very great. Gun-cotton per se is not much in favour in ordinary mining ; but mixed with some nitrate or mixture of nitrates, such as the nitrates of barium and potassium, and known as cotton powder, tonite, and potentite, it is employed extensively. Though not quite so powerful as dynamite, nitrated gun-cotton possesses the important advantage of not requiring to be thawed in cold weather. As in the case of dynamite, accidents have been caused by remnants of charges ; and with both explosives it is neces sary to examine carefully the bottoms of all holes after blasting, and to destroy any possible remnants by firing off a detonator in any bottom or &quot;socket&quot; which cannot with certainty be pronounced free from danger. The commonest method of firing a charge is by means Safety- of the safety-fuse, a cord containing a core of gunpowder ^ use - introduced during the process of manufacture ; it may be rendered waterproof by tar or gutta-percha. In blasting in the ordinary way the charge of gunpowder is put in either loose or enclosed in a paper bag, and it is pressed down to the bottom of the hole with a wooden stick, whilst a piece of fuse also is inserted extending from the charge well beyond the hole. If the powder is loose the miner carefully wipes down the sides of the hole with a wet swab stick (a wooden rod with the fibres frayed at one end), or with a wisp of hay twisted round the scraper, in order to remove any loose grains adhering to the fuse or the sides of the hole, and then presses in a wad of hay or paper. A little fine tamping, often the dust from boring a dry hole, is now thrown in and rammed down with the wooden charging stick, and the same process is repeated, and when harder tamping is required the metal bar is brought into operation, until the hole is completely filled. As the safety fuse burns slowly, at the rate of about 2 or 3 feet a minute, the miner can secure ample time for retreat by taking a sufficient length. It is usual to ignite the fuse by a caudle-end fixed under it by a piece of clay, and it takes a little time for the candle to burn through the fuse. The old plan of firing a charge, which is still in use in many