INGERSOLL-SERGEANT STRAIGHT LINE AIR COMPRESSOR.
Soon after the machine drill became a success dynamite was invented, and these two have been the greatest factors in bringing about that rapid development and production which is the most pronounced attribute of modern mining. Dynamite alone has doubled the amount of ore which can be extracted from a face in a given time. Le Neve Foster, in his work on mining, gives the rate of advance in driving a tunnel by fire setting at two fathoms per month. Compare with this the Niagara Falls tunnel, driven with power drills and high explosives, 342 feet in four weeks.
It is probably to the power drill more than to anything else that we are indebted for the development of the air compressor; the exhaust from a steam drill and the heat emitted from the pipes being very disagreeable under ground. As early as 1800 a Welsh engineer had attempted to run a blast by means of a water power a mile and a half distant, but it was not until 1865 that machines were operated to any extent by compressed air. The great difficulty had been the loss of efficiency, owing to the clearance spaces and the heating of the air. In driving the Mont Cenis tunnel but 16 per cent of the power developed was available, and up to 1880 the efficiency was extremely low; but to-day as high as 80 per cent is obtained. The air compressor is simply a force pump with ingenious devices to overcome the loss of energy. For ordinary use the air is compressed to a pressure of from 60 to 80 pounds per square inch. This is done in a single cylinder for low pressures, but for high pressures two cylinders are used. From the compressor the air is conducted to a reservoir, from which it is piped to the machine which it is to run.
INGERSOLL-SERGEANT DUPLEX STEAM-DRIVEN AIR COMPRESSOR.
One of the advantages of air-driven machines under ground is that the exhaust furnishes fresh air to the miners and cools the atmosphere. The result has been that in metal mines, where there are no noxious gases escaping from the ground, the exhaust from the air-drills, together with the natural air currents, has supplied sufficient ventilation. In the coal mines, however, it has been necessary to employ other means. After it was found that, even with the safety-lamp, gas would be exploded if a large amount of it had accumulated, more attention was paid to ventilation. Levels and shafts were divided to produce a natural current; the size of the drifts was carefully figured in order to regulate it; doors were put in to compel it to follow the faces; devices were adopted to split it, a part going to one room, the remainder to a second; and boxes were built to carry one current across another. Early in the century hand fans run by a wheel and pinion had been employed for forcing the air down the shaft, but it was soon found that the circulation produced in this way was inferior to the result of eduction. Large furnaces were then constructed at the bottom of the upcast shafts, in order to cause a strong upward current. Again, huge air pumps, run by machinery, were tried for exhausting the air. By 1850 exhaust fans were coming into use, and these, occasionally replaced by blowers, also used for exhausting, are now generally employed. The Guibal, which has been the most prominent of the fans, has been made as large as forty-six feet in diameter. The Capell, which is an improved form of the Guibal, has six curved veins, or blades, and is made from eight feet to fifteen feet in diameter. It is driven quite rapidly, making from one hundred and eighty to three hundred revolutions, and having a capacity of from one hundred thousand to three hundred thousand cubic feet of air, per minute. The result of this thorough ventilation is that the gas is removed from the mine almost as rapidly as it enters, and often the safety-lamp is no longer needed by the common miner. Nevertheless, it has by no means become useless, since as an indicator of the presence of gas it is invaluable. The action of the different lamps in the presence of gas varies, but in general the size of the flame increases in direct proportion to the increase in the amount of gas mixed with the air. Each morning, before the men go to work, the fire boss takes his safety-lamp and makes the round of the mine. When he goes into a room he watches the flame, and if it burns up to the point which indicates that it would not be safe to enter with a naked light, he makes a mark on the wall which serves as a danger line beyond which the men do not go.
Another machine, which, like the fan, has been developed by the demands of the coal mines, is the coal-cutting machine. Probably the lot of no man was as hard as that of the coal-digger at the beginning of the century. After he had performed the dangerous task of exploding the accumulated gases, he was often forced to work all day lying in the most constrained attitude. Applied in this manner, his power was largely wasted, and much useless dust and small coal was produced. The first effort at relief was a machine which imitated the miner, striking a blow with a pick worked by a lever, and making as high as seventy blows a minute. These have been generally replaced by quite another type of machine, one which depends on the action of either a rotary bar, a rotary wheel, or a chain cutter. These machines are operated by either air or electricity. The Jeffrey rotary bar cutter will undercut a block of coal thirty-nine inches by fifty-four inches in six minutes. The chain-cutter is an endless chain carrying cutting knives and traveling horizontally. It is claimed that these machines will effect a saving of about ten cents a ton in the cost of mining.
When in 1848 the finding of gold in California was reported, followed in 1851 by the discovery of the Australian fields, large numbers of men were attracted to the placer mines, who later, as the placers became exhausted, turned their attention to vein mining. Nor did hydraulic mining itself fail to progress. When the placers were first discovered, the miner, standing in the shallow stream, washed the gravel, a panful at a time, and secured from fifteen to twenty-five dollars a day. As the placers became poorer he built sluices, and, shoveling in his gravel, turned the stream in to wash off the light rock, while the heavy gold was caught in the interstices between the blocks with which he had paved the bottom. If the ground became clayey, he brought part of the water through a hose and used it to break up the lumps in his sluice box. Then as he gradually removed the gravel and the banks about him became higher, he turned his hose toward the bank and brought more water from a higher level, until, to quote Bowie, “a forty-inch wrought-iron pipe has been substituted for canvas hose and a stovepipe, and an inch stream replaced by a river of water discharged through a nine-inch nozzle under a four-hundred-foot pressure.” By this means, at North Bloomfield, Cal., nearly a million yards of gravel, containing but two and nine tenths cents per cubic yard, was moved in a single season, and at a profit.
ELECTRIC COAL-MINING MACHINE.