Coal-min′ing ma-chine′.
One form of the coal-cutting machine has an engine with a reciprocating piston driving a massive steel pick, in any desired direction, and at a very material saving in hewing, or kirving. The motive-power of the engine is highly compressed air, condensed by the steam-engine at the mouth of the pit, and this elastic air is conveyed by slender pipes down the shaft and along the mine to the breast where the coal is being worked. The compressed air is pumped by the steam-engine into a receiver at the pit-head during its otherwise idle hours, or by its surplus power when drawing up the coal, or pumping out the water from the mine, and is condensed to a tension of forty or fifty pounds to the square inch. It is conducted in metallic pipes, 4 1/2 inches in diameter, down to the bottom of the shaft, and thence in pipes of a smaller diameter to the workings, tubes of 1 or 1 1/4 inch caliber bringing it to the cylinder of the machine. This compressed air, when set free at each alternating stroke of the piston, imparts to the adjacent portions of the mine a pure, dry, cool atmosphere, from a well-known law of all air and [581] gases, that when compressed they develop heat, and when expanded under a relaxation of pressure, they are relatively cool.The machine is supported by a cast-metal frame of great solidity, and is of a size and weight proportioned to the character of the coal to be cut. It is constructed to give the blow of the pick, either by the pull or push of the piston.
The engine has an oscillating cylinder which has the merit of combining compactness of shape with but little complication of working parts. The machine rests upon flanged wheels and is propelled either backward or forward by a wheel and screw on a rachet and pinion, attached to one side of the engine. On the opposite side is a valve-screw for regulating by the hand the access of air to the engine. When working, the man seated upon the little stool in the rear of it moves the rachet-screw connected with the gearing of the under-carriage, and thereby propels the whole machine along the little railway or tram laid parallel to the front of the coal-seam, a small distance equal to the longitudinal nip or bite of the pick.
One machine, working 90 blows of the pick per minute, discharges, of condensed air, about 100 cubic feet per minute, which immediately becomes 300 cubic feet of cold air at the normal density, and each machine is competent to supply from 12 to 15 per cent of the ventilation required at the heading; the air being perfectly fresh, pure, and cool, and afforded precisely at the localities where the workmen are most in need of such an atmosphere. When working at 120 picks per minute, the machine cuts an inch at each stroke, 20 inches deep, 2 1/2 inches wide; a second traverse deepens it to 30 inches, and a third to 36 inches.
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