Perhaps the most simple, economical, and safe mode of raising coal or water from deep mines is the pneumatic process, or by the use of compressed air. We think the first cost of establishing this process would not be much greater than the modes now in use ; while the constant operation thereafter would be permanent and cheap.
The mode consists in laying two air-tight tubes down the main slope or shaft, of a size sufficient to receive the cars or boxes designed to carry the coal. If boxes are used, the size of the tubes may be small,—say five feet in diameter; but if cars are used they could not well be less -than six feet in diameter, in slopes where the cars ascend end and end ; in shafts where the cars go up as they stand, the tubes should be five by ten.
Boxes are to be preferred for several reasons in pneumatic tubes, since they require much less space, and the load to be lifted is reduced by the weight of the car, or two tons. If a tube of five feet diameter be used, the area is 25 square feet, or 3600 square inches: this, at a pressure of five pounds to the square inch, will lift a weight of 18,000 pounds. In order to obtain the pressure, two blowing-cylinders of a capacity of 250 cubic feet, to be run at 25 revolutions or 25 double strokes per minute, and at seven or eight pounds pressure—say 71- pounds—per square inch, are erected on the surface, and two direct-acting high-pressure steam-engines of 30-inch cylinder are placed perpendicu larly below them, so that the pistons of the engines connect with the pistons of the blowing-cylinders. This will give 21 pounds per square inch excess of air for escape and contingencies.
Thus prepared, the cages may be made of sufficient size to carry five tons of coal and three tons of water, leaving a surplus of 900 pounds to operate in favor of balancing the boxes, which are connected by a small rope capable of suspending the empty cage from a pulley over the tubes. At 25 revolutions per minute, the blowing-cylinders will
produce a third more compressed air than is necessary, by computation, to raise this amount of coal and water 1000 feet high every minute.
This power will raise 1000 tons of coal in ten hours with perfect ease,—giving three minutes time to each load, or one minute for loading, one for ascending, and one for unloading,—and 600 tons of water. While the box is standing the air is accumulating, and therefore compressed ready for instantly throwing up the boxes from the bottom of the mine. In case larger tubes are used, larger loads may be raised with the same pressure per square inch, or the same load with less pressure. The water-boxes are suspended beneath the coal-boxes, and fill themselves from the "sump" while the coal is being loaded into the coal-boxes at the bottom of the shaft or mine, and the water is discharged on the top at the same time the coal is being unloaded.
The whole operation is simple, and governed by valves : there are no pumps or chains or heavy ropes required, and a serious accident is impossible. We do not think the first cost of erecting machinery of this character can be greater than that of the machinery now in use, since the power of the engines would be one-half less than that of the hoisting- and pumping-engines required to do the same work at the same depth ; and, although the tubing would be expensive, there would be no outlay for pumps, rods, and timbers, since they would not be required.