We will snake our calculations, however, without reference to the unequal poise, and estimate the weight of cars and cages as equal, and, therefore, as counterbalanced. In shafts of great depth it seems proper to raise as much coal as practicable at once: therefore we calculate cars of 4 tons as the maximum; but the size of the cars muse depend on the size and dip of the seam. If the seam is small, large cars cannot be used; if the dip is considerable, yet not steep enough to run, it would not be conve nient to take large cars into the breasts or chambers. The size of the car, and the amount of coal to be raised each time through the shaft, depend on these conditions. A capacity of four tons may be considered a maximum, and one ton a minimum, for a car. " The load in a shaft 1000 feet deep, provided the amount of coal be four tons, may be estimated at 25,000 pounds ; viz.: coal 8960 pounds, car and cage 8960 pounds, and a rope of 2 inches diameter, weighing 7.05 pounds to the foot,-7050 pounds,—giving a total weight of nearly 25,000 pounds to be lifted by the rope. But the load on the engine is only the coal and the rope, if we suppose the car and cage to be in equipose: therefore the load which the engines have to start from the bottom of the shaft is 16,000 pounds, in the middle of the shaft 8960, and at the top of the shaft only 1910 pounds. A single engine of 200 horse-power, running at 20 revolutions per minute, would have enough to do in starting such a load and raising it at the rate of 500 fees per minute ; but two 100 horse-power connected engines, running at 40 revolutions per minute, would do the work with ease.
In order to equalize the load, and give, consequently, additional power to the engines, flat ropes, winding on narrow drums, increasing the raising drum and decreasing the descending drum, are the best means. The great difference between the power re quired to start the load from the bottom, or that required from the middle up, is evident. Two fifty horse-power engines would be more effective with flat ropes and narrow increasing and decreasing drums than two one hundred horse-powers with the round and common stationary drum would be.
The same thing, however, may be accomplished by cone drums, having grooves in which the rope must wind in the form of a screw, increasing in diameter on the raising side and decreasing in diameter on the descending side. Thus, the rope might start to raise the load on a six-feet drum and land it on a twelve-feet drum. The objection to this mode is the size of the rope which must be used to raise the load required, and the small dimensions of the drum. A two-inch wire rope should not be wound on a drum of less than 12 feet, particularly on starting with 25,000 pounds weight : consequently, to use the cone drums effectively, the starting or smallest diameter should be 12 feet, and the largest diameter 24 feet, which would be ponderous and costly. But by the other mode, using flat ropes and narrow drums, with the rope winding on itself, this objection would not exist. The decrease in the size of the ropes allows of a corre sponding decrease in the diameter of the drums. If the load is started on a drum 7 feet in diameter instead of one 12 feet diameter, with the same proportions of pinions or leverage, the power to do the work is proportionately increased ; but, in addition to this advantage, the counterbalance is increased in nearly equal ratio, since the drum is over ten feet in diameter on the descending to seven feet on the ascending side, and the balance in favor of the load is correspondingly increased. The descending car and cage, weighing 8960 pounds, hang 5 feet from the centre of the drum, while the ascending load of 25,000 pounds hangs only 31 feet from its centre : this would increase the descending weight as a counterbalance by making it equal to 11,684 pounds, and the ascending load to be lifted by the engine would be 13,352 pounds. Of course some thing must be allowed for friction, and the loss of equilibrium effected through a set of ponderous machinery ; but the above calculation is nearly correct ; and hence we gain by this mode of constructing drums and connecting engines over the old mode of single engines and stationary drums not only in first cost, but in effectiveness of ma chinery and in durability of both ropes and machinery.