System of Pullies.
To carry Guns &c.
Heights and distances.
In the system of pulleys (vide [plate], Mechanics) the Power is shown at the hooks of the moveable blocks, which are to be applied to the bodies, or weights, requiring to be moved or raised. The strain is also shown at the fixed blocks.
In [Fig. 3], there are three parts of the rope engaged in supporting the weight—viz., the parts marked 1, 1, 1. Each of them, hence, sustains one-third of it, and the fall of the rope to which the power is to be attached requires the Power = 1, if weight = 3. The same principle of calculation is applicable to all systems of pulleys having one fixed block, any number of moveable wheels, and a single rope over all the wheels. Hence, in such a system of pulleys, gravity being applied, there will be an equilibrium, when the weight is as many times the power as there are portions of the rope employed in sustaining the weight. For example, in a system consisting of six moveable sheaves, the same rope going over them all, there will be 12 portions of the rope engaged; and to produce an equilibrium the power must be equivalent to 1/12 the weight, no allowance being made for friction.
From the foregoing observations, and by referring to the plate, it will be seen that each tackle has two applications, differing in power one from the other; for example, if the double block of a luff tackle is fixed to a weight to be moved, and the single block to a picket, or other fastening, [Fig. 6], then, if one man haul on the fall, the power of four men will be applied to the weight (4 to 1), and the power of three men to the picket; but if the double block be fixed to the picket, [Fig. 5], and the single, block to the weight, then the force of only three men will be applied to the weight (3 to 1), and a power of four men to the picket, or fastening.