When hoisting with tackles, they should never be allowed to twist. If they show a tendency to do so, insert a bar in the block or sling, and use it as a lever to hold it straight.
It frequently happens that the men cannot apply their full strength in the direction in which it would be most effective. In such cases hook a single block to some object about two feet above the ground and reeve the end of the fall through it, so that the men can add their strength to their weight, and more men can apply themselves.
Never trust the suspension of a weight to holding it by the unaided strength of men. If it is possible to get a turn around any fixed object, even in raising or hauling a weight, it is best to take a turn, as all that is gained is then saved.
Always select such blocks that the fall will run freely through them, and not ride upon the edges of the sheaves. If it does, it will be certain to cut. The rope should not quite fill the groove on the sheave. In this way excessive friction is avoided.
The power gained by using tackle is as follows:
Two single blocks, or gun-tackle: nearly doubled.
Luff-tackle, double and single block: doubled. If the double block is movable, trebled.
Two double blocks: power × 3⅓.
Double and treble blocks: power × 4.