BLOCK AND TACKLE, one of the most useful and ex tensively applied of mechanical devices. The principle is repre sented at A in the diagram, showing the suspension of a load \'V by two "parts" or "falls" of rope, so that a weight of I lb. applied at P will pull a load of 2 lb. at W ; this assumes friction to be ab sent, and the rope perfectly flex ible. By increasing the number of pulleys and consequently falls of rope, in the manner shown at B, the mechanical advantage is increased, according to:— W =nP n being the number of parts of rope that spring from the mov able block. If, for instance, there are three pulleys at the top, and two on the moving block, it will be evident that the weight is sup ported by five parts of rope, therefore a force of 120 lb. will hoist a load of 600 lb. Blocks are used for rigging, tightening guys, lifting loads, and hauling.
The objection that the load is not held, as it is in the differential pulley-blocks, and worm-geared types, may be obviated by fitting a friction brake. The block B is of this self-sustaining form, the act of moving the hand-rope outwards or to one side applying the brake.