A pulley (also called a block) is a mechanism composed of a wheel (called a sheave) with a groove between two flanges around the wheel’s circumference. A rope, cable or belt usually runs inside the groove. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.
Benson John Lossing, ed. The New Popular Educator (London, England: Cassell & Company Limited, 1891)