"Worm" is another word for the thread of a screw. This device combines a screw with a gear. As the gear turns, a heavy weight can be pulled up by turning a crank. When using a pulley, if you let go, the weight falls down again. With a worm gear, you can let go and the weight will stay suspended at the same height. You can rest and then continue pulling up the load. The English clock maker Henry Hindley reinvented the worm gear in the 18th century.
