Shortly after the Sumerians invented the wheel (perhaps 6,000 years ago), engineers worked out how to multiply the brute force of men or beasts by rotating smaller wheels against bigger ones. Wooden gear wheels with pegs for cgs served for centuries, first with levers and pulleys to lift heavy materials in building, then for tensioning catapults and controlling anchor hoists.
Better metalworking techniques in the 18th century allowed a vast increase in metal gearing, used in the tiniest watches and in huge steam locomotives. Metal-toothed wheels could move faster than the old wooden peg system, and inaccuracies in the relationships between wheel ratios could be offset by matching the number of teeth in a wheel.
For more on gears try www.howstuffworks.comor www.efunda.com Victoria Neumark