tractor, in agriculture, vehicle used to pull such equipment as plows, cultivators, and mowers; to power stationary devices such as saws and winches; and to push snowplows and earth-moving implements. Early tractors were steam-driven; now they are generally powered by gasoline or diesel engines. The two main types are wheeled tractors and crawlers, or caterpillars, which move on treads. Advances in technology resulted in the number of tractors in the United States increasing from 600 in 1907 to 3.4 million by 1950. The power takeoff, which transmitted power from the tractor engine to an implement via a shaft, was introduced in 1918. The small general-purpose tractor followed c.1924. Pneumatic rubber tires, affording increased speed, easier operation, lower fuel consumption, and longer wear, were introduced in 1932. Four-wheel drive and diesel power increased pulling power in the 1950s and 60s. These advances led to today's very large, double-tired tractors with enclosed, air-conditioned cabs, capable of pulling several gangs of plows.
See R. Leffingwell, The American Farm Tractor (1991).