Traction engine's gear. Close-up of a worm gear on the steering column of an 1886 Marshall agricultural steam traction engine. The worm gear consists of a toothed wheel (at lower right) which is worked by a short, revolving cylinder (at upper left) known as the worm. These types of gear transmit power between perpendicular shafts. The vehicle was powered by the expansion of steam in an engine above a boiler. Agricultural steam engines were used for powering harvesters and threshing machines, and as tractors, diggers and a means of transporting heavy loads. This engine is in the Engineerium, Hove, England.