TRACTION, in Mechanics, is the act of drawing a body along a plane, usually by the power of men, animals, or steam ; as when a vessel is towed on the surface of water or a carriage moved upon a road. The power exerted in order to produce the effect is called the force of traction.
Numerous experiments have been made for the purpose of ascertaining the value of a force so exerted. When men are employed to draw laden boats on canals, it is found that if the work be continued for several days successively, of eight hours each, the force Of traction is equivalent to a weight of 311 lbs., moved at the rate of two feet per second, or 11 mile per hour (it being understood that such weight is imagined to be raised vertically by means of a rope passing over a pulley, and drawn in a horizontal direction). The force of traction exerted when, without moving from his place, a man pulls horizontally against a weight so suspended, is estimated at 701bs. Mr. Tredgold considers that a horse exerts a force of fraction expressed by 125 lbs. raised at the rate of 81 feet per second, or 21 miles per hour. A man or a horse can however double his power of traction for a few minutes without being injured by the exertion; and when the carriage is in motion, so that the friction on the ground is alone to be overcome, a horse can draw, during a short time, on a level road, a weight exceed ing 1600 lbs.
Experiments have shown that when the angle of traction, as it is called, that is, the angle which the plane of the traces makes with the road on which a carriage is moving, is 15 or 16 degrees, a horse pulls with good effect ; and the height of the points at which the traces are attached to a horse's collar being about 4 feet 6 inches from the ground, it follows that, in order to obtain this incli nation, the lower extremities of the traces or shafts should be 2 feet 3 inches from the ground. In general however, in two-wheeled carriages, the height of these extremities is about 3 feet, As an example of the force of traction exerted by steam, it may be stated that on a level line of railway, an engine with an 11-inch cylinder, and having an effective pressure of 50 lbs. per square inch in the boiler, drew 50 tons at the rate of 30 miles per hour, working 10 hours daily; and that the same engine, with an equal pressure in the boiler, drew 160 tons at the rate of 151 miles per hour.