The combination of auto-tractor and trailer is very convenient, for it not only enables the tractor to deposit its load without any delay, but it can be used to haul all sorts of trailing cars, and often a number of them in a train. The method of connecting the tractor and trailer by a fifth-wheel device is shown in the illustration. Tracks T are mounted on the rear frame F of the tractor, and a swivel S is set between the rails. The fifth-wheel device con sists of an axle centrally mounted on this swivel, and two light wheels W W that run around on the tracks, and can be swung about in a half circle. The forward end of the trail ing car C is mounted and fixed on this fifth wheel device. The fifth-wheel device of course takes its name from the fifth wheel on the front axle of an ordinary wagon, though it has been developed into a mechanism that is much more than a wheel. Some manufacturers use a swiveling pair of bolsters instead of this fifth-wheel device. When a trailer is loaded ready to start, the tractor is backed up to it, and hitched on by simply guiding the two small wheels of the fifth-wheel device up the tracks; or, if an automatic coupler is used, the method of connecting may be very similar to that used in coupling railway cars. It is necessary to provide trailers with brakes, else they might create trouble in going down hill, and these have to be connected with the tractor, so as to be operable in unison with the brakes when set by the driver. The connection between tractor and trailer must also be such as to permit a rocking motion, so that when passing over uneven roads all the wheels shall travel on the road without straining the framework.
Military Tractors.— The military ((tank" which attained fame in the World War was based on a peculiar form of tractor designed to work in a rough country, as in logging opera tions, canal building, etc. The very heavy end less chains, bearing feet on each link, fitted these machines for the most arduous pulling work under the most trying conditions, and some military man among the British recog nized,their possibilities when armored, and the military "tank" was the result, introducing this type of machine to untold thousands who other wise would never have heard of it. Though too slow of operation to compete with the ordi nary tractor of auto-truck type, yet the tank tractor has such superiority in marshy, stony and very rough ground, that it has obtained considerable use.
The Synmotor.— Another peculiar form of tractor that has been invented by a New Jersey genius is called the synmotor. It is an adapta tion of the motor-cycle to agricultural traction. Instead of pneumatic tire wheels, this machine is provided with steel rims and angle grips, like the wheels of big farm tractors, and is designed to draw cultivators, seeders and small mechanism used in truck farming. Its peculiar ity lies mainly in its being attached by a wire to a central post, around which it circles in re ducing spirals. When started on a circle of ground it does its work automatically until the entire circle of 100 or more feet in diameter is plowed or cultivated, excepting a small por tion in the centre. It operates in a four- to seven-foot swath, and uses gasoline fuel.
The drawing or pulling device of a rope way or wireway is also termed a tractor. See MOTOR TRUCK ; TANK.