FIG. 17.-Holt Caterpillar Tread Tractor. A Distinctive Design Suitable for Soft Soils or Any Other Condition Where Wheel Tractors Would Work at a Disadvantage.

the gear case which in turn operates the largest gear on the differential casing. When the largest gear on the main shaft is engaged with the smallest gear on the differential box the high speed forward is obtained.

The differential mechanism, the action of which will be described more in detail later, is necessary with all forms of self-propelling vehicles where two wheels are driven from a common source. Without it the automobile or traction engine would not be practical. It permits the outer wheel to revolve faster than the inner wheel when the machine is turning a corner. Leading from the differential gear are two shafts which carry bull pinions at their outer ends. These engage with the bull gears which drive the large rear wheels of the tractor. The rear axle is a substantial fixed member on which the rear wheels revolve.

The cut previously described shows clearly the relation of the power plant to the driving wheels and outlines the various gears through which the engine power is transmitted and by which the engine speed of four or five hundred revolutions per minute is reduced to a few revolutions per minute of the driving wheels. The side elevation outlined on the large folding plate, Fig. 16, outlines clearly all the parts of a large capacity tractor of the Caterpillar tread type. As the key clearly defines the various parts their functions will be readily apparent.

Some Distinctive Designs.-Special designs of tractors have been devised to do work more efficiently than the conventional forms. For example, for overcoming the disadvantage of very soft ground no device appears to be more successful than the substitution of a Caterpillar tread for the ordinary type of traction wheel. A tractor of this form is shown at Fig. 17 and has proved very satisfactory in practice. It has been used on

swampy ground where wheel tractors could not turn a wheel and it has found a field of usefulness in hauling lumber on the soft snow-covered roads of the great Canadian forests. This idea is several generations old but it is only recently that tractors having this type of transmission have been put on the market in large numbers.

The weight of the tractor is supported on steel rollers, which run on the inside of a continuous belt made of steel plates, joined together so they can run around wheels at the front and rear of the truck. This means that the tractor lays a track and picks it up as fast as it is no long needed. The front of the tractor is supported by a single wheel, so under actual working conditions nearly all the weight of the tractor is carried on the rear where it is distributed over such an area of ground eontact that the weight carried is but five or six pounds per square inch as compared to the forty to ninety pounds ordinarily supported by each square inch of wheel surface in contact with the ground on the conventional type of tractor. This Caterpillar tread and driving mechanism are described in detail in the chapter dealing with the tractor frame, wheels and axles.

Some experiments have been made abroad with powerpropelled machines carrying a large drum at the rear driven by chain connection from the rear wheels. These drums carry a number of teeth or small spades, which dig into the ground and break up the surface in the same way that a multitude of picks would do. Other forms have rotary ploughs carried at the rear end.

A tractor that has been well received by many farmers is built on the order of the motor truck used in cities for hauling merchandise. It will carry a load of three tons in the wagon bed with which it is provided

and has a pulley for driving stationary machines. A drawbar is provided for pulling ploughs and other machines requiring draft. The weight of this truck is carried on four wheels, therefore when used for ploughing ballast must be put over the driving wheels in order to provide proper traction. As the machine is spring mounted and adapted to speeds from two to fifteen miles

FIG. 19.-An English Tractor With Three Bottom Gang Plough. This Machine Driven by Two Cylinder Vertical Motor Utilizes Front Wheels for Steering and Traction Duty as Well.

per hour it can be used for a wide variety of work. Instead of using rubber tires as the ordinary motor truck does, the wheels are provided with wooden plugs to adapt the tractor to hard roads. Traction on soft ground is provided by an extension rim with a number of mud lugs attached, which is so fastened to the wheel that these automatically grip the soil when the wheels sink to a certain depth. It is also possible to move a hand