shifting, and practically all the variations of speed that are desired on average highways may be obtained by simply manipulating the throttle lever which supplies more gas to the engine. The speed changing lever is left in its direct drive position and is not moved to the lower gear ratio except when starting the car or when ascending steep grades. The modern gasoline automobile is just as suitable for city work as either the steam or electric cars and is much superior as a touring conveyance.

Fig. 2.-Side View of Typical Heavy Truck Design With Power Plant Placed Under Operator's Seat.

Q. Into what classes may automobiles be divided?

A. The two main classes of automobiles are pleasure vehicles and commercia cars.

Q. How do they differ from each other?

A. Pleasure cars generally are built for high speed and are much lighter than the commercial vehicles which must carry heavy loads. In a pleasure car as a rule, everything is sacrificed to speed and comfort. In a commercial vehicle, the parts are large and substantial and everything is designed with a view to maximum carrying capacity and minimum operating expense. The parts of automobiles are the same in general principle, whether used in a touring car or truck, except that they are made heavier, and of greater strength when used in the vehicle carrying the greatest loads. Pleasure cars

are usually mounted on pneumatic tires, as air filled tires are called, while motor trucks which travel at slower speeds use solid rubber tires.

Motor trucks may be divided into two classes: those which utilize chassis patterned after pleasure types, but fitted with delivery bodies, as shown at Fig. 1, and those types of which a typical example is shown at Fig. 2, in which the motor is carried under the driver's seat and where the general design is such that the chassis can be used only for commercial purposes. There are classes of commercial cars, such as taxicabs, light delivery wagons, ambulances and vehicles of this nature, which are practically pleasure car chassis fitted with forms of bodies adapting it to a commercial use.

Q. What are the essential elements of the automobile?

A. The principal parts of automobiles are shown at Figs. 3, 4, 5, these outlining the chassis of pleasure cars and a typical truck. The foundation for the entire mechanism is the frame, which is attached to and supported by springs which are bolted to the axles on which the wheels are mounted. The power plant, which consists of the gasoline engine and a number of other parts, is usually carried at the front end of the frame, where it is housed under a hood, or in a special compartment under the body where it can be easily reached.

The clutch and gear set form part of the transmission system, as does the chain or propeller shaft used to transmit the engine power from the change speed gearing to the rear axle. A steering gear by which the direction of car is controlled and levers for shifting the change speed gears, for controlling the brake and for operating the spark and throttle of the engine, complete the chassis assembly. An automobile may be said to consist of the following main groups: the power plant; the clutch and change speed gearing; the power transmission system; the supporting frame and springs; the axle and wheels; the controlling elements; and the load carrying or body member. (Figs. 4 and 5.)

Q. What is the function of the power plant?

A. The purpose of the power plant is to generate enough power so that when this is transmitted to the rear wheels thru the medium of suitable reduction gearing, it will propel the vehicle.