Transportation on Land 265 



the radiator and cylinders. In fact, some motors are wholly 

 air-cooled by means of large fans which suck air past the cylin- 

 ders whose exteriors are so shaped as to give a large radiating 

 surface. 



The motions of the pistons in the gasoline motor are carried 

 to a crank shaft and changed from back and forth motion to cir- 

 cular motion. By means of a clutch, which is a device for connect- 

 ing or disconnecting parts of machinery at will, this motion is 

 continued through a transmission rod, or main shaft, to the rear 

 axle and thence to the rear wheels. Speeds are in part regulated 

 by the gear shift, which is a collection of interlocking cog wheels 

 of various sizes. By means of the gear shift, great power can be 

 applied at slow speed or, by shifting to other combinations, less 

 power at higher speeds. Most gear shift systems allow three speeds 

 forward and one back. 



The motor constitutes the power plant. The various connec- 

 tions which drive the car constitute the transmission system. In 

 addition, there is an electric system, consisting of the generator 

 which charges the battery while the motor is running; the battery 

 itself for the storage of electricity; the wiring and spark plugs 

 to fire the gasoline ; the lighting system ; and the self-starter, the 

 motor which cranks the engine for starting. 



There are other systems as well, such as the control system 

 which causes the car to stop and to change its direction. 

 It includes the steering device and the brakes. There is also 

 the cooling system, consisting of the radiator, water pump and 

 entire circulatory apparatus for keeping down heat. There is the 

 carburetor system, which consists of a vacuum tank to draw gaso- 

 line from the fuel tank, and a carburetor in which gasoline is 

 mixed with air to form an explosive vapor. 



The gasoline tractor is also finding a steadily wider use. It is 

 employed in road construction and in hauling loaded cars. In 

 many sections it is widely used in agriculture. A modern tractor 

 will draw a series of plows or harrows, preparing many acres of 

 ground a day for crop planting. It hauls reapers, harvesters, and 

 cultivators. On larger farms where the soil is free from boulders, 

 it is rapidly taking the place of horses. 



