262 Our Surroundings 



High-speed locomotives frequently use two large drive wheels 

 on each side. Heavy freight locomotives use three or four smaller 

 ones, and tremendous compound locomotives may use as many as 

 twelve drive wheels on each side, divided into two or more sets, 

 each set having its own cylinder. 



Great strain comes on a locomotive in starting a heavy train. 

 Once it is started, momentum helps to keep it going. If it happens 

 that tracks are oily or frosty, the drive wheels may simply spin 

 when the engineer attempts to start the train. This trouble is 

 usually overcome by sanding the rails, that is, by letting a small 

 amount of sand flow through a tube from a sand box on the top 

 of the locomotive to the track immediately in front of the 

 wheels. 



Owing to momentum, caused by their exceptionally heavy 

 weight and speed, modern trains are exceedingly difficult to stop, 

 once they are started. In the early days, with lighter trains, hand 

 brakes were used. Today all trains are equipped with modern 

 air brakes. The locomotive runs an air pump which compresses 

 the air for the brakes, storing it in a reservoir from which it flows 

 through an air hose to the car reservoirs. When it is desired to 

 set the brakes, the engineer opens a valve in the air hose, reducing 

 the pressure. At once the air in each car reservoir forces open 

 a control valve and exerts its pressure through an air cylinder 

 against the brake shoes, forcing them against the wheels. When 

 the engineer restores the pressure, the control valve is reversed 

 and the brakes are released. 



The Gas Engine. Just as the invention of the stationary 

 steam engine led to the locomotive, so the invention of the gas 

 engine led to the remarkable modern development of the auto- 

 mobile. In the steam engine, water is heated to steam and this 

 steam drives the piston back and forth. The problem in steam 

 engines is that of keeping the cylinders hot, so that the admitted 

 steam will not cool and lose its power before it fully expands. 

 In the gas engine, the fuel used is burned right in the cylinders, 

 and the real problem has always been to keep the cylinders cool 

 and so to prevent the expanding of the metal pistons to a point 

 where they would bind in the cylinders. 



