THE LOCOMOTIVE 253 



with the rail, and so there is less slipping when the train 

 starts. This allows better time to be made when there are 

 frequent stopping places. 



When the piston rod is in line with the center of the driv- 

 ing wheel, it has no driving force and is said to be at dead 

 center. The wheels on the two sides of the locomotive are 

 so arranged that when those on one side are at dead center, 

 those on the other side receive the maximum driving power 

 of the piston. Speeds of more than a mile a minute have 

 been maintained for a period of eight hours. For short 

 distances, rates of over 90 miles an hour have been made. 



LABORATORY EXERCISE 26 



Purpose. To study the parts of a steam engine. 



Apparatus. Model of steam engine or toy engine. 



Directions. Operate the model and notice carefully how 

 each part works. Make a drawing and label the parts. By 

 means of arrows show the path of steam. Explain the use of 

 each part. 



FIELD EXERCISE 5 



Purpose. To see how the locomotive works. 



Directions, i. If there are locomotive shops in town, arrange 

 with the superintendents to visit them with the class.. Or arrange 

 with some engineer or fireman to explain to the class the working 

 of the locomotive (during the stop of the locomotive at the sta- 

 tion). Notice also the operation of the brakes. Have some 

 of the station men explain the operation of the block signals. 



2. It may be possible also to visit a stationary steam engine 

 in some power hoiase in town. 



Brakes. Modern trains are supplied with powerful brakes, 

 which make it possible to stop the train quickly. One of 

 the most common types is the Westinghouse brake, which 

 is worked by compressed air. On the engine is a tank kept 

 filled with compressed air under high pressure. Under each 

 car is a smaller tank connected by a pipe with the main tank 



