Transportation on Land 261 



and another opens admitting steam on the opposite side of the 

 piston, driving it to its first position and forcing out the expanded 

 steam through another valve that opens at the same time. This 

 process causes the reciprocal, or back and forth, motion of the 

 piston. The piston is connected with a shaft running through 

 the end of the cylinder and joined with a connecting rod which 

 is coupled with the drive wheels of the locomotive near their 

 rims. The back and forth motion of the piston shaft is thus 

 converted into circular motion, causing the drive wheels to turn 

 and so move forward by reason of their friction with the rails. 



The weights of the connecting rods destroy the balance of the 

 drive wheels to which they are attached. This would tend to cause 

 the wheels to pound the track. To restore this balance weights 

 are built into each drive wheel near the rim. The heavier an object 

 is, and the faster it is moving, the harder it is to stop it. This com- 

 bined effect of weight and speed is called momentum. The weights 

 built into the drive wheels add to the momentum of the locomo- 

 tive, and tend to make less steam pressure necessary in drawing 

 loads after a train is once started. The drive wheels on the two 

 sides of the locomotive are so coupled that the connecting rods 

 will not both be horizontal at the same time. Otherwise, it might 

 be impossible to start the engine. 



By means of a steel rod, or "link," which the engineer con- 

 trols by a lever, it is possible so to shift the adjustments of the 

 driving machinery of a locomotive that the motion of the con- 

 necting rods forces the drive wheels to revolve in the opposite 

 direction and so sends the locomotive backward. 



There must be sufficient friction between the rims of the drive 

 wheels and the rails on which they run to make the wheels cling 

 to the rails and cause forward motion, instead of slipping under 

 a load. This is accomplished in part by the weight of the locomo- 

 tive itself which makes its wheels press heavily upon the rails 

 and so increase the friction grip upon them. It is also accom- 

 plished by using two or more drive wheels on each side. This 

 not only increases the "gripping" power of the locomotive, but 

 distributes its tremendous weight which, if centered at very few 

 points of contact, might be too heavy for rails and roadbed. 



