THE LOCOMOTIVE, 



13. The pair of wheels to which revolution is imparted by the 

 piston-rod, through the intervention of the connecting-rods, are 



Fig. 3. 



called the DRIVING-WHEELS, since it is by their immediate action 

 that the engine draws the train which is attached to it. They 

 are generally of greater diameter than the supporting- wheels, in 

 order that the engine may be propelled through a greater space 

 by each stroke of the piston, since the space through which it 

 moves by each double stroke is equal to the circumference of 

 the driving-wheels. 



The actual dimensions of such an engine as is represented, arc 

 indicated on the diagram. 



In some engines of more recent construction the driving-wheels 

 are placed in the hindermost part of the engine, the cylinders 



Fig. 4. 



Diameter of piston f 1 * 4-' 

 Slro/ce 1?*' 



being between the intermediate and foremost pairs of wheels, as 

 118 



