DIFFERENT FORMS OF LOCOMOTIVES. 



represented in fig. 4. In these the driving-wheels are of greater 

 dimensions, and the engine is adapted to attain greater speed. 



A lighter and less powerful class of locomotive, supported on 

 two pairs of wheels, is shown in fig. 5, the hinder pair being the 

 driving-wheels. 



Fig. 5. 



14. When locomotives are intended to draw very heavy loads 

 with less speed, as in the case of goods engines, the driving- 

 wheels have less dimensions, and, in order to give them a greater 

 hold upon the rails, it is usual to connect two pair of side wheels, 

 of exactly equal dimensions, so that the piston shall act at once on 

 both by means of the connecting-rods. The two pair of driving- 

 wheels thus connected are said to be COTTPLED, and the engine is 



called a coupled-engine. Such an engine is shown in fig. 6, 

 where the hinder and intermediate pairs are coupled, the connect- 



119 



