458 



FARM MOTORS 



the horse power they will develop at the brake. Conse- 

 quently when one speaks of a 15 H.P. gasoline engine he 

 refers to an engine which will develop only about the 

 same horse power which a commercially rated 7 H.P. 

 steam engine will develop. For this reason when com- 

 paring the powers of the two engines it is always well at 

 least to double the size of the gasoline engine to do the 

 work which a commercially rated steam traction engine 

 has been doing. 



FIG. 338 TRACTION ENGINE WHICH REVERSES IN THE CLUTCH 



Regulation of speed. A gasoline traction engine oper- 

 ated by means of friction gearing, as illustrated in 

 Fig. 337, can have any speed required of it at the expense 

 of slippage between the gears. But a positively driven 

 traction engine must have other methods of changing 

 the speed. These methods generally amount to changing 



