76 SCIENCE OF SUCCESSFUL THRESHING 



time, but it is being gradually brought about. If we look 

 into history and causes, we find that the early method of 

 driving threshing machines was by horses, and when en- 

 gines were first used for threshing, a ten horse-power en- 

 gine was supposed to supply about the same amount of 

 power as a lever-power driven by ten horses. From the 

 time of those early engines to the present, the competition 

 of different manufacturers, all endeavoring to furnish 

 the most powerful engine of a given rating, and the raising 

 of the steam pressure from 60 to 130 or even 160 pounds 

 (which was done without reducing the size of the cylinder 

 of a given rating), has caused the rating of engines of 

 this class to become more and more confusing. The re- 

 lation which the rated horse-power bears to the actual size 

 of the engine varies so greatly that, in reality, the "rated 

 horse-power" gives only a very indefinite idea of the 

 actual size of an engine. There are reasons why it is 

 preferable to indicate the size of an engine by size of its 

 cylinder, instead of by its rated horse-power; for example, 

 to say a "nine by ten" rather than a "fifteen horse" engine. 

 However, besides the cylinder size, the steam pressure 

 carried and the speed are also important factors in de- 

 termining the amount of horse-power an engine will de- 

 velop, and therefore a brake rating based on the actual 

 load the engine is capable of carrying is the only satis- 

 factory method. English engines are more under-rated 

 than any of those built in the United States, but in compar- 



