328 AGRICULTURAL ENGINEERING 



differs from all mechanical motors in that it is self-feeding, 

 self-controlling, self -maintaining, and self-reproducing. 



Classification. The horse in one sense is a heat motor, 

 burning fuel in the shape of feed, and as such is a prime mover. 

 The thermal efficiency of a horse exceeds that of an average 

 steam engine, but does not equal that of a gas or internal- 

 combustion engine. 



Capacity of the Horse. The amount of power that a horse 

 can develop depends largely upon its size and muscular devel- 

 opment. Experiments indicate that a horse exerts a pull on 



Fig. 212. Testing the draft of a horse. Also studying the effect of 

 the height and length of the hitch. 



his traces equal to from 1-10 to 1-8 of his weight when the 

 working day is not allowed to extend over eight to ten hours. 

 The speed at which the horse is able to produce the largest 

 day's work is from 2 to 2J^ miles per hour. Thus a 1500- 

 pound horse walking 2 3^ miles per hour and exerting a pull 

 of 150 pounds, will develop one horsepower; and, furthermore, 

 he will be able to continue this for a period not longer than 10 

 hours. An increase in the rate of travel, or an increase of 

 the effort or draft, must result in a corresponding decrease in 

 the length of the working day. 



