THE CONQUEST OF NATURE 



horse-power has not greatly changed. The vast major- 

 ity of the many millions of horses that are employed 

 every day in helping on the world's work, use their 

 strength without gain or loss through leverage, and 

 with only the aid of rolling friction to increase their 

 capacity as beasts of burden. 



To a certain extent horse-power is still used with 

 the aid of the modified treadmill just referred to- 

 consisting essentially of an inclined plane of flexible 

 mechanism made into an endless platform, which the 

 horse causes to revolve as he goes through the move- 

 ments of walking upon it. In agricultural districts this 

 form of power is still sometimes used to run threshing 

 machines, cider mills, wood-saws, and the like. An- 

 other application of horse-power to the same ends is 

 accomplished through harnessing a horse to a long 

 lever like the spoke of a wheel, fastened to an axis, 

 which is made to revolve as the horse walks about it. 

 Several horses are sometimes hitched to such a mech- 

 anism, which becomes then a wheel of several spokes. 

 But this mechanism, which was common enough in 

 agricultural districts two or three decades ago, has 

 been practically superseded in recent years by the per- 

 ambulatory steam engine. 



It is obvious that the amount of work which a horse 

 can accomplish must vary greatly with the size and 

 quality of the horse, and with the particular method 

 by which its energy is applied. For the purposes of 

 comparison, however, an arbitrary amount of work 

 has been fixed upon as constituting what is called a 

 horse-power. This amount is the equivalent of raising 



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