POWER OP HOUSES. 109 



the power of a single horse ; hence the common expres- 

 sions of twenty, or fifty, or a hundred horse-power 

 engines. The strength of difierent horses varies greatly, 

 but the expression, as commonly understood, indicates a 

 force equivalent to raising or pressing with a force equal 

 to 150 pounds 20 miles a day, at the rate of two and a 

 half miles an hour. This is the same as 33,000 pounds 

 raised one foot in one minute. The results of numerous 

 experiments in different places give the actual power of 

 the average of horses at somewhat less than this ; and 

 there is no doubt that, for most of the farm-horses of this 

 country, the result would be considerably less. The 

 power of a strong English draught-horse has been ascer- 

 tained to be about 143 pounds for 22 miles a day, at 2} 

 miles an hour. Many American horses are scarcely more 

 than half as strong. The strength of a man, working at 

 the best advantage, is estimated at Cne-fifth that of a 

 horse. 



As the speed of a horse increases, his strength of draught 

 diminishes very rapidly, till at last he can move only his 

 own weight. This is owing to three reasons : first, the 

 load moves over a greater space in a given time, and if, 

 for instance, the speed be doubled, liaLt' the load only can 

 be carried with the same quantity of power, according to 

 the law of virtual velocities ; secondly, the horse has to 

 carry the full weight of his body, whatever his speed may 

 be, and the force expended for this purpose alone must, 

 therefore, be doubled as the speed is doubled ; thirdly, a 

 very quick and unaccustomed motion of the muscles is in 

 itself more fatiguing than the ordinary or natural velocity. 



The following table show^ the amount of labor a horse 

 of average strength is capable of performing in a day at 

 different degrees of speed, on canals, rail-roads, and on 

 turnpikes. The force of draught is estimated at about 83 

 pounds. This is considerably less than the horse-power 

 used in estimating the force of machinery, but it is as much 



