168 MECHANICS. 



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 ascertained 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 one fifth that of a horse. As the speed 

 of a horse increases, his strength of draught diminishes 

 very rapidly, till at last he can only move 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, half 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 mo- 

 tion of the muscles is in itself more fatiguing than the 

 ordinary or natural velocity. 



The following table shows 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 machin- 

 ery, but it is as much as an ordinary horse can exert 

 without being improperly fatigued with continued ser- 



