170 MECHANICS. 



will exert a considerably greater force when working 

 horizontally than up a steep inclined plane. On a 

 level, a horse is as strong as five men, but up a steep 

 hill he is less strong than three ; for three men, carry- 

 ing each 100 pounds, will ascend faster than a horse 

 with 300 pounds. Hence the obvious waste of power 

 in placing horses on steeply-inclined tread-wheels or 

 aprons. The better mode is to allow them to exert 

 their force more nearly horizontally, by being attached 

 to a fixed portion of the machine. For the same rea- 

 son, the common opinion is erroneous that a horse can 

 draw with less fatigue on an undulating than on a level 

 road, by the alternations of ascent and descent calling 

 different muscles into play, and relieving each in turn ; 

 for the same muscles are alike exerted on a level and 

 on an ascent, only in the latter case the fatigue is much 

 greater than the counterbalancing rehef. Any person 

 may convince himself of the truth on this subject by 

 first using a loaded wheel-barrow or hand-cart for one 

 day on a level, and for the next up and down a hill ; 

 bearing in mind, at the same time, that the human 

 body is better fitted for climbing and descending than 

 that of a horse. 



A draught-horse can draw 1600 pounds 23 miles in 

 a day on a good common road, the weight of the car- 

 riage included. On the best plank-road he will draw 

 more than twice as much. 



A man of ordinary strength exerts a force of 30 

 pounds for 10 hours a day, with a velocity of 2 J feet 

 per second. He travels, without a load, on level ground, 

 during 8^ hours a day, at the rate of 3.7 miles an hour, 

 or 31^ miles a day. He can carry 111 pounds 11 



