THE MODES OF ESTIMATING ENERGY. 519 



serve to illustrate this point : As just mentioned, a man of 

 150 Ib. weight, walking 20 miles a-day, makes an effort equal 

 to raising 792,000 Ib. to the height of one foot ; a man of the 

 same weight carrying a load of 60 Ib. makes an effort equal to 

 raising, in walking 20 miles, 1,103,800 Ib. to the height of one 

 foot an effort, however, greater than could be continued for 

 several successive days without injury to health. A horse 

 weighing 900 Ib., led without any load over 20 miles of road, 

 on the same assumption as to the difference between horizontal 

 carrying and vertical elevation, performs a task equal to the 

 raising of 4,797,000 Ib. to the height of one foot, or more than 

 six times the effort made by a man of 1 50 Ib. weight in walk- 

 ing 20 miles. If, again, a horse weighing 900 Ib. for 20 miles 

 carry a weight proportionate to the weight of 60 Ib. carried by 

 a man of 150 Ib., or 360 Ib., then the effort made equals the 

 elevation of 6,652,800 Ib. to the height of one foot. Lastly, to 

 illustrate what is commonly said as to a man out-journeying a 

 horse in a few days if a horse do 20 miles a-day with a rider 

 weighing 1 50 Ib., his daily effort should not be too much for 

 him, since it is no more than equal to the exertion of raising 

 5,016,000 Ib. to the height of one foot. If however, this exer- 

 tion were made, not at the rate at which a horse walks, or 

 short of 5 miles an hour, but at the rate of 8 or 10 miles an 

 hour, the exertion would be very different. Lastly, when a 

 man climbs by the inclined side of a mountain to the top, it is 

 a mistake to regard his effort as equal to that of raising his 

 own weight through as many feet as denote the height of the 

 mountain ; for if the inclined side of the mountain be twice as 

 long as the mountain is high, then the effort is only one-half 

 that of raising a load equal to the man's own weight that 

 height. 



The whole of this subject thus requires careful revision. 



