KXERGY AND ITS RELATION TO USE OF MACHINES 553 



weight. The work performed by horses of different weights 

 would, then, be about as follows: 



WORK PERFORMED PER DAY BY HORSES OF DIFFERENT WEIGHTS 



WALKING AT THE RATE OF 20 MILES PER DAY, 



OR 2> MILES PER HOUR 



W ho?8e f Pul1 exerted Rate of work at 2 ^ mile8 per hour 



800 Ib. 80 to 100 Ib. . 53 to . 67 horse-power. 



1000 Ib. 100 to 125 Ib. 0.67 to 0.83 horse-power. 



1200 Ib. 120 to 150 Ib. 0.80 to 1.00 horse-power. 



1400 Ib. 140 to 175 Ib. . 93 to 1 . 17 horse-power. 



1600 Ib. 160 to 200 Ib. 1 . 07 to 1 . 33 horse-power. 

 (The student should verify these figures.) 



658. Why the Horse can not Compete with the Steam 

 Engine as a Motor. We have seen that a strong horse can not 

 work steadily at a rate faster than 1 horse- power. We shall 

 presume that 8 hours, working at full capacity, makes a full 

 length day for a horse to labor. Now, although a horse can 

 convert 20 per cent, of the energy in its food into effective 

 work, it still is true that the food of the horse is so much more 

 expensive than the fuel of the steam engine that the horse is 

 quite unable to compete with the engine. 



The principal food of the horse is corn or oats. Smith, in 

 Profitable Stock Feeding, says that a horse weighing 1200 Ib. 

 at severe labor, needs 16 Ib. of oats and 12 Ib. of hay per day. 

 Other authorities give the following rule for feeding a working 

 horse: 1J^ Ib. of oats or corn and 1 Ib. of hay per day for each 

 100 Ib. of weight. The energy in a pound of oats is about 

 1500 Cal. (see Table XIV, Art. 391. Remember that the 

 husk of the oat is removed in making oat meal; that the 

 energy in a pound of corn is about 1650 Cal.). 



1. We shall consider that a 1200-lb. horse is able to do 1 horse- 

 power of work for eight hours per day. We shall consider his food 

 as 18 Ib. of oats per day, and omit any consideration of the hay or 

 "roughage" consumed because the available energy in it is not great. 

 What is the efficiency of this horse as a motor? 



First: 18 Ib. of oats contain 18 X 1500 Cal. of energy, or 27,000 



