50 THE ECONOMICS OF FEEDING HORSES 



This amount of heat — namely, that to raise the tempera- 

 ture of 2*2 lbs. of water 1° C. — is called a (large) Calorie/ 

 If, then, we know the heat value of a food, and the work 

 to be done by a horse, we can calculate the amount of 

 that food necessary to provide for that amount of work. 

 Now, it has been proved experimentally that 1 lb. of 

 starch when completely digested produces 1,710 Calories 

 of heat, and this is therefore equivalent to 3,057 x 1,710 



foot lbs. of work, or ' = 2,334 foot tons. But 



the whole of this energy is not available for external 

 work, for, like every other machine for the transforma- 

 tion of potential energy into work, the body is only able 

 to utilize a part of the power supplied to it. In a good 

 type of steam-engine it is not possible to get more than 

 15 per cent, of the total energy of the steam out in 

 useful work, whereas the body is much more economical, 

 and gives out as work about 30 per cent, of the energy 

 it receives. Kellner describes a series of experiments 

 with a horse made to turn a braked capstan. The exact 

 amount of work done was ascertained, and of the energy 

 supplied in the digestible food the animal used from 

 29 to 38 per cent., or on an average about one-third. 

 Thus the actual work-value of 1 lb. of digestible starch 

 is about 778 foot tons. Now, the average maximum 

 amount of work that a horse weighing 1,000 lbs. can 

 do daily is about 6,500 foot tons, so that the amount 

 of digestible starch to be given for work production is 



' , or approximately 8| lbs. This amount must, 



* In English equivalents the amount of heat required to raise the 

 temperature of 1 lb. of water 1° F. is equivalent to 772 foot lbs. of 

 work. 



