DYNAMICS 147 



than those of Fahrenheit's thermometer, so the energy of 

 772 x i "8= 1389-6 foot-pounds of work will raise the tem- 

 perature of T Ib. of water i C. Therefore, 1,389,600 foot- 

 pounds of work = i kt. 



Now, although the whole of the work can be obtained 

 as heat, the converse is not true; the whole of the energy 

 cannot be obtained as work. In the case of steam engines 

 there is great loss of energy in the form of heat radiated 

 from the boilers, and there is a certain amount of internal 

 work of the ^machine itself due to the friction of the working 

 parts, and so on. At best, steam engines do not give out as 

 work more than about one-seventh part of the energy produced 

 by combustion of the fuel. 



Animals are much more economical not necessarily in 

 the pecuniary sense in this respect. The temperature of 

 their bodies is much lower than that of steam boilers, and 

 they do not lose so much heat by radiation. What they do 

 lose in this way is provided for in the' maintenance rations. 

 There is practically no internal friction, but there is a con- 

 siderable increase in the internal work of circulation, etc., and 

 the amount of water evaporated is much greater when the 

 animal does work (p. 144). 



Numerous experiments show that, under favourable con- 

 ditions, animals can give out as work about one-third of the 

 total available energy l of the food added to a sufficient main- 

 tenance ration. That is to say, that for each kilo-pound-unit 

 of such energy in this added food, the animal can produce 

 about [1,389,600 -f- 3 =] 463,200 foot-pounds of work. Ex- 

 pressed as a formula, this would read 



W = 0-46320 or D = 2 'i6W 



where D is the amount (kt.) of dynamic energy in the food, 

 and W is the amount (in million foot-pounds) of work. 



The " power " of a machine or of an animal is measured 



by the amount of work it can do in a given time. The engineers' 



unit of 550 foot-pounds per second is called a "horse-power," 



because it was based on an estimate of the supposed capacity 



1 Energy available for work (cf. p. 156). 



