12 APPLIED PHYSIOLOGY 



are expended upon actual mechanical labour (exclusive 

 of increased internal work), then only one-sixth of the 

 sum expended on food is returned in work i.e., lOd. 

 worth a week represents the external work done, or 

 4s. 2d. worth, if one assumes that four-fifths have to be 

 added to this to allow for increased heat production. In 

 other words, only one-fifth of a man's wages really go to 

 produce work if one regards him as a ' hired machine.' 

 One pound of coal a day consumed in a steam-engine 

 would have produced more than as much work, assuming 

 that the combustion of the coal yields 3,000 Calories of 

 energy, and that the engine converts 15 per cent, of this 

 into work. 



It will be obvious from this that the larger part of 

 a man's earnings must always be spent simply in keep- 

 ing the machine ' alive,' apart from whether it does any 

 work or not. If, on the other hand, one regards the 

 machine as merely a ' peripatetic residence for the soul,' 

 which has to be kept both alive and warm, then the 

 only waste in it is the heat given off from the surface, 

 and its efficiency from this point of view is advanced 

 to about 50 per cent, of the energy consumed, which 

 is better than that of any human invention.* 



2. Income of Energy. To balance its expenditure, 

 the body is dependent for its income of energy upon the 

 chemical constituents of the food.f Of these, the pro- 



* Thurston, ' The Animal as a Prime Mover ' (Smithsonian 

 Eeport for 1896). 



-j- It is possible that the body may derive some energy from 

 sources other than food. Radiant heat e.g., from the sun may 

 certainly be regarded as such a source, and there may conceivably 

 be others of which as yet we know nothing. 



