94 THE POPULAR SCIENCE MONTHLY. 



Animals have been looked upon as machines for converting the 

 vegetable products of the farm into animal products of greater 

 value, and this in the light of the law of the conservation of 

 energy may be interpreted as the conversion of the potential 

 energy of field crops into the useful work of an animal machine. 

 Considered as machines for the manufacture of definite products, 

 the efficiency of animals must depend upon the amount of work 

 performed for a given supply of energy in their food. 



An ordinary steam-engine formerly converted less than one 

 tenth of the potential energy of the fuel consumed into useful 

 work, and the attention of engineers has been directed to improve- 

 ments in construction to secure greater economy and efficiency in 

 the work performed, by a more complete utilization of the poten- 

 tial energy supplied in the form of coal or other fuel. The re- 

 markable industrial development of the past few years, resulting 

 in a material reduction of the cost of production and transporta- 

 tion, is largely owing to improvements in the steam-engine 

 which have been brought about by a more intelligent application 

 of the principle of the conservation of energy. 



There are good reasons for the belief that the animal machine 

 works with greater economy than the steam-engine, even in its 

 improved form, but, according to the most favorable estimates, 

 only a small proportion of the potential energy of foods is utilized 

 in useful work, and there is a broad margin for improvement, 

 even in what we call our improved breeds, to secure a more 

 efficient expenditure of energy. 



The problem of paramount interest in animal husbandry is 

 essentially the same the mechanical engineer has been dealing 

 with in his efforts to improve the steam-engine. It is simply to 

 obtain the largest net returns in useful work from the potential 

 energy of the food consumed. It is evident that improvements in 

 the animal machine itself must be the leading object to receive 

 attention, and the breeders of pure-bred stock must recognize this 

 principle in their efforts for improvement. The form and pro- 

 portions in which the chemical constituents of food are provided 

 are of far less importance than the inherited capacity and capa- 

 bilities of the animal machine to utilize and economize energy in 

 the work involved in the manufacture of animal products. 



When speaking of foods we should bear in mind the fact that 

 there is but a limited demand in the animal economy for the 

 so-called nutritive constituents, aside from their agency in the 

 transformations of energy involved in the metabolism of the 

 system. But a small proportion of the chemical constituents of 

 foods are stored up in the body, even during the period of growth, 

 when the demands for new materials in constructive metabolism 

 are most active, while an abundant supply of energy in an avail- 



