484 PHYSIOLOGY AT THE FARM. 



horse, of the breed to which he belongs, and of the mode and 

 degree of exertion to which he is at any time to be subjected, 

 but also of his own special temperament and constitution ; and 

 the same things are true of all the other animals of the farm. 



Among the circumstances which require special attention in 

 regard to varying the kind of food and method of treatment, 

 in respect to all the animals of the farm, one of the most 

 familiar is the period of breeding. The consideration of this 

 subject, indeed, in some degree of detail would afford the 

 means of illustrating the importance and utility of the whole 

 range of our treatise. But our plan claims precedence for 

 another subject. 



Theory of the conservation of energy applicable to show 

 the difference between the diet of an animal at rest and an 

 animal under exertion. A brief view of a subject, which is 

 most appropriately taken in connection with the horse, will 

 give a wider illustration of the kind of circumstances under 

 which variations should be made on the mode of feeding 

 animals, namely, the different proportions respectively of flesh- 

 forming, and of calorific and fat-giving proximate principles, 

 required to maintain health in an animal at rest or only 

 moderately exercised, and in the same animal fully or exces- 

 sively worked. Allied inseparably with this question is the 

 new field of inquiry opened up by the discoveries falling under 

 the head of what is termed the " conservation of energy," respects 

 ing which it will be enough for our purpose, in the first instance, 

 to remark the principle of the alternate convertibility of heat 

 into mechanical force, and of mechanical force into heat, so 

 that numerical equivalents have come to be employed, signifi- 

 cant of each reciprocal effect. 



First, then, of animal heat in its relation to the "con- 

 servation of energy." It is known that the higher tempera- 

 ture of the animal body, as compared with that of the sur- 



