252 VETERINARY PHYSIOLOGY 



9. The Efficiency of the Horse as a Machine. 



The total amount of energy liberated by the horse in 

 performing a measured amount of work has been estimated 

 by the indirect method of calorimetry (p. 260). By this 

 means the efficiency of the horse as a machine has been 

 determined by Zuntz and others. 



The total energy expended in work, as in drawing a load, 

 is the sum of three separate items : — 



(1) The amount for maintenance purposes. This is 

 required whether the animal is working or at rest. 



(2) The amount spent in moving the body of the 

 animal. 



(3) The amount spent in moving the load. 



The proportion which the amount of useful work done bears 

 to the total energy liberated (1 + 2 + 8) is called the gross 

 efficiency. The proportion which the work done bears to the 

 part of the energy expended in moving the load is called the 

 net efficiency. The amount spent solely in moving the load is, 

 of course, the total output (1 + 2 + 3) minus the output 

 when the animal is moving without a load (1 + 2). The 

 net efficiency under the most favourable conditions of load 

 and speed is found to be 30 to 35 per cent., which is about 

 equal to that obtained in man. 



10. Capacity of the Horse for Work. 



Draught. — The amount of work performed by the 

 draught horse has been measured by means of the moving 

 platform (p. 246). Over two million kilogram-metres (about 

 7000 foot tons) has been registered in an experimental day's 

 work, and the amount of work which the horse is capable 

 of performing daily has been estimated as high as 6000 

 foot tons (1,854,720 kilogram-metres). F. Smith, however, 

 considers that 5000 foot tons (1,545,600 kilogram- metres) 

 is a severe day's work, and that 3000 foot tons (927,360 

 kilogram-metres) is a fair average. The capacity of the 

 draugfht horse for work is doubtless being increased bv 

 selective breeding designed to increase the weight of the 



