198 UTILISATION OF FOOD BY THE ANIMAL [chap. 



surface, and also because of their higher temperature; 

 a sheep weighing lOO lb. requires digestible food which 

 will develop about 2000 calories, Le, 1000 lb. of sheep 

 requires 20,000 calories, whereas 1000 lb. of lean cattle 

 only require about 11,000 calories. All these figures, 

 however, refer only to maintenance diets, when the 

 animal is doing no work and not putting on flesh. As 

 soon as work has to be performed the number of calories 

 required jumps up : for example, a horse weighing iioo 

 lb., and carrying harness weighing about 20 lb., will 

 perform about 1150 foot-tons of work in walking 10 

 miles at a pace of 2\ miles an hour ; this will be 

 increased to about 1440 foot-tons at 3 J miles an hour, 

 and to about 2200 foot-tons when trotting at 7 miles an 

 hour. Foot - tons can be transformed into calories 

 directly on the basis that 1-4 foot-tons of work will 

 yield i calorie when degraded into heat by friction, 

 but the converse change cannot be made so readily, 

 because a large proportion of the heat must always be 

 left unutilised in its conversion into work. A steam 

 engine, for example, is a machine for transforming heat 

 energy into mechanical energy, yet at its utmost 

 efficiency we can barely get out in work one-seventh of 

 the energy contained in the coal, the rest being evolved 

 in waste, low-grade heat From this point of view the 

 animal is a much more efficient machine than a steam 

 engine, for it is able to convert into mechanical work 

 about one-third of the available energy it derives from 

 its food, i.e. of the "dynamic" energy of the food. 

 According to Zuntz's experiments, a horse can turn out 

 about 770 foot- tons of work for each pound of available 

 food, reckoned as usual as starch, given in addition to 

 the animal's maintenance diet — the available food being, 

 however, the equivalent of what we have hitherto called 

 dynamic energy, or the value of the food for work, i.e. 



