THE ANIMAL AS A PRIME MOVER. 317 



Joule thus, at the middle of the century, had found the animal, taken 

 as a prime mover, to be two and a half times as efficient as the best 

 engines of his day, one and a quarter times as efficient as the best 

 engines of our day, the close of the nineteenth century. 



This figure is corroborated by many independent experiments and 

 computations. Joule, as above, put the work of horse and man at about 

 one-fourth, sometimes as low as one-sixth, the dynamic equivalent of 

 the food supply. Hirn found substantially the same figure as is above 

 deduced by measuring the work and the exhalations of carbon-dioxide 

 and of moisture by his inclosed treadmill operators. Helmholtz 

 deduced 20 per cent, also, from the same experiments. Joufret 

 takes the work of the man at 280,000 kilogrammeters and obtains 21 

 per cent, by the day, rising to 37 per cent, short intervals and actual 

 expenditures of energy and proportional supply being taken. 



We may thus, without much doubt, conclude: 



The efficiency of the animal machine, assuming that only external, 

 so-called useful work is reckoned ;is the product, and the full dynamic 

 equivalent of the energy latent in the food supply being taken as unity, 

 is about 20 per cent. 1 Hirn's experiments with his inclosed treadmill 

 gave efficiencies from 17 to 25 per cent, the lowest being given by "a 

 lymphatic youth of eighteen," the highest by a strong laborer of the 

 age of forty-seven. The average is precisely that computed by the 

 method pursued above. 2 



Our computation, however, should be checked by the introduction 

 of the quantity of rejected, uuassimilated food, if we are to learn the 

 real maximum possible efficiency of the animal machine motor. 



The factor of digestibility is probably with the animal machine, 

 human or other, when in good health and normal, between 75 and 90 

 per cent, averaging not far from 80 or 85 with the customary healthful 

 foods. With domestic animals Professor Woods finds this factor to 

 range all the way from about 50 to approximately 90 per cent. :i This 

 is confirmed by many other investigators, and the assumption of 85 

 percent as a fair maximum is probably perfectly justifiable, with all the 

 familiar forms of the vital machine in good order. 



In the case of Weston, the pedestrian — studied by Dr. Flint — the 

 proportion of food utilized being taken as measured by the ratio of 

 nitrogen absorbed in food to that excreted in chemically different com- 

 binations, the efficiency of nutrition, the "factor of digestibility," in 

 one sense, was, when quietly training without excessive exertion and 

 with very moderate exercise, and as an average for five days, 8G.6 per 

 cent. 4 It is probable that a good digestion and assimilation should be 

 expected to attain an efficiency of 90 per cent, and that 10 per cent of 



1 Tne Animal as a Prime Motor. 



2 Ibid., p. 43. 



"Reports of Connecticut Agricultural Experiment Station. 

 'Muscular Power, page 84. 



