316 THE ANIMAL AS A PRIME MOVER. 



that it performs one-fifth of a horsepower of commercially valuable 

 labor, and, assuming all work concentrated into the working-day, three- 

 fifths of one horsepower, mainly, in the work of the organism itself. 



A day's work, even for the average workman of full working t)ower, 

 varies greatly with the nature of the work and the method and facilities 

 of its performance, and the same is true of the horse and all other 

 animals, but usually in less degree. The most powerful horses may be 

 expected to develop, as an average, two-thirds of a horsepower for 

 eight hours a day, or 12,000,000 foot-pounds per day, very nearly, 

 under favorable circumstances. The work of a man is variously given 

 by different writers, but it is usually stated to be not far, at best, from 

 2,000,000 foot-pounds per day in the treadmill, ascending mountains 

 and stairways when his own weight is the useful load, and carrying 

 burdens on a level. Weisbach gives as maximum figures 1,935,360, 

 Kankine gives 2,088,000, while Euhlmau states the work of a Prussian 

 soldier, carrying a knapsack and other accouterments weighing a total 

 of 64 pounds, as about 3,000,000 foot-pounds. We may safely take 

 2,000,000 foot-pounds per day as a figure to be compared with the 

 10,000,000 foot-pounds of energy supplied, and as giving a fair maxi- 

 mum for the efficiency of the animal considered as a prime motor. 1 It 

 is 0.125 horsepower for eight hours, 0.04 for the day. 



The efficiency of the animal machine as an apparatus for the per- 

 formance simply of external work is on the basis here taken, 



E = 2,000,000 / 10,000,000 = 0.20, 

 20 per cent. This happens to be just the efficiency of the best recorded 

 steam-engine performance to date. 



The efficiency of the vital machine considered as a motor is high. 

 Haller, about 1840, had applied the then new principles of thermody- 

 namics to the action of the vital elements of the system, and had 

 suggested that the heat of the body was at least in part due to the 

 friction of the circulation. Joule found by experiment that the passage 

 of fluids through tubes gave rise to heat, and gave the correct explana- 

 tion as early as 1843. In the now famous paper of Joule, dated 1846, 3 

 he concludes : 



The duty of a Daniell battery, per grain of zinc, is 80 pounds raised 1 

 foot high — about one-half the theoretical duty — i. e., its efficiency is 

 about 0.50. 



The duty of a Cornish engine, per grain of coal, is 143 pounds raised 

 1 foot high, an efficiency of 0.10. 



The duty of a horse, per grain of food, is 143 pounds raised 1 foot high, 

 "which is one-quarter the energy due its combustion," an efficiency of 

 0.25. 



'The Animal as a Prime Motor. R. H. Thurston, section 17, page 50; section 18, 

 page. 53. 



-Philosophical Magazine, 1846. Memoir of Joule, hy Oshorne Reynold, 1892, page 

 100. 



