THE ANIMAL AS A PRIME MOVER. 315 



probably 500 foot-pounds per pound, or at the rate of 0.015 horsepower 

 per pound, 66 pounds of bird per horsepower. This flying machine is 

 proportioned to give the higher power at starting; the lower in steady 

 working. Their emergency performance is thus probably three or four 

 times the average for the day's work. This is also a fact illustrated in 

 common experience with men, who, developing one-eighth of a horse- 

 power for an average day's work, can exert a half horsepower for two 

 or three minutes, a full horsepower for a few seconds. The aggregate 

 power of the machine varies from an indefinitely small quantity with 

 the smaller creatures to 140 horsepower, as estimated for the whale 

 swimming 10 knots per hour. 



The heart is, perhaps, the most powerful and enduring of all the 

 muscular structures of the system. Helmholtz has computed that this 

 organ can, on the average, raise its weight at the rate of 6,670 meters 

 per hour. He found that the locomotive, climbing heavy gradients, in 

 the cases investigated by him, at that time, could rise 800 meters in the 

 hour, unloaded. He therefore concluded that the heart, considered 

 as a machine, was eight times as effective as the locomotive. Presum- 

 ing that his locomotive had an efficiency of 10 per cent, this would 

 make the heart, were it self-contained and a prime motor, exhibit an 

 efficiency of energy- conversion of about 80 per cent. This supposition 

 is, however, by no means correct. 



Fick estimates the efficiency of energy transformation in the useful 

 work of the muscle at one-third to one-quarter, the remainder of the 

 total energy supplied being consumed in internal work and wasted 

 directly or indirectly as heat. 1 



Chauveau points out the fact that the efficiency of the muscle varies 

 enormously with time and method of contraction and extension and 

 magnitude of load. In the case of the muscles of the automatic — the 

 vital — system as those of the heart and lungs and digestive organs, it 

 is probable that the conditions are those of constant maximum 

 efficiency, and the figure attained would seem, from other considera- 

 tions, to be likely to be found comparatively large, and, for the machine 

 as an energy transformer, immensely greater than is ever attained in 

 nonvital motors of the thermodynamic class. The energy transforma- 

 tion is presumably never thermodynamic, but is, directly or indirectly, 

 dynamo-thermic, and the heat of the muscular system is a product, not 

 a source, of useful work, and an excretion, not a food. 2 



It has been seen that the food required by the average workingman 

 contains about 12,000 British thermal units of energy when resting or 

 doing little work, and about 16,000 when at hard work. It would thus 

 appear that the work of the laborer, for which he is paid, represents 

 about 25 per cent of all the energy expended by the vital machine in 

 external and internal work, heat-producing and wastes. This means 



1 Experimeuteller Beitrag, 1869 ; Ueber die Wiirmeentwicklung, 1878. 

 -Chauveau. Travail Museulaire, page 233. 



