YIELD OF THE HUMAN MACHINE 



195 



G. Weiss and Chauveau found a similar result in hydraulic 

 motors ( 114). The analogy between the muscles and heat 

 engines, however tempting it may be, cannot be pressed, since 

 the cause which limits the effects of speed cannot be the same. 

 An extreme value of the speed prevents the reparation of the tissue 

 and arrests the muscular contraction by fatigue. 



In a cannon a yield of 44% is obtained, far above that of other 

 heat engines, the reason being that the combustion is so complete, 

 in spite of the rapidity of the explosion, whilst only a slight 

 thermic loss takes place through the barrel owing to the speed of 

 discharge. Perhaps this is somewhat analogous to the contrac- 

 tion of the muscles. 



138. The variations of the yield are also due to other causes. 

 In normal continuous work, without excessive fatigue, the mus- 

 cular fibre becomes more irritable and the fibres and nervous 

 cells more excitable. This is shown by an increase of force 2 ) 

 and by a smaller consumption of oxygen. ( 3 ) 



Voluntary work, with a suitable load and at a suitable speed 

 they vary according to the active organs leads to a better 

 energetic yield. 



139. Finally the loss of work due to the passive resistances of 

 the tools or apparatus must be considered. To run the bicycle 

 dynamometer unloaded necessitates an expenditure of energy 

 which is dissipated as heat in Atwater's experiments. Benedict 

 and Carpenter ( 4 ) found this to be 6-50% of the measured mechani- 

 cal work. It is a fraction of the work done for which the muscles 

 have made a very appreciable expenditure of energy. It is 

 obvious that, when using the bicycle dynamometer, or, indeed, 

 any other ergometric instrument, we must know accurately the 

 frictional losses in the apparatus so that we can add them to the 



() Aim6 Witz (Demise Evolution du Moteur & Gaz, p. 144, 1910. Paris. 

 ( a ) Manca (Arch. Ital. Biol., vol. xvii., p. 390) ; Kronecker and Cutter 

 (Comptes Rendus Sciences, vol. cxxix., p. 492, 1900). 



( 3 ) Griiber (Zeitsch. f. Biol., vol. xxviii., p. 466, 1891). 



( 4 ) Benedict and Carpenter (Bulletin, No. 208, p. 27, 1909) ; (Benedict 

 and Cad y (Publication, No. 167, 1912). 



