548 NUTRITION OF FARM ANIMALS 



This method of computation is unlike any usually employed by 

 the engineer, and Schreber 1 has criticized it severely. The engineer 

 is accustomed to estimate the losses due to the internal resistance of 

 an engine by a comparison of brake horse power and indicated horse 

 power. No method exists, however, for determining the indicated 

 horse power of the animate motor, if indeed it permits of any cor- 

 responding conception, and only the method of comparison just out- 

 lined is available. It is as if the engineer had no indicator and esti- 

 mated the efficiency of his engine by deducting from the total steam 

 consumption that required to run the engine empty and compared 

 the remainder with the external work done. The internal work of 

 the animal, however, like that of the engine, is largely mechanical. 

 If, on the basis of Zuntz's computation of the efficiency in locomo- 

 tion (652), it may be assumed that this internal work is performed 

 with approximately the same efficiency as the external work, then 

 the net efficiency of the animal will be somewhat analogous to the 

 efficiency of the steam in the cylinder of the engine. 



649. Gross efficiency variable. It should be observed that 

 while the net efficiency may be regarded as .substantially con- 

 stant under a considerable variety of conditions, the gross effi- 

 ciency will vary with the ratio of work done (load) to main- 

 tenance requirements. Thus if Benedict and Cathcart's 

 subject had done only half as much work per minute with the 

 same net efficiency, his gross efficiency would have been only 

 13.7 per cent instead of 16.1 per cent. 



Total energy expended per minute. 



For mechanical work, 0.51 Cal. -r- 0.195 = 2-62 Cals. 



For maintenance 1.09 Cals. 



Total 3.71 Cals. 

 Recovered in work done 0.51 Cals. 



Gross efficiency 13.7 per cent 



Up to the point at which the net efficiency begins to be affected, 

 the gross efficiency will increase with increasing load as Bene- 

 dict and Cathcart show experimentally to be the case. This 

 influence of the maintenance requirement upon the computa- 

 tion of the utilization of energy is identical with that to which 

 attention has already been called in connection with the utili- 

 zation for material products. If the useful work performed 



1 Arch. Physiol. (Pfliiger), 159 (1914), 276, 



