YIELD OF THE HUMAN MACHINE 187 



and the vertical height ascended. The work done by a cyclist is 

 the product of the distance covered, and the passive resistances 

 of rolling, friction, etc. The work done by the arms or legs in 

 operating tools, or the work done in producing locomotion are 

 also measurable more or less exactly. It is found, nevertheless, 

 that this mechanical quantity is more difficult to measure in 

 some cases than in others, and when the movements are diverse 

 or complicated, the problem becomes acute, all the more so 

 because any static effort of the workman escapes the measure- 

 ments, although the whole muscular activity consists of static 

 efforts. 



Hence, in a given occupation or trade, it is only possible to 

 compare the quantities of useful work done by several workmen 

 if the conditions are approximately identical. It is impossible 

 to make comparison between different occupations 01 trades. 



130. It must be stated, in fact, that the sum of human activity 

 bears more relation to the muscular work done than to the 

 mechanical work performed by the tool. The manufacturer may 

 estimate the latter quantity, but that will not prevent the work- 

 man from estimating the former on the purely subjective but 

 pertinent indications of fatigue. Take, for example, a man 

 occupied in filing ; the useful work that he performs is the result 

 of his effort to overcome the resistance of the metal in displacing 

 the file. But this result is only obtained by an effoit which 

 presses the file on the metal and obliges it to bite, by the return 

 stroke of the tool, by the speed, by an appropriate attitude at 

 the bench, by the swaying of the body, by a certain inclination 

 of the vertebral column and a tension of the muscles of the 

 upper limbs. These multiple muscular efforts vary in direction 

 and extent in different trades. All, frcm the greatest to the 

 smallest, necessitate an expenditure of energy, a consumption 

 of oxygen ; and their effects must be added in determining the 

 fatigue. Thus, in any given occupation man's maximum output 

 is conditioned by his maximum expenditure of eneigy, an ex- 

 penditure whose value will be given later ( 342), and trie quanti- 

 ties of work performed by two different workmen are proportional 

 to their respective expenditures^ 1 ) 



131. We will now consider furthei the net yield of the muscles. 



T 



In the equation : R = , 



T stands for the product Ph of the weight of the subject and of 

 the height h of a staircase mounted by him. Now D^ does not 

 correspond solely to this amount of work, because the ascension 



( x ) If it is a case of the same occupation and conditions of work as 

 similar as possible. 



