424 THE HUMAN MOTOR 



solved not by theory, but by practical experiment on truly 

 scientific lines. 



The above experiments are only true for tools of the specified 

 nature and dimensions. They are, indeed, generally appliable 

 to files of the same general dimensions, but of different cuts, 

 but they are not correct for small files where a much higher speed 

 of working is required which, in the case of a triangular file 17 

 cm. long, was found to be about 150 strokes per minute. 



To recapitulate : With a flat bastard file, total length 34 cm., 

 and effective length used 26 cm., the muscular work expended 

 amounted to 65,000 kgm. in 8i hours, out of which 7 hours were 

 actually spent in work. The corresponding weight of metal 

 removed was, at most, 600 grammes of brass. The expenditure 

 of energy was -023 Calories per kilogrammetie. This gives a 

 net yield of 



1 . _ i O.QO/ 



U23 X 425 ' 



This small yield shows that filing is not a profitable method of 

 utilising muscular activity. 



We have also seen that the workman should hold himself up- 

 right but without stiffness, that he should stand at 20 cm. from 

 the vice with his feet in the relative positions shown in fig. 287, 

 that the left arm should be fully extended and should exert some- 

 what more pressure than the right arm, the respective figures 

 being 8-5 kg. and 7-5 kg., for a total useful effort of 8 kg. We 

 have also seen that no pressure should be exerted daring the re- 

 turn stroke and that best economy is attained at a speed of 70 

 strokes per minute. 



If the above conditions are complied with work can be carried 

 out for continuous periods of five minutes followed by periods 

 of complete rest for one minute. The rates of respiration and 

 pulse do not then increase more than 25% and 20% respectively 

 above the normal, the local fatigue of the forearm is bearable, 

 and the general fatigue of the body is but slight. 



The above output is at least double that obtained from the 

 majority of workmen. The regularity of the graphical records 

 of the muscular action of skilled workmen has been already 

 pointed out by Imbert^ 1 ) The irregularity and discontinuity 

 exhibited in the work of unskilled apprentices and beginners, has 

 already been referred to. 



320. The Use of the Saw. No definite experiments have been 

 made in regard to the use of saws, on wood, metal or stone. We 

 may, however, infer, from some workshop observations, that a 



( J ) A. Imbert, Revue Generate de Sciences, June, 1911. p. 485. Comptes 

 Rendus A cad. Sciences, July 10th, 1911, p. 128. 



