INDUSTRIAL LABOUR 419 



regular and, in general, the mechanical and physiological condi- 

 tions favourable. The expenditure of energy per kilogram- 

 metre, and per gramme of metal removed, do not exceed -023 Cal. 

 and 2-5 Cal. respectively. The results will, of course, be affected 

 by any variations in the weight of the file, the effective length 

 thereof which is actually used, and the shape and nature of the 

 surface operated upon. 



316. Effects of the Return Stroke and the Intervals of Rest. 



It will* be observed that the economical rate, 70 strokes per 

 minute is about one quarter of the maximum " rhythm " of the 

 shoulder joint ( 91). We have seen that, in the return stroke, 

 the right arm exerts an effort of -55 kg., an almost negligible 

 amount compared with the total effort. The relative duration 

 of the working and return strokes varies with the speed of 

 working, as is shown by the following table showing the times 

 taken, in seconds, for the working and retain strokes for various 

 speeds of working : 



Strokes per minute 32 40 44 48 54 70 

 Time of working 



stroke -72 -59 -58 -51 -36 -44 



Time of return stroke 1-15 -91 -78 -74 -77 -42 

 Percentage of Total 

 Period of Cycle : 



Working stroke 38 39 42 41 32 51 



Return stroke 62 61 58 59 68 49 



The 'results are somewhat irregular, but it will be noted that, 

 at the economical working speed of 70 strokes per minute, the 

 duration of the forward and return strokes is approximately 

 equal. 



The frequency and duration of the periods of entire rest natur- 

 ally depend on the speed of working. At the speed of 70 strokes 

 per minute continuous working for 3-J- minutes caused fatigue of 

 the right arm that was actually painful. At 54 strokes per 

 minute the fatigue was much less and work could be carried on 

 without inconvenience for continuous periods of as much as 10 

 minutes, if followed by two or three minutes of complete rest. As 

 a rule, the ratio of rest period to working period should be about 

 1 to 5. Under this condition no disturbance of the physiological 

 functions of the organism is generally to be observed after an 

 hour's work other than an increase of about. 20% above the normal 

 in the rate <tf respiration and pulse. 



Observations at some large Paris workshops show that a man 

 can work at the vice for about 8^- hours per diem. Assuming the 

 above proportion of actual working time to time of repose, this 

 means 7 hours of actual working time and 1 J hours of rest inter- 



