436 



THE HUMAN MOTOR 



pushed like a barrow. The average co-efficient of traction 

 reaches -025 ( 255). 



An examination of the instantaneous photographs reproduced 

 in figs. 294, 295 and 296 will show that the labourer makes con- 

 siderable use of his weight, that the heavier the load the more 

 he bends forward (vide figs. 294 and 295). Often the knees are 

 bent which, as previously pointed out, advantageously diminishes 

 the vertical oscillations of the body (vide fig. 296). 



331. The Bicycle. In fig. 297 let P be the pressure exerted 

 on the pedal and p the force trans- 

 mitted by the chain, then 



A 



where oo ' is the radius of the crank 



and oA the radius of the chain wheel, 



experience shows that to obtain the 



best results the radius of the crank F '- 297- 



should not exceed 19 cm., while the 



speed of pedaling should be about 90 strokes per minute. 



For any given distance the number of strokes of the pedal is 



inversely 'proportional to the gear of the cycle. A high gear 



calls for increased effort. 

 The gear should therefore 

 be proportional to the age 

 and strength of the rider. 

 Leo Zuntz f 1 ) measured 

 the expenditure of energy 

 during a bicycle ride of 

 5 or 6 minutes duration. 

 He employed the method 

 described' in para. 288, 

 mounting the gas meas- 

 uring apparatus (which 

 weighed 7-5 kg.) on the 

 handle bars of the 

 machine (vide fig. 298). 

 The weight of the rider 

 was 65 kg. The total 

 weight of rider, plus load, 

 was therefore 72-5 kg. 



At a speed of 15 kilo- 



F|0 298 metres per hour, the ex- 



penditure of energy per 



Leo Zuntz, Untersuchungen uber d. Gaswechsel und Enereieumsaz des 

 ***** m, *-,!, icnm. Atwater> Sherman, and Carpenter 



