240 THE HUMAN MOTOR 



some characteristics. It may then cause wood to split, bodies to 

 become electrified, f 1 ) and the skin of the hands to peel in places. ( 2 ) 



Finally, garments absorb water ( 3 ) in varying quantities, 

 according to the nature of the material, thereby moderating the 

 evaporation and preventing too rapid cooling. 



Rubner found that the maximum quantity of water retained, 

 per gramme of material was : 



Flannel 10-30 gr. Silk tricot 3-80 gr. 



Flannelette 6-00 Linen 2-10 



Wool tricot 4-80 Cotton 0-80 



Cotton 4-20 



Wool, from this point of view, is equally as valuable in summer 

 as in winter ; the Arabs, for instance, always wear it. 



175. The Influence of Air Currents. The air acts on man and 

 animals by its temperature ; but it also affects them by its speed 

 and its bulk. In a still atmosphere the resistance of the air is 

 due to the speed of movement. Th^ speed of a pedestrian, walk- 

 ing at a good pace, is 1-50 metres per second. The resistance 

 offered by the air, that is to say, the pressure that it brings to bear 

 on the body in movement, is : 



R = K x S x V 2 , 



^ R being in kilogrammes, V in metres, and the surface S in square 

 ^ metres. The co-efficient of resistance, K ( 4 ), is 0-079 up to a speed 



of 42 metres. A man's effective surface, S, when walking, is 



about 0-75 m 2 . Hence : 



R = 0-079 X f50 2 X 0-75 = 0-133 kg. 

 This represents, per second, a useless labour of : 



0-133 X 1-5 =0-20 kgm. 



In reality, the speed to be considered is that of the wind, which 

 can vary between 2 and 40 metres, according to the momentum 

 and also according to the altitude, increasing with the latter. 

 Of course the speed of the pedestrian is, in these cases, negligible. 

 Exceptions must be made for races, on bicycles, in motors, etc., 

 or even on foot. 



We can take 6 metres as the normal speed on the level. There 

 is, therefore, a pressure, R = 2-133 kg. and the work done is 

 12-80 kgm. per second. 



(*) Mile, de Harven (Bull. Soc. Astron. de France, 1904, p. 173). 

 ( a ) Jean Mascart (Revue Generate des Sciences, 1910, p. 906). 

 ( 8 ) Rubner (Arch, fur Hygiene, vol. xv., p 64 1892) 

 () G. Eiffel, La Resistance de I' Air, 2nd Edn 1911 



