198 THE HUMAN MECHANISM 



this is only the perspiration which has not evaporated ; the 

 true measure of the cooling effect would be the perspiration 

 which has evaporated and of which we are not conscious. 



It is important to note that the evaporation of perspira- 

 tion (or of water from the lungs and air passages) is the 

 only means of cooling the body when objects around it are 

 warmer than the body itself. In this case the agents of heat 

 transfer only add heat to the body, but even their combined 

 action may often be overcome by an abundant evaporation 

 of perspiration. Men have remained for some time in rooms 

 whose temperature was as high as 260 F., or 48 above the 

 boiling point of water, without any marked rise of the body 

 temperature and without severe discomfort, the temperature 

 of the body being kept down solely by the evaporation of 

 perspiration from the skin. In order to make this means of 

 cooling possible, it is absolutely essential that the air be dry 

 and capable of taking up moisture. No one can survive long 

 at such temperatures in moist air. 



13. The effect of stagnant versus moving air ; the aerial 

 blanket. On a perfectly still day the layer of air about the 

 body becomes warmed by the skin and, so long as it is not 

 removed, forms an air-blanket which goes far to keep the 

 skin warm; for air is a poor conductor of heat. As soon, 

 however, as a breeze springs up, convection comes into play 

 and the skin is cooled more rapidly. In stagnant air, more- 

 over, the evaporation of the perspiration tends to saturate 

 this air-blanket with water vapor, so that further evaporation 

 is rendered difficult. Accordingly, when perspiration is not 

 being secreted, moving air cools the body by increasing con- 

 vection ; and when the skin is moist it cools the body both 

 by increasing convection and by facilitating the evaporation 

 of perspiration. The breeze which in winter is an unwhole- 

 some draft, in summer is often absolutely essential to working 

 power as well as to bodily comfort, for without it we are 

 clothed in this aerial blanket. 



