106 THE ATMOSPHERE AND ITS SERVICE TO MAN 



FIGURE 52. DESERT 

 WATER BAG 



provides for keeping the human body and the bodies 



of some other animals at the right temperature by this 



process of evaporation. The 

 warmer the healthy body gets 

 the more it perspires, and the 

 evaporation of the perspiration 

 keeps down the temperature. In 

 case of fever, the pores of the 

 body close up, perspiration ceases, 

 and the temperature immediately 

 rises. The physician often has to 

 use ice packs to do the work of 

 normal evaporation until perspira- 

 tion resumes. 



Plants are also kept cool by the 



evaporation of water which exudes from their leaves. This 



is called transpiration. 



Humidity and Comfort. The humidity of the air has 

 much to do with our bodily comfort. In quiet warm air, 

 nearly saturated with moisture, the perspiration cannot 

 readily evaporate and cool the body. Thus a temperature 

 of 80 F. with a high relative humidity may be more un- 

 comfortable than a temperature of 95 F. with a low relative 

 humidity. On a humid day the perspiration that evap- 

 orates brings the air that is near the body closer to the 

 saturation point, and we fan ourselves to move it away 

 and.to allow the less moist air to take its place. Any breeze 

 gives relief because it keeps changing the air around the 

 body. An electric fan, although it in no way cools the air, 

 helps evaporate the perspiration by keeping the air in mo- 

 tion. Crowded rooms often become " close " because of a 



