372 



HUMAN PHYSIOLOGY 



By radiation we mean the very swift departure of heat 

 through space. We radiate heat through the air without 

 heating the air itself to any appreciable extent. Heat 

 may pass from the human body across a room to be 

 absorbed by the frosty window panes. Outdoors it 

 may go into interminable space. The heat which we 

 radiate is often compensated by the heat radiated to us. 

 TWO rooms may have the same temperature as deter- 

 mined by thermometers hanging in central locations, but 

 one may seem much cooler than the other if it has cool 

 walls and windows. In this room we radiate heat which 



FIG. 77. Cats in hot and cold environments. (See text.) 



is not returned; in the other the give and take are more 

 nearly equal. 



Radiation is hindered by humidity. All things cool 

 down more rapidly at nightfall if the sky is cloudless 

 and the air dry than when it is foggy. It is in regions 

 of low humidity, like Arizona, that the greatest differ- 

 ences between night and day occur. But the humidity 

 that limits radiation favors conduction and this gives 

 cold, damp air an added chill. Winter on our Atlantic 



