370 



HUMAN PHYSIOLOGY 



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 



FIG. 69. Cats in hot and cold environments. (See text.) 



coast is declared to be more trying than the same season 

 inland although the thermometer goes much lower in the 

 interior. For the same reason that water feels colder 

 than air of the same temperature moist air feels colder 

 than dry. High humidity makes us warmer in summer 

 and colder in winter. 



In discussing the conditions affecting the metabolism 

 we have shown how the body meets external cold. The 

 skeletal muscles are called into extra activity. Without 

 special coverings to keep the heat pent in the body one 

 cannot be still. A rise in the metabolism is secured 

 which is, as we have insisted, essentially a case of exer- 



