200 



THE HUMAN MECHANISM 



II. MOVEMENT OF AIR 



A. INFLUENCE ON HEAT 

 TRANSFER 



Movement of air increases heat 

 transfer to the atmosphere by re- 

 placing the " aerial blanket " of 

 warmed air with colder air, to 

 which heat is transferred more 

 rapidly. 



B. INFLUENCE ON EVAPO- 

 RATION 



When perspiration is evapo- 

 rating into stagnant air in contact 

 with the skin, this air becomes 

 more nearly saturated with water 

 vapor, and its power of absorbing 

 water vapor is lessened. By re- 

 placing the "aerial blanket" of 

 muggy air with dry air, the out- 

 put of heat by evaporation is 

 greatly favored. 



III. HUMIDITY OF THE ATMOSPHERE 



A. INFLUENCE ON HEAT 

 TRANSFER 



Humidity increases the rate of 

 transfer of heat, as explained on 

 page 197. This is of little impor- 

 tance on warm days, because little 

 heat is then transferred either by 

 dry or by moist air. On cooler 

 days it is of great importance. 



B. INFLUENCE ON EVAPO- 

 RATION 



Humidity diminishes the out- 

 put of heat by evaporation, because 

 the water vapor which the atmos- 

 phere can take up is limited and 

 a humid atmosphere is one already 

 largely saturated. This influence 

 of humidity is of no consequence 

 unless perspiration is being se- 

 creted, but it is a very important 

 matter on warm days. 



15. How the heat balance when disturbed is restored by 

 the body. In these ways changes in the activities of daily 

 life and changes of weather tend to change the heat balance 

 of the body that is to say, they tend to change the tem- 

 perature of the body. And they would do this, did not the 

 body possess the power, within certain limits, of changing 

 both its rate of heat loss and its rate of heat production. 



The rate of heat loss may be changed in two ways: 

 (1) by changing the quantity of blood flowing through the 

 skin. Obviously the more the warmed blood is kept within 



