1 66 ELEMENTARY SCIENCE 



is absorbing heat, the other half is giving it off. While 

 the sunny side of earth absorbs heat, the shaded side 

 radiates it. The shadow of earth we call night, and the 

 nights would all be intensely cold were it not for the giv- 

 ing off from land and water surfaces of heat which they 

 absorb by day. 



The capacity of air to absorb heat is obviously far less 

 than that of land or water. What capacity it has depends 

 largely upon the amount of moisture in it. Thus we see 

 why in our dry southwest, or in the Desert of Sahara, 

 there is much greater variation in temperature between 

 day and night than there is in regions of greater humidity. 



Now if day be longer than night, as in spring or summer, 

 evidently the amount of heat absorption will be greater 

 than the amount of heat radiation. The land, the water, 

 and the "weather" all become warmer. But if night 

 be longer than day, as in fall and whiter, more heat is lost 

 than is gained, and everything gets colder. So we see that 

 the principal cause of the seasons is the variation in the 

 lengths of day and night. But, you ask, what causes these 

 variations of length? The answer to that we must post- 

 pone till the chapter on seasons. 



Conduction. This method of heat transfer you con- 

 sidered in connection with the freezing of ice-cream. Con- 

 duction requires the actual contact of substances of differ- 

 ent temperature. It is by conduction that our hand is 

 cooled when we touch ice, or warmed when we touch a 

 stove or a radiator. In nature, conduction operates to 

 transfer heat into air which is in contact with warmer 

 land or water surfaces, and thus it is an important factor 

 in the general distribution of heat which we are considering. 



