BOIL, SITUATION AND ASPECT. 37 



least not enough to enable us in all cases to reconcile 

 the anomalies which occur. Enough is known, how- 

 ever, to cause us to avoid the tops of hills and the 

 bottoms of valleys, and it may be worth our while to 

 consider a few of the principles which regulate tem- 

 perature in these situations. During the night, the 

 cold air, being heavy, settles down into the valleys 

 and hollows, thus producing in such locations a 

 temperature several degrees lower than is found on 

 the sides of the adjacent hills. And no influence is 

 then at work to disturb this state of things, for the 

 earth itself is becoming rapidly cooled by radiation ; 

 and if a small quantity of the air should become 

 warmed by contact with it, it immediately ascends, 

 and cool air takes its place. 



At daybreak, however, an agency is introduced 

 which reverses this condition of things. Then the 

 dense air in the valleys concentrates and absorbs the 

 heat of the sun's rays and increases their effect upon 

 the soil, which in turn imparts heat to the stratum of 

 air lying next it. This lower stratum of air being 

 warmed and consequently rendered much lighter than 

 the colder portion above it, it ascends, but as it rises 

 it also expands still more, which in some measure 

 compensates for the heat which it received from the 

 earth. The same process keeps going on until night 

 comes, when the lower stratum of air being no longer 



