PRIMER OF PRIMERS. 201 



to take in heat, either directly from the sun, or in- 

 directly from the heated earth. 



Forests prevent sudden changes of temperature 

 throughout the year. In early autumn they de- 

 crease the frequency of destructive frosts by pre- 

 venting the temperature of the air from rapidly 

 falling. 



The presence of forests over extended areas pre- 

 vents the occurrence of sudden changes of tem- 

 perature. 



1. By permitting such areas to more thoroughly 

 absorb the sun's heat, on account of the greater 

 surfaces they possess. 



2. By keeping the air over the forests moister 

 than over the open fields, thus enabling it more 

 readily to absorb the sun's heat. 



3. By acting as a screen to the lands lying to the 

 leeward of cold winter winds. 



4. By preventing the frosts from penetrating 

 great distances into the ground, and, therefore, in- 

 creasing the chance of winter snows falling on 

 unfrozen ground. 



The atmosphere is composed of a mixture of 

 about seventy-seven per cent, by weight of nitro- 

 gen and twenty-three per cent, of oxygen. It also 

 contains a nearly constant quantity of carbonic 



