10 LOCALITY AND FOIIKST VJ-KiKTATlOX. 



Sheets of water (the sea, lakes, rivers, etc.) and moist bodies 

 evaporate moisture, which, as vapour, rises in the atmosphere, 

 until it is again condensed into water. It either settles as 

 dew on cool objects, or falls as rain, snow, or hail from the 

 cloud region to the ground. Plants, being moist bodies, take 

 part in the circulation of moisture ; they receive it from the 

 soil through the roots, and evaporate it through the leaves. 



In this perpetual circular motion of moisture, several points 

 are of special interest to the forester. By the action of 

 heat water is converted into vapour, and consequently 

 evaporating bodies become drier and cooler, and reduce the 

 temperature of the surrounding layers of air in their effort 

 to replace the expended heat. It follows that the rate of 

 evaporation is, amongst other influences, governed by the 

 temperature, which depends upon the climate. There is, how- 

 ever, another reason why the rate of evaporation depends on 

 the temperature : — The maximum of vapour which saturated 

 air can hold rises at a more rapid rate than the increase in 

 temperature. If, for instance, one cubic foot of air is saturated 

 with vapour when it contains "15 grams of water at freezing 

 jjoint (32^ F.), it can hold "28 grams at a temperature of 

 54^ degrees, and "(U grams at 77 degrees. It follows that air 

 of a high temperature can hold more vapour than at a low 

 temperature, and yet the relative, humidity may be smaller in 

 the former case. Hence evaporation is more rapid in summer 

 than in winter : it is generally also greater during the day 

 than at night. 



4. Solid JUhUcs. 



Tlie atmosphere always contains a certain (juaiitity of 

 organic and inorganic solid ])odies, which are kept in suspen- 

 sion in conseijuence of their minute size and lightness. AVhen 

 vapour is condensed and falls to the ground as rain, snow, or 

 hail, it carries with it a certain ipiantity of these solid bodies, 

 which differs according to locality ; the mineral part of these 

 deposits is not inconsiderable compared with that wliicli 



