EFFECT OF WOODS ON CLIMATE 



155 



temperature. It has been found that the mean annual tem- 

 perature of forest soil is decidedly lower than that of soil in the 

 open ; the temperature of the surface soil varies with that of the 

 air, therefore to get a right estimate the temperature must be 

 taken at different depths. On the continent, and in America, 

 exact records of soil temperature have been kept for some time, 

 and of late years in this country also. Another important use 

 of the humus is to prevent evaporation of water from the soil. 

 It has been calculated that the evaporation from forest soil 

 with a deep layer of leaf-mould is less than one-fourth of that 

 evaporated in the open, and about one-half of that evaporated 

 from forest soil without a thick layer of humus, thus the 

 roots of the trees get a more constant supply 

 of water than plants in the open. Here the 

 mosses do a valuable work. The Feather-Moss ( O 

 absorbs five times its own weight of water, the 

 Bog-Moss seven times its own weight. 



EFFECT OF WOODS ON CLIMATE 



It follows, from what has been said, that 

 woodlands have a considerable effect on the 

 climate of a district. Owing to the taking in 

 of carbon dioxide by the leaves of the trees for 

 the purposes of assimilation the air is purified. 

 In decomposing this carbon dioxide the oxygen 

 is returned to the atmosphere. Observations 

 show that the air in a well-wooded district has 

 more ozone than that of the open country. 

 The ozone is greatest in winter and at the 

 edges of the woods. 



They also tend to make the air moister. 

 The degree of humidity in air may be measured 

 by a wet and dry bulb hygrometer ; readings 

 should be taken at least three times a day ; 

 at 6 a.m., i p.m., and 6 p.m. The difference in 

 the temperature indicated by the two thermo- FlG 4 , _w e t and d 

 meters is noted, and the degree of humidity bulb hygrometer. 



