410 



FORESTS AND THEIR CLIMATIC INFLUENCE. 



fonntlation which retains the water, because the roots do not pierce the 

 subsoil or do so with difficulty, than in sandy formations which favor 

 the infiltration of water. In the latter case, the humidity proceeds 

 solely from the transpiration of the leaves. 



What occurs when a country is cleared of its woods, supposing the 

 soil either i)ervious or im])ervious? The effects which result depend on 

 the composition of the soil and on its absorbent, radiating, and conduct- 

 ing powers. Of these an idea may be formed from the researches of 

 Schubler. Commencing with the calefaction of lands exposed to the 

 sun, the following are the relations which are found to exist between 

 different soils : 



Design atiou of eartlis. 



Maximum temperature of the up- 

 per stratum, the meau tem- 

 perature of the air beiug 100°. 



Humid soil. 



Dry soil. 



Siliceous sautl, yellowish gray . 

 Calcareous saud, whitish gray 



Pure gypsnm 



Poor jrellowish clay 



Fertile claj^ 



White calcareous earth 



Grayish-black humus 



Grayish-black garden mold 



Degrees. 

 37.25 

 37.38 

 36.55 

 36.75 

 37.25 

 36.63 

 39.75 

 37.50 



Degrees. 

 44.75 

 44. .50 

 43. 62 

 44.12 

 44.50 

 43. 60 

 47. 37 

 45. 25 



It will be seen that color and humidity are the causes which exert the 

 greatest influence. The differences of temperature tlue to these causes 

 and that of the ambient air may, for the same soil, amount to 14° or 15°. 



If we pass to the capacity of retaining heat, it will be found that all 

 else being equal, the siliceous and calcareous sands, comjiared in equal 

 volumes with the difl'ereut argillaceous earths, with lime finelj" commi- 

 nuted, with humus, with arable and garden earths, are the soils which 

 conduct heat more imperfectly. This is the reason why sandy formations, 

 in summer, preserve, even during the night, an elevated temperature. 

 We may conclude from this that when a sandy tract is cleared of wood 

 the local temi)erature must be raised, and with the greater reason, inas- 

 much as the cause of refrigeration exists no longer. After the sands come 

 in succession argillaceous, arable and garden soils, and tiually humus, 

 which occupies the last rank. Kepresenting by 100^ the capacity which 

 calcareous sand possesses of retaining heat, the following are the ratios 

 observed : 



Degrees. 



Calcareous sand 100 



Siliceous sand 95. 6 



Argillaceous earth 68. 4 



Garden soil 64. 8 



Humus 49. 



It has been further established that the capacity of retaining heat is 

 proportional to the bulk of the particles. It is on this account that 

 land covered with siliceous stones grows cool more slowly than siliceous 

 sand, and that i)ebbly soils are better adapted for maturing the grape 

 than chalky and argillaceous formations which cool rapidly. From this 



