FORESTS AND THEIE CLIMATIC INFLUENOE. 399 



ation of these questious requires that we should take into consideration 

 the folloAvinp: data: 



Schubler lias ])roved that all earths do not possess the same property 

 of imbibition. In 100 parts of earth desiccated to 40° or 50°, the num- 

 bers fountl, for the quantities of water absorbed, are these : 



Siliceous sand 2r> Pure clay 70 



Calcareous sand 29 Fine calcareous earth 85 



Barren clay 40 Humus 190 



Calcareous and siliceous sands are the substances, thel^efore, which 

 have least affinity for water, Avhile humus is that which has most. The 

 state of division, as will be seen, has an intluence on the conditions of 

 fine calcareous earth. It is impossible to separate, in the present case, 

 the i)roperty of imbibition from that of ai)ti>ade for desiccation, to 

 which regard must be had in evaporation. Experiment proves that 100 

 parts of the water of saturated earth lose in four hours, at 13°.75 

 (57° F.) of temperature: 



Siliceous sand 83.0 Argillaceous earth 34.9 



Calcareous sand 75.9 Pure clay 31.9 



Barren clay 52. Lime in line powder 28. 6 



Fertile clay 45. 7 Humus 20. 15 



It will be seen from this that siliceous sand is the substance which 

 allows water to escape most easily, while humus is that which retains 

 it longest. Calcareous saud loses water less easily than silicious sand. 



We will further mention the results obtained by Melloni in experi- 

 ments relating- to the refrigeration undergone by certain substances 

 under the iuliuence of nocturnal radiation, and which should be taken 

 into consideration : 



Substances. 



Eatio in the 

 effects of re- 

 friseratioQ. 



Plants with smooth leaves . 



Siliceous saud 



Vegetable earth 



103 



103 



92 



Now, the absorbing i)Ower being- equal to the emissive power, it must 

 be admitted that the substances will in the same time acquire heat in 

 the same ratio. Such are the elements which enter into the solution of 

 the question, or rather questions, proposed by M. Gay-Lussac. 



When rain falls on the soil, the upper strata first become saturated, 

 then the excess of water passes to the lower stratum, which likewise 

 becomes saturated until the excess of one stratum completely saturates 

 that which is beneath it. When the upper stratum becomes dry through 

 evaporation in the air, it resumes, from that which is below, what it has 

 lost, as does the latter from the third stratum, until all the water pri- 

 marily absorbed is dissipated. As to the evaporation, it is evidently 

 less, all else being- equal, on a wooded surface than on a merely sodded 

 one. 



M. Gasparin {Tralte cVAf/ricuIturc, t. II, p. IIG) has made experiments 

 on the suljject we are considering-, and finds, on comparing- the evapor- 

 ation of a surface of water with that of a surface of earth completely 

 saturated, in the month of August, and at a temperature of 23° to UG°, 

 (73° to 79° F.,) the following results as the ratios of one to the other. 



