400 



FORESTS AND THEIR CLIMATIC INFLUENCE. 



First clay... 

 Second (lay. 

 Third day'. . 

 Fonrtli day. 

 Fifth day.'.. 

 Sixth day.., 

 Seyeuth day 



Evaporation 

 from the 

 water. 



2IiUimcfrcs. 

 15. 

 1:3. 7 

 ILf) 

 12.0 

 11.7 

 11.0 

 !.). 4 



Evaporation 

 from the 

 earth. 



MiUimcfrcs. 

 4.1 

 2.5 

 L8 

 1.3 

 1.3 

 1.2 

 1.3 



Evaporation, therefore, proceeds rapidly iu the earth at first, but, as 

 we see, becomes finally very feeble. 



The series of experiments we have above reported show that evapor- 

 ation must vary considerably with the nature and physical state of the 

 soil, a consideration to which no regard has heretofore been paid ; thus, 

 lands covered with low vegetation, or with woods, and wliose soil is 

 composed of humus inixed with sand, lime, or clay, absorb more water 

 than those which do not contain humus, and retain it consequently 

 longer than the latter. The effects vary according to the proportions of 

 the different elements which compose the soil. The infiltrations are 

 greater iu wooded lands than iu sodded, the roots dividing them to a 

 greater depth, and thus facilitating the passage of the waters which 

 are not arrested excei)t by some imi)ervious stratum. The branches of 

 trees ]irovided with leaves not only form an obstacle to the evaporation 

 of the water resting on the surface, but the leaves are further constantly 

 surrounded by an atmosphere of vapor proceeding from exhalation, and 

 which prevents evaporation, in so far as this exhaled water suffices for 

 the saturation of the air ; during all this time the infiltration continues 

 iu the earth. Herbaceous plants, being deficient in mass, do not pro- 

 duce the same effect ; iu fact, whoever has been in i>laces wooded in 

 part, and in part covered with sod, must have remarked that, after rain 

 and exposure for some time to the sun, the sodded spaces had become 

 dry, while the wooded were still damp. 



Let us sj^eak now of the water taken up by the roots, and of that 

 which is exhaled in the air. The roots of trees, as is proved by the 

 experiments of Hales, Dutrochet, Mirbel, and M. Chevreul, draw iu a 

 great fiuantity of water charged with the divers elements which go to 

 constitute the sap; the surplus of the water is exuded by the leaves, 

 and continually maintains in the ambient medium a humid atmosphere. 

 The water imbibed proceeds not only from the superficial strata, but, 

 moreover, from the more or less deep strata into which the roots pene- 

 trate, and which furnish little or no water to herbaceous vegetation; 

 these strata are fed by subterranean currents of water often coming from 

 a great distance. Further, this water which existed in the deeper strata, 

 being effused into the atmosphere, afterwards falls as dew, fog, or rain, 

 and thus augments the quantity of water which the surface of the soil 

 receives eveu at remote distances. 



The quantity of water imbibed by the roots is such that it is difficult, 

 in fact, to make anything grow near trees. Several causes prevent it. 

 The earth which envelopes the roots nearest the surface is often in a 

 certain state of desiccation ; it loses by degrees its nutritive principles, 

 its lime, &c. ; it becomes dense, and, no longer containing aught else 

 than clay and sand, acquires compactness, and is then more perjueable. 



