M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FOEESTS. 317 



water than those which contain no humus, and consequently retain it 

 longer than the latter. These eifects vary according to the proportions 

 of the various elements of which the soils are composed. The iniiltra- 

 tions are greater in wooded lands than in those covered with sod. The 

 roots penetrate deeper, and thus facilitate the passage of waters, which 

 would be only stopped by an impervious stratum. 



The branches of trees in leaf, not only oppose the evaporation of the 

 water in the soil, but the leaves themselves are constantly yielding a 

 vapor from exhalation, and which tends to reduce the evaporation of 

 waters, so far as the moisture exhaled goes to saturate the air, the infil- 

 tration at the same time going on into the soil. Herbaceous plants not 

 in masses, do not produce similar effects; in fact, whoever has been in 

 places partly wooded and partly sodded must have observed after a rain 

 and a rest of some duration, that the sodded grounds were dry while the 

 wooded soil was always damp. 



We will now speak of the water absorbed by the roots, and that which 

 is exhaled into the atmosphere. 



The roots of trees, as shown by the experiments of Hales, Dutrochet, 

 Mirbel, and Chevreul, absorb a large amount of water charged with 

 various elements constituting the sap. The surplus water is evaporated 

 from the leaves, which are constantly surrounded by a humid atmos- 

 phere. The water thus evaporated is drawn not only from the upper 

 strata, but likewise from the deeper layers of the soil into which the 

 roots penetrate, and which supply little or no water to herbaceous vege- 

 tation. These lower strata are fed by subterranean sheets of water that 

 often come from a distance. Furthermore, this water remaining in these 

 lower strata, being thus given to the atmosphere, fall again as fog, dew, 

 or rain, and thus increase the quantity of water that the surface of the 

 soil receives from some distance away. 



The amount of water absorbed by the roots is so great, that it is 

 practically difficult to make much of it remain near the trees, several 

 reasons for preventing it occurring. The soil in contact with the roots, 

 and for a little distance away, is in a certain state of desiccation, little by 

 little it loses its nutritive properties, the lime, &c., and when these ele- 

 ments are gone, the soil contains little but sand and clay, which then 

 becomes more permeable. It is, therefore, well demonstrated — 



(1.) That a difference exists between the evaporation from a naked 

 soil and a soil covered with sod. 



(2.) That there is a like difference between a aoil covered with sod 

 and one that is wooded, with the further advantage of the latter in 

 facilitating the infiltration of water. 



(3.) That the amount of water absorbed by the roots does not produce 

 drought in the soil, since it is returned after evaporation in the condition 

 of fog, dew, or rain. The drought does not take place till the soil is 

 exhausted- 

 Let us now see to what extent the conclusion of M. Beugnot is well 

 founded, that the clearing of the woods is never injurious to health. 

 This conclusion is true if the soil is siliceous or calcareous, and the sub- 

 soil permeable ; but it is not if either one or the other of these is argil- 

 laceous, because in this the roots are no longer able to facilitate infil- 

 tration, as we see in Sologne, la Brenne, and la Dombes, which cannot 

 generally be drained except by drawing off their stagnant waters. It 

 is the same if the woods are cleared in the vicinity of pestilential miasms, 

 as the Pontine Marshes. 



Let us now pass to consider the thermal influence of forests. This in- 



