62 



" It has been claimed that the presence of forests, like mountains, have the effect' of 

 lowering the temperature, and by this means of increasing the abundance of rains as well 

 as of diminishing their violence. It cannot be doubted that forests have the effect of 

 sheltering the surface from solar heat, and of causing a cutaneous exhalation from the 

 leaves, while they multiply, by the spreading of their branches, the amount of surface 

 cooled by this evaporation, and thus have a cooling effect ; but this, in fact, is far from 

 being general, and especially in our climate it is often marked, and even neutralized, by 

 local circumstanced, such as the physical properties of the soil, the topographical situation 

 of the place, the direction of prevailing winds, etc. If it is certain that the mean tempe- 

 rature of our country is higher than was in Gaul in the days of Csesar, when it was 

 covered with forests, we must nevertheless admit that while a forest protects the surface 

 from cold winds it does not tend to raise the temperature, and that if cut away a refri- 

 gerator would not be thereby necessarily produced. Thus, for example, it has been 

 proved that the department of VArdeche, which is now without a single considerable piece 

 of woods, has shown during the last thirty years a perturbation of climate, of which late 

 spring frosts, formerly unknown in the country, are among the saddest effects. A similar 

 remark may be made in the plains of Alsace, since the denudation of several of the crests 

 of the Vosges." 



" Tropical Forests. — On the contrary, in countries within the tropics, where the nights 

 are usually very serene, the radiating power of plants is sensibly increased, and the energy 

 of other frigorific causes are developed in the same proportion, so that the presence of 

 forests tends uniformly to reduce the temperature. This fact was proved by numerous 

 observations given in M. Boussingault's work on the region included between the eleventh 

 degree of north and fifth degree of south latitude, and it effectually explains the reason 

 why AmeHca is not so hot as Africa within these latitudes. 



" The action of forests upon rainfall, through the influence which they exert upon 

 the temperature, is therefore very difficult to determine in our country ; but it is dis- 

 tinctly marked in warmer climates, as proved by numberless examples. M. BoussLngault 

 reports that in the region comprised between the Bay of Cupica and the Gulf of Guaya- 

 quil — a district covered with immense forests — the rains are almost contimbal, and that 

 the mean temperature of this humid country is scarcely above 79° F. M. Blanqui, in his 

 travels in Bulgaria, mentions that at Malta the rains have become so seldom, since the 

 trees have been cut away to make room for cotton, that at the time of his visit in October, 

 1841, not a drop of rain had fallen during three years. The fearful dryness which has 

 desolated the Cape Verde Islands may be, in like manner, attributed to the cutting off of 

 forests. On the island of St. Helena, where the wooded surface has considerably increased 

 within the last few years, they observe that the amount of rain increases in the same pro- 

 portion, and it is now double that which fell annually at the time of Napoleon's sojourn 

 there. Lastly, in Egypt, the recent plantations have brought rains where they were 

 almost unknown before. 



" In the midst of this uncertainty in which our climate is left, by the study of mete- 

 orology — for the hygrometrical operations made at different points in France have yielded 

 results too diverse to serve as the foundation of any theory — we will come to limit our 

 •study of the action of forests to the regulation of the water courses in the single point of 

 view which their mechanical and physical laws present." 



" Bains, how disposed of in Forests. — The rains which fall upon our continents are 

 ■disposed of as follows : — A part runs from the surface into the streams that carry it back 

 to the sea. Another part is evaporated soon after its fall and returns to the atmosphere, 

 and another part is absorbed by the ground. The first and third of these exclusively go 

 to feed the springs and rivers, while the second is wholly withdrawn from our calculation. 

 This feeding of the water courses is more or less regular or constant, according as it finds 

 a superficial or underground passage-way, and therefore depends not only on the physical 

 properties and the topographical contours of the soil, but also upon the vegetation with 

 ^hich it is covered. 



" Under ordinary circumstances, the superficial flow produces no effect except upon 

 -soil where the slope is considerable and quite impervious to the water, such as denuded 



