FORESTS AND THEIR CLIMATIC INFLUENCE. 401 



It has now been sTiown, 1st, tliat there exists a difference between eva- 

 poration on a naked and on a sodded soil; 2d, that there is such a differ- 

 ence also in regard to a sodded soil and a wooded one, with the advantage 

 in favor of the" latter that it better facilitates infiltration of the w'ater; 

 3d, that the quantity of water imbibed by the roots does not parch the 

 soil, since, after exhalation, it again falls in the state of fog, dew, or 

 rain. Desiccation only takes place when the soil is exhausted. 



Let us see now to what point the conclusion of M. Beugnot, that the 

 clearing away of woods is never prejudicial to salubrity, has its founda- 

 tion in fact. ' This conclusiou is true if the soil is siliceous or calcareous 

 and the subsoil pernie.ible; but it is not so if both are argillaceous, for 

 the roots are no longer i>resent to facilitate infiltration, unless indeed 

 drainage is used to remove the stagnant water. Of this, Sologue, La 

 Brenuo, and Dombes are examples. Xor is it true if the woods which 

 are removed existed in the proximity of swamps producing pestilential 

 miasms, like the Pontine marshes. 



Let us next consider the calorific influence of forests. This influence 

 has been established as follows by Humboldt and the meteoroh^gists : 

 They screen the soil from solar radiation, preserve a greater humidity 

 therein, and promote the decomposition of the leaves and twigs, which 

 are converted into humus; they produce frigoriflc effects through the 

 strong aqueous transpiration of the leaves, and by multiplying, through 

 the expansion of the branches, the surfaces, which, acquiring heat by 

 the action of the solar radiation, are again chilled by the nocturnal 

 radiation. In relation to this latter action, positive experiments demon- 

 strate that the stratum of air in contact with a prairie or a field covered 

 with grass or with leafy plants is cooled, all else being equal, under the 

 nocturnal radiation, to the amount of several degrees, sometimes from 

 6° to 7° or 8° (10° to 15° F.) below the temperature of the air at some metres 

 higher up ; while nothing similar takes place on a denuded surface, which 

 grows warm or cool according to the nature of the parts composing the 

 soil. We will add, however, that, inasmuch as the leaves as well as the 

 trunk and branches acquire heat under the solar influence and preserve 

 during the night a portion of that heat, this effect must naturally coun- 

 terbalance that resulting from the nocturnal radiation. Until now, no 

 account has been taken of this effect of solar radiation upon trees, 

 although it exerts a considerable influence on the temperature of the 

 air beyond the woods as within them. To explain the calorific influence 

 of ti-e'es on the temperature of the air, it is necessary, therefore, to join 

 to the older observations those which have been more recently made on 

 the temperature of the air at different heights in the neighborhood and 

 at the periphery of trees. 



Humboldt and Bonpland, recumbent on the grass during the fine 

 nights of the tropics in the plain of Venezuela and the lower Orinoco, 

 experienced a humid coolness when the strata of air at a height of 1 or 

 2 metres (3 or 7 feet) had a temperature of 20° to 27°, (79° to 80° F.) 

 In the equatorial and tropical regions, where the nocturnal radiation 

 acts with so much force by reason of the serenity of the sky, the increase 

 of temperatm^e, as we ascend, is manifested as in middle latitudes, but 

 to a much greater height. Hence, in the equatorial zone, no change is 

 observed in the vegetation from the level of the sea to the height of GOO 

 metres, (1,969 feet;) and beyond this, even to an altitude of 1,200 metres, 

 (3,937 feet,) we still recognize the flora of the tropical zone. 



We can now explain why, under our latitudes, certain products of 

 culture fail in the depressions of the siuface and succeed upon hills, 

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