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Show ~00 EFFECTS OF FORESTS ON CLIMATE. [Ch.XII. have produced on the surface of certain parts of En~lancl, we need not revert to periods w ben the heat of the climate was tropical. . . In the torrid zone the degradatiOn of land 1s generally more rapid, but the waste is by no means propo~·tioned to the superior quantity of rain or the suddenness of its fall, the transporting power of water being counteracted by a greater luxuriance of vegetation. A geologist who is no stranger to tropical countries observes, that the softer rocks would speedily be washed away in such regions, if the numerous roots of plants were not matted together in such a manner as to produce considerable resistance to the destructive power of the rains. The parasitical and creeping plants also entwine in every possible direction so as to render the forests nearly impervious, and the trees possess forms and leaves best calculated to shoot off the heavy rains, which when they have thus been broken in their fall are quickly absorbed by the ground beneath, or when thrown into the drainage depressions give rise to furious torrents*. The fellinO' of forests has been attended, in many countries, 5 by a diminution of rain, as in Barbadoes and Jamaica t· For in tropical countries, where the quantity of aqueous vapour in the atmosphere is very great, but where, on the other hand, the direct rays of the sun have immense power, any impediment to the free circulation of air, or any screen which shades the earth from the solar rays, becomes a powerful cause of humi· dity, and wherever dampness and cold have begun to be generated by such causes, the condensation of vapo~r continues. The leaves moreover of all plants are alembics, ~nd some of those in the torrid zone have a remarkable power of distilling water, thus contributing to prevent the earth from becoming parched up. There can be no doubt that the state of the climate, espe· cially the humidity of the atmosphere, influences vege~ation, and that, in its turn, vegetation reacts upon the chmate i "' De la Beche, Geol. Man. p. 184, t Phil, Trans., vol. ii., P• 294· Ch. XII.] DISTRIBUTION OF AMERICAN FORESTS. ~01 but some writers seem to have attributed too much importance to the influence of forests, particular} y those of America, as if . they were the primary cause of the moisture of the climate. The theory of a modern author on this subject, "that forests exist in those parts of America only where the predominant winds carry with them a considerable quantity of moisture from the ocean," seems far more rational. In all countries, he says, " having a summer heat exceeding 70o, the presence or absence of natural woods, and their greater or less luxuriance, may be taken as a measure of the amount of humidity, and of the fertility of the soil. Short and heavy rains, in a warm country, will produce grass, which, having its roots near the surface, springs up in a few days, and withers when the moisture is exhausted; but transitory rains, however heavy, will not nourish trees, because, after the surface is saturated with water the rest runs off, and the moisture lodged in the soil neithe; sinks deep enough, nor is in sufficient quantity, to furnish the giants of the forest with the necessary sustenance. It may be assumed, that twenty inches of rain falling moderately, or at intervals, will leave a greater permanent supply in the soil than forty inches falling, as it sometimes does in the torrid zone, in as many hours * .'' "In a1 1 regw. ns, "he contm. ues, '' wI 1 ere ranges of moun-tains intercept the course of the constant or predominant winds, the country on the wind ward side of the mountains will be moist, and that on the leeward dry, and hence parched deserts will generally be found on the west side of countries within the tropics, and on the east side of those beyo~d them,. the .pre~ailing winds in these cases being generally 111. opposite dtrectiOns. On this principle, the position of forests 111 North and South America may be explained. Thus~ for. example, in the region within the thirtieth parallel, the mmsture swept up by the trade-wind from the Atlantic is pre- * Maclaren, Art. America, Encyc. Britannica. |
