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Page:Popular Science Monthly Volume 19.djvu/192

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THE POPULAR SCIENCE MONTHLY.

utes; hardly bad it begun when the torrent of St. Phalez became awful; it filled the ravine from bank to bank, seizing and carrying off rocks which had been used to form a road, which was considered safe against all contingencies. At the same time, that of Combe d'Yeuse and all those traversing wooded lands remained dry or carried a comparatively insignificant quantity of water.

The forest conservator describes a second scene on the same spot in the following October. In a few minutes after the rain began, the torrent of St. Phalez gushed forth with the same destructive effect as before. But after an entire day of rain a small stream appeared coming down the Ravine d'Yeuse, which increased for three days and then for two declined. The only damage done was to a little footpath. "Thus we have," says the reporter, "two torrents very near to each other, and exposed to the same conditions, except that the basin drained by the one comprises fifty hectares of cultivated lands, that of the other two hundred and fifty hectares of woodlands. The first receives and allows to flow away the waters of the greater part of a storm in a few hours at most, causing thereby considerable damage; the second, which has received a greater quantity of rain, stores it—keeps it for two days—evidently retaining a portion of it, and takes three or four days to yield up the surplus, which it does in the form of a limpid and inoffensive stream."

So also in the colder latitudes, where during the wintry months the moisture of the atmosphere is precipitated in the form of snow and accumulates often to a great depth, the conservative influence of the forest is very obvious. The temperature of the woods is warmer in winter, as it is cooler in summer, than that of the open ground. Sheltered from the winds and, to a considerable degree, from the cold, the snows themselves forming a protecting covering, the earth seldom freezes in the forest, and the warmth from the ground below gradually melts the snow and so feeds the springs and streams as to maintain in them an equable flow. As the warmer sun and wind of advancing spring-time begin to heat the surface of the ground, the screen of the trees prevents their influence from being so sensibly felt in the woods as in the open fields. The result is, that the snows dissolve gradually, and the resultant water, sinking in the first place and for the most part into the spongy, leafy soil, flows away gently, as do the rains of summer-time, into the valleys and fields below. But, when the forests have been removed, the case is very different. The snows, no longer screened from the sun's rays and the warm winds by the interposed foliage or even the naked trunks of the trees, are rapidly dissolved, often before the earth beneath or the ground over which the waters must flow has been unlocked from the wintry frosts. As a necessary result, thousands of rivulets are formed almost at once, which are precipitated into the adjacent streams, whose rapidly increased volumes are hurried to the larger streams below, and thus we