Page:Popular Science Monthly Volume 19.djvu/191

Rh drying muddy roads as being greater than that of sunshine, while we fail to recognize the fact that the desiccating effect is the same upon the fields as upon the roads.

Forests have a very obvious influence also in preventing the occurrence of floods and droughts. When the rains fall upon the open, unwooded country, unless it is of a quite level character, they flow off at once into the beds of the neighboring streams, and pour their united flood into the larger rivers, swelling their volume rapidly to such an extent that their waters can not be confined within their banks, but break out and overspread the adjacent lands, carrying destruction oftentimes to the growing crops, covering fertile fields with masses of gravel and rubbish of various sorts, interfering with manufacturing interests, and often destroying life itself. These floods are succeeded by periods of drought. The flow of water in the streams shrinks away, often leaving their beds almost dry. As a consequence, crops and herds suffer, the mill-wheels are stopped or turn but slowly and feebly, the transportation of merchandise is impeded, and the various industries of life suffer. The forests prevent such a deplorable condition of things. The spongy soil formed by their fallen leaves, accumulated for years, retains the rain which falls upon it as in a great reservoir, and obliges it to flow off gradually instead of with a sudden flood. The difference in the operation in the two cases may be likened to that between the flow of the rain from a smoothly shingled house-roof and from one covered with thatch. In the one case the water runs at once to the ground without any impediment. In the other it sinks into the straw to a considerable depth and trickles thence for days perhaps after the rain has ceased to fall. So, our hillsides and mountain-slopes, where the forests are most usually found, are the world's great roofs or water-sheds, from which, if they are thatched with trees, the water flows off slowly and in the most desirable manner into the streams and upon the lands of the regions below, but, if stripped of this protecting covering, then with sudden and disastrous flood which no art of man can withstand.

This is well illustrated by the report of the effect of a storm in two neighboring ravines in the valley of the Durance in southeastern France, the Ravine de St. Phalez and the Ravine de la Combe d'Yeuse. St. Phalez runs north and south, has a basin of reception fifty hectares (one hundred and twenty-five acres) in extent, is well cultivated and has an argillaceous soil. Combe d'Yeuse is much more steep, has a basin of reception of two hundred and fifty hectares (seven hundred and twenty-five acres), and is covered with pines and oaks. In other respects the two ravines are alike.

In September, 1864, an abundant rainfall took place. On the morning after the rain the ravine of St. Phalez was flowing with a small stream. The Combe d'Yeuse was dry. During the day a waterspout struck the mountain and prevailed for not more than forty