Page:Popular Science Monthly Volume 31.djvu/75

Rh frosts, and permit the safe continuance of navigation during tardy winters.

In all countries where the snow forms deep masses in winter, the rivers rise at the time of its melting; the quantity of water produced hy the melting of snow and ice is so great, and the evaporation is so little, as to produce much greater floods than ever can arise from rains. This phenomenon is therefore of a character to affect some of the sides of practical life; yet the way in which it operates has never been sufficiently well observed. The results of the thaw depend upon its rapidity as well as upon the quantity of snow that may be upon the ground when the frost breaks up. If the snow melts rapidly, inundations may ensue, while the duration of the high water will be too brief for it to be utilized for navigation; and the contrary will take place if the thaw is slow and gradual.

It is a popular saying in Russia that when there is little snow the waters will be high, and there will be little water when much snow falls. This kind of paradox is justified in the case of the smaller rivers. When little snow falls in winter, the ground freezes to a great depth. The first water that is spread over the surface also freezes, and a crust of ice is formed, over which the water flows as over a rock, without penetrating it. It therefore reaches the rivers quickly and swells their waters. When, on the contrary, the snow is abundant, it protects the ground in such a way that the thaw can begin from below, and the formation of a crust of ice on the surface is not possible. The melted snow penetrates the soil, and does not reach the ravines and rivers till after some time—that is, till after the ground has been saturated. The Russian peasants call this ground-water. Observation teaches that it proceeds from the forests rather than from the fields, because the snow accumulates there to a greater depth, and is less scattered by the winds than in open places.

The melting of snow from below was observed in 1884, at the Agricultural Academy of Petrovsky, near Moscow. Observations were made at the surface and at various depths down to two metres. At seventy-five centimetres the temperature reached the freezing-point on the 5th of March, and it rose to a greater height sooner from this point than at fifty or at twenty-five centimetres. As similar conditions have been observed at various points in the valley of the Volga, the high waters of that river in 1884 did not rise above the mean, notwithstanding the great depth of the snow; but the sources of the stream were so well supplied with water by the gradual melting of the snow that navigation was unimpeded during the whole of the summer and fall. The contrary took place in 1880, when a colder winter with less snow was favorable to a rapid thaw; the freshets were among the highest that had been observed, but the water soon fell off, and from August till October navigation everywhere above the mouth of the Kama was precarious.