Page:Popular Science Monthly Volume 1.djvu/683

Rh this is no more than might have been expected from a correct consideration of the meteorological circumstances under which rain is produced.

It is the belief of European meteorologists that the mean rainfall on the western portions of that continent varies little, if at all, when periods of many years are considered. In England there are rainfall records reaching back to 1677. Since 1725 these records are unbroken; at present there are more than 1,500 rain-gauge stations in that country. The Scotch observations extend to 1731, the Irish to 1791.

A discussion of the observations made at the Royal Observatory at Greenwich, in 1859, led to the conclusion that the annual fall of rain, as compared with that previous to 1815, was becoming smaller; but more extended observations, taken from gauges at stations widely separated, led to the opposite conclusion, viz., that there was a perfect compensation, the decrease at one place being compensated by the increase at another.

This conclusion was strikingly illustrated by the Continental observatories. The rainfall at Paris was found not to have altered in 130 years, and, though the observations of 50 years at Marseilles gave a decrease, those for 54 years at Milan gave an increase.

Even in the same locality this principle of compensation may be noticed. Thus the rainfall in England, in the ten years from 1850 to 1859, was found by Mr. Symons to be five per cent, less than during the previous 40 years, but during the following six years it was five per cent, above the mean of the preceding ten.

It may, however, be supposed, that conclusions which apply to the old settled countries of Europe, in which but few important topographical changes through agricultural or other operations have taken place for many years, will scarcely apply to America, wherein the clearing of land and agricultural surface changes have been occurring on a very extensive scale. The foregoing conclusions, however, show us how insignificant is the meteorological result which these variations produce.

The actual supply of water does not depend on rainfall alone. It is diminished by evaporation and also by percolation. When the condition of the atmosphere is such that, either by reason of the heat, the prevalence of dry winds, and other such causes, the water that has fallen is exposed to rapid vaporization, the available supply necessarily becomes less.

As regards percolation, much depends on the rate at which the rain falls, and the contemporaneous condition of the surface of the ground. The supply may come so rapidly that there is not time for it to soak into the earth. In this manner the quantity that properly belongs to a whole month may fall in the course of a few hours, and, rushing over