Page:Popular Science Monthly Volume 1.djvu/684

668 the surface, may be lost. Again, if the surface be frozen, it may be impossible for the water to percolate into the ground, and, though it may descend in a more moderate manner, it may, in this as in the preceding case, be lost. Obviously, there are many causes of the kind which might be referred to; these, however, are sufficient to indicate the principle involved.

We have shown that agricultural conditions do not perceptibly affect the rainfall; they do, however, very powerfully influence what may be designated as the rain-waste. Thus, a growing plant vaporizes from its leaves an immense amount of water which its roots have abstracted from the ground. A sunflower will thus remove twenty ounces of water in a single day. There is in this respect a waste which varies in the different months, being greatest in those during which general vegetation is most rapid, and less in those—the winter months—when it is torpid. For these and other such reasons the monthly distribution of rain influences the actual supply.

It is interesting to remark that the rainfall in New York greatly exceeds that of London. Here it is 47.62 inches, in London it is but 25 inches, and the mean for all England is estimated at 31.25 inches.

But these considerations of the amount of rainfall are only a portion of a far more general and most important problem, viz.:

In this case, as in the preceding, there is a popular belief that clearing of land, drainage, and other agricultural operations, tend to produce such a result. Land that has been ploughed and exposes a dark surface to the sun, absorbs more heat, that is, becomes hotter, than land covered with forest-growth. It does not seem unreasonable, then, to suppose that, where thousands of square miles of surface have been submitted to such operations, the corresponding effect should be traceable, at least in the temperature of certain seasons of the year.

Moreover, there are some interesting facts which are matters of public observation and constant remark. Thus, as every one knows, in the city of New York itself, there are no longer the deep snows which characterized the winter seasons years ago. The large sleighs, often drawn by very many horses, used in those times as the public conveyances, have altogether disappeared from the streets. .It would seem, therefore, that the winters have become milder. In like manner, though in support of this conclusion we have less palpable evidence, there is a very general opinion that the intolerable and long-continued heats, which formerly made the summer months almost unbearable, have greatly moderated, and, that, though the thermometer may occasionally rise as high as it formerly did, the continuance of the hot weather is shorter.