Page:Popular Science Monthly Volume 7.djvu/355

Rh the summits of the mountain-ridge, from the valley of the Connecticut, and also from the sea. This moist air, meeting with the general current from the southwest, piles up an immense mass of cumulus cloud, of many square miles in extent. So long as the intense heat prevails, this cloud increases in size; grows black and blacker with its dense vapor, and casts a gloomy, lurid glare over the face of Nature, darker than that of any eclipse. The vapor, pushed up by the ascending currents of heated air, attains to a great height above the sea, where the temperature is very low. But finally, at that hour of the afternoon when the heat begins to decline, the accumulated vapors, no longer augmented or sustained by heated air from the valleys below, fall in rain.

The effect of large cold drops of water, or perhaps of ice, making altogether millions of tons in weight, falling from a great height into a deep, narrow valley, is, not only to beat down the air into that valley, but to chill the air there; and the cold air, seeking the lowest level, tends to rush down the valley, at first near the surface of the earth, but growing deeper and deeper, until the cloud itself is borne away on the swift-rushing air-freshet of its own making.

The land beginning to cool with the declining sun and the cooling rain, causes the southerly breeze to slacken and die away, and the storm-cloud rushes on unobstructedly down the West River and the Connecticut, deluging and fertilizing the fields along its course, while its quick lightning and oft-repeated claps of thunder flash and resound among the reverberating hills.

The cloud passes on, and often the sinking sun comes out from behind it; the late hushed and frightened birds gush forth with new song; myriad drops hang glittering on the spray; the green is flushed with a brighter, fresher hue, and the glowing rainbow smiles serenely from the dark, retiring, and still grumbling storm.

This storm is followed the next day by delightfully clear weather, with a cool, exhilarating breeze from the northwest; though this is not always the case, the cloud sometimes overspreading the sky, losing its motion, and leaving the air damp and murky.

The thunder-shower, as we have thus described it, though limited to a small district of country, may be regarded as the type of all similar showers that occur in mountainous regions everywhere. Numerous modifications, however, of local origin will occur, due to various causes; and it would be a highly-interesting and valuable study to ascertain these causes for every particular case.

Yet as to whether the moving force of the thunder-gust is limited wholly to the causes here given, may well admit of a question. It is not improbable that a cloud, from its great height, may penetrate a high upper current from the northwest, and that both this upper and the lower current may contribute to its rapid motion of translation. It is well that these thunder-showers are movable, instead of being