are the sources of man's mechanical powers when compared with those employed by nature in moving machinery which brings the seasons round and preserves the harmonies of creation!
275. Physical adaptations.—The amount of heat required to reconvert these 513 cubic miles of rain-water into vapour and bear it away, had accumulated in the Mississippi Valley faster than the earth could throw it off by radiation. Its continuance there would have been inconsistent with the terrestrial economy. From this stand-point we see how the rain-drop is made to preserve the harmonies of nature, and how water from the sea is made to carry off by re-evaporation from the plains and valleys of the earth their surplusage of heat, which could not otherwise be got rid of without first disturbing the terrestrial arrangements, and producing on the land desolation and a desert. Behold now the offices of clouds and vapour—the adaptations of heat. Clouds and vapour do something more than brew storms, fetch rain, and send down thunder-bolts. The benignant vapours cool our climates in summer by rendering latent the excessive heat of the noonday sun; and they temper them in winter by rendering sensible and restoring again to the air, that self-same heat.
276. Whence come the rains for the Mississippi.—Whence came, and by what channels did they come, these cubic miles of water which the Mississippi River pours annually into the sea? The wisest of men has told us they come from the sea. Let us explore the sea for their place and the air for their channel. The Gulf of Mexico cannot furnish rain for all the Mississippi Valley. The Gulf lies within the region of the north-east trades, and these winds carry its vapours off to the westward, and deliver them in rain to the hills, and the valleys, and the rivers of Mexico and Central America. The winds that bring the rains for the upper Mississippi Valley come not from the south; they come from the direction of the Rocky Mountains, the Sierra Nevada, and the great chain that skirts the Pacific coast. It is, therefore, needless to search in the Gulf, for the rain that comes from it upon that valley is by no means sufficient to feed one half of its springs. Let us next examine the Atlantic Ocean, and include its slopes also in the investigation.
277. The north-east trades of the Atlantic supply rains only for the rivers of Central and South America.—The north-east trade-wind region of this ocean extends (§ 210) from the parallel of 3° to the equator. These winds carry their vapour before them, and,
