rainless regions of the northern hemisphere.—According to the views presented in § 358 and Plate VII., the south-east trade-winds, which reach the shores of Brazil near the parallel of Rio, and which blow thence for the most part over the land, should be the winds which, in the general course of circulation, would be carried, after crossing the Andes and rising up in the belt of equatorial calms, towards Northern Africa, Spain, and the South of Europe. They might carry with them the infusoria of Ehrenberg (§ 358), but according to this theory, they would be wanting in moisture. Now, are not those portions of the Old World, for the most part dry countries, receiving but a small amount of precipitation? Hence the general rule: those countries to the north of the calms of Cancer, which have large bodies of land situated to the southward and westward of them, in the south-east trade-wind region of the earth, should have a scanty supply of rain, and vice versâ. Let us try this rule: The extra-tropical part of New Holland comprises a portion of land thus situated in the southern hemisphere. Tropical India is to the northward and westward of it; and tropical India is in the north-east trade-wind region, and should give extra-tropical New Holland a slender supply of rain. But what modifications the monsoons of the Indian Ocean may make to this rule, or what effect they may have upon the rains in New Holland, my investigations in that part of the ocean have not been carried far enough for final decision; though New Holland is a dry country.
366. Each hemisphere receives from the sun the same amount of heat.—The earth is nearer to the sun in the summer of the southern hemisphere than it is in the summer of the northern; consequently, it has been held that one hemisphere annually receives more heat than the other. But the northern summer is 7⋅7 days longer than the southern; and Sir John Herschel has shown, and any one who will take the trouble may demonstrate, that the total amount of direct solar heat received annually by each hemisphere is identically the same, and therefore the northern hemisphere in its longer summer makes up with heat for the greater intensity but shorter duration of the southern summer. But though the amount of heat annually impressed by the sun upon each hemisphere be identically the same, it by no means follows that the amount radiated off into space by each hemisphere again is also identically the same. There is no reason to believe that the earth is growing warmer or cooler, and
