The power of the solar rays depends much upon the manner in which they fall, as we readily perceive from the different climates on our globe. Although the sun is about three millions of miles nearer to the earth in winter than in summer, his rays strike the atmosphere in the northern hemisphere so obliquely that it absorbs a much greater quantity of heat than when they are more direct (N. 217). Indeed it is so great that, when the sun has an altitude of 30°, one half of his heat is absorbed by the atmosphere, and it increases very rapidly as he sinks towards the horizon. However, that heat is not lost: it is most beneficial to the earth, being really the heat which supplies the greatest part of that which is radiated into space during the absence of the sun. Professor Dove has shown, by taking at all seasons the mean of the temperatures of points on the earth’s surface diametrically opposite to each other, that the average temperature of the whole earth’s surface in June, when we are farthest from the sun, considerably exceeds that in December, when we are nearest to him, owing to the excess of water in the southern hemisphere, and that of land in the northern, which gives a general insular climate to the former, and a continental climate to the latter. The observations of the north polar navigators, and those of Sir John Herschel at the Cape of Good Hope, show that the direct heating influence of the solar rays is greatest at the equator, and that it diminishes gradually as the latitude increases. At the equator the maximum is 48-3/4°, while in Europe it has never exceeded 29-1/2°. M. Pouillet has estimated with singular ingenuity, from a series of observations made by himself, that the whole quantity of heat which the earth receives annually from the sun is such as would be sufficient to melt a stratum of ice covering the whole globe 46 feet deep. Part of this heat is radiated back into space; but by far the greater part descends into the earth during the summer, towards the zone of uniform temperature, whence it returns to the surface in the course of the winter, and tempers the cold of the ground and the atmosphere in its passage to the ethereal regions, where it is lost, or rather where it combines with the radiation from the other bodies of the universe in maintaining the temperature of space. The sun’s power being greatest between the tropics, the heat sinks deeper there than elsewhere, and the depth gradually diminishes towards the poles; but the heat is also transmitted laterally from the warmer to the colder strata north and south of the equator, and aids in tempering the severity of the polar regions.