344 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



for the opposite hemisphere; the length of the winter will there be 165.5 

 days, and that of the summer 199.5 days. But as the long summer will 

 receive the same amount of heat as the short summer, on account of the 

 distance from the sun, and as the same unequal distribution of the heat 

 of the winters is balanced by an inverse unequal proximity to the cen- 

 tral luminary, the sum of annual heat received from the sun is the same 

 for the two hemispheres. 



In like manner, the intensity of the aerial currents should be equal m 

 the two hemispheres, if we except all accessory influence. Thus the 

 northern hemisphere has a very warm summer, while the winter of the 

 opposite hemisphere is of mean intensity. The difference which exists 

 between these two correlative seasons of the two hemispheres should, 

 by virtue of the compensation mentioned, be exactly equal to the dif- 

 ference which exists between the rigorous winter of the northern, and 

 the temperate summer of the southern, hemispheres. The aerial cur- 

 rents depend, then, entirely upon the course of the sun. for generally the 

 warmest place is that where the sun is at the zenith, and toward this 

 place or this line the currents will be directed. It is evident that wLen 

 the earth is at the equinoxes the two hemispheres are very nearly 

 equally heated, consequently the aerial currents will be of the same in- 

 tensity in the two parts of the globe. Let us suppose summer to com- 

 mence in the southern hemisphere, the northeast trade-winds will blow 

 from the beginning to the end of the season to the south of the equa- 

 tor, and displace, consequently, the line of greatest heat toward Tbe 

 austral pole. The upper currents, which sweep over an extent greater 

 than the corresponding winds of the opposite hemisphere, will equally 

 transport a greater quantity of humidity toward the north pole, where 

 it will be precipitated, under the form of snow, on account of the groat 

 cold which reigns there. This phenomenon will occupy 199.5 days. 



Now comes the summer of the boreal hemisphere. Its burning heat 

 will contrast with the winter of the opposite hemisphere as tbe two jire- 

 ceding seasons, contrasted with each other; consequently from the spring 

 equinox to the autumn equinox, the southeast trade-winds will blow to 

 the north of the equator, as in the preceding case the northeast tra tie- 

 winds blew to the south of the equator, and for 1G5.5 days an equiva- 

 lent of humidity will be carried to the austral hemisphere. But these 

 vapors are, in the first place, warmer than those of the austral summer; 

 then the temperature of the austral winter, which, moreover, is the 

 shorter, is elevated on account of the proximity of the sun. The 

 vapors, therefore, carried to the south by the upper currents are resolved 

 into rain instead of falling in the form of snow, as in tbe boreal hemis- 

 phere, and in consequence the solar beat of tbe next summer will be able 

 without interruption to heat the surface of the earth, and raise the mean 

 temperature of the climate. In the boreal hemisphere, on the contrary, 

 a part of the heat is absorbed by fusion of the accumulated snow and 

 ice, and the action of the sun weakened by the constant precipitatioaa^ 



