AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 645 



16. Having thus considered the influence which an atmosphere like that of the Earth is 

 capable of exerting on the superficial temperature of a planet, I shall proceed to consider the 

 influence of solar heat on that temperature. It has been already stated that the Earth's 

 atmosphere is nearly diathermanous for the heat radiating directly from the Sun. It is not 

 probable, however, that it is entirely so, and if not, it must absorb some heat which must 

 elevate in some degree the temperature of its constituent particles. This augmentation of 

 temperature, however, must be too small to render it of any importance in a discussion like the 

 present, and may therefore be omitted. We may regard, then, nearly the whole of the solar 

 heat which radiates towards the Earth as penetrating by direct radiation to the lower region of 

 the atmosphere, and, when not intercepted by clouds, to the surface of the Earth. That 

 portion which reaches the surface is nearly all absorbed by the terrestrial objects on which it 

 falls, which, after thus becoming heated to a temperature above that of the surrounding 

 atmosphere, give it forth again, and in such degree that the mean quantity thus given back 

 in the course of a year is equal to the mean annual supply derived from the Sun. This 

 terrestrial heat, as already explained, not possessing the power of direct solar heat to permeate 

 the atmosphere without loss of intensity, passes into it either by conduction or partial 

 radiation, and, being absorbed, elevates the temperature of the atmosphere. But the atmo- 

 sphere when thus heated gives back again to the Earth itself a portion of the heat which it 

 has thus acquired ; and this action and reaction is continued till the ultimate equilibrium of 

 temperature is established. The direct solar heat, intercepted and absorbed by the clouds, 

 is given forth again in all directions, and thus contributes also to elevate the temperature of 

 the Earth's surface and of the stratum of the atmosphere reposing upon it. 



We should expect that the mean annual temperature of that portion of the atmosphere 

 which is constantly in contact with the Earth's surface, would be very nearly the same as that 

 of the surface itself; and, as far as observation has extended, this has been verified; for it has 

 been found, where the necessary observations have been made, that the mean annual tempera- 

 ture of the air, deduced from indications of a thermometer in the shade, and that of the 

 surface of the Earth, deduced from a thermometer of which the bulb was placed just beneath 

 the surface, are nearly identical. Hence if any assigned cause produce a certain effect on the 

 mean superficial temperature of the Earth itself, we know that it must produce the same effect 

 on the mean temperature of the adjacent atmosphere as a direct and immediate consequence. 

 Thus Poisson has calculated the increase of the Earth's mean superficial temperature due to the 

 quantity of heat which radiates directly upon it from the Sun. The mean atmospheric tem- 

 perature receives directly on this account an equal augmentation. There then remains to be 

 calculated the additional effect above mentioned arising from the mutual interchange of heat 

 between the Earth and its atmosphere. Poisson has likewise given formula? for this purpose; 

 but experimental data are altogether wanting to render them of the slightest practical value. 

 I shall endeavour in a different manner to estimate, in the sequel, this less direct effect of the 

 solar heat in elevating the Earth's superficial temperature and the temperature of that portion 

 of the atmosphere which determines the climatal conditions under which we live. 



The statement now made, it will be observed, respecting the equality of the temperatures 

 of the surface of the Earth itself and of the contiguous stratum of its atmosphere, applies 



