AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 651 



Thus, in any latitude n, the mean annual temperature of the Earth's surface will 

 be equal to 



I + Qh, 

 where Q depends on the latitude n. 



The temperature denoted by £ depends upon the heat which the Earth receives from 

 stellar space, and on that which it receives by radiation from the atmosphere*. Supposing 

 the former source to be constant, the second will vary with all the causes which may 

 locally affect the mean temperature of the air in the shade, at any proposed place. If 

 the condition of the Earth were the same at every point of its surface, the mean annual 

 temperature of its surface would be a function only of the latitude of the place. The 

 principal deviations from this simple law arise from local causes, and are comprised in f. 

 The value of h also may vary, as above explained, though probably in a very much less 

 degree, with local influences. 



Poisson shews that the value of Q is 

 ,95910 when fi = 0, 

 ,66662 ... n = 48°50' (at Paris), 

 ,398 ... n = 90 . 



Also, h = 35°,925 (C). 



Consequently, the direct effect of the solar heat is to increase the mean temperature 

 of the Earth's surface by 



34°,45 (C.) at the equator, 



23°,948 at Paris (lat. = 48°50'), 



14<>,996 at the pole. 



On account of the general purer state of the atmosphere, however, at the equator, 

 and its more clouded state at the pole, the first of the above numbers ought probably 

 to be somewhat increased, and the last somewhat diminished, as above mentioned. 



20. The second term in the expression for u has a year for its period, and is the 

 principal annual inequality in u , unless the latitude (^) be that of a place near the 

 equator. 



The difference between its greatest and least values 



b 

 = h — . ir sin 7 sin p.. 



D is determined from the expressions for D sin 8 and D cos S (Art. 17) ; whence 



D = 1,44573, 

 6 



Also 7 = 23°28', 



.-. sin 7 = ,398. 



* That which it receives from the air by actual contact has been omitted, in omitting the constant \, as compared with X 

 (Art. 17). 



