AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 671 



confounded with the temperature heretofore denoted by ^ (Art. 21), and which I have 

 estimated at — 39°,5 (C). We have no means, as already stated, of determining the value 

 of £. The above expression for u only tells us that the mean equatorial temperature of the 

 Moon at points not far beneath her surface, must be augmented by the temperature h, 

 derived from solar radiation ; i. e., by the same quantity as that by which the mean 

 equatorial temperature of the Earth would be augmented supposing the equator and ecliptic 

 to coincide, and the intensity of solar radiation to be the same in the two cases. If we 

 assume it to be somewhat greater for the Moon, and estimate h, for example, at 40° (C). 

 then the Moon's mean equatorial temperature, in the case supposed respecting the position 

 of her axis and the plane of her orbit, would be greater or less than zero (Centigrade), 

 according as £ should be greater or less than - 40° (C). At the pole the value of h would 

 be reduced to zero, and the temperature to £. With the actual positions of her orbit and axis 

 of rotation, the equatorial temperature would be somewhat less and the polar temperature 

 rather greater. 



The first periodical term expresses a monthly inequality. To find its greatest value we 



must know the value of — . 



Now, 



D' 1 2\/7r(l2,37) 2tt(12,37) 



— 1 J. J - _1_ J — 



V ab a 2 b- ' 



so that the value of — ; depends on that of — (or — = as before explained Art. 27), and 

 D ab p 



can only be determined, therefore, with some assumed value of this fraction. If we suppose 



a and b to have the same values as for the Earth, we shall have 



b 



and, therefore, 



D' " ,4 °' 



7T 



-,.--,628. 

 D 2 



Hence the term in question becomes at an equatorial point of the Moon's surface, 



h . (,628) cos {n(t - t) - ^} ; 

 and consequently its greatest value with our assumed value of h, will 



= 25°,12 (C). 



(b 2\ 

 h — Tl - ] of the next term is nearly 



= 8° (C). 

 The values of ^ and 5" are respectively 26.34' and 32°. 13' nearly, so that, though these 

 two terms cannot attain their respective maxima at the same time, it is easily seen that the 

 greatest value of their sum will amount to more than 30° (C). The succeeding terms are 

 much smaller, and may be neglected in a rough approximation like the present. 



Voi,. IX. Part IV. 86 



