44i Phijsiog^nomy of the Mooii's Surface. 



decide the very difficult question, whether the objects which 

 this luminary presents are subject to true physical changes, 

 which we have the power of recognising, a question regarding 

 which, in our opinion, nothing at all is known at the present 

 time. The circle above referred to, may be extended some addi- 

 tional eight or ten degrees, by taking the libration into account, 

 and profiting by it properly. Within these limits an ex- 

 perienced, attentive, and persevering observer, especially if free 

 from preconceived bias, and if supplied with powerful instru- 

 ments, which clearly define the objects, may be preserved from 

 optical illusions. Whatever is seen beyond the limits we have 

 just mentioned, may be very useful in determining the general 

 relations of the different parts of the moon ; but the region 

 near the margin is, for a variety of reasons, unsuitable for any 

 conclusions which are to be founded upon the accurate appre- 

 ciation of minute details. 



Those mountains which are situated in the interior of the 

 different circular zones which we have been describing, vary 

 considerably. Sometimes they are solitary, and at other times 

 they are conglomerated in masses, and more rarely in moun- 

 tain chains. Their summits never attain the elevation of 

 the zone, and sometimes not even that of the surrounding 

 surface. The shadow of the zone usually covers them a long 

 time before the sun sets on that locality in which they are 

 situated ; and it is not a little curious to witness them reappear, 

 as very minute luminous points, in the midst of the deepest 

 shade which envelopes them. 



From what has now been said, it will appear that the forms 

 of the craters of which we have been speaking, have very little 

 external conformity with analogous appearances in our planet. 

 The largest craters of our volcanos can scarcely be compared 

 with the smallest pits on the surface of the moon. We can always 

 perceive the ground at the bottom of the lunar craters ; whilst 

 many of those of the earth are real abysses. The Laacher lake, 

 near Andernach, the mountain Albano near Rome, and other 

 localities which might be mentioned, exhibit but the feeblest and 

 most obscure type of these lunar craters ; and these formations 

 are moreover confined in this planet to some very special regions, 

 whilst they decidedly predominate over the moon's surface. On 



