4^ Physiognomy of the MoorCs Surface. 



have a diameter which varies from 10 or 12 to 2 or 3 miles. 

 Their form is nearly circular. They exhibit a conspicuous mar- 

 gin, which usually determines their outline. They are met with 

 in nearly all possible situations. They very often exhibit a 

 central mountain in their interior, which is rarely connected 

 with the principal and outer envelope. The internal surface 

 enclosed by some of these circular mountains is upon the 

 same level with the exterior ; and the encircling envelope pre- 

 s?nts something like gates of communication from the exterior 

 parts to the interior. Frequently two of these mountains may 

 be observed very near to each other, and very similar in form and 

 size. This fact is of importance, as it indicates a common origin, 

 whether we regard their nature or the epoch of their formation. 



In these annular mountains the elevation of the exterior en- 

 velope has generally a direct relation and correspondence with 

 the interior hollow. The external'slope ranges between a third 

 and a half of the extent of the internal one ; and this has led 

 Schroeter to imagine that were the envelope again levelled, 

 it would precisely fill up the vacuity. Were this demonstrated, 

 it would explain how the equilibrium between the different 

 parts of the moon is still maintained, notwithstanding the very 

 great differences in their altitude, and this would also go to 

 prove that it is to the agency of volcanic eruptions that we 

 must attribute the appearances which are met with on the sur- 

 face. Schroeter has modelled some annular mountains ; of 

 these he has compared the projections with the excavations, and 

 has often found a great similarity in the volume, though some- 

 times there have been sensible differences. However, it is as- 

 suredly a very difficult matter in this point to reach any thing 

 like certainty, more especially when we consider the obstacles 

 presented by a distance of 50,000 Gervian miles in exactly ap- 

 preciating all the physical dimensions of an annular mountain. 



No kind of proportion exists between the altitude and the 

 diameter of these mountains, and it would rather appear that 

 the smaller have the greatest absolute depths in their interior. 



These annular mountains and the analogous formations are 

 found throughout every region of the moon's surface ; but they 

 are more numerous and more closely connected with each other 

 in its southern than in its northern parts. In this latter region 



