204 The Moon. 



certainly not easy to account for the infrequency and uncer- 

 tainty of such observations. B. and M. could occasionally 

 perceive, in very clear air, a multitude of the minutest points 

 just on the night-side of the terminator, indicating a surface 

 less perfectly level than it might otherwise appear. They 

 could trace also, from their shadows, ridges of about 60 feet 

 in height, and 2i to 4 or 5 miles wide. There are many 

 more considerable ridges in the plain, running generally from 

 N. to S., branching and reuniting, rising to knolls at their 

 intersections, and sometimes enclosing circular hollows. The 

 remarkable peculiarity to which Schroter paid so much attention, 

 as existing everywhere in the moon, that these ridges form 

 lines of communication between more conspicuous objects, is 

 not without examples here. 



A few other features remain to be described. Olbers dis- ' 

 covered, with a 3f-inch Dollond, in 1794, two minute craters 

 between Picard and Condorcet. Five more of very trifling depth 

 are mentioned by B. and M. in the same region, but not 

 drawn, having been detected after the publication of this part 

 of the map. The Mare Grisium, indeed, is executed altogether in 

 an inferior way, as though it had been an early attempt. They 

 have omitted a few minute craters figured by Schroter near the 

 W. and N. border ; and many of these objects are of such dif- 

 ficult visibility that discrepancies must here cause no surprise. 

 Nevertheless, insignificant as they may appear, they are of 

 much value to the selenographer, as they admit of close com- 

 parison with regard to relative size, and consequently afford 

 an especially fair prospect of discovering the progress — should 

 it exist — of eruptive action. 



I 'Irani <l, a crater discovered by Cassini at the -S. end of 

 the serpentine ridge, was noticed by Schroter to be of extra- 

 ordinary depth, deeper than l'lcarcl, from its long retention of 

 shade.* Immediately S. of it lies a curious object first per- 

 ceived by him, an ancient looking ring with a central mound, 

 resembling much a walled plain — such as wo shall bo introduced 

 to hereafter — in ;i state of degradation and decay. He could 

 ttol always find it afterwards, though under corresponding 

 circumstances^ and licnce was led to infer some atmospheric 

 obscuration. Although l'>. said M. bare introduced abundant 

 details in this <lisl rid, somo of which I have not seen, they 

 railed to notice thai the curve which the winding ridge, so fre- 

 quently mentioned, tonus towards the E. is in reality the half 

 of a circle (projected of course as an ellipse), of which I have 

 distinctly made out the remainder, as sketched in the diagram. 



* This observer lias noticed that many of the smaller class of craters are so 

 remarkable on this account that there is cause to suspect some illusion, as true 

 shado could hardly remain so long. This muy be a point worthy of attention. 



