208 The North-West Lunar Limb. 



Schroter, who had supposed that what he calls the central hill 

 was of uniform height,, found with his 27 foot reflector, 1794, 

 Jan. 6, that it had two small summits, one towards each end ; 

 June 4, however, nothing could be perceived of these, but in 

 the centre there was a little bright elevation, that, perhaps, 

 included a minute indistinct crater. What is very remarkable 

 is a very dark spot of a roundish form in its southern part ; on 

 the other hand, a region just outside the wall to the W. con- 

 taining a small mountain, Atlas a, of no peculiar height or 

 steepness, glitters with 9° of light in the full moon. At a short 

 distance N. lies a small crater, Hercules A, worthy of notice 

 for its extraordinary steepness, through which it retains its 

 shadow longer than its colossal neighbours. 



Hercules is about 46 miles in diameter, encompassed by a 

 ring equally elevated with that of Atlas, and doubled in places, 

 especially on the E. side, where there is a secondary interior 

 and little less lofty chain, crowned like the principal range 

 with peaks arranged at almost regular intervals. This exhibits 

 a magnificent appearance about three days before the Last 

 Quarter. In the interior, but not in the centre, lies a remark- 

 ably distinct crater of 8° to 9° in Full Moon. Another may be 

 occasionally detected in the S. side of the ring. 



Notwithstanding the immediate vicinity of these two grand 

 foci of eruption, it is observable that neither has produced any 

 disturbance in the other's form. 



I have seen these objects very well at different times, 

 varying from three to six days after New Moon, and about four 

 days after Full. But it is to no purpose to affect precision in 

 such estimates, as they are liable to so much interference from 

 libration. The aspect of this region under a high illumination 

 is especially curious, and deserves a more detailed examina- 

 tion than appears to have been given to it by B. and M. j in 

 fact, the topographical study of what we may for convenience 

 sake term the " local colouring" of the moon, has hardly as 

 yet been begun. As far back as the year 1826, I remarked 

 that the interior crater in Hercules was larger than as it has 

 been twice drawn by Schroter ; but subsequent experience has 

 taught me to be cautious as to comparisons of this nature. 

 This object, however, affords an excellent example of an 

 arrangement so frequent that it may be termed a general 

 law upon the lunar surface, that among the products of erup- 

 tions whose successive date is self-evident, the more recent 

 are also the more reflective. 



This is a very suggestive fact, and deserves attentive con- 

 sideration. One obvious explanation would be the chemical 

 action of an atmosphere which may be presumed to cxisl, 

 however rare and difficult of detection. 



