The North-West Lunar Limb. 205 



20,000 feet ; several times afterwards, however, lie could not 

 see so great a proportion of shade, nor was the object distinct. 

 1794, Jan. 5, he found as broad a shadow, but only in one part 

 of the depth. 1795, March 25, it was again much as when he 

 saw it first. Hence he inferred, not as B. and M. say for him, 

 a mistake, but some occasional atmospheric obscuration within 

 that wonderful gulf. However, as they very justly remark, 

 there must be great difficulty in the measurement of so narrow 

 and so strongly foreshortened a cavity ; and we may observe 

 in general that the question of possible atmospheric variations 

 would be very unfairly tried in any oblique situation of this kind. 



A line drawn through the centres of Qeminus and Bemouilli 

 will point out a widely extended plain surrounded by a circular 

 wall, but thrown into a narrow ellipse by a position so near 

 the limb that it will be greatly affected by lib ration, and can 

 only be well seen when favourably circumstanced in this respect. 

 It is, however, a grand object, aud well worth looking for. Its 

 extent is no less than 110 miles, and its interior contains a chain 

 of mountains, such that scarcely more than one parallel instance 

 occurs on the visible lunar hemisphere. B. and M., whose 

 descriptions are usually of a very unimaginative character, 

 cannot refrain from enlarging on the beautiful effect of the 

 sunset in Gauss, as this region is termed, when, shortly after 

 the full moon, the illumination of the great wall gradually breaks 

 up into separate portions, and these again dwindle into mere 

 islands of light in the midst of the encompassing mass of dark- 

 ness — a spectacle which is repeated on the B. nine hours after 

 its termination on the W. side of the ring. They observe, too, 

 what a magnificent view must be obtained from the central 

 range of mountains in a suitable state of phasis and Hbration, 

 when the enormous plain all round is sunk in night, and the 

 far horizon encompassed by illuminated peaks, above which 

 the Sun on the one side, and the still almost full Earth on the 

 other, are slowly ascending into the sky. 



As our object is not to weary our readers by a repetition 

 of similar forms marked by little characteristic difference, but 

 rather to make a selection of the most interesting points, we 

 shall pass over a good deal of ground, full as it is of the 

 gigantic products of forces now quiescent, merely remarking 

 the singular darkness of a spot named Struve, which lies at 

 some distance N. of Gauss, but further from the limb. It is 

 not a crater, but a slightly depressed surface bordered by hills. 

 Impossible as it may be ever to ascertain the cause of this 

 remarkable darkness, the attention cannot rest upon it without 

 a strong feeling of interest and wonder. Beer and Madler 

 observes that near its NW. edge is found a small point of T of 

 brightness, but no especial elevation. 



