148 The Lunar Cdssini. 



and M., three small craters ; one (A) very deep, precipitous, 

 and conspicuous ; a smaller one adjoining it, S.W., and a third 

 close to the S.E. main wall. They speak of having measured 

 A ten times, and their book contains an additional diagram of 

 Cassini on a much larger scale, not referred to in the text, but 

 taken apparently with the great Berlin 9* 6 -inch achromatic, and 

 dated 1836, Sept. 1. And in neither of their delineations, nor in 

 the text, is there the least indication of anything in the interior 

 of A. This might appear conclusive. Yet ten years before 

 that elaborate drawing, and eleven before their description, I 

 had seen something there with a 5 -ft. achromatic which I drew 

 and described as a " bright object with shadow, whether cen- 

 tral hill, or projection on the declivity, or small encroaching 

 crater, I could not make out." I have since frequently noticed 

 it, and any one may see it ieven with a small telescope under 

 suitable illumination. I have doubted whether it might be an 

 eccentric hill, or the edge of an interior and deeper crater ; but 

 rather inclined to the latter. A sketch obligingly sent me by 

 E. Brodie, Esq., represents it from an 8J-in. object-glass, as a 

 defective wall between two confluent craters. I have not as 

 yet brought to bear upon it the sharp definition of my 9J-in. 

 silvered mirror. Should it be a deeper interior cavity, it might 

 add a third to a rare class, of which two instances are pointed 

 out by Schmidt, where a subsequent eruption has produced a 

 ring contiguous to, but distinct from, that of the more ancient 

 formation. 



But whatever may be its nature, we may well ask how it 

 could have escaped the notice of B. and M., especially at the 

 time of a special delineation so minute in detail as to show even 

 a deviation from circularity in the form of A, as well as varia- 

 tions in the height of its ring ? So marked an omission in the 

 midst of so much display makes us feel how unsatisfactory it 

 is to argue the question of lunar changes on no other data than 

 those furnished by the most seemingly careful drawings of 

 previous authorities. It is to the labours of such men as Birt 

 and Schmidt that we must look for the solution of this 

 difficulty. 



But we have not yet ended the story of Cassini. B. and 

 M. have spoken of a second considerably smaller crater enclosed 

 by an imperfect ring, as lying close to the S.W. side of A: — 

 S in the detailed drawing, where it is much smaller in propor- 

 tion. I have seen what corresponded exactly with this ; but 

 further examination has led me to believe that it is not a crater 

 but a mere gorge or hollow on the E. side of the spot where a 

 long buttress, as it were, lies against the S.W. wall of A. 

 This ridge was well seen and drawn by Schr. in 1798, when he 

 also saw two mountains casting shadows outside the N. main 



