56 The Lunar Arctic Region. 



exhibit to the human eye, if it could look down beneath it 

 from the inner slope of their rings into such deep basins shaped 

 by nature, and could see the day break in these natural moun- 

 tain cauldrons, that in point of depth could receive our St. 

 Gothard into them." These measurements are accepted by 

 Lohrmann, but B. and M. do not even allude to this extraor- 

 dinary depth, and have only thought the inner of the two 

 craters worthy of a letter of. reference (B). 



This great ring-plain, and its subordinate members, afford 

 an instructive instance of two leading facts pointed out by 

 Schroter, and exemplified more or less in every part of the 

 moon, that the lesser are in general the more recent eruptions, 

 and that the smaller craters, when not actually deeper than 

 their larger neighbours, as is frequently the case, are almost 

 universally so in proportion to their aperture. These evident 

 workings of general laws require the especial attention of 

 every student who wishes to investigate the early history of 

 the moon. 



It is somewhat singular that Lohrmann has assigned a 

 brightness of 9° (the maximum being 10°) to our central 

 crater, Posidonius A, which B. and M. rate only as 6° to 7°. 

 Had the observers not been nearly contemporary, we might 

 have supposed some alteration in reflective power. B. and M. 

 speak of two other points as reaching 7°, and therefore by 

 inference exceeding A in brightness ; the one, the deep crater 

 B in the N.W. ring ; the other, a summit on the ring where it 

 abuts upon the adjacent smaller ring-plain Posidonius F, on its 

 S.W. side. So little is really known respecting what may be 

 called the local colouring of the lunar surface that landmarks 

 of this kind are deserving of record and re- examination from 

 time to time, and we are not without some reason, as will be 

 seen hereafter, for believing that a careful investigation of this 

 subject might meet with its reward. 



The cavity Posidonius F, just mentioned, is distinguished 

 by a minute crater in its centre ; a position much more fre- 

 quently occupied by a solitary peak, or group of small hills. 



I have observed that Lohrmann has considerably exag- 

 gerated the size of a small crater (C in his notation — unlettered 

 by B. and M.), lying at some distance E. from theN. end of 

 Posidonius, in the M. Serenitatis. Too little care seems to 

 have been in general taken in the delineation of small craters 

 in the same neighbourhood according to their proportional 

 size ; it is to be hoped that future contributors to selenography 

 will be more scrupulous as to this point, which only requires a 

 moderate degree of attention, and a little caution not to be 

 misled by any apparent enlargement of the ring near the 

 terminator. It is only by means of carefully proportioned 



