Lunar Details. 29 



trifling breadth, bat, according to Schroter, more than ninety 

 nriles in length, proving that the latter plain, in this region at 

 least, must he at a lower level : its remaining boundaries are 

 more distinctly made out by its hue than by any strong, natu- 

 ral configuration of mountain or cliff. Its colour is a clear 

 grey, without any trace of green or other distinguishable hue. 

 The surface is diversified and " marbled" by a multitude of 

 very minute streaks of light, perceptible only under the most 

 favourable circumstances. It is intersected by a number of 

 long, low banks, mauy of which unite themselves in a broad 

 central mass of slight elevation. To this plain belongs the 

 conspicuous crater Plinius (13), a cavity thirty- two* miles in 

 diameter, with an interior full of small inequalities, a ring 

 built up in terraces, and a luminous central hill ; the whole 

 forming a very confused, though brilliant mass in the Full 

 Moon. The wall, which is much blocked up by its exterior 

 adjuncts, is, according to Schroter, more than nine miles broad, 

 nearly 1400 feet above the neighbouring region, and 7300 feet 

 above the interior (1000 feet more than is given by B. and M.). 

 On two occasions, with two different instruments, and with an 

 interval of more than twenty-five years, I have seen the two 

 summits of the central hill figured by B. and M. as minute 

 craters. Such illusions may easily take place when the sha- 

 dows of small eminences fall among other elevations of a similar 

 character, and show the necessity of caution in forming conclu- 

 sions under these circumstances. Plinius A, a crater of four- 

 teen miles in diameter, just to the W. of Plinius, is remarkable 

 as the centre of a large white area, considerably brighter than 

 the surrounding level. Its W. side has 8° of brightness, the 

 E. wall is 2200 feet above the outer plain, and the depth, 

 according to Schroter, is at least 5700 feet. 



S. of Plinius, in the region between it and the equator, lie 

 three pairs of craters near the E. edge of the plain : — Boss, 

 4100 feet deep according to Schroter, and Boss A — Arago and 

 the much smaller Arago A — and Bitter and Sabine : the four 

 first remarkable, as well as other craters in the district, for 

 the corresponding position in each case of a ' c wall-peak" or 

 tower on the S. edge of the ring; the two last as forming a 

 double ring of which the components are nearly similar. The 

 W. wall of Bitter (the more easterly and higher of the two) is 

 4000 feet above the interior. Nearly W. of this last pair, 



* Schroter gives twenty-five miles ; L. thirty-five. The discrepancy may in 

 this case probably be ascribed to the circumstance that the ring is of an oval form. 

 But, generally speaking, those who are accustomed to lunar observations will 

 feel no great surprise at variations of this kind, which may naturally occur when 

 the wall is of a terraced construction, without any very dominant crest, and the- 

 shadow may consequently be cast from different ridges under different angles of 

 illumination. 



