The Lunar Aristillus and Autolycus. 201 



existence of an atmosphere whose denser portion does not 

 extend much above the lower regions. 



THE LUNAB AKISTILLUS AND AUTOLYCUS. 



At a short distance S.W. from our last object, Gassini, and 

 near the foot of the Caucasus, with which, however, it has no 

 apparent connection, we shall find a smaller, but very obvious 

 crater, Thecet-etus, so deep, that it is wholly free from shadow 

 for about five days only during the whole lunation. The W. 

 wall rises 7600f. above the interior, and higher still in one 

 bright peak: the E. side is 3500f. above the plain; Schr. 

 had given 33001*. — a close agreement. The latter made the 

 depth 10,000f. From the data of B. and M., Schmidt found 

 the ratio of the outer and inner heights of the wall as 1 to 2 "6, 

 and the depth T '-r of the diameter, this latter being a not 

 unusual proportion in craters of between 12,000f. and I9,000f. 

 in breadth. The character, therefore, of extreme relative 

 depth — the goblet-like impression — which might be easily 

 received in such cases from the appearance of the shadow 

 near the terminator, is thus shown to be somewhat decep- 

 tive when checked by actual measurement. But though 

 our lunar cups may thus be said to be turned into 

 saucers, enough remains in their proportions to fill us with 

 astonishment. 



A double ridge rising only some 100f., and throwing off 

 four parallel branches to the S.W., leads from Thewtetus towards 

 Aristillus (21), a crater from its size, depth, and position, be- 

 longing to the most striking class. The mass of wall, enormous 

 in itself, but still more impressive as the result of eruptive 

 action, though less lofty N. and S., rises W. nearly to 8900f., 

 and with a steeper peak to fully ll,000f., the height of the 

 great Pyrenean Maladetta (the latter measured, however, from 

 the sea, and producing, therefore, a less imposing effect). 

 Schr. gives the E. side 6750f. above the surrounding Pains, 

 the depth 94l)0f. A formation here lies beneath our eye, to 

 which nothing terrestrial makes more than a very distant 

 approach. What would be our feelings if transported to the 

 midst of this huge cavity, and gazing upon its colossal boun- 

 dary at a distance of seventeen miles on every side ! The 

 central hill on which Ave should have to take our stand is a 

 commanding one, and of a complex character, which has not 

 been well exhibited in the published designs. It really con- 

 sists, as I found, 1861, Dec. 10, with 5£ inches of aperture, and 

 power 170, of three parallel ridges separated by narrow ravines, 

 rising towards S.W., and terminated, especially the outer ones, 

 by bluffs at that end. The S.E. ridge, which is the longest, is 



