202 The Lunar Aristillus and Autolycus. 



also enlarged at the opposite extremity; and as they pro- 

 gressively and rapidly decrease in length, the whole mass 

 approaches to a triangnlar form. The most remarkable feature, 

 however, connected with Aristillus, is the system of radiating 

 ridges, which extend in all directions, especially S., for a dis- 

 tance of ten, twenty, or even thirty miles from the foot of the 

 ring. These were discovered by Schr. in 1796, who describes • 

 them as an " innumerable multitude of little, very flat, low 

 hills, for the most part connected and forming longish ridges, 

 which have collectively received their general direction from 

 the centre of the crater." This remarkable arrangement is not 

 a difficult object, but must of course be looked for under alow 

 illumination. I have seen it extremely well when the termi- 

 nator has bisected the ring of Archimedes (33) a little way S.E. 

 It is deserving of especial examination by those possessed of 

 superior instruments, as in no part of the moon, perhaps, can 

 this peculiar branch of the evidence of explosive action be 

 studied to more advantage. Similar systems exist in other 

 quarters; we shall recall the grand one of Aristoteles ; we shall 

 hereafter meet with others on a large scale ; and I believe that 

 careful search would detect them in places where they have not 

 as yet been clearly described. But in the case before us, the 

 distinctness and comparative simplicity of the radiation, as well 

 as its favourable position with regard to our eyes, mark it out 

 as a leading instance peculiarly adapted for the prosecution of 

 the interesting inquiry, What was the nature of the force by 

 which these cavities were produced, and through what pro- 

 cesses did they receive their present form? The -time is now 

 coming when such investigations may be carried on upon more 

 reasonable grounds. M. Chacornac has already distinguished 

 himself by his original and ingenious speculations, and as 

 observations are multiplied, selenological theories will be pro- 

 posed, discussed, and some of them, no doubt, abandoned in 

 their turn. On the present occasion, I venture to submit the 

 following entry from my observing-book : the date being 1863, 

 Dec. 17, the terminator being in progress from bisection to 

 inclusion of the ring of Aristillus; instrument, my 5| in. 

 achromatic, power 170; air unsteady ; definition " imperfect 

 and uncomfortable/'' 



" The lava streams finely seen ; they are confined- to the 

 lower part of the glacis, as in Aristarchus. The wall proper is 

 a narrow ridge, showing no sign whatever of having been over- 

 flowed or broken down, but clearly of a horizontal character 

 like a terrace; beneath its foot on the S.W. are two sharp, 

 steep, narrow terraces ; W. and JT.W. there are irregular and 

 rounded mounds ; but in each case the source of the lava lies 

 beneath, as though it had squeezed its way through beneath 



