The Lunar Aristillus and Autolycus. 203 



(in which case the wall would scarcely have been so con- 

 tinuous and regular), or more probably had flowed previously, 

 the upper regions being elevated in a cooler and less fluid 

 condition/'' 



Upon so imperfect a ground no theory ought to be, or is 

 attempted to be, put forward. The extract is given merely as 

 a specimen of the ideas which might naturally occur to any 

 observer, as they did to myself, on examining this instructive 

 formation as the sun was rising upon it. They may be very 

 unfounded : possibly a further exploration might lead me to 

 reject them for fresh notions, in the opinion of other observers 

 equally baseless. But it is by a tentative process of this kind 

 that we are most likely to approach the outskirts of that truth 

 which the great Creator has perhaps willed to be in its fulness 

 an impenetrable mystery. • This we may plainly see, that such 

 a radial arrangement must have been determined, like the 

 circular form of the crater, by a central force : but in what 

 manner, it is less easy to conjecture. The divergent matter 

 might have been extruded in a semi-fluid condition analogous 

 to lava ; a condition again which might have originated either 

 in igneous or aqueous action. It might have been poured out 

 before the final formation of the ring', or have pierced the 

 flanks of the mountain, as frequently happens in terrestrial 

 eruptions, or have flowed over the ridge subsequently without 

 leaving any trace perceptible to our vision, from the already 

 hardened condition of the wall, and its standing up so steeply 

 as to cast off the viscid matter to the more gradual slopes 

 below. These radiations may be streams of enormous blocks 

 in a non-coherent state, like beds of cinders on the flanks of a 

 terrestrial volcano ; and such torrents might have had their 

 source either in the downpour of ejected fountains of stones, 

 or in the overflow of a great mass of similar matter collected 

 within the crater, and making its escape chiefly through gaps 

 in its highest ledge. Such speculations may be, some of them, 

 very improbable, but perhaps are none of them to be rejected 

 for their dynamical impossibility. Each of them, therefore, 

 might be compared with the observed appearances ; and from 

 rough hints of this kind, well applied here, and extended 

 to other formations in distant quarters, some inference 

 might, perhaps, result which might be worthy of serious 

 consideration. 



Aristillus is also the centre of a system of those mysterious 

 light-streaks so familiar to every observer, and so difficult of 

 explanation. Many of these diverge on the W., N.W., and 

 N. sides; on the E. they extend to some distance through 

 the M. Imbrium. It is well worthy of notice, that, 

 except in a few instances, they do not coincide with the 



