If, as seems most probable, these gigantic cracks are due to contractions of the moon’s surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now “a changeless world,” that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also.  Should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for.  Many instances might be adduced tending to confirm this supposition, to one of which, as coming under my notice, I will briefly refer.  On the evening of November 11, 1883, when examining the interior of the great ring-plain Mersenius with a power of 350 on an 8 1/2 inch reflector; in addition to the two closely parallel clefts discovered by Schmidt, running from the inner foot of the north-eastern rampart towards the centre, I remarked another distinct cleft crossing the northern part of the floor from side to side.  Shortly afterwards, M. Gaudibert, one of our most experienced selenographers, who has discovered many hitherto unrecorded clefts, having seen my drawing, searched for this object, and, though the night was far from favourable, had distinct though brief glimpses of it with the moderate magnifying power of 100.  Mersenius is a formation about 40 miles in diameter, with a prominently convex interior, containing much detail which has received more than ordinary attention from observers.  It has, moreover, been specially mapped by Schmidt and others, yet no trace of this rill was noted, though objects much more minute and difficult have not been overlooked.  Does not an instance of this kind raise a well-grounded suspicion of recent change which it is difficult to explain away?

To see the lunar clefts to the best advantage, they must be looked for when not very far removed from the terminator, as when so situated the black shadow of one side, contrasted with the usually brightly-illuminated opposite flank, renders them more conspicuous than when they are viewed under a higher sun.  Though, as a rule, invisible at full moon, some of the coarser clefts—­as, for example, a portion of the Hyginus furrow, and that north of Birt—­may be traced as delicate white lines under a nearly vertical light.

For properly observing these objects, a power of not less than 300 on telescopes of large aperture is needed; and in studying their minute and delicate details, we are perhaps more dependent on atmospheric conditions than in following up any other branch of observational astronomy.  Few indeed are the nights, in our climate at any rate, when the rough, irregular character of the steep interior of even the coarser examples of these immense chasms can be steadily seen.  We can only hope to obtain a more perfect insight into their actual structural peculiarities when they are scrutinised under more perfect climatic circumstances