, and at other times sought for it
in vain; though on each occasion the conditions of illumination,
libration, and definition were practically similar. I have sometimes
found this cleft an easy object with a 4 inch achromatic. Again, many
rills described by Madler as very delicate and difficult to trace, may
now be easily followed in "common telescopes." In short, the more direct
telescopic observations accumulate, and the more the study of minute
detail is extended, the stronger becomes the conviction, that in spite of
the absence of an appreciable atmosphere, there may be something
resembling low-lying exhalations from some parts of the surface which
from time to time are sufficiently dense to obscure, or even obliterate,
the region beneath them.

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 difficul