t 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 than they have
been hitherto. When observing the Hyginus cleft, Dr. Klein noticed that
at one place the declivities of the interior displayed decided
differences of tint. At many points the reflected sunlight was of a
distinctly yellow hue, while in other places it was white, as if the
cliffs were covered with snow. He compares this portion of the rill to
the Rhine valley between Bingen and Coblentz, but adds that the latter,
if viewed from the moon, would probably not present so fresh an
appearance, and would, of course, be frequently obscured by clouds.

Since the erection of the great Lick telescope on Mount Hamilton, our
knowledge of the details of some of the lunar clefts has been greatly
increased, as in the case of the Ariadaeus cleft, and many others.
Professor W.H. Pickering, also, at Arequipa, has made at that ideal
astronomical site many observations which, when published, will throw
more light upon their peculiar characteristics.

A few years ago M.E.L. Trouvelot of Meudon drew attention to a curious
appearance which he noted in connection with certain rills when near the
terminator, viz., extremely attenuated threads of l