rm hollows or sudden expansions in their
course, and deep sinuous ravines, which render them still more
unsymmetrical and variable in breadth. With regard to their distribution
on the lunar surface; they are found in almost every region, but perhaps
not so frequently on the surface of the Maria as elsewhere, though, as in
the case of the Triesnecker and other systems, they often abound in the
neighbourhood of disturbed regions in these plains, and in many cases
along their margins, as, for example, the Gassendi-Mersenius and the
Sabine-Ritter groups. The interior of walled plains are frequently
intersected by them, as in Gassendi, where nearly forty, more or less
delicate examples, have been seen; in Hevel, where there is a very
interesting system of crossed clefts, and within Posidonius. If we study
any good modern lunar map, it is evident how constantly they appear near
the borders of mountain ranges, walled-plains, and ring-plains; as, for
instance, at the foot of the Apennines; near Archimedes, Aristarchus,
Ramsden, and in many other similar positions. Rugged highlands also are
often traversed by them, as in the case of those lying west of Le Monnier
and Chacornac, and in the region west of the Mare Humorum. It may be here
remarked, however, as a notable fact, that the neighbourhood of the
grandest ring-mountain on the moon, Copernicus, is, strange to say,
devoid of any features which can be classed as true clefts, though it
abounds in crater-rows. The intricate network of rills on the west of
Triesnecker, when observed with a low power, reminds one of the wrinkles
on the rind of an orange or on the skin of a withered apple. Gruithuisen,
describing the rill-traversed region between Agrippa and Hyginus, says
that "it has quite the look of a Dutch canal map." In the subjoined
catalogue many detailed examples will be given relating to the course of
these mysterious furrows; how they occasionally traverse mountain arms,
cut through, either completely or partially (as in Ramsden), the borders
of ring-plains and other enclosures, while not unfrequently a small mound
or similar feature appears to have caused them to swerve suddenly from
their path, as in the case of the Ariadaeus cleft, and in that of one
member of the Mercator-Campanus system.

Of the actual nature of the lunar rills we are, it must be confessed,
supremely ignorant. With some of the early observers it was a very
favourite notion that they are artificial wo