tion was made to this number by the
discoveries of Gruithuisen, and, a short time after, by those of
Lohrmann, who in twelve months (1823-24) detected seventy. Kinau, Madler,
and finally Schmidt, followed, till, in 1866, when the latter published
his work, _Ueber Rillen auf dem Monde_, the list was thus summarised:--

In the 1st or N.W. quadrant 127 rills
In the 2nd or N.E. quadrant 75 rills
In the 3rd or S.E. quadrant 141 rills
In the 4th or S.W. quadrant 82 rills

or 425 in all. Since the date of this book the number of known rills has
been more than doubled; in fact, scarcely a lunation passes without new
discoveries being made.

The significance of the word _rille_ in German, a groove or furrow,
describes with considerable accuracy the usual appearance of the objects
to which it is applied, consisting as they do of long narrow channels,
with sides more or less steep, and sometimes vertical. They often extend
for hundreds of miles in approximately straight lines over portions of
the moon's surface, frequently traversing in their course ridges,
craters, and even more formidable obstacles, without any apparent check
or interruption, though their ends are sometimes marked by a mound or
crater. Their length ranges from ten or twelve to three hundred miles or
more (as in the great Sirsalis rill), their breadth, which is very
variable within certain limits, from less than half a mile to more than
two, and their depth (which must necessarily remain to a great extent
problematical) from 100 to 400 yards. They exhibit in the telescope a
gradation from somewhat coarse grooves, easily visible at suitable times
in very moderately sized instruments, to striae so delicate as to require
the largest and most perfect optical means and the best atmospheric
conditions to be glimpsed at all. Viewed under moderate amplification,
the majority of rills resemble deep canal-like channels with roughly
parallel sides, displaying occasionally local irregularities, and fining
off to invisibility at one or both ends. But, if critically scrutinised
in the best observing weather with high powers, the apparent evenness of
their edges entirely disappears, and we find that the latter exhibit
indentations, projections, and little flexures, like the banks of an
ordinary stream or rivulet, or, to use a very homely simile, the serrated
edges and little jagged irregularities of a biscuit broken across. In
some cases we remark craterifo