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 crateriform 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,