Page:The Moon (Pickering).djvu/75

Rh the sake of clearness, yet they are much more hazy when seen upon the Moon, and the observation, especially in the case of Censorinus, is not an easy one except under favourable atmospheric conditions. It has recently been found that the outlines are much more distinct when viewed through blue glass. The drawings of Abulfeda e are on a scale of $1⁄2,000,000$, or about thirty-two miles to the inch; those of Censorinus are on a scale of eight miles to the inch.

For a few days about the time of full moon a curious fountain-like structure or whorl may be photographed, proceeding south from Tycho, 10C [2.4, 5.6], toward the limb, then turning westerly and northward. It is well shown in the photograph, but is inconspicuous in the telescope.

There are no extensive unbroken snow-fields on the Moon which are brilliant at all altitudes of the Sun, unless they occur near the poles, where they cannot be well seen. The most conspicuous permanently bright regions generally occur within the lunar craters. Of these, Aristarchus, 13C [2.6, 4.7], is the most striking example. Outside of the craters the permanent snow-fields are usually small. On either side of Stevinus, 4B [1.6, 5.4], a bright region is found which is brilliant most of the time; there are two such regions to the east of the Mare Nectaris, 6C [0.8, 2.7], but perhaps the most conspicuous and permanent snow area found outside of a crater lies a few miles to the west of Hell, 10C [2.1, 4.4]. Another is found about three times as far to the east of it.

There is one other formation that may be mentioned in this connection that should have special interest for amateur astronomers, since it can be readily studied with even a four-inch telescope, and yet has gone through a series of changes that no astronomer has as yet been able fully to explain. I refer to the pair of craters known as Messier, Plate 4E [1.1, 1.1], and Messier A, [1.2, 1.1]. Their history, in brief, is as follows: Schroter first suspected some change in them, and represents Messier as the larger of the two. Beer and Madler state that in size, shape and brightness they are precisely alike, and that the striking resemblance between them is most extraordinary. Webb pronounced them markedly dissimilar, A being now the larger of the two and of an entirely different shape. Neison describes A as elliptical; Elger says it is triangular, with curved sides.

Here would indeed seem to be sufficient evidence of a physical change on the Moon's surface, excepting for the fact that, as Neison justly observes, "it does not seem possible