Page:The Moon (Pickering).djvu/109

Rh shelving. This difference of slope, which is very marked, is very unusual upon the Moon. If the crevices which gave off the vapour were located at the foot of, and therefore nearer to, the western walls, we should expect them to be more heavily coated with frost.

The studies described in this and in the previous chapter will serve to illustrate what may be called "the new selenography"—the selenography which consists, not in a mere mapping of cold dead rocks and isolated craters, but in a study of the daily alterations that take place in small, selected regions, where we find real, living changes—changes that cannot be explained by shifting shadows or varying librations of the lunar surface.