Page:The Moon (Pickering).djvu/62

40 between 1869 and 1872, and thirty-six craterlets in all were mapped. They were next studied by A. S. Williams and three other English astronomers acting in cooperation, who published a second map, showing thirty-eight craterlets. A few years later another map was published by the same astronomers. Finally, the craterlets were studied by the author in 1892, when he succeeded in mapping forty-two of them. The accompanying maps, however, show only the location of the ten most prominent craterlets in each of these three periods, the order of their prominence being in each case indicated by numerals.

It will be noted that the central craterlet is the most conspicuous throughout, but only three others are common to the three maps. The craterlet which was third in brilliancy on the first map was replaced in 1892 by a hazy patch of light. No. 5, as it is called on the first map, was so faint in 1892 that it could only just be seen. Nos. 6 and 9 could not be found at all, although a much larger telescope was used and the work was carried on under much more favourable atmospheric conditions. No. lo, on the other hand, was so conspicuous in 1892 that it was assigned the third place. The craterlet which was given the sixth place in 1892 may perhaps have been newly formed, as it had not been detected by any previous observer, while the seventh craterlet in 1892, which had been No. 13 in 1870, was entirely invisible in 1881, although seen as a very faint object a few years later by the same observers.

The next object to claim our attention on the Moon is the deep, winding cleft which has been named after its discoverer, Schröter's Valley, Plate 15E [0.7, 4.1]. At my first view of it in Arequipa I was struck by the strong resemblance of the crater which forms its source to the crater of a terrestrial volcano in a state of active eruption. Dense clouds of white vapour were apparently arising from its bottom and pouring over its southeastern wall in the direction of Herodotus. So striking, indeed.