Page:The Moon (Pickering).djvu/61

 CHAPTER VI

is no doubt that in former times volcanic forces played a very important part in the history of the Moon. At the present time these forces have greatly diminished in intensity. All astronomers are agreed on these two points. The question is whether all volcanic action has ceased. The best-known example of possible volcanic activity within historic times is the little crater known as Linné, 7B [1.7, 4.2], after the great botanist Linnæus. Here our earliest evidence depends on a map constructed by Riccioli in 165 1, where Linné is represented as a deep crater of moderate size. (See Plate I, facing page 84.) It is next noted by Schröter in 1788, who described it as "a very small, roimd, brilliant spot, containing a somewhat tmcertain depression." It is certain that if the crater of Linné had been no larger then than it is now it could not have been detected by either of these astronomers with the imperfect telescopes of their times.

With more modern instruments, however, the testimony becomes much more precise. Thus, early in the last century Lohrmann described Linné as being very deep and as more than four miles in diameter. Maedler observed it seven times, and described it as very distinct under the oblique illumination of the Sun, when the contrast of shadow was strongest, and as measuring six miles in diameter. Schmidt drew it eight times, and represented it as being seven miles in diameter and one thousand feet deep. Schmidt, in 1843, was the last astronomer, apparently, to see it with any such dimensions, and in 1866 he announced that it had disappeared. A few months later, however, he found in its place a small craterlet about one-quarter of a mile in diameter, which, in the course of a couple of years, gradually increased to a mile and a half. Although still visible, its diameter has now sunk to three-quarters of a mile.

Another equally interesting but perhaps less well-known instance of lunar volcanic activity is the large crater known as Plato, 9 A [2.3, 2.2]. The floor of this crater is a smooth, nearly level plain, some sixty miles in diameter, but studded over with numerous small volcanic cones. These range from about a mile in diameter down to a few hundred feet only. They were first carefully studied by a committee of the British Association 39