Page:Popular Science Monthly Volume 10.djvu/589

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the crater named Messier by Beer and Mädler. In its equatorial region, on the westernmost of the great lunar plains, close to one another, are two small crater-plains about nine miles in diameter, surrounded by very low ridges and mounds and crater-like depressions. These two formations, named Messier and Messier A, were discovered and described by Schröter, who regarded Messier as slightly the larger. Beer and Mädler examined these formations most carefully on more than three hundred distinct occasions between 1829 and 1837. They declared that the two crater-plains were exactly alike in every particular. Both were circular and of the same size, with bright, grayish white walls and a yellowish-gray interior. The walls were of the same height, with wall-peaks situated in the same relative position. In diameter, form, height of walls above the surrounding surface, and depth and color of the interior of the walls, Beer and Mädler declared that they were completely alike. Some years after this, a slight dissimilarity between the forms of the two craters was noticed, and in November, 1855, the Rev. T. W. Webb, one of the best living lunar observers, discovered that the eastern crater-plain, Messier A, appeared the larger of the two.

In March of the following year he observed that not only was Messier the smaller, but that it was elliptical. He confirmed these observations on repeated occasions, and in 1857 made drawings showing Messier A unchanged, while Messier had an elliptical form, with a long diameter of about 10 or 11 miles and a short diameter of about 712 to 8 miles. The matter attracted little further attention until 1870 to 1875. During these years Messier and Messier A were studied with the aid of powerful telescopes, and during the past year the long diameter of Messier appears to be 12.2 miles and the short diameter 6.9. The difference between the form and dimensions of these two formations is now obvious in the smallest astronomical telescope. It is inconceivable that Beer and Mädler could have failed to recognize these differences with their fine Fraunhofer equatorial, with which, on hundreds of different occasions, they carefully scrutinized them in search of differences.

This slow squeezing out of shape of an immense crater-plain is a change that seems to defy explanation. Nothing analogous now exists on the surface of the earth, and it is not surprising that there should be a strong reluctance to admit that such a change has occurred. A careful examination of Messier and its neighborhood, however, suggests that, instead of a bodily compression of the entire crater, where has been a gradual sliding of the north and south walls into the interior, and a pushing of the entire western wall outward and westward down an incline existing there. Stenographers could point to a hundred cases where a like circumstance has occurred. As far as is at present known, this explanation accords with the condition of the formations around Messier, but further observations with