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 in action. The walls of Aristarchus are also conspicuously terraced and rise about 4000 feet above its floor.

South of Aristarchus and east of Copernicus may be seen Kepler, a 22-mile crater noted for its extended system of glistening streaks. Its surrounding wall, like that of Tycho and Copernicus, seems to be covered with the same shining substance. The wall of Kepler is very low but its crater is about 10,000 feet below the exterior plain.

Southeast of the two bright craters, Kepler and Copernicus, and almost at the edge of the moon, lies the darkest and also one of the largest of the moon's craters, or walled-plains, as the large craters are now more frequently called. This huge crater, Grimaldi, extends north and south for a distance of 148 miles and covers an area of 14,000 square miles. It is very noticeably dark in comparison to all surrounding objects.

Gassendi, a little southwest of Grimaldi, contains a great variety of detail both on its rim and on its floor. Its plain is 54 miles in diameter and includes a number of central peaks.

Again starting from Copernicus and letting the eye travel northwest along the Apennine mountains almost to the Caucasus, one arrives at a group of three beautiful craters which rest conspicuously on the level floor of Mare Imbrium. The largest of these craters is named Archimedes after the most celebrated of ancient mathematicians. It is to be noted that lunar craters have been given the names of great men—Copernicus, Tycho, Kepler, great astronomers; Aristotle, Plato, great philosophers,—a very fine recognition of scholarly worth. Archimedes, 52 miles in diameter, has no interior mountain, but the shadows of its tall