"Yes," I said, "this part of the lunar surface is simply riddled with tiny craterlets, and some of them are utilised as tests for the definition of our telescopes. I have heard it remarked that a map of this part of the moon presents almost the appearance of the froth on a glass of stout when it has settled down, the very numerous tiny air-bubbles of different sizes representing the craterlets; and really it does bear such a resemblance.
"Almost due east of Copernicus is another bright and isolated ring-plain named Kepler, after the celebrated astronomer. This is some twenty-two miles in diameter and surrounded by very bright streaks of light, extending in some directions over seventy miles, the whole nimbus of light covering an area of nearly ten thousand square miles. These really are streaks, not ridges, for, as you will see, nearly all the surface surrounding this formation is flat and level.
"Some of the streaks from Kepler radiate in the direction of Aristarchus, others towards Copernicus, cutting right through the rays from those formations. From this it is gathered that Copernicus was formed first, then Aristarchus, and Kepler still later on in the moon's history.
"The surrounding wall of Kepler is comparatively low with respect to the lunar surface level, but the depth of the crater is nearly ten thousand feet below the mountain peaks. The whole formation is covered with the same light-reflecting material as the streaks which surround it."
CHAPTER VI
THE SCENERY OF THE MOON'S SOUTHERN HEMISPHERE
I now directed M'Allister to steer across the lunar equator into the southern hemisphere, and our attention was soon attracted by a very large walled plain on the eastward side of our course.