"At all events, Professor," continued John, "whatever may be M'Allister's actual temperature, I'm simply burning to know something about that very striking formation with the steel-grey coloured flooring which is situated not very far down from the North Pole, and a little to the east of the central meridian." "That," I said, "is a large walled plain called Plato, and, being on a receding curve of the moon, it is seen from the earth foreshortened, so that it appears to be elliptical in shape. It is about sixty miles in diameter, and encloses an area of 2700 square miles, which is just about the area of Lincolnshire. The general height of the mountain walls is over 3600 feet; one mountain on the east is nearly 7500 feet high, and others on the north and west are but little lower. "You will notice that there are several breaks in the walls, and a large one on the south-west; whilst on the inner slope of the mountains you can see where a great landslide has occurred. "It is rather singular, John, that in your first selection you have chosen a formation which is one of the lunar mysteries!" "Ah! Professor," said John, smiling, "I always was lucky! What is this dreadful mystery?" he asked, with an assumed expression of awe. "Oh, it's not a ghost story, John, nor anything to make your flesh creep," I said rather grimly. "Usually the floor of a walled plain becomes brighter as the sun rises higher and higher in the sky, but Plato actually becomes darker under a high sun. By some it has been thought that this is merely the effect of contrast with the very bright surroundings of this formation, and that there is no actual darkening of the tint. This is certainly not the case, for I have examined it carefully myself with the telescope--shutting out all the bright surroundings from the field of view, but the floor still appeared equally dark. "Others have suggested that the hot sun causes the growth of some kind of vegetation all over the plain, the ripening of which makes the floor darker in tint. As regards this suggestion, it is the fact that upon Mars the old sea-beds are the places where vegetation is most luxuriant at the present time; so, if Plato were at one time an enclosed sea, it might not be impossible that vegetation in some low form might grow and be nourished by the crude gaseous remains of a former atmosphere. A greenish tint has occasionally been noticed by some observers, also several light streaks across the floor, as well as several small craterlets, which have been duly noted on the maps. "But before we go any further we will have a better means of seeing, for it is rather uncomfortable looking directly down upon the moon. So, John, just lend a hand and we'll fetch one of those large mirrors."