e
natural features of the moon down yonder."

"Yes do, please, Professor," said John; "M'Allister's own temperature is
evidently rising rapidly. Strange, isn't it, that a douche of cold facts
should make our friend so warm!"

"Well, not altogether," I replied laughingly; "there should always be a
healthy reaction after a cold douche. Much depends on the intensity of
the cold applied, and you know that if you touch extremely cold metal it
burns you like hot iron!"

"Professor," chimed in M'Allister, "maybe I _was_ a bit warm, but really
your facts were not so cold as to make me hot."

"I'm glad to hear you say so," I answered.

"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