ding at this rate for nearly two hours,
M'Allister came hurrying into our compartment in a state of great
excitement.

"Professor," he exclaimed with a gasp, "something's gone wrong
altogether, and I don't know what to do!"

"Gone wrong!" I repeated. "Why, what is the matter?"

"Mon," he answered, "everything is the matter! A while back we were
rushing towards the moon, but just now when I looked ahead there wasn't
any moon to be seen. I happened to go round to the other window and look
back and, my word! if there wasn't the moon right behind us! We have
been travelling so very fast that we must have run past it without
knowing we had done so."

"Oh, we could not possibly have done that!" I exclaimed.

"But there's more to come, Professor," continued M'Allister. "When I
last saw the moon it was nearly full and not so very much bigger than
when we saw it at starting, but now this moon behind us is an enormous
thing; yet it is only a new moon, or rather what folks call a new moon
with the old moon in its arms!"

"Oh, now I understand," I replied. "It's all right, M'Allister, and you
can make your mind quite easy. You were not able to see the moon when
you first looked through the window because it was nearly in a direct
line with your course, and therefore just hidden by the prow of the
vessel. It's still ahead of us and still nearly full: if you had looked
out of the conning tower or used the periscope you would have seen it."

"Heh, Professor," he interjected, "I know I couldn't see the moon if it
was straight ahead of our course, but then what about that enormous new
moon that's behind us? I saw that right enough."

"That enormous new moon, M'Allister, is only our own little world which
we left a few hours ago," I replied.

He stared at me as though bewildered, and after pondering a while,
exclaimed, "Losh, mon, you surely don't mean to say that our own little
world changes about in the same way as the moon does--sometimes new and
sometimes full?"

Here John interposed. "Yes, M'Allister, you can take it from me that it
is just what our world does do. I think you are aware that, like the
moon, our world simply reflects the light it receives from the sun, and
does not shine by its own light. So one side is light and the other side
is dark, according to its position in regard to the sun. From our
present position we are only able to see a small portion of the lighted
side, the remainder being dark except for t