and you will I
think understand that in these circumstances the people on that part of
the planet where the moon is visible must necessarily keep it in view
for a longer period than would be the case if the planet were not
revolving in the same direction.

"You have been used to being on board a ship; so suppose your vessel was
steaming twelve miles an hour and there was another vessel at anchor
just twelve miles ahead of you, you would reach it in just one hour,
would you not?"

"Yes, certainly I should," replied M'Allister.

"Now," I continued, "suppose that the other vessel, instead of being at
rest, was moving away from you at the rate of six miles an hour; after
you had steamed one hour it would still be six miles ahead of you, and
it would take you exactly another hour to catch it up. So you would be
just double the time reaching it when moving as compared with the time
required to do so when it was at anchor. This is very similar to the
cases of the satellites of Mars, and much the same thing happens in
regard to Mars and the earth. If they are opposite to each other at a
certain point, Mars will have taken much more than one revolution round
its orbit before they will be opposite to each other again, because they
are both moving in the same direction. Do you see it now?" I asked.

"Yes, Professor," he replied. "I know now, because you have cleared it
all up. It's simple enough when one understands it."

Merna then asked me if I would like to see some of their astronomical
instruments, and, on my replying that I should very much like to do so,
he took us to an observatory where Corontus was at work.

I was at once struck by the small size of the telescopes; and, on
inquiring about them, Corontus told me that very large instruments had
long become obsolete, for these small ones could be used for all the
purposes for which a large one had been required, and gave better
results.

I examined one of them and found, to my surprise, that it embodied the
very ideas that I had long been trying to carry into effect. With this
view I had made many experiments, as it seemed to me that it ought to be
possible to construct an instrument of moderate and convenient
dimensions which would show as much as our monsters will show, and yet
be capable of being used with low powers when occasion required. I had
endeavoured to attain this result by the aid of electricity, but failed
to do so. Evidently I had missed something, bu