ece of work, and shake hands with him; he must be a rare clever
fellow!"

"Yes," said John, "and I would like to see the drill he used; for such a
long and extremely slender tool, to be effective, must be as clever a
piece of work as the steel tube."

"I may tell you," I proceeded, "that success was at last attained; and
as a result of the comparison of our moon's light with that of Deimos,
it was shown that if the general surface brightness of the latter were
equal to that of our moon, then Deimos must be only 18 miles in
diameter, or about a 15,000th part of the area of our moon's disc.

"To state the matter in another way--supposing our moon were only 18
miles in diameter, and was removed to the same distance as Deimos is
from us, then it would appear only the very faint point of light that
Deimos appears when viewed through the telescope.

"By the same means Phobos, the satellite nearest to Mars, was estimated
to be about 22-1/2 miles in diameter. These dimensions, however, depend
on the brightness of these satellites being exactly the same as the
general brightness of our moon; and later experiments have fixed the
sizes as 36 miles for Phobos, and 10 miles as the diameter of Deimos.

"I will not detain you much longer on this subject, as we shall be able
to discuss it further when we arrive upon Mars; but I may now mention
that, in one respect, the little satellite named Phobos is unique. It is
the only satellite we know of which revolves round its primary planet in
less time than it takes the planet itself to make one revolution on its
axis.[6]

"Mars revolves on its axis in 24 hours, 37 minutes, and 22 seconds, thus
the 'day' on Mars is nearly 38 minutes longer than our 'day.' Phobos
revolves round the planet in the very short period of 7 hours, 39
minutes, and 14 seconds, and therefore makes more than three complete
revolutions round the planet in the course of a single Martian day. The
peculiar phenomena to which this very rapid motion gives rise, and the
numerous eclipses which occur, will be matters of great interest to us
all when we reach Mars. Our moon, as you know, takes a month to make one
revolution round the earth."

"Professor," said John, "when we get to Mars, it will be rather a
curious experience for us to see two moons shining in the sky at the
same time!"

"My word!" exclaimed M'Allister, "two moons shining at once! If I go out
and see such a sight as that, I shall think the whisky has b