erature. But once arrived at the Moon, during those
nights fifteen days long, which each of her faces experiences
alternately, we shall have plenty of time to make an experiment with
every condition in our favor. To be sure, our Satellite is at present
moving in a vacuum."

"A vacuum?" asked Ardan; "a perfect vacuum?"

"Well, a perfect vacuum as far as air is concerned."

"But is the air replaced by nothing?"

"Oh yes," replied Barbican. "By ether."

"Ah, ether! and what, pray, is ether?"

"Ether, friend Michael, is an elastic gas consisting of imponderable
atoms, which, as we are told by works on molecular physics, are, in
proportion to their size, as far apart as the celestial bodies are from
each other in space. This distance is less than the 1/3000000 x 1/1000',
or the one trillionth of a foot. The vibrations of the molecules of this
ether produce the sensations of light and heat, by making 430 trillions
of undulations per second, each undulation being hardly more than the
one ten-millionth of an inch in width."

"Trillions per second! ten-millionths of an inch in width!" cried Ardan.
"These oscillations have been very neatly counted and ticketed, and
checked off! Ah, friend Barbican," continued the Frenchman, shaking his
head, "these numbers are just tremendous guesses, frightening the ear
but revealing nothing to the intelligence."

"To get ideas, however, we must calculate--"

"No, no!" interrupted Ardan: "not calculate, but compare. A trillion
tells you nothing--Comparison, everything. For instance, you say, the
volume of _Uranus_ is 76 times greater than the Earth's; _Saturn's_ 900
times greater; _Jupiter's_ 1300 times greater; the Sun's 1300 thousand
times greater--You may tell me all that till I'm tired hearing it, and I
shall still be almost as ignorant as ever. For my part I prefer to be
told one of those simple comparisons that I find in the old almanacs:
The Sun is a globe two feet in diameter; _Jupiter_, a good sized orange;
_Saturn_, a smaller orange; _Neptune_, a plum; _Uranus_, a good sized
cherry; the Earth, a pea; _Venus_, also a pea but somewhat smaller;
_Mars_, a large pin's head; _Mercury_, a mustard seed; _Juno_,
_Ceres_, _Vesta_, _Pallas_, and the other asteroids so many grains
of sand. Be told something like that, and you have got at least the tail
of an idea!"

This learned burst of Ardan's had the natural effect of making his
hearers forget what they had been arguing about, and t