"Not just now exactly," observed Barbican, "for the solar rays, striking our Projectile directly, would give us a very elevated instead of a very low temperature. 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!"