much are absent next door. And so the list of our globe's peculiar attributes might be run through to the finding of diversity to our familiar ways at every turn. But, as we shall see later, these differences from one planet to the next are not only not incompatible with a certain oneness of the whole, but actually help to make the family relationship discoverable. Analogy alone is a dangerous guide, but analogy crossed with diversity is of all clews the most pregnant of understanding. The very fact that we can tell them apart when we see them together, as the Irishman remarked of two brothers he was in the habit of confusing, points to their brotherly relation.
Proceeding still further, we come to Mars at a mean distance of one hundred and forty-one million miles. Smaller than ourselves, his diameter is but a little over half the Earth's, or forty-two hundred miles, his mass one-ninth of ours, and his density about seven-tenths as much. Here, again, but in a different way, we find a planet unlike ourselves, and we know more about him than of any body outside the Earth and Moon. So much about him has been set forth elsewhere that it is enough to mention here that no oceans diversify his surface, no mountains relieve it, and but a thin air wraps it about,—an air containing water-vapor, but so clear that the surface itself is almost never veiled from view.
