space, or gap, between the outermost of them, Mars, and
the innermost of the jovian planets, Jupiter, is nearly two and a half
times as broad as the entire radius of the circle within which they are
included. And not only is the jovian group of planets widely separated
from the terrestrial group, but the distances between the orbits of its
four members are likewise very great and progressively increasing.
Between Jupiter and Saturn is a gap 400,000,000 miles across, and this
becomes 900,000,000 miles between Saturn and Uranus, and more than
1,000,000,000 miles between Uranus and Neptune. All of these distances
are given in round numbers.

Finally, we come to some very extraordinary worlds--if we can call them
worlds at all--the asteroids. They form a third group, characterized by
the extreme smallness of its individual members, their astonishing
number, and the unusual eccentricities and inclinations of their orbits.
They are situated in the gap between the terrestrial and the jovian
planets, and about 500 of them have been discovered, while there is
reason to think that their real number may be many thousands. The
largest of them is less than 500 miles in diameter, and many of those
recently discovered may be not more than ten or twenty miles in
diameter. What marvelous places of abode such little planets would be if
it were possible to believe them inhabited, we shall see more clearly
when we come to consider them in their turn. But without regard to the
question of habitability, the asteroids will be found extremely
interesting.

In the next chapter we proceed to take up the planets for study as
individuals, beginning with Mercury, the one nearest the sun.

CHAPTER II

MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS

Mercury, the first of the other worlds that we are going to consider,
fascinates by its grotesqueness, like a piece of Chinese ivory carving,
so small is it for its kind and so finished in its eccentric details. In
a little while we shall see how singular Mercury is in many of the
particulars of planetary existence, but first of all let us endeavor to
obtain a clear idea of the actual size and mass of this strange little
planet. Compared with the earth it is so diminutive that it looks as if
it had been cut out on the pattern of a satellite rather than that of an
independent planet. Its diameter, 3,000 miles, only exceeds the moon's
by less than one half, while both Jupiter and Saturn, amon