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