these polar changes.

[19] Quite recently, however, Professor Lowell has announced that his
observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of
water vapour in the Martian atmosphere.

[20] In a somewhat similar manner the term "crater," as applied to the
ring-mountain formation on the moon, has evidently given a bias in
favour of the volcanic theory as an explanation of that peculiar
structure.

[21] Mr. Slipher's results (see note 2, page 213) were not then known.

CHAPTER XVIII

THE SUPERIOR PLANETS--_continued_

The planets, so far, have been divided into inferior and superior. Such
a division, however, refers merely to the situation of their orbits with
regard to that of our earth. There is, indeed, another manner in which
they are often classed, namely, according to size. On this principle
they are divided into two groups; one group called the _Terrestrial
Planets_, or those which have characteristics like our earth, and the
other called the _Major Planets_, because they are all of very great
size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
and Neptune. As the earth's orbit is the boundary which separates the
inferior from the superior planets, so does the asteroidal belt divide
the terrestrial from the major planets. We found the division into
inferior and superior useful for emphasising the marked difference in
aspect which those two classes present as seen from our earth; the
inferior planets showing phases like the moon when viewed in the
telescope, whereas the superior planets do not. But the division into
terrestrial and major planets is the more far-reaching classification of
the two, for it includes the whole number of planets, whereas the other
arrangement necessarily excludes the earth. The members of each of
these classes have many definite characteristics in common. The
terrestrial planets are all of them relatively small in size,
comparatively near together, and have few or no satellites. They are,
moreover, rather dense in structure. The major planets, on the other
hand, are huge bodies, circulating at great distances from each other,
and are, as a rule, provided with a number of satellites. With respect
to structure, they may be fairly described as being loosely put
together. Further, the markings on the surfaces of the terrestrial
planets are permanent, whereas those on th