t for himself the evidence
which may properly lead a person, even without mathematical
considerations of a formal kind, to construct a theory as to the
relation of the planets to the sun. It is not likely that he can go
through all the steps of this argument at once, but it will be most
useful to him to ponder upon the problem, and gradually win his way to
a full understanding of it. With that purpose in mind, he should avoid
reading what astronomers have to say on the matter until he is
satisfied that he has done as much as he can with the matter on his
own account. He should, however, state his observations, and as far as
possible draw the results in his note-book in a diagrammatic form. He
should endeavour to see if the facts are reconcilable with any other
supposition than that the earth and the other planets move around the
sun. When he has done his task, he will have passed over one of the
most difficult roads which his predecessors had to traverse on their
way to an understanding of the heavens. Even if he fail he will have
helped himself to some large understandings.

The student will find it useful to make a map of the heavens, or
rather make several representing their condition at different times in
the year. On this plot he should put down only the stars whose places
and names he has learned, but he should plot the position of the
planets at different times. In this way, though at first his efforts
will be very awkward, he will soon come to know the general geography
of the heavens.

Although the possession or at least the use of a small astronomical
telescope is a great advantage to a student after he has made a
certain advance in his work, such an instrument is not at all
necessary, or, indeed, desirable at the outset of his studies. An
ordinary opera-glass, however, will help him in picking out the stars
in the constellations, in identifying the planets, and in getting a
better idea as to the form of the moon's surface--a matter which will
be treated in this work in connection with the structure of the earth.

CHAPTER IV.

THE EARTH.

In beginning the study of the earth it is important that the student
should at once form the habit of keeping in mind the spherical form of
the planet. Many persons, while they may blindly accept the fact that
the earth is a sphere, do not think of it as having that form. Perhaps
the simplest way of securi