of ``Jerusalem the Golden.''

If there were a discoverable center of predominant gravitative power,
to which the motions of all the stars could be referred, those motions
would appear less mysterious, and we should then be able to conclude
that the universe was, as a whole, a prototype of the subsidiary
systems of which it is composed. We should look simply to the law of
gravitation for an explanation, and, naturally, the center would be
placed within the opening enclosed by the Milky Way. If it were there
the Milky Way itself should exhibit signs of revolution about it, like
a wheel turning upon its hub. No theory of the star motions as a whole
could stand which failed to take account of the Milky Way as the basis
of all. But the very form of that divided wreath of stars forbids the
assumption of its revolution about a center. Even if it could be
conceived as a wheel having no material center it would not have the
form which it actually presents. As was shown in Chapter 2, there is
abundant evidence of motion in the Milky Way; but it is not motion of
the system as a whole, but motion affecting its separate parts.
Instead of all moving one way, the galactic stars, as far as their
movements can be inferred, are governed by local influences and
conditions. They appear to travel crosswise and in contrary
directions, and perhaps they eddy around foci where great numbers have
assembled; but of a universal revolution involving the entire mass we
have no evidence.

Most of our knowledge of star motions, called ``proper motions,''
relates to individual stars and to a few groups which happen to be so
near that the effects of their movements are measurable. In some cases
the motion is so rapid (not in appearance, but in reality) that the
chief difficulty is to imagine how it can have been imparted, and what