way of our sun,
as this earth is obliged to do. The stars will, therefore, act as the
external objects by which we can test whether our system is voyaging
through space.

With the stars as our beacons, what ought we to expect if our system be
really in motion? Remember that when the ship was approaching the
harbour the lights gradually opened out to the right and left. But the
astronomer has also lights by which he can observe the navigation of
that vast craft, our solar system, and these lights will indicate the
path along which he is borne. If our solar system be in motion, we
should expect to find that the stars were gradually spreading away from
that point in the heavens towards which our motion tends. This is
precisely what we do find. The stars in the constellations are gradually
spreading away from a central point near the constellation of Lyra, and
hence we infer that it is towards Lyra that the motion of the solar
system is directed.

There is one great difficulty in the discussion of this question. Have
we not had occasion to observe that the stars themselves are in actual
motion? It seems certain that every star, including the sun himself as a
star, has each an individual motion of its own. The motions of the stars
as we see them are partly apparent as well as partly real; they partly
arise from the actual motion of each star and partly from the motion of
the sun, in which we partake, and which produces an apparent motion of
the star. How are these to be discriminated? Our telescopes and our
observations can never effect this decomposition directly. To accomplish
the analysis, Herschel resorted to certain geometrical methods. His
materials at that time were but scanty, but in his hands they proved
adequate, and he boldly announced his discovery of the movement of the
solar system.

So astounding an announcement demanded the severest test which the most
refined astronomical resources could suggest. There is a certain
powerful and subtle method which astronomers use in the effort to
interpret nature. Bishop Butler has said that probability is the guide
of life. The proper motion of a star has to be decomposed into two
parts, one real and the other apparent. When several stars are taken, we
may conceive an infinite number of ways into which the movements of each
star can be so decomposed. Each one of these conceivable divisions will
have a certain element of probability in its favour. It is the business
of t