motion we may call it--of twenty-one miles per second. Some stars
may move more slowly than this to any extent; others more rapidly.
In two or three cases the speed exceeds one hundred miles per
second, but these are quite exceptional. By taking several
thousand stars having a given proper motion, we may form a general
idea of their average distance, though a great number of them will
exceed this average to a considerable extent. The conclusion drawn
in this way would be that the stars having an apparent proper
motion of 10" per century or more are mostly contained within, or
lie not far outside of a sphere whose surface is at a distance
from us of 200 light-years. Granting the volume of space which we
have shown that nature seems to allow to each star, this sphere
should contain 27,000 stars in all. There are about 10,000 stars
known to have so large a proper motion as 10". But there is no
actual discordance between these results, because not only are
there, in all probability, great numbers of stars of which the
proper motion is not yet recognized, but there are within the
sphere a great number of stars whose motion is less than the
average. On the other hand, it is probable that a considerable
number of the 10,000 stars lie at a distance at least one-half
greater than that of the radius of the sphere.

On the whole, it seems likely that, out to a distance of 300 or
even 400 light-years, there is no marked inequality in star
distribution. If we should explore the heavens to this distance,
we should neither find the beginning of the Milky Way in one
direction nor a very marked thinning out in the other. This
conclusion is quite accordant with the probabilities of the case.
If all the stars which form the groundwork of the Milky Way should
be blotted out, we should probably find 100,000,000, perhaps even