concentric spheres each of whose surfaces is at the distance of
six light-years outside the sphere next within it. The inner is at
the distance of six light-years around the sun. The surface of the
second sphere will be twelve light-years away, that of the third
eighteen, etc. The volumes of space within each of these spheres
will be as the cubes of the diameters. The most likely conclusion
we can draw from measures of parallax is that the first sphere
will contain, beside the sun at its centre, only Alpha Centauri.
The second, twelve light-years away, will probably contain,
besides these two, six other stars, making eight in all. The third
may contain twenty-one more, making twenty-seven stars within the
third sphere, which is the cube of three. Within the fourth would
probably be found sixty-four stars, this being the cube of four,
and so on.

Beyond this no measures of parallax yet made will give us much
assistance. We can only infer that probably the same law holds for
a large number of spheres, though it is quite certain that it does
not hold indefinitely. For more light on the subject we must have
recourse to the proper motions. The latest words of astronomy on
this subject may be briefly summarized. As a rule, no star is at
rest. Each is moving through space with a speed which differs
greatly with different stars, but is nearly always swift, indeed,
when measured by any standard to which we are accustomed. Slow and
halting, indeed, is that star which does not make more than a mile
a second. With two or three exceptions, where the attraction of a
companion comes in, the motion of every star, so far as yet
determined, takes place in a straight line. In its outward motion
the flying body deviates neither to the right nor left. It is safe
to say that, if any deviation is to take place, thousands of years