stone, or other hard substance each by a cord, the series so arranged
that they touch each other. Then striking a blow against one end of
the line, we observe that the ball at the farther end of the line is
set in motion, swinging a little away from the place it occupied
before. The movement of the intermediate balls may be so slight as to
escape attention. We thus perceive that energy can be transmitted
from one to another of these little spheres. Close observation shows
us that under the impulse which the blow gives each separate body is
made to sway within itself much in the manner of a bell when it is
rung, and that the movement is transmitted to the object with which it
is in contact. In passing from the sun to the earth, the light and
heat traverse a space which we know to be substantially destitute of
any such materials as make up the mass of the earth or the sun. Judged
by the standards which we can apply, this space must be essentially
empty. Yet because motions go through it, we have to believe that it
is occupied by something which has certain of the properties of
matter. It has, indeed, one of the most important properties of all
substances, in that it can vibrate. This practically unknown thing is
called ether.

The first important observational work done by the ancients led them
to perceive that there was a very characteristic difference between
the planets and the fixed stars. They noted the fact that the planets
wandered in a ceaseless way across the heavens, while the fixed stars
showed little trace of changing position in relation to one another.
For a long time it was believed that these, as well as the remoter
fixed stars, revolved about the earth. This error, though great, is
perfectly comprehensible, for the evident appearance of the movement
is substantially what would be brought about if they really coursed
around our sphere. It was only when the true nature of the earth and