not to revolve on its axis at all. Then we should always see the
stars at rest and the star which was in the zenith of any place,
say a farm-house in New York, at any time, would be there every
night and every hour of the year. Now the zenith is simply the
point from which the plumb-line seems to drop. Lie on the ground;
hang a plummet above your head, sight on the line with one eye,
and the direction of the sight will be the zenith of your place.
Suppose the earth was still, and a certain star was at your
zenith. Then if you went to another place a mile away, the
direction of the plumb-line would be slightly different. The
change would, indeed, be very small, so small that you could not
detect it by sighting with the plumb-line. But astronomers and
surveyors have vastly more accurate instruments than the plumb-
line and the eye, instruments by which a deviation that the
unaided eye could not detect can be seen and measured. Instead of
the plumb-line they use a spirit-level or a basin of quicksilver.
The surface of quicksilver is exactly level and so at right angles
to the true direction of the plumb-line or the force of gravity.
Its direction is therefore a little different at two different
places on the surface, and the change can be measured by its
effect on the apparent direction of a star seen by reflection from
the surface.

It is true that a considerable distance on the earth's surface
will seem very small in its effect on the position of a star.
Suppose there were two stars in the heavens, the one in the zenith
of the place where you now stand, and the other in the zenith of a
place a mile away. To the best eye unaided by a telescope those
two stars would look like a single one. But let the two places be
five miles apart, and the eye could see that there were two of