des of the dial, because the sun will shine on opposite sides
in the summer and in the winter months, changing at each equinox.

_To find the Meridian Plane._--We have, so far, assumed the meridian
plane to be accurately known; we shall proceed to describe some of the
methods by which it may be found.

[Illustration: FIG. 5.]

The mariner's compass may be employed as a first rough approximation.
It is well known that the needle of the compass, when free to move
horizontally, oscillates upon its pivot and settles in a direction
termed the magnetic meridian. This does not coincide with the true
north and south line, but the difference between them is generally
known with tolerable accuracy, and is called the variation of the
compass. The variation differs widely at different parts of the
surface of the earth, and is not stationary at any particular place,
though the change is slow; and there is even a small daily oscillation
which takes place about the mean position, but too small to need
notice here (see MAGNETISM, TERRESTRIAL).

With all these elements of uncertainty, it is obvious that the compass
can only give a rough approximation to the position of the meridian,
but it will serve to fix the style so that only a small further
alteration will be necessary when a more perfect determination has
been made.

[Illustration: FIG. 6.]

A very simple practical method is the following:--

Place a table (fig. 6), or other plane surface, in such a position
that it may receive the sun's rays both in the morning and in the
afternoon. Then carefully level the surface by means of a
spirit-level. This must be done very accurately, and the table in that
position made perfectly secure, so that there be no danger of its
shifting during the day.

Next, suspend a plummet SH from a point S, which must be rigidly
fixed. The extremity H, where the plummet just meets the surface,
should be somewhere near the middle of one end of the table. With H
for centre, describe any number of concentric arcs of circles, AB, CD,
EF, &c.

A bead P, kept in its place by friction, is threaded on the plummet
line at some convenient height above H.

Everything being thus prepared, let us follow the shadow of the bead P
as it moves along the surface of the table during the day. It will be
found to describe a curve ACE ... FDB, approaching the point H as the
sun advances