umb-line from some point of the style, then the style will be
properly adjusted if, at the very instant of noon, its shadow falls
exactly on the plumb-line,--or, which is the same thing, if both
shadows coincide on the dial.

This instant of noon will be given very simply, by the meridian plane,
whose position we have secured by the two permanent marks on the
ground. Stretch a cord from the one mark to the other. This will not
generally be horizontal, but the cord will be wholly in the meridian
plane, and that is the only necessary condition. Next, suspend a
plummet over the mark which is nearer to the sun, and, when the shadow
of the plumb-line falls on the stretched cord, it is noon. A signal
from the observer there to the observer at the dial enables the latter
to adjust the style as directed above.

_Other Methods of finding the Meridian Plane._--We have dwelt at some
length on these practical operations because they are simple and
tolerably accurate, and because they want neither watch, nor sextant,
nor telescope--nothing more, in fact, than the careful observation of
shadow lines.

The Pole star, or _Ursae Minoris_, may also be employed for finding
the meridian plane without other apparatus than plumb-lines. This star
is now only about 1 deg. 14' from the pole; if therefore a plumb-line
be suspended at a few feet from the observer, and if he shift his
position till the star is exactly hidden by the line, then the plane
through his eye and the plumb-line will never be far from the meridian
plane. Twice in the course of the twenty-four hours the planes would
be strictly coincident. This would be when the star crosses the
meridian above the pole, and again when it crosses it below. If we
wished to employ the method of determining the meridian, the times of
the stars crossing would have to be calculated from the data in the
_Nautical Almanac_, and a watch would be necessary to know when the
instant arrived. The watch need not, however, be very accurate,
because the motion of the star is so slow that an error of ten minutes
in the time would not give an error of one-eighth of a degree in the
azimuth.

The following accidental circumstance enables us to dispense with both
calculation and watch. The right ascension of the star [eta] _Ursae
Majoris_, that star in the tail of the Great Bear which is farthest
from the "pointers," happens to differ