MARYLAND GEOLOGICAL SURVEY 519
some two or three months before and' after. Six months later the star
Zeta (£) in the tail of the Great Bear will supply its place.
2. Using the apparatus just described under II, place the " peep
sight " in the line with the plumb-line and Polaris, and move it to the
west as Polaris moves east, until Polaris and Delta, for example,
appear upon the plumb-line together, and carefully note the time by a
clock or watch; then by moving the peep sight, preserve the align-
ment with Polaris and the plumb-line (paying no further attention
to the other star); at the expiration of the small interval of time given
below the peep sight and plumb-line will define the true meridian,
which may be permanently marked for future use.
According to Mr. Schott the interval of time before Polaris will
be exactly on the meridian is:
For Zeta (C) Ursae Majoris in J o. 35 m.
For Delta (6) Cassiopeiae in 3 4 i 0.33 m.
The method given in this article for finding the true meridian can-
not be used with advantage on account of the haziness of the atmo-
sphere near the horizon, at places below about 38° north latitude.
The foregoing methods for the determination of the true meridian
are excellent in themselves when available, as they answer the require-
ments of the surveyor and give results with all desirable precision.
They do not require an accurate knowledge of the time, which is
their principal advantage. The relative motion of the stars em-
ployed, when near the meridian and the unchangeable azimuth of
Polaris at elongation (so far as the surveyor is concerned), indicate
with sufficient exactness the moment when the observation should be
made. Stormy weather, a hazy atmosphere, or the presence of clouds,
may interfere or entirely prevent observation when the star is either
at elongation or on the meridian, and both events sometimes occur
in broad daylight or at an inconvenient hour of the night. Under
such circumstances a simple method applicable at any time (Polaris
being visible), may be acceptable, and can often be used by the
surveyor when other methods fail.