MARYLAND GEOLOGICAL SURVEY 517

hundred feet from the peep sight and exactly in range with it and
the plumb-line; carefully measure this distance.

Take from the table XVI the azimuth of Polaris correspond-
ing to the latitude of the station and year of observation; find the
natural tangent of said azimuth and multiply it by the distance from
the peep sight to the rod; the product will express the distance to be
laid off from the rod exactly at right angles to the direction already
determined (to the west for eastern elongation or to the east for
western elongation) to a point which with the peep sight will define
the direction of the true meridian with a fair degree of accuracy.

III.——TO DETERMINE THE TRUE MERIDIAN BY OBSERVING THE TRANSITS
OF POLARIS AND ANOTHER STAR ACROSS THE SAME VERTICAL PLANE.

This simple method for tracing out on the ground a true north and
south line, one demanding only a very slender instrumental outfit,
was given in Lalande's Astronomy, published more than a century
ago. It was used by Andrew Ellicott in 1785 in his boundary
survey work of Pennsylvania and was again brought to notice in the
present century by Dr. Charles Davies. It consists in watching for
the time when Polaris and a given bright star come to the same
vertical, and then after a short lapse of time, given in a table, Polaris
will be found exactly on the meridian, and hence can be referred to
the horizon and to any meridian mark placed there.

The vertically may be ascertained by a plumb-line or by the ver-
tical thread of a transit instrument; the method demands neither a
graduated circle, nor a chronometer, nor any exact knowledge of the
local time, an ordinary watch being sufficient to measure the short
tabular interval.

Early in the present century the star Alioth (e Ursae Majoris) was
favorably situated for the use of the method; however in 1850 the
interval between times of vertically and of culmination already
amounted to 17 minutes, which interval in 1893 had grown to 28.5m.
for lower culmination and to 29.5m. for upper culmination, hence
this star is no longer suitable. Zeta (z) Ursae Majoris or Delta (d)
Cassiopeiae should now be substituted for it, both these stars being