MARYLAND GEOLOGICAL SURVEY 525

when such conditions exist by comparing the time given in the table
with his astronomical time of observation. The upper culmination
to be used at any time will always be the LAST one that occurs before
the observation.

When an hour angle comes out within one minute of either 0h. 0m.,
or 23h. 56m.1, the observation may be regarded as having been taken
with the star on the meridian, above the pole; if within one minute of
11h. 58m., Polaris may be considered on the meridian below the pole
at the time of observation.

At elongation Polaris is nearly 5h. 55m. west (or east) of its posi-
tion at iipper culmination; consequently if the hour angle for any
observation comes within five minutes of 5h. 55m., or 18h. 1m., the
star may be assumed to be at elongation, west for the first and east
for the second hour angle, and its azimuth may be taken from a pre-
ceding table (No. XVI), which gives its value at elongation from
1890 to 1910 inclusive.

Should the surveyor wish the time of lower culmination, for use
with the plumb-line method (No. II), described on page 516, or for
any other purpose, he will first determine the time of upper culmina-
tion for the date (Table XV) and then subtract llh. 58m. for
the preceding lower culmination, or add 11h. 58m. for the lower cul-
mination following the derived time for upper culmination, attend-
ing to the addition or subtraction of 23h. 56.1m., as directed in an
example (1) below.

The time to be used when making observations on Polaris off the
meridian should be as accurate as can be obtained. Looking at Table
XVII, near the top of the page, the surveyor will observe that for a
difference of four minutes in the time argument there is a change of
about two minutes in azimuth; consequently, to obtain the azimuth
to the nearest whole minute of are, the local mean time, upon which
all depends, should be known within two minutes. When the sur-
veyor uses a solar instrument, he can readily determine the time for
himself during the afternoon before observing Polaris, or in the morn-
ing after observation, and, without moving the hands of his watch,
apply the necessary correction to his observed watch time. When