526 FIRST REPORT UPON MAGNETIC WORK

the surveyor uses standard railroad time he will correct the same for
the difference of longitude between his station and the standard
meridian for which the time is given, at the rate of four minutes
of time for each degree of the difference in are (see instructions, p. 514).
Thus, if the difference of longitude for the station be 1° 45' = 1°.75,
the equivalent in time will be 4x1.75 = 7 minutes. Since table XV
applies to longitude 75°, the correction will always be subtracted
from the standard railroad time of observation to obtain local time.
It is immaterial where the surveyor obtains the standard time, pro-
vided he gets it right; a result which will be determined in the most
satisfactory manner by a direct comparison at a telegraph office.

Generally the surveyor will have only two or three simple additions
or subtractions to make, and ten minutes will be ample time in which
to make the observation and perform the little computation required.