MARYLAND GEOLOGICAL SURVEY 529

THE DETERMINATION OF THE MAGNETIC DECLINATION BY THE

SURVEYOR.

In conclusion, it may be remarked that when the surveyor deter-
mines the value of the magnetic declination himself it would be well
for him to make the observations on several days, if possible. Prob-
ably the best time of day for making the observations would be
towards evening, about 5 or 6 o'clock.¹ At this time the declination
reaches, approximately, its mean value for the day (see Table II).
The observations on any one day should extend at least over one-half
of an hour, preferably an hour, and the readings should be taken
every ten minutes. Before each reading of the needle it would be
well to tap² the glass plate lightly with the finger or a pencil so as to
slightly disturb the needle from the position of rest it may have
assumed. The accurate time should be noted opposite each reading
and a note entered in the record-book as to the date, the weather and
the kind of time the observer's watch was keeping. A brief descrip-
tion of station and of method employed in determining the meridian
line and declination should be added to the record. It is very essential
that the surveyor should have some knowledge as to the error³ of his
compass. He can determine this by making observations at one of
the magnetic survey stations. He should reduce his value to January
1st, 1900, by allowing an annual change of 3' per annum, as explained
elsewhere, and then compare his value with that obtained in the mag-
netic survey. This correction can best be determined at the county-
seats where meridian lines have been established. It would not be
amiss to determine the compass correction before and after the deter-
mination of the magnetic decimation.

¹ Or the surveyor may make his observations in the morning and early in
the afternoon, at about the times of minimum and maximum values of the
magnetic declination. He may regard the mean of the two extreme values
as corresponding closely to the mean value for the day (24 hours).

² Great care must be taken not to electrify the needle by rubbing the glass
plate in any manner. Remarkable deflections of the needle can thus be
produced.

³ I have found surveyor's compasses to differ at times as much as 1° from
the readings with the C. and G. S. magnetometer. The error may be due to
a variety of causes, such as an imperfect pivot, non-coincidence of magnetic
axis of needle with the geometric axis, and loss of magnetism of the needle.