MARYLAND GEOLOGICAL SURVEY 449

making generally two independent determinations at every station.
Since entire reliance had to be put upon this one needle, it was neces-
sary to examine whether any constant error, due, for example, to
irregularity of shape of pivots, which the method of observation would
not eliminate, was inherent in the results obtained with it. This
matter was tested in two independent ways:

(a) By observing the dip in different azimuths. Here different
parts of the pivots came into play while making the dip observations.
This test was made in about the middle of the period of the survey.

(b) By comparing the results obtained with my dip circle and
Needle No. I with the results obtained with new Kew dip circles,
just received by the Coast and Geodetic Survey, which had as yet
seen no field use and which had been examined and certified to at the
Kew Observatory. The comparison was made in the Coast and
Geodetic Survey office at Washington at the close of the magnetic
survey in 1896. The result of both tests was that no constant cor-
rection need be applied to the results with the dip needle used in the
magnetic survey and that furthermore any correction that could be
applied to refer the dip results to the Kew standard would lie within
the probable error of a dip determination.

THE EPOCH SELECTED FOR THE MAGNETIC MAPS.

The epoch to which the results of the magnetic survey are to be
reduced has been selected as January 1st, 1900. This was done for
three reasons:

(a) The Coast and Geodetic Survey has undertaken to issue new
magnetic charts of the United States for the year 1900.

(b) The secular variations of the magnetic elements have been so
thoroughly determined by the researches of the Coast and Geodetic
Survey that it is possible to make very accurate reductions for a
period of from five to ten years.

(c) There is every reason to suppose that the attempt is going to
be made to issue new magnetic charts for the year 1900 for the entire
earth. In this event the results of this magnetic survey will be in
shape directly available.