424 FIRST REPORT UPON MAGNETIC WORK

local disturbances, he is the very one who should be made aware of
the fact and be obliged by law to discard his compass entirely for
accurate surveying over the disturbed region.

Now there are disturbances which manifest themselves principally
in the vertical plane and which would hence largely fail to be revealed
by a survey with a horizontal needle alone, i. e., a declination survey.
Such disturbances likewise demand attention.

Coming now to the geological standpoint, there can be no question
whatsoever that if the geologist hopes to gain anything from geo-
magnetic investigations conducted under his auspices, all the elements
must be observed, not simply declination. The declination survey
can at its best only indicate the presence of the phenomena to be
studied, and even at that, only a part of them. To frame any
hypotheses with reference to the relationship between geological for-
mations and disturbances in the distribution of terrestrial magnetism,
as based upon declination data alone, can be of but little scientific
value, and may even retard real progress in this direction. If, on
the other hand, inclination and intensity data are added to the declina-
tion data, it is possible to approach the matter under discussion from
a scientific and, therefore, practical standpoint. The magnitude of
the deflecting forces and true direction of line of action can then be
computed, and thus the real data to be utilized in the correlation of
disturbances in geological structure and of magnetic distribution can
be obtained.

From the purely economical standpoint, likewise, I believe experi-
ence will at once teach the desirability of a survey that is complete.
The results of the Maryland magnetic survey will bear out the state-
ment that it need not cost much more, either in time or money, to
observe the three elements than to observe simply one. Fully 75 per
cent of the time and money (if not more) are consumed in the
occupying of a station. After the observer has once reached his sta-
tion, it is comparatively a mere trifle to observe a little longer, and
the additional observations will entail practically little extra expense.
The heaviest part of a magnetic survey consists in the determination
of the magnetic declination.

Again, experience will teach that it is just as economical to make