MARYLAND GEOLOGICAL SURVEY 421

are established, magnetic surveys are begun, etc., etc. The magnitude,
frequency, and simultaneity in time over large areas, of the magnetic
disturbances or storms become known for the first time. It is per-
ceived that the earth's magnetism is in sympathetic touch with a
variety of hitherto unsuspected forces, and that gravitation is not the
only bond that binds us in friendly union with our sister planets and
our parent Sun. To quote from one of the most active workers of
that period, Sabine:

" We have hitherto known magnetism only as a telluric force of
much less apparent simplicity than gravity, and greatly requiring a
key to enable us to apprehend its laws, as well as its purposes in the
general economy of nature. But if, as we appear to have reason to
believe, we are to view the magnetism not only as a telluric but also
as a cosmical force, the interest and importance of our inquiries
regarding it must undoubtedly be viewed as greatly augmented. "

The total force, F; of the earth's magnetism is equal to the product
of the horizontal component, H, into the secant of the magnetic
inclination, I. ,

THE ELEMENTS INVOLVED IN MAGNETIC SURVEYS.

There are then three elements involved in a complete magnetic survey
—the declination, the dip, and the intensity. These three quantities
completely define the direction and magnitude of the earth's mag-
netic force. The determination of one element alone, for example,
declination, does not any more constitute a magnetic survey than the
determination of a number of latitudes would constitute a geograph-
ical survey. A latitude survey might fulfil certain definite and prac-
tical purposes, just as a declination survey will suffice for the practical
needs of the surveyor. The more complete the geographical survey
in all its details, the more complete the magnetic survey in all its
details, the more to be obtained of direct benefit from such surveys.

In view of the fact that there is every evidence that the era of
detailed magnetic surveys has set in in this country, it will not be
amiss to set forth fully the desirability, if not indeed the absolute neces-
sity, of making the survey a complete one at the very start, that is to