MARYLAND GEOLOGICAL SURVEY 425

observations with instruments especially designed for magnetic work
as to employ instruments the results from which need a continual
careful inspection. The observation data must be of such a degree of
precision that the phenomena revealed by them can be accepted as
having a physical existence, and therefore not to be ascribed to errors
of observation. Data only fairly accurate may completely hide the
very phenomena to be studied.

HISTORICAL SUMMARY OF MAGNETIC SURVEYS.

The first complete magnetic survey in which the three magnetic
elements—declination, dip and intensity—were determined, and
which was executed as a national piece of work and was co-extensive
with the limits of the country surveyed, was that of the British Islands,
corresponding to the epoch of January 1, 1837. This survey was
undertaken in 1836 at the request of the British Association for the
Advancement of Science and was completed in 1838. The example
set by Great Britain was speedily followed by the execution of similar
surveys in various portions of the globe—in Austria, Bavaria, Ger-
many, Holland, Belgium, France, Canada, etc. At the present time
nearly every civilized country has been surveyed magnetically to a
greater or less extent.

But such surveys must be repeated after the lapse of a number of
years on account of the slow, yet appreciable, change forever going
on with regard to the earth's magnetic state, which change, as one of
the most noted physicists has truly said, is a warning " that we must
not suppose that the inner history of our planet is ended. " Thus
after the lapse of twenty years Great Britain—again at the instance
of the British Association for the Advancement of Science¹—repeated
its original magnetic survey. ² The observations were taken between

¹ Doubtless no national organization has done so much for the advance-
ment of the subject of terrestrial magnetism as this most distinguished body
of scientific men. Money grants have been freely made; committees on
magnetics composed of the most eminent physicists have been formed from
time to time, and co-operation has been extended and encouragement given
to magnetic enterprises in many ways.

² Report on the Repetition of the Magnetic Survey of England by Major-
General Edward Sabine. Report of the British Association for the Advance-
ment of Science for 1861.