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MARYLAND GEOLOGICAL SURVEY 411
and in the depths below us—at the bottom of the sea. Let us hope
that the means will soon be forthcoming to permit the making of
such observations, so that continued progress, another step forward,
may be made in the subject of the earth's magnetism!
THE SECULAR VARIATION OF THE MAGNETIC DECLINATION.
The year 1634 witnessed the discovery of the next remarkable fact
with regard to the magnetic declination, the so-called secular varia-
tion, whereby the earth's magnetism suffers most remarkable changes
in the course of time. It was an accepted fact by this time that the
needle's deviation from the true north was not the same at all places,
but varied with geographical position. But now an entirely new
fact was clearly demonstrated. Henry Gellibrand, a professor of
mathematics at Gresham College, after a careful determination made
on June 12, 1634, at Deptford, about three miles southeast of London
Bridge, found that the needle pointed 4° 6' east, while Gunter, on
June 13, 1622, had found the value of 5° 56-J' east, and, as above
cited, Boroughs found the declination to be in London in 1580, 11° 15'
east. It was therefore clearly apparent that the declination or varia-
tion was changing and getting smaller. Gellibrand gives his con-
clusion thus:
" Hitherto, according to the tenets of our magnetical philosophers,
we have supposed the variation of all particular places to continue one
and the same.... But most diligent magnetical observations have
plainly offered violence to the same, and proved the contrary, namely,
that the variation is accompanied with a variation. "
He embodied his observations in a little treatise published in 1635. '
This fact of the " diminution of the variation " was likewise shown
by the Paris observations. It seemingly was ascribed, however, to
1 The title was, Henry Gellibrand: A Discourse Mathematical on the
Variation of the Magneticall Needle, together with its admirable diminution
lately discovered. London, 1635.
This interesting book has just been reprinted in facsimile under the
editorship of the well-known meteorologist and bibliographer, Prof. Hell-
mann, of Berlin. It constitutes No. 9 of Hellmann's " Neudrucke von
Schriften, und Karten iiber Meteorologie und Erdmagiietismus. " A. Asher &
Co., Berlin. Price, 3 marks.
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