BOOK SIXTH.
CHAP. I.
ON THE GLOBE OF THE EARTH, THE
great magnet.
itherto our subject hath been the
loadstone and things magnetical: how they conspire together, and are acted upon, how they conform themselves to the
terrella and to the earth. Now must we consider separately the globe itself of the earth. Those experiments which have
been proved by means of the terrella, how magnetick things conform themselves to the terrella, are all or at least the
principal and most important of them, displayed by means of the earth's Body: And to the earth things magnetical are in
all respects associate. First, as in the terrella the æquator, meridians, parallels, axis, poles are natural boundaries,
as numerous experiments make plain: So also in the earth these boundaries are natural, not mathematical only (as all
before us used to suppose). These boundaries the same experiments display and establish in both cases alike, in the earth
no less than in the terrella. Just as on the periphery of a terrella a loadstone or a magnetick piece of iron is directed
to its proper pole: so on the earth's surface are there turnings-about, peculiar, manifest, and constant on either side
of the æquator. Iron is indued with verticity by being extended toward a pole of the earth, just as toward a pole of the
terrella: By its being placed down also, and cooling toward the earth's pole after the pristine verticity has been annulled by fire, it acquires new verticity, conformable to its position earthward. Iron rods also,
when placed some considerable time toward the poles, acquire verticity merely by regarding the earth; just as the same
rods, if placed toward the pole of a loadstone, even without touching it, receive polar virtue. There is no magnetick
body that in any way runs to the terrella which does not also wait upon the earth. As a loadstone is stronger at one end
on one side or other240 of its æquator: so is the same property displayed by a
small terrella upon the surface of a larger terrella. According to the variety and artistick skill in the rubbing of the
magnetick iron upon the terrella, so do the magnetick things perform their function more efficiently or more feebly. In
motions toward the earth's body, as toward the terrella a variation is displayed due to the unlikeness, inequality, and
imperfection of its eminences: So every variation of the versorium or mariners' compass, everywhere by land or by sea,
which thing has so sorely disturbed men's minds, is discerned and recognized as due to the same causes. The magnetick dip
(which is the wonderful turning of magnetick things to the body of the terrella) in systematick course, is seen in
clearer light to be the same thing upon the earth. And that single experiment, by a wonderful indication, as with a
finger, proclaims the grand magnetick nature of the earth to be innate and diffused through all her inward parts. A
magnetick vigour exists then in the earth just as in the terrella, which is a part of the earth, homogenic in nature with
it, but rounded by Art, so as to correspond with the earth's globous shape and in order that in the chief experiments it
might accord with the globe of the earth.