56
ESSEX WORTHIES.
discussion of the differences between magnets of loadstone and
masses of iron the first book is brought to a close with a remarkable
chapter which gives the key-note to the rest of the work. Its title
advances the proposition that the terrestrial globe is magnetic, and is
a magnet. "Our new and unheard-of opinion concerning the earth"
is his way of emphasizing his discovery that the earth is itself also a
great magnet—a big loadstone : for it was by this hypothesis that he
proposed to explain the puzzling facts of the several variations of the
compass needle. It has poles, he says, not mathematical points but
natural terminals, and between them lies an equator, not a mathe-
matical circle but a natural separation between the two polar regions.
The whole of the remainder of the work is devoted to sustaining this
remarkable generalisation.
Book II. of the volume, the longest of all the six sections, deals
with magnetic motions and forces. Almost immediately, however,
he introduces a digression upon the attractions which can be set up
by rubbed amber and other electric bodies, a digression which though
itself of immense importance has little to do with the development of
his theme. We will deal separately with this interpolated chapter,
merely observing here that he comes to the conclusion that electric
actions are comparable with cohesion whilst magnetic actions are
comparable with gravity. In his opinion the globe of the earth is
collected together and coheres electrically, though it is directed and
turned about magnetically. This obscure saying becomes more
intelligible by the light of later passages. The next chapters of Book
II. are occupied with a discussion of the opinions of philosophers
about the nature of magnets and the origin of magnetic attractions,
followed by Gilbert's own views thereon and an account of his experi-
ments on the effects, upon the attractive power, of varying the exter-
nal shape of the loadstone. Here again we meet with his spherical
loadstones, specially constructed for these observations. He points
out that iron chips and small magnets arrange themselves in particu-
lar directions, dipping towards the magnetic poles of the terrella, as
the dipping needle does towards the poles of the earth. He con-
ceived the magnetic power as extending within a certain limited region
external to the stone ; and he indicates in his simple woodcuts with
external curved lines the orbits of the magnetic virtue. In one
woodcut, here reproduced in reduced facsimile, (Fig. 1), compass
needles are shown pointing variously over various regions of the
terrella. In another, (Fig. 2), the terrella is shown enclosed within a