In De magnete,
William Gilbert describes numerous experiments with the magnet or
lodestone. Gilbert's treatise is a key text in the development of
the experiment as a means of discovering truths about the natural
world. In twentieth-century scholarship, De magnete is
the focal point of a discussion about the origins of the methodology
that became central to the experimental philosophy in the seventeenth
century. One issue involves the extent to which Gilbert derived
his methods and views from earlier treatises such as the thirteenth-century
"Letter on the Magnet" by Pierre de Maricourt, first published
in 1558, and The Newe Attractive by the retired mariner
and compass maker, Robert Norman, first published in 1581.
For most of his experiments
Gilbert chose a natural lodestone, which he shaped into a sphere
by turning the stone on a lathe. He called this round stone a terrella,
or little earth. The many images of this "little earth"
(of which the image shown here is the first) thoroughly prepare
the reader to accept Gilbert's conclusion that the earth itself
is a magnet. The
illustration shows how to find the poles of the terrella
similar to the earth's poles. Gilbert explains how to do this using
an instrument called a versorium. The versorium
is a small compass needle made with a piece of magnetized iron that
can freely turn on its base. The versorium is placed on
various spots on the terrella and the point to which it
turns is marked. Gilbert explains that after marking a number of
points, you can discover the point at which all the points converge
(A and B) which are the poles. A versorium placed near
but not on the terrella (as D) will point directly at the
Nine years later,
Galileo created strikingly earth-like images of the moon which he
claimed to have viewed through his new instrument, the telescope.
Galileo was acquainted with Gilbert's De magnete and may
have noticed how effectively Gilbert used visual images of magnets
that looked like small earths to help persuade his readers that
the earth itself was actually a magnet.
Freudenthal, Gad. "Theory of Matter
and Cosmology in William Gilbert's De Magnete."
Isis 74 (1983): 22-37.
Henry, John. "Animism and Empiricism:
Copernican Physics and the Origins of William Gilbert's Experimental
Method." Journal of the History of Ideas 62 (January
Pumfrey, Steven. "William Gilbert's
Magnetical Philosophy, 1580-1674: The Creation and Dissolution
of a Discipline." Ph.D. diss., University of London, Warburg
Zilsel, Edgar. "The Origins of
William Gilbert's Scientific Method." Journal of the
History of Ideas 2 (January 1941): 1-32.