WILLIAM GILBERT.
57
certain orbit of magnetic virtue, as by a surrounding atmosphere. In
further experiments loadstones were cut into two parts, the parts being
floated on water in little vessels to observe their mutual attractions
and repulsions. All experiments which Gilbert considered as being
original he claimed as his own by affixing an asterisk, large or small
according to the importance of the matter, in the margin of the text.
He suggest mapping out the lines of magnetic virtue upon the surface
of his terrella as the parallels of latitude. The tropics, the arctic circles
and the meridians are marked out upon a geographical chart or
terrestrial globe. The fact that a magnet of elongated shape—a
magnetic rod—is more powerful than one of spherical or cubical or
any other shape of equal weight (the horseshoe shape not being dis-
covered until many years later) is announced. The screening effect
of a sheet of thin iron, and the failure of other metals to screen off
magnetic action, are noted. Then comes a series of studies on the
effect of capping loadstones with armatures of iron, and on the
strengthening of the power of loadstones. Chapter xxxii. of Book II.
is a notable one, containing a number of magnetic aphorisms, each
tersely summing up some result of experiment or observation. In it the
principle of equality of action and reaction is illustrated by the experiment
of floating a magnet in a little skiff and showing that it attracts itself to
a piece of iron, just as the iron, if placed in the skiff, will be attracted
to the magnet, thus furnishing an illustration of the principle of action
and reaction. Several experiments are also described illustrative of
the mutual repulsions of similar poles, north repelling north and south