60
ESSEX WORTHIES.
regions, being that shown in the accompanying reduced woodcut.
Several others are depicted in his book. He particularly discusses
the effects of masses of iron ore in mountains and continents in
producing local perturbations or variations of the compass ; a matter
which has quite lately received fresh attention recently from the
magnetic surveys of Professors Rucker and Thorpe, in which they
have measured the perturbing effects of mountain-chains such as the
Malvern Hills, and have even been led to discover the existence of
underground mountains, one of which, for example, is in the neigh-
bourhood of Reading. Book VI. is of a more speculative character,
dealing with magnetic motions and cosmical systems : the main point
of interest in it being its frank acceptance of the astronomical
doctrines of Copernicus.
These contributions to purely magnetic knowledge were of great
importance ; but far transcending them in interest is a short digres-
sion interpolated in the Second Book. This is the famous chapter
on Electricity which laid the foundation of that science. Prior to
his time the only known electrical facts were two isolated observa-
tions of prehistoric date. The mineral amber, or electron, then of
great rarity and regarded as a gem, was known to acquire, when
rubbed, the magical property of attracting straws and other light
objects. A similar property had been recognised to exist in jet.
Amber was a substance about which there was something uncanny.
It was clear like glass, when of good quality, but was often found to
contain flies and other insects enclosed within itself—"shining,"
says Gilbert, "in eternal sepulchres." Much had the ancients,
including Theophrastus and Pliny, written about it and the magical
properties which it exhibited after being rubbed. This peculiar
phenomenon was submitted to examination by Gilbert with an
industry and experimental sagacity thoroughly characteristic of the
man. He devised for facilitating the observation of feeble attractions,
a simple instrument, consisting of a light, stiff arm of metal,
resembling in shape, a compass needle, pivoted like a compass upon