the magnet. Its properties had lain uninvestigated for two thousand
years, except in China, where the observation had been made that its
qualities may be imparted to steel, and that a little bar or needle so
prepared, if floated on the surface of water or otherwise suspended,
will point north and south. In that manner the magnet had been applied
in the navigation of ships, and in journeys across trackless deserts.
The first European magnetical discovery was that of Columbus, who
observed a line of no variation west of the Azores. Then followed the
detection of the dip, the demonstration of poles in the needle, and of
the law of attraction and repulsion; the magnetic voyage undertaken by
the English government; the construction of general variation charts;
the observation of diurnal variation; local perturbations; the influence
of the Aurora, which affects all the three expressions of magnetical
power; the disturbance of the horary motion simultaneously over
thousands of miles, as from Kasan to Paris. In the meantime, the theory
of magnetism improved as the facts came out. Its germ was the Cartesian
vortices, suggested by the curvilinear forms of iron filings in the
vicinity of magnetic poles. The subsequent mathematical discussion was
conducted upon the same principles as in the case of electricity.

[Sidenote: Electro-magnetism.] Then came the Danish discovery of the
relations of electricity and magnetism, illustrated in England by
rotatory motions, and in France adorned by the electrodynamic theory,
embracing the action of currents and magnets, magnets and magnets,
currents and currents. The generation of magnetism by electricity was
after a little delay followed by its converse, the production of
electricity by magnetism; and thermoelectric currents, arising from the
unequal application or propagation of heat, were rendered serviceable in
producing the most sensitive of all thermometers.

[Sidenote: Of light and optics.] The investigation of the nature and
properties of light rivals in interest and value that of electricity.
What is this agent, light, which clothes the earth with verdure, making
animal life possible, extending man's intellectual sphere, bringing to
his knowledge the forms and colours of things, and giving him
information of the existence of countless myriads of worlds? What is
this light which, in the midst of so many realities, presents him with
so many delusive fictions, which rests the coloured bo