collects the opposed currents which are
developed in them by rotation across the terrestrial lines of magnetic
force. He varies the shapes of his rectangles while preserving their
areas constant, and finds that the constant area produces always the
same amount of current per revolution. The current depends solely on
the number of lines of force intersected, and when this number is kept
constant the current remains constant too. Thus the lines of magnetic
force are continually before his eyes, by their aid he colligates his
facts, and through the inspirations derived from them he vastly expands
the boundaries of our experimental knowledge. The beauty and exactitude
of the results of this investigation are extraordinary. I cannot
help thinking while I dwell upon them, that this discovery of
magneto-electricity is the greatest experimental result ever obtained by
an investigator. It is the Mont Blanc of Faraday's own achievements.
He always worked at great elevations, but a higher than this he never
subsequently attained.

Footnotes to Chapter 13

[1] He compares the interpenetration of two atoms to the
coalescence of two distinct waves, which though for a moment
blended to a single mass, preserve their individuality, and
afterwards separate.

[2] In this form the experiment is identical with one made
twenty years earlier. See page 34.

Chapter 14.

Unity and convertibility of natural forces: theory of the
electric current.

The terms unity and convertibility, as applied to natural forces, are
often employed in these investigations, many profound and beautiful
thoughts respecting these subjects being expressed in Faraday's memoirs.
Modern inquiry has, however, much augmented our knowledge of the
relationship of natural forces, and it seems worth while to say a few
words here, tending to clear up certain misconceptions which appear to
exist among philosophic writers regarding this relationship.

The whole stock of energy or working-power in the world consists of
attractions, repulsions, and motions. If the attractions and repulsions
are so circumstanced as to be able to produce motion, they are sources
of working-power, but not otherwise. Let us for the sake of simplicity
confine our attention to the case of attraction. The attraction exerted
between the earth and a body at a distance from the earth's surface is
a source of working-power; because the body can be move