might give one an advantage over the other, and that thus a residual
or differential current might be obtained. He combined wires of
different materials, and caused them to act in opposition to each
other, but found the combination ineffectual. The more copious flow
in the better conductor was exactly counterbalanced by the
resistance of the worse. Still, though experiment was thus emphatic,
he would clear his mind of all discomfort by operating on the earth
itself. He went to the round lake near Kensington Palace, and
stretched 480 feet of copper wire, north and south, over the lake,
causing plates soldered to the wire at its ends to dip into the
water. The copper wire was severed at the middle, and the severed
ends connected with a galvanometer. No effect whatever was observed.
But though quiescent water gave no effect, moving water might.
He therefore worked at London Bridge for three days during the ebb
and flow of the tide, but without any satisfactory result. Still he
urges, 'Theoretically it seems a necessary consequence, that where
water is flowing there electric currents should be formed. If a line
be imagined passing from Dover to Calais through the sea, and
returning through the land, beneath the water, to Dover, it traces
out a circuit of conducting matter one part of which, when the water
moves up or down the channel, is cutting the magnetic curves of the
earth, whilst the other is relatively at rest.... There is every
reason to believe that currents do run in the general direction of
the circuit described, either one way or the other, according as the
passage of the waters is up or down the channel.' This was written
before the submarine cable was thought of, and he once informed me
that actual observation upon that cable had been found to be in
accordance with his theoretic deduction.[1]

Three years subsequent to the publication of these researches--