rfectly clean bright copper plates, each exposing four square
feet of surface, were soldered to the extremities of a copper wire; the
plates were immersed in the water, north and south of each other, the wire
which connected them being arranged upon the grass of the bank. The plates
were about four hundred and eighty feet from each other, in a right line;
the wire was probably six hundred feet long. This wire was then divided in
the middle, and connected by two cups of mercury with a delicate
galvanometer.

186. At first, indications of electric currents were obtained; but when
these were tested by inverting the direction of contact, and in other ways,
they were found to be due to other causes than the one sought for. A little
difference in temperature; a minute portion of the nitrate of mercury used
to amalgamate the wires, entering into the water employed to reduce the two
cups of mercury to the same temperature; was sufficient to produce currents
of electricity, which affected the galvanometer, notwithstanding they had
to pass through nearly five hundred feet of water. When these and other
interfering causes were guarded against, no effect was obtained; and it
appeared that even such dissimilar substances as water and copper, when
cutting the magnetic curves of the earth with equal velocity, perfectly
neutralized each other's action.

187. Mr. Fox of Falmouth has obtained some highly important results
respecting the electricity of metalliferous veins in the mines of Cornwall,
which have been published in the Philosophical Transactions[A]. I have
examined the paper with a view to ascertain whether any of the effects were
probably referable to magneto-electric induction; but, though unable to
form a very strong opinion, believe they are not. When parallel veins
running east and west were compared, the general tendency of the
electricity _in the wires_ was from north to south; when the comparison was
made between parts towards the surface and at some depth, the current of
electricity in the wires was from above downwards. If there should be any
natural difference in the force of the electric currents produced by
magneto-electric induction in different substances, or substances in
different positions moving with the earth, and which might be rendered
evident by increasing the masses acted upon, then the wires and veins
experimented with by Mr. Fox might perhaps have acted as dischargers to the
electricity of the mass of