suitable they may be for land lines, they are next to
useless for long cables. They require a certain definite strength of
current to work them, whatever it may be, and in general it is
very considerable. Most of the moving parts of the mechanism are
comparatively heavy, and unless the current is of the proper strength to
move them, the instrument is dumb, while in Bain's the solution requires
a certain power of current to decompose it and leave the stain.

In overland lines the current traverses the wire suddenly, like a
bullet, and at its full strength, so that if the current be sufficiently
strong these instruments will be worked at once, and no time will be
lost. But it is quite different on submarine cables. There the current
is slow and varying. It travels along the copper wire in the form of
a wave or undulation, and is received feebly at first, then gradually
rising to its maximum strength, and finally dying away again as slowly
as it rose. In the French Atlantic cable no current can be detected
by the most delicate galvanoscope at America for the first tenth of
a second after it has been put on at Brest; and it takes about half
a second for the received current to reach its maximum value. This
is owing to the phenomenon of induction, very important in submarine
cables, but almost entirely absent in land lines. In submarine cables,
as is well known, the copper wire which conveys the current is insulated
from the sea-water by an envelope, usually of gutta-percha. Now the
electricity sent into this wire INDUCES electricity of an opposite kind
to itself in the sea-water outside, and the attraction set up between
these two kinds 'holds back' the current in the wire, and retards its
passage to the receiving station.

It follows, that with a receiving instrument set to indicate a
particular strength of current, the rate of signalling would be very
slow on long cables compared to land lines; and that a different form
of instrument is required for cable work. This fact stood greatly in
the way of early cable enterprise. Sir William (then Professor)
Thomson first solved the difficulty by his invention of the 'mirror
galvanometer,' and rendered at the same time the first Atlantic cable
company a commercial success. The merit of this receiving instrument
is, that it indicates with extreme sensibility all the variations of
the current in the cable, so that, instead of having to wait until
each signal wave sent into the cab