ther
impediment which the rays encounter if they pass from the air into the
sea. In the main the causes which hinder a pulse committed to a cable
are two: induction, and the electrostatic capacity of the wire, that is,
the capacity of the wire to take up a charge of its own, just as if it
were the metal of a Leyden jar.

Let us first consider induction. As a current takes its way through the
copper core it induces in its surroundings a second and opposing
current. For this the remedy is one too costly to be applied. Were a
cable manufactured in a double line, as in the best telephonic circuits,
induction, with its retarding and quenching effects, would be
neutralized. Here the steel wire armour which encircles the cable plays
an unwelcome part. Induction is always proportioned to the conductivity
of the mass in which it appears; as steel is an excellent conductor, the
armour of an ocean cable, close as it is to the copper core, has induced
in it a current much stronger, and therefore more retarding, than if the
steel wire were absent.

A word now as to the second difficulty in working beneath the sea--that
due to the absorbing power of the line itself. An Atlantic cable, like
any other extended conductor, is virtually a long, cylindrical Leyden
jar, the copper wire forming the inner coat, and its surroundings the
outer coat. Before a signal can be received at the distant terminus the
wire must first be charged. The effect is somewhat like transmitting a
signal through water which fills a rubber tube; first of all the tube
is distended, and its compression, or secondary effect, really transmits
the impulse. A remedy for this is a condenser formed of alternate sheets
of tin-foil and mica, _C_, connected with the battery, _B_, so as to
balance the electric charge of the cable wire (Fig. 60). In the first
Atlantic line an impulse demanded one-seventh of a second for its
journey. This was reduced when Mr. Whitehouse made the capital discovery
that the speed of a signal is increased threefold when the wire is
alternately connected with the zinc and copper poles of the battery. Sir
William Thomson ascertained that these successive pulses are most
effective when of proportioned lengths. He accordingly devised an
automatic transmitter which draws a duly perforated slip of paper under
a metallic spring connected with the cable. To-day 250 to 300 letters
are sent per minute instead of fifteen, as at first.

[Illustration: Fig.