co-operate in effecting an enormous enhancement both
in the insulation and in the carrying capacity of the wire. As an
example of recent work in ocean telegraphy let us glance at the cable
laid in 1894, by the Commercial Cable Company of New York. It unites
Cape Canso, on the northeastern coast of Nova Scotia, to Waterville, on
the southwestern coast of Ireland. The central portion of this cable
much resembles that of its predecessor in 1866. Its exterior armour of
steel wires is much more elaborate. The first part of Fig. 59 shows the
details of manufacture: the central copper core is covered with
gutta-percha, then with jute, upon which the steel wires are spirally
wound, followed by a strong outer covering. For the greatest depths at
sea, type _A_ is employed for a total length of 1,420 miles; the
diameter of this part of the cable is seven-eighths of an inch. As the
water lessens in depth the sheathing increases in size until the
diameter of the cable becomes one and one-sixteenth inches for 152
miles, as type _B_. The cable now undergoes a third enlargement, and
then its fourth and last proportions are presented as it touches the
shore, for a distance of one and three-quarter miles, where type _C_ has
a diameter of two and one-half inches. The weights of material used in
this cable are: copper wire, 495 tons; gutta-percha, 315 tons; jute
yarn, 575 tons; steel wire, 3,000 tons; compound and tar, 1,075 tons;
total, 5,460 tons. The telegraph-ship _Faraday_, specially designed for
cable-laying, accomplished the work without mishap.

Electrical science owes much to the Atlantic cables, in particular to
the first of them. At the very beginning it banished the idea that
electricity as it passes through metallic conductors has anything like
its velocity through free space. It was soon found, as Professor
Mendenhall says, "that it is no more correct to assign a definite
velocity to electricity than to a river. As the rate of flow of a river
is determined by the character of its bed, its gradient, and other
circumstances, so the velocity of an electric current is found to depend
on the conditions under which the flow takes place."[2] Mile for mile
the original Atlantic cable had twenty times the retarding effect of a
good aerial line; the best recent cables reduce this figure by nearly
one-half.

In an extreme form, this slowing down reminds us of the obstruction of
light as it enters the atmosphere of the earth, of the fur