Until very recently gutta-percha was the only
material used to insulate submarine cables,
although experience had shown that even when
most carefully manufactured it could not resist
the heat of tropical climates. At a temperature
of a little over two hundred and twenty
degrees Fahrenheit gutta-percha is entirely spoiled.
While the committee were sitting, improvements
in the manufacture of gutta-percha were
completed, by which it can be rendered a perfect
insulator, and delivered in commercial quantities.
Important improvements have also been made
in the manufacture of india-rubber, by which all,
or almost all, the defects of manipulation, which
obstructed its previous use, have been removed.
Under Silver's process the raw india-rubber is
masticated, that is, torn into small fragments
by steel teeth, and re-kneaded into a solid mass
from which eventually thin adhesive ribands
of almost unlimited length can be obtained.
As india-rubber bears a very high degree of
temperature without loss of insulating power,
and is of extreme if not unlimited durability
when protected by water from the action of
the air, it may be expected that this new
manufacture will come into use for those hot climates
where gutta-percha fails, and where submarine
telegraphic communication is most urgently
required.

There is also a patent material, partly
composed of gutta-percha, called "Wray's
compound," of the insulating properties of which
the committee, after careful experiments, speak
highly. All the other substances to which their
attention was directed by divers patentees were
inferior in insulating qualities to gutta-percha.

In order to strengthen the cable against the
strain which inevitably occurs in paying-out
over a ship's stern, and on many other
occasions, various expedients are adopted. In the
first cable laid successfully between Dover and
Calais, the core was first surrounded with a
considerable thickness of hemp steeped in
Stockholm tar and tallow, and then with a covering
of iron wire laid on spirally. The weight of the
cable was made so great that it was effectually
protected from injury from the anchors of coasting
vessels. This arrangement was admirably
suited to the circumstances in which that cable
was placed; it has consequently been very
durable, and been the type of other cables laid
in very different situations. All experience has
since shown that in shallow waters a large heavy
cable is the most economical arrangement. But
in dealing with long distances and deep seas it
is impossible to use cables of proportionate
weight unless ships of enormous capacity were
specially constructed for the purpose. The
Atlantic cable, which was very slim as compared
with the Dover cable, considering the work it
had to do, required two vessels of three thousand
two hundred tons to carry each half.

Wire, however small, will break with its own
weight at a length of about three miles. An
iron rod, however thick, will break at the
same length. Therefore, in dealing with deep
sea cables, the strain can only be relieved by an
alteration of the proportion between the absolute
weight of the rope in water and the absolute
strength of the cable, and this is not always
obtained by a simple decrease of the specific
gravity of the rope, as many imagine. Thus,
to take an instance from the evidence of one
of the witnesses, "suppose a wire-covered cable
be served round with hemp, giving the spiral
of the hemp a very short lay, the hemp will not
take its strain with the iron, and can
consequently add nothing to the strength of the cable.
Hemp is, however, heavier than water, the cable
will not, therefore, support so great a length of
itself as it did before the hemp was added, as
it has no greater strength, and yet has a greater
weight per yard run. The specific gravity of
the whole rope is, however, less than it was
before the addition of the hemp; for, as the
specific gravity of the hemp is less than that of
the iron-covered cable, the mean specific gravity
of the two taken together must be less than the
iron-covered cable taken alone. Here, then,
the modulus of tension is actually decreased by
a decrease of specific gravity." On the other
hand, steel, with about the same specific gravity
as iron, has greater strength, and gutta-percha
wire, being of but little greater specific gravity
than water, might be out any length with any
increase of strain. A cable of the specific gravity
of a gutta-percha wire, and strong enough to
resist the rubbings and strains to which a
submarine cable must be subject in deep waters,
would be perfect. But all attempts have as yet
failed to produce a covering strong enough to
resist strains, durable enough to resist decay, and
of less specific gravity than iron or steel. There
are, however, before the telegraphic public a
crowd of patentees, each certain that his
combination and arrangement of iron or steel and
copper wire, with its coverings, is the one thing
needed for a perfect cable, for the greatest
distances, and the deepest seas, whose earnest
statements must be received with the greatest
caution.

Supposing a suitable cable decided on, the
bottom of the sea where it is about to be laid
should be surveyed and analysed. Hitherto,
the course of a cable has been too much left to
hap-hazard; in future, it is to be hoped that a
careful investigation of the route will be the first
step. The failures in laying submarine cables
have generally been due to unsuitable ships and
defects in the paying-out apparatus. "The ship,"
say the committee, "should be of large capacity,
to admit of the cable being easily coiled without
injury; care should be taken to isolate the hold
from the engine-room, as the heat would
injuriously affect a gutta-percha covering. The hold
should be of a form to allow the cable to be paid-
out without materially altering the trim of the
ship, which should have sufficient power to
maintain a speed of from four to six knots per
hour, in the direction in which it is proceeding,
in any weather, and it should be very steady in
a rough sea." Therefore, to do the work of
laying submarine cables in the best manner,
ships must be built for the special purpose, and