[8] Reproduced in abridged form from the _Electrical Review_ and the
cuts from _La Lumiere Electrique.--Science_.
The weight of the conductors, says Henry Vivarez in _La Lumiere
Electrique_, plays an important part in submarine telegraphy, not
merely as a heavy item in the outlay, but as one of the principal
factors in laying down the lines, and in taking them up in case of
damage. When the conductor is being raised, the grappling-irons which
lift it have to resist not merely the vertical component of the weight
of the cable, but also the considerable effects resulting from friction
against the water. It thus frequently happens, when working at great
depths, that the conductor may be exposed to a strain greater than it
is able to bear, and we are forced to have recourse to stratagems to
bring it to the surface. These artifices consist in the use of two or
more ships in raising, which is done as shown in Figs. 2 and 3, or, in
the most simple cases, with the aid of an auxiliary buoy, as in Fig. 4.
In any event, we see that the difficulties, and of course the cost of
raising, must be considerable.
[Illustration: Fig. 1.]
Hence to decrease the weight of the cables would be an important step
in advance. If the weight is in general very great, it is because the
copper core does not take any part in the strain which the entire
cable has to resist. We know, indeed, that copper cannot bear a
breaking-strain greater, at most, than 28 kilos per square millimeter.
Besides, it would be elongated by such a strain by a very considerable
fraction of its initial length; and, if the core were made to take
part in any manner whatever in the strain which the entire cable has
to support, it would be drawn out beyond its limit of elasticity, and
would remain permanently elongated, while the substances in which it is
inclosed would return to their natural length. It would result that,
being no longer able to find room in a sheath which had become too
short, the copper wire would take a sinuous form in its gutta-percha
envelope, and would occasion at certain points ruptures, the effect
of which would be to decentralize the wire, to perforate the layer of
insulating matter, and finally to open out a fault in the cable.
But there exists an alloy (silicium bronze) which can be drawn out into
wires having a conductivity equal to that of copper, and a mechanical
resistance equal to that of the best iron. The use of this alloy would
render it possible to set free the coating of the cables from a part of
the strain which it now has to resist, and to diminish, consequently,
their dimensions and weight. Wires are now made of this alloy, having
a conductivity of from ninety-seven to ninety-nine per cent. of the
standard, which at 0°C., and with the diameter of a millimeter, have a
resistance of 20.57 ohms per kilometer. These wires do not break with a
less strain than from 45 to 48 kilos. per square millimeter, and, which
is a very precious property, their increase in length at the moment of
rupture does not exceed one or one and a half per cent.
Let us consider the deep-sea section of cable of the French company
from Paris to New York--the so-called "Pouyer-Quertier" cable,
constructed and laid in 1879 by Siemens Brothers of London.
The respective weight of each of its component elements is, per
nautical mile, copper core, 220 kilos; gutta-percha, 180 kilos;
hemp, or an equivalent, 80 kilos; 18 wires of galvanized iron of 2
millimeters in diameter, 860 kilos; external hemp and composition,
400 kilos; total, 1,740 kilos. Total diameter, 30 millimeters. Total
mechanical strength, 3,000 kilos, the wires of the covering being
supposed to be of iron. Weight under water, 450 kilos. It can support
its own weight without breaking for a length of from six to seven miles.
[Illustration: Fig. 2.]
The Atlantic presents from north to south, and at about an equal
distance from each continent, a sort of longitudinal ridge, in which
the depths vary from 300 to 400 meters. This ridge spreads out, in 50°
north latitude, into the region which has received the principal wires
connecting England and France with the United States. On both coasts
there are depressions in which the bottom is at the depth of from 4,000
to 6,000 meters. The one on the east extends from the south point of
Ireland to the latitude of the Cape of Good Hope, and its left-hand
boundary follows the general outlines of the west coasts of Europe and
Africa. The two others, the northwestern and the southwestern, form two
basins, bordering respectively on the United States and the Antilles
and South America.
In these depressions soundings have shown certain zones in which the
depths exceed 6,000 meters, the principal of which are found to the
west of the Canaries, to the south of Newfoundland, between Porto Rico
and the Bermudas, and to the right of the Isle of Marten-Vaz.
[Illustration: Fig. 3.]
The great depths of the Pacific are differently distributed. Between
Japan and California, between 40° and 50° north latitude, there is the
Tuscarora depression, which has depths of from 6,000 to 8,000 meters.
Parallel to Japan and the Kuriles there is a depression in which has
been found the greatest known depth--8,513 meters.
We see, therefore, that any new great submarine line, having to extend
into another zone than that which has received the present Atlantic
cables, must traverse depressions in which the bottom reaches a maximum
depth of 4,000 meters. The possibility of raising a damaged cable
would be very problematical under such conditions, and it would become
certainly impossible in case of a cable from San Francisco to Japan.
Under these conditions, we are forced to conclude that the use of the
present cables limits strikingly the progress of submarine telegraphy,
which must remain confined to certain zones of the Atlantic, to inland
seas, and to lines along the coasts. But if we consider the daily
progress of applied science, and the constantly increasing demand for
rapid communication between nations, it is certain that we must shortly
undertake the study of new cables intended to traverse the greatest
depths of the ocean for long distances. Necessity, therefore, compels
us to investigate the new solutions of the problem, which may furnish
us with light cables, easy to lay, and possible to repair.
[Illustration: Fig. 4.]
A cable made by Mr. J. Richards is composed as follows: core of
silicium bronze equal in weight to that of the Pouyer-Quertier cable,
or, per nautical mile, 220 kilos; gutta-percha, 180 kilos; layer of
hemp, 80 kilos. The sheathing is formed of 28 wires of galvanized
iron of 1.25 millimeters in diameter, each covered with hemp, and all
twisted into a rope around the dielectric; the wires, 500 kilos: the
hemp covering them, 250 kilos. The weight of the cable is, therefore,
1,230 kilos in the air, and 320 kilos in the water. Its diameter is 25
centimeters, and its resistance to fracture 2,800 kilos, of which the
core supports one-half. Under these conditions, the cable can support
from eight to nine nautical miles of its length, and can be raised from
the greatest depths. The results of this comparative examination are
self-evident.
For an equal conductivity and an approximately equal mechanical
strength, the new cable is in weight and bulk equal to about two-thirds
of the Pouyer-Quertier cable. It would cost about $165 less per mile,
and would require, for laying, a ship and engines of less power,
and therefore cheaper. The reduced armature will suffice to resist
friction and the attacks of animal life in the deep sea; but for the
shore ends we must keep to the types generally employed. Such as
it is, and although it may undergo modifications in detail from a
more complete study and from experience, it merits the attention of
competent engineers.