[PRIVATE
PRESS]
SESSION 1857-58.-No. 12.
INSTITUTION OF
CIVIL ENGINEERS,
FEBRUARY 16, 1858.
JOSEPH LOCKE, ESQ., M.P.,
PRESIDENT,
IN THE CHAIR.
THE Paper read was "On Submerging
Telegraphic Cables," by Mr. J. A. Longridge, M. Inst. C.E., and Mr. C. H. Brooks. The
Authors desired their attempt to investigate the laws, to which the operation of
submerging Telegraphic Cables were subject, to be considered only as a partial solution of
an interesting and somewhat complicated problem. It was evident that much misapprehension
existed on the subject, and it had been stated in the Journals relating the proceedings at
the meeting of the British Association at Dublin, in the year 1857, that "it seemed
to be universally admitted that it was mathematically impossible, unless the speed of the
vessel, from which the cable was payed out, could be almost infinitely increased, to lay
out a cable, in deep waters, say two miles, or more, in such a way as not to require a
length much greater, than that of the actual distance, as from the inclined direction of
the yet sinking part of the cable, the successive portions payed out, must, when they
reach the bottom, arrange themselves in wavy folds, since the actual length is greater
than the entire horizontal distance."
It was desirable to ascertain how far
such a proposition was correct, and if correct, what amount of "slack," or of
surplus cable, should be provided to meet the waste, in varying depths of water.
The questions discussed in the Paper, and
of which the mathematical investigations were given in an Appendix, were:-
1. The possibility of laying out a cable straight along the bottom, in deep water, free
from the action of currents.
2. What degree of tension would be required in the process?
3. What would be the effect, as regarded strain, under the varying circumstances of the
depth of water, of the specific gravity of the cable, and of the velocity of the
paying-out vessel?
4. What would be the relative velocities of the cable and of the paying-out vessel,
requisite to reduce the strain, or tension to any given amount, and what would be the
consequent waste of cable?
5. The effect of currents, and the consequent waste of cable.
6. How far it would be necessary, or safe, to check the velocity of paying out, when
passing currents, so as to avoid, as far as possible, waste of cable?
7. Would it be safe, and if so, under what circumstances, to stop the paying-out, and to
attempt to haul in the cable from great depths?
8. The effect of the pitching of the vessel in a heavy sea.
9. The principal desiderata in the paying-out apparatus.
10. The effect of floats, or resisters.
11. The best means for saving the cable, in case of fracture.
12. The best mechanical construction of a submarine cable.
After investigating the laws of bodies,
such as cables, sinking in a resisting medium, the Paper proceeded to show the great waste
of cable attendant upon paying out free from tension at the ship. The form of the curve
assumed by a descending cable was then examined, and the amount of tension at the
paying-out vessel, requisite to lay the cable without slack along the bottom, estimated
under various conditions. The effect of the friction of the water in decreasing this
tension, and the result, as regarded the tension, of increasing the velocity of the cable
beyond that of the ship, were then pointed out. It was shewn, that the decrease thus
obtained was of small amount, unless the speed of the paying-out vessel was considerable,
and that a decrease of tension should rather be sought in a diminution of the specific
gravity of the cable.
The tension at the ship in 2,000 fathoms
water was stated to be about 35 cwt. for a cable similar to the Atlantic cable, but with a
cable of the specific gravity of 1.5 it would not exceed 7¾ cwt.
The effect of currents was then
considered, and it was maintained that they did not bring any additional strain upon a
cable, and involved only a small loss of length on first entering them. In a hypothetical
case of a current extending to a depth of 200 fathoms, and running with a velocity of 1½ foot per second, at right angles to the ship's course, it was calculated that the extra
length of cable due to the deflecting action of the current would not exceed 28 fathoms,
the velocity of the ship being 6 feet per second.
The effect of stopping the paying out was
next treated of, and it was shown that it would be to bring a very heavy catenarian strain
on the cable, depending upon the depth of water, and the velocity of the paying-out
vessel. The amount of this strain for the Atlantic cable in a depth of 2,000 fathoms, and
at a velocity of the paying-out vessel of 6 feet per second, was calculated at above seven
tons.
The question of hauling in the cable was
then adverted to, and the conditions under which it might be safely attempted, were
pointed out.
After discussing, briefly, the effect of
the pitching of the vessel upon the strain of the cable, the paying-out apparatus was
referred to; and the importance of reducing its inertia, and of so constructing the brakes
that they should act freely, was maintained. Two plans were then mentioned for saving the
end of the cable in case of fracture, and tables were given, showing the velocity and
direction taken by the end of the cable under such circumstances.
The Authors then proceeded to offer some
remarks upon the mechanical structure of the cable, and strongly advocated a light cable.
The distinguishing feature of this system of construction was, that the whole of the
metallic portion was placed in the centre, and was surrounded by the insulating material;
whereas, in the Atlantic Cable, there was an outer sheathing of wire rope twisted spirally
round the insulating medium. It was shown that whilst the absolute weights of the two
cables were as 21½ to 10, their relative strengths were as 11 to 25, so that the light
cable, weighing scarcely one-half of the heavy one, had nearly two and half times its
relative strength.
The effect of compression and tension on
the two constructions was then referred to, and it was maintained, that in this respect
also, the light cable possessed advantages over the other.
In conclusion, the Authors, while
disclaiming any intention to find fault, expressed their strong conviction, that though
the Atlantic Cable was a step in the right direction, as compared with the heavier cables
of former days, it yet fell far short, in mechanical structure and condition, of the light
system recommended by Mr. Allan and others.
The practicability of safely submerging
the present Atlantic Cable was not denied, but it was strongly urged, that with a cable of
its specific gravity, success would be greatly dependent upon the nature of the paying-out
apparatus, and the sedulous attention of those in charge of the brakes.
It was considered advisable to postpone
the Discussion on this subject until the following Paper, which was announced to be taken
at the next meeting, Tuesday, February 23rd, had been read, "On the Practical
Operations connected with the Paying Out and Repairing of Submarine Telegraph Cables, by
Mr. F. C. Webb, Assoc. Inst. C.E.
