Popular Science Monthly/Volume 3/May 1873/Ocean-Cables
THIS is by no means a new subject for investigation, but in the present day I am certain that it will be instructive to many among the thousands who are now interested in this class of property to have their attention briefly called to all that has been done to make submarine cables a sound property.
Eleven years ago there was a joint committee appointed by the "Lords of the Committee of Privy Council for Trade and Atlantic Telegraphy, to inquire into the construction of submarine cables, together with other evidence." Attention is called in the report to the "remarkable fact that in almost all cases small cables had been found liable to mishaps, while the heavier the cable had been the greater had been its durability." The report is full and complete, and establishes principles which up to the present time have uniformly guaranteed success, while the neglect of them has as uniformly resulted in partial loss or failure.
The loss of cables was found to be attributable to the following causes: First, and the most important of all, from imperfect manufacture, resulting without doubt, prior to this date, from inexperience of the materials for insulating the copper wire, and from ignorance of the fact discovered by Prof. Thomson about 1856, viz., that some kinds of copper wire were no better than iron for the purpose of conductivity, and that it required carefully-selected copper to give the desired standard, which may be represented by a copper wire one-tenth of an inch in diameter, being equal to an iron wire one-third of an inch in diameter for electrical purposes. All cables manufactured previous to this date had no advantage from this discovery.
There appear to have been mechanical difficulties in keeping the copper conductor in the centre of the insulating medium, so that the copper was sometimes found to be almost visible under the light film of gutta-percha which covered it. The electric current soon weakened this film, stronger currents were used to overcome the weakness of the signals, and the cable was soon destroyed. Experience about this time had established that a cable from the commencement of its manufacture to the time of its being laid should be tested under water and under pressure, and kept as much as possible under all the conditions in which it was meant to continue.
Attempts to lay cables from sailing-ships towed by steamers was another source of failure. The ships had not enough steerage-way when met with strong head-winds, and too much slack was paid out. It was difficult under such circumstances to steer a straight course, and sailing-ships possessed no power of being readily stopped when a fault or accident occurred.
Many accidents happened from inexperience in the method of paying out cables; at the present day the wonder is, that they should have succeeded so well with the rude methods and inexperience which then existed, and not that there should have been many failures and much recrimination. Reading the history of these first attempts to place a network of cables at the bottom of the ocean fifteen and twenty years ago, is a good deal like reading the old stories of the early voyages of discovery. There are difficulties and disasters peculiar to every attempt, and the grand result is that, one way or another, they were overcome, or else they suggested such modifications that their recurrence was avoided, and an accident to a well-manufactured cable no longer constitutes a loss.
The first Atlantic cable failed principally on account of imperfect manufacture, in a great measure arising from undue haste and urgency, but largely owing to insufficient experience. The cable was not tested under water, for fear of rusting the small steel wires of the external covering, and small wires have never since been used; large wires, the larger the better, is now a principle. The copper was not all good. It had often been coiled and uncoiled, and had been exposed to the strong heat of the sun, and to many changes of temperature. Any of these conditions would nowadays be regarded as enough to condemn the most carefully-manufactured cable.
The Red Sea and Indian cables are said to have been imperfectly manufactured and laid too taut, but they were not tested under water from the time of manufacture until they were placed at the bottom of the sea, and this one grand omission, largely due to inexperience, is enough, without the recriminatory points, to condemn to loss and failure any cable whatever.
The cables laid from Cagliari to Malta and Malta to Corfu are said to have failed from imperfect manufacture. One experienced gentleman in his evidence said these cables were "such as nobody should have laid in deep water." It is sufficient at present to know that they have failed from neglect or inexperience, and that they, among other failures, have established the principles which have since insured success.
The want of constant supervision by engineers, exclusively in the interests of the purchasers of the cable, has been a great cause of defective cables. There may often be minute defects in the core itself, or a slightly defective splice which may reduce the electrical condition of a comparatively short length; this may easily be raised above the average standard required by the contract, by the next length being more carefully manufactured. These minute defects must, however, kill the cable in more or less time, and the principle is established that every inch should be tested in course of manufacture, and rejected if there is any irregularity of condition to cause suspicion. There should be constant supervision, and a record of all the tests kept for the purchasers of the cable from the commencement of the contract to its final completion, and continued ever afterward by the purchasers.
The principal sources of injury to cables are—first, moving water, either currents or tides, chafing the cables upon rocks or shingle. Experience has given many costly lessons of the effect of moving water.
Ten years ago it was generally believed that water had very little motion below 50 fathoms, and 100 fathoms was considered a point of great safety. We now know that there are exceptional localities where there is motion in the water at a depth of 500 fathoms. The Falmouth cable was chafed and destroyed at this depth from this cause. The Channel Islands cable was also destroyed from the same cause. The first cable ever manufactured with due regard to the principle of careful supervision, testing under water, and being retained quietly in that condition until it was laid, was the Malta and Alexandria cable, laid in 1861. This cable was submerged in too shallow water, for many miles in less depth than 20 fathoms; the result was the frequent recurrence of fracture from being rolled about by the surf, and yet this cable was only finally abandoned last year; not because it could not be kept in repair, but because it was too expensive to keep in order. These and many other examples have established the principle that no cable should be laid without first obtaining an accurate survey of the approach to the coast and landing-places, with accurate soundings over the intended route, and as much knowledge as possible of the nature of the bottom. Currents and anchorage should be avoided, and, where that is impossible, the heaviest cable that can be laid should be provided. Heavy cables should be laid out to depths of 400 fathoms, where there are tide-ways. Where a current exists, a position should be sought for as far removed from it as possible. A great cause of injury to cables is the corrosion of the external wires, caused by moving water or marine vegetation, etc., and this has established the general practice of covering the external wires with tarred yarn saturated with a mixture of pitch and silica. There is still great room for improvement upon the present method of protecting the external covering; of cables, and I commend it to the further careful study of telegraph-engineers as a subject of vital importance.
Another enemy of submarine cables is the teredo of all kinds; there is one kind which has proved destructive by boring through the core, but that has only occurred in shallow water; there is another kind which destroys the hemp in a few months, and is then satisfied to fix itself upon the gutta-percha and remain there. Cables have been recovered from depths of 1,200 fathoms with all the hemp eaten away, and the core pitted with these marine animals. The recovery is then only possible by the strength of the external wires.
All the experience we. have points to the value of protection, first, of the core, then of the external covering, and, if those responsible for the safety and maintenance of submarine cables could be allowed to dictate the most desirable conditions of safety, they would select, besides the strongest possible cable to be manufactured, and laid with extreme care, a depth of water of about 500 fathoms, and a bottom of sand or mud; but, as this cannot always be secured, nothing should be omitted in the direction of strength and quality.
Lightning is still another source of injury to cables; this is, however, so readily guarded against that we no longer hear of injury from this cause: it is said to have destroyed three cables. Mr. Siemens produced before the committee a piece of the core of the Corfu cable injured by lightning; the land-line had been struck, and, from the absence of any lightning-guards, the cable was damaged. Mr. Preece described the Jersey cable as having been destroyed by lightning. Mr. Fleeming Jenkin has seen a fault 18 inches long due to this cause, and it is asserted that the same cause destroyed the Toulon-Algiers cable, which was connected to the land-lines without lightning-guards.
We are every now and then startled by the announcement that light cables are to be preferred to the present iron-clad type, and the object of this investigation has been to discover what data there are to justify any preference to one form of cable over another. I have said already that the committee called attention to the remarkable fact that, in almost all cases, small cables have been found liable to mishaps, while the heavier the cable the greater had been its durability.
Mr. Newall, in his evidence, said that the hemp-covered cable which he attempted to lay in 1859, between Candia and Egypt, had the hemp eaten off by the teredo in a very short time, and it was too weak to recover for repairing. The same firm laid an unprotected core from Varna to the Crimea, and it lasted until the winter set in; it is frequently said that it was cut by order of the French commander-in-chief, but there is no proof of this, and I am not disposed to believe it. Mr. Woodehouse, the engineer who laid this core, said in his evidence he "should not advise anybody to lay so light a cable across the Atlantic, because so small a strain would break it. If it is once safe at the bottom, perhaps it may rest." Mr. Newall said he thought it folly to lay any thing excepting unprotected core. Consistently with this conviction, he laid in 1869 several lines of unprotected India-rubber core, connecting the Grecian islands with the main-land; they were protected only near the shore. The sea is quiet and tideless in those parts; no better spot could be wished for the experiment, yet they every one of them gave out within two years.
The Red Sea cable, covered externally with light wires, and unprotected with bituminous compound, was so rusted in a short time that it could not be lifted for repairs.
Notwithstanding, Mr. Newall's partiality for light cables, he suggests at the close of his evidence what I assume he would consider the most perfect form of cable. He would cover the copper with India-rubber, protect this core with steel wires vulcanized, the whole then passed through heat; thus insulating all the wires, he would make the cable in one length, and have no joints. Mr. Fleeming Jenkin, in his report to the International Exhibition of 1862, says:
Every word of this can be written at the present moment, that is, ten years later, with exactly the same significance. All cables which have been manufactured and laid upon the principles which were established in 1859 are yet in good working order, and every divergence from these principles has been at best but a costly experiment or utter failure. There is no instance yet of a well-manufactured heavy cable breaking or giving out in deep water after it has been carefully laid free from defects; but there may be much due to the external covering keeping it quiet; there has assuredly been a great deal due to the external covering in the successful submerging, and there is no experience whatever to justify the assumption that an unprotected core would last, even if laid.
It has been urged that an iron-covered cable, suspended from one point to another, gradually becomes weaker, that rust and marine growth or deposit accumulate and break the cable with their weight; but I do not know of any instance in support of the assumption, nor is it at all certain that a simple unprotected core would exist for any length of time, or be in any way better adapted for the supposed conditions. Mr. Latimer Clark, in his evidence, says: "You want a certain degree of weight to enable your cable to sink steadily to the bottom, especially when it has to fall into hollows and cavities, and not lie loosely across elevations."
Again, it is urged that experiments with light cables have been tried in factories or sheds, and the result proves that there are many advantages in their favor; but I am of opinion that no experiments which can be made on shore will sufficiently resemble the exigencies which may occur over a period of several days and nights at sea in storms and darkness, and still less will they prove their fitness for the unknown conditions which may exist at great ocean-depths. I desire to write with great respect for the opinions of the talented men who urge the adoption of light cables; it is my special duty to weigh well and without prejudice all they have to advance; but I think a careful investigation into the experience and practice of the last twenty years establishes conclusively that all light cables have been short-lived, and that all heavy cables have continued working, often under most adverse conditions. It is my own opinion, and I am authorized to say that it is also the opinion of my friend Captain Halpin, who has laid all the cables from Suez to Australia, besides the French Atlantic cable (11,000 miles), and has also recovered and repaired cables from a great variety of depths, that a cable should be as heavy as it can be laid with safety, and admit of being recovered in case of accident. Multiply every precaution which shall increase the strength and keep that strength intact as long as possible.
The best form of light cable I have seen is the copper-covered core invented by Mr. Siemens (No. 8). I should have anticipated that, if any light cable could have been successful, this one would have met all the conditions, excepting that of extreme cheapness, but it has not been so uniformly successful as the heavy iron-clad cables. The very light cable invented by Mr. Varley (No. 21) admits of being laid by having the strain taken off the core by the two hempen strands, the core itself being the third strand of the cable. As a light cable, to be manufactured in a great hurry, and laid to meet some emergency, it has a good deal of merit, but for a deep-sea cable I am of opinion that it would be found too incomplete and unfinished, and that difficulties would be experienced in laying which are not at once foreseen, and that there would be no durability even if successfully laid.
Every day of my experience in watching over the permanence of the 10,000 miles of cable under my care, confirms me in the opinion that too great caution and vigilance cannot be exercised in the making and laying a thread which is to be removed from all human vision forever, and designed to earn dividends by continuing a perfect conductor of electricity. Upward of 30,000 miles of cable have been laid since the report of the committee was printed, eleven years ago, and much experience has been gained of the exigencies incidental to submerging, buoying, grappling, and repairing; but no fact has resulted from all that experience which has established that any one precaution recommended in the report has been superfluous, whereas much has occurred, which I will not particularize, proving that any attempt to disregard any single precaution has resulted in great pecuniary loss or utter failure.
We have many reasons to confirm the belief that a submarine cable, manufactured and laid with strict attention to all known principles, may be regarded as a substantial property, likely to last for any length of time; for there is no evidence whatever upon record which shows any decay of the insulating medium or copper conductor of a well-manufactured cable, i. e., there is no decay inherent in the nature of a cable; all deterioration is external; nor is there any experience whatever to establish that this insulated copper wire will enjoy any durability if unprotected with an external covering.
A light cable or unprotected core must therefore be regarded at best as an experiment, with the chances against the successful laying, and still more against its existing as a permanent property.
I have written enough to illustrate that the present submarine cable is not a haphazard idea, but one which has grown out of many failures and thousands of experiments; all the principles of manufacture and laying down have been established by great anxiety and reflection on the part of the able men who gave their energies to this kind of enterprise prior to 1865. We who have come upon the stage since that date have only discovered that we may not neglect one of all the known principles, but elaborate every one of them, and even then the duty of laying and maintaining this class of property has enough of risks and anxieties to make one heartily dislike any experiment which can only be advocated for the sake of cheapness in the first cost. I believe this economy would be at the expense of security, and that the cable of the future will be even heavier, more perfect, and more costly, than the cable of the present day.—Abstract of Address before the Statistical Society.
- See article, in this number, on the "Borers of the Sea."