again with the insulating material. The conducting power of steel is very low as compared with copper; hence the compound wire would have but little conducting capacity above that of the internal copper wire; but the induction, which varies with the area of the conductor, would be largely increased in a coated wire of this construction, as compared with a copper wire of the same conducting capacity, and this would necessitate a corresponding increase in the thickness of the insulating covering. It has been also suggested as possible, that in a considerable length of this compound wire an electric current might become divided, and that a portion would pass rapidly along the copper conductor, whilst the remainder would lag behind in the steel conductor, and that thus two currents would be exhibited at the opposite end. This objection could only be tested on a length of from 500 to 1000 miles of a cable so formed.

The conducting power of copper has been shown by Professor Thomson to vary with the purity of the metal, and it also changes with the temperature. The ordinary coppers of commerce are found to vary from pure copper as much as forty per cent. Indeed, in the case of copper wire a much greater variation has been found to exist, as is shown by experiments recently made by Mr. Mathiessen. If, for instance, the conducting power of pure copper be considered equal to 100, the conducting power of copper from Lake Superior, which contains traces of iron, silver and sub-oxide of copper, will be 92.5; that of copper from the Burra Burra mines in Australia, which contains traces of iron and sub-oxide of copper, will be 88; whilst that of Russian (Demidoff) copper, which contains arsenic, iron, nickel, and sub-oxide of copper, will be 59; and that of Spanish (Rio Tinto) copper, which contains two per cent. of arsenic, traces of lead, iron, nickel, and sub-oxide of copper, will be only 14, and is thus of lower conducting power than iron. The presence of suboxide of copper is especially injurious to the conducting power of copper, and the presence of the metalloids is as a rule more injurious than that of foreign metals. Mathiessen's valuable report on this subject, concludes by showing that no substance added to copper increases its conducting power, and that the purest obtainable copper should therefore be used in the manufacture of submarine cables. To secure this, the contract for the cable should specify a given resistance per knot, in which case every failure in quality would have to be compensated, at the manufacturer's expense, by extra thickness. Copper is not, however, a good metal to employ as a standard, because it oxydises easily, and the conducting power varies much with the temperature.

Mr.

Γ

The material which has hitherto been almost exclusively used for the insulating covering is gutta percha. This substance is a good non-conductor of electricity, and from its viscous character when warmed it adheres easily to a wire. In order to coat the wires, the gutta percha is forced out of a circular die, through the centre of which the wire is passed, and draws away with it the gutta percha covering thus forced on to it. A compound made of Stockholm tar, resin, and gutta percha, is placed on the wire before the gutta percha covering is placed on it, and also between each coating of gutta percha. In placing the gutta percha on the wire, the causes of injury to be guarded against are: 1st, air bubbles; 2nd, the eccentricity of the wire, in which a thinner layer of gutta percha exists between the wire and the surface, than at other parts; 3rd, porosity of the gutta percha; 4th; the presence of foreign bodies which connect the copper wire inside the gutta percha with the water outside; 5th, bad joints at the places where different lengths of wire are joined together; 6th, small punctures. Any of these permit the electric current flowing through a wire to pass through the gutta percha at the place where the injury occurs more easily than at other places, and the passage of the electricity generated by strong battery power, produces a chemical action which gradually destroys the gutta percha and exposes the copper wire, until in turn it also becomes eaten away. Mr. Jenkin in one of his papers observes:-

Accident put me in possession of a fault caused by an air bubble, which, I think, throws light on many recent failures of cables which tested well when first laid down. I traced it, until I ascertained its position in the gutta percha to within one inch. There were signs that the gutta percha had been heated during manufacture. Nevertheless I was unable to find any visible flaw in the gutta percha. A little white speck looked suspicious, but on wiping it away I could see no hole. During the tests (in fresh water) the fault got a little worse, but not much except perhaps for an instant on the first admission of a negative current. The current was continually reversed during the tests. I put the fault into a wineglass of salt water and tested it once more (with a negative current). I at once saw a little row of bubbles rise from the spot where the white mark had been. I took the fault out of the water almost immediately, and a little hole could now be distinctly seen. I replaced the fault in the wineglass, putting the bulb of a thermometer close to the little hole. Bubbles escaped rapidly from the fault, the mercury in the thermometer rose 5°, and in three minutes the fault had lost almost all resistance. A positive current as well as a negative current was used. The hole was now th in. long, and th broad. A hollow extending to some distance on each side, indicated the presence of an air bubble. A second hole had begun

to form a quarter of an inch off from the same cavity. The copper was visible, well placed in the centre of the gutta percha. The sudden and complete opening burnt in the gutta percha in this case was obviously due to the body of water held in the bubble; after the slight fault in the outer skin had increased to a certain point, the water was so heated by the passage of the current from the internal wire to the water outside, as to melt the surrounding gutta percha. The transition from fresh to salt water probably made the action more sudden, but would not, I think, change the sequence of facts. With such a fault as this, eighty cells are, therefore, sufficient to destroy a cable. Nevertheless, the deterioration, had the cable been laid, would not have been immediate. Probably in the two days during which I tested to find the exact position of the fault, it was subjected to as severe a trial as would be caused by a month's signalling, where reverse currents were not used. Nevertheless, in a few months the cable would have been rendered useless.'

Gutta percha varies very materially in its quality, and although termed a good non-conductor, is very far from being perfect as an insulator, inasmuch as the leakage of electricity through the material is always very large. Moreover temperature has a most important influence upon the insulating properties of gutta percha: at 32° the insulation is comparatively perfect, that is to say, the leakage is very small, at 52° the leakage is three times as great as at 32°, at 72° it is nearly six times as great, at 92° it is ten times as great. Mr. Gisborne says in his evidence before the Submarine Telegraph Committee that in laying the Red Sea cable the temperature on board ship was 92° in the hold, and the insulation was so bad, that they could not speak through the cable; but when it reached the bottom, where at 300 fathoms the temperature was 73°, the insulation materially improved.

The leakage of electricity through gutta percha renders it extremely difficult to detect faults in a great length of gutta percha covered wire (unless the fault be a very serious one), as it is in many cases almost impossible to discern whether the observed loss of electricity is due to the material or to some accidental injury. India rubber and compounds, such as Wray's, which have been proposed as substitutes for gutta percha, are comparatively unaffected by heat, until the temperature is raised to considerably over 100°, and their insulation, as compared with that of gutta percha, is almost absolutely perfect. We should, however, be cautious in discarding gutta percha for these new and untried materials. We know the faults of gutta percha from long experience. We know that india rubber does not possess these faults, but we do not know yet whether it may not possess others equally serious.

Light and air seriously affect the durability of gutta percha; chemical science shows that the deterioration which has been observed in land lines coated with gutta percha or india rubber, is due to the oxidation of the material, and that when the air is carefully excluded, as is the case in the submarine wires, no decay from natural causes need be apprehended. Indeed the gutta percha in pieces of the Dover and Calais cable laid in 1852, which were taken up during repairs in 1860, was quite as good as when laid down, and in the same manner pieces of india rubber covered wire, immersed in water for ten years by Jacobi at St. Petersburgh showed no deterioration when brought up. Mr. Hooper, who has patents for manufacturing telegraph cables of india rubber, possesses specimens of pure bottle india rubber cut into fine threads, which have lain in his office for from fifteen to twenty years, the material of which is apparently uninjured.

Gutta percha has not generally suffered from the ravages of marine animals when laid in an exposed position at the bottom of the sea. Mr. Newall mentions that in the case of a hempcovered cable which he had laid in the Mediterranean, the hempen covering was completely eaten away by the xylophaga, a species of teredo; but the gutta percha, though marked by the animal, had evidently not been found palatable, and its inroads had not been proceeded with. In the case of another cable, however, a teredo penetrated the gutta percha. But on the other hand, this curious substance is very subject to injury from friction or from pressure. The greatest care must therefore be exercised in protecting a cable from chances of injury before it is laid, and also during the process of paying out, as well as from abrasion or other mechanical injury after it has been laid down. It is moreover necessary that a cable, when laid in moderate depths, should have sufficient strength to enable it to be raised for the repairs of such faults as may have developed themselves after it has been working for a short time.

Besides the covering of gutta percha for the purpose of insulation, a telegraphic wire must be enclosed in a protecting sheath made either of iron or steel wires, or of hemp. Its form depends upon the position in which the cable is to be laid. In shallow water, where it is liable to injury from anchors or to abrasion from rocks, in consequence of storms or currents, it is necessary to make the outer covering very strong; in deep water, a lighter outer covering has been adopted. A heavy cable weighs from three or four tons to eight tons per mile, and even more. Light cables weigh under one-and-half or two tons per mile. The covering of a heavy cable consists of a

serving of hemp steeped in tar laid over the gutta percha, the hemp covering acting as a bed for large iron wires laid spirally round the core. Cables of this class have great strength, and have been successfully laid in great depths, as for instance between Spezzia and Corsica, where the depths are at least 800 fathoms. Between Cagliari and Algeria, Mr. Brett laid a cable weighing nearly four tons per mile, over a depth of 1,600 fathoms, but on reaching a depth of 400 fathoms, it became necessary, in consequence of the cable having run short from some error in the reckoning, to cease paying out, and to hold on by the cable until means to buoy the end of it could be procured from Algiers. The ship held on for five days, but a storm having then arisen, the cable broke. Had it not been for this misfortune, this heavy cable would have been laid successfully, and a light cable would have broken much sooner under similar circumstances. But whatever may be the advantage in point of strength and security in laying of heavy cables over light ones, it is obvious that when a great length of submarine cable has to be laid in deep water, its weight would be so great as to act practically as a bar to their use with ordinary ships. Moreover, in great depths, such as from 1000 to 2000 fathoms, all the evidence which has been collected goes to show that the cable, when laid, will remain undisturbed by anchors, and that probably it will not be materially affected by currents.* Whatever the strength of the cable, it would be hopeless to raise it for purposes of repair from such depths as we are now considering; there are, therefore, no reasons of this nature to call for the large extra expenditure which the heavy cable entails

* In Dr. Wallich's Notes on Animal Life in the Deep Sea,' that naturalist observes: 'It has been repeatedly laid down as a law, that, along the entire bed of the sea, wherever the depth is great, the dis'turbing influence of currents cannot take place. The evidence derived; from some of our recent soundings proves, however, that this law, although correct in a general sense, admits of exceptions. I would more particularly mention two instances which indicate that currents do occur. In a sounding taken in lat. 59° 45′ N. and 'long. 46° 30′ W., at a depth of 1204 fathoms, basaltic gravel was brought up, the pieces of which were so rounded and smooth that it is difficult to assign any other agency by which they could have 'assumed this aspect." In another sounding, taken in lat. 61° 35′ N. and long. 24° 9′ W., at 871 fathoms, it was found that lava dust must have been drifted to this position, probably from the Blinde Skier' rocks, which were about midway between it and the mainland of Iceland, by the current thus shown to be in operation.