Now, as to the prospect of success in another attempt to lay a telegraph across the ocean. The most erroneous opinions prevail as to the difficulties of laying submarine telegraphs in general, and securing them against injury. It is commonly supposed that the number of failures is much greater than of successes; whereas the fact is, that the later attempts, where made with proper care, have been almost uniformly successful. In proof of this I will refer to the printed "List of all the Submarine Telegraph-Cables manufactured and laid down by Messrs. Glass, Elliot, & Co., of London," from which it appears that within the space of eight years, from 1854 to 1862, they have manufactured and laid down twenty-five different cables, among which are included three of the longest lines connecting England with the Continent,—namely, from England to Holland, 140 miles, to Hanover, 280 miles, and to Denmark, 368 miles,—and the principal lines in the Mediterranean,—as from Italy to Corsica and thence to Toulon, from Malta to Sicily, and from Corfu to Otranto, and besides these, the two chief of all, that from France to Algiers, 520 miles, laid in 1860, and the other, laid only last year, from Malta to Alexandria, 1,535 miles! All together the lines laid by these manufacturers comprise a total of 3,739 miles; and though some have been lying at the bottom of the sea and working for eight years, each one of them is at this hour in as perfect condition as on the day it was laid down, with the exception of the two short lines laid in shallow water along the shore between Liverpool and Holyhead, 25 miles, and from Prince Edward's Island to New Brunswick, 11 miles; the latter of which was broken by a ship's anchor, and the former by the anchor of the Royal Charter during the gale in which she was wrecked, both of which can be easily repaired.
Where failures have occurred in submarine telegraphs, the causes are now well understood and easily to be avoided. Thus with the first Atlantic cable, its defects have all been carefully investigated by scientific men, and may be easily guarded against. When this cable was in process of manufacture in the factory of Messrs. Glass, Elliot, & Co., in Greenwich, near London, it was coiled in four large vats, and there left exposed, day after day, to the heat of a summer sun, which was intensified by the tarred coating of the cable to one hundred and twenty degrees. This went on, day after day, with the knowledge of the engineer and electrician of the company, although the directors had given explicit orders that sheds should be erected over the vats to prevent the possibility of such an occurrence. As might have been foreseen, the gutta-percha was melted, so that the conductor which it was desired to insulate was so twisted by the coils that it was left quite bare in numberless places, thus weakening, and eventually, when the cable was submerged, destroying the insulation. The injury was partially discovered before the cable was taken out of the factory at Greenwich, and a length of about thirty miles was cut out and condemned. This, however, did not wholly remedy the difficulty, for the defective insulation became frequently and painfully apparent while the cable was being submerged. Still further evidence of its imperfect condition was afforded when it came to be cut up for charms and trinkets.
The first cable was, to a great extent, an experiment,—a leap in the dark. Its material and construction were as good as the state of knowledge at that time provided, and in many respects not unsuitable; but the company could not avail itself, at that time, of the instruments or apparatus for testing its conducting power and insulation, in the manner since pointed out by experience. The effects of temperature, as we have seen, were not provided for. The vast differences in the conducting power of copper were discovered only by means of that cable, when made. The mathematical law whereby the proportions of insulation to conduction are determined had not been fully investigated; and it was even argued by some of the pretended electricians in the employ of the company, that, the smaller the conductor, the more rapidly the current could pass through it. No mode of protecting the external sheath from oxidation had then been discovered; and the kind of machinery necessary for submerging cables in deep water could only be theoretically assumed.
Looking back to that period, and granting that there was too much haste in the preparations, and that other mistakes were committed which could now be foreseen and avoided, it is not too much to say, that, if that cable could be laid and worked, as was done, after one failure in 1857, and the consequent uncoiling and storage of it in an exposed situation, and after three attempts in 1858, under the most fearful circumstances as to weather, it would be an easy task to lay a cable constructed and submerged by the light of present experience.
[Illustration: The Cable laid in 1858.]
[Illustration: The proposed New Cable.]
The above cuts, representing sections of the cable laid in 1858 and the proposed new cable, will serve to show the difference between the two, and the immense superiority of the latter over the former. In the old Atlantic cable the copper conducting-wire weighed but ninety-three pounds to the mile, while in the new cable it weighs five hundred and ten pounds to the mile, or more than five times as much. Now the size, or diameter, of a telegraphic conductor is just as important an item, in determining the strength of current which can be maintained upon it with a given amount of battery-force, as the length of the conductor. To produce the effects by which the messages are expressed at the end of a telegraphic wire or cable, it is necessary that the electric current should have a certain intensity or strength. Now the intensity of the current transmitted by a given voltaic battery along a given line of wire will decrease, other things being the same, in the same proportion as the length of the wire increases. Thus, if the wire be continued for ten miles, the current will have twice the intensity which it would have, if the wire had been extended to a distance of twenty miles. It is evident, therefore, that the wire may be continued to such a length that the current will no longer have sufficient intensity to produce at the station to which the despatch is transmitted those effects by which the language of the despatch is signified. But the intensity of the current transmitted by a given voltaic battery upon a wire of given length will be increased in the same proportion as the area of the section of the wire is augmented. Thus, if the diameter of the wire be doubled, the area of its section being increased in a fourfold proportion, the intensity of the current transmitted along the wire will be increased in the same ratio. The intensity of the current may also be augmented by increasing the number of pairs of the generating plates or cylinders composing the galvanic battery.
All electrical terms are arbitrary, and necessarily unintelligible to the general reader. I shall, therefore, use them as sparingly as possible, and endeavor to make myself clearly understood by explaining those which I do use.
All telegraphic conductors offer a certain resistance to the passage of an electric current, and the amount of this resistance is proportional to the length of the conductor, and inversely to its size. In order to overcome this resistance, it is necessary to increase the number of the cells in the battery, and thus obtain a fluid of greater force or intensity.
On aërial telegraph-lines this increase in the intensity of the battery occasions no particular inconvenience, other than by tending to the more rapid destruction of the small copper coils, or helices, employed; but upon submarine lines it has the effect of increasing the static electricity, or electricity of tension, which accumulates along the surface of the gutta-percha covering of the conducting-wire, in the same manner as static electricity accumulates on the surface of glass, or of a stick of sealing-wax, by rubbing it with a piece of cloth. The use of submarine or of subterranean conductors occasions, from the above cause, a small retardation in the velocity of the transmitted electricity. This retardation is not due to the length of the path which the electric current has to traverse, since it does not take place with a conductor, equally long, insulated in the air; but it arises from a static reaction, caused by the passage of an intense current through a conductor well insulated, but surrounded outside its insulating coating by a conducting body, such as sea-water or moist ground, or even by the metallic envelope of iron wires placed in communication with the ground. When this conductor is presented to one of the poles of a battery, the other pole of which communicates with the ground, it becomes charged with static electricity, like the coating of a Leyden-jar,—electricity which is capable of giving rise to a discharge-current, even after the voltaic current has ceased to be transmitted. Volta showed in one of his beautiful experiments, that, in putting one of the ends of his pile in communication with the earth, and the other with a non-insulated Leyden-jar, the jar was charged in an instant of time to a degree proportional to the force of the pile. At the same time an instantaneous current was observed in the conductor between the pile and the jar, which had all the properties of an ordinary current. Now it is evident that the subaqueous wire with its insulating covering may be assimilated exactly to an immense Leyden-jar. The glass of the jar represents the gutta-percha; the internal coating is the surface of the copper wire; the external coating is the surrounding metallic envelope and water. To form an idea of the capacity of this new kind of battery, we have only to remember that the surface of the wire is equal to fourteen square yards per mile. Bringing such a wire into communication by one of its ends with a battery, of which the opposite pole is in contact with the earth, whilst the other extremity of the wire is insulated, must cause the wire to take a charge of the same character and tension as that of the pole of the battery touched by it.