Every two miles of the completed core were wound upon channelled drums with deep flanges, iron shod at the Tins, so that they could be rolled about and made to perform their own locomotion. When the contents of these drums were used in supplying the cable with more core, one of the ends was attached to the outgoing core of the com pressed cable, and so the contents were unrolled from the drum as the external metallic wires were spun round the core. During the unrol ling, a serving of hemp, saturated with a mixture of pitch and tar, was compactly wound round the core to act as a bed for the external me tallic sheath. Then eighteen strands, each of seven wires of charcoal iron, were twisted firmly round the core. The strands and the cable -were made by precisely analogous machinery. A large horizontal table, containing seven bobbins on the circumference in the case of the strand machine, and eighteen in the case of the closing or finishing machine, was whirled round' by steam-power with great rapidity. A central wire, or the core, was drawn up through a hole In the middle of the table, and so invested with a twisted whorl of wires or strands, given off from the bobbins as the table revolved. The strands were used in completing the cable, instead of solid wires, because by this means greater flexibility and strength, for the material used, were obtained. The external Investment of iron was solely designed to pro tect the coated core from mechanical violence during the act-of sub mergence, and to confer upon it a convenient amount of weight for effecting its sinking in the sea.
Each strand-machine, during the manufacture of the cable, was worked day and night, and In twenty-four hours spun ninety-eight miles of wire into fourteen miles of strand. The several etrand-nuichines at work simultaneously every twenty-fonr hours transformed 2058 miles of wire into 294 miles of strand. Aa much as thirty miles of cable have been made within twenty-four hours. At one time all the wire4lrawers in England proved to be unable to supply the exacting demand:, of the machinery, and the works had to pause for a short spaee. The entire length of wire, Ion and copper, spun into this wonderful structure, amemited to 332,500 miles ; a length sufficient to engirdle the earth thirteen times I The completed cable weighed from nineteen hundredweight to one ton per mile, and proved to be able to bear with impunity a direct strain of five tons. In the salt-water the weight of the cable would, however, not exceed fourteen hundredweight per Mile; and as the greatest depth of the Atlantic in which it would have to be laid is only a little more than two miles, and a certain por tion of the weight would necessarily be borne by friction against the particles of the water as the rope sank, it was anticipated that the cable would never, under any circumstances, be required to meet a strain of more than one ton and a half.
The failure of the Atlantic cable was due, in some respects, to the haste with which it was manufactured. The company had obtained, in 1854, the exclusive right, for fifty years, of landing cables on the shores of Newfoundland and Labrador, end in 1856 the British as well as the American government made a grant of 14,0001. a year, con ditional on success, the company pledging themselves to make the first attempt to lay the cable in 1857. It is now admitted that the con struction of the cable was hastily commenced, before the experiments necessary to ascertain the proper form and other conditions had been completed. " The roan ufacturo was, however, not fairly commenced till February, 1857, and 2500 miles were completed in June, 1857.
Half was made at Messrs. Newall'a works at Birkenhead, and half at Messrs. Glass and Elliot's works at Greenwich. The manufacture was very. much hurried. The portion made by :Mews. Glass and Elliot, not being under cover, suffered from exposure to heat. The testing of each coat of gutta pereha in water was recommended by Mr. White house, but this could not be performed on account of the speed at which the cable was required to be made. In the manufacture of the last 400 miles a system of testing the copper wire for its conducting power was introduced, by which an improvement of from 20 to 25 per cent in the conductivity of the cable was obtained. Messrs. Newall's half of the cable was put on board the United States' ship Niagara, and Messrs. Glass and Elliot's half wasput on board the Agamemnon ; but so backward were the preparations, that the machinery for laying the cable was being put together as the ships went round to Valentin. The break machinery was novel and cumbrous." (` Edinburgh Review,' 1861.) The object for which all these complicated arrangements are made is for the purpose of indicating at the distant station certain electric signals by visible motions or by sounds, or by marks on a ribbon of paper. In all these cases there must be a contrivance for connecting the voltaic battery or other clectroanotor with the line, in such a way as to be able to send along it a positive or a negative current at will, so as to be able to produce variety in the signals at the distant station, and having performed its work there, the current returns back to the battery by way of the earth. In this way, as already explained, the circuit is completed without the necessity of a return wire. Although a line of earth, as a conductor of electricity, is many thousand times infe rior in power to a line of metal of the same diameter, yet as the conduct power of bodies increases in proportion to the area of the section of the conductor, and the area of the conducting portion of the earth between the two stations may be indefinitely extended, we thus have a return line offering less resistance than the wire which conveys the current. All that is necessary, therefore, instead of the return wire, is to bury a large copper plate in the earth, and to connect with it a wire from the telegraphic apparatus at the end of the line. Instead of this copper plate, or rartle as it is called, the gas-pipes and water-pipes of towns are used as earths. In the Electric Telegraph Company's Cen tral Station at Lothbury, an earth was formed ley digging a hole in the foundations until moist ground was arrived at, and into this a cylinder of copper, 40 lbs. in weight, was sunk, and this was covered up with crystals of sulphate of copper. All the earth-wires of the establishment are connected with this earth-plate. The general arrangement will be made clearer by means of fig. in which Y and r. represent two telegraphic instruments, one stationed in York, another in London, connected by an insulated air-line. r Q are two earth-plates, connected by means of wires to each instrument A message, we will suppose, is being transmitted from London to York. if e z represent the bat tery at the London Station, the current will pass from c, to a wire connected with the earth-plate Q, thence through 200 miles of earth ; when at the York earth-plate, r, the current will be taken up again, and, by means of the wire, will act upon the instrument v, from which it will peas to the line, return back to London, peas through the coil of the instrument, delleCting the needle, and so back to the cud z of the battery, where the current will be completed.