In 1865, new capital having been raised, preparations were made for another expedition. It was now decided to use only one vessel for laying the cable, and the Great Eastern was chosen for the task. This vessel had been lying idle for close on ten years, owing to her failure as a cargo boat, but her great size and capacity made her most suitable for carrying the enormous weight of the whole cable. In July 1865 the Great Eastern set sail, under the escort of two British warships. When 84 miles had been paid out, a fault occurred, and after drawing up about 10½ miles it was found that a piece of iron wire had pierced the coating of the cable. The trouble was put right, and the paying-out continued successfully until over 700 miles had been laid, when another fault appeared. The cable was again drawn in until the fault was reached, and another piece of iron was found piercing clean through. It was evident that two such pieces of iron could not have got there by accident, and there was no doubt that they had been inserted intentionally by some malicious scoundrel, most likely with the object of affecting the company’s shares. A start was made once more, and all went well until about two-thirds of the distance had been covered, when the cable broke and had to be abandoned after several nearly successful attempts to recover it.
In spite of the loss, which amounted to £600,000, the energetic promoters contrived to raise fresh capital, and in 1866 the Great Eastern started again. This effort was completely successful, and on 28th July 1866 the cable was landed amidst great rejoicing. The following extracts from the diary of the engineer Sir Daniell Gooch, give us some idea of the landing.
“Is it wrong that I should have felt as though my heart would burst when that end of our long line touched the shore amid the booming of cannon, the wild, half-mad cheers and shouts of the men?... I am given a never-dying thought; that I aided in laying the Atlantic cable.... The old cable hands seemed as though they could eat the end; one man actually put it into his mouth and sucked it. They held it up and danced round it, cheering at the top of their voices. It was a strange sight, nay, a sight that filled our eyes with tears.... I did cheer, but I could better have silently cried.”
This time the cable was destined to have a long and useful life, and later in the same year the 1865 cable was recovered, spliced to a new length, and safely brought to land, so that there were now two links between the Old World and the New. It was estimated that the total cost of completing the great undertaking, including the cost of the unsuccessful attempts, was nearly two and a half millions sterling. Since 1866 cable-laying has proceeded very rapidly, and to-day telegraphic communication exists between almost all parts of the civilized world. According to recent statistics, the North Atlantic Ocean is now crossed by no less than 17 cables, the number of cables all over the world being 2937, with a total length of 291,137 nautical miles.
Before describing the actual working of a submarine cable, a few words on cable-laying may be of interest. Before the cable-ship starts, another vessel is sent over the proposed course to make soundings. Galvanized steel pianoforte wire is used for sounding, and it is wound in lengths of 3 or 4 nautical miles on gun-metal drums. The drums are worked by an engine, and the average speed of working is somewhere about 100 fathoms a minute in descending, and 70 fathoms a minute in picking up. Some idea of the time occupied may be gained from a sounding in the Atlantic Ocean which registered a depth of 3233 fathoms, or nearly 3½ miles. The sinker took thirty-three minutes fifty seconds in descending, and forty-five minutes were taken in picking up. The heavy sinker is not brought up with the line, but is detached from the sounder by an ingenious contrivance and left at the bottom. The sounder is fitted with an arrangement to bring up a specimen of the bottom, and also a sample of water; and the temperature at any depth is ascertained by self-registering thermometers.
When the soundings are complete the cable-ship takes up her task. The cable is coiled in tanks on board, and is kept constantly under water to prevent injury to the gutta-percha insulation by overheating. As each section is placed in the tank, the ends of it are led to a test-box, and labelled so that they can be easily recognized. Insulated wires run from the test-box to instruments in the testing-room, so that the electrical condition of the whole cable is constantly under observation. During the whole time the cable is being laid its insulation is tested continuously, and at intervals of five minutes signals are sent from the shore end to the ship, so that a fault is instantly detected. The cable in its tank is eased out by a number of men, and mechanics are posted at the cable drums and brakes, while constant streams of water cool the cable and the bearings and surfaces of the brakes. The tension, as shown by the dynamometer, is at all times under careful observation. When it becomes necessary to wind back the cable on account of some fault, cuts are made at intervals of a quarter or half a mile, tests being made at each cutting until the fault is localized in-board. As soon as the cable out-board is found “O.K.,” the ends are spliced up and the paying-out begins again. If the cable breaks from any cause, a mark-buoy is lowered instantly on the spot, and the cable is grappled for. This may take a day or two in good weather, but a delay of weeks may be caused by bad weather, which makes grappling impossible.
The practical working of a submarine cable differs in many respects from that of a land telegraph line. The currents used in submarine telegraphy are extremely small, contrary to the popular impression. An insulated cable acts like a Leyden jar, in the sense that it accumulates electricity and does not quickly part with it, as does a bare overhead wire. In the case of a very long cable, such as one across the Atlantic, a current continues to flow from it for some time after the battery is disconnected. A second signal cannot be sent until the electricity is dissipated and the cable clear, and if a powerful current were employed the time occupied in this clearing would be considerable, so that the speed of signalling would be slow. Another objection to a powerful current is that if any flaw exists in the insulation of the cable, such a current is apt to increase the flaw, and finally cause the breakdown of the line.
The feebleness of the currents in submarine telegraphy makes it impossible to use the ordinary land telegraph receiver, and a more sensitive instrument known as the “mirror receiver” is used. This consists of a coil of very fine wire, in the centre of which a tiny magnetic needle is suspended by a fibre of unspun silk. A magnet placed close by keeps the needle in one position when no current is flowing. As the deflections of the needle are extremely small, it is necessary to magnify them in some way, and this is done by fixing to the needle a very small mirror, upon which falls a ray of light from a lamp. The mirror reflects this ray on to a sheet of white paper marked with a scale, and as the mirror moves along with the needle the point of light travels over the paper, a very small movement of the needle causing the light to travel some inches. The receiving operator sits in a darkened room and watches the light, which moves to the right or to the left according to the direction of the current. The signals employed are the same as those for the single-needle instrument, a movement to the left indicating a dot, and one to the right a dash. In many instruments the total weight of magnet and mirror is only two or three grains, and the sensitiveness is such that the current from a voltaic cell consisting of a lady’s silver thimble with a few drops of acidulated water and a diminutive rod of zinc, is sufficient to transmit a message across the Atlantic.
The mirror receiver cannot write down its messages, and for recording purposes an instrument invented by Lord Kelvin, and called the “siphon recorder,” is used. In this instrument a coil of wire is suspended between the poles of an electro-magnet, and to it is connected by means of a silk fibre a delicate glass tube or siphon. One end of the siphon dips into an ink-well, and capillary attraction causes the ink to fill the siphon. The other end of the siphon almost touches a moving paper ribbon placed beneath it. The ink and the paper are oppositely electrified, and the attraction between the opposite charges causes the ink to spurt out of the siphon in very minute drops, which fall on to the paper. As long as no current is passing the siphon remains stationary, but when a current flows from the cable through the coil, the latter moves to one side or the other, according to the direction of the current, and makes the siphon move also. Consequently, instead of a straight line along the middle of the paper ribbon, a wavy line with little peaks on each side of the centre is produced by the minute drops of ink. This recorder sometimes refuses to work properly in damp weather, owing to the loss of the opposite charges on ink and paper, but a later inventor, named Cuttriss, has removed this trouble by using a siphon kept constantly in vibration by electro-magnetism. The ordinary single-needle code is used for the siphon recorder.