described, and also in the other receiving instrument for submarine work, the zero is not fixed but movable. The vertical line on the screen is only the nominal zero. The continuous current underlying the ripples which form the message, deflects the spot from the zero-line; but this slow deflection can be disregarded by the clerk, for over and above it there are smaller quicker movements of the spot to right and left corresponding to the ripples, and these are the proper signals of the message. It requires long practice to make a good 'mirror' clerk, one who can follow the gleam with his eye through all its quick and intricate motions, and distinguish between those due to the shifting zero and those due to the various signals sent. Even this compound-ripple difficulty, however, is now got rid of by the use of an apparatus called a 'condenser,' the effects of which are that continuous currents are neutralised, and the pulsations of the signals sent are alone seen in the movements of the light upon the scale.

The other instrument is the siphon recorder, which permanently records in ink the signals which the 'mirror' only shews transiently. The principle of the siphon recorder is the converse of that of the mirror. In the mirror there is a large fixed coil and a light suspended magnet. In the siphon recorder there is a large fixed magnet and a light suspended coil. When the current passes through this coil, the latter moves much in the same way as the magnet moves in the 'mirror;' that is, it rocks to right or left according as the current flows. This rocking motion is communicated, by a system of levers and fibres, to a very fine glass capillary siphon, which dips into an ink-bottle and draws off ink upon a strip of running paper. The ink is highly electrified, so as to rush through the siphon and out upon the paper, marking a fine line upon it as it runs. When no current passes in the coil, this zero-line is straight; but when currents are passing, the line becomes zigzag and wavy; and the right and left waves across the paper constitute the message. Both of these instruments are very beautiful and ingenious applications of well-known electric, optical, and mechanical principles. The great merit of the recorder is that if a false signal is accidentally made by the sending clerk, the whole word need not always be lost by the receiving clerk, but may be made out from the rest of the word written down. Thus much repetition of messages is saved. There is some advantage too in having a written message for purposes of after reference.

A singularly ingenious system of telegraphy, termed the duplex, has recently been extended to long submarine cables, and is likely to become of general, if not universal application. It is effected by constructing an artificial line, in this case representing an artificial cable, which shall have the same influences on the current that the actual cable has. The signalling current from the battery is then split up at each station between the actual cable and the artificial cable, so that half flows into one and half into the other. And there is placed a receiving instrument in such a way between these two halves of the current that they exactly counterbalance each other's effect upon it; and so long as sending is going on from a station, the receiving instrument at that station is undisturbed. But the sending currents from the other station have the power to disturb this balance and cause signals to be made. Thus then, while the sending at a station does not affect the receiving instrument in connection with the cable there, the currents sent from the distant station cause it to mark the signals. Each station is thus enabled to send a message and receive one at the same time; and this is what is called duplex or double telegraphy.

In ordinary telegraphy, one station is receiving while the other is sending; but in duplex working, both stations are sending together and receiving together, so that there is little or no delay in the traffic, and the carrying power of a busy cable is practically doubled.

In case of accident to the cables each Company maintains a repairing-ship ready to go to sea at shortest notice. Some 'faults' are of a nature not seriously to interfere with the working of a cable; but it cannot be expected that they will remain always in the same comparatively harmless state. When a flaw occurs in the insulator it tends to enlarge itself, and more of the current escapes to the sea, until so much escapes that the current which reaches the distant station is too feeble to work the instruments there. All traffic therefore ceases. The electrician's tests having localised the fault so many miles from shore, the repairing-ship proceeds to the spot. Here she lowers her grapnel a mile or two on one side or other of the supposed line of the cable, and when enough rope has been let out, she steers very slowly under steam, or drifts with the tide across the cable's track. The grapnel is simply a great iron hook, one approved form being like a compound fish-hook, with five or six flukes starting from the shank. A weight of chain drags behind it, to keep it well down on the bottom. The rope, which is generally of wire and yarn, passes under a dynamometer, which indicates its tension, and thence to the steam winch used for hauling in. Often the grapnel catches in rocks, or mud, and gives rise to false hope of the cable having been found. The ship is brought to, and hauling in commences; but soon the obstruction 'gives,' or the grapnel itself breaks, and the true nature of the 'catch' is found out. When the cable is hooked, the greatest skill and care are needed, especially when the ship's head lifts with the waves, to bring up the bight carefully without breaking the cable. When brought to the surface the cable is cut, and each end is brought on board in turn and tested. The fault, as we have previously shewn with the paying-out ship, may prove to be but a few miles distant. The sound end is thereupon buoyed, and the ship proceeds to pick up or haul in the faulty end until it is thought the fault must have been picked up. The electrician then cuts off the piece which contains the fault, and then he has only to join on a sound piece of cable in its place, and lay it back to the end that was buoyed, so filling up the gap. But if it should not contain the fault, the tests are again applied, until finally the fault is detected and cut out. Repairing is arduous and trying work; now giving rise to hopes, now crushing them, and anon deferring them. A great responsibility rests on those who undertake them, as the gain or loss of a week or two may represent an enormous sum of money to the Company.