which the current flows, and at the one side or the other of the ribbon, according to the spring by which the electricity passes: and these blue marks or lines may be made to represent letters, according to their length and position on the paper. Their variations in both respects are determined either by the movements of a handle at the station sending messages, by means of which the current from a battery is interrupted, renewed, or reversed at pleasure; or by a mechanical arrangement of great ingenuity which we have not left ourselves room to describe.

Lastly, it may be mentioned, on this topic, that, from the first, much attention has been directed to the arrangement of an apparatus which should print as well as signal its messages. Many beautiful contrivances for this purpose have been devised and tried-and in no long time we may expect to see some of them in use. Descriptions of them, however, would scarcely be intelligible without drawings; and their consideration may be deferred till their adoption is ratified by public approval. The question, What is the best method of applying electricity to produce signals? is at present undergoing the keenest discussion; nor will it be speedily settled. The telegraph has not been long enough in use to enable us to decide what arrangement is best; but all competent parties are satisfied that, wonderful as its achievements are, they will yet be greatly exceeded. Our immediate object, however, is to record its present condition, not to speculate on its future improvements.

In the preceding description we have purposely referred to the simplest and most easily understood form of electric telegraph, where there is a wire reaching from the terminus at the one end of the telegraph-line, to the terminus at the other, and back again. In actual practice, however, one half of the wire is now commonly dispensed with, and its place supplied-by the earth! A century has elapsed since the very curious discovery was made, that the electricity of a charged Leyden jar or battery will pass instantaneously through a great length of moist earth. Voltaic electricity has more recently been discovered to possess the same power; and advantage has been taken of it in the following way. A wire is led from the last copper plate of a battery placed, let us suppose at London, along the telegraph posts in the way already described, to Edinburgh, and is there bent backwards towards London. Instead, however, of being carried along the posts a second time, the wire is now cut short and soldered to a large plate of metal, which is buried in the ground at some little depth. A comparatively short wire is also attached to the last zine of the London battery, and soldered to a metallic plate which is likewise buried in the ground. The arrangement

is equivalent to a great gap or breach several hundred miles long in the double wire, filled up by moist earth. When the battery is in action, the electricity (positive) flows from the copper along the wire to Edinburgh, descends there to the one earthplate (as it has been called), passes from it through the earth to the similar plate near the London station, and from it reaches the zinc of the London battery. The circulation of the electricity in this way, is found to be even more rapid than when the double wire is furnished for its passage.

Good people have perplexed themselves with speculations as to why the electricity never wanders, misses its road, or fails to find its way back. But, as has been implied already, in the case of the double wire, electricity, like a prudent general, always takes care that a retreat be provided for, before it begins its march. Till an unbroken circuit of conductors connect the terminal plates of the battery, no electricity can be set free. It is not essential, however, that those conductors should be metallic; a column or stratum of moist earth, we have seen, will do quite as well as an iron or zinc wire. One half in length of the connecting conductors must however be insulated; so that the electricity may be compelled to travel to the farthest point to which messages are to be telegraphed. But the other half of the conductors need not be insulated, and cannot be too large. quicker the current can pass the better; and it will pass most quickly when conveyed by one or other of the two great electrical conductors which man has at his disposal-the solid mass of the globe, and the ocean with its tributary waters.

The

The last allusion leads us directly to the Marine Telegraph. It requires, however, no detailed description—as it differs from the Land Telegraph only in having the space between the buried plates occupied by water instead of by earth. Broad estuaries or channels do not permit the insulated wire to be carried across by bridges. The wire therefore proceeding from the copper end of the battery is embedded in gutta percha, or any other waterproof insulator, and sunk in the waters to a depth sufficient to secure it against fishing-nets, ships, anchors, or large sea animals.

In this way it is conveyed from one shore to the other, and bending backwards after being connected with the index needles, terminates in a broad plate of metal sunk in the waves, close to the further shore. A second uninsulated wire proceeds from the zinc end of the battery to a metal plate sunk below lowwater mark, at the side from which the insulated wire set off. Between the immersed plates on the opposite shores, the mass of water, though ever changing, acts in relation to electricity as if it were an undisturbed gigantic metallic wire. Theoretically,

there is no limit to the ocean spaces which electricity may traverse in this way. Already, accordingly, schemes for telegraphing across the Atlantic and the Pacific have been triumphantly expounded to the wonder-loving public.

One of these, whether hopeless or not for immense distances, is so very ingenious, and so likely to succeed across limited spaces, that we cannot pass it unnoticed. It dispenses, except to a very trifling extent, with wires, and carries the current both ways through moist earth and water. It is desirable, for example, to telegraph from the right to the left bank of a broad river. From the copper end of a battery on the right bank, a wire is carried to the shore (on the same side) and soldered to a plate buried in the river below water mark. A wire is also led from the zinc end to a long coil of wire which ends in a metallic plate. This likewise is buried in the river below water mark on the same right bank-but at a distance from the battery considerably greater than the breadth of the river across which signals are to be sent. On the left bank two plates are immersed opposite those on the right bank, and connected by a wire. The electricity on leaving the battery has therefore the choice of two paths. It may either keep entirely on the right bank, passing from the one buried plate on that side to the other, and so back to the battery by the long coiled wire. Or it may cross to the left bank through the water, traverse the wire on that side, return across the water to the right bank, and regain the battery by the shorter coiled wire. The Thames, as we learn, has been actually crossed by electric currents in this way; the resistance to their passage by the water between the banks being less than that between the ends of the wires on the right and left bank respectively. A wire stretched from Land's End to John O'Groat's House, would indeed measure but a small portion of the breadth of the Atlantic, but by twisting the wire into coils, we might include in a short space an enormous length.

It remains to consider some of the imperfections which attend the electric telegraph, and considerably limit its useful applications. When it was first suggested as a substitute for the optical telegraph, which was useless in dark nights and in fogs or snow-storms, it was confidently anticipated that the system of electric signals would be available in all states of the weather. But this expectation has proved fallacious. For hours together the telegraph will not work. This failure is sometimes owing to the insulation of the wires along the poles having for the time been destroyed by moisture. The porcelain insulating tubes, however, are now made of such a shape, and so well protected from rain by sloping covers, that non-insulation from moisture occurs much

more rarely than might be expected. There are certain damp fogs, however, or mists, which penetrate every where; and so thoroughly wet the porcelain tubes, that they become conductors of electricity. In those circumstances it travels from the battery no further than the first wet post, down which it passes to the earth, and returns, re infecta, to the battery.

But a much more troublesome cause of inaction, or of irregular action in telegraphs, is the influence of atmospheric electricity upon them. The door left open that the friend may enter, stands open also for the foc. The insulated wires stretched along the telegraph-posts for hundreds of miles, in order that a special current of electricity evolved by a battery shall travel only in one direction, cannot, like a private road, be barred against electricity evolved from other sources. Nor is this all. When the electrician wishes to collect atmospheric electricity, he insulates a metallic wire, and suspends it in the air. In other words, he acts exactly, as the constructor of the telegraph does, though with a very different object in view. The latter, much against his will, finds that his wires not only permit, but invite atmospheric electricity to employ them as a highway. They act, in short, as lightning-conductors; and lead the formidable meteor into every station, where it deranges or destroys the coils and magnets, and occasionally menaces buildings, and even life, with destruction.

To guard against these serious evils, lightning-rods, descending to the ground, are fixed at intervals to the telegraph-posts, and at the station-houses. The sharp spikes in which these rods terminate above, being elevated considerably beyond the telegraph-wires, present points of attraction to the electricity of the clouds, so that it is determined to them rather than to the less exalted and unprojecting wires. It is thus transferred from the atmosphere to the earth without affecting the telegraph. The rods in question, however, only protect the wires in their immediate neighbourhood, and that ineffectually.

An additional and more effectual mode of protection is to place a knob of metal on each wire where it crosses the posts. A second and lower knob is then placed close to the first, but without touching it, and connected with a wire led down the post to the ground. If the lightning discharge ran along the wire, it would be cut off at the first knob it reached on the line, on reaching which it would leap across to the lower knob, and descend to the ground-while the current from the battery is found not to have sufficient intensity to overleap the space between the knobs, and hence does not descend the wire-as it would do if the knobs touched.

VOL. XC. NO. CLXXXII.

I I

An additional and very ingenious device against lightningshocks injuring the station-houses, consists in making one part of the wire which is led off to them from the main line very thin. If a powerful electrical discharge reach this, it melts it; so that the lightning, like an enemy too hasty in pursuit, burns the only bridge by which it could cross to make an attack, and remains on the safe side,— out-generalled by itself.

By one or other, or all of the methods described, sufficient protection can, on the whole, be secured, against the more familiar and more perilous effects of atmospheric electricity. Electrical disturbances, however, of a kind which do not manifest themselves in discharges of lightning, or involve life or ordinary property in danger, are quite sufficient to derange the operations of the telegraph. During snow and hail-storms, whilst dry fogs are prevailing, when the aurora borealis appears, and in truth during most meteorological changes, much electricity is developed in the atmosphere. It is sometimes directly transferred to the telegraph-wires, but as frequently its action is only indirect. A body in which free electricity is in any way developed determines a similar electrical condition in an insulated mass of metal near it, exactly as a magnet induces magnetism in pieces of iron placed in its neighbourhood. Thus an electrical cloud floating along above the extended wires generates a current of electricity in them; or, to speak more strictly, causes the electricity naturally present in a latent state in the wire, to become free and move along the metal. The currents which thus travel, as well as those which are directly transferred from the atmosphere, have the same effects on the index-needles and signal bells, as the electricity purposely sent along the wires from the battery. The needles are swung unceasingly to and fro, or remain for hours deflected to one side. The bells ring violently at irregular intervals, or stop only when their weights are run down. Signals cannot be transmitted at all when atmospheric electricity is thus largely developed; and they become more or less confused whenever it is sufficiently powerful to affect the index-needles.

Apart altogether from its practical importance, there is something exciting in the contemplation of these strange atmospheric influences. It must be not a little startling to the drowsy occupant of some solitary telegraph station, to be roused from his midnight slumber by the spectral clanging of his signal bell, bidding him quail at the wild quiverings of the magnets, now swayed plainly by no mortal hands. An imaginative man might then well recall the legends which tell of disembodied souls sent back to this earth, to divulge some great secret