Within the last few years Electric Telegraphy has received some developments which seem wonderful even by comparison with those other wonders which had before been achieved by this method of communication. In reality, all the marvels of electric telegraphy are involved, so to speak, in the great marvel of electricity itself, a phenomenon as yet utterly beyond the interpretation of physicists, though not more so than its fellow marvels, light and heat. We may, indeed, draw a comparison between some of the most wonderful results which have recently been achieved by the study of heat and light and those effected in the application of electricity to telegraphy. It is as startling to those unfamiliar with the characteristics of light, or rather with certain peculiarities resulting from these characteristics, to be told that an astronomer can tell whether there is water in the air of Mars or Venus, or iron vapour in the atmosphere of Aldebaran or Betelgeux, as it is to those unfamiliar with the characteristics of electricity, or with the results obtained in consequence of these characteristics, to be told that a written message can be copied by telegraph, a map or diagram reproduced, or, most wonderful of all, a musical air correctly repeated, or a verbal message made verbally audible. Telegraphic marvels such as these bear to the original233 marvel of mere telegraphic communication, somewhat the same relation which the marvels of spectroscopic analysis as applied to the celestial orbs bear to that older marvel, the telescopic scrutiny of those bodies. In each case, also, there lies at the back of all these marvels a greater marvel yet—electricity in the one case, light in the other.

I propose in this essay to sketch the principles on which some of the more recent wonders of telegraphic communication depend. I do not intend to describe at any length the actual details or construction of the various instruments employed. Precisely as the principles of spectroscopic analysis can be made clear to the general reader without the examination of the peculiarities of spectroscopic instruments, so can the methods and principles of telegraphic communication be understood without examining instrumental details. In fact, it may be questioned whether general explanations are not in such cases more useful than more detailed ones, seeing that these must of necessity be insufficient for a student who requires to know the subject practically in all its details, while they deter the general reader by technicalities in which he cannot be expected to take any interest. If it be asked, whether I myself, who undertake to explain the principles of certain methods of telegraphic communication, have examined practically the actual instrumental working of these methods, I answer frankly that I have not done so. As some sort of proof, however, that without such practical familiarity with working details the principles of the construction of instruments may be thoroughly understood, I may remind the reader (see p. 96) that the first spectroscopic battery I ever looked through—one in which the dispersive power before obtained in such instruments had been practically doubled—was of my own invention, constructed (with a slight mechanical modification) by Mr. Browning, and applied at once successfully to the study of the sun by Mr. Huggins, in whose observatory I saw through this instrument the solar spectrum extended to a length which, could it all have been seen at once, would have234 equalled many feet.27 On the other hand, it is possible to have a considerable practical experience of scientific instruments without sound knowledge of the principles of their construction; insomuch that instances have been known in which men who have effected important discoveries by the use of some scientific instrument, have afterwards obtained their first clear conception of the principles of its construction from a popular description.

It may be well to consider, though briefly, some of the methods of communication which were employed before the electric telegraph was invented. Some of the methods of electric telegraphy have their antitypes, so to speak, in methods of telegraphy used ages before the application of electricity. The earliest employment of telegraphy was probably in signalling the approach of invading armies by beacon fires. The use of this method must have been well known in the time of Jeremiah, since he warns the Benjamites “to set up a sign of fire in Beth-haccerem,” because “evil appeareth out of the north and great destruction.” Later, instead of the simple beacon fire, combinations were used. Thus, by an Act of the Scottish Parliament in 1455, the blazing of one bale indicated the probable approach of the English, two bales that they were coming indeed, and four bales blazing beside each other that they were in great force. The smoke of beacon fires served as signals by day, but not so effectively, except under very favourable atmospheric conditions.

Torches held in the hand, waved, depressed, and so forth, were anciently used in military signalling at night; while in the day-time boards of various figures in different positions indicated either different messages or different letters, as might be pre-arranged.

Hooke communicated to the Royal Society in 1684 a paper describing a method of “communicating one’s mind235 at great distances.” The letters were represented by various combinations of straight lines, which might be agreed upon previously if secrecy were desired, otherwise the same forms might represent constantly the same letters. With four straight planks any letter of this alphabet could be formed as wanted, and being then run out on a framework (resembling a gallows in Hooke’s picture), could be seen from a distant station. Two curved beams, combined in various ways, served for arbitrary signals.

Chappe, in 1793, devised an improvement on this in what was called the T telegraph. An upright post supported a cross-bar (the top of the T), at each end of which were the short dependent beams, making the figure a complete Roman capital T. The horizontal bar as first used could be worked by ropes within the telegraph-house, so as to be inclined either to right or left. It thus had three positions. Each dependent beam could be worked (also from within the house) so as to turn upwards, horizontally, or downwards (regarding the top bar of the T as horizontal), thus having also three positions. It is easily seen that, since each position of one short beam could be combined with each position of the other, the two together would present three times three arrangements, or nine in all; and as these nine could be given with the cross-bar in any one of its three positions, there were in all twenty-seven possible positions. M. Chappe used an alphabet of only sixteen letters, so that all messages could readily be communicated by this telegraph. For shorter distances, indeed, and in all later uses of Chappe’s telegraph, the short beams could be used in intermediate positions, by which 256 different signals could be formed. Such telegraphs were employed on a line beginning at the Louvre and proceeding by Montmartre to Lisle, by which communications were conveyed from the Committee of Public Welfare to the armies in the Low Countries. Telescopes were used at each station. Barrère stated, in an address to the Convention on August 17, 1794, that the news of the recapture of Lisle had been sent by236 this line of communication to Paris in one hour after the French troops had entered that city. Thus the message was conveyed at the rate of more than 120 miles per hour.

Various other devices were suggested and employed during the first half of the present century. The semaphores still used in railway signalling illustrate the general form which most of these methods assumed. An upright, with two arms, each capable of assuming six distinct positions (excluding the upright position), would give forty-eight different signals; thus each would give six signals alone, or twelve for the pair, and each of the six signals of one combined with each of the six signals of the other, would give thirty-six signals, making forty-eight in all. This number suffices to express the letters of the alphabet (twenty-five only are needed), the Arabic numerals, and thirteen arbitrary signals.

The progress of improvement in such methods of signalling promised to be rapid, before the invention of the electric telegraph, or rather, before it was shown how the principle of the electric telegraph could be put practically into operation. We have seen that they were capable of transmitting messages with considerable rapidity, more than twice as fast as we could now send a written message by express train. But they were rough and imperfect. They were all, also, exposed to one serious defect. In thick weather they became useless. Sometimes, at the very time when it was most important that messages should be quickly transmitted, fog interrupted the signalling. Sir J. Barrow relates that during the Peninsular War grave anxiety was occasioned for several hours by the interruption of a message from Plymouth, really intended to convey news of a victory. The words transmitted were, “Wellington defeated;” the message of which these words formed the beginning was: “Wellington defeated the French at,” etc. As Barrow remarks, if the message had run, “French defeated at,” etc., the interruption of the message would have been of less consequence.

237 Although the employment of electricity as a means of communicating at a distance was suggested before the end of the last century, in fact, so far back as 1774, the idea has only been worked out during the last forty-two years. It is curious indeed to note that until the middle of the present century the word “telegraph,” which is now always understood as equivalent to electric telegraph, unless the contrary is expressed, was commonly understood to refer to semaphore signalling,28 unless the word “electric” were added.

The general principle underlying all systems of telegraphic communication by electricity is very commonly misunderstood. The idea seems to prevail that electricity can be sent out along a wire to any place where some suitable arrangement has been made to receive it. In one sense this is correct. But the fact that the electricity has to make a circuit, returning to the place from which it is transmitted, seems not generally understood. Yet, unless this is understood, the principle, even the possibility, of electric communication is not recognized.

Let us, at the outset, clearly understand the nature of electric communication.

In a variety of ways, a certain property called electricity can be excited in all bodies, but more readily in some than in others. This property presents itself in two forms, which are called positive and negative electricity, words which we may conveniently use, but which must not be regarded as representing any real knowledge of the distinction between these two kinds of electricity. In fact, let it be remembered throughout, that we do not in the least know what electricity238 is; we only know certain of the phenomena which it produces. Any body which has become charged with electricity, either positive or negative, will part with its charge to bodies in a neutral condition, or charged with the opposite electricity (negative or positive). But the transference is made much more readily to some substances than to others—so slowly, indeed, to some, that in ordinary experiments the transference may be regarded as not taking place at all. Substances of the former kind are called good conductors of electricity; those which receive the transfer of electricity less readily are said to be bad conductors; and those which scarcely receive it at all are called insulating substances. The reader must not confound the quality I am here speaking of with readiness to become charged with electricity. On the contrary, the bodies which most freely receive and transmit electricity are least readily charged with electricity, while insulating substances are readily electrified. Glass is an insulator, but if glass is briskly rubbed with silk it becomes charged (or rather, the part rubbed becomes charged) with positive electricity, formerly called vitreous electricity for this reason; and again, if wax or resin, which are both good insulators, be rubbed with cloth or flannel, the part rubbed becomes charged with negative, formerly called resinous, electricity.

Electricity, then, positive or negative, however generated, passes freely along conducting substances, but is stopped by an insulating body, just as light passes through transparent substances, but is stopped by an opaque body. Moreover, electricity may be made to pass to any distance along conducting bodies suitably insulated. Thus, it might seem that we have here the problem of distant communication solved. In fact, the first suggestion of the use of electricity in telegraphy was based on this property. When a charge of electricity has been obtained by the use of an ordinary electrical machine, this charge can be drawn off at a distant point, if a conducting channel properly insulated connects that point with the bodies (of whatever nature)239 which have been charged with electricity. In 1747, Dr. Watson exhibited electrical effects from the discharges of Leyden jars (vessels suitably constructed to receive and retain electricity) at a distance of two miles from the electrical machine. In 1774, Le Sage proposed that by means of wires the electricity developed by an electrical machine should be transmitted by insulated wires to a point where an electroscope, or instrument for indicating the presence of electricity, should, by its movements, mark the letters of the alphabet, one wire being provided for each letter. In 1798 Béthencourt repeated Watson’s experiment, increasing the distance to twenty-seven miles, the extremities of his line of communication being at Madrid and Aranjuez. (Guillemin, by the way, in his “Applications of the Physical Forces,” passes over Watson’s experiment; in fact, throughout his chapters on the electric telegraph, the steam-engine, and other subjects, he seems desirous of conveying as far as possible the impression that all the great advances of modern science had their origin in Paris and its neighbourhood.)

From Watson’s time until 1823 attempts were made in this country and on the Continent to make the electrical machine serve as the means of telegraphic communication. All the familiar phenomena of the lecture-room have been suggested as signals. The motion of pith balls, the electric spark, the perforation of paper by the spark, the discharge of sparks on a fulminating pane (a glass sheet on which pieces of tinfoil are suitably arranged, so that sparks passing from one to another form various figures or devices), and other phenomena, were proposed and employed experimentally. But practically these methods were not effectual. The familiar phenomenon of the electric spark explains the cause of failure. The spark indicates the passage of electricity across an insulating medium—dry air—when a good conductor approaches within a certain distance of the charged body. The greater the charge of electricity, the greater is the distance over which the electricity will thus make its escape. Insulation, then, for many miles of wire, and still240 more for a complete system of communication such as we now have, was hopeless, so long as frictional electricity was employed, or considerable electrical intensity required.

We have now to consider how galvanic electricity, discovered in 1790, was rendered available for telegraphic communication. In the first place, let us consider what galvanic or voltaic electricity is.

I have said that electricity can be generated in many ways. It may be said, indeed, that every change in the condition of a substance, whether from mechanical causes, as, for instance, a blow, a series of small blows, friction, and so forth, or from change of temperature, moisture, and the like, or from the action of light, or from chemical processes, results in the development of more or less electricity.

When a plate of metal is placed in a vessel containing some acid (diluted) which acts chemically on the metal, this action generates negative electricity, which passes away as it is generated. But if a plate of a different metal, either not chemically affected by the acid or less affected than the former, be placed in the dilute acid, the two plates being only partially immersed and not in contact, then, when a wire is carried from one plate to the other, the excess of positive electricity in the plate least affected by the acid is conveyed to the other, or, in effect, discharged; the chemical action, however, continues, or rather is markedly increased, fresh electricity is generated, and the excess of positive electricity in the plate least affected is constantly discharged. Thus, along the wire connecting the two metals a current of electricity passes from the metal least affected to the metal most affected; a current of negative electricity passes in a contrary direction in the dilute acid.

I have spoken here of currents passing along the wire and in the acid, and shall have occasion hereafter to speak of the plate of metal least affected as the positive pole, this plate being regarded, in this case, as a source whence a current of positive electricity flows along the wire connection to the other plate, which is called the negative pole. But I241 must remind the reader that this is only a convenient way of expressing the fact that the wire assumes a certain condition when it connects two such plates, and is capable of producing certain effects. Whether in reality any process is taking place which can be justly compared to the flow of a current one way or the other, or whether a negative current flows along the circuit one way, while the positive current flows the other way, are questions still unanswered. We need not here enter into them, however. In fact, very little is known about these points. Nor need we consider here the various ways in which many pairs of plates such as I have described can be combined in many vessels of dilute acid to strengthen the current. Let it simply be noted that such a combination is called a battery; that when the extreme plates of opposite kinds are connected by a wire, a current of electricity passes along the wire from the extreme plate of that metal which is least affected, forming the positive pole, to the other extreme plate of that metal which is most affected and forms the negative pole. The metals commonly employed are zinc and copper, the former being the one most affected by the action of the dilute acid, usually sulphuric acid. But it must here be mentioned that the chemical process, affecting both metals, but one chiefly, would soon render a battery of the kind described useless; wherefore arrangements are made in various ways for maintaining the efficiency of the dilute acid and of the metallic plates, especially the copper: for the action of the acid on the zinc tends, otherwise, to form on the copper a deposit of zinc. I need not describe the various arrangements for forming what are called constant batteries, as Daniell’s, Grove’s, Bunsen’s, and others. Let it be understood that, instead of a current which would rapidly grow weaker and weaker, these batteries give a steady current for a considerable time. Without this, as will presently be seen, telegraphic communication would be impossible.

We have, then, in a galvanic battery a steady source of electricity. This electricity is of low intensity, incompetent242 to produce the more striking phenomena of frictional electricity. Let us, however, consider how it would operate at a distance.

The current will pass along any length of conducting substance properly insulated. Suppose, then, an insulated wire passes from the positive pole of a battery at a station A to a station B, and thence back to the negative pole at the station A. Then the current passes along it, and this can be indicated at B by some action such as electricity of low intensity can produce. If now the continuity of the wire be interrupted close by the positive pole at A, the current ceases and the action is no longer produced. The observer at B knows then that the continuity of the wire has been interrupted; he has been, in fact, signalled to that effect.

But, as I have said, the electrical phenomena which can be produced by the current along a wire connecting the positive and negative poles of a galvanic battery are not striking. They do not afford effective signals when the distance traversed is very great and the battery not exceptionally strong. Thus, at first, galvanic electricity was not more successful in practice than frictional electricity.

It was not until the effect of the galvanic current on the magnetic needle had been discovered that electricity became practically available in telegraphy.

Oersted discovered in 1820 that a magnetic needle poised horizontally is deflected when the galvanic current passes above it (parallel to the needle’s length) or below it. If the current passes above it, the north end of the needle turns towards the east when the current travels from north to south, but towards the west when the current travels from south to north; on the other hand, if the current passes below the needle, the north end turns towards the west when the current travels from south to north, and towards the east when the current travels from north to south. The deflection will be greater or less according to the power of the current. It would be very slight indeed in the case of a needle, however243 delicately poised, above or below which passed a wire conveying a galvanic current from a distant station. But the effect can be intensified, as follows:—
Fig. 1.

Suppose a b c d e f to be a part of the wire from A to B, passing above a delicately poised magnetic needle N S, along a b and then below the needle along c d, and then above again along e f and so to the station B. Let a current traverse the wire in the direction shown by the arrows. Then N, the north end of the needle, is deflected towards the east by the current passing along a b. But it is also deflected to the east by the current passing along c d; for this produces a deflection the reverse of that which would be produced by a current in the same direction above the needle—that is, in direction b a, and therefore the same as that produced by the current along a b. The current along e f also, of course, produces a deflection of the end N towards the east. All three parts, then, a b, c d, e f, conspire to increase the deflection of the end N towards the east. If the wire were twisted once again round N S, the deflection would be further increased; and finally, if the wire be coiled in the way shown in Fig. 1, but with a great number of coils, the deflection of the north end towards the east, almost imperceptible without such coils, will become sufficiently obvious. If the direction of the current be changed, the end N will be correspondingly deflected towards the west.

The needle need not be suspended horizontally. If it hang vertically, that is, turn freely on a horizontal axis, and the coil be carried round it as above described, the deflection of the upper end will be to the right or to the left, according to the direction of the current. The needle actually seen, moreover, is not the one acted upon by the244 current. This needle is inside the coil; the needle seen turns on the same axis, which projects through the coil.

If, then, the observer at the station B have a magnetic needle suitably suspended, round which the wire from the battery at A has been coiled, he can tell by the movement of the needle whether a current is passing along the wire in one direction or in the other; while if the needle is at rest he knows that no current is passing.
Fig. 2.
Fig. 3.

Now suppose that P and N, Fig. 2, are the positive and negative poles of a galvanic battery at A, and that a wire passes from P to the station B, where it is coiled round a needle suspended vertically at n, and thence passes to the negative pole N. Let the wire be interrupted at a b and also at c d. Then no current passes along the wire, and the needle n remains at rest in a vertical position. Now suppose the points a b connected by the wire a b, and at the same moment the points c d connected by the wire c d, then a current flows along P a b to B, as shown in Fig. 2, circuiting the coil round the needle n and returning by d c to N. The upper end of the needle is deflected to the right while this current continues to flow; returning to rest when the connection is broken at a b and c d. Next, let c b and a d be simultaneously245 connected as shown by the cross-lines in Fig. 3. (It will be understood that a d and b c do not touch each other where they cross.) The current will now flow from P along a d to B, circuiting round the needle n in a contrary direction to that in which it flowed in the former case, returning by b c to N. The upper end of the needle is deflected then to the left while the current continues to flow along this course.

I need not here describe the mechanical devices by which the connection at a b and c d can be instantly changed so that the current may flow either along a b and d c, as in Fig. 2, circuiting the needle in one direction, or along a d and b c, as in Fig. 3, circuiting the needle in the other direction. As I said at the outset, this paper is not intended to deal with details of construction, only to describe the general principles of telegraphic communication, and especially those points which have to be explained in order that recent inventions may be understood. The reader will see that nothing can be easier than so to arrange matters that, by turning a handle, either (1), a b and c d may be connected, or, (2), a d and c b, or, (3), both lines of communication interrupted. The mechanism for effecting this is called a commutator.

Two points remain, however, to be explained: First, A must be a receiving station as well as a transmitting station; secondly, the wire connecting B with N, in Figs. 2 and 3, can be dispensed with, for it is found that if at B the wire is carried down to a large metal plate placed some depth underground, while the wire at c is carried down to another plate similarly buried underground, the circuit is completed even better than along such a return wire as is shown in the figures. The earth either acts the part of a return wire, or else, by continually carrying off the electricity, allows the current to flow continuously along the single wire. We may compare the current carried along the complete wire circuit, to water circulating in a pipe extending continuously from a reservoir to a distance and back again to the reservoir. Water sucked up continuously at one end could be carried through the pipe so long as it was continuously246 returned to the reservoir at the other; but it could equally be carried through a pipe extending from that reservoir to some place where it could communicate with the open sea—the reservoir itself communicating with the open sea—an arrangement corresponding to that by which the return wire is dispensed with, and the current from the wire received into the earth.

The discovery that the return wire may be dispensed with was made by Steinheil in 1837.

The actual arrangement, then, is in essentials that represented in Fig. 4.
Fig. 4.

A and B are the two stations; P N is the battery at A, P′ N′ the battery at B; P′ P′ are the positive poles, N′ N′, the negative poles. At n is the needle of station A, at n′ the needle of station B. When the handle of the commutator is in its mean position—which is supposed to be the case at station B—the points b′ d′ are connected with each other, but neither with a′ nor c′; no current, then, passes from B to A, but station B is in a condition to receive messages. (If b′ and d′ were not connected, of course no messages could be received, since the current from A would be stopped at b′—which does not mean that it would pass round n′ to b′, but that, the passage being stopped at b′, the current would not flow at all.) When (the commutator at B being in its mean position, or d′ b′ connected, and communication with c′ and a′ interrupted) the handle of the commutator at A is turned247 from its mean position in one direction, a and b are connected, as are c and d—as shown in the figure—while the connection between b and d is broken. Thus the current passes from P by a and b, round the needle n; thence to station B, round needle n′, and by b′ and d′, to the earth plate G′; and so along the earth to G, and by d c, to the negative pole N. The upper end of the needle of both stations is deflected to the right by the passage of the current in this direction. When the handle of the commutator at A is turned in the other direction, b and c are connected, as also a and d; the current from P passes along a d to the ground plate G, thence to G′, along d′ b′, round the needle n′, back by the wire to the station A, where, after circuiting the needle n in the same direction as the needle n′, it travels by b and c to the negative pole N. The upper end of the needle, at both stations, is deflected to the left by the passage of the current in this direction.

It is easily seen that, with two wires and one battery, two needles can be worked at both stations, either one moving alone, or the other alone, or both together; but for the two to move differently, two batteries must be used. The systems by which either the movements of a single needle, or of a pair of needles, may be made to indicate the various letters of the alphabet, numerals, and so on, need not here be described. They are of course altogether arbitrary, except only that the more frequent occurrence of certain letters, as e, t, a, renders it desirable that these should be represented by the simplest symbols (as by a single deflection to right or left), while letters which occur seldom may require several deflections.

One of the inventions to which the title of this paper relates can now be understood.
 
Fig. 5.

In the arrangement described, when a message is transmitted, the needle of the sender vibrates synchronously with the needle at the station to which the message is sent. Therefore, till that message is finished, none can be received at the transmitting station. In what is called duplex telegraphy,248 this state of things is altered, the needle at the sending station being left unaffected by the transmitted current, so as to be able to receive messages, and in self-recording systems to record them. This is done by dividing the current from the battery into two parts of equal efficiency, acting on the needle at the transmitting station in contrary directions, so that this needle remains unaffected, and ready to indicate signals from the distant station. The principle of this arrangement is indicated in Fig. 5. Here a b n represents the main wire of communication with the distant station, coiled round the needle of the transmitting station in one direction; the dotted lines indicate a finer short wire, coiled round the needle in a contrary direction. When a message is transmitted, the current along the main wire tends to deflect the needle at n in one direction, while the current along the auxiliary wire tends to deflect it in the other direction. If the thickness and length of the short wire are such as to make these two tendencies equal, the needle remains at rest, while a message is transmitted to the distant station along the main wire. In this state of things, if a current is sent from the distant station along the wire in the direction indicated by the dotted arrow, this current also circuits the auxiliary wire, but in the direction indicated by the arrows on the dotted curve, which is the same direction in which it circuits the main wire. Thus the needle is deflected, and a signal received. When the direction of the chief current at the transmitting station is reversed, so also is the direction of the artificial current, so that again the needle is balanced. Similarly, if the direction of the current from the distant station is reversed, so also is the direction in which this current traverses the auxiliary wire, so that again both effects conspire to deflect the needle.

249 There is, however, another way in which an auxiliary wire may be made to work. It may be so arranged that, when a message is transmitted, the divided current flowing equally in opposite directions, the instrument at the sending station is not affected; but that when the operator at the distant station sends a current along the main wire, this neutralizes the current coming towards him, which current had before balanced the artificial current. The latter, being no longer counterbalanced, deflects the needle; so that, in point of fact, by this arrangement, the signal received at a station is produced by the artificial current at that station, though of course the real cause of the signal is the transmission of the neutralizing current from the distant station.

The great value of duplex telegraphy is manifest. Not only can messages be sent simultaneously in both directions along the wire—a circumstance which of itself would double the work which the wire is capable of doing—but all loss of time in arranging about the order of outward and homeward messages is prevented. The saving of time is especially important on long lines, and in submarine telegraphy. It is also here that the chief difficulties of duplex telegraphy have been encountered. The chief current and the artificial current must exactly balance each other. For this purpose the flow along each must be equal. In passing through the long wire, the current has to encounter a greater resistance than in traversing the short wire; to compensate for this difference, the short wire must be much finer than the long one. The longer the main wire, the more delicate is the task of effecting an exact balance. But in the case of submarine wires, another and a much more serious difficulty has to be overcome. A land wire is well insulated. A submarine wire is separated by but a relatively moderate thickness of gutta-percha from water, an excellent conductor, communicating directly with the earth, and is, moreover, surrounded by a protecting sheathing of iron wires, laid spirally round the core, within which lies the copper conductor. Such a cable, as Faraday long since showed,250 acts precisely as an enormous Leyden jar; or rather, Faraday showed that such a cable, without the wire sheathing, would act when submerged as a Leyden jar, the conducting wire acting as the interior metallic coating of such a jar, the gutta-percha as the glass of the jar (the insulating medium), and the water acting as the exterior metallic coating. Wheatstone showed further that such a cable, with a wire sheathing, would act as a Leyden jar, even though not submerged, the metal sheathing taking the part of the exterior coating of the jar. Now, regarding the cable thus as a condenser, we see that the transmission of a current along it may in effect be compared with the passage of a fluid along a pipe of considerable capacity, into which and from which it is conveyed by pipes of small capacity. There will be a retardation of the flow of water corresponding to the time necessary to fill up the large part of the pipe; the water may indeed begin to flow through as quickly as though there were no enlargement of the bore of the pipe, but the full flow from the further end will be delayed. Just so it is with a current transmitted through a submarine cable. The current travels instantly (or with the velocity of freest electrical transmission) along the entire line; but it does not attain a sufficient intensity to be recognized for some time, nor its full intensity till a still longer interval has elapsed. The more delicate the means of recognizing its flow, the more quickly is the signal received. The time intervals in question are not, indeed, very great. With Thomson’s mirror galvanometer, in which the slightest motion of the needle is indicated by a beam of light (reflected from a small mirror moving with the needle), the Atlantic cable conveys its signal from Valentia to Newfoundland in about one second, while with the less sensitive galvanometer before used the time would be rather more than two seconds.

Now, in duplex telegraphy the artificial current must be equal to the chief current in intensity all the time; so that, since in submarine telegraphy the current rises gradually to251 its full strength and as gradually subsides, the artificial current must do the same. Reverting to the illustration derived from the flow of water, if we had a small pipe the rapid flow through which was to carry as much water one way as the slow flow through a large pipe was to carry water the other way, then if the large pipe had a widening along one part of its long course the short pipe would require to have a similar widening along the corresponding part of its short course. And to make the illustration perfect, the widenings along the large pipe should be unequal in different parts of the pipe’s length; for the capacity of a submarine cable, regarded as a condenser, is different along different parts of its length. What is wanted, then, for a satisfactory system of duplex telegraphy in the case of submarine cables, is an artificial circuit which shall not only correspond as a whole to the long circuit, but shall reproduce at the corresponding parts of its own length all the varieties of capacity existing along various parts of the length of the submarine cable.

Several attempts have been made by electricians to accomplish this result. Let it be noticed that two points have to be considered: the intensity of the current’s action, which depends on the resistance it has to overcome in traversing the circuit; and the velocity of transmission, depending on the capacity of various parts of the circuit to condense or collect electricity. Varley, Stearn, and others have endeavoured by various combinations of condensers with resistance coils to meet these two requisites. But the action of artificial circuits thus arranged was not sufficiently uniform. Recently Mr. J. Muirhead, jun., has met the difficulty in the following way (I follow partially the account given in the Times of February 3, 1877, which the reader will now have no difficulty in understanding):—He has formed his second circuit by sheets of paper prepared with paraffin, and having upon one side a strip of tinfoil, wound to and fro to represent resistance. Through this the artificial current is conducted. On the other side is a sheet of tinfoil252 to represent the sheathing,29 and to correspond to the capacity of the wire. Each sheet of paper thus prepared may be made to represent precisely a given length of cable, having enough tinfoil on one side to furnish the resistance, and on the other to furnish the capacity. A sufficient number of such sheets would exactly represent the cable, and thus the artificial or non-signalling part of the current would be precisely equivalent to the signalling part, so far as its action on the needle at the transmitting station was concerned. “The new plan was first tried on a working scale,” says the Times, “on the line between Marseilles and Bona; but it has since been brought into operation from Marseilles to Malta, from Suez to Aden, and lastly, from Aden to Bombay. On a recent occasion when there was a break-down upon the Indo-European line, the duplex system rendered essential service, and maintained telegraphic communication which would otherwise have been most seriously interfered with.” The invention, we may well believe, “cannot fail to be highly profitable to the proprietors of submarine cables,” or to bring about “before long a material reduction in the cost of messages from plac............