Fig. 287 is a view of the Morse transmitting key. A B is a brass lever, moving in bearings at C, and provided at the end of its longer arm with a large knob or button of some insulating material. Steel pins are screwed in at B and D, and they are so adjusted that while that at B is pressed against the projection, E, by the action of the spring, F, when the knob, K, is pressed, contact is broken at B, and established at D. D and E are each provided with a binding-screw, so that wires may be attached in the manner indicated in Fig. 285. When the key is in the position shown, a current arriving by the line-wire passes from the fulcrum, C, of the lever through the contacts into the apparatus. When the knob is pressed down the battery current enters the lever by the contact at D, and passes into the line from the fulcrum, C. The clerks who are called upon to transmit messages usually soon learn to time the contacts very accurately in accordance with the code of signals, so as to produce the dashes and lines with accuracy. However, with certain persons some difficulty was found in acquiring the requisite uniformity, and to obviate any objection on this score, Morse invented an arrangement for facilitating the signalling, which is represented in Fig. 288. This is a smooth tablet of a non-conducting substance, such as ivory, except the shaded portions, which are plates of metal having their surfaces even with that of the ivory, and all soldered to a plate of metal beneath the ivory, which places them all in communication with each other and with the binding-screw, C. The lengths of the strips of metal and those of the spaces between them correspond with the dots and dashes of the Morse alphabet as marked on the tablet. The battery wire is connected with the binding-screw, C, and the line-wire terminates in an elastic and flexible coil of insulated wire, which is attached to a short rod having an insulated handle and terminated by a blunt platinum point. This the transmitter takes in his hand and draws uniformly along the line of metal strips belonging to the letter which he wishes to telegraph. The circuit is closed while the point of the style is passing across the metallic strips. This arrangement appears to be but little used, but it is nevertheless admirable for its simplicity, and is described here as a good illustration of the mode in which the varied duration of the contacts is able to produce the signals of the Morse alphabet. With the ordinary transmitting key a clerk is able to telegraph, on the average, twenty or twenty-five words in a minute, but the receiving apparatus is capable of recording three times as many. Morse also invented a system of transmitting the messages automatically, by setting up the message in a kind of type, just as ordinary letters are arranged for printing. The type, if it may be so called, had simple projections like the slips of metal, corresponding with each letter in Fig. 288. The lines of the message were drawn under a contact-lever, which closed the circuit when lifted up by the projections. Thus the speed of transmission could be very greatly increased, and a single wire and apparatus had its capacity of conveying a great number of messages in a given time proportionately enlarged.

We have now to ask the reader’s attention to the details of the apparatus in Fig. 286, the use of which has not already been pointed out. The electro-magnet, O O´, and the parts immediately connected with it, form what is called a relay. The object of this may be illustrated by supposing that the instrument is at one end of a long line, such as that between Edinburgh and London. Let us suppose it is at Edinburgh: the currents sent from London by a battery of convenient size might not be powerful enough to magnetize the soft iron of A with sufficient intensity to give clearness to the signals. They are, therefore, made to circulate in the electro-magnet, O, where they act by attracting the armature, W, which has the form of a split tube of soft iron, attached to a very light lever, Q, adjusted with great delicacy, and so that it moves by little magnetic force. The end of the lever works between two adjustable screws, R and S, which are electrically insulated, except that R is in communication with one extremity of the coils of the electro-magnet, A. Q is in metallic communication through the pillar, T, and the binding-screw, U, with the zinc end of a battery at Edinburgh, which is called the local battery, the other pole of which communicates with the other ends of the coils, A, through the screw, U´. When no current from London is passing through O, Q is held down by the spring, W´, and the circuit of the local battery is broken; but the instant the line-current passes, the armature, W, is attracted, and Q makes contact with R, the current from the local battery rushes through the coils, A, and the appropriate movements of the printing lever are effected by its action. X is a spring for drawing down the lever, and it is provided with a screw for adjusting its tension, and Y, Z, are screws for limiting the extent of motion of the lever; under P is the little projection by which the band of paper is pressed against the inking-disc; l and e are respectively the screws for the line and earth connections.

An extremely ingenious system of signalling, by which the speed could be greatly increased, has been devised by Sir Charles Wheatstone, and is largely adopted by the British postal authorities. In this system the message is first translated into telegraphic language by a machine, which punches certain holes in a strip of stiff paper. The apparatus originally designed for this purpose by the inventor is thus described by him in the Juror’s Report, International Exhibition of 1862:

“Long strips of paper are perforated by a machine constructed for the purpose, with apertures grouped to represent the letters of the alphabet and other signs. A strip thus prepared is placed in an instrument associated with a source of electric power, which, on being set in motion, moves it along, and causes it to act on two pins in such a manner that when one of them is elevated the current is transmitted to the telegraphic circuit in one direction, when the other is elevated it is transmitted in the reverse direction. The elevations and depressions of these pins are governed by the apertures and intervening intervals. These currents, following each other indifferently in these two opposite directions, act upon a writing instrument at a distant station in such a manner as to produce corresponding marks on a slip of paper, moved by appropriate mechanism.

“The first apparatus is a perforator, an instrument for piercing the slips of paper with the apertures in the order required to form the message. The slip of paper passes through a guiding groove, at the bottom of which an opening is made sufficiently large to admit of the to-and-fro motion of the upper end of a frame containing three punches, the extremities of which are in the same transverse line. Each of these punches, the middle one of which is smaller than the two external ones, may be separately elevated by the pressure of a finger-key.

“By the pressure of either finger-key, simultaneously with the elevation of its corresponding punch, in order to perforate the paper, two different movements are successively produced: first, the raising of a clip which holds the paper firmly in its position; and secondly, the advancing motion of the frame containing the three punches, by which the punch which is raised carries the slip of paper forward the proper distance. During the reaction of the key consequent on the removal of the pressure, the clip first fastens the paper, and then the frame falls back to its normal position. The two external keys and punches are employed to make the holes, which, grouped together, represent letters and other characters, and the middle punch to make holes which mark the intervals between the letters.

“The second apparatus is the transmitter, the object of which is to receive the slips of paper prepared by the perforator, and to transmit the currents in the order and direction corresponding to the holes perforated in the slip. This it effects by mechanism somewhat similar to that by which the perforator performs its functions. An eccentric produces and regulates the occurrence of three distinct movements: 1. The to-and-fro motion of a small frame which contains a groove fitted to receive the slip of paper, and to carry it forward by its advancing motion. 2. The elevation and depression of a spring-clip, which holds the slip of paper firmly during the receding motion, but allows it to move freely during the advancing motion. 3. The simultaneous elevation of three wires placed parallel to each other, resting at one of their ends over the axis of the eccentric, and their free ends entering corresponding holes in the grooved frame. These three wires are not fixed to the axis of the eccentric, but each end of them rests against it by the upward pressure of a spring; so that when a light pressure is exerted on the free end of either of them, it is capable of being separately depressed. When the slip of paper is not inserted the eccentric is in action; a pin attached to each of the external wires touches during the advancing and receding motions of the frame a different spring; and an arrangement is adopted, by means of insulation and contacts properly applied, by which, while one of the wires is elevated, the other remains depressed; the current passes to the telegraphic circuit in one direction, and passes in the other direction when the wire before elevated is depressed, and vice versâ; but while both wires are simultaneously elevated or depressed the passing of the current is interrupted. When the prepared slip of paper is inserted in the groove, and moved forward whenever the end of one of the wires enters an aperture in its corresponding row, the current passes in one direction, and when the end of the other wire enters an aperture of the other row, it passes in the other direction. By this means the currents are made to succeed each other automatically in their proper order and direction to give the requisite variety of signals. The middle wire only acts as a guide during the operation of the current.

“The wheel which drives the eccentric may be moved by the hand, or by the application of any motive power. Where the movement of the transmitter is effected by machinery, any number may be attended to by one or two assistants. This transmitter requires only a single telegraphic wire.

“The third apparatus is the recording or printing apparatus, which prints or impresses legible marks on a strip of paper, corresponding in their arrangement with the apertures in the perforated paper. The pens or styles are elevated or depressed by their connection with the moving parts of the electro-magnets. The pens are entirely independent of each other in their action, and are so arranged that when the current passes through the coils of the electro-magnet in one direction, one of the pens is depressed, and when it passes in the contrary direction the other is depressed; when the currents cease, light springs restore the pens to their elevated points. The mode of supplying the pens with ink is the following: A reservoir about an eighth of an inch deep, and of any convenient length and breath, is made in a piece of metal, the interior of which may be gilt in order to avoid the corrosive action of the ink; at the bottom of this reservoir are two holes, sufficiently small to prevent by capillary attraction the ink from flowing through them; the ends of the pens are placed immediately above these small apertures, which they enter when the electro-magnets act upon them, carrying with them a sufficient charge of ink to make a legible mark on a ribbon of paper passing beneath them. The motion of the paper ribbon is produced and regulated by apparatus similar to those employed in other register and printing telegraphs.”

The mode by which Wheatstone proposed to indicate the letters was novel, consisting in dots only, the numbers and positions of which in two lines along the paper ribbon distinguished the letters—the system of combining the symbols being still identical with the Morse code, only the dash was replaced by a dot in the lower lines:

A single dot in the upper line stood for E, in the lower line for T; a dot in the upper line, followed by one in the lower line a little to the right, represented A; one in the lower line, followed by another in the upper line, indicated N; and so on. By the dot printing it is said that Wheatstone would signal 700 letters per minute. There were, however, objections to the new code of signals: all the world had agreed to use the Morse alphabet, and it was perhaps less liable to incorrect reading; and for other reasons this more rapid signalling was unsuitable for submarine lines. The apparatus has therefore been modified to suit the dot and dash system of signals, and great improvements have been effected by Sir Charles on the original instruments, with a view of increasing the rapidity of transmission as much as possible. The paper as punched for the Morse signals shows a row of equidistant holes in the middle, by which the paper is guided uniformly forward, and in the outer rows are holes arranged in pairs, either exactly opposite to each other or obliquely—the former produce dots at the receiving station, the latter dashes. From 60 to 100 words can thus be sent and printed in one minute, and the automatic transmitting system can be applied to the needle, or any other form of telegraph.

After a clerk has for some time been habituated to working with the Morse instrument, he is able to read the message from the different sounds made by the armature, as dashes or dots are respectively marked, and he usually listens to the message, and transcribes it at once into ordinary language by the ear alone. This observation soon led to the adoption of sound alone as the means of signalling, and an instrument on this plan has already been referred to.

Among the more remarkable forms of recording telegraphs, that of Hughes may be mentioned, in which the message is printed at the receiving station in distinct Roman characters; and as only a single instantaneous current is required to be sent for each letter, the speed with which a message can be dispatched is about three times as great as with the Morse instrument. These advantages are, however, obtained only at the cost of great delicacy and complexity in the apparatus, so that it is unfit for ordinary use, although it is much employed on important lines, where competent operators and skilled mechanics and electricians are at hand to keep it duly regulated. This machine is too complicated for a full description in these pages, although it is the best form of type-printing telegraph, and possesses a special feature in the fact that the printing is done whilst the wheel carrying the types is in rapid rotation. The reader will find full and untechnical descriptions of this and of all the more important forms of telegraphic apparatus in Mr. R. Sabine’s useful “History and Progress of the Electric Telegraph,” or in Lardner’s work as edited by Sir Charles Bright.

From the numerous forms of dial telegraphs we select two for description. All these instruments are characterized by what is called the “step-by-step” movement, and differ in their mechanical details, and in the nature of the apparatus for producing the currents, some being driven by electro-magnets and others by galvanic batteries. Their principle may be easily explained. Suppose that a ratchet-wheel, having twenty-six teeth, is mounted on an axis carrying a hand over a dial having the letters of the alphabet inscribed upon it. A simple arrangement in connection with an electro-magnet, somewhat like the escapement of a clock, will serve to advance the wheel by one tooth each time a current passes. The diagram, Fig. 289, will at once make this principle clear. E is the electro-magnet, F the armature, separated by the spring, S, from the magnet, except when the current passes, when the catch, C, draws down the tooth in which it is engaged, so that a tooth passes under the point at D; and when the current ceases, the spring, S, brings up the catch to engage the succeeding tooth, and thus the hand moves step by step in the direction of the arrow, advancing each time the electric circuit is closed by one twenty-sixth of a revolution. In Fig. 290 is represented lecture-table models of a step-by-step indicating and transmitting instrument, as constructed by M. Froment, of Paris. These instruments are supposed to be at the extremities of a long line of wire. The left-hand figure is the manipulator, or sending instrument, in which the operator has merely to quickly turn round the index in the direction of the hands of a watch, by means of the knob, P, until it points to the desired letter, pause at the letter for an instant, and then quickly continue the movement until his index points to the cross at the top of the dial, where he pauses if the word is spelt out, and, if not, continues the rotation until he arrives at the next letter, and so on. All these movements and pauses the hand on the indicator will accurately repeat, and the reason of this may be seen by observing that the battery contacts are made by the projections on the metallic wheel, R, which turn with the index. The spring, N, is always in contact with the wheel, but the spring, M, has such a shape that contact is alternately made and broken as the projections and spaces pass it. It is obvious that the needle of the indicator will therefore advance over the same letters as the index of the communicator.

A very elegant dial instrument has been invented by Sir Charles Wheatstone, in which magneto-electric currents are made use of. In Fig. 291 communicator and indicator are represented mounted in one case, or small box. The larger dial is the communicator, and its circumference is divided into thirty equal spaces, in which are the twenty-six letters of the alphabet, three punctuation marks, and a +. In an inner circle are two series of numerals and other signs. About the circumference of the dial are thirty small buttons or projecting keys, conveniently arranged, so as to be readily depressed by the touch of a finger. Inside of the box a strong permanent horse-shoe magnet is fixed, and near its poles a pair of armatures of soft iron cores with insulated wire coils revolve when the handle, A, is turned, as in the machines described in the last article. In this manner a series of waves or short currents of electricity are produced in the conductors when the circuit is complete, and the currents are alternately in opposite directions, so that fifteen revolutions of the coils will produce fifteen currents in one direction and fifteen in the other. A pinion on the same spindle as the coils works with a wheel on the axis carrying the pointer on the dial, so that the pointer makes a complete revolution as often as the handle, A, makes fifteen turns. Each of the thirty currents will pass through the indicator, I, and through the line to the distant station, where they will, by a step-by-step movement, advance the needle of the indicator. So that the hand of the dial and the needle of the indicator at the sending station, and that of the indicator at the distant station, will all simultaneously be pointing to the same letter on their respective dials; and they would continue to move round these, ever pointing to the same letter, so long as the handle, A, is turned. How, then, is the sender to cause the needle of his correspondent’s instrument to pause at any desired letter? Not by stopping the revolution of the handle, A, for that could not be done so as to send just the right number of currents, inasmuch as the rotating armatures could not be instantly stopped. The mode of causing the indicators to pause at any required letter is as simple as it is ingenious. It has been already mentioned that the step-by-step movement takes place at every current which passes through the line, including the two indicators, and that thirty such currents pass at each revolution of the pointer of the communicator. But when these currents no longer flow, the indicators, of course, stop; and the stoppage of the movements is reconciled with the continuous production of the currents by having a series of little levers, each connected with one of the buttons, and so arranged that when one of these has been pushed down, the lever stops the revolution when it has come round of an arm on the same central axis as the pointer, and riding loosely on a hollow spindle, which bears the toothed wheel, driven by the pinion already spoken of. The projecting arm is provided with a spring, which falls between the teeth of the wheel, so that the arm is with certainty carried round with the wheel. But where a button has been pushed down, its lever catches the arm, lifting its spring away from the teeth of the wheel. So long as the key remains down, the arrested arm makes a short metallic circuit by its contact, and no currents pass into the line, for they take the shortest path. The key is raised only when another is depressed, and then the arm and the pointer immediately resume their revolution until they again become stationary at the letter corresponding with the key which has been pushed down. Suppose the key of +, the zero of the dial, to be down, which is the proper condition of the apparatus when a message has to be dispatched. The operator having rung a bell at the distant end, to call the attention of the person who receives his message, begins to turn the handle, A, at the rate of about two revolutions per second. In this state of affairs no current is passing into the line, and the fingers of both his communicator and indicator remain stationary, as does also that of the indicator at the distant end of the line. Now, suppose he has to spell the word “FOX.” He turns the handle A continuously with his right hand the whole time he is sending the message; and, manipulating the keys with his left, he depresses that opposite to the letter F. By this action the key opposite + is raised, for the levers are pressed into notches against a watch-chain, which has just enough slack to allow one lever to enter a notch, and therefore the pressure of another lever always raises the key last depressed. When the operator presses down the F key, the + rises, the radial arm is instantly released, and with the index is carried on to F, where it stops; and the contacts will have, during that movement, sent six currents into the line, so that the fingers of both indicators will also point to F. When the pointer of the communicator has made just a visible pause at F, he pushes down the key of O, and all the three pointers recommence their journeys towards that letter. The operator must, of course, wait until they have reached it and paused an instant, when he depresses the button opposite X; and when the index has pointed at that, he pushes down the + key, whereby the fingers all arrest their movements at that point, indicating that the word is completed. In the case supposed the word is completed by a single revolution of the pointers; but this is, of course, not usually the case; thus, in indicating the syllable “PON,” nearly three complete revolutions would be required.

This admirable little instrument was designed for the use of private persons, and is largely used in London and elsewhere. Its great compactness and simplicity of operation render it highly suitable for this purpose. There is no battery required, and all the inconvenient attention demanded by a battery is therefore dispensed with. On the other hand, the magnets gradually lose their power, and after a time must be re-magnetized; and the electro-motive force developed in these instruments is insufficient for lengths of line much exceeding 100 miles. For shorter lines, and for the purposes for which they are designed, these instruments are perfection.

Very interesting forms of telegraph are those in which a despatch is not merely written or printed, but actually transcribed as a facsimile of the writing in the original; and in this way it is possible for a design to be drawn telegraphically at the distance of hundreds of miles. Like the Hughes’ printing telegraph, the instruments which produce these apparently marvellous results require synchronous movements at the two stations. But although they are scientifically successful, there appears to be no public demand for these copying telegraphs. One of the best known is Bonelli’s, which dispatches its messages automatically when they have been set up in raised metal types precisely similar to the Roman capitals in the type of the ordinary printer. In Bonelli’s and most other copying telegraphs the impressions are produced by chemical decompositions—effected at the receiving station on the paper prepared to receive the message. By Bonelli’s instrument it is said that when the type has been set up, messages can be sent at the extraordinary rate of 1,200 words in one minute of time! The action of this system is such that it is proved to be possible to reproduce in a few seconds—at York, say—the very characters of a page of type the moment before set up in London. The limits of our space will not admit of details of this invention; but we here place before the reader a facsimile of the letters printed by it at the receiving stations.

We have to describe two other forms of instruments for receiving telegraphic signals, both contrived with consummate skill by Sir William Thomson, and, though exhibiting no new principle in any of their parts, both fine examples of beautiful adjustment of materials for a desired end. In these forms of apparatus, the delicacy of the mechanical construction, and the accurate relations of one part to another, have produced results of the greatest practical importance. Fig. 292 represents the mirror galvanometer, an instrument which has not only proved of the highest value in scientific researches, but is of the first importance in submarine telegraphy. It is in principle nothing more than the single-needle telegraph, and it is exceedingly simple in construction. A very small and light magnet, such as might be formed by a fragment of the mainspring of a watch, ⅜ths of an inch long, say, is attached to the back of a little circular mirror, made of extremely thin silvered glass, also about ⅜ths of an inch in diameter. The mirror and magnet are suspended by a single cocoon-fibre, so fine as to be almost invisible, in the centre of a coil, A, of fine silk-covered copper wire. In front of the suspended mirror, in the axis of the coil, is placed a lens of about four feet focal distance, and opposite to this is a screen having a slit, B, in the centre, behind which is placed a paraffin lamp, D. The screen is provided with a paper scale, C, divided into equal parts, and is placed at the distance of about two feet from the little mirror. It follows, from this arrangement, that when the light passing through the slit falls upon the mirror, it is reflected again through the lens, and an image of the slit is seen on the scale. This image is immediately above the slit when the beam falls perpendicularly upon the mirror, and this condition may be brought about by properly placing the apparatus with regard to the magnetic meridian. The directive power of the earth over the little suspended magnet is, however, almost annulled by properly fixing the steel magnet, E, which slides upon the upright rod, so that the suspended magnet is thus free to obey the least force impressed upon it by a current passing through the coil. And when the mirror is deflected through a certain angle, the image on the scale will be deflected to twice that angle, and thus the smallest movements of the suspended magnet are readily recognized; not only by reason of the length of the beam of light, which forms a weightless index, but because they are doubled by this increased angular deflection.

When the signals are being rapidly transmitted through a long submarine line, the currents at the receiving station are much enfeebled and retarded, and the result is that the movements of a suspended needle have by no means the decided character which is seen in the instruments connected with land lines. The signals through a submarine cable could not therefore be received by any apparatus which required a certain strength of current; but the mirror galvanometer indicates every change in the currents, and the apparently irregular motions of the spot of light can be interpreted by a skilled clerk, who, by long experience, recognizes, in quite dissimilar effects, the same signal sent by the clerk at the other end in precisely the same way. Thus a first contact, corresponding with a dot of the Morse alphabet, may cause the light to move some distance on the scale, a second contact immediately succeeding moves it but a little way farther, and a third may occasion a movement hardly perceptible.

The messages sent by the mirror galvanometer must be read as they are received; and, as a telegraphic instrument, it is wanting in the manifest advantages attending a recording instrument. Sir W. Thomson has, however, devised another receiving instrument of great delicacy, which is termed the syphon recorder. We cannot here describe its admirable mechanical and electrical details, but the chief feature is that the attractions and repulsions of the currents are made to produce oscillations in a syphon formed of an extremely fine glass tube, the shorter branch of which dips in a trough of ink, and the longer branch terminates opposite to, but not touching, a band of paper, which is continuously and regularly drawn along by clockwork while the message is being received. The tube is a mere hair-like hollow filament of glass, and the ink, which would not itself flow from a tube of so fine a bore, is squirted out by electrical repulsion when the insulated reservoir in which it is contained is electrified at the receiving station by an ordinary machine. The message as written by this instrument appears thus:

The reader, on comparing these signals with the Morse code on page 560, will have no difficulty in discovering their relation to it.

TELEGRAPHIC LINES.

It now remains to give some account of the line, that is, the conductor by which the sending and receiving instruments are united, and along which the currents flow. Overhead lines are nearly always constructed with iron wires, which are usually ⅙ in. in diameter, and are coated with some substance to protect them from oxidation. Zinc is often used for this purpose, the wire being drawn through melted zinc, by which it becomes covered with a film of this metal—a process known as “galvanizing” iron. Another mode is to cover the wires with tar, or to varnish them from time to time with boiled linseed oil, and this must be done in populous places, where the gases in the air are liable to act upon the zinc. Sometimes underground wires are used, and these are often made of copper, covered with gutta-percha, and are laid in wooden troughs, or in iron pipes. They are protected by having tape or other material, saturated with tar or bitumen, wound round them. The poles employed to suspend the overhead wires are generally made of larch or fir, of such a length that when securely fixed in the ground they rise 12 ft. to 25 ft. above it, and at the top have a diameter of about 5 in. About thirty poles are required for each mile, and every tenth pole forms a “stretching-post,” being made stronger than the others and provided with some appliance by which the wires can be tightened when required. The wires are attached to the posts by insulating supports; but at every pole there is always some “leakage,” the amount of which depends on the form, material, and condition of the insulators. Glass is quite unsuitable, because its surface strongly attracts moisture, which thus forms a conducting film. All things considered, porcelain is found to be the best insulating material for this purpose, since moisture is not readily deposited on its surface, and even rain runs off without wetting it; and it is durable, strong, and clean. Fig. 293 shows a telegraph post, with brown salt-glazed stoneware insulators, shaped like hour-glasses, with a perspective view and section of one of them. Another form of insulator, shown in Fig. 294, has a stalk or hook of porcelain, with a notch, into which the wire is simply lifted, and is protected above by a porcelain bell. This form, or some modification of it, is that most generally used.

It need hardly be remarked that only a single wire is required with most of the modern instruments for communication between any two places. Each of the many wires often seen attached to the telegraph posts along a road or railway represents a distinct line of communication—that is, one wire may connect the two termini, another may join an intermediate station and a terminus, a third may belong to two intermediate stations, and so on. We have already alluded to the discovery by Steinheil of the apparent conducting power of the earth; and if we must continue to think of complete circuits, we must regard the earth as replacing for telegraphic purposes the second or return wire, which was at first supposed essential. For instance, when a battery current had to be sent from Station A, Fig. 295, which we may suppose to be London, to Station B, which we may call Slough, it was at first thought requisite to provide a wire for the return of the current after it had traversed the coils at the receiving station. But now the connections are made as shown in Fig. 296, where the return wire is dispensed with, except a small portion at each end, which is connected with a large plate of copper buried in the earth; the arrows show the direction of the current, according to the commonly received notion. By this plan the current is increased in intensity, for the “earth circuit” appears to offer less resistance than the copper wire. The view, however, which regards the earth not as a conductor in the same sense as the wire, but as the great reservoir or storehouse of electricity, accords better with known facts.

The spread of telegraph lines, and the extent to which this mode of communication is used by the public, may be illustrated by a few particulars regarding the Central Telegraphic Office in London. The management of all the public telegraph lines in Great Britain is now in the hands of the Post Office authorities, and the arrangements at the central office in London are an admirable specimen of administrative organization. The Central Telegraph Office occupies a very large and handsome building opposite the General Post Office, St. Martin’s-le-Grand. In one vast apartment in this building, containing ranges of tables, in all three-quarters of a mile long, may be seen upwards of six hundred telegraph instruments, besides a number of stations for the receipt and transmission of bundles of messages by pneumatic dispatch. The number of clerks employed in working the instruments is 1,200, and about three-fourths of these are females. The wires from each instrument are conducted below the floor of the apartment to a board where they terminate in binding-screws, marked with the number of the instrument. The same board has binding-screws, with battery connections, and others which form the terminals of the telegraph lines, and thus the requisite connections are readily made. The batteries are placed in a lower room, which contains about 23,000 cells of Daniell’s construction, formed into nearly 1,000 distinct batteries, in each of which the number of cells varies according to the length of the line through which the current has to pass. Thus, the battery which supplies the currents that are sent through the coils of the instrument at Edinburgh consists of 60 cells, but one-sixth of that number suffices for some of the short lines. The instrument almost exclusively used is the Morse recorder, and Wheatstone’s automatic punching machine and transmitters are in constant employment. There are also some examples of other instruments to be seen in operation, such as the Hughes type printing telegraph, the American sounder, a few A, B, C, dial instruments, and a solitary specimen of a double-needle instrument. Upwards of 30,000 messages pass through this office each day.

But the most striking achievements in connection with telegraphy are the great submarine lines which unite the Old and New Worlds. Morse and Wheatstone about the same time (1843) independently experimented with sub-aqueous insulated wires, and their success gave rise to numerous projects for submarine lines. How far any of these might have been practical need not here be discussed, but it fortunately happened that some years after this, the electrical properties of gutta-percha were recognized, and this material, so admirably adapted for forming the insulating covering of wires, was taken advantage of by Brett and Co., who obtained the right of establishing an electric telegraph between France and England, and they succeeded in laying down the first submarine cable. This cable extended from Dover to Cape Grisnez near Calais, and the experiment proved successful; but, unfortunately, the cable was severed within a week by the sharp rocks on which it rested near the French coast. It proved, however, the excellent insulating property of the new material, and demonstrated the possibility of submarine telegraphic communication. Another cable was manufactured, in which the gutta-percha core was protected by a covering of iron wires laid specially on the exterior, and thus combining greater security with a far larger amount of tenacity. A view and section of this—the first practically successful submarine cable—are given in Fig. 297 of the real size. It has four separate copper wires, each insulated with a covering of gutta-percha, and the whole was spun with tarred hemp into the form of a rope, and protected with an outer covering of ten of the thickest iron wires wound spirally upon it. The cable when complete was 27 miles in length, and each mile weighed 7 tons. This cable was laid in 1851, and from that time it has been in constant use, with the exception of a few interruptions from accidental ruptures. Its success immediately led to the construction of other cables connecting England with Ireland, Belgium, Holland, &c. In 1855 the practicability of an Atlantic cable was no longer doubted, and £350,000 were soon subscribed by the public for the project. A cable was manufactured weighing 10 tons to the mile, and in August, 1857, 338 miles of it had been successfully paid out by the ships when the cable parted. Better paying-out apparatus was now devised—self-releasing brakes were constructed, so that the cable should not be exposed to too great a strain; and in 1858 another cable, requiring a strain of 3 tons to break it, was manufactured, and the laying of it commenced in mid-ocean—the Mægera and Agamemnon going in opposite directions, and paying out as they proceeded. Twice the cable was severed, twice the ships met and repaired the injury; but the third time, when they were 200 miles apart, the cable again broke. But again the attempt was repeated, and this time success crowned the effort; for on the 5th of August the two continents were telegraphically connected. Unfortunately the electric continuity failed after the cable had been a month in use.

Seven years elapsed before another endeavour was made; but the experience gained in the unsuccessful attempt was not lost; and in 1865 another cable had been constructed, and the Great Eastern was employed in laying it. In this the conductor was composed of seven copper wires twisted into one strand, covered with several layers of insulating material, and covered externally with eleven stout iron wires, each of which was itself protected by a covering of hemp and tar. This cable was 2,600 miles long, and contained 25,000 miles of copper wire, 35,000 miles of iron wire, and 400,000 miles of hempen strands, or more than sufficient to go twenty-four times round the world. It was carefully made, mile by mile, formed into lengths of 800 miles, and shipped on board the Great Eastern in enormous iron tanks, which weighed, with their contents, more than 5,800 tons. This cable was manufactured by Messrs. Glass and Elliot, at Greenwich, to whom the iron wire for the outer covering was furnished by Messrs. Webster and Horsfall, of Birmingham. Fig. 298 represents the workshops with the iron wire in process of making. The great ship sailed from Valentia on the 23rd of July, 1865, and the paying out commenced. Constant communication was kept up with the shore, and signals exchanged with the instrument-room at Valentia, which is represented in Fig. 299, where, among various instruments invented by Sir W. Thompson, may be seen his mirror galvanometer. After several mishaps, which required the cable to be raised for repairs after it had been laid in deep water, the Great Eastern had paid out about 1,186 miles of cable, and was 1,062 miles from Valentia, when a loss of insulation in the cable was discovered by the electricians on board. This indicated some defect in the portion paid out, and the usual work of raising up again had to be once more resorted to. During this process the cable parted, and Fig. 300 shows the scene on board the Great Eastern produced by this occurrence, as represented by an artist of the “Illustrated London News” who accompanied the expedition. The broken cable was caught several times by grapnels, and raised a mile or more from the bottom, but the tackle proved unable to resist the strain, and four times it broke; and after the spot had been marked by buoys, the Great Eastern steamed home to announce the failure of the great enterprise. For this 5,500 miles of cable had altogether been made, and 4,000 miles of it lay uselessly at the bottom of the ocean, after a million and a quarter sterling had been swallowed up in these attempts.