passing, takes a position which would be exactly perpendicular to the wire if no other magnetic or electromagnetic force acted on it; that end or pole of the needle which has the same magnetic quality as the north polar regions of the Earth, and which is, therefore, properly called its north magnetic pole, (although generally marked with an S. by British instrument makers,) is driven to the south if the line of current be from east to west and below the line of the needle, or if from west to cast and above it. And if the point of support of the needle, and the wire conveying the current, be held fixed, while the direction of the current is reversed, the needle is as much deflected in the opposite direction. The amount of this electro-magnetic deflecting force producible by a current of fixed strength, may be greatly multiplied by bending the wire round and round so as to convey the current in one direction below the needle, and in the opposite direction above it.

To carry out this idea, the philosophical instrument maker took advantage of the silk-covered wire of the milliners; only using, instead of the iron wire which had served their purpose, copper wire, as being five or six times as good a conductor of electricity. This silk-covered copper wire came to be made into coils, which, with magnetic needles properly supported in their interior, constituted what was called the galvanometer, an instrument for measuring the strength of galvanic action, that is, simply the strength of a continuous electric current. The silk covering of the wire played the important part of insulating its several turns from one another, and preventing the current from taking any short cut, so as to compel the whole of it to pass through each turn of the wire. The needle, balanced on a point, or hung by a fine thread or single silk fibre, remained always with its own length horizontal, and was free to turn round into any horizontal position. When no electric current flows, the needle takes a definite direction, either in virtue of terrestrial magnetism, or of the action of steel magnets fixed in its neighbourhood. When the current passes through the coil the needle is deflected to a greater or less degree, according to the greater or less strength of the current, and in one direction or the other according to the direction of the current in the wire. Oersted's discovery of this grand law of nature early brought in a rich harvest of knowledge, through the labours of Ampère, Seebeck, Cumming, and Faraday; and within twenty years had an obvious practical application in the Electric Telegraph. In every kind of electric telegraph which has been practically successful, the magnetic virtue possessed by a metal wire during the flow of a current of electricity through it, is used in the receiving instrument to exercise force and thereby cause motions by which visible effects are produced. In the simplest forms of telegraph these motions are themselves read directly, as in the so-called needle telegraphs still used so much on our railways; sometimes, again, as in one of the very early telegraphs, that of Steinheil, and in the

system now generally followed by the British and Irish Magnetic Telegraph Company, the motion directly produced by the electro-magnetic force of the current causes (by aid of a "relay," as described below) one or other of two bells to be struck, according as the flow of signal current is through the wire, nominally from or towards the sending station. Or again, as in the Morse and Bain telegraphs, currents in only one direction are used, and signals of the requisite variety are produced by making them of longer or shorter duration. In the Morse system a long ribbon of paper, kept running at a uniform rate by wheelwork, is embossed with longer or shorter marks by a point pressed against it by electro-magnetic action as long as the current flows through the line of telegraph. Various forms of inkmarkers have been devised, and some have come into tolerably extensive use of late. These long or short ink-marks are produced with less electromagnetic force, but yet with greater and more easily legible distinctness than the embossed long or short lines of the original Morse system.

The Morse alphabet, as printed on a long narrow slip of paper by any of the recording instruments, is as follows::

Any system in which the alphabet is made of two different signals, by positive and negative electric pulses, is called, after the name of its inventor, Steinheil. The Steinheil alphabet, most commonly used now by practical telegraphers, is letter for letter the same as the Morse, but with positive currents, producing, let us say, a deflection to the right in the receiving instrument, instead of a dot, and an equally short negative or left-hand deflection instead of the dash of the Morse alphabet.

In the Bain system, as now used on some lines, the signals, and the ribbon of paper receiving them, are arranged precisely as in the Morse system. The ribbon of paper is moistened with solution of nut galls, or ferro-cyanide of potassium: and a fixed steel wire or thin spring presses it against a metal roller, as it runs through the wheelwork. As long as an electric current passes from the steel, through the paper, to the metal cylinder, the steel is eaten away in very minute quantities, which combine with the moist chemical preparation on the paper, making ink or prussian blue, as the case may be, and marking the paper with it.

But the electric currents used for marking, whether electro-magnetically or electro-chemically, are not, in any long line of telegraph, produced by the current through the line. This current produces a minute, scarcely visible, motion in a magnet, through the electro-magnetic influence discovered by Oersted.

These minute motions serve to make and break the circuit of a special battery, placed to work the marking instrument at the receiving station, and kept separate from the battery used in the line circuit. A receiving instrument which acts thus is called a relay, from its being used to give, under the guidance of the spent currents from the distant station, a fresh relay of power as it were, to carry forward the message, whether over a farther stage of telegraph line, or merely through the conducting wire of the marking instrument. The relay, with one form or another of marking instrument, and Morse signals of longs and shorts, or "dots and dashes," as they are technically called, has hitherto been used in every great submarine line, until the example of a different mode of receiving messages has been set in the practical working of the Atlantic Cable, which, with the reasons for introducing it, I shall now briefly explain.

As early as 1849, or little more than ten years after the practical realisation of the electric telegraph, a principle then new to the scientific world was pointed out by Dr. Werner Siemens, of Berlin, according to which it has since been found that long || submarine lines lose the sharp quick action characteristic of the electric telegraph as known at that time. Experimental and mathematical investigations on the subject, which have since been carried out by various workers, allow me to explain the comparative sluggishness of a long submarine telegraph, by an analogy which, though apparently gross and mechanical, is suggestive of the deeper relations and properties of electricity and of speculations as to its nature and way of action, learned from Faraday. A water telegraph, which has sometimes been used on a small scale, consists of a metal pipe filled with water, and communicating at each end with a short cylinder stopped by a piston. If at one end (the sending station) the piston is pressed forward through a small space into its cylinder, the piston at the other end will be driven outwards through an equal space, and, if the former is again driven back, the other piston will return again to where it stood before. Letters and words may of course be sent and read, by proper combinations of such signals. If the pipe were perfectly rigid and the water incompressible, the siguals would be absolutely sharp and definite. Each slightest motion of the piston at the one end would produce, at exactly the same instant, a precisely equal motion of the piston at the her end. But if, instead of being of absolutely rigid material, (which does not exist in nature,) or of very rigid material, as iron, the tube consisted of a soft yielding yet elastic material, as india-rubber, it would, if the piston at one end is pressed forward, swell out, under the influence of increased pressure at that end, as the water is thrust forward through the tube; and would shrink to less than its ordinary diameter when water is sucked back as it were by drawing back the piston. And thus a considerable time, which may be a few seconds, a few minutes, or a few hours, according to circumstances, will

elapse before the piston at the receiving end moves through any sensible space.

Not only the general characteristics of the result, but the mathematical law of the whole action, will be precisely the same as that of a submarine cable, if we suppose the motion of the water through the tube to be obstructed by a porous material, as chips or minute particles of india-rubber, packed uniformly into it through its whole length, and of enough of elasticity to press out and fill the tube, or to yield to the tube when contracting, during the very small changes of diameter it experiences in the signalling. Siemens' phenomenon of "electric charge along a line" is here represented by the swellings and contractions of the india-rubber pipe.

The copper wire of the submarine cable, admitting, but not without resistance, the flow of electricity through it, is represented by the hollow spaces of the tube occupied by the porous material filling the tube and resisting, but not absolutely preventing, the flow of water through it. The law of this resistance is identical with Ohm's celebrated law of the resistance of metal wires to the electric current.

The electric charge upon the conductor of the cable under electric pressure, as we may call it for the moment (or difference of potentials, as the scientific world learned, long after his death, from George Green, the Nottingham miller's son, to call it), is less and less the thicker the gutta-percha is; just as an india-rubber pipe will be less and less affected by pressure the greater the thickness of its substance. One and the same mathematical law applies to the two phenomena, even including the relation between thickness and practical rigidity in one of them, and smallness of electric charge in the other.

If, now, instead of our india-rubber tube, we convert the whole atmosphere of the earth into an incompressible elastic material, and make a long bore or tunnel through it, in place of the telegraph wire, the circumstance of the ordinary long line of telegraph stretching through the air would be hydraulically represented with like completeness. The greatness of the solid mass surrounding the bore would make the yielding under pressure very much less than in an india-rubber pipe comparable with the dimensions of the gutta-percha in a submarine cable; and the signals would, accordingly, be sharper and quicker. There is, indeed, additional gain of sharpness in the air telegraph, in virtue of a remarkable and most startling discovery made by Faraday, of what he calls "specific inductive capacity" of insulating materials. The specific inductive capacity of gutta-percha is, according to measurement made by Mr. F. Jenkin, from two to three times that of air. Thus in our analogy, the elastic solid taking the place of air must be specifically from two to three times as rigid as that which we imagine substituted for the gutta-percha of a submarine cable, to give a just comparison of the telegraphic speeds of air and submarine lines.

As the galvanic battery is a subtle machine for maintaining, by almost innumerable molecular pumps, driven by chemical energy, a continuous flow of electricity through the circuit of which it forms a part, and the galvanometer is an instrument for measuring the rate of flow of electricity at any moment; the two pistons do not form a perfect analogy for the direct use of the battery in sending, and galvanometer in receiving messages.

But it is interesting to remark that the piston for sending signals in the water arrangement is an absolutely perfect scientific analogy for Mr. Varley's application of the condenser described below; and the piston to receive and indicate signals is pre

cisely analogous to an electrometer, or instrument for measuring electric potentials, which may be substituted, although not advantageously, for the galvanometer, or current measurer, as receiving instrument. But notwithstanding this, the mathematical law to which the deviations of the piston at one end, following gradually after the sudden impression on the piston at the other end, are subject, is identical with that of the increase of electric current at the receiving end of the submarine telegraph wire, when the galvanic battery is suddenly applied and kept in action at the other end.

The accompanying sketch represents the simplest possible arrangement of sending and receiving in

The keys (K K'), with the battery, constitute the sending instrument; the receiving instruments are indicated by RR'. Each key consists of two metal springs, set to press on a fixed metal bar. Wires connected with them in the manner indicated in the sketch, keep up the metallic connection between the telegraph line and the earth plate, unless either key is depressed by the finger of the operator. When this is done, a positive or a negative current is sent into the line, according as one spring or the other is depressed. But before beginning to send a message, he puts his own receiving instrument out of the circuit, by a simple movement of a metal piece, not shown in the sketch. If the Morse system is to be used, a single spring key at each end, instead of the double one shown in the sketch, is sufficient.

From the preceding explanation, it will be readily understood, that if the moveable magnet of the receiving instrument is, as in a relay, limited to a very narrow range in its motion by stops, and if the indication by which the message is sent is merely an indication of deflection or no deflection, an extremely slow rate of working will be entailed when the line is submarine and of great length. But if, on the other hand, the receiving instrument shows truly, by the amount of its deflection, the actual strength of the current, the sudden effect of depressing either key or letting it rise again will be perceptible even although the large varying

in the line from previous signals.

It was for this purpose that the telegraph mirror galvanometer was devised, eight years ago. It consists of a very light mirror, with a magnet cemented to its back, suspended by a single fibre of unspun silk, in the hollow core of a bobbin, wound round with fine silk-covered copper wire.

The adjuncts for its use consist simply of a lamp and a proper screen (a scale printed on white paper) to receive an image of the flame reflected from the mirror. The mirror may be slightly concave; and if so, it of itself brings the rays to a focus on the screen. Or if the mirror is plane, a convex lens in front of it, through which the light passes and repasses, produces the same effect. Thus an observer sees, as it were, an inverted image of the flame freely traversing the scale, and showing excessively minute deflections of the mirror, by large motions, easily followed and read off as signals.

The mirror is a little more than one-third of an inch in diameter, and weighs one-third of a grain (that is, about half the diameter of a threepennypiece and about one-sixtieth of its weight). The attached magnet is of about the same weight as the mirror; so that the whole suspended moving mass is less than three-quarters of a grain. A powerful steel magnet fixed outside the coil causes the suspended magnet to return with extreme rapidity to its middle or undeflected position, when the deflecting force ceases. This instrument may be used for either Morse signals or Steinheil signals. Thus

equal rapidity, but continuing with a diminishing speed for a longer time before the return impulse becomes sensible. This was the system employed in reading the messages which passed through the 1858 cable.

The Steinheil system, which, all circumstances being the same, admits of a higher speed than the Morse, in consequence of saving the extra time consumed by the dashes, has been used hitherto in working the cables completed in 1866. The positive current throws the luminous image to the right, from which it begins quickly to return; but before it has returned by one-tenth of the distance to its andeflected or "zero" position, a second positive signal may follow and give it a second impulse a little further to the right, a third may give it immediately after a third impulse to the right. If, after this, its return to zero is undisturbed, the reader understands that the letter s (rrr) is meant.

But if it suddenly begins going much further to the left than corresponds to the s, and then settles towards zero, the reader understands the letter v rrrl). Thus, in one case, three positive currents at the sending end, and in the other, three positives followed by one negative, are read with ease by a practised eye, even though the luminous image never returns to zero after its first deflection to the right. All that a relay could have indicated in such & case, would have been the first contact of the moving magnet on the stop limiting its excursion. The plan which has been actually used in the Atlantic signalling of 1866, differs from that illustrated in the preceding sketch by the introduction, due to Mr. Varley, of a "condenser" at one end of

the line, by which a very considerable increase of speed is obtained. The term "condenser" has long been used among electricians, to denote an arrangement in a moderate compass equivalent to a Leyden jar of enormous capacity. One coating of this Leyden jar is put in direct communication with the conductor of the cable. The other is joined to the sending key, or the receiving instrument, according as the message is being sent from or received at the condenser station. To attempt to explain the principle of this valuable application, for diminishing the embarrassments experienced in working through a submarine cable, would carry us into details regarding electrical principles beyond the scope of the present paper.

By the very simple arrangements I have now described, in the hands of remarkably skilful and apt operators, speeds of thirteen or fourteen words a minute were attained through the 1866 cable, within ten days after its completion; indeed, before the Great Eastern had left Heart's Content to recover the lost cable of 1865. It is quite certain that, as soon as a higher speed is demanded for the public service, a higher speed may with great ease be obtained; by means of sending instruments constructed to carry into effect methods which mathematical theory long ago indicated for applying a battery at one end in such a manner as to produce the sharpest and shortest possible effect at the other.

The question is often put-"What is the velocity of electricity?" or, in more familiar language, sometimes, "How long does electricity take to go across the Atlantic?" In truth, we know of no limit to the velocity of electricity. In 1834, Professor Wheatstone found that the effect of an electric impulse, on a circuit of copper wire on which he experimented, produced a spark in a distant part of the circuit so quickly, that the influence must have travelled along the wire at a velocity one and a half times greater than that of light. This would correspond to a distance of 250,000 nautical miles (or about twelve times round our globe) in one second of time. At this rate, the first effect of an electric impulse would reach the remote end of the 1850 nautical miles of either Atlantic cable in from seven to eight thousandths of a second, or something less than a hundredth of a second. In all probability a real electric effect does reach the remote end of the cable with some such almost inconceivable rapidity as this. But the mathematical theory proves that, after the instant of applying the battery at one end of the cable, an interval of from one to two tenths of a second of time elapses before the effect becomes sensible to the most delicate instrument at the other end. But the explanations which I have given above show that these questions are irrelevant to telegraphic signalling through an Atlantic cable; and therefore, interesting as their subject is, I refrain from more than alluding to them in the present article.

WILLIAM THOMSON.

IN the spring and early summer of the year now past, the year 1866,-the hatred felt by Venetians towards the Austrian soldiers who held their city in thraldom, had reached its culminating poiut. For years this hatred had been very strong;-how strong can hardly be understood by those who never recognised the fact that there had been, so to say, no mingling of the conquered and the conquerors, no process of assimilation between the Italian vassals and their German masters. Venice as a city was as purely Italian as though its barracks were filled with no Hungarian long-legged soldiers, and its cafés crowded with no whitecoated Austrian officers. And the regiments which held the town, lived as completely after their own fashion as though they were quartered in Pesth or Prague or Vienna, -with this exception, that in Venice they were enabled,—and indeed from cir- | cumstances were compelled,-to exercise a palpable ascendancy which belonged to them nowhere else. They were masters, daily visible as such to the eye of everyone who merely walked the narrow ways of the city or strolled through the open squares; and as masters they were as separate as the gaoler is separate from the prisoner. The Austrian officers sat together in the chief theatre,-having the best part of it to themselves. Few among them spoke Italian. None of the common soldiers did so. The Venetians seldom spoke German; and could hold no intercourse whatever with the Croats, Hungarians, and Bohemians, of whom the garrison was chiefly composed. It could not be otherwise than that there should be intense hatred in a city so ruled. But the hatred which had been intense for years had reached its boiling point in the May preceding the outbreak of the war.

Whatever other nations might desire to do, Italy at any rate was at this time resolved to fight. It was not that the King and the Government were so resolved. What was the purpose just then of the powers of the state, if any purpose had then been definitely formed by them, no one now knows. History perhaps may some day tell us. But the nation was determined to fight. Hitherto all had been done for the Italians, and now the time had come in which Italians would do something for themselves! The people hated the French aid by which they had been allowed to live, and burned with a desire to prove that they could do something great without aid. There was an enormous army, and that army should be utilised to the enfranchisement of Venetia and to the great glory of Italy. The king and the ministers appreciated the fact that the fervour of the people was too strong to be repressed, and were probably guided to such resolutions as they did make by that appreciation. The feeling was as strong in Venice as it was in Florence or in Milan; but in Venice only,-or rather in Venetia only-all outward signs of such feeling were repres

sible, and were repressed. All through Lombardy and Tuscany any young man who pleased might volunteer with Garibaldi; but to volunteer with Garibaldi was not, at first, so easy for young men in Verona or in Venice. The more complete was this repression, the greater was this difficulty, the stronger of course arose the hatred of the Venetians for the Austrian soldiery. I have never heard that the Austrians were cruel in what they did; but they were determined; and, as long as they had any intention of holding the province, it was necessary that they should be so.

During the past winter there had been living in Venice a certain Captain von Vincke, Hubert von Vincke,- --an Austrian officer of artillery who had spent the last four or five years among the fortifications of Verona, and who had come to Venice, originally, on account of ill health. Some military employment had kept him in Venice, and he remained there till the outbreak of the war; going backwards and forwards, occasionally, to Verona, but still having Venice as his headquarters. Now Captain von Vincke had shown so much consideration for the country which he assisted in holding under subjection as to learn its language, and to study its manners; and had by these means found his way more or less into Italian society. He was a thorough soldier, good looking, perhaps eight-and-twenty or thirty years of age, well educated, ambitious, very free from the common vice of thinking that the class of mankind to which he belonged was the only class in which it would be worth a man's while to live, but nevertheless imbued with a strong feeling that Austria ought to hold her own, that an Austrian army was indomitable, and that the quadrilateral fortresses, bound together as they were now bound by Austrian strategy, were impregnable. So much Captain von Vincke thought and believed on the part of his country; but in thinking and believing this, he was still desirous that much should be done to relieve Austrian Italy from the grief of foreign rule. That Italy should succeed in repelling Austria from Venice was to him an absurdity.

He had become intimate at the house of a widow lady, who lived in the Campo San Luca, one Signora Pepé, whose son had first become acquainted with Captain von Vincke at Verona. Carlo Pepé was a young advocate, living and earning his bread at Venice, but business had taken him for a time to Verona; and when leaving that city he had asked his. Austrian friend to come and see him in his mother's house. Both Madame Pepé and her daughter Nina, Carlo's only sister, had somewhat found fault with the young advocate's rashness in thus secking the close intimacy of home life with one whom, whatever might be his own peculiar virtues, they could not but recognise as an enemy of their country.