all yielded the palm to electricity, it is curious to review Dr. Guyot's ideas, of only a few years ago, and he was only one of thousands,-who foresaw the difficulty of erecting and insulating wires, of protecting them from malevolence; of keeping up supplies of electric force; of sending the force to a distance; the expense of training and maintaining an enormous staff; of stores, too, and apparatus, and the "thousand other practical difficulties which unite to terrify the boldest imagination." But these apparent difficulties have vanished before the persevering ingenuity of man, and have settled themselves down into the mere routine of every-day duty.

8. As soon as electricity had revealed the great velocity with which it moves, and this was towards the latter part of the last century, the idea had birth of causing it to produce distant signals; and many curious suggestions were made but seeing that, in those days, friction was the only practical source of electricity, and tension the only form in which it was developed, it may readily be supposed that no great progress was made towards a true and useful telegraph. The difficulty of generating free electricity at command, and the extreme precautions required for its insulation, were serious obstacles in the way to success; so that we can scarcely regard the telegraphs that were then constructed or proposed, in any other light than as philosophical apparatus to illustrate the idea of electric-telegraphy.

9. Sparks and repulsions were the two forms under which it was first proposed to employ free electricity. One or more sparks, variously combined, were to constitute the signals; or the letters were to be traced in tinfoil, which was so cut away in the course of the discharge, that the group of sparks occurring at the intervals showed an illuminated letter. Attention was in some cases to be gained by the spark occurring in hydrogen gas and air, and so causing explosion. As late as the year 1844, Highton proposed piercing papers with sparks; and he reports that he tried the plan with success, by means of a small Leyden jar, along a distance of ten miles; but it has never been turned to practical account. 10. One philosopher proposed a couple of dozen pairs

of pith-balls or gold leaves, and as many wires, a pair for each letter, and whichever wire was electrized, its leaves diverged. Some proposed fewer pairs, and conventional signals. In 1816 Mr. Ronolds insulated eight miles of wire, on his lawn at Hammersmith, with silk and dry wood. He had a clock at each end, and each clock carried a disc with letters on the circumference; a screen was in front of this disc, and, through a hole in the screen, letter by letter was seen as the disc revolved; and as the clocks were uniform in motion, the same letter appeared at the same time at each station. A gas pistol was fired to call attention. Then, as soon as the letter came in sight that was wanted, the sender electrized the wire, and a pair of pith-balls at the distance diverged; when the next required letter came, he did the same, and so on till his message was spelled out. The receiver made a note of the letters that were visible whenever he saw the balls diverge. The same experiment was tried through 525 feet of wire buried in glass tubes in a trough of pitch.

11. These and other experiments firmly established the idea of applying electricity for telegraphic purposes; and the discovery of current electricity, at the opening of the nineteenth century, was soon followed by attempts to carry out more successfully the well-recognised idea. Many of the early attempts ended in producing various ingenious philosophical toys, but no apparatus worthy of being dignified by the name of a telegraph. About ten years after the discovery of the voltaic pile, Soemmering proposed thirty-five wires, terminating in gold points in acid water, each to represent a letter, sign, or figure; the water was decomposed, and gas given off at the two points, through which a current of electricity was sent; and thus two letters were indicated, the point at which hydrogen was given off indicating the first letter. He called attention, by holding on the current till enough gas collected in a receiver to produce motion and liberate a ball and let it fall on a bell. About the same time other cases of decomposition were proposed, so as to produce change in colour. Nearly thirty years later, it was proposed to reduce the thirty-five wires to two, and by strong

or weak currents, and long or short intervals, to form an alphabet. About the same time it was elsewhere suggested to put the ten fingers upon ten wires, each finger to represent a signal, and to discharge electricity by one or other wire, and thus give a shock to one or other of the fingers, according to the signal required. Dr. O'Shaughnessy suggested administering shocks, according as the required signal was brought to view by discs revolving by aid of sympathetic clocks.

LESSON II.

ELECTRO-MAGNETIC TELEGRAPHS.

1. IN 1819 Ersted laid the foundation of electromagnetism, having discovered the relation between current electricity and a magnetized needle. And in the following year it occurred to Ampère that a set of galvanometers might be prepared, each with its own wire and one for each letter, and that their deflections at a distance, by means of current electricity, would constitute a practical electric telegraph. There was evidently something manageable in the galvanometer; and Ampère's ideas were re-echoed and expanded. In succeeding years we find propositions of one coil and two needles; thirty wires and needles to carry screens ; thirty-six wires and needles; twelve needles and screens ; one needle; two dozen needles; five needles; then we have crescent needles, horse-shoe needles, and suspended needles.

2. The electro-magnet has also been very extensively introduced into the construction of telegraphic apparatus, either for moving an index, or for liberating clockwork, or for making marks or punctures, and in many other ways. It would, indeed, require volumes adequately to describe the various applications of the fundamental principles of electricity to the construction, not merely of impracticable apparatus, but of real working electric telegraphs. It will serve our present purpose to select from the telegraphs now in use, and that have gained a reputation, some striking illustrations of the manner in which the principles are carried out.

3. Cooke and Wheatstone's double-needle instrument

is familiar to every one who has seen a telegraph station in England. It has been from the first established in this country, and has well maintained its ground: the instrument of to-day differs but little from its predecessors, and the difference is only in detail; the galvanometers are made more delicate, the needles more sensitive; the mechanical parts firmer, and the whole more compact. It consists of two vertical galvanometers, to each of which a separate wire is devoted. An axis carries two needles, one within the coil of the galvanometer, and out of sight, and which, from its position, is most concerned in producing the action, and the other in front of the coil, outside a dial, serving as the index; the inner needle has its north pole downward, the outer, its south. A current passing in one direction through the coil deflects the index-needle to the right; in the other direction, to the left. The letters of the alphabet are made by one, two, or three, and in some cases four movements, of one or other or both needles. The letters begin with the left needle; and when its combinations are exhausted, proceed to the right needle, and finish with both needles. For instance, A and F, I and O, S and X, are made by two motions; B and G, K and O, T and Y, by three motions. The first pairs are made with the left needle, the second pairs with the right, and the third pairs with both. The first of each pair is made by left-hand deflections; the second of each by right-hand. When two needles are used, the deflection of the bottom half of the needles is read off.

These signals and all others are made and read with extreme rapidity by the initiated, but naturally puzzle a stranger. Wires are led from instrument to instrument in the same group, in such a manner as to cause the signals on each to correspond.

4. The current employed for actuating these instruments is obtained from the simplest form of voltaic battery. A trough of wood, gutta-percha, or glass is divided into cells; the cells are furnished with amalgamated zinc and copper plates, and then filled with sand, saturated with sulphuric acid and water; or, which is better, with plates of amalgamated zinc and graphite, no

sand being used, and the zinc being retained by contact with mercury in a permanent state of amalgamation. The number of cells employed depends on the distance along which the signals must travel. We have thus, in the galvanometer and the battery, the essence of the telegraph; and the only portions at all complex are the mechanical arrangements for bringing these two things together-for guiding, that is, the voltaic current at will in this or that direction through either or both coils.

5. This operation may be very well traced in fig. 2.

It represents a telegraph needle at London and another at Dover; it shows how the wires are arranged for establishing communication between the two; and it further shows the mechanical operation by which a signal, made at Dover, is shown both there and at London. It shows, indeed, the actual section of an instrument, except that, instead of a coil of wire round the needle, which would make the matter less simple, we have merely introduced a single wire between the spectator and the needle; for what is true of one wire is true of every wire of the coil. The needle here represented is the one that is within the coil, and invisible to the eye in the actual instrument. The mechanical parts of the instrument consist of two vertical curved springs, which