earth may evidently be regarded as a conductor, of which the diameter is infinite, compared with its length, and we might therefore be led to expect the result mentioned above.

To return, however, to the description of the apparatus, the key or handle by means of which the connection of the battery is effected and varied, consists of a cylinder, in which is a middle zone, e, of hard wood or ivory, while the ends are of metal. One of these ends, c, extending through the case in front, forms the actual handle; while the other, d, is turned down to a shoulder and forms a pivot, which rests in a brass collar, p. The end d carries a steel pin, f, projecting upward, and c, a similar pin g, directed downward. The battery being connected with the terminals z and c, and thence by the brass strips k and m, of which the extremities rest as springs on the metallic ends of the cylinder, may in fact be considered as having its poles at the two pins, ƒ and g, which are fixed in those ends. Two springs, h and h', are fixed by broad feet to the base of the instrument, and rest by their upper extremities on two studs or points projecting from a brass rod, i,

screwed into the case in front. These springs form the circuit between the terminal 71, with which one end of the conducting wire is connected, and 14. Between this

latter terminal and the external connection, 12, the coil itself, and the brass strip extending from 13 to 72, are interposed. When a signal is being sent through the instrument (supposed to hold the place of No. 2 in fig. 5), the current from No. 1 being considered to enter from the long wire on the line, by the terminal 2, passes along the brass strip to the terminal 13, thence through the coil to 14; then to the spring h, across the pin i, down the spring h', and to the terminal 1 in connection with the wire leaving the station. Were the instrument in question situated as No. 1 or No. 3, it will be seen that one of the terminals, 1 or 12, according to its position, would be joined to the wire coming from the earth-plate.

When a signal is to be given by the instrument, the handle being turned, as in figs. 3 and 4, one battery-pin, f, is brought in contact with the spring h', which is bent back and released from its contact with i, while the other battery-pin g, is pressed against the foot of the other spring h, which at this part is turned up to act as a stop. The current being then supposed to start from the pole f, proceeds down the spring h', to the coil and terminal 2, as before; thence along the line through the other instruments, and into the earth at the further extremity of the line. Re-ascending from the earth by the wire connected with the terminal 71, it gains the foot of the spring h, with which the second battery pole g, is in contact. It will be seen that the direction of the current is different, according as the key is turned to the right or left. The position of the battery wires is such, that the needle shall be deflected in parallelism to the handle. A comparison of fig. 5, in which three instruments are shown in series, with the figures 1, 2, 3, and 4, will render

the method of connection, both of the two terminal and of the intermediate instruments, sufficiently obvious. The wires on entering a station are designated as 'up' or 'down' wires, according to the portion of the line from which they come. Care must be, of course, taken that all the instruments in one series are similarly joined to the up and down wires, so that the course of the current may be alike in all. At the extreme 'up station' the earth connection becomes the 'up wire,' and at the opposite terminus it takes the place of the 'down wire.'

The electric fluid is represented as starting from one pole of the battery only, and, after traversing the circuit, returning to the other pole. It is, however, more consonant with the theories deduced from the observation of electrical phenomena, to suppose that force is developed equally at both poles of the battery.

The second principle is that of the temporary magnetization of soft iron by the electric current, applied for the purpose of sounding an alarum at a distant station, in order to summon the attendant to his instrument. The same principle has been employed from an early period of the invention, both by Mr. Cooke and by Prof. Wheatstone, to transmit visible signals by causing the rotation of a disc, bearing letters or figures, or of a hand or index pointing to characters on a fixed dial; but in England the needle telegraph has been universally adopted, in preference to any other form. Many improvements have, however, been effected within the last few years by Prof. Wheatstone in the construction of the mechanical telegraph, as it has been named in contradistinction to the needle instrument. The same gentleman has also succeeded in substituting for the voltaic battery in the working of this telegraph,

the magneto-electric machine, in which the current is derived by induction from a permanent mag

net. The improvement which was effected by this adoption of a source of power, alike energetic and unalterable, will be immediately perceived. Telegraphs on this plan of construction have been erected in Prussia and France, and, were it not for some very marked advantages connected with the use of the needle signals, they could hardly fail to become general.

The ringing of the alarum was originally effected by the direct action of the voltaic magnet upon the hammer of the bell; but this method has been long superseded. The apparatus generally in use for this purpose is shown in a front and back view. in figs. 11 and 13, and in a side view at figure 12. An electro-magnet is formed by coiling fine insulated copper wire around two cylinders of very soft pure iron. These coils, c c, are then connected by two of their ends, in such a manner that the direction in which the wire is wound about the iron cores may be alike in both. The iron cylinders are joined together at one end by a cross-piece d, likewise of soft iron, so that the whole then forms a horse-shoe magnet, having however its two sides parallel. In front of the free ends or poles of this magnet, which is fixed on the top of the plates of the alarum, an armature a, of soft iron, is placed at such a distance that it may be strongly attracted by the electromagnet when the circuit is completed through its coils. The armature moves on an arbor, upon which a detent or catch e, is fastened, and so arranged that it is disengaged from a small fly v, whenever the attraction takes place. This disengagement allows a train of clock-work, impelled by a spring or weight, to run down, and by the action of a scape-wheel and pal

lets, seen in fig. 11, a hammer, h, rapidly strikes a small bell g. Immediately, however, that the ourrent ceases to flow through the coils, the iron within them loses its magnetism, a small re-acting spring, s, draws back the armature, and interposes the detent so as to stop the clock-work. It is necessary that the iron of the magnet should be quite pure and soft, as otherwise the magnetization is to a greater or less degree permanent; and this may be the case to such an extent as to keep the armature attracted, even after the cessation of the electric current. The bell would then continue to ring until the disengagement of the armature were caused by the hand of the attendant.

The bell or alarum may form part of the telegraphic system in two modes. In the first, and most economical, its coil is made to form part of the circuit of one of the needle wires, in conjunction with a key or rheotome. In the second and more complete method of introducing the alarum, a distinct wire is employed for it, in the course of which the magnet coils at the several stations are interposed. A key of different construction (shown in section at fig. 14) is then employed. The body is of brass, but two stout wires, z and c, are conducted through ivory tubes, and terminated in studs, pp, at the top and bottom of the cylindrical end of the key; the wires and studs being insulated from each other, and from the key, by the ivory in which they are fixed. The collar, t, and nut, t, serve to secure the key to the side of the case or box in which it is placed, the former, t, also containing the spring, by which, after use, the key is brought back to its quiescent position. Two springs, not shown in the figure, then rest against the metal of the end, one on each side, and while in this position, merely

complete, by the intervening metal, the circuit of the bell-wire which is connected to the foot of one spring directly, and to the foot of the other by the intervention of the bell coil. The wires, z and c, are joined to the two poles of the battery by pieces of thin wire, which will offer no resistance to the revolution of the key on its axis. In the quiescent position, the course of a current entering from a distant station would be, from the linewire on one side, along one spring, across the body of the key to the other spring, and thence through the alarum coil to the continuation of the line-wire, or to the earth connection. When, however, the bell is to be rung to call the attention of the clerk at another station, the key is for a moment turned onequarter round. This brings the battery studs, pp, into the circuit, instead of the body of the key; and the current then proceeds from the battery of the ringing station by the spring and wire on one side, passes along the line, and returns by the earth through the other spring again to the battery.

The relative advantages of these two methods may be briefly stated. The first has the recommendation of economy, inasmuch as no additional wire is necessary for the bell. If, however, the clerk in charge of an instrument, after turning off his bell preparatory to sending or transmitting intelligence, should chance to leave his telegraph, and omit to turn the key so as to replace the bell in the circuit, no means are left to other stations of calling his attention, except by working the needles, with the chance of their movement meeting his eye. By the adoption of the second method, the expense of an additional wire is incurred, but the bells at all stations are constantly in a position to be rung if necessary. In addition to this, it may be remarked, that by keep