through a piece, M, that serves as a safeguard, and from thence reaches the wire, L, which passes to the station where the message is to be delivered. We see the same wire entering at the top of Fig. 302, whence the current passes through a safeguard, M, then into the galvanometer, from which it goes to the electro-magnet of the receiver. After passing through the electro-magnet, it passes through the wire, T, and is lost in the earth.

Morse's Manipulator and Receiver.

437. MORSE'S MANIPULATOR is shown in Fig. 303. It consists of a wooden stand, upon which is a metallic lever, kh, turning about a horizontal axis. One end of this lever is raised up by a spring, r, and the other is traversed by a

stem, a, which rests upon a copper button, and this in turn communicates through the stand with the wire, A. Fig. 303 represents the manipulator at the instant when it receives a dispatch. The current arrives by the wire, L, which is the wire of the line, rises into the lever, kh, and descends by the wire, A, to the receiver.

When it is desired to transmit a signal, it becomes necessary for the current from the battery, P, to enter the manipulator. This is not effected when the latter is disposed as in Fig. 303, for the lever, kh, does not touch the button in which the wire, P, terminates. By pressing the

button, B, the lever, kh, is lowered; a contact is established, the current passes immediately into the wire, L, which leads to the other station.

The RECEIVER, Fig. 304, is composed of an electro-magnet, E, which, whenever a current is transmitted, acts by attraction upon an armature of soft iron, m, fixed at the extremity of a lever, mn, and movable about an axis. At its extremity, n, the lever carries a point, 2, which may be made to press against a movable fillet of paper, ab. When the current does not pass through the electro-magnet, the point, x, does not press against the fillet; but as soon as the current passes, the point is pressed against the paper, and traces upon it either a point, or a line more or less elongated,

according to the length of time during which the current is uninterrupted.

The fillet of paper is kept in motion uniformly by means of a train of clock-work, V, which turns the cylinder, Z (Fig. 301). The fillet of paper moving uniformly in the direction from a to b, the operator at the other station, by pressing the button of the manipulator, and maintaining the pressure for greater or lesser periods of time, causes a succession of points and marks to be made upon the fillet at

Describe tho Receiver in detail. Its use. How is the fillet of paper kept in motion? How are the letters recorded?

pleasure. These marks are, by convention, made to stand for the letters of the alphabet, as shown in the following

It only remains to explain the PROTECTOR, M. Experience has shown that the wires may, from atmospheric influences, accumulate upon themselves sufficient quantities of electricity to prove troublesome to the operators of the telegraph. The piece, M, is destined to prevent any injurious action of this kind. It is composed of two toothed pieces of metal, disposed so that the teeth are nearly in contact. The current passes into one of these pieces, whilst the other is in communication with the earth. If, from any atmos. pheric change, electricity accumulates upon the wires or apparatus, it is given off by the points to the piece which is in communication with the earth, and shocks are thus avoided.

In what has been said, only a single wire, L, has been spoken of as running from station to station. It would seem to be necessary, in order to complete the circuit, that a second wire should be em ployed; such however is not the case. The employment of a second wire is avoided by connecting the two ends of the single wire with the earth. The parts T, Figs. 301 and 302, are for this purpose

Explain the Protector. Its use. Why is it possible to operate a line of telegraph with a single wire?

prolonged from the instruments till brought into free communication with the earth. The fluid then continues to circulate just as though a return wire had been used.

438.

Velocity of Electricity.-Submarine Cables.

It has been found by experiment that the velocity of electricity is such as to carry a current around the earth in about a quarter of a second. For short distances, then, we may regard the transmission as instantaneous.

Since the invention of the telegraph, a complete net-work of lines has been established over both continents. Not only have thousands of miles of wires been stretchfed on land, but submarine wires have been laid, connecting places separated by hundreds of miles of water. Telegraphic wires connect England and Ireland, England and France, France and Algiers, and so on. Finally, an attempt has been made to connect the two continents, and although it has thus far failed to be successful, there is good reason to anticipate a complete success at no distant day. Signals and messages have been transmitted from Ireland to Newfoundland, and the possibility of the connection has thus been fully demonstrated.

Electro-Magnetic Motor.

439. Many attempts have been made, and with partial success, to employ electro-magnetism as a motor for the propulsion of machinery. JACOBI, of St. Petersburg, constructed an engine of this kind in 1838, which was capable of propelling a boat containing twelve persons. Many other machines have since been constructed, but in all cases the expense of moving them has been so great as to preclude their economical use.

Fig. 305 represents an electro-magnetic machine, constructed according to the design of M. FROMENT. It is composed of four electro

(438) What is the velocity of an electrical current? Give an account of some of the submarine lines of telegraph. (439.) Has electricity been used as a motor? Describe M. FROMENT's machine in detail.