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.

Velocity of Electricity.-Submarine Cables.

438. 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 stretched 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.

magnets, acting in pairs upon two pieces of soft iron, P, only one of which is seen in the figure. These pieces, attracted by the electromagnets, EF, transmit the motion by means of a working-beam, to a crank, m, fixed at the extremity of a horizontal arbor. The latter bears an iron fly-wheel, which regulates the motion. Finally, the same arbor supports a piece of metal, n, of a greater diameter, the use of which will be explained presently.

The current from the battery, P, entering A, passes into a platform of cast-iron, B, then, through different metallic pieces, it reaches the arbor and the piece n. From thence the current flows alternately to the electro-magnets, EF and ef. The manner in which this alternate flow is effected, is shown in Fig. 306, which represents a section of the piece, n, and its accessories. Upon the piece, n, is a projection, e, called a cam, which in the course of one revolution touches successively two pallets, a and b; these transmit to the electro-magnets the current, whose course is indicated by the unfeathered arrows. The feathered arrows in the figure show the direction in which the parts of the machine move.

The current passing alternately into the two pallets, a and b, and thence into the systems of electro-magnets, EF and ef, the piece, P, is first attracted, and then a similar piece at the other extremity of the arbor of the fly-wheel is attracted, and so on. The result is a continuous rotary motion, which is transmitted by a driving-band to a train of wheels, and so on to the pumps, which it is destined to work.

III. INDUCTION. —APPLICATION то MEDICINE.

Induction by Currents.

440. We have seen that the electricity of the machine acts upon bodies by induction. The electricity of the battery acts in a similar manner, but only when the currents begin to flow and when they cease.

To show this, take two copper wires, covered with silk, and wind them side by side upon a bobbín. Then fasten the two ends of the

Its mode of action. (440.) Does galvanic electricity act by induction? How is

this shown?

first wire to the two binders, m and n, of the galvanometer, Fig. 297. Next connect one end of the second wire with one pole of a feeble galvanic battery: If the other end of the second wire be brought into contact with the second pole of the battery, at the instant of contact, the needle of the galvanometer will indicate the production of a current in the first wire, flowing in an opposite direction to that of the battery. If the contact is kept up, the flow of the induced current ceases, as is shown by the needle of the galvanometer returning to its position of rest. If the current of the battery is broken, the needle of the galvanometer is again deviated, but in a contrary direction, indicating an induced current flowing in the same direction as that of the battery.

The battery current is called the inducing current; the other current is called the induced one. These currents conform to the following laws:

1. At the instant when the inducing current begins to flow, an induced current is developed flowing in a contrary direction.

2. The inducing current continuing to flow, the induced

current ceases.

3. At the instant when the inducing current ceases to flow, an induced current is developed flowing in the same direction.

Properties of Induced Currents.

441. Experiment has shown that induced currents possess all the properties of other electrical currents. Like them, they give sparks, produce violent shocks, decompose water, salts, and the like, and act upon the magnetic needle.

Induced currents are the more powerful, the longer the wires em

What is the direction of the induced current at the instant of closing the circuit? Of breaking it? What is the inducing current? The induced one? What are the laws that govern the induced currents? (441.) What are the properties of induced currents?

ployed. Hence, in practice it is usual to wind the wires upon bobbins, as shown in Fig. 307.

The coil shown in Fig. 307, consists first of a cylinder of pasteboard, upon which is wound about three hundred coils of coarse copper wire. This is the inducing coil. Over it is a finer wire, making several thousand coils. These wires are not only covered with

silk, but also with an insulating varnish of gumlac. At the extreme left of the stand on which the coil rests, are two binders in connection with the two poles of a battery. From one of them proceeds a plate of copper, going to a toothed wheel, moved by clockwork, and communicating with one of the ends of the inducing coil;

How are the wires wound? How insulated? How are the battery communications made?