disturbances it may be subjected to when in place.

Consisting, as a cable does, of a long conductor surrounded by a thin layer of insulating material, and then again by a conductor, it is electrically similar to a great condenser, or Leyden jar, as was observed by Faraday in connection with some of the earlier short cables. Reference has already been made to what is called the "static capacity" of a land line: that of a cable of the same length is generally many times as great. If a well-insulated wire, a few hundred or a thousand feet in length, be coiled in a vessel of water, with one end projecting into the air or sealed over with guttapercha, the current from a single battery-cell, in rushing in to charge the wire, will cause a violent deflection of the needle of a sensitive galvanometer. A considerable length of time will be consumed in completely charging a cable, and, of course, time will be occupied in dis, charging it. The result of this is a great retardation of signals and a correspondingly less speed of transmission. It is said, that, if the attempt were to be made to use ordinary Morse instruments on one of the Atlantic cables, hardly more than one word per minute could be transmitted. The signals are not only retarded: they are altered in character, becoming

less sharp and distinct as the length of the cable increases.

As already remarked, the use of strong currents is extremely objectionable, and thus there are several reasons why ordinary methods of operating prove insufficient when applied to ocean cables. Scarcely any modification is required in the sending apparatus: a single key for closing the circuit may be used, or a double

Ocean cable system; a a, condensers; g, galvanometer or receiving instrument.

key, by means of which either positive or negative electricity may be sent to the line. It is found to be advantageous, however, not to connect the battery with the line at all; that is to say, not directly, but only indirectly through a condenser, one branch of which is connected with the line, and the other with the earth through the battery, key, and receiving instru

ment. The condenser is prepared by insulating sheets of tinfoil from each other, as already described. The surface of foil used in one of these condensers is only slightly less than one acre, although it occupies a space of less than three cubic feet; and, for the purpose of "duplexing" the cable, a condenser of more than two acres of surface has been used. The use of a condenser increases the speed of transmission, besides offering other advantages. The signals are received by an extremely sensitive galvanometer, devised for the purpose by Sir William Thomson, to which reference has already been made. In this the wire is very fine, and the number of turns very great. The needle is extremely small, consisting of several short magnets fastened to the small circular mirror, the whole often weighing less than half a grain. This needle is suspended by a single fibre of silk in the centre of the coil, which is wound as closely to it as the necessary freedom of motion will allow. A beam of light falls on the mirror, and is reflected upon a screen, where a spot of light is seen. The movements of the needle are indicated by and magnified in the motion of this spot, and the alphabet is made up of to-and-fro movements.

This beautiful instrument has been used on many cable lines, but it has been largely super

seded by the "siphon recorder," devised by the same distinguished electrician. In this a light, rectangular coil of fine wire is suspended be

Thomson's siphon recorder for ocean cables.

tween the poles of a powerful electro-magnet. Advantage is taken of the fact that a coil of wire through which a current is passing tends

to place itself in a particular position in a magnetic field. A fine glass siphon tube is attached to the coil, and moves with it. The short arm dips into a vessel of ink, which is insulated and capable of being electrified. The long arm has its open end very near to a small plate or table, over which a strip of paper is moved regularly by clock-work, as in the Morse register. The whole system (tube and coil) moves with great

freedom, and is deflected from its normal position by very feeble currents. The electrification of the ink causes it to be projected from the end of the tube in minute drops, so that the movements of the coil are recorded on the moving slip of paper in very fine dots very near to each other. An actual record of the message is thus made, which can be read at leisure and preserved. As noticed by Mr. Prescott in his valuable work on the telegraph, it is curious to