If the inclination of the cable had been exactly 6° 45' when the speed of the Great Eastern was exactly 61⁄2 miles per hour, the value of p for the Atlantic cable of 1865 would be exactly 61÷8478, or 765 of a mile per hour.

[From Transactions of the Institution of Engineers and Shipbuilders in Scotland, .18th March, 1873.]

ART. LXXXV.-ON SIGNALLING THROUGH SUBMARINE CABLES.

THE Lecturer began by explaining the terms "electrostatic capacity " as applied to a submarine telegraph cable, and, referring to the phenomena of the transmission of an electric impulse through such a cable, he described how that quality effected the speed of signalling, by giving rise to "Retardation." These points were illustrated by signals sent through a model cable-an ingenious apparatus devised and constructed by Mr Cromwell Varley, and consisting of a combination of his condensers with resistancecoils, embodying in an instrument convenient for experiment, the conditions under which electric signals are transmitted through a submarine cable.

The instrument first used for receiving signals through a long submarine cable (the short-lived 1858 Atlantic cable) was the Mirror Galvanometer, which consisted of a small mirror with four light magnets attached to its back (weighing, in all, less than halfa-grain), suspended by means of a single silk fibre, in a proper position within the hollow of a bobbin of fine wire: a suitable controlling magnet being placed adjacent to the apparatus. The action of this instrument is as follows. On the passage of a current of electricity through the fine wire coil, the suspended magnets with the mirror attached, tend to take up a position at right angles to the plane of the coil, and are deflected to one side or the other according as the current is in one direction or the other.

Of various other forms of receiving instruments which he had devised, the lecturer referred specially to the Spark Recorder, both on account of the principles involved in its construction, and because it in some respects foreshadowed the more perfect instrument, the Siphon Recorder, which he introduced some years later. The action of the spark recorder was as follows. An indicator, suitably

supported, was caused to take a to and fro motion, by means of the electro-magnetic actions due to the electric currents constituting the signals. This indicator was connected to a Ruhmkorff coil or other equivalent apparatus, designed to cause a continual succession of sparks to pass between the indicator, and a metal plate situated beneath it and having a plane surface parallel to its line of motion. Over the surface of this plate and between it and the indicator, there was passed, at a regularly uniform speed in a direction perpendicular to the line of motion of the indicator, a material capable of being acted on physically by the sparks, either through their chemical action, their heat, or their perforating force. The record of the signals given by this instrument was an undulating line of fine perforations or spots, and the character and succession of the undulations were used to interpret the signals desired to be sent.

The latest form of receiving instrument for long submarine cables, is that of the Siphon Recorder, for which the lecturer obtained his first patent in 1867. Within the three succeeding years he effected great improvements on it, and the instrument has, since that date, been exclusively employed in working most of the more important submarine cables of the world-indeed all except those on which the Mirror-Galvanometer method is still in use.

In the siphon recorder (a view of which is shewn in Fig. 1), the indicator consists of a light rectangular signal-coil of fine wire, suspended between the poles of a powerful electro-magnet, so as to be free to move about its longer axis which is vertical, and so joined up that the electric currents constituting the signals through the cable, pass through it. A fine glass siphontube is suitably suspended, so as to have only one degree of freedom to move, and is connected to the signal-coil so as to move with it. The short leg of the siphon-tube dips into an insulated ink bottle, which permits of the ink contained by it being electrified, while the long leg is situated so that its open end is at a very small distance from a brass table, placed with its surface parallel to the plane in which the mouth of this leg moves, and over which a slip of paper may be passed at a uniform rate as in the Spark Recorder. The effect of electrifying the ink is to cause it to be projected in very minute drops from the open end of the siphon-tube, towards the brass table or on the

Fig. 1.

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S. MILLER

paper-slip passing over it. Thus when the signal-coil moves in obedience to the electric signal currents passed through it, the motion then communicated to the siphon, is recorded on the moving slip of paper by a wavy line of ink marks very close together. The interpretation of the signals is according to the Morse code; the dot and dash being represented by deflections of the line to one side or other of the centre line of the paper.

[Addition of December, 1883. Within the last five years I have brought into use a very much simpler form of siphon recorder. In this form of the instrument, instead of the electromagnets, I use two bundles of long bar-magnets of square section and made up of square bars of glass-hard steel. The two bundles are supported vertically on a cast-iron socket, and on the upper

end of each is fitted a soft iron shoe, so shaped as to concentrate the lines of force and thus produce a strong magnetic field in the space within which the signal-coil is suspended. I have made instruments of this kind to work both with and without electrification of the ink. Without electrification the instrument, as shewn in Fig. 2, is exceedingly simple and compact, and in this

Fig. 2.

S.MILLER

form is capable of doing good work on cables of lengths up to 500 or 600 miles. When constructed for electrification of the ink, as shewn in Fig. 3, it is of course available for much longer lengths of cable, but for cables such as the Atlantic cables, the original form of the siphon recorder is that still chiefly used. The strongest magnetic field hitherto obtained by permanent magnets (of glass-hard steel) is about 3000 c. G. s. With the electromagnets of the original form of siphon recorder as in ordinary use a magnetic field of about or over 5000 C. G. S. is easily attained.