arc.

Facility and rapidity of priming and lighting.

Possibility of instantly obtaining the required length of

The carbons can be withdrawn as much as the speed and power of the motor will allow, and can thus furnish the highest possible luminous intensity.

Independence of the apparatus of the movements of the projector and the vessel.

Possibility of pressing the carbons against one another, so as to remove their excrescences.

The mechanism is simple and solid and comparatively inexpensive (£12, instead of £32 for the automatic lamp); the repairs can easily be executed on board.

The carbon-holders need not be vertical; they can be held in an inclined position, and this reduces to a considerable extent the necessary displacement of the upper carbon.

The ironclad ought, therefore, to be provided with one hand-regulated lamp for each projector, one reserve lamp of the same kind, and, if occasion requires, one automatic lamp for special cases where the lighted beacon has to be maintained in a perilous position which would endanger the life of the person in charge. The electric current is generated by a Gramme machine (type D) driven by a Brotherhood three-cylinder engine, to which it is coupled as shown in Fig. 217. From experiments made at Chatham, the Gramme machine has proved the most advantageous, as well as regards the total quantity of light produced (27,500 candles for fifteen horse-power expended), as also for good utilization of work. As a light apparatus, Mangin's projector is in universal use.

Mangin's Projector. This projector has been adopted in

this country as well as in France, and is represented in Fig. 218. The hand-worked lamp described above is placed inside the apparatus; the reflecting spherical surface of the glass mirror is on the inside, and the ray must therefore

twice traverse the

its final direction.

thickness of the mirror before receiving The two surfaces are not parallel either, the interior surface being a sphere, which has not the same radius as the other. The different thicknesses which the

respective rays have to traverse, according to the angle they form with the axis, modify their direction so as to bring them back to strict parallelism, provided the radii of the two spherical surfaces are of proper proportion. As regards the inclined position of the carbons, it has been found that, on employing a continuous current, the ray of maximum luminous intensity does not lie in the plane perpendicular to the line of the carbons, but about 30° below it. Fig. 219 shows the electric light projected by this apparatus on the Isle of Tabarca, in the Tunisian war.

But not only for men-of-war, but also for our transatlantic steamers the use of an electric light-beacon and projector might be of the greatest value. It is well known that, towards the autumn equinox, immense masses of ice are detached from the northern coast of Greenland and carried south by a powerful current. The track of the outgoing transatlantic steamers takes them very near to Cape Farewell, the southern extremity of Greenland, and their meeting with the floating icebergs is by no means a rare The approach of these formidable enemies at night constitutes a great danger for the vessel, and this danger could easily be averted by the employment of an electric light-beacon.

occurrence.

NIGHT SIGNALS.

The signals actually in use are of two different kindssignals intended to ensure the safety of the vessel, and signals intended to form a system of optical telegraphy, enabling the vessels to communicate during the night, either with one another or with the shore, as easily as they communicate in the day-time by means of the flag-signals. The

former have been described in the preceding article, and we have here only to occupy ourselves with the latter kind of signals. Successful experiments have lately been made on board the French ironclad Colbert, with electric light signals.

In the ordinary system ten oil or petroleum lamps are employed, which are hoisted in two groups of five along the mainmast-the first group to the masthead, the second below the mizzen-yard; and the different signals constituting the international code are obtained by varying the number of lighted lamps or their respective positions.

In order, therefore, to give a signal, a certain number of lamps must be lighted and hoisted, which requires a certain time; then, since a signal must not be changed until it has been repeated by the other vessel, a certain time must be allowed for the answer. For the second signal the lamps must be lowered again, the new signal prepared, the lamps hoisted, and again time allowed for the answer, and so on. It will easily be understood that this mode of correspondence is very slow, and it is therefore only employed for communication with vessels at a great distance, and when one and the same signal has to be transmitted to several vessels, as, for instance, in squadron manoeuvres, and in this case the transmission is even slower, each vessel having to repeat the signal.

The electric installation on board the Colbert has been carried out by De Méritens. The source of electricity is a small De Méritens magneto-electric machine placed on the hind quarter-deck and driven by eight men. The current of the machine is conveyed to a commutator placed in the pavilion of the foot-bridge, and thence can assume three different directions.