Page:Popular Science Monthly Volume 3.djvu/608

592 electric current has ever before been conducted for a similar purpose. The signalling apparatus is placed in a lantern five feet high, four feet wide, and having a semicircular glazed front, which projects from the lantern of the belfry on the north side of the tower, or that overlooking the Victoria Embankment. It consists—first, of a fixed table, in which is inserted a flat brass ring 16 inches diameter and one inch broad, which serves as a roller-path for the apparatus carrying the lamp and reflector; next, there is a circular revolving table, having bearings on the roller-path, and which is moved around a central pivot projecting from the fixed table, being actuated by a worm wheel and screw. By means of this arrangement the light can be directed horizontally from side to side through an arc of 180°. It could, of course, be made to sweep the whole of the horizon, but the position of the lantern with regard to the clock-tower is such as to enable the light to be seen through the range of a semicircle only. Upon the revolving table, and hinged to it at the front, is the elevating table, which has a screw adjustment to the rear by which the light can be raised or depressed, being capable of vertical training through an arc of 25°. On the elevator is placed the lamp-table, upon which again is a sliding platform, on which the lamps themselves stand. There are two lamps, which are in use alternately, the carbon-points lasting but four hours, while the House frequently sits for ten.

The copper conductors terminate at the fixed part of the machine, and the method of carrying the current from them to the lamps is very ingenious, the moving parts of the apparatus forming in themselves conductors. The negative conductor is placed in metallic contact with one hinge of the elevator-table through the centre-pin on which the table revolves, and the positive conductor with the other hinge by means of the brass roller-path. The currents from those points are conducted to the lamp-table, and thence through the traversing platform to the lamps, metallic contact being obtained throughout the whole circuit by means of flat springs moving over flat surfaces. The changing of the lamps is effected, without any appreciable break of continuity in the light, by means of the traversing platform on which they stand, and which has a sliding motion from side to side. When the carbon-points in one lamp are nearly consumed, the traverser is quickly shifted from right to left, or vice versa, as may be necessary. The break of contact is but momentary, and only exists during the time required to move the traverser rapidly through a space of six inches. The light will not become extinct during that period, as there is not sufficient time to allow the incandescence of the carbon to entirely subside. The springs under the lamp thrown out of use are by this action removed from the metal plate in the lamp-table, and the springs under the fresh lamp are brought into contact, and the light is at once produced anew.

The intensifying apparatus at present in use is a holophole lent by