Above the accumulator-room is the storeroom and a general workshop, followed by a bedroom and above that the service-room. As only one keeper is on duty at a time, he is provided with ample devices whereby he can summon his comrade in times of emergency; the generating machinery is also controllable from this floor. From the service-room the lower light-room is entered. This is a secondary or back light in the range, the front light being in the tower half a mile away. Each of these two light-rooms is fitted with two 150 candle-power incandescent electric lights, but only one is burned in each set at a time: the second is a reserve. Should the light in action fail from any cause, although the keeper is warned of the occurrence, he does not have to stir a finger to bring the reserve light into service. The short-circuit produced by the accident to the light automatically revolves the table upon which the lamps are mounted, swings the reserve light into focus, and then sets it going. Above the secondary light in the main tower is the principal beacon, comprising a brilliant rapidly-flashing light, the characteristic of which is groups of two flashes alternating with four flashes, the cycle being completed once in thirty seconds. The optical apparatus has been devised especially for the “differential arc-light,” as it is called, with a reflecting lens having a focal distance of 250 millimetres (10 inches), the lens itself being 1,180 millimetres (approximately 47 inches) in diameter. In front of the lens is placed a disperser, having a diameter of 1,200 millimetres (48 inches) whereby the ray of light is dispersed through an arc of 10½ degrees. Before the disperser is the means for producing the characteristic flash. This comprises a blind, or shutter, which is opened and closed by mechanism adjusted to requirements; while the rotating mechanism, instead of being weight-driven, is actuated by an electric motor. The “differential arc,” which is utilized in this installation, is considered by German engineers to be the best system that has yet been devised for the exacting purposes of lighthouse engineering, and the description has arisen from the disposition of the carbons. While the positive carbon is held horizontally, the negative carbon is placed at an angle of 70 degrees thereto, and only the crater of the positive carbon is considered for the lighting effect, this being placed in the focus of the apparatus. The positive carbon is 3/5 inch, and the negative carbon 2/5 inch, in diameter, although both have a common length of 19 inches, which is sufficient for nine hours’ service. The beam emitted is of some 5,000,000 candle-power. This is one of the cheapest electric stations at present in operation, the annual running charges averaging less than £300, or $1,500. CHAPTER XVIII SOME LIGHTHOUSES IN AUSTRALIAN WATERS