The searchlights throw their rays from a massive conical tower, the focal plane of which is 272 feet above sea-level. In average weather the rays are visible at a distance of twenty-three nautical miles, and under the most advantageous weather conditions visibility is limited only by the curvature of the earth, although on a clear night the light is seen from Büsun, which is about thirty-five miles away. The Heligoland electric light ranks as a remarkable development in the application of electricity to lighthouse illumination, but it never has been duplicated. The cost of maintenance—about £1,400, or $8,000, per annum—is an insuperable handicap.
On the other hand, the Hornum electric light, which is the most modern of its type in Germany, is more economical, although by no means so powerful. The tower is of cast-steel, and carries two electric lights; while about half a mile distant is a second tower, which throws a third electric light. In the main tower, on the ground floor, is installed the electric generating plant (in duplicate), together with all accessories, such as switchboards, etc. The floor above is devoted to housing 100 accumulators, which are charged during the day. This task can be completed by one generating set in about six hours. A single charge is sufficient to keep the three lights going for ten or eleven hours, and the lights are controlled by a simple throw-over switch. By this arrangement the cost of the maintenance of the light is reduced very appreciably, as only one keeper is on duty at a time, the station being equipped with two men, who have proved adequate for the purpose.
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
Although the waters washing the Australian continent are not so thickly intersected with steamship lanes, and the mercantile traffic is not so dense there as in the seas of the Northern Hemisphere, yet, owing to the activity in emigration from Great Britain, as well as to the increasing prosperity of the various rising industries under the Southern Cross, they are becoming more crowded with each succeeding year. The efficient lighting of the coasts is an inevitable corollary of this expansion. Lighthouse engineering, however, is unavoidably expensive, especially when sea-rocks demand indication.
From time to time severe strictures are passed by European shipping interests upon the apparent lack of coastal lights in Australasian waters, and the various Government departments concerned with this responsibility are often accused of parsimony and neglect. Unfortunately, the greater number of these critics are apt to consider the situation through European glasses; to take the countries of the Old World and the United States as a basis for their arguments, and to ignore local conditions. It has taken a century or more for Europe and the United States to develop their respective organizations, and in the majority of instances there are ample funds from which expenses in this direction may be met, especially when passing shipping is mulcted a small sum in light-dues for the purpose. When the shipping is heavy, these levies are certain to represent in the aggregate a large sum every year.