Subsequently a fourth searchlight was introduced into the apparatus, for the purpose of practical experiments and observations concerning the duration of the light-flash. This fourth unit was mounted above the three searchlights, but in the axis itself. It is so disposed that its flash comes midway between any of the two below, and it is arranged to rotate three times as quickly as the main group of lights. Accordingly, the duration of the flash thrown from the fourth searchlight is only one-third of the flash thrown by the others--that is, one-thirtieth of a second. This lamp is provided with all the necessary mechanism for keeping it in steady rotation at the increased speed, and for drawing current from its feed-cable. Before the installation was placed in the lighthouse at Heligoland, it was submitted to searching tests at the Nuremberg works of the builders. These trials proved that with a current of only 26 ampères the average intensity was as high as 34,000,000 candle-power, with a maximum of nearly 40,000,000 candle-power; while with 34 ampères the average intensity rose to approximately 40,000,000, with a maximum of nearly 43,000,000 candle-power. Accordingly, the terms of the contract were fulfilled completely. 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.