This sun-valve is exposed. At the break of dawn, under the gathering intensity of daylight, the central black rod absorbs the luminosity, the amount of which is increased by the light thrown from the gold-burnished outer rods, and, converting it into energy, expands longitudinally. In so doing it forces the lever at the base downwards, closing the opening through which the gas flows to the flasher. In a short while, when the day has broken fairly and there is no further need for the beacon’s services, the gas-feed is cut off entirely, only the pilot burner remaining alight, the gas-supply to this not being affected by the sun-valve. In order to bring the greatest possible pressure upon the lever, the blackened rod is so arranged that it can expand only in one direction--namely, downwards. Upon the approach of evening, owing to the daylight becoming weaker, the blackened rod contracts, and, the pressure upon the lever being released, the gas commences to flow once more to the burner. It is a small stream at first, but as the darkness gathers, and the shrinking continues, the valve opens wider and wider, until at last, when night has settled down and the copper central rod has fully contracted, the gas-valve is opened to its fullest extent, permitting the greatest pressure of gas to flow to the burner, so that the beacon throws its most brilliant light. This automatic action continues infallibly every dawn and dusk, and is the simplest and at the same time most reliable means of economizing gas during the day that has yet been devised. There is another feature of this system which must not be overlooked. Suppose, for some reason or other, that the sea becomes shrouded in suffused light, such as might arise from the obscuring of the sun by an overhanging bank of fog or smoke, the beacon comes automatically into service, as the cutting off of the daylight must bring about a contraction of the blackened copper rod controlling the valve. The central rod can be adjusted to any degree of sensitiveness, by means of a screw, while protection of the vital parts is insured by enclosure within a heavy glass cylinder. The first apparatus of this character was tested by the Swedish authorities in 1907, and proved so successful that it is now in service at all the exposed unattended lighthouses in Swedish and Finnish waters; while it has been adopted, also, very extensively by the United States, more particularly for the lighting of the lonely stretches of the Alaskan coastline and of the Panama Canal.