they might be rendered useless by headland fogs--at a height varying between 150 and 250 feet. The base of the tower forms a space for the accommodation of the gas-accumulators, wherein the illuminating medium is stored under pressure, surmounted by the lantern which carries the requisite optical apparatus, and the flasher whereby the characteristic visual warning is given. Although adoption of the flasher enabled the consumption of gas to be reduced very appreciably, there was one noticeable drawback: the light had to burn both night and day, unless clockwork mechanism were introduced to extinguish the light at sunrise and to ignite it at twilight. Some authorities, however, do not place trust in clockwork mechanism. Certainly it is liable to fail at a critical moment, and in the case of an isolated light, several hundred miles from the nearest base, this would be a serious calamity, intimation of the fact not being available until several weeks after the disability had been observed. In order to overcome the fallibility of clockwork, and to insure a still further marked decrease in the consumption of gas, Mr. Gustaf Dalén devoted his energies to the perfection of a device which should achieve the self-same end, but be operated by Nature herself. His efforts were crowned with complete success by the invention of the “light-valve,” but which has become more widely known as the “sun-valve.” [Illustration: THE GAS ACCUMULATORS EMPLOYED IN THE DALÉN AUTOMATIC SYSTEM. The size of the storage cylinder varies according to the work, character, and position of the beacon.] This device is based upon a well-known principle. If two objects, fashioned from the same metal, and identical in every respect except that one is made light-absorbing and the other light-reflecting, are exposed to daylight, while the former will expand, the latter will remain unaffected. This result is due to the fact that the one which absorbs light transforms it into energy. The acting part of the “sun-valve” therefore is a light-absorber. It consists of a central rod, the surface of which is coated with lampblack, so that its light-absorbing qualities are enhanced as much as possible. The lower part of this rod is connected to a small lever, which opens and shuts an orifice through which the gas passes to the flasher in the lantern above. Around this central black copper rod are three other copper rods, disposed equidistantly. They resemble the former in every respect except that they have no light-absorbing qualities, but they are given polished gold surfaces, so that their light-reflecting properties are raised to the maximum.