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LIGHTHOUSE. 261 LIGHTNING. BiBUOGBAPHY. For a general description of a large number of lighthouses, consult: Heap, Ancient and Modern Liyhlhuuses (Boston, 1889) ; also Johnson, Modern Lighthouse Service (Wash- ington, 1889) ; and annual reports and various special reports of the Lnited States Lighthouse Board, Washington, U. C. LIGHTHOUSE BOARD OE THE UNI- TED STATES. A body organized in accord- ance «ith an act of Congress, approved August 31, 18o2, and having the control and manage- ment of all lights, buoys, beacons, etc., on the coasts of the United States, and the custody of all archives, drawings, apparatus, and other prop- erty pertaining to the lighthouse establishment. This board consists of the Secretary of the Treas- ury as ex-officio president, two naval officers of high rank, two army officers, and two civilians, all appointed by the President. For administra- tive purposes the lnited States is divided ( 1903) into IG districts, to each of which is assigned a naval officer as inspector, who has charge of all the floating aids to navigation, the supplies of the light stations, the salaries of keepers, and the disbursement of funds relating to the above ob- jects. The inspectors are required to inspect the lights at least once every three months, at which time they ascertain the condition of the station and report it to the board. Every district has also an engineer officer of the army as district engineer : his duty is to superintend the construc- tion and renovation of the fixed aids to naviga- tion. When the Lighthouse Board receives pe- titions for the erection of new lights the matter is referred to both the district officers for their opinions as to the necessity of the light and its proper location ; the district engineer also sub- mits plans and estimates of the cost of the pro- posed structure. The board then decides as to what should be done and makes a report to Con- gress through the Secretary of the Treasury. Should Congress make an appropriation, the dis- trict engineer is charged with the erection of the light, which must be done by contract after due advertisement. When the lighthouse is completed the keepers are appointed and the structure placed in charge of the district inspector. The Light- house Board has offices in Washington, depots for supplies in various districts, and a number of vessels for conveying supplies and perform- ing other necessary duties. LIGHTNING {'ME. liyhlnyng, illumination, from lighten, light). A brilliant flash of light between clouds, or between a cloud and the earth. These flashes were shown by Benjamin Franklin to be simply enoimous electrical discharges. The length of such a flash may be several miles. When the flashes are between the lower clouds and the earth they are comparatively narrow and brilliant and are accompanied by thunder, but when they take place in the upper cloud region they become more diffuse and thunder is rarely heard. Three general classes of lightning are recognized. The first of these is the forked or zigzag lightning, which is a line of light of in- tense brilliancy, appearing to the eye as a single blinding streak of light, which sometimes breaks into one or two branches and is often of a rosy or violet tint. The second class of lightning is called sheet lightning: this has no definite form. It is generally of a rosy nr red tint and appears in the distant horizon lighting up the clouds, dust, or haze in the atmosphere. A true sheet-lightning discharge is rare, but the ordinary appearance of sheet lightning is very common, being due to the illumination of the clouds and haze by flashes of lightning which may be be- yond the horizon, and thus become manifest al- though distant one or two hundred miles. The third kind of lightning is the so-called ball lightning, which is said to appear like a small globe of brilliant light moving slowly through the air at a short distance above the ground or even rolling along the ground itself. Reliable observers have stated that on some occasions ball lightning has been seen to roll slowly into a house through an open door or window; it gen- erally breaks up with an explosion, which, how- ever, is not very destructive or dangerous. The descriptions of this phenomenon present many points difficult to understand, and it is only of late years that electricians have been willing to admit that we have here a true but peculiar form of electric discharge that demands further ex- perimental investigation. Very minute discharges analogous to ball lightning have been produced in the physical laboratory. The silent electric discharges known as Saint Elmo's fire (q.v.) are not usually included under lightning. T!ie thunder that accompanies lightning seems to be satisfactorily explained by the fact that the electric discharge in forcing its way through the atmosphere heats the air and the vapor lying in its path to a very high temperature, causing a very violent expansion along the whole length of the flash, simultaneously followed by an equally rapid contraction and the production of a wave of expansion and compression, or what is the same thing, a noise. Giving to the refraction of sound as it travels through the atmosphere, and especially the irregilar refraction due to the temperature and wind, thunder does not travel very far before it begins to rise above the ground, so that observers frequently see flashes of lightning without hearing the thunder which has passed over their heads. Thunder is there- fore rarely heard at a distance of 15 or 20 miles, whereas discharges of artillery may be heard 30, 50, or 100 miles. Beginning with the researches of the late Prof. Ogden X. Rood, of Columbia College, and with the results obtained from the recent applications of photography, a few definite facts have been added to our knowledge of the nature of light- ning. Professor Rood was able to show that a so-called single flash which may last several tenths of a second is simply an irregular suc- cession of elementary flashes each of which lasts but a few thousandths of a second, or even less. Prof. .John Trowbridge, of Harvard I'ni- versity, has constructed a storage battery of minute cells, by means of which he has been able to imitate true lightning flashes of sev- eral yards in length. Prof. .Joseph Henrj- was able "to show that all electric discharges are of an oscillating or alternating nature. The indi- vidual oscillations take place in a few millionths of a second and rapidly die away in intensity so that the whole discharge is accomplished in a very short time, depending on the size and dis- tance of the electrified bodies. Now as lightning is evidentiv a simultaneous discharge from mvriads of' electrified drops toward numerous spots on the earth's surface, or toward a similar electrified cloud at a distance, it is therefore a rational hypothesis to assume that the numerous