Page:The American Cyclopædia (1879) Volume X.djvu/469

 LIGHTNING 463 officio president of this board, and its decisions are in all cases subject to his control. The lighthouse establishment is therefore a branch of the treasury department, and its annual ex- penses are estimated for by that department. For the proper administration of the affairs of the establishment, the coasts of the United States are divided into 13 lighthouse districts. To each of these districts is assigned an inspec- tor, who is detailed from the officers of army engineers and the navy. These inspectors have the control of the operations of the establish- ment in their respective districts (with the ex- ception of the appointment of light keepers), and correspond directly with the lighthouse board. They are furnished with steamers in which they make quarterly inspections of the light stations in their districts, and which are also used for taking care of buoys. They are re- quired to make annual reports of the condition of their districts, in which are embodied their recommendations of new lights, &c., for the action of the lighthouse board. The construc- tion of new lighthouses and important repairs of old ones are carried on under the direction of officers of the corps of army engineers, who are detailed for this service. The routine duties of the board are discharged by two secretaries, one of whom is an officer of the navy, and the other an officer of the corps of army engineers. Meetings of the board are held weekly for the transaction of the routine or any other business that may be brought before it by the action of the treasury depart- ment. The meetings are held at Washington, where the office of the board is situated. LIGHTNING, the illuminating flash produced by the discharge of atmospheric electricity, either between two clouds, or between a cloud and the earth, usually accompanied by a noise called thunder. It manifests itself in various forms, which have been called forked, zigzag, ball, sheet, and heat lightning. Zigzag light- ning is produced by the discharge of a large quantity of electricity from a cloud through a resisting medium, which becomes compressed at various points and thus turns the current aside. As shown in the article ELEOTEICITY, experiments demonstrate that the more air is rarefied, other things being equal, the more readily will it permit the passage of the electric current, and the more it is compressed the more resistance it offers. Ball lightning has the appearance of a ball of fire, accompanied by a terrible explosion, and results from an unusually intense charge of electricity which forces a direct path. It has been supposed by some that the ball is an agglomeration of particles of ponderable matter which have been carried along with the current. Sheet lightning has the appearance of a diffuse glare of light, sometimes illuminating the edges and sometimes the whole surfaces of clouds. It may be caused by a stroke of zigzag lightning at a great distance, sending its light through great thicknesses of clouds, so as to give it the 497 VOL. x. 30 appearance of diffuseness; or it may result from the passage of electricity of no great ten- sion from particle to particle, like that pro- duced in discharging an electrical machine over the surface of a bedewed pane of glass. Heat lightning differs but little from sheet lightning, being produced in the same two ways. When produced by the reflection or transmission of zigzag lightning, thunder is not heard on account of the distance. The color of lightning varies like that of the spark of the electrical machine when passed through the receiver of an air pump in various states of exhaustion, or when it contains different gases and vapors. Of the nature of lightning the ancients knew nothing. Its disastrous effects were associated rather with the terrific sound of the thunder than with the flash, and the Greeks and Romans attributed them to the thunderbolt hurled by Jupiter to the earth. The Hebrews often represented them as direct exhibitions of divine power, and frequently in the Old Testament, as in Job xxxvii., the thunder is spoken of as the voice of the Lord. Even the earlier electricians did not suspect the identity of lightning and electricity. The abbe" Nollet in 1746 first drew attention to the similarity of effects exhibited by thunder clouds and the prime conductor of an electri- cal machine. Winckler next argued that the principle of the powers of each was identical. Franklin established the fact first by enumera- ting in a clear and methodical manner the va- rious points of resemblance, and the similar effects produced by each, and finally by actually conducting the lightning to the earth in his well known experiment with the kite in Phila- delphia. The following quotations from letters of Franklin written in 1749-'50, and contained in the "Observations on Electricity" published in London in 1869, are full of interest : " Where there is a great heat on the land in a particular region (the sun having shone on it perhaps for several days while the surrounding countries have been screened by clouds), the lower air is rarefied and rises ; the copier, denser air above it descends ; the clouds in the air meet from all sides and join over the heated place, and if some are electrified, others not, lightning and thunder succeed and showers fall. Hence thun- der gusts after heats, and cool air after gusts. ... As electrical clouds pass over a country, high hills and high trees, lofty towers, spires, and masts of ships, chimneys, &c., as so many prominent points draw the electrical fire, and the whole cloud discharges there. Dangerous is it, therefore, to take shelter under a tree during a thunder gust. It is safer to be in the open fields for another reason. When the clothes are wet, if a flash in its way to the ground should strike your head, it may run in the water over the surface of your body, whereas if your clothes were dry it would go through the body. . . . Sulphurous and inflam- mable vapors arising from the earth are easily kindled by lightning. Besides what arise from