LIGHTNING 482 LIGNUM VITiB their coninion focus, and be made to revolve, a dioptric revolving light is pro- duced. The electric light was first shown uenT SECTION OF EDDYSTONE LIGHTHOUSE to the mariner in 1858 from the Fore- land h'ghthouse. LIGHTNING, the dazzling light emitted by a large spark darting from clouds charged with electricity. In the lower regions of the atmosphere it is white, in the upper one somewhat violet, as is the spark of an electrical machine fti a vacuum. In the upper regions of the sky, where the air is rarefied, it tends to take the form of sheet-lightning, sometimes^ called heat lightning; in the lower regions it becomes more concen- trated and moves in lines. As a rule hghtnmg strikes objects from above, though occasionally ascending lightning has been seen. When it sinks deeply into the ground it sometimes vitrifies the rocks, producing fulgurites. Light travels with such speed that a flash is seen the instant it occurs. Thunder, which is simply the noise of the explo- sion, takes about five second:: to travel a mile, hence the distance of any flash, followed by thunder, may be calculated. If a mile distant, the danger is but slight. LIGHTNING ARRESTEE, a con- trivance used in telegraphy for guarding against passage of atmospheric elec- tricity through the instruments. The line wires are attached to a plate of brass, usually serrated on the under side. This plate rests on another plate con- nected with the ground, the two being separated by a thin layer of insulating material. LIGHTNING CONDUCTOR, an appli- ance designed to protect a building and its inmates against destruction or dam- age by lighting. It was invented by Benjamin Franklin in 1755. In general an iron rod rises vertically from the roof of the building which it is designed to protect. Its height is from 6 to 10 feet, its thickness at the base two or three inches. Its top is of platinum or gilded copper. From the rod runs a wire cord, or a bar of iron, or strip of copper, to the ground, terminating in general in ramifications 18 to 21 feet below the surface, in many cases in a well. Gen- eral opinion now favors a large number of rods with many points, and held off from the building by means of supports. LIGHTSHIP, or LIGHTBOAT, a vessel serving as a lighthouse in positions where a fixed structure is impracticable. Over 40 lightships guard the coast of the United States. The new South Shoal lightship, 26 miles from Nantucket, is farther out to sea than any other light- ship in the world. LIGHT THERAPY. See PHOTO Therapy, and Radium Therapy. LIGNITE, fossil wood, generally of Tertiary age, converted into a kind of coal. It is usually dull or brownish- black, and has not the glistening luster of Carboniferous coal. Occasionally it is pitch black. As the name lignite implies, it, as a rule, retains the form and aspect of the original wood. It con- tains a larger proportion of hydrogen than wood does. If decomposition goes on, it discharges carburetted hydrogen and becomes changed into common or bituminous coal. LIGNUM VITJE, the wood of Guaiacum officinale. It is a very hard and heavy wood, brought from Cuba and
