Page:The American Cyclopædia (1879) Volume X.djvu/463

LIGHTHOUSE 457 valve, covering the flow-hole. This flow-hole is about three eighths of an inch below the top of the burner, measured vertically. Figs. 4, 5, and 6 give an elevation of the lamps and reflectors in a revolving apparatus, a plan of the same apparatus, and a plan of the lamps FIG. and reflectors in a fixed light: n n n is the frame ; o o 0, the reflectors ; p p p, the lamp fountains ; q, the central shaft ; r r r, the tubes which lead off the smoke ; s s s, the braces supporting the shaft; t t, supports of the shaft ; u u, a pan for catching drops from the roof; Z, a bracket supporting the foot of the shaft ; m m, bevelled wheels con- ducting the motion from the apparatus which FIG. 7. moves the system. The introduction of the Fresnel or dioptric method of illumination of lighthouses has superseded the reflector sys- tem, so that reflectors are now only used as range lights, or on light vessels, or in light- houses built with too little money to permit the purchase of a lens. Fig. 7 shows a plan and elevation of a Fresnel lens of a small or- der. It consists of thirteen rings of glass of va- rious diameters arranged vertically one above the other. The five middle rings are ll^f in. in internal diameter, and by two refractions throw the rays which they receive from the flame F to the horizon. The five upper and three lower rings throw their rays to the hori- zon by two refractions and one total reflection. The dotted lines show the courses of the rays after they leave the lamp. F is the flame ; r r, cylindrical refractors ; x x, catadioptric rings acting by refraction and reflection. The external lines r' r' represent a system of ver- tical prisms which revolve around the appara- tus described above, and by deviating the rays as represented in the plan, so that they leave the prisms parallel to each other, cause the variation in the light described in the lists as a "fixed light varied by flashes." The first cost of a lens of the first order is four times as great as that of 20 reflectors, which is the greatest number placed in a lighthouse. III. AUXILIARY SAFEGUARDS TO NAVIGATION. These consist of light vessels, beacons, fog signals, and buoys. Light vessels are ves- sels moored to point out dangers or sl}ow the entrances of channels, turning points, &c., by exhibiting a light at night. They are strong, and built for riding easily at anchor, and are well manned, to provide against accidents to which their exposed positions render them pe- culiarly liable. The lighting apparatus is con- tained in a lantern which at night is hoisted to the masthead. It consists of lamps and reflect- ors like those described above for lighthouses, except that they are smaller. The lamps are hung on gimbals, so that their positions may be affected as little as possible by the motion of the vessel. During the past ten years Fresnel lenses have been successfully applied to light vessels. Some of the light vessels of the United States are in positions exposed to the full force of the Atlantic, and one moored off the Nan- tucket New South shoal is 22 m. from the nearest land. On account of the great ex- pense of maintaining light vessels, and the little dependence that can be placed on them as signals when they are the most needed, they are only placed near dangers which it is impossible to point out by lighthouses. A beacon is a structure of stone, iron, or wood, placed upon the shore or upon a rock or shoal in the water to designate a danger. Beacons are built at points where lighthouses cannot be built, or which are not of sufficient impor- tance to justify the constant expense of keep- ing up lighthouses or light vessels, but which nevertheless require to be pointed out. A fog signal is an aid to navigation placed on board a light vessel or near a lighthouse to give warn- ing to vessels in time of foggy or thick weath- er. They are of the greatest importance, hardly if at all inferior to lighthouses. In fogs no light can be seen far enough to be of