Light, frame, flashes once every five seconds, which is one of the most striking of all the dis tinctions, was first introduced by the late Mr. Robert Stevenson, the engineer of the northern light-houses, in 1822, at Rhinns of Islay, in Argyleshire. The same engineer also introduced what has been called the intermittent light, by which a stationary frame with reflectors is instantaneously eclipsed, and is again as suddenly revealed to view by the vertical movement of opaque cylinders in front of the reflectors. The intermittent is distinguished from the revolving light, which also appears and disappears successively to the view, by the suddenness of the eclipses and of the reappearances, whereas in all revolving lights there is a gradual waxing and waning of the light. The late Mr. Wilson introduced at Troon harbor an intermittent light which was produced by a beautifully simple contrivance for suddenly lowering and raising a gas-flame. Mr. Robert Louis Stevenson has proposed an intermittent light of unequal periods by causing unequal sectors of a spherical mirror to revolve between the flame and a fixed dioptric apparatus (such as that shown in fig. 1). The power of the light is increased by the action of the spherical mirror, which also acts as a mask in the opposite azimuths. The number of distinctive light-house characteristics has not yet been exhausted in practice, for various other distinctions may be produced by combination of those already in use; as, for example, revolving, flashing, or intermittent lights might be made not only red and white alternately, but two red or white, with one white or red. Similar combinations could of course be employed where two lights are shown from the same or from separate towers.

Dioptric System.-Another method of bending the diverging rays proceeding from a lamp into such directions as shall be useful to the mariner is that of refraction. If a flame be placed in the focus of a lens of the proper form, the diverging rays will be bent parallel to each other, so as to form a single solid beam of light. M. Augustin Fresnel was the first to propose and to introduce lenticular action into light-house illumination, by the adoption of the annular or built lens, which had been suggested as a burning instrument by Buffon and Condorcet. He also, in conjunction with Arago and Mathieu, used a large lamp having four concentric wicks. In order to produce a revolving light on the lenticular or dioptric system, a different arrangement was adopted from that which we have described for the catoptric system. The large lamp was now made a fixture, and four or more annular lenses were fitted together, so as to form-a frame of glass which surrounded the lamp. When this frame is made to revolve round the lamp, the mariner gets the full effect of the lens whenever its axis is pointed toward him, and this full light fades gradually into darkness as the axis of the lens passes from him. In order to operate upon those rays of light which passed above the lens, a system of double optical agents was employed by Fresnel. These consisted of a pyramid of lenses with mirrors placed above at the proper angle for rendering the rays passing upward parallel to those which came from the annular lens. But Fresnel did not stop here, for, in order to make the lenticular system suitable for fixed as well as revolving lights, he designed a new optical agent, to which the name of cylindric refractor has been given. This consisted of cylindrical lenses, which were the solids that would be generated were the middle vertical profile of an annular lens made to circulate round a vertical axis. The action of this instrument is obviously, while allowing the rays to spread naturally in the horizontal plane, to suffer refraction in the vertical plane. The effect of this instrument is, therefore, to show a light of equal intensity constantly all round the horizon, and thus to form a better and more equal light than that which was formerly produced for fixed lights by parabolic reflection. It is obvious, however, from our description that the diverging rays which were not intercepted by this cylindric hoop, or those which would have passed upward and been uselessly expended in illuminating the clouds, or downward in uselessly illuminating the light-room floor, were lost to the mariner; and in order to render these effective Fresnel ultimately adopted the use of what has been called the internal or total reflection of glass; and here it is necessary to explain that one of the great advantages of the action by glass over reflection by metal is the smaller quantity of light that it absorbs. It has been ascertained that there is a gain of nearly one-fourth (.249) by employing glass prisms instead of metallic reflectors for light-house illumination. There were, therefore, introduced above and below the cylindric refracting hoop which we have described, separate glass prisms of triangular section, the first surface of each of which refracted to a certain extent any ray of light that fell upon it, while the second surface was placed at such an angle as to reflect, by total reflection, the ray which had before been refracted by the first surface; and the last or outer surface produced another refraction, which made the rays finally pass out parallel with those refracted by the central cylindric hoop. The light falling above the cylindric hoop was thus by refractions and reflections bent downward, and that falling below was bent upward, so as to be made horizontal and parallel with that proceeding from the refracting hoop. Fig. 1 represents in vertical section this, which is the most perfect of Fresnel's inventions in light-house illumination, especially when made in pieces of the rhomboidal form, and used in connection with the diagonal framing introduced by Mr. Alan Stevenson. In the fig., p shows the refracting and totally reflecting prisms, and R the cylindric refractor.

From what has been stated, it will be readily seen that, in so far as regards fixed lights, which are required to illuminate constantly the whole of the horizon with equal IX.-1a.

Light.

Fig. 1.

intensity, the dioptric light of Fresnel with Mr. Alan Stevenson's improvements is a perfect instrument. But the case is different as regards revolving lights, or those where the whole rays have to be concentrated into one or more beams of parallel rays. To revert to the parabolic reflector, it must be obvious that all rays which escape past the lips of the reflector, never reach the eye of the mariner, while, if we return to the dioptric revolving light of Fresnel, we find that those rays which escape past the lens are acted on by two agents, both of which cause loss of light by absorption. The loss occasioned by the inclined mirrors, and in passing through the pyramidal inclined lenses, was estimated by Fresnel himself at one-half of the whole incident rays. In order to avoid this loss of light, Mr. Thomas Stevenson proposed, in 1849, to introduce an arrangement by which the use of one of these agents is avoided, and the employment of total reflection, which had been successfully employed by Fresnel for fixed lights, was introduced with great advantage for revolving lights. "This effect may be produced in the case of metallic reflectors by the combination of an annular lens, L (fig. 2); a parabolic conoid, a, truncated at its parameter, or between that and its vertex; and a portion of a spherical mirror, b. The lens, when at its proper focal distance from the flame, subtends the same angle from it as the outer lips of the paraboloid, so that no ray of light coming from the front of the flame can escape being intercepted either by the paraboloid or the lens. The spherical reflector occupies the place of the parabolic conoid which has been cut off behind the parameter. The flame is at once in the center of the spherical mirror, and in the common focus of the lens and paraboloid. The whole sphere of rays emanating from the flame may be regarded as divided into two hemispheres. Part of the anterior hemisphere of rays is intercepted by the lens, and made parallel by its action, while the remainder is intercepted by the paraboloidal surface, and made parallel by its action. The rays forming the posterior hemisphere fall on the spherical mirror behind the flame, and are reflected forwards again through the focus in the same lines, but in opposite directions to those in which they came, whence passing onwards they are in part refracted by the lens, and the rest are made parallel by the paraboloid. The back rays thus finally emerge horizontally in union with the light from the anterior hemisphere. This instrument, therefore, fulfills the necessary conditions, by collecting the entire sphere of diverging rays into one beam of parallel rays without employing any unnecessary agents."

Fig. 2.

What has been just described is what Mr. Stevenson terms a catoptric holophote. What follows is a description of his dioptric holophote, in which total reflection, or the most perfect system of illumination, is adopted. The front half of the rays is operated upon by totally reflecting glass prisms, similar in section to those applied by Fresnel for fixed lights; but, instead of being curvilinear in the horizontal plane only, they are also curvilinear in the vertical plane, and thus produce, in union with an annular lens, a beam of parallel rays similar to what is effected by the parabolic mirror. The rays proceeding backwards fall upon glass prisms, which produce two total reflections upon each ray, and cause it to pass back through the flame, so as ultimately to fall in the proper direction upon the dioptric holophote in front, so that the whole of the light proceeding from the flame is thus ultimately parallelized by means of the smallest number and the best kinds of optical agents. It is a remarkable property of the spherical mirror that no ray passes through it, so that an observer, standing behind the instrument, perceives no light, though there is nothing between him and the flame but a screen of transparent glass.

Where the light is produced by a great central stationary burner, the apparatus assumes the form of a polygonal frame, consisting of sectors of lenses and holophotal prisms, which revolves round the flame, and each face of which produces a beam of parallel rays. Hence, when the frame revolves round the central flame, the mariner is alternately illuminated and left in darkness, according as the axis of each successive face is pointed toward him or from him. In the revolving holophotol light one agent is enabled to do the work of two agents in the revolving light of Fresnel, as total reflection, or that by which least light is lost, is substituted for metallic reflection. The dioptric holophotal system, or that by which total reflection is used as a portion of the revolving apparatus, was first employed on a small scale in 1850 at the Horsburg light

Light. house, and on the large scale in 1851 at North Ronaldshay in Orkney. Since that date this system has been all but universally introduced into Europe and America.

Azimuthal Condensing Light.-The above is a description of the general principles on which light-houses are illuminated. In placing a light in some situations, regard, however, must be had to the physical peculiarities of the localities; the following plans of Mr. Thomas Stevenson may be cited as examples. In fixed lights of the ordinary construction, the light is distributed, as already explained, equally all round the horizon, and is well adapted for a rock or island surrounded by the sea. But where it is only necessary to illuminate a narrow sound, it is obvious that the requirements are very different. On the side next the shore, no light is required at all; across the sound, a feeble light is all that is necessary, because the distance at which it has to be seen is small, owing to the narrowness of the channel; while up the sound and down the sound the sea to be illuminated is to be of greater or lesser extent, and requires a corresponding intensity. If the light were made sufficiently powerful to answer for the greater distance, it would be much too powerful for the shorter distance across the sound. Such an arrangement would occasion an unnecessary waste of oil, while the light that was cast on the landward side would be altogether useless. Fig. 3 represents (in plan) the condensing light, by which the light proceeding from the flame is allocated in the different azimuths in proportion to the distances at which the light requires to be seen by the mariner in those azimuths. Let us suppose that the rays marked a require to be seen at the greatest distance down the sound, and

those marked B to a somewhat smaller distance up the sound. In order to strengthen those arcs, the spare light proceeding landwards, which would otherwise be lost, is intercepted by portions of holophotes, B and C, subtending spherical angles proportioned to the relative ranges and angular spaces of the arcs a and B. The portions of light thus intercepted are parallelized by the holophotes, and fall upon straight prisms a, a, and b, b, respectively, which again refract them in the horizontal plane only; and, after passing through focal points (independent for each prism), they emerge in separate equal beams, and diverge through the same angles as a and respectively. In this way, the light proceeding up and down the sound is strengthened in the required ratio by utilizing, in the manner we have described, the light which would otherwise

DARK ARC-
Fig. 3.

have been lost on the land. These instruments were first introduced at three sound lights in the w. of Scotland, in 1857, where apparatus of a small size, combined with a small burner, was found to produce, in the only directions in which the great power was required, beams of light equal to the largest class of apparatus and burner. The saving thus effected in oil, etc., has been estimated at about £400 or £500 per annum for these three stations.

Apparent Light.-At Stornoway bay, the position of a sunk rock has been sufficiently indicated by means of a beam of parallel rays thrown from the shore upon certain optical apparatus fixed in the top of a beacon erected upon the rock itself. It was suggested that the light-house should be built on the outlying submerged reef, but the cost would have been very great, and Mr. Stevenson's suggestion of the apparent light was adopted. By means of this plan the expense of erecting a light-house on the rock itself has been saved, and all the purposes of the mariner served. It has been called an apparent light from its appearing to proceed from a flame on the rock, while the light in reality proceeds from the shore, about 650 ft. distant, and is refracted by glass prisms placed on the beacon.

Floating lights are vessels fitted with lights moored at sea in the vicinity of reefs. Prior to 1807 the lantern was hung at the yard-arm. The late Mr. Robert Stevenson then introduced the present system of lanterns, having a copper tube in the center capable of receiving the vessel's mast, which passed through the tube, the lights being placed all round. In this way proper optical appliances can be employed, and the lantern can be lowered on the mast so as to pass through the roof of a house on the deck, where the lamps are filled or trimmed. In 1864 six floating lights were constructed for the Hoogly under the directions of Messrs. Stevenson, in which the dioptric principle

Light.

was applied. Eight half-fixed light apparatus of glass with spherical mirrors behind were placed in the lantern round the mast, so as to show in every azimuth rays from three of them at once.

Differential Lens.-This is an annular lens, curved to different radii on both sides, so as to increase the divergence in any given ratio. The small arc of about 6°, which is unequally illuminated by the lens as presently constructed, may be made of equal intensity throughout by the differential form, or by means of separate straight prisms placed at the sides.

Sources of Light.-The descriptions which have already been given have all had reference to the best means of employing a given light. Many attempts have from time Magneto-electric Light.-The electric light, which has of late been greatly developed and improved, and especially adapted to light-house purposes, was introduced under the auspices of the Trinity house of London.

to time been made to increase the power of the radiant itself.

Gas.-The uncertainty and other objections attending the manufacture and use of gas in remote and inaccessible places have, with some exceptions, as yet prevented its adoption at light-house stations, but it has been successfully used at many harbor-lights. Oil and Paraffine.-The oil which is chiefly employed in Great Britain is that which goes by the name of colza, and the quantities annually consumed at the northern lighthouses may be stated at 40 galls. for an argand 1 in. in diameter, and 800 galls. for the four-wick burner, which is used in dioptric lights of the first order. Capt. Doty's burner for paraffine, which is the best which has as yet been suggested, has been introduced into the French and the Scotch light-houses. Paraffine has been found to give a more intense light than colza at half the cost.

Visibility of Lights.-The distance at which any light can be seen, of course depends on the height of the tower, and varies with the state of the atmosphere. The greatest recorded distance at which an oil light has been visible is that of the holophotal light of Allepey at Travancore, which has been seen from an elevated situation at a distance of 45 miles. The holophotal revolving light at Baccalieu, in Newfoundland, is seen every night in clear weather at cape Spear, a distance of 40 nautical miles.

Power of Light-house Apparatus.-The reflector (25 in. diam.) used in the northern light-houses, with a burner of 1 in. diam., is considered equal to about 360 argand flames. The cylindric refractor, used in fixed lights, with a four-wick burner, has been estimated at 250; while the annular lens in revolving lights, with the same burner, is equal to about 3,000 argand flames. See LIGHTING OF BEACONS AND BUOYS AT SEA.

LIGHT-HOUSE (ante). Light-houses were not constructed until some advance ment was made in navigation, but beacon-fires were lighted for the guidance of the early mariners. The most celebrated ancient light-house was the Pharos (q.v.) of Alex andria, built upon a rocky point of that name which had been an islet, but was connected by Alexander the great with Alexandria by a roadway called the seven-mile mole, or heptastadium. The light-house was commenced by Ptolemy Soter, and finished about 280 B.C., and was regarded as one of the wonders of the world. It was about 400 ft. high, and the light which was kept burning on its top could be seen, according to Josephus, at a distance of 40 miles. It is thought to have been destroyed by an earthquake after having stood 1600 years. It was constructed in the form of the frustrum of a square pyramid, having an immense base whose dimensions are not known. The tower of Cordouan, at the mouth of the Garonne, in the bay of Biscay, is another celebrated light-house, but of modern date and still standing. It was commenced in 1584 and finished in 1610 by Louis de Foix. It stands upon a rocky ledge, which is under water except at low tide. The base is the frustrum of a cone, 135 ft. in diameter at the bottom, 16 ft. high, and 125 ft. in diameter at the top; built solid of cut stone, with the exception of a chamber in the center, 20 ft. square and 8 ft. high, containing a water cistern. A wall 12 ft. high and 11 ft. thick stands upon the margin of the upper surface of the base. The tower is 50 ft. in diameter at its base, is 115 ft. high, and is the frustrum of a cone, surmounted by a lantern dome. The entire height from the rock is 162 ft., the whole height of the tower, including the dome, being 146 feet. The first Fresnel lens ever manufactured was placed in this light-house in 1823. The Eddystone light-house in the English channel is described under the title EDDYSTONE. The Bell rock light-house, off the e. coast of Scotland, is built upon a reef of rocks in the German ocean, 11 m. from the coast, nearly opposite the Tay firth. The rock upon which it stands is a red sandstone, from 12 to 15 ft. below spring tide, with from 2 to 4 ft. exposure at low tide. The structure is also of sandstone, but the outer tiers for 30 ft. high are of granite. It was designed by the celebrated Scotch engineers, Robert Stevenson and John Rennie, and constructed by the former. The erection of the second Eddystone light-house had given Smeaton much study, and his experience was taken advantage of by Stevenson in the structure at Bell rock. In form it resembles the Eddystone. The diameter at the base is 42 ft., while at the top, beneath the cornice, it is 15 feet. The stone-work is 1024 ft. high, and the whole structure, including the lantern, 115 feet. See BELL ROCK. The Skerryvore light-house, built upon the Skerryvore rocks, which lie in the tracks of vessels going around the north of Ireland

Light.

or Scotland from the Clyde and Mersey, was constructed by Alan Stevenson, the son of Robert. See SKERRYVORE. There are many very fine light-houses in the United States) the most noted of which was erected upon Minot's ledge, off the town of Cohasset, Massachusetts bay, about 20 m. e.s.e. of Boston, and one of the most dangerous places in the world without a signal. The difficulties in the construction of a light-house upon this rock were immense. An iron structure was first erected, being completed in 1849, which stood till April, 1851, when it was demolished by a terrific storm. The iron piles, 10 in. in diameter and sunk 5 ft. into the rock, were twisted off near the surface. In 1852 money was appropriated by congress for a new light-house, and work was commenced in 1855, but it was not till the latter part of 1857 that the first stone was laid. Four stones were laid in this year; six courses were, however, laid in 1858; and in 1859 the stone-work was completed. The whole was finished in 1860. It is a granite tower in the form of the frustrum of a cone, having a base 30 ft. in diameter, and a height of stone-work of 88 ft., the lower 40 ft. being solid. The courses are dovetailed, and are fastened together with wrought-iron dowels. The defect in the iron Minot's ledge light-house was owing to the stinted outlay. Had three or four times as much money been expended on it, so that it could have been much broader at the base as well as higher, it would doubtless have been standing to-day. The present stone structure is a fair model of engineering, and will probably resist the waves for centuries. It possesses the advantage, which all solid or almost solid stone structures must have over iron framework, of a vastly greater amount of inertia, an important element of resistance to the waves. Its construction is said to have offered a more difficult problem than that of Bell rock or Skerryvore, one reason being that its foundation is deeper beneath the surface. The light-house at Spectacle reef, in the northern part of lake Huron, was built not only to resist waves, but ice fields, often covering thousands of acres and moving at the rate of 2 or 3 m. per hour. That the structure should be able to withstand this force it was so designed as to cause the ice to be broken and piled into a protecting barrier. The tower is the frustrum of a cone, 32 ft. in diameter at the base, and 18 ft. just beneath the cornice at a height of 80 feet. The whole height of stone-work is 93 ft. above the base, which is 11 ft. below the surface of the water. The tower is solid as high as 34 ft., above which it contains 5 stories, each 14 ft. in diameter. The work was commenced May 1, 1870, and the light was first used June 1, 1874. The cost was $375,000. The first cast-iron light-house ever erected was at Point Morant, Jamaica, in 1842. The tower is built of 9 tiers of plates three-quarters of an in. thick and 10 ft. high, held together by bolts and flanges on the inside. The tower is filled in with masonry and concrete to the height of 27 feet. It rests upon a foundation of granite and rises to a height of 96 feet. It is 18 ft. in diameter at the base, and 11 ft. at the top. A modern form of light-house is constructed on what is called the "screw pile" system, an invention of Alexander Mitchell, who, with his son, laid the foundation of the lighthouse on Maplin sand, at the mouth of the Thames, England. Two similar structures followed, Chapman head in 1849 and Gunfleet in 1850, also near the mouth of the Thames. Other screw-pile lights were afterwards erected in different parts of the kingdom. The great feature of the screw-pile is that the piles upon which it rests are in the form of screws and are driven in the sand or soil to a sufficient depth in the manner of a corkscrew. The first screw-pile light-house erected in the United States was by by col. Hartman Bach, U. S. E., at the mouth of Delaware bay, 8 m. from the ocean, in 1847-50, where it stands at the present time in good condition, although in an exposed place, being often acted against by immense cakes or fields of ice which come down the Delaware and move to and fro with the ebb and flow of the tide. It is surrounded by an ice-breaker composed of screw-piles driven independently of the tower. The screwpile light-house at Sand Key, Florida reefs, is supported on 16 piles, with an auxiliary pile in the center to support the staircase, making in all 17. They are 8 in. in diameter, with a screw of 2 ft. in diameter at the lower ends, which are bored 12 ft. into the reef. The framework of the tower consists of cast-iron tubular columns framed together, having wrought-iron ties at each joint, and braced diagonally on the faces of each tier. The keeper's house is supported by cast-iron girders and joists 20 ft. above the foundation. The structure is 120 ft. above the level of the water. The foundation is 50 ft. in diameter. Over 50 such light-houses have been erected in various parts of the United States; whole number of light-stations on or near the coast, 722.

LIGHT-HOUSE BOARD OF THE U. S., a body organized in accordance with an act of congress, approved Aug. 31, 1852, and having the control and management of all lights, buoys, beacons, etc., on the coasts of the United States. It consists of eight persons, viz., two officers of high rank in the navy, two officers of the corps of engineers, two civilians of high scientific attainments, an officer of the navy, and an officer of the corps of engineers-the two latter serving as secretaries. The board as thus constituted is attached to the office of the secretary of the treasury, who is ex-officio president of the same. A chairman, elected by the members from their own number, is chosen to preside in the absence of the president ex-officio. The board is required to meet four times a year, and the secretary of the treasury is empowered to call it together whenever, in his judgment, the exigencies of the service may require a meeting. It actually meets almost every week in the year. The coast and the waters of the country