Chapter D: D, an apparatus for holding a small tube, that the spectrum given by (9)

In passing from the invention of the musical to that of the speaking telephone, Mr. Bell passed from the more complex to the more simple instrument, for of all apparatus by which communication can be carried on at a distance, the Bell speaking telephone is one of the simplest. He had only to make its vibrating disc of Scott’s phono-autograph into a magnetized body, capable of producing currents in an electro-magnet coil in the same way as did the vibrating plates in his musical telephone. The Bell speaking telephone was publicly exhibited for the first time at Philadelphia, in 1876, and was shown the same year to the British Association by Sir William Thomson, who pronounced it the wonder of wonders. For the first time in England, the instrument in a still simpler form was exhibited by Mr. Preece, at the Plymouth meeting of the British Association in 1877, and of nearly the same construction as is still often used, although, as we shall presently see, for battery telephones the transmitting apparatus is now made of larger dimensions, of a different shape and on a different principle. We shall describe the simple form in which transmitter and receiver are identical, each consisting externally of a small cylindrical wooden or ebonite box, and with a handle three or four inches in length of the same material. Fig. 302_d_ is a section of the instrument where N S is a cylindrical steel magnet, on one end of which is wound the small coil B, made of fine silk covered copper wire, the extremities of which pass through the handle M at _f f_, and are connected by the binding screws _I I´_ with the line wire C C´. Close to the coil covered end of the magnet is a very thin diaphragm of iron, L L´, and when this is thrown into vibration by the voice speaking into the trumpet-mouth opening, R R´, its movements produce currents in the coil according to the principles that have already been explained, for it will be observed that the iron disc is magnetized by the inductive action of the permanent magnet N S. These currents passing through the coil of the receiving instrument raise or lower the intensity of the magnetic force in it, so that the distant disc reproduces the vibrations of the transmitter. Such is at least an obvious explanation of the action of this very simple arrangement; but from a number of experiments and observations that have been made with modifications of the instruments, it would appear that other and much more complex phenomena concur in producing the effects. It has indeed been suggested—and the idea is supported by numerous experiments—that, in these telephonic transmissions of speech, vibrations are concerned which are not at all of the mechanical kind we have been dealing with in these explanations, but are _molecular_.

The Bell telephone is used by speaking distinctly before the mouth-piece of the transmitter, while the listener at the other end of the line applies the mouth-piece of his instrument to his ear, and one wire is sufficient with good earth connections, although sometimes a second wire is employed to complete the circuit. It is also found advantageous to have two instruments in the circuit at each end, so that one may be held to the ear while the operator is speaking through the other. In this way, a rapid conversation can be carried on with the greatest ease, or again, an instrument may be held at each ear, by which arrangement the words are more distinctly heard. It is not necessary to shout, as this has no effect, but to speak with a clear intonation, and some voices are found to suit better than others. The vowel sounds are best transmitted, except that of the English _e_, which, with the letters _g_, _j_, _k_, and _q_, are always somewhat imperfectly transmitted. A song is very distinctly heard, both in the words and the air, and the voice of the person singing is readily recognized. Several instruments may be included in one circuit at different stations, so that half a dozen persons may take part in a conversation, and questions and answers may be understood even when crossing each other. If two distinct telephone circuits have their wires laid for a certain distance (two miles) near each other, say a foot or more apart, and without any connection whatever, listeners at the end of the one line will hear the conversation exchanged through the other line. Other forms of the instruments have been arranged, by which a large audience may hear sounds produced at a distance, as, for instance, when a cornet-à-piston was played in London, it was heard by thousands of people assembled in the Corn Exchange at Basingstoke.

It would be impossible within our limits to even briefly describe the great number of improvements and modifications of Bell’s system that were devised by various persons soon after the invention was brought out, and many additional complications were introduced into some of the arrangements. Advantage was also taken to a greater or less extent of another principle affecting the strength of electric currents, to which we have now to call the reader’s attention, and to exemplify by one of the simplest instruments, leaving detailed accounts of the various forms in which it has been applied to be found in special treatises. The reader should first turn back to page 400, where he will see an expression of the strength of a battery current. It will be observed that the current may be increased or diminished by diminishing or increasing R, the external resistance, without changing the other terms. Now M. Du Moncel discovered, as far back as 1856, that an increase of pressure between two conductors in contact, and conveying a current, caused a diminution of the electrical resistance, and this discovery was utilized for telephonic purposes by Mr. Edison in his invention of the carbon transmitter (1876). In this there is no magnet, and a stretched membrane may take the place of the metallic plate, although a circle of photographers’ ferro-type plate gives better results. A pad of india-rubber, cork, or other material is fixed on the plate, and rests upon a carbon disc, which again is in contact with a metallic conductor. Between the latter and the carbon the current from a constant battery passes. When the plate is thrown into vibration by speaking into the mouth-piece, the variations of pressure conveyed to the carbon cause variations in the resistance of its electrical contact, and thus a series of undulations are produced in the current, and these affect the electro-magnet of a Bell receiving instrument in the circuit as before, so that the sounds are reproduced. It is now time to say a word about the share in the invention of the speaking telephone which has been claimed by Mr. Elisha Gray, also of the United States, who, at the time Mr. Bell applied for the patent for his instruments, produced drawings and descriptions of a plan he had devised for transmitting speech by undulating electrical currents, and it has been admitted that the plan he had conceived was perfect in principle. He proposed to use a battery current, and his receiving instrument was nearly the same as Bell’s. The undulations of the current were also determined, as in Edison’s telephone, by changes in the external resistance, but this was effected in a different, though equally simple manner. To a membrane stretched across the lower end of a short wide tube that formed the mouth-piece of the transmitter, and was placed vertically, was attached a piece of platinum wire, conveying the current and dipping into a liquid of moderate conductivity, but not quite touching another platinum electrode fixed at the bottom of the vessel containing the liquid. The space of liquid traversed by the current being thus varied by the oscillations of the membrane, the resulting variations of the resistance produced the requisite undulations in the intensity of the current. Both Mr. Bell and Mr. Gray applied for patents on the 14th February, 1876, but the American Patent Office recognized the claim of the former as prior.

FIG. 302_e_.—_Mr. Hughes’ Microphone._

(B _and_ R _are merely diagrammatic_.)
]

Du Moncel’s observation was applied by Mr. Hughes in the construction of an instrument, which he named the _microphone_. This was in the same year that Edison had brought out his carbon telephone, and a certain similarity, resulting from the identity of the principle employed, led to an acrimonious controversy on what were supposed to be rival claims. But the microphone differs so much in arrangement and performance from the other instrument as to constitute a distinct invention. The instrument, if it may be so called, is simplicity itself, in the form represented in Fig. 302_e_, which is one of the most sensitive. There, C and C´ are two small blocks of carbon, fixed on a small upright piece of wood. Two cup shaped cavities are hollowed out in the carbon blocks, and these serve to hold loosely, in a nearly vertical position, a small rod of gas retort carbon pointed at the ends. This rod is only about one inch in length, and the lower end merely rests on the bottom of the cup in C´, while the other is capable of moving about in the upper cavity, the vertical position being nearly maintained in a state of unstable equilibrium. The carbons are in the circuit of a voltaic cell or small battery, B, in the line through a Bell receiving instrument, which may be at a distance. When the microphone is to be used, it is placed on a table with a cushion or several folds of wadding beneath its base. If the receiver be applied to the ear of a listener, he will distinctly hear every word pronounced by one speaking near the microphone, even in a low tone; but a loud voice may be heard when the speaker is 20 or 30 feet from the instrument. The minutest vibrations conveyed to the stand are perceived at the receiver as loud noises. The tread of a fly walking over the board, S, is heard like the tramp of a horse, and the ticks of a watch are audible in the receiver when the ear is several inches away from it. The slight touch of a feather on the stand is distinctly audible, and a current of air impinging upon it is reproduced as the noise of a stream of water. The microphone is, in fact, the most sensitive detector of vibrations that is known, and its employment as a transmitter has brought the telephone to its present perfection. It has been constructed in an endless variety of forms, according to the purposes for which it is intended, and its simplicity is as wonderful as its extreme sensitiveness. We will further illustrate these qualities by an experiment of Mr. Willoughby Smith’s on the same principle. Instead of the two carbon blocks, he laid on the table, in parallel positions, two small rat tail files, and completed the circuit by a third file, laid across the others at right angles. This arrangement constituted so sensitive a transmitter that the listener at the distant Bell receiver could hear even the faint sound of the speaker’s breathing. Even three common pins, similarly crossed, make an effective transmitter. The feebleness of the variations in the current requisite to make the Bell receiver produce sounds is extraordinary, and a very weak battery current is sufficient, even under the circumstances of ordinary practical use. Still more remarkable is the fact that in favourable conditions the microphone is capable of transmitting sound without any battery at all, but merely with connections to earth, when the ticking of a watch placed upon the stand has been distinctly heard at the distance of nearly one-third of a mile, and speech, also, has been transmitted with unusual distinctness with the battery left out and merely a few drops of water placed at the carbon contacts; indeed, it is said that, even without the water, the voice may be heard. This effect has been attributed to the carbons and water forming a battery themselves, and in the latter to the moisture of the speaker’s breath supplying the fluid element. But, again, the microphone will not only transmit speech, but, under certain arrangements, it will reproduce it (when one of the carbon electrodes is attached to a membrane), although the result is less distinct than with the Bell receiver. It is, however, not so easy to explain how mere variations of current intensity can produce the effect where there can be no magnetic attractions and repulsions. We must, no doubt, look for the cause in some other property of electric currents. The transmitters used in various lines of telephonic communication, erected by the Post Office or by companies in Great Britain, are generally applications of the principle of the microphone, and not of that of either Mr. Bell’s or Mr. Edison’s original instrument. But more recently, Mr. Edison has most ingeniously adapted variations of sliding friction, as modified by the action of the undulatory current on a liquid electrolyte between the sliding surfaces to the production of a loud speaking telephonic receiver—that is, one by which the sounds are made audible to a large assembly. From this instrument, the notes of a cornet-à-piston, played in Brighton, have been distinctly heard throughout a large hall in London.

Another curious transmitter is formed of a fine jet of water traversed by an electric current. Acoustic vibrations are easily set up in the jet, and these modify its conductivity so as to produce corresponding undulations of current intensity.

It would take long to point out the many scientific applications of so sensitive an instrument as the microphone with its Bell receiver. As a medium for conveying speech to a distance, whether for purposes of peace or war, its use is sufficiently obvious. Some curiosities of musical transmission have been noticed, and such experiments are repeated from time to time with increasing success. It has been applied to many purposes in surgery and medicine. In many cases of deafness it has made conversation easy. Even the passage of the molecules of gases, when diffusing through porous partitions, Mr. Chandler Roberts has by its means made audible. The distances to which speech can now be transmitted are considerable, as conversations have been carried on by persons nearly 300 miles apart.

LIGHTHOUSES.

Who does not regard with interest the lighthouses which at night throw their friendly beams across the sea, to guide the mariner in his course, and warn him of perils from sunken rock or treacherous shoal? The modern lighthouse, with its beautiful appliances, is entirely the result of the applied science of our age; and it affords a fine example of the manner in which experiments, carried on to determine natural laws apparently of an abstract character and without any obvious direct utility, give rise to inventions of the highest importance and most extended usefulness. The lofty structures which were erected near certain ancient harbours, and of which the Pharos of Alexandria is the most memorable example, burned on their summits open fires of wood; and whatever beacons existed from that time down to the end of last century were merely blazing fires of wood or coal. The lighthouses of the South Foreland, which were established in 1634, displayed coal fires until 1790, and the lighthouses in the Isle of Man were first illuminated with oil only in 1816. Down to the beginning of the present century, therefore, the modern lighthouses showed no improvement on the ancient plan. Even the Tour de Cordouan, at the mouth of the Garonne river, which was completed in 1610, and is one of the most famous of modern lighthouses, from its great height (200 ft.), and the care which has always been given to render it efficient, showed down to 1780 merely a fire of billets of wood, the upward loss of the light being diminished by a rude reflector in the form of an inverted cone. In the improved means of obtaining artificial light, and in the admirable optical apparatus by which that light is utilized, we find the vast superiority of modern lighthouses. But these are sometimes erected on isolated, and almost submerged, rocks, exposed to the fury of the waves. The difficulties which have to be overcome in their construction cause some lighthouse towers to rank among the best specimens of engineering skill. We may, therefore, consider under the present head—the towers; the sources of light; the optical apparatus and its accessories.

FIG. 303.—_The Eddystone Lighthouse._
]

One of the best-known lighthouses on the English coast is that on the Eddystone Rock, about 14 miles S.S.W. from Plymouth. The structure which now[9] stands upon this rock was the work of Smeaton, and was completed in 1759. The stones forming the lower courses of this tower, which is represented in Fig. 303, half in section and half in elevation, are dovetailed into the rock itself and into each other. The masonry is carried up in a solid mass for about 12 ft., the stone used being granite, which also constitutes the whole of the exterior masonry. The four upper apartments are formed with arched roofs, the side-thrust of which is counteracted by iron chains surrounding the tower. These chains, which are bedded in lead, were placed in their positions while hot, and by their contraction bound the structure together with great force. The masonry of the tower is 68 ft. high, and this is surmounted by the light-room, the total height from the lowest course of stonework to the gilt ball at the top being 94 ft., or nearly half that of the London Monument. The diameter at the base is 26 ft., and that at the top 15 ft. The light-room is of an octagonal shape, and is made of iron framework, glazed with thick plate glass. Below this are two store-rooms, a kitchen, and a bed-room. The Eddystone has now breasted the storms of more than a hundred years, and it remains as firm as the rock it is built on. Fig. 304 is a picture of this noble lighthouse, with the British fleet passing close to it, during a furious gale on the 22nd of October, 1859, or exactly a century after the completion of the structure. The incident of the man in the water, which occupies the foreground, is not an imaginary one, for it is recorded that the _Trafalgar_ stopped in the midst of the storm to pick up a man who had fallen overboard. For eighteen hours the ships encountered the fury of the tempest, keeping out at sea in open order throughout the night. They wore in at dawn, came up the Channel in line of battle, steamed into Portland, and took up their anchorage without the loss of a sail, a spar, or a rope-yarn.

Footnote 9:

Smeaton’s tower proving unsafe, has since been taken down and
replaced, in 1882, by one from Mr. Douglass’ design.

FIG. 304.—_The Eddystone in a Storm._
]

The lighthouse tower on the Bell Rock is 100 ft. high, 42 ft. in diameter at the base, and 15 ft. at the top. The Inchcape Rock, on which it is placed, is the scene of Southey’s ballad of “Ralph the Rover,” and the lighthouse here is one of the most serviceable on the Scottish coast, for the dangerous spot on which it is placed lies in the direct track of all vessels entering the Firth of Tay from the German Ocean. The rock is submerged at spring tides to the depth of 12 ft. The tower bears a close resemblance in shape to that of the Eddystone: it is circular and faced with massive blocks of granite. The lower part, to the height of 30 ft., is solid, and the door is reached by a bronze ladder. The building contains five apartments, and a cistern for storing fresh water for the use of the keepers, who have sometimes to remain in their solitary situation for six or eight weeks together, the weather preventing the possibility of any communication with the shore.

Still loftier than the tower on the Bell Rock is that which rises in the midst of the Skerryvore Reef, 12 miles from Tyree, a small island off the coast of Argyleshire. This building may be taken as a typical specimen of a detached lighthouse, and the difficulties overcome in its construction attest the skill of the engineer, Mr. Alan Stevenson, who has written a highly interesting account of the work. The rocks here are of _gneiss_, an extremely hard formation, and their surfaces are worn as smooth as glass by the action of the water. On one of a numerous series of these small islets, where only a narrow strip of rock, a few feet wide, remains above the surface at high water, and this divided by rugged lumps into narrow gullies, through which the sea constantly rushes, the lighthouse is built. The work was commenced in 1838 by the erection of a temporary wooden barrack on piles at a little distance from the site chosen for the foundation. In a gale during the winter the whole of this structure was swept away in one night. Another, more strongly secured, was built the following summer, and in this Mr. Stevenson and his men remained sometimes for fourteen days together, the weather preventing any passage to or from the shore: here the men were sometimes awakened from their hard-earned repose by the water pouring over the roof, and by its rushing through the crevices, while the erection swayed and reeled on its supports. Mr. Stevenson relates that one night the men became so alarmed for the stability of their shelter that some descended, and sought in cold and darkness a firmer footing on the rocks. Two summers were occupied in cutting the foundations, and the blasting of the rock in so narrow a space was an operation attended with no little danger. A small harbour had to be formed at the rocks for the vessels bringing the ready-prepared stones of the building from the quarries, where also piers were built expressly for the shipment of the materials. In designing his tower, the engineer preferred to oppose the force of the waves by the weight of his structure, rather than to rely on dovetailed or joggled-jointed stones. Measurements were made of the force of the waves, which at Skerryvore was sometimes equivalent to a pressure of 4,335 lbs. on the square foot; and calculations based on these measurements showed that the mere weight of the superstructure would amply suffice to keep the stones immovable. Nearly 59,000 cubic feet of stone were used, or about five times the quantity contained in the Eddystone Lighthouse, and the total cost of the building was £87,000.

The use of iron, as a building material advantageously replacing stone, has extended to lighthouses, and many have been constructed entirely of cast and wrought iron, or partly of iron and partly of gun-metal, which is not readily acted on by the sea spray. Such lighthouses are cheap, easily and quickly erected, strong enough to bear shocks and vibrations, and proof against fire, lightning, and earthquakes. The lighthouse on Morant Point, Jamaica, is made of iron, cast in England; and it was erected in a few months at a cost of one-third of that of a stone tower of the same altitude. Its height is 105 ft., and the shaft is formed of iron plates in segments of 10 ft. high, which are bolted together at their flanges. At Gibbs Hill, Bermuda, is a lighthouse 130 ft. high, constructed in the same manner.

So inefficient, inconvenient, and uncertain were the lamps or other means of artificial illumination known up to nearly the beginning of the present century, that nothing better could be found for the Eddystone Lighthouse for forty years after its erection than tallow candles stuck in a hoop—a means of illumination which would scarcely now be tolerated even in a booth at a village fair. To M. Argand, a Frenchman, we are indebted for the first great improvement in lamps. The admirable invention which bears his name is, as everybody knows, an oil lamp with a tubular wick, which occupies the annular space between two metallic tubes, in such a manner that a current of air rises through the inner tube, and thus reaches the interior of the flame. This current, and the current which supplies the exterior, are increased by surrounding the flame with a tall glass chimney; and a contraction of the chimney, just above the flame, aids greatly in distributing the air, so as to insure the complete combustion of the oil. In the original lamp the supply of oil to the flame depended on the capillary attraction in the meshes of the wick. M. Carcel applied clockwork to continuously pump up the oil into the burner, so that, by overflowing, it was maintained at an invariable level. This arrangement added greatly to the intensity and steadiness of the light; and, on account of the uniformity of its flame, the Carcel lamp has been selected as a standard to which, in France, photometric determinations are referred.

The power of the Argand lamp, as employed in lighthouses, is greatly increased by the plan of employing several concentric wicks instead of one. Between these wicks there are, of course, open spaces, through which the air obtains access to the flame, and the current of air is made more rapid by the use of a very tall chimney. The large amount of heat produced by the combustion of so much oil in a small space is partly carried off by the excess of oil which is made to overflow the burner—about four times the quantity consumed being constantly pumped up into the burner for this purpose. Lighthouse lamps are made with two, three, and four wicks; and the oil is forced up in the burners either by clockwork or by the pressure of a piston loaded with a weight. The following table gives the sizes of the burners and the illuminating powers of the lamps:

┌──────┬──────┬────────┬─────────┐
│Order │Number│Diameter│Intensity│
│ of │ of │ in │of Light │
│Light.│Wicks.│inches. │in Carcel│
│ │ │ │ Lamps. │
├──────┼──────┼────────┼─────────┤
│ 1 │ 4 │3½ │ 23│
│ 2 │ 3 │2–15/16 │ 15│
│ 3 │ 2 │1¾ │ 5│
└──────┴──────┴────────┴─────────┘

The quantity of oil consumed in these lamps is less than that proportional to the increase of the light:—for example, although the four-wick lamp gives twenty-three times the light of the simple Argand, it only consumes nineteen times the quantity of oil. The oil used in these lamps is colza; but experiments have been made with a view of introducing petroleum, which has the advantages, not only of being cheaper and uncongealable by cold, but of giving a whiter and more brilliant light. Hitherto, however, this substance has answered only with lamps of one wick.

Coal-gas has been applied to the illumination of lighthouses, and as it gives a light of great brilliancy and steadiness, when consumed in proper burners, it has certain advantages over oil lamps, which have caused it to be employed in situations where a supply can be readily obtained. The light produced by lime, ignited by the combustion of coal-gas or hydrogen mixed with oxygen, has also been suggested; but this plan is not without risk of interruptions and of dangerous accidents, and it has been considered inadvisable to entrust the apparatus to the persons who commonly take charge of lighthouses.

The electric light has been very successfully applied in certain lighthouses, since the mode of producing steady currents by magneto-electric[10] machines has come into use. The lighthouses at the South Foreland have been thus illuminated by a machine constructed by Mr. Holmes in 1862. A very powerful electric light is exhibited from the lighthouse on Cape Grisnez; and the adoption of this source of light has been extending, as it is far more intense than any other artificial light, and can be sent in more concentrated beams across the sea, on account of its being emitted from a space which is practically a point. These circumstances cause the beams of the electric light to possess greater power of penetrating the atmosphere than those from any other source.

Footnote 10:

Now superseded by the dynamo-electric machines.

But it is perhaps in the optical apparatus of lighthouses that the greatest improvements and most admirable inventions are to be found. When only the blaze of an open fire furnished the guide to the mariner, the means resorted to in order to throw across the sea the light which issued from the flames upwards or landwards, appear to have been of the rudest kind, even where such attempts were made at all. The inverted cone on the Tour de Cordouan has been already mentioned, and we read of cases in which screens of sheet brass were placed on the landward side, to throw back the light seaward.

Here it may be proper to examine the conditions which determine how the light can be made most available for the guidance of the mariner. Everybody knows that the light from a luminous body spreads out from it in all directions equally. Thus, if we simply place an electric light on a tower such as that on the Bell Rock, but few of the luminous rays can benefit the mariner: namely, those which fall upon the sea or are directed to the horizon. A much larger portion of the light will stream upwards and be lost in space; another part will descend towards the base of the tower, and be equally wasted. Again, if the situation of our lighthouse were on the shore of the mainland, all the light which passes landwards, whether horizontally or not, would be entirely lost for our purpose. Even if, in the case of an isolated lighthouse, we can send out all the light in a nearly level zone over the sea to the horizon, the intensity of the illumination will diminish, on account of the widening space, as the distance increases. The question, therefore, arises whether it is possible to send the whole of the light in one unbroken beam, not liable to this kind of enfeeblement, so that the only loss it can experience may be absorption by the imperfectly transparent atmosphere.

There are two means of gathering up all the otherwise useless beams, and sending them in such a direction as to reach the eye of the distant mariner. The one is by reflection from mirrors, and the other by refraction through lenses. The apparatus employed in the first process is termed _catoptric_, and in the latter _dioptric_.

When a luminous point is placed at the focus of a parabolic mirror, all the rays which fall upon the mirror are reflected by it in a direction parallel to its axis, so that they form a cylindrical beam. This is the method which was adopted in the first improvements effected in lighthouses. The parabolic reflector was first used at the Tour de Cordouan in 1780, and soon afterwards metallic reflectors became the ordinary appliances of lighthouses, and they are still largely used. Such reflectors are made of sheet copper, thickly plated with silver, about 6 oz. of this metal being applied to 16 oz. of copper. They are formed by carefully beating a circular sheet of the plated copper into a concave shape, which is finally brought to the exact curve by the aid of gauges, and is then turned and polished. The largest of these reflectors have a diameter of 2 ft. at the _mouth_, as it is termed, for the reflector comes forward in advance of the lamp, the chimney and burner passing through openings in the metal. The flame of the lamp occupies such a position that its brightest part is in the focus of the mirror; but since the focus is a point merely, whereas the flame has a certain magnitude, it follows that the want of coincidence of the other luminous points with the focus produces a certain divergence in the reflected rays, so that the beam is not accurately cylindrical. This, however, is far from being a disadvantage practically, for it has the effect of widening a little the strip of sea illuminated by the beams. But all that portion of the light which escapes from the mouth of the mirror without being reflected is radiated in the ordinary manner, and is practically lost. We shall presently see how even this light may be gathered up and brought into the main beam.

Let us suppose a number of such reflectors, each with its own lamp, placed in a horizontal circle, so as to throw their beams towards different points of the compass. If eight lamps were so placed, eight beams of light would stream out across the water, like eight spokes of a wheel; eight sectors would, however, be left unilluminated, and for ships in these spaces the lighthouse would be virtually non-existent: its rays could only reach vessels within the eight narrow strips traversed by the beams. If we double the number of reflectors in the circle, or if we arrange another series of eight in a circle above or below the others, so that a lamp in the second circle coincides vertically with an interval in the first, the effect will be that we shall have sixteen beams, and sixteen dark sectors, instead of eight; that is, only a very small part of the expanse of water will receive the benefit of the light. It must be remembered that the breadth of the cylindrical beam would not be greater than the diameter of the mirrors, and that the space illuminated by it has the same breadth at all distances; or rather, that this is nearly the case, for the light does not all issue precisely from the focus of the mirror. Thus, even if we use a very great number of mirrors, we shall succeed in illuminating but an extremely small proportion of the sea horizon. This evil is met by giving a horizontal rotatory motion to the reflectors, causing the beams to sweep over the whole expanse of the waters; and thus from every ship the light will be visible for an instant. The rotation is produced by clockwork, duly regulated, so that an uniform motion is obtained. The regular appearances and eclipses of the light prevent the mariner from mistaking for a lighthouse a bright star near the horizon or an accidental fire on the coast; and, further, it being necessary that the lighthouses along any particular coast should be readily distinguishable from each other, it becomes easy, by assigning to each lighthouse a different period of revolution, to individualize them, so that the mariner shall be in no danger of confounding one with another.

But when the lighthouses on a certain extent of coast are numerous, this mode of distinguishing them becomes inconvenient, as mistakes might easily be made in small differences of time; and it would be inexpedient to keep long intervals of darkness. Hence other methods have been resorted to in addition—such as red lights, or lights alternately red and white. The following are the distinctions made use of among the Scottish lighthouses, including the double lighthouses, which give a leading line to the navigator:

1. Fixed lights.
2. Revolving lights.
3. Revolving, with red and white beams alternately.
4. Revolving, with alternately two white beams and one red.
5. Revolving, with alternately two red beams and one white.
6. Flashing, in which the light increases and decreases at regular
intervals.
7. Intermittent, in which, by means of a revolving screen, the light
is abruptly cut off and exhibited.
8. Double fixed lights.
9. Double revolving lights, which appear and disappear at the same
instant.

The efficiency of reflectors depends on the state of polish of the surface, and even with the most brilliant polish there is a very large loss of light: in the ordinary condition of lighthouse reflectors, it is found that one-half of the light is lost at the surface of the mirrors. An attempt was made in England, about the beginning of the present century, to substitute glass lenses for mirrors. But it was found that, in spite of the loss occurring in reflection, the mirrors produced a more intense beam. No doubt the person who made the attempt did not observe the true conditions of the problem. It was Fresnel, the illustrious Frenchman, whose name has already been mentioned in these pages, who successfully solved the problem. He saw that it would be necessary to give the lenses a short focal length, and at the same time to have their diameters very great. The dimensions required by these conditions far exceeded any that could be given to lenses formed in the ordinary manner; and even if they could be so formed, the great thickness of glass which would be necessary would diminish the transparency, and unduly increase the weight of the apparatus to the detriment of the revolving apparatus. An idea now occurred to Fresnel’s mind, which, although similar to previous projects, he conceived independently, and was undoubtedly the first to carry out. This was the idea of the _lentille à échelons_, or “lens in steps.” The construction of this will be understood from Fig. 305, where _a b_ is a section of a lens in steps, and the dotted line, _c_, shows the thickness an ordinary lens of the diameter _a b_ would have. Fresnel kept only the marginal part of such a lens; and inside of the ring formed by this, he fitted the margin of a second large lens having the same focal distance; inside of this another ring, and so on; and in the centre a large lens of moderate thickness. He also placed above and below the lens the concentric prisms, _e e´_ and _f f´_, which, by refraction and total reflections (see page 399), send the rays parallel to the axis of the lens. Fresnel also contrived methods of economically grinding such lenses and prisms with precision.

FIG. 305.—_Revolving Light Apparatus._
]

Fresnel saw that it would be useless to apply lenses in lighthouse illumination unless the intensity of the light given out by the single-wick Argand lamps then in use could be considerably increased, without much enlarging the flame. Accordingly he devoted himself, in conjunction with his friend Arago, to this preliminary consideration. Their studies and experiments led them to the construction of the lamp with several concentric wicks—by which a brilliancy of light is obtainable twenty-five times greater than that of the single-wick Argand. The light which the improved lamp, when combined with Fresnel’s lenses, could send to the horizon, was equivalent to that which would be given by the united beams of 4,000 Argand lamps without optical apparatus; and it was eight times greater than any which could be produced by the reflectors then in use. The first apparatus constructed on Fresnel’s plan was placed on the Tour de Cordouan in July, 1823.

France led the van in the erection of the most perfect lighthouses in the world, and it was not until 1835 that, by the strenuous advocacy of Mr. Alan Stevenson, a dioptric apparatus was employed in a British lighthouse; but at the present time Fresnel’s principle has been adopted in the majority of British lighthouses. Fig. 305 is a part elevation, with the section, of a catadioptric apparatus of the first class. In plan it is a regular octagon, and it sends out eight beams, which are directed to the horizon, and made to sweep over the sea by its regular rotation, produced by clockwork contained in the case, A. The whole frame is very accurately balanced, and turns on its bearings, and the rollers, _h_, _h_, with great smoothness and steadiness. The moving power is given by the descent of a weight attached to a chain or cord, which is wound round a barrel. One train of wheels is connected with apparatus for regulating the speed, and to this an indicator is attached which registers the number of revolutions made in an hour. There is also a contrivance of some kind for maintaining the motion while the weight is being wound up. The reader will observe that all the light of the lamp, L, is utilized, except that which is directed towards the base and top of the apparatus—a quantity less than one-fifth of the whole. About 45 per cent. of the light emitted by the lamp falls on the refracting lenses; 22½ on the upper reflecting prisms; and 13½ on the lower reflecting prisms. The brightest part of the flame is placed so that the beams from it are directed towards the sea horizon, and the space between the horizon and the neighbourhood of the lighthouse receives ample light from the other parts of the flame. Thus a ship, or any part of the sea within the range of the lighthouse, will see the light appearing at regular intervals, as one after another of the eight beams passes across it, the intervals being one eighth of the time in which the apparatus completes its revolution. The zones of totally reflecting prisms, shown at _e e´_, _f f´_, Fig. 305, were not adopted in British lighthouses until 1844, when the Skerryvore light was exhibited with the complete apparatus represented in the drawing.

The optical apparatus for lighthouses is constructed of certain sizes, adapted to the different situations in which it is to be used. The apparatus we have just described is made in six forms, according to the _order_ of light required. The first three orders are for sea lights, the rest for harbour lights; and the following are the dimensions of the apparatus for each order of revolving or fixed lights:

┌─────────────┬─────────────┬─────────────┬───────────────────────────┐ │ Order. │ Height in │ Internal │ Number of Reflecting │ │ │ Inches. │ Diameter in │ Prisms. │ │ │ │ Inches. │ │ ├─────────────┼─────────────┼─────────────┼─────────────┬─────────────┤ │ │ │ │ In Upper │ In Lower │ │ │ │ │ Zone. │ Zone. │ ├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤ │ 1 │ 106½│ 72½│ 18 │ 8 │ │ 2 │ 83½│ 55 │ 16 │ 4 │ │ 3 │ 61½│ 39½│ 11 │ 4 │ │ 4 │ 29 │ 19¾│ 5 │ 4 │ │ 5 │ 21¾│ 14¾│ 5 │ 4 │ │ 6 │ 17½│ 12 │ 5 │ 4 │ └─────────────┴─────────────┴─────────────┴─────────────┴─────────────┘

When a revolving apparatus of the above description is erected on shore, a reflector of suitable shape and dimensions is placed on the landward side of the lamp, so as to throw its rays back upon itself and towards the lenses which are directed seaward.

Fresnel also constructed glass apparatus for fixed lights. If we require to send the light equally towards the horizon in all directions at once, the problem is capable of solution, either by a proper form of glass apparatus or by a proper form of mirrors. Suppose the section, _e c f_, Fig. 305, to revolve about a vertical axis passing through the lamp, it would sweep out a form which, when executed in glass, would spread out all the light falling upon it into one horizontal sheet. Fresnel was obliged to content himself with an approximation to this shape, formed by a prismatic frame of many sides, containing straight horizontal bars of glass, having the section _e c f_. The light is not quite uniformly distributed by such apparatus, but the difficulty and expense attending the formation of prismatic rings were very great when Fresnel constructed this apparatus. Such rings can now be produced economically and accurately, and therefore the fixed-light apparatus is now constructed of circular glass rings, mounted in sections in such a manner that a vertical section through the axis of the apparatus would cut them in the form represented at _e c f_. Instead of forming the metal framework in which the glass is mounted with vertical ribs, it is made with the ribs placed somewhat diagonally, in order that the dark sectors which would be produced by the shadows of upright ribs may be avoided. It should be understood that the forms of the glass in each side of the octagonal apparatus represented in the figure are produced by the revolution of the same section, _e c f_, about the horizontal axis, _d g_.

An ingenious promoter of the catoptric system has contrived to solve the same problem by mirrors. The form of these may be understood by the aid of Fig. 306, which, however, relates to another contrivance. Suppose that the lines A B, A´ B´, are turned about C D as an axis, all three preserving their relative positions, A B and A´ B´ would sweep out two parabolic cones, which would have the property of reflecting in a horizontal direction all rays falling upon them from a lamp placed at L. But glass, as a material for lighthouse apparatus, has so many advantages over metal that it is probable that metallic reflectors will soon be entirely obsolete. The polish of the metal is very readily destroyed, and as it is constantly liable to be tarnished, the frequent cleaning required is apt to produce a scratched state of the surface, even when great care is used. Far greater accuracy of form can be imparted to glass than to metal reflectors. And then there is the great loss of light occurring at even the most highly polished surfaces of metal: a loss which is far greater than that occasioned by the refraction and reflections of the glass apparatus. There are cases, however, in which it is desirable to throw the whole of the light into one beam, and this cannot be done without reflecting the light from one side. Mr. Alan Stevenson contrived an excellent apparatus for this purpose, and the diagram, Fig. 306, will explain its nature. L is a point representing the source of light, A B, B´ A´, a parabolic metallic mirror. All the rays between L A and L B, and all between L A´ and L B´—that is, all those which fall upon the mirror—will be reflected parallel to L G; but those between L B and L B´ would escape from the mouth of the mirror, B B´, as a diverging cone. This is prevented by placing the lens, H I, the focus of which is at L, so as to convert the diverging cone, I L H, into the cylindrical beam, E H I F; and thus half the light emitted from the luminous point is sent in one direction. A hemispherical reflector, C K D, of which L is the centre, receives the other half, which is thus thrown back through L, and then follows the same course as the direct rays. For the metallic reflector, C K D, Mr. Stevenson afterwards substituted a system of glass zones; of which O P Q represents the sections. These had the same effect as the metallic reflectors, without the loss of light occasioned by the latter. The inner surface of the glass, C K D, is hemispherical, and the prismatic zones are such as would be produced by turning the section about L K (or C D) as an axis. The dotted lines show the course of a ray of light, L _m_, which, meeting the hemispherical surface perpendicularly, passes straight through it, and is totally reflected at _m_ by the inclined surface, and again at _n_, so that it returns to L by the path _n_ L. Reflecting glass prisms were also substituted for the metallic mirror, A B, B´ A´, and thus the use of metal has been entirely dispensed with in this apparatus. This light has been termed by Mr. Stevenson the _holophotal_ (ὁλο, _entire_, φως, _light_). Such an apparatus will form the intensest beam that a given source of illumination can yield. On the other hand, when a fixed light is distributed to the whole horizon simultaneously, the illuminating power of the source is taxed to the utmost. These two cases may be considered the extreme modes of disposing of the light, while the parcelling of it into several beams, as effected by the apparatus represented in Fig. 305, is an intermediate mode.

FIG. 306.—_Stevenson’s Holophotal Light._
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It may be interesting to mention that the holophotal light at Baccalieu, in Newfoundland, is visible in clear weather from another point 40 miles distant. So long a range as this is seldom possible at sea, on account of the rounded form of the earth rendering it necessary to raise the light nearly 1,000 ft. above the water, if it is required to be visible at 40 miles’ distance. A shorter distance generally suffices for the requirements of the navigator; and therefore lighthouse towers rising from the water are seldom carried to a greater height than something between 100 ft. and 150 ft. A light elevated 100 ft. above the water would be seen from the deck of a vessel 14 miles distant, and from the masthead a much greater distance.

The optical apparatus of a lighthouse is protected by an outer metal framework glazed with thick plate glass. This framework is made of iron, or of gun-metal—the latter being preferred on account of the frequent painting which iron needs in order to preserve it from corrosion. The glass is carefully fitted into the framework, so as to avoid exposure to strains from the shocks and vibrations to which a lighthouse is exposed. The keepers are always provided with a store of panes of glass, ready for fitting into their places in case of accidents. Sometimes the glass is broken by large sea-birds dashing against it, and by pebbles which are thrown up by the waves, or driven by the wind against the panes. It is the interior of this lantern which forms the light-room already spoken of. Great pains have been bestowed on the proper ventilation of these light-rooms, as not only must the air have access to the lamp to supply the flame, but the carbonic acid which escapes from the chimney of the lamp must be promptly removed. Another serious inconvenience of an ill-ventilated light-room would be the condensation, in the inner surface of the plate glass, of the aqueous vapour, which is also a product of the combustion.

The lenses and circular prisms for lighthouses are usually made of crown glass, and are ground by fixing them on a large revolving iron table, on which they are bedded in plaster of Paris and cemented by pitch—great care being taken to place them in the exact position required, for only about one-eighth of an inch is allowed for grinding down to shape the glass as it comes from the moulds. Sand, emery, and finally rouge, are used with water for the grinding and polishing processes. The cost of the optical apparatus alone of a light of the first order, like that shown in Fig. 305, amounts to upwards of £1,500. The lenses and prisms are very carefully adjusted in their framework after this has been fixed, and no plan of testing the adjustment has been found more efficient than that of viewing the sea horizon through them from the position which the flame will occupy.

The men to whom the charge of a lighthouse is confided undertake a duty involving the gravest responsibilities, and demanding unremitting care. In those lighthouses where a number of reflectors are hung upon a revolving frame, the extinction of one lamp may not be a matter of much consequence; but where only one lamp is used, life and death depend upon its burning. To isolated lighthouses—such as those of Skerryvore and the Bell Rock—four keepers are appointed, and one of these is always on shore on leave, so that the men may be relieved at intervals; for it has been found that a residence in these lonely towers cannot be continued long together without bad effects. The duties of the lighthouse-keepers must be performed with the greatest regularity. The glasses of the light-room and the optical apparatus are carefully cleaned every morning; the lamps are supplied with oil, the wicks trimmed or renewed, the machinery oiled and adjusted, and everything prepared in readiness for the evening. At sunset the lamps are lighted, and one keeper takes his watch until midnight, when he is relieved by another, who maintains the vigil till sunrise, when the lamps are extinguished.