Imágenes de páginas
PDF
EPUB

between the combustible gas and the of an inch high, and consuming about surrounding air. If this surface be too six-sovenths of a gallon of oil hourly. limited, relative to the generation of The mechanical part of this lightgaseous vapors, or if, for lack of an air- generating problem has reference current, the burned gas is not rapidly. chiefly to maintaining a continuous removed, the unburned gaseous vapors flow or supply of the oil or other must pass off as smoke. Hence, only fluid at a constant level, so as to presmall, solid wicks can be used. Argand serve unaltered the conditions of commet this difficulty by making the wick bustion, and thus prevent unsteadiness hollow, and by providing for an interior of flame. The well-known Carcel or air-current, as well as an exterior one. mechanical lamp is a common type of By this means, not only is a large de- this arrangement. An expenditure of velopment of combustion surface ob- ingenuity has been bestowed upon this tained, but, by means of Argand's glass problem, which can best be appreciated chimney, the air currents are made to by inspecting the great number of flow rapidly and uniformly, giving patent specifications for this object in steadiness, as well as amplitude of fame. the French Description des Machincs. The increased heat of burning, by reason Innumerable specifications for burners, of the enlarged flame, also adds essen oil-cisterns, oil-pumps, oil-pump valves, tially to the intensity of the light. lamp-regulators, oil-supply alarins, etc., Flaine being really transparent, the —these give evidence to the difficulties light produced on the inner suface is and importance of the problem. The mostly given out and utilized.

main result is, that there are now in uso As it is indispensable for light-houses for light-houses, the clock-work movethat the luminous flame should be quite ment lamp; the lamp of Henry Lepante; limited in volume, in order that a proper the Wagner lamp; the hydraulic lamp, direction may be given to the generated and the pneunatic lamp. We must light, the Argand arrangement was an not enter the maze of specifications ; absolute prerequisite to any intense for scarcely has the watch itself outrun illumination for this purpose, whether the lamp in varieties of movement, from sperin, colza, or olive oil, or from model, and mechanisın. The lamps any other known lighting, solid or fluid. used on board light-vessels are, or at Argand, at once, produced his lamp least ought to be, simple Argand burnnearly in perfection; and the chief, al ers, properly mounted, and supplied for most the only, subsequent augmenta bearing the instability of a floating mass, tion of its power was due to the memo and furnished with reflectors. The rable research in which Arago and same principles govern the shaping, Augustin Fresnel, acting as associates, formation, support, and adjustment of produced the burner with two, three, lamp-chimneys for light-houses, as in four, and five concentric hollow wicks the common Argand lamps; though with an air-current passing up through the enlarged wicks require them tu aseach of the open, ring-like tubes or sume proportionate dinensions, and spaces between the concentric wick or make a free air-draft or ventilation of oil tubes. Thus, any number of con increased importance. The mechanical centric wicks might burn together, until arrangements are usually so adjusted as the imperfect transparency of the flame to cause a flow, over the wicks, of about should indicate a limit. The effect of five times the amount of oil burned, this invention has been to gather the which serves to prevent charring of the previous cluster of separate lamps into wicks, and obviates the need of freå single central lamp, of decidedly quent triinming and raising. The surgreater power; and which may be made plus is caught in a dripper, strained, to meet fully all proper demands of the and again used. highest existing towers. In such a Practically, the chief materials used case, the limit of range prescribed by for light-house illumination, are sperm the earth's sphericity can be effectively oil and colza, or rapeseed oil. The reached in ordinary states of the atmog former is still used in this country ; the phere, by a burner with four or five latter, which is derived from a species concentric wicks, producing a flaine of wild cabbage, is used entirely in from four to four and a half inches France, chiefly in Great Britain, and is, high, and four and a half inches in indeed, altogether the main reliance of diameter—the wicks being one-fourth the European lights. Olive oil has

been pretty extensively employed, but rent which is likely to change this fact, is nearly gone out of use. Colza nil though, of course, none can foretell gives the intensest light; produces less what inventions or discoveries are imcharring of the wick; is less affected by pending in this inventive and inquiring cold; breaks fewer chimneys, and is, in age. most places, very much cheaper than We would call special attention to sperm oil.

Indeed, so decided is the the great benefits attainable, by estabsuperiority of colza over all other ma lishing in our own country the colza terials for light-house illumination, that culture, for the production of colza or very few lights in Europe are now rape-seed oil. It is clear that sperm burning anything else. Lard oil, cot oil cannot much longer maintain its ton-seed oil, olive oil, various patent place in our light-houses; but we must oils, both animal and vegetable, besides soon resort, us England already has, to sundry organic compounds, evolved by the use of colza oil. The whale fishemodern chemistry, have been experi- ry is growing precarious, and the absomented on for this particular purpose, lute supply of sperm oil is sensibly with results adverse to their relative diminishing. Meanwhile machinery is availability, reliability, and economy. devouring it at an increasing rate, and Carbureted hydrogen or coal gas, from the price is rapidly advancing In coal, rosin, or oil, has been on trial in 1811–2, the light-house sperm vil cost various light-houses here and elsewhere; $0 55 per gallon ; in 1847-8, do. $1 07 ; but it is found to be quite too precari- in 1850-1, do. $1 17; in 1854, do. $1 39 ous, and the apparatus for its produc- to $1 58; and at the close of 1855, the tion and thorough purification is costly, cost of oil, delivered at the lights, was and too complex to be worked by a $2 25. It is, therefore, almost iinperacommon keeper. The same objections tive that colza oil should be introduced apply with increased force to the three into our service as soon as an adequate intensest known sources of light. The supply can be procured. To becomo electric light from the brush between dependent on importations for this purtwo charcoal points in a powerful cir- pose, is a very unpalatable contingency; cuit, is entirely at the mercy of bat but unless the colza culture is develtery derangeinents, and of the wasting, oped in our country, such importation breaking, or maladjustment of the char must soon be begun. There is ample coal points. Tho Drummond light reason to believe that this culture might from a ball of lime, made incandescent be made very profitable, if judiciously in the oxyhydrogen blow-pipe flame, is and vigorously undertaken. Throughpeculiarly difficult to maintain, becauso out France, Belgium, Holland, and Gerthe incandescent ball is constantly many. it occupies an important agriprone to fly to pieces under the exces cultural rank, and is in some parts the sive heat. In Gurney's Bude light, staple production. In England, and to oil is burned in an oxygen jet, instead some extent in this country, colza is of the atmospheric mixture; hence an cultivated for fertilizing and grazing oxygen generating laboratory, with all purposes, the oil from the seed being its essential resources and skill, is mostly neglected. The German popurequired; and this, in the hands of a lation in Texas raise the colza, and excominon keeper, cannot at all be relied press enough oil from its seeds to meet on. Besides other economical and me their domestic wants. In Mexico, its chanical objections to this trio of in- production is carried so far, that the tense lights, they would be too small to lighting of streets and houses, in many give the required vertical and horizontal villages and cities, is effected by domesdivergence without bringing the para tic colza oil. It may, therefore, be rebolic reflectors or dioptio lenses too garded as proven, that colza oil can be near for their safety. The sum of the readily produced in our country for whole matter is, therefore, that as light domestic as well as public purposes, houses are all isolated, and kept by men and that a large family consumption of moderate skill, and as they yet re

could be relied on at remunerating quire the utmost certainty of lightingrates. Even at present European rates colza oil and sperm oil are the only for this oil, the colza culture would probilluminating substances which can now ably be quite as profitable as that of be safely and economically used for this our chief agricultural staples. But for purpose. Nothing is at present appa- light-house uscs, a much higher rate

would now be justified, and the expenses reflection, the whole is also called a of importation would by no means in- cata-dioptric apparatus—the two terms terfere with the economy of introduc- being currently applied to the same ing it, at least in our lens-lights. As arrangement. Both reflection and resoon as the growth and manufacture fraction, in this connection, take place should become systematically establish- only at the limiting surfaces of solid, ed, an enormous consumption for house- homogeneous masses, shaped for their lighting might be anticipated. Shall special ends. The line perpendicular we not, then, make haste to introduce so to a surface at any point is called the important a branch of production; one normal at that point. so needful for our light-houses, in case The law governing reflection at surof not improbable failures in the whale faces is, that the incident and reflected fishery, and so essential, should we be rays always make equal angles with involved in a commercial war? Would the normal, at the point of incidence. it not, even, be quite as wise for Con- By means of this geometrical law, all gress to offer bounties for its initiation the rays from any radiant point could as it was thus to favor hemp and the be reflected in obedience to any given fisheries?

condition, if we were only able to make, We must now very concisely present with accuracy, mirrors of any geometrithe main features of the optical portion cal form, and free from absorption. In of the problem of light-house illumina- fact, only spherical and paraboloidal tion. If, then, we conceive a simple, mirrors are used. A mirror presenting naked light, burning on the summit of the hollow surface of a spherical seg. a tower on an ocean headland, the rays ment, is the one most easily made ; but would issue in all directions from the this is only an approximate instrument flame as a centre, though .only those for throwing out the rays veceived from portions of them which proceed in di a light placed in front of it, over the rections where they might reach a water spread out before it. The focal navigator's eye could be of

any service. radii and the diameters at each point of Not only would all the rays in the be a paraboloid, make equal angles with misphere-whose centre is the light, and the normal at that point: also, all tho which lies above a horizontal plane diameters of a paraboloid are parallel to through the light-be without useful each other. Hence, the paraboloid (this effect, but all the rays which proceed surface is shaped like a shallow washlandwards, and which strike the ground, bowl) is characterized by the property would be thrown away. In ordinary that all the rays proceeding from its cases, not one-eighth of the light gene- focus will be reflected at its concave rated would be so emitted as, without surface in a beam of parallel rays. If artificial direction, to be of any use. a light-house lamp be placed at the foous Calling the range of a light the greatest of a paraboloid mirror whose axis is bodistance at which the earth's curvature, rizontal, the reflected light will constiatmospheric refraction, and its own ele tute an approximate beam of parallel vation, would generally permit it to be rays, in the precise direction for the best seen, it is evident that what is wanted effect at the limit of visibility. Unforis, so to direct all the light generated as tunately, the reflected rays make only that it shall entirely fall on, or pass just a limited portion of the whole radiation. over, the water within a sector traced But, what is worse still, if the light and around tho light, with the range as a mirror are stationary, the luminous beam radius, and limited by the extreme radii having theoretically no divergence, and passing over navigable areas. The practically not over 15%, it would require question, then, is how to direct all the twenty-four lamps set in a circle to generated light within these limits of illuminate the entire horizon by parauseful effect, and especially along the boloidal reflection. Besides this, all meextreme water horizon limit.

tallic reflections cause a great absolute There are two modes of changing the loss of light, by absorption ; even silver, direction of a ray of light, leading to with the best polish Lord Rosse is able the two species or systems of light-house to give it, absorbing from 7 to 10 per apparatus, called the catoptric, or re cent. His great six-feet refleotor gives flecting system, and the dioptric, or not much clearer illumination than the refracting. As the usual dioptric ap- fifteen-inch Cambridge equatorial reparatus has parts which use internal fractor possesses. Thus, paraboloid

VOL. VIIJ.-14

mirrors are very far from being a cor therein can be accurately traced. Given, rect theoretical solution of this prob then, glass of a known index of refraclem; and the practical departures from tion, how can all the rays of a central theory, in so far as they correct the light-house lamp be strictly utilized by abstract faults, annul the abstract merits its use, and what shapes and positions of the arrangement. Both in theory must be given it? and practice, every simple metallic re To Augustin Fresnel the world owes flector is radically faulty; for it must enduring gratitude, for his elegant and waste much of the reflected light, and almost faultless solution of this practimust leave the front radiation wholly un cal problem. Prior to his research, corrected. Except for this loss of light lenses had been tried for giving direction and front radiation, a series of lamps to light-house illumination ; but these on faces of a revolving frame, each lamp trials were very faulty, either optically with its paraboloid mirror attached, or mechanically. In England and Irewould make up a satisfactory revolving land, simple spherical lenses were placed light; but no satisfactory fixed light is before lights a hundred years ago; but thus possible.

their great thickness, and the bad As might be supposed, it was not till quality of the glass, made them, on the the Argand lamp had given an intense whole, injurious to their effect. Buffon, concentrated light that spherical or pa who was much engaged in forming raboloidal mirrors were to any extent burning-glasses by which the sun's used in light-houses. Some rude trials of parallel rays are focalized, proposed to plane mirrors, and paraboloids built cut away the central mass of glass, and of plane glass facets, preceded Borda's to reduce the lens to a series of rings arrangement of Argand burners with placed around the central lens in echelon paraboloid reflectors on a revolving order. As his idea was to make all these frame, first set up in the Corduan in a single connected piece of glass, its tower, in 1784; but, practically, the supreme mechanical difficulty made it great merit of this combination belongs virtually impracticable. Condorcet was either to Teulère or to Borda, who, aided the first to indicate the plan of a sepaby Lenoir's skill, really initiated the rate formation of the rings, with which existing catoptric system. Probably no large annular lenses might then be built essential advance from Borda's arrange up. Brewster, when treating of burnment will ever be made by using me ing-glasses, in 1811, presented a clear tallic reflection only. None has thus exbibit of the composition and action of far been realized; and, from the nature annular lenses, and advocated their use of the case, all metallic catoptric ar in the inverse problem of parallelizing rangements must leave much of the the rays diverging from a light-house light unutilized. Borda's plan, though lamp flame as a focus. As he did not still much in use, only survives by vir- fully embody his ideas in practical tue of organic inertia, and it is now forms, and as he, apparently inspired rapidly giving place to one vastly su

with less than his accustomed ardor, perior.

failed to procure responsive action by A strict geometric law also governs the inert light-house commissioners, no the refraction of light at the surfaces fruit resulted from his advanced conof transparent bodies. The sines of ceptions. Brewster, of all men living. the two angles made by the incident and can best afford to spare a single optical refracted portions of a ray with the laurel, but even this he is not bound normal at the point of incidence, bear wholly to forego. In 1819, Arago ofa constant ratio to each other, for each fered to undertake for the light-house substance, whatever be the angle of commission a systematic series of reincidence. Each refracting medium, searches, with the express object of implaced in vacuo, is characterized by its proving light-house illumination, and own special value of this ratio, called for this he applied to have Mathieu and its index of refraction, which may once Fresnel assigned as co-laborers. It was for all be experimentally determined for through the acceptance of this proposal each substance. Knowing the indices that Fresnel, being duly detailed as an of refraction for the various media of a officer of Ponts et Chaussées, was led to given combination, as a telescope, a that brilliant train of researches and inmicroscope, or a light-house refracting ventions so admirably detailed in his system, the entire course of any ray Memoir, read before the Academy July

29, 1822, in which, not knowing of glass to be penetrated is so small that Brewster's conceptions, he takes up the absorption produces only a slight loss. whole problem de novo. He was already. The middle ring, at the level of the recognized as the profoundest optical flame, is plane-convex in cross-section, philosopher of his age, and as a perfect with the convexity outwards, and is of master of the most difficult analytical considerable breadth above and below implements. Among the many illus the focal level. The rings just above trious opticians since Newton and Huy- and below this have a four-sided, apgens, we think not one has possessed so proximately-trapezoidal section, and excellent a blending of all the qualities with the precise curvature on the exand powers needed for fruitful and com. terior for parallelizing and horizontalizplete research as Augustin Fresnel. ing the emerging rays. The several As with Snellius and Malus, his brilli- rings, above and below, are similarly ant career of research was prematurely determined. All these rings are limited closed, yet each of this illustrious trio by horizontal top and bottom surfaces, made fundamental discoveries which and their interior surfaces together only Young has equaled since Newton make up a vertical cylinder: thus all and Huygens. To then we may apply the curvatures are thrown into the outer Newton's saying when Coates died: surfaces. The horizontal glass sur“ Had these men lived, we should have faces in contact are cemented, and the known something.” Huygens originated segments of the rings are sustained by the watch, and the undulatory theory metal ribs placed radially, and connectof light; Fresnel approached his merit, ed with the main supporting-frame. by inventing his light-house apparatus, This cylindrical refractor receives the and by discovering the formulæ of in- rays for about 300 above and 30° beterference, double refraction, and polar-low the horizontal plane, through the ization. Before the trained powers of focus. It is surmounted by a domesuch a man, the difficulties of light like arrangement of prismatic zones, so house optics vanished forever. Not con adjusted as to receive and horizontalize tent with vaguely indicating desirable the rays between about 30° and 80° combinations, he determined, with pre above the horizontal. These zones give a cision, their exact form, dimensions, and spherical triangle in cross-section. The modifications. He left but few improve light enters at the under side, passes to ments to be made, and even these he the superior face, where it is intervally had indicated the mode of effecting. It reflected, and, after a second refraction is not amiss, therefore, to call the diop. at the outer surface, it emnerges duly tric or catadioptric light-house appara- horizontalized. It is fortunate that tus, now in general use, the Fresnel interval reflection is attended with far lens-a name than which no worthier less loss of light than in metallic reflecor more enduring monument could be tion, provided it take place as in these erected. We will now indicate briefly zones, within the angle called the angle the prevailing forms of these lenses. of total reflection. The cylinder of the

The light is produced by a single main cylindrical refractor is extended central lamp-flame, proceeding from downward by several zones of spherical concentric wicks, varying in number triangular cross-section, operating like from one to five, the focus being the the upper zones, and receiving the light central point of flame. Around this are from 30° to 52o below the horizontal. arranged, for a fixed light of the first Thus all the rays, except an upper cono order, horizontal hoops or rings of glass, of 10° angle, and a lower one of 38° so shaped and placed as to throw out in angle, are received on glass rings, and a horizontal direction the light re thrown out horizontally and uniformly ceived on them. Thus while the hori. in each azimuth. The upper opening zontal divergence is duly preserved, the is needed for the chimney, and the vertical divergence is counteracted, and lower for the lamp and lamp-attendnearly all the rays are brought into a ance-very little light being thrown flat, star-like horizontalism (as when a down in that space any way, because chestnut burr is pressed flat), and the it is eclipsed by the lamp and wick. illumination is equally diffused over all Such, in general terms, is a fixed catapoints in the horizon. The number of dioptric lens--the cylindrical refractor these rings varies with the order of the simply bending the rays horizontally, light, and, in all cases, the thickness of while the upper and lower zones combine

« AnteriorContinuar »