網頁圖片
PDF

and first used in the Tour de Corduan. They are made of thin sheet copper, lated over with a thin film of silver. #. their manufacture, as conducted in this country, the plater rolls out a disc of copper, coated with a lamina of silver. This disc of metal, varying in diameter according to the size of the reflector, is handed over to a coppersmith, who hammers it to the requisite concavity, the curvature being ascertained by the use of a wooden mould. Next, the silversmith burnishes the silver surface, in the usual way; and, when this is done, the reflector is complete. An argand burner is then placed in the focus, and supplied with .." from a fountain-lamp placed behind. The form of the reflector is supposed to be a parabola. During the animated discussion of 1833 and '4, concerning the Light-House System of Great Britain, an able writer in the Edinburgh Review, supposed to have been Sir David Brewster, alludes to imperfections rendered unavoidable in these reflectors, by difficulties of mechanical execution; and his objections,” as every scientific person must see, are well founded, and prove the inefficiency of reflectors in cases where the best possible lights are required. They have been found, however, extremely useful in ordinary cases, and indeed, until a more perfect apparatus was invented, they were used in all the Light-Houses of France and Great Britain as well as in those of this country. But for many years it has been felt that, in principle, and according to all optical laws, lenses would be far better concentrators of light than reflectors. The law quoted by the Edinburgh Reviewer, from Sir Isaac Newton, that “every inequality in a reflecting surface makes the rays stray five or six times more out of their due course, than the like inequalities in a refracting one,' announces, indeed, the decided superiority, of the dioptric to the catoptric principle in its application to lighthouse as well as to telescopic illumination. And, as early as the year 1780, attempts had been made to introduce a polyzonal lens in France, by the Abbe Rocheu, and in Scotland by Messrs. Cooksen of Newcastle-upon-Tyne. Difficulties of construction, however, prevented the success of the experiment, though it was clearly established in theory. The great difficulty to be encountered, arose from the thickness of the glass,

which was found to absorb more light than it refracted; and though before 1750 Buffon the naturalist had suggested a method of grinding away a part of the useless surface, this had been found quite impracticable, and the plan of using a lens had gradually been abandoned. In 1811, Dr. Brewster invented a method of building lenses of any magnitude required, of separate pieces; and in 1812, he minutely described this method, in his “Treatise on Burning Instruments, containing the method of building large Polyzonal Lenses.” This important invention, however, excited little notice in England, until it had first been applied, with the most decided success, in France. In 1822, M. Fresnel, a distinguished member of the French Light-House Commission, published a “Memoire sur un Noveau Système d’Eclairage des Phares,” in which he earnestly recommended the substitution of lenses for reflectors in the Light-Houses of France. He does not claim originality in this idea of the superiority of lenses, for he says distinctly, that “this application of lenses to the illumination of Light-Houses cannot be a new idea, for it readily suggests itself to the mind; and there exists in reality a lens LightHouse in England.” This was the lower Light-House in the Isle of Portland, fitted up with lenses in 1789; owing to defective construction, however, and a general ignorance of the best methods of applying them, the experiment as already stated, had not proved success. ful. In consequence of Fresnel's recommendation, the Light-House at Corduan was fitted up with the polyzonal lens; and the result was so satisfactory, that the French Government, in 1825, adopted his improvements throughout the *House Establishment of the kingOnn. The success of this experiment induced Mr. Robert Stevenson, Engineer to the Scottish Light-House Board, to go to France in 1825. He saw M. Fresnel and bought one of his polyzonal lenses; but no ". were immediately taken to test its efficacy by experiment. Sir David Brewster, in the early part of 1826, addressed to the Board a memorial on the subject; and in 1827, corresponded with the Trinity House and the Irish Board in reference to the substitution of polyzonal lenses for reflectors. It was not, however, until 1831 that any efficient measures were taken to test the superiority of the new invention. In February of that year the Commissioners appointed a committee “for the purpose | conducting experiments on the comparative merits of lenses and reflectors.” From a variety of causes these experiments were not made until February, 1833.” They took place upon Gulan Äi. the Coast of Haddingtonshire: the lenses and reflectors being placed in temporary cabins erected for the purpose, about one hundred yards apart, were obsevred from Calton Hill, twelve miles distant, by Sir David Brewster and other scientific persons. Mr. Stevenson, the Reporter of the Committee, in his account of these experiments, says:

* Edinburgh Review, Vol. 57, p. 180.

“The result of the experiments, in the judgment of the leporter, was that the lens light had a more brilliant appearance than the reflected light till the number of reflectors were increased to about seven or eight, when both lights seemed equal to the naked eye; but when seen through the medium of a telescope, the lens light appeared to more advantage in point of brilliancy or intensity, while the body of the reflected light appeared larger to the naked eye.”

In ...i up his report on the experiments of the third night, Mr. Stevenson says:– “It also appears from the experiments with the lens and reflectors, that from seveN to NINE of the reflectors now in use at the Northern Lights produce a light equal to that of the lens.” These experiments seem very clearly to establish the fact that a single polyzonal lens with one Argand burner of four concentric wicks, gave a light equal to that of nine reflectors, each carrying a single Argand burner. Sir David Brewster, in a communication addressed to the committee, expressed in very nearly the terms we have used, his judgment of the result, and quoted a similar conclusion reached by the Academy of Sciences of St Petersburgh more than five years before, as well as an opinion to the same effect, expressed by Sir John Herschell in his Discourse on Natural Philosophy. In his examination by the

Committee, Lieut. Drummond also stated, that “the light given by the lens was found by the experiments at the Trinity House in London, to be equal to that of nine reflectors.” He also spoke of an additional apparatus, to which we have not yet alluded. It is simply a combination of reflectors placed above and below the lens, to collect the light that would otherwise pass beyond instead of through them. ith this, he considered a single lens fully equal to TEN reflectors.

In consequence of these representations, the Light-House on the |. of Inskerth, in the Frith of Forth, a few miles from Leith, was fitted up, in 1835, with a lenticular apparatus; and the increased brilliancy of the new light was so apparent, that a number of others have since been established in the kingdom and by the government in the colonies. The French light at Barfleur is probably the most splendid in any LightHouse in the world; and exemplifies better than any other the real efficiency of the lenticular method of illumination. It is lighted, like all others of this kind, by a single lamp, having four concentric wicks, .. largest being three and onehalf inches in diameter. These wicks are raised on cylinders, separated so that the air can pass between them, and produce a flame six inches high. This lamp, being placed in the centre, is surrounded by sixteen lenses, in oblong frames, 34 inches high and 14; wide, standing side by side on one ring and steadied by another laid on top and screwed fast to the frames, thus forming a sixteen sided prism, of about six feet in diameter and thirty-four inches high. Each lens is composed of several separate pieces of glass, put together in the manner invented by Sir David Brewster and afterwards by Fresnel and Arago; the center piece being a perfect plano-convex lens, having the flat side towards the light, and the others being portions of circular prisms concentric § the lens. The same effect, of course, is produced as from a lens the entire size of all these pieces taken together, except that it is greatly increased by the inferior thickness of the central glass. The backs of all these pieces being in the same plane, a vertical cross-section through the centre would

The official account of these experiments, which were made on the nights of the 12th, 13th, and 14th of February, 1833, may be found in the Appendix to the Report of the Select Committee of Parliament on Light-Houses, in 1831; No. 130, page 127 of the show a straight line on the side towards the light, and a serrated edge on the opposite. These sixteen lenses, thus secured between two rings, the lower one is supported on eight bars that curve out around the lower mirrors, and then curve in again to another ring, which is connected with the machinery by which the revolving motion is secured. Below the lenses are four circles, and above them seven, on each of which are 28 mirrors. Of this magnificent light the writer from whom this description has been condensed,” says,

Report.

“To the stranger who visits the Barfleur light, this assemblage of 308 mirrors and 16 large lenses, surrounded by 16 windows of late-glass, nore than 10 feet high, all poished to the highest degree of perfection, and all concentrated within the small compass of the lantern, presents one of the most brilliant exhibitions that the arts can furnish, especially when, in addition to this, he feels the effect of standing 236 feet above the level of the ocean, without any thing to prevent falling out; for the plateglass of the windows is scarcely perceptible, although so strong that the largest sea-birds cannot break it, but frequently fall dead by the blow when, flying towards the light, they come with full force against the glass. The stranger on entering from the darkness below, is taken by surprise, and, for a while, afraid to move, lest he may touch on one side or the other, and the apparently frail fabric crumble under his hands.”

The French lenticular lights, which are thus recommended by their superior illuminating power, are also represented as being far more economical than the English reflectors. The writer in the Edinburgh Review, to whose able article on this subject we have already referred, shows by a comparative statement of the exense of each, that a revolving Lightouse could be originally fitted up with lenses for £1025, while a revolving light

with reflectors would cost £1500—making a difference of £475 in favor of the lenticular system. But even were the original cost to be greater instead of less, it is evident that lenses wouid he far more desirable than reflectors; that trimming and cleansing them would be far less expensive; and that, in this way, the amount annually saved in current expenses and repairs, would soon overbalance the original loss. But there is ood reason to believe that a still more ecided saving would be effected in the expense of oil, by the substitution of lenses for reflectors. It will be recollected that upon testimony already introduced, one Argand burner with a single lens, gives a light equal to nine reflectors, fitted with nine argand burners. At the same time Mr. Stevenson, in his Report on the experiments at Gulan Hill, says that “a powerful lamp is used for the lens, which consumes oil equal to the supply of fourteen Argand burners.” The saving effected here will readily be seen. In a Light-House of the first order furnished with reflectors, there are ten on each of the three sides, making thirt reflectors, to be supplied, of course, wit thirty lamps. In one of the same order furnished with lenses, a single burner, consuming only the oil of fourteen lamps, would be sufficient. Here, then, we should have the light of thirty lamps, at the erpense of fourteen, and the economy would increase with the brilliancy of the light.f The opinion of Sir David Brewster with regard to the expediency of substituting lenses for reflectors, was expressed in the most decided terms in his communications to the lens committee of Parliament. In one of them dated February 23, 1833, after stating that the superiority of lenses was “no longer a matter of opinion,” since it had been proved that a single lens, was equal to at least nine reflectors, he applies this result to the

[ocr errors][merged small]

House Doc. No. 190, 25th Congress, 3d Session.

f In 1840 Hon. John Davis, on behalf of the Committee of Commerce, submited to the Senate a comparative table of the annual consumption of oil by the Light-Houses with reflectors, and those with lenticular glasses of corresponding range and brilliancy, preared by the Superintendent of the construction of lenticular Light-Houses in France, r. Henry Lepaute. ... Taking each Light-House in the United States, he gave first its range in nautical miles, the number of its sockets and its annual consumption of oil; and then gave the range in nautical miles, and the annual consumption of a lenticular lantern which might be substituted for it. The aggregate result showed that in 164 LightHouses with reflectors which consume 60,673 gallons of oil, lenses might be substituted so as to give a much greater average range, with a consumption of only 32,575 gallons. The table is incorrect in some minor particulars, but is well worth examining. —See Senate Doc. 474, 26th Congress, 1st. Session.

case of a revolving light, and thus states the advantages of the lenticular system:

“The revolving light with lenses will consist of two lenses, placed opposite to each other, and illuminated by a single lamp between them. “The revolving light with reflectors will consist of 18 reflectors with argand burners, nine reflectors, being substitutes for each lens. “It being admitted that these two pieces of apparatus will give the same light, let us consider their comparative advantages. “1. The lens apparatus will be decidedly the cheapest in its first cost, and the lenses will never require to be renewed. “2. The lens apparatus will not require one-third of the labor in cleansing and arranging them daily for use. “3. The lens apparatus will not require so strong and powerful a piece of machinery to move it, from its inferior weight and greater compactness. “4. The lens apparatus may be placed in a much smaller light-room, the is reflectors requiring a very large space; and economy might thus be introduced in the erection of future Light-Houses. “5. The 18 argand burners will decidedly consume more oil than the simple compound burner used for the lenses; hence, it follows that the lens apparatus is in every respect better and more economical than the reflector apparatus.”

Under testimony so explicit and authoritative as this, (and much more might be introduced had we space,) no doubt can well be entertained that the lenticular system is the farthest advance science has yet made in perfecting the methods of Light-House illumination. It concentrates, far more perfectly than any other method, the rays of light which have been created, and gives them, more nearly than any other, precisely that brilliancy and direction which will best answer the purpose for which such beacons are erected. One of its most prominent recommendations is, that it affords special advantages for establishing what we greatly need, systematic and efficient distinguishing lights, which the mariner can so readily distinguish from one another as to be able at once to name the beacon, and thus to ascertain his precise position.

This is, evidently, a matter of the first importance; and the utter, lack of any such provision in the Light-Houses of this country, has led to many very sad disasters. In 1840 the schooner Delaware stranded on Scituate beach, on the Massachusetts coast, from having been unable to distinguish Scituate Light from that at Boston; and in the same year the schooner Perse ran ashore in the same spot, and from precisely the same mistake. Utter confusion pervades the arrangement of our Lights on nearly every portion of our extended coast. Many methods have been devised to give an individual and easily recognizable character to each individual light. The method in general use in Great Britain, at least until very recently, was that of giving different colors to the lights, by coloring the glass through which the rays were obliged to pass; but this was found seriously to impair the brilliancy and effect of the light itself. Dr. Brewster, in a communication to the Parliamentary Committee, dated March, 29, 1833," speaks of a discovery he had made, whereby a numeri character could be impressed on any light, which nothing could change, and which could easily be recognized by looking at the light through a small and cheap apparatus made for the purpose. This, in its theory, would evidently best answer the end desired; but its practicability has never, we believe, been demonstrated; and the French method is probably the best now in use. It consists o so arranging the lenses, in the revolving apparatus, that eclipses shall occur at regular intervals; and the intervals are of different duration in different Light-Houses. Thus, in one a brilliant flash may be visible twice in a minute, in another three times in the same interval, &c. A master, therefore, has only to inform himself of the character of each light, and then determine, by his watch, the duration of the eclipses, to understand at once his precise position. This is the method adopted by Fresnel; and it is undoubtedly much the best ever used.f. It is due to our commercial and maritime interests,

* See Parliamentary Report of 1834, Ap. p. 135. f Though not strictly involved in our subject, it may not be unimportant to remark

that our system of buoyage is quite as defective as our Light-House Establishment. England the utmost care is taken to designate every buoy in every channel.

In Thus, on

entering the port of Liverpool, if the master of a ship sees a red buoy marked “F.1,” he knows at once that it is the first buoy of the Formby channel, and is to be left on

the starboard side, going in.

In the same way every one of them can be instantly recognized. There is no chance of mistaking them; and a ship in a fog, in falling in with

that it should be more generally adopted in the Light-Houses upon our coast.

With this general view of our LightHouse System, we must leave the subject. Every person, acquainted with its character, must concede that it requires improvement. In a few cases, new lights, and in many cases better lights, are greatly needed. Lenticular lights of the second order, were imported from France in 1840, and set up at the Highlands of Neversink. Their economy has never been fairly tested, as the results of their use have been cautiously kept from the public eye; but every shipmaster who has entered this port since their erection, will bear prompt testimony to their superior brilliancy and efficiency. A few others of the same kind, are greatly needed along our coast. At Cape Hatteras, on Carysfoot Reef, and especially at Tortugas, the great turning point of all the navigation in and out of the Gulf of Mexico, lenticular lights of the first order should be erected. One of the second class should be placed on Cape Canaveral, another on Cape Florida, and a third on the Key Sombrero.

But our whole System lacks method; and nothing can ever supply this radical defect, until Science and Experience are systematically introduced into its supervision. The establishment is not adapted either to the wants of our commerce, or to the advanced state of science and of art. And yet, in all other branches of industry and social economy, we are prompt to seize upon all improvements. We use the very best steam engines, the best machinery in manufactures, and even the Magnetic Telegraph, the latest achievement of science in the transmission of intelligence, is usurping the place of the mail coach and the locomotive. In these departments we should rightly deem it niggardly and narrow to reject the new because the old was cheaper. There, certainly, is no reason why similar improvements should be rejected or neglected, in so important a branch of the public service as our Light-House Establishment, a branch on the efficiency and perfection of which depend, not only the wealth with which our ships are freighted, but the lives of the thousands who follow the sea.

one of them knows precisely her position and what belongs to it.

We have no such

method. The only difference that prevails is that of color, and this is without system,

and often an embarassment rather than a guide.

A naval officer, in writing information

concerning a channel, once wrote—“do not describe the color of the buoys, as they paint them of whatever color the Custom House contains.”

« 上一頁繼續 »