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so formed and placed as to bring them to a distinct and accurate focus. It is difficult to make and preserve a good lens in the way proposed by Buffon. I do not know of any lens made after Buffon's manner, except one made lately at Newcastle for the Commissioners of Northern Lighthouses."

"Will you describe the difference between that suggested by Buffon and that suggested by Sir David Brewster?"-" The one suggested by Buffon must be one piece of glass, and one side may be either plain or convex; but the other side must be formed into zones, in such a manner as to cause parallel rays of light to converge to the same focus. On the other hand, the poly zonal lens proposed by Sir David Brewster, is composed of a single lens in the centre, and of several concentric zones built round it, and each zone is composed of several pieces."

"Of what diameter and focal distance have the polyzonal lenses generally been made?"—" About thirty inches diameter, and three feet focal distance."

"With that diameter, what lantern would be large enough to illuminate as many points of the compass as are required? Would there be room in any lighthouse to put up a sufficient number of these lenses to illuminate the sea in as many points of the compass as are required ?""Yes."

"Take twenty-four-these may be so many inches in diameter-what size lighthouse, or what space would enable you to do that?"-" The focus of a polyzonal lens being about three feet, of course the quadruple argand lamp put in the centre would be three feet each way, which would make a polygon of six feet in diameter."

"Have the built lenses any other advantage over the solid lens?"— "Yes; they can be made to fill up square spaces, by completing the angles with portions of additional zones. The manner of Buffon requires the glass to be very annealed and ground."

"Were those built lenses first suggested by Sir David Brewster?" "Yes, so far as I know, in 1811.”

"Was it first adopted by the French ?"-" Yes; by the French, at the Corduan lighthouse, about 1822.”

"How many lighthouses in France are lit in that way not know how many."

y?"—" I do

"Sir David Brewster's plan is carried into effect in France?" -"Yes."

"Are you aware whether, in any of the French lighthouses, there is a different plan from that?"—" None that I am aware of."'

The only other witness whom we shall cite in favour of the lens system, is Capt. J. H. Pelly, who has been eleven years an elder brother of the Trinity House. The examination of Capt. Pelly's evidence would not lead us to adopt his opinions on these matters; but we appeal to him as an eyewitness capable of determining whether one light is brighter than another; and his testimony is the more valuable, because he is placed in a position. where his interest and views are opposed to his opinion.

'Lieut. Drummond's light,' says Captain Pelly, is a beautiful

Now, as the

light, and NEXT TO IT IS THE Corduan light.' Corduan light is produced by lenses, we have Capt. Pelly's evidence that the lens system produces a light decidedly more brilliant than any light in England under the Trinity House.

It would be a waste of time to pursue this subject any farther. The opinion of this man or of that, upon a question of prac tical science capable of being determined by infallible tests, can be of no use. Comparisons, indeed, of lenses and reflectors have been made partially by the Trinity Board, and on a greater scale by the Scotch Commissioners; but they were conducted by interested persons who had the strongest motives to misdirect and to misinterpret them; and so forcibly was this felt by the commercial public, that Mr Robert Dow Kerr, chairman of the Chamber of Commerce of Greenock, whose evidence is highly instructive, complains of the manner in which the experiments were conducted. He states it as his own opinion, as well as that of the Chamber of Commerce, that the experiments made in Edinburgh by the Lighthouse Board, should be repeated under the direction of neutral scientific men, not connected with lighthouses or lighthouse boards. Although we entirely concur in opinion with Mr Kerr, yet we do not recommend any farther enquiry into the subject; because decisive experiments have been already made, an account of which we shall now submit to our readers.

In order to compare the reflector light of the Eddystone with the lens light of the Tour de Corduan, we cannot place the one lighthouse beside the other; but we can place an apparatus similar to that used at the one beside an apparatus similar to that used at the other, and measure, by accurate and well-known methods, the relative brilliancies of the two lights.

Now, this has been actually done with great labour and care by Mr L. Fresnel, whose elaborate detail of the results is now before us, and of which we shall endeavour to give a popular and

condensed account.

The object of the experiments was to compare a revolving reflector apparatus made in England, with a revolving lens apparatus made in Paris; and a fixed reflector apparatus of forty parabolic reflectors, with a fixed lens apparatus of thirty-two lenses. These pieces of apparatus are compared in reference to their useful effect, and also to their economical effect. The useful effect is represented by the product arising from the mean lustre of the light, multiplied by the number of the angular divisions of the horizontal section which it illuminates; and the economical effect may be represented by the ratio between the useful effect and the expense of oil; or more exactly by the ratio between the seful effect and the annual expense of the lighthouse, compre

hending the interest and the original expense of apparatus and erection.

1. In the REVOLVING APPARATUS of metallic reflectors, consisting of twenty-four reflectors, each reflector had an aperture of

m

0.52 metres, and a depth of 0.215; the lamp burned forty grammes of oil per hour, and the reflectors are supposed to be distributed equally on the four faces of a rectangular frame. The following were the results of experiment :

Maximum lustre of a reflector,
Total lustre of six reflectors,
Amplitude of lustre about 20°, or
USEFUL EFFECT of a reflector,
USEFUL EFFECT of the apparatus,

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400 2400

1200 minutes.

=127 × 1200=152,400 =24 × 152,400=3,657,600

ECONOMICAL EFFECT in reference to expense of oil, ditto, in reference to total outlay,

Ditto,

=3810

=384

2. In the LENS APPARATUS of eight revolving lenses and eleven zones of fixed mirrors, the diameter of the lenses was 0.76 metres,

m

and their focal length 0.92; and the central lamp with four concentric wicks, burned per hour 750 grammes of oil, or 24.12 oz. Troy.

The USEFUL EFFECT was
The ECONOMICAL EFFECT

Hence, Mr Fresnel concludes

7,269,880 826

1st, That the USEFUL EFFECT of the lens apparatus is equal to about TWO TIMES that of the reflecting apparatus; and 2dly, That the ECONOMICAL EFFECT of the lens apparatus is from Two and to Two and 10 times that of the reflecting apparatus.

Again

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1. In the FIXED LENS APPARATUS, the drum of lenses had 32 sides or quadrilateral lenses, each 1.05 metres in height, and there were besides 11 zones of mirrors.

The USEFUL EFFECT of these was,

8,046,680

The ECONOMICAL EFFECT, in reference to expense of oil, 10,729 Do. do., in reference to total cost, &c. .

969

2. In the FIXED REFLECTOR APPARATUS there were 40 parabolic reflectors, each with an aperture of 0.275 metres, and each burning 35 grammes of oil per hour.'

Mr Fresnel remarks that these small reflectors are inferior, in an economical point of view, to the English reflectors, but are more suitable for a fixed light; in so far as they distribute the light more equally in different azimuths.

The USEFUL EFFECT of 40 reflectors is,

The ECONOMICAL EFFECT, in reference to expense of oil is,
Do. do., in reference to total cost is,

2,376,000

1,697

208

Hence, Mr Fresnel concludes

1. That the USEFUL EFFECT of the fixed light with lenses is THREE and Ad times that of the fixed light with reflectors; and, 2. That the ECONOMICAL EFFECT of the fixed lens light is equal to from SIX and d to FOUR and 3d times that of a fixed light with reflectors, according as we consider the expense of oil, or the total annual expense.'

These results, which demonstrate the vast superiority of lens lighthouses, in reference both to their useful effects to the navigator, and their economical effects for the rate-payer, put an end to all farther discussion on the subject; and we hope that Lieutenant Drummond will be induced to reconsider the opinion he has unfortunately given in reference to the application of lenses to fixed lights.

The next subject which demands our attention is that of occasional lights, which are required only in particular states of the atmosphere, when all ordinary lights disappear. For such lights we recommended the lime-ball light of Lieutenant Drummond as a separate instrument for every lighthouse; and we have pointed out various methods of giving additional brilliancy to lights in cases of emergency. We were not aware that there existed any person who could suppose that the occasional use of such a splendid light would operate injuriously to navigation. The following opinion, however, is given by Mr A. Stevenson:

Are you aware of any propositions on the part of Sir David Brewster, that have not met with attention on the northern lights?No; no propositions of Sir David Brewster have been overlooked. He suggested the introduction of gas, and an enquiry was set on foot with regard to the propriety of using it; the commissioners at present think that its use is not quite safe. He made a proposition with regard to lenses which are at present in course of being adopted at Inchkeith. Sir David also suggested the occasional exhibition of certain lights at particular times, or in certain states of the weather. It is quite obvious, however, that this recommendation deserves no attention, and it could not come from any one acquainted with seamanship. Its adoption would, in fact, render the lights worse than useless; for unless the characteristic appearance of a light be invariably the same, it could not answer the great purpose of distinguishing the place where it stands from the other parts of the coast; and the mariner could never tell where he was, or what lights he had in view. The great object of a light is to indicate, by its certain and invariable characteristic appearance, the point of land where it stands, in the same way that a beacon of a particular form points out its own locality by day. To change the appearance of a light, therefore, at particular times, would deprive it of all its use, and might be the means of leading vessels directly to danger, by causing the seamen to mistake one place for another.'

We dare not decide between the presumption and the ignorance

which this opinion involves. The occasional light was proposed to be used only in hazy weather, when the characteristic appearance of every light is entirely lost. If a ship comes into the vicinity of the Bell Rock in a thick haze, the red light is, of course, completely lost from its faintness, and the character of the light from that moment disappears. What injury then could the mariner suffer if, in this predicament, a lime-ball light were exhibited to warn him, by a brighter ray, of the dreaded reef? Or if the white light had disappeared along with the red, the sailor might owe the existence of his vessel and crew to the occasional light that had penetrated the gloom. It would insult the reader to argue this point any farther; but it may be useful to show, that Mr Robert Stevenson, the engineer of the board, contradicts the opinion of Mr Alan Stevenson, the clerk of the works.

6

Important advantages,' says he, might doubtless be obtained by using this light (the lime ball light) during hazy weather; and the Reporter is resolved to spare no pains on his part to bring about its introduction into lighthouses."

This is precisely the opinion pressed upon the board eight years ago by Sir David Brewster; and it is amusing to find that the Premier of the lighthouse Cabinet, rather than quit office, will now consent to govern on the principles of his opponents.

From these observations, we learn the true value of the lime ball light of Lieut. Drummond; or of any other brilliant light, such as that produced by galvanic action. The general system of illumination by oil or gas lamps and lenses is adequate, in ORDINARY WEATHER, to every want of the navigator; and nothing could be more irrational than to introduce the lime ball light into lighthouses as a general mode of illumination; unless it could be done as cheaply, as safely, and as effectually, as the present improved system with oil or gas lights. Our streets are all lighted in reference to the ordinary state of the atmosphere; and it would be a strange piece of management if lime ball lights were to be introduced into the street illumination of the metropolis, because the ordinary lights become useless in a haze. What would we think of a man who slept with his spectacles on, in order that he might better recognise the midnight robber; or of him who should wear his great-coat in the dog-days, lest he should be caught in a hail-storm.

We have been induced to make these remarks, and we earnestly invoke the public attention to them, because there appears throughout the whole examination of the witnesses a disposition on the part of the committee to consider the introduction of a very brilliant light as a great desideratum. The range of the Corduan light is, in the opinion of navigators, amply sufficient for all the purposes of a sea light; and if any case shall occur of a very

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