THE N E W • Y O R IC EXHIBITION ILLUSTRATED.

THE FRESNEL LENS,

OK

DIA-CATOPTRIC ILLUMINATING APPARATUS FOR LIGHT-HOUSES.

N connection -with the beautiful specimen of art and-workmanship catalogued under this title, it seems ap-propriate, and may be interesting to the student ofthe results of the Exhibition to have here somecondensed memoranda upon Light-Houses; the modesof illuminating them; and the development whichthese essential aids to navigation have received in'modern times. For in ancient times, commerce,

. chiefly carried on by land, stood less in need ofthem, and the mariner with his unfrequent freightsclung closely to the shore. The light-houses there-of which industrious antiquarians believe they find atrace, could hardly have been more than beacon marks toassure the voyage by day, and by their very size, to loomlarge and dark by night; and even if lighted at all, theycould not have surpassed the rank, if they at all equalled theefficiency, of our harbor lights of the present day. Such,

• instance, was the Colossus of Rhodes, familiar to our school-daysa marvel; such the tower at Alexandria, whose locality (theisland of Phra, or Pharaoh or Pharos), has been perpetuated tothis day in the classic epithet for structures of more undoubted purpose and effect;that of Corunna, consecrated in Milesian tradition; and that of Capio, on or near theAndalusian Tartessus. Other structures of smaller size appear to have been placedat several prominent points on the coasts of Gaul and Britain, probably to guide thetransport of Roman invaders and colonists; and FJamborough Head on the EastCoast of England, seems to preserve in its name an allusion to a luminous beacon thatstood upon it. Bjit it was not until after the use of the compass and the improve-ments (or rather discoveries) in Nautical Astronomy of the 15th and 16th centuries,that lights in aid of navigation came to be systematically demanded, or formedpart of the policy of even the most commercial people.

When this occurred, and as the towers, to answer their purpose best, hadgenerally to be placed on points of land, salient or otherwise exposed to the windsand fliry of the sea, cases often would arise where difficulties had to be encounter-ed and outlay made, either of money or of skill on the part of the engineer, or ofboth. Some such cases, from the bold ingenuity exhibited or the happy nar-rative of the steps of the undertaking, have come to mark classical epochs in thehistory of Light-Houses.

Among these may be mentioned (first in point of date, and as having beenkept up sedulously ever since, abreast or ahead of all its class, so that a detailedaccount of it would be also a tolerable index of the improvements in the archi-tecture and illumination of lights), the Tower of Cordouan, at the mouth ofthe Gironde on the Bay of Biscay. Begun in 1584, the troubles and tumultsof the League of France, and the often anxious counsels of Henry IV., pre-vented its completion until the death of that monarch in 1610. More than fiftyyears afterwards, in the magnificent period of Louis XIV., additions were madeto it and the structure partly rebuilt. Its cost would be equivalent to nearly orquite a half a million of our money. Its historian is Belidor.

Great Britain has alw'ays contributed most largely to these provisions of com-merce • and the Eddystone Light, remarkable for its vicissitudes; that of the BellRock whose seat is hardly ever dry even for a few hours; and that of Skerryvore,the latest triumph of the art, are places of pilgrimage for engineers who wish tonote how apparent impossibilities may be overcome.

The first of these, built on a rock in the English Channel, opposite themouth of Plymouth Sound, and some ten miles from land, goes back originally asfar as 1698, when a light was shown from a wooden tower. But the water whichrages there at times, and can submerge an object sixty feet in height, soon render-ed an enlargement indispensable, and, after that was made, a fresh repair. It wasupon a last occasion of this sort, deep in November, 1703, that the Engineer Win-stanley, and all hands employed, and all visible preceding prints of their labor,were swept away at once. Five years afterwards it was replaced, still of wood,and so continued until 1755, when a new agent, fire, destroyed afresh what thewind and waves for nearly half a century had failed to move. Immediatelyafter, and now almost a hundred years ago, the real Eddystone of mason work,artificially and curiously designed and laid, and which reciprocally immortalizes,and is immortalized by the name of Smeaton, began to be built, and was shortlyafter finished. It is to be hoped that art has now triumphed over nature, and thatcare will exclude accident, so that in a distant future the inquirer into the historyof Light-nouses, more fortunate than ourselves, may have an existing and un-doubted specimen of the art as it -was with us.

The Bell Rock, a lonely and never long uncovered reef, some twelve or fifteenmiles from land, opposite the firth of Tay on the East coast of Scotland, testifiesin its name to the honor of the monks of Aberborthwiek, whose pious charity

devised a floating bell tolled by the moving waves, and rung with more appallingenergy the higher rose the storm, to warn the mariner of what he was approach-ing. But with the decay of the Abbey went also the bell. Twice afterwardsprivate beneficence provided there a wooden beacon that speedily followed thebell; but about the beginning of this century (in 1811) after nearly four yearsdangerous labor, Robert Stevenson (of a family whose members have to be de-signated by their Christian names, since it has furnished so many names ofeminence), under the authority of the Commissioners of Northern Lights, com-pleted the present stone structure. In this the old tradition is revivified; andthe song of the Bell, now moved by the revolving machinery of the lamps, is stillheard,—a warning adjunct in foggy weather,

Skerryvore, the last that need be mentioned, is on a reef of rocks lying on thewestern coast of Scotland, and among the Isles, celebrated in one of Sir WalterScott’s metrical romances, the materials for which, in fact, he gathered during areconnaissance in 1814, when he accompanied the Northern Light-House Boardand their Engineer, Alan Stevenson, for the object, among others, of examiningthis very site. So difficult, however, appeared the work, and so faint the chance,that twenty years had passed by before the question of placing a light there wasseriously taken into consideration. Then, in 1834 a minute survey was begun, andin 1838, the work of building was commenced, and the tower lit up in 1844.This Light-House is remarkable not only for its dangerous position, and for its size,in which it more than doubles that on the Bell Rock, and nearly quintuples theone on the Eddystone, but also for the extreme theoretical and practical skill inboth its architectural and optical relations developed there by the distinguishedEngineer, Alan Stevenson, who may justly be regarded as among the very firstauthorities, living or dead, on the subject of Light-Houses.

In building a Light-House, these two relations, optical and architectural, orits brilliancy and its permanence, are the chief things to be considered. In re-spect to the latter, both theory and practice seem to agree in showing that it is tobe obtained by the weight or inertia of the insisting building, with an externalshape, of course, the best calculated for presenting the least resistance to thewaves, and for allowing them the soonest to expend themselves, rather thanby any complicated mechanical framing, which, in some other works, is the mostapproved mode of obtaining the required strength of resistance. Hence stone,which is twice and a half as heavy as water, has been found of far more success-ful application than wood, which last material, indeed, has become only tradi-tional in dangerously exposed situations. Lately, a substitute has been pro-posed of broken stone concreted with crude iron, which claims also this advan-tage, of being moulded in any form, and especially with the Smeaton dove-tailsand joggles, that render the various parts of the building incapable of beingmoved by any force short of what transcends the tensile strength or cohesion ofthe material. Wrought iron has also been proposed and used; but in generalwith a sacrifice of the idea of inertia to that of a mechanical fastening to therock foundation of a tripod or multiple-legged frame, bearing the necessary lanternand chambers above and beyond the reach of the sea. Oast iron, whose weight isnearly triple that of stone, and seven times that of water, would, in cases wherethe risk is great, and the expense at all considerable, be most likely the bestresort both for economy and permanence. The use of this material in plateshowever, as has been proposed, docs not appear to be the most eligible; it shouldrather be cast in solid rings, or in segments to be bolted together, and thus formthe entire periphery of the tower.

Suggestions of this kind, however, although they relate to what must beadmitted as the most important point in Light-Houses, namely tho maintenanceof the building, and so its very existence as a Light, do not constitute the mostattractive speciality of the subject. This is rather the optical part, and whatconcerns the mode of illumination and the light itself.

Down to nearly the beginning of the present century, the only illuminationknown, was from the combustion of wood or coal in suitable grates or chauffers.Next to the glare of this open fire, which was as expensive and troublesome as itwas variable and inefficient, came the light from tallow candles. For fortyyears after so much thought and labor had been expended in erecting the Eddy-stone, the light it gave was from no other source than these. Other lights else-where, it is true, used the flame of an oil lamp at an earlier date, but the wicks ofthe lamps were all solid, and the combustion of the oil, or rather of the inflamma-ble gas from it, very imperfect, as is seen in the quantity of smoko evolved. Andeven when an improvement was made by flattening and thinning the wick, thosupply of atmospheric air essential to combustion, was only external, and a con-siderable quantity of carbonaceous matter on the inside necessarily went offwithout contact of air, and therefore unconsunied.

It was reserved for Argand, about the year 1784, to invent the well-knownlamp with a circular hollow wick and burner, that still bears his name. In thisthe wick is thin, and the air supplied on both sides, without and within. Theaddition of the glass chimney to tho burner makes tho combustion nearly per-fect; and to this day, although there have been some slight modifications ofarrangement, nothing has been devised to supersede his original idea. The effi-

ciency of the double current thus furnished, any one may easily test, who has