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THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[July, 



pieces, seems to be due to Condnrcet, who in his FJoge tie Buffon, puhlisheil 

 so far back as 1773, enumerates the advantages to lie derived from this 

 method. Sir David 15renster also described this mode of building lenses in 

 1811, in the Kdinburgh Encyclopcedia ; and in 1822, the late eminent 

 Fresnel, unacquainted with the suggestions of Condorcet or the description 

 by Sir David Brewster, explained, with many ingenious and interesting 

 details, the same mode of constructing those instruments. To Fresnel 

 belongs the additional merit of having first followed up his invention, by the 

 onstruction of a lens and, in conjunction with MM. Arago and Matbieu, of 

 placing a powerful lamp in its focus, and indeed of finally applying it to the 

 jiractical purposes of a lighthouse." 



To sliow Fresiiel's system fully would take more room than we 

 can trive, and many wood-cuts, and we are sorry that we must leave 

 it alone, and send our readers to Mr. Stevenson's book, where 

 everythinfi is s'^e" i" f"'!) — Fresnel's brother having put his 

 papers in Mr. Stevenson's hands. 



( )ne of Mr. Stevenson's own works was with the Isle of JMay 

 liirht. 



'• Having been directed by the commissioners of the Northern lighthouses 

 to convert the fixed catoptric light of the Isle of May, into a dioptric light 

 of the first order, 1 proposed that an attempt should be made to form a true 

 cylindric, instead of a polygonal belt for the refracting part of the apparatus ; 

 and this task was successfully completed by Messrs. Cookson of Newcastle 

 in the year 1836. The disadvantage of the polygon lies in the excess of the 

 radius of the circumscribing circle over that of the inscribed circle, which 

 occasions an unequal distribution of light between its angles and the centre 

 of each of its sides ; and this fault can only be fully remedied by constructing 

 a cylindric belt whose generating line is the middle mixtilinear section of an 

 annular lens, revolving about a vertical axis passing through its principal 

 focus. This is, in fact, the only form which can possibly produce an equal 

 difl'usion of the incident light over eveiy part of the horizon. 



" I at first imagined that the whole hoop of refractors might be built 

 between two metallic rings, connecting them to each other solely by the 

 means employed in cementing the pieces of the annular lenses ; but a little 

 consideration convinced me that this construction would make it necessary 

 to build the zone at the lighthouse itself, and would thus greatly increase 

 the risk of fracture. I was therefore reluctantly induced to divide the whole 

 cylinder into ten arcs, each of which being set in a metallic frame, might be 

 capable of being moved separately. The chance of any error in the figure of 

 the instrument has thus a probability of being confined within narrower 

 limits ; whilst the rectification of any defective part becomes at the same 

 time more easy. One other variation from the mode of construction at first 

 contemplated for the Isle of May refractors, was forced upon me by the 

 repeated failures which occurred in attempting to form the middle zone in 

 one piece ; and it was at length found necessary to divide this belt by a line 

 passing through the horizontal plane of the focus. Such a division of the 

 central zone, however, was not attended with any appreciable loss of light, 

 as the entire coincidence of the junction of the two pieces with the horizontal 

 plane of the focus, confines the interception of the light to the fine joint at 

 which they are cemented. With the exception of those trifling changes, the 

 idea at first entertained of the construction of the instrument was fully 

 realised at the manufactory of Messrs. Cookson." 



Speaking of Fresnel's lamp, used in tlie French lighthouses, Mr. 

 Stevenson writes : — 



'■ The only risk in using this lamp arises from the liability to occasional 

 deriingtments of its leathern valves that force the oil by means of clockwork ; 

 ami several of the lights on the French coast, and more especially the 

 Corduan, have been extinguished bythe failure of the lamp for a few minutes, 

 an accident which has never happened, and scarcely can occur with the 

 fountain lamps which illuminate the reflectors. To prevent the occurrence 

 of such accidents, and to render their consequences less serious, various pre- 

 cautions have been resorted to. Amongst others, an alarum is attached to 

 the lamp, consisting of a small cup pierced in the bottom, which receives 

 part of the overflowing oil from the wicks, and is capable, when full, of 

 balancing a weight placed at the opposite end of a lever. The moment the 

 machinery stops, the cup ceases to receive the supply of oil, and, the remainder 

 running out at the bottom, the equilibrium of the lever is destroyed, so that 

 it falls and disengages a spring which rings a bell sufliciently loud to waken 

 the keeper should he chance to be asleep. It may justly be questioned whether 

 this alarum would not prove a temptation to the keepers to relax in their 

 watchfulness and fall asleep ; and I have, in all the lamps of the dioptric 

 lights on the Scotch coast, adopted the converse mode of causing the bell to 

 cease when the clockwork stops. There is another precaution of more im- 

 portance, which consists of having always at hand in the light-room a spare 

 lamp, trimmed and adjusted to the height for the focus, which may be sub- 

 stituted for the other in case of accident." 



In the French lights, "these distinctions depend upon the periods of 

 revolution, rather than upon the characteristic appearance of the light ; and 

 therefore seem less calculated to strike the eye of a seaman, than those 

 employed on the coasts of Great Britain and Ireland. In conformity 

 with this system, and in consideration of the great loss of light which 

 results from the application of coloured media, all distinctions based upon 

 colour have been discarded in Freuch lights. 



"The distinctions are, in fact, only four in number, viz.: fixed ; fixed 

 varied by flashes; revolving, with flasbes once a miaule ; and revolving 



with flashes every half minute. To those might be added, revolving, with 

 bright periods once in two minutes, arul perhaps ftus/tin^ once in Jire 

 seconds (as introduced by mi- at the Liille Koss, but 1 cannot say with 

 such complete success as would induce me to recommend its general adop- 

 tion). My own experience would also lead me to reject the distinction 

 called ' fixed, varied by flashes,' which I do not consider as possessing a 

 marked or eflicient character." 



For putting lights on a shore, Mr. Stevenson lays down a few 

 laws for the engineer, which will be found very useful for those of 

 our readers who may have to build lighthouses in our settlements 

 abroad: — 



" 1. The most prominent points of a line of (oast, or those first made on 

 oner-sea voyages, should be first lighted ; and the most powerful lights 

 should be adapted to ihem, so that they may be discovered by the niunuer 

 as long as possible before his reaching land. 



2. So far as is consistent with a due attention to distinction, revolving 

 lights of some description, which are necessarily more powerful than fixtd 

 lights, should be employed at the outposts on a line of coast. 



3. Lights of precisely identical character and appearance should not, if 

 possible, occur within a less distance tliau 100 miles of each other on the 

 same line of coast, which is made by over-sea vessels. 



4. In all cases, the distinction of colour should never be adopted except 

 from absolute necessity. 



5. Fixed lights and others of less power, may be more readily adopted 

 in narrow seas, because the run^e of the lights in such situations is geue- 

 lally less than that of open sea-lights. 



6. In narrow seas also, the distance between lights of the same appear- 

 ance may often be safely reduced within much lower limits than is de- 

 sirable for the greater sea-light; and there are many instances iu which 

 the distance separating lights of the same character need not exceed 50 

 miles, and there are peculiar cases in which even a much less separation 

 between similar lights may be suflicieut. 



7. Lights intended to guard vessels from reefs, shoals, or other dangers, 

 should in every case be placed, where practicable, to the seaward of the 

 danger iiself, as it is desirable that seamen be enabled to make the lights 

 with confidence. 



8. Views of economy in the first cost of a lighthouse should never be 

 permitted to interfere with placing it in the best possible position ; and, 

 when funds are deficient, it will generally be found that the wisest course 

 is to delay the work until a sura shall have been obtained suflicieut for the 

 erection of the lighthouse on the best site. 



9. The elevation of the lantern above the sea should not, if possible, for 

 sea-lights, exceed 200 feet; and about 150 feet is suflicieut, under almost 

 any circumstances, to give the range which is required. Lights placed 

 on high headlands are subject frequently to be wrapped in fug, and are 

 often thereby rendered useless, at times when lights on a lower level might 

 be perfectly eflicient. But this rule must not, and indeed cannot, be 

 strictly followed, especially on the British coast, where there are so many 

 projecting clifl's, which, while they siabject the lights placed on them to 

 occasional obstruction tiy fog, would also entirely and permanently hide 

 from view lights placed on the lower laud adjoining them. In such cases, 

 all that can be done is carefully to weigh all the circumstances of the lo- 

 cality, and choose that site for the lighthouse which seems to atford the 

 greatest balance of advantage to navigation. As might be expected, in 

 questions of this kind, the opinions of the most experienced persons are 

 often very conflicting, according to the value which is set on the various 

 elements which enter into the inquiry. 



10. The best position for a sealight ought rarely to be neglected for the 

 sake of some neighbouring port, however important or influential ; and the 

 interests of navigation, as well as the true welfare of the port itself, will 

 generally be much better served by placing the sea-light witere it ought to be, 

 and adding, on a smaller scale, such subsidiary lights as the channel 

 leading to the entrance of the port may require. 



11. It may be held as a general maxim, that the fewer lights that can 

 be employed in the illumination of a coast the better, not only on the score 

 of economy, but also of real efficiency. Every light needlessly erected 

 may, in certain circumstances, become a source of confusion to the 

 mariner, and, in the event of another light being required in the neigh- 

 bourhood, it becomes a deduction from the means of distinguishing it from 

 the lights which existed previous to its establishment. IJy the needless 

 erection of a new lighthouse, therefore, we not only expend public treasure, 

 but waste the means of distinction among the neighbouring lights. 



12. Distinctions of lights, founded upon the minute estimation of inter- 

 vals of time between flashes, and especially on the measurement of the 

 duration of light and dark periods, are less satisfactory to the great ma- 

 jority of coasting seamen, and are more liable to derangement by atmo- 

 spheric changes, than those distinctions which are founded on what may 

 more properly be called the characteristic appearance of the lights, in 

 which the times for the recurrence of certain appearances diller so widely 

 from each other as not to require for their detection any very minute ob- 

 servation in a stormy night. Thus, for example, flashing lights of five 

 seconds interval, and revolving lights of half a minute, one minute, and 

 two minutes, are much more characteristic than those which are distin- 

 guished from each other by intervals varying according to a slower series 

 of 5", 10", 20", 40", &c. 



13. Harbour and local lights, which have a circumscribed range, should 

 generally be fixed instead of revolving; and may often, for the same 



