6'i 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Febbuarv,' 



similar struftnre to tlie E'ldystone ; it was Ijuilt by Stevenson at a cost of 

 4,'C0,(l00. The most recent erection of this description is on tlie Skerryvore 

 rock, wliicli cost .€90,700. 



Tlie iiiithor next alluded to the difficulty of constructing permanent light- 

 houses in exposed situations, and the advaiitay;es of thetn over floating lights, 

 as well as the much smaller annual expenditure requited to maintain an effi- 

 cient light. The fitst floating light was the well known .Nore light-vessel, 

 moored in 173 1. In order lo insure slahilily in a lighthouse, Mr. Findlay 

 slated that it is necessary that the structure should he capahle of afl^urding 

 resistance to a pressure of not less than 6,000 Ih. to each square foot of 

 surface exposetl to the action of waves. This assertion was founded on 

 experiments made hy Mr. .\lan Stevenson, who ascertained and registered 

 the force of the waves at the Skerryvore rock, on March 2.>tli, 184.i, during 

 a westerly gale, when it was found to he C08i Ih. per square foot ; this, the 

 greatest force hitherto registered, was ciied with many others. He next 

 proceeded to point out the inapplicability of iron to the construction of 

 lighthouses wliere the metal was immersed in the sea water, which has the 

 effect of reducing it to a hndy similar in its chemical properties to tdack- 

 lead ; and instanced the ettects produced on a caiuiou-hall taken from the 

 Mary Hose, after having been sunk oil" Spithead for a period of 150 years : 

 the iron shot upon being exposed to the air gradually became red hot, and 

 then fell into a red powder resembling burnt clay. — The author next de- 

 .scribed the methods which have been suggested (or overcoming the difficulty 

 of exposing large surfaces to the action of the force of the waves, and also 

 for obtaini;.^ a firmer foundation on a sand, and especially Mr. Alexander 

 Mitchell's screwpilc lighthouse erected on the Maplin Sand, and Dr. I'utts's 

 method of driving piles by atmospheric pressure, as applied at the South 

 Calliper beacon on the Goodwin sands, in 1847, and to other beacons on 

 various slinals at the mouth of the Thames, as on the Blyth saiul, and on 

 the shingles in the Prince's channel. Another plan for the erection of 

 lighthouses has been carried into effect at the Point of Ayr by Mr. Walker; 

 it consists in constructing hollow cylinders, which are filled with concrete 

 and then sunk, and from them the piles rise. Capt. Sir S. Brown has also 

 proposed a plan for the erection of lighthouses in deep water upon bronze 

 standards, and a modification of bis plan was adopted by Captain Bullock. 

 The author further alUided to Mr. Bush's Light of all Nations, atid to Mr. A. 

 Gordon's iron ligbtbuuses at Jamaica and the Bermudas, in which the cases 

 are filled with a solid mass of concrete ; and alluded to the fact that Rennie 

 had proposed iron for this purpose as early as the year 180.') for the Bell 

 Rock. 



Having thus shown the difl^erent methods employed in the construction 

 and erection of lighthotises, Mr. Findlay proceeds to remark on the various 

 plans of illumination which have been eiuployed : of these the earliest was 

 the coal fire and the Cordouan billets of oak. In 1752 the South Foreland 

 lighthouse, previously illuminated v\ith an open coal fire, was covered with a 

 lantern with large sash windows, and the fire was kept bright by means of 

 large bellows; the lantern was subsequently removed, and afterwards, at the 

 commencement of the present century, fifteen large lenses with separate 

 lamps were placed in it. In 1790, the only exception to the coal fire was 

 the Eddystone lighthouse, which had a chandelier with 24 wax candles, and 

 the Liverpool lighthouses with oil lamps and rude parabolic reflectors. An 

 interestitig historical fact was then metjtioned — viz.. that parabolic reflectors 

 were used at the Liverpool lighthouses (huilt in 1763), as Mr. W. Hutchin- 

 son, in his " Practical Seamanship,'' published that year, describes the appa- 

 ratus then in \!se — the larger reflectors of wood lined with small pieces of 

 looking-glass, the smaller of polished tin : this was the more curious, as it 

 had been claimeil by the French for M. Teulere in 1783, and first used in 

 Scotland in 1780. The parabolic reflectors, of which some beautiful speci- 

 mens were shown to the meeting, are now constructed upon the formula of 

 the celebrated Captain lluddart. Having explained the catoptric or reflect- 

 ing principle of illumination, which received so great an improvement in the 

 invention of the Argand lamp in 1780 or 85, several other lights were ex- 

 hibited and described — viz., the Drummond light, the voltaic light, and the 

 causes of their inapplicability. The present mode of lighting is from lamps 

 constructed on a modification of the Argand principle. A first-order pneu- 

 matic lamp with four concentric wicks, showing a most powerful light, was 

 exhibited. The dioptric principle, in which the rays of light etnanate from 

 a central lamp, atid are controlled and directed liy a series of lenses placed 

 before atid around it, next occupied attention. The author claims the 

 priority of its suggestion for an optician in Loudon, as tuentioned hy Sinea- 

 tun, who proposed, in 1759, to grind the panes of the Eddystone lighthouse 

 into a sphere of 15 feet diameter. The present form of lens, generally 

 known as Fresuel's, was fiist suggested by the celebrated Buffon, to whom it 

 is probable the catoptric system owes its origin. Sir David Brewster, in 

 1811, showed the prarticability of constructing a lens of separate pieces, 

 and this was first used in France by Fresnel, and has since become universal 

 in French lighthousps. A comparative view of the catoptric and dioptric 

 systems is aft'orded hy the fixed lights of the South Foreland, the higher 

 being from the dio|)tiic principle and the lower from lliidriart's reflectors, 

 which to a distant observer appear equally bright — the only test of their 

 efficiency. The cata-dioptric principle was illustrated by a beautiful fouiih- 

 order apparatus, lent by Messrs. Wilkins, in which, above and below the 

 light, a system of totally reflecting prismatic zones is arranged, the sugges- 

 tion of Mr. A. Stevenson. Mr. Alexander Gordon's cata-dioptric system, a 

 union of the reflector and refractor, was also described. — Some particulars 

 respecting the power of light in penetrating mist were also brought forwaid. 



During fogs the attendants of light vessels sound a bell at intervals, or, as 

 now used by the Trinity Board, a Chinese gong. Instead of this, Lii ut. 

 Sheringham, R.N., proposed, in 1842, to use a whistle wurked by bellows, 

 and Mr. Gordon priiposeil to place the whistle in the focus of a parabcdic 

 reflector, to direct the scmud. Mr. Findlay concluded bis paper by suggest. 

 ing the use of Mowbray's chemical whistle, which was exhibited and Oc- 

 scribed. 



ROYAL SCOTTISH SOCIETY OF ARTS. 



Jan. 10. — GiiORGE Buchanan, Esq., F.R.S.E., President, in the Chair. 

 The following communications were tuade : — 



1. Descriji/ion wi/h Draiohif/s of a Porlahle Cofferdam, a(lnpti A specially 

 for tlie use of Harbour and other Marine Works m exposed situations. By 



Thomas Stevenson, Esq., C.E. 



This coflferdam was used at Ilynish harbour works, Argyllshire, for exca- 

 vating rock which was seldom left dry by the tide, and was covered with 

 two feet of sand. It was found impossible to form a comiuon cofferdam, 

 owing to the shallowness of the sand, which could not afl'ord any suppor'; to 

 piles, and to the violence of the sea, which would in a single tide either 

 wholly break it up or render it leaky. The cofferdam aihipted being porta- 

 ble, was moved from one compartment of the cutting, when firdshed, to 

 another. It consisted of two dnuble frames of timber, each complete in 

 itself, being bound together with iron rods, forming a dam about 10 feet by 

 14, and 3 feet high. One of these double frames (being somewhat less than 

 the other) was placed inside of the larger, so as to admit two piles being 

 driven between thetu. In this way the piles could, from the depth of the 

 frames, be driven perfectly straight, and were also quite independent of sup- 

 port from the sand. As eacbcoiupartment of the excavation was completed, 

 and before the dam was removed, one row of piles was driven liown to the 

 bottom of the pit and left standing, so as to be a guide for again super- 

 imposing the frames over them, and in this way it was impossible for any of 

 the rock to escape being removed. The peculiar advantages are its porta- 

 bility — its ready adaptation to a sloping or to an irregular bottom — the ease 

 and certainty with which the partitions between each section of the rock 

 were removed, and the double-framed walings that supported and directed 

 the diiving of the piles. Whenever excavations require to be made in i: 

 rocky beach covered by a stratum of sand, however thin, this form of dam 

 may be used, as there is no kind of lateral supports such as shore wanted, 

 the structure containing within itself the elements necessary for its stability. 

 It possesses, indeed, the properties of a caisson, with the additional advan- 

 tage of accommodating itself to an irregular bottom. 



2. Description of a Cast-iron SItnii Bridge, of two arches — of Wf) feet 

 span each —now being erected to carry f/ie Leeds, Dewsbury, and Manchester 

 Railway, over the River Colder at Ravenswharfe, near Dewsbury. By 

 Thomas Grainger, Esq., C.E. 



This bridge is a skew at an angle of 56 deg., and consists of two arches of 

 100 feet span, with a pier about the centre of the river ; each arch is formed 

 with six cast-iron segmental ribs, having a rise of 12 feet, — each rib is cast 

 in five pieces, having flanges or lugs at the joinings, and bolted together 

 with 2-inch bolts ; the section of the ribs at the aliutments is 3 feet ileep, 

 the web 2^ inches thick, the top and bottom moulding or flanges 8 inches 

 by 3 inches, presenting an area of 123 inches; the section at the crown is 

 2 ft. 9 in. deep, and otiierwise the same as at the abutments, and presents an 

 area of 1 15^ inches. The spaudrils are cast along with the ribs — the joints 

 being formed at the uprights instead of at the intermediate spaces, as shown 

 on the model. The ribs have dovetailed sockets cast upon them to receive 

 the cast-iron braces which are keyed into them ; these braces, 10 in number, 

 stretch across the bridge at right angles to the ribs ; there are also 8 wrought- 

 iron tie-rods, 2 inches diameter, placed parallel to the line of the abutments, 

 to connect the whole structure together. The ribs abut against and are 

 keyed into massive iron bed-plates sunk into the stone-work of the a'uit- 

 raents. The roadway is supported by transverse timber beams 12 inches hy 

 9 inches, bolted to the top of the spandrils at intervals of 3 feet from centre 

 to centre ; the planking is 3 inches thick, and is laid diagonally across tliese 

 beams, and spiked to them with 6-inch spikes; and over the planking a 

 coating of asphalte is to be laid. The outside ribs are surmounted by a 

 cast-iron cornice to correspond with the luasoury. and having a cast-iron 

 railing on tlie top. The eslimated weight of the cast-iron in the bridge is 

 603 tons 4 cwt., ami the expense of fitting up the iron and timber work has 

 been contracted for at 8,598/. 



3. Observations on the means hy which Time may be communicated by 

 Signal Balls from one Station to anollier. By John Adie, Esq., F.U.S.E. 

 The author of this paper remarked, that the distance of the Nelson Monu- 

 ment from Leith, and more so from Leitb Roads, would allow a time-ball 

 placed on the Monument to be distinctly seen only in very clear weather, 

 which is confined to a liiniled number of days, rendering it of little use to 

 the shipping in the Frith of Forth. He ne.xt described a method by which 

 the ball on the Monument, and one at Leith, might be dropped at the same 

 second of time, by a person in charge at the Royal Observatory, Calton Hill. 

 This he proposed to do by making use of the great force induced on artificial 

 iron magnets, the wires surrounding these magnets being brought into con- 

 tact with the poles of a galvanic battery placed in the Ubservatory, and em- 



