2:,2 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



LAlGVST, 



weight of the upper portion of the roof resting on the posts at 

 the points </, e. It is consequently manifest that there will be a 

 very considerable vertical pressure on the posts rf,/; e,^, unsup- 

 ported by any counterpoise at the outer ends of the hammer-beams, 

 and tending- to upset tlie frames rf, 6,/, e,g,c, inwards, by turning 

 them on the points A, e, as centres; and this would undoubtedly be 

 the result, unless this force were otherwise counteracted. Now, 

 this pressure, I conceive, is carried by the great arch rib at the 

 points At,/, where the jiosts intersect it; and it thus affords five 

 principal points of support to the weight of the roof — one at the 

 <entre of each hammer-beam, one at each post, and one at its 

 apex jn, at the collar-beam under the king post, which I conchule 

 in this case fulfils the duty, not of a suspending tie, but really of 

 a post. The lower arches of foliation will also assist in sustaining 

 this weight. The pressure thus thrown on the arch ribs is dis- 

 charged at their feet on the wall; and their rise being so high in 

 proportion to their span, they probably exert a comparatively 

 small side pressure against the wall. On the supposition of the 

 upper part of the arch rib not receiving the pressure of the posts 

 (/, k. e, /, this pressure would of course be transmitted to the inner 

 ends of the hammer-beams. We should then have it acting verti- 

 cally, and resolving itself into a horizontal compressing force 

 against the collar- beam at the points rf, e, and an oblique force 

 against the points A, i, of the arch ribs. In this case the direction 

 of this pressure will be very nearly a tangent to the rib at the 

 points h,i, and will be transmitted by the arch rib to the wall. As 

 the wall is of considerable thickness, the direction of this pressure 

 probal)ly does not pass outside, but readies the foundation within 

 the thickness of the wall. The wall has also the weight of its 

 upper portion above the springing of the ribs to assist it. The 

 oblique pressure at /i, », certainly tends to force the arch ribs out- 

 wards, turning them on their springing as centres; but against 

 this is opposed the weight of the rib itself, and also that of the 

 arch of foliation, which acts in the opposite direction.— Offering 

 these remarks to your notice, if you think them worth a place in 

 your journal, I am, &c., 



Jidy lltli, 1850. 



J. A. Davies. 



LIGHTHOUSE IN THE SKERKI CHANNEL. 



^^^E extract the following description from a very able report by 

 Mr. Alexander Gordon: — 



"Admiral Sir Francis Beaufort suggested to me the importance 

 of a lighthouse in the Skerki Channel, for Keith's Reef. I am now 

 able to assert confidently, that a lighthouse can be erected on 

 Keitli's Reef, showing a light of the greatest power 100 feet or 

 more above the sea, capable of containing stores, provisions, and 

 water for four men, or even more, for six months, and at an expense 

 not exceeding 17,000/. The rocks are compact limestone, in extent 

 nearly half-a-mile long, and one-third of a mile broad, with a small 

 space of about six feet square, even with the water's edge. 



''The sketch is partly an elevation and partly a section of such 

 a lighthouse as I would propose for Keith's Reef, to show a light 

 of the most powerful character at the height of 93 feet above the 

 water level, and the tower sufficiently spacious to contain even six 

 men (if such should be required by stress of weather) for six 

 mouths, and all necessary lighthouse stores, provisions, and water, 

 witli ample space for exercise, and for live stock. 



".\ strong wrought-iron bar should be at once jumped into the 

 centre of the area of the peak, and immediately succeeded by other 

 wrouglit-iron bars, on which to form a crow's nest, or an open 

 platform; from tliis the surface and edges of the rock could be 

 cleaned of the large quantity of sea-weed now upon it, and a small 

 crane would enable us to have the edges of this rock trimmed and 

 undercut, and a circular seat rudely prepared about six feet under 

 water. It may not, however, be necessary to go to such a depth. 

 A few sections of this seat will enable us to cast lead slabs weighing 

 a ton and a-half or two tons, such as can be dovetailed into each 

 otfjcr, and fastened together with lead joggles and dowels driven 

 ill hard, and the wliole of the seams closely chinced or caulked 

 with lead. This wall of lead will then be carried up about 15 inches 

 thick, with the seams all chinced together below and above the 

 wiiter line, so as to prevent percolation of water. Tliis lead wall 

 w fiere above water, will have its seams all run together with a 

 powerful blow-pijie. We may even maiuige to cast the ujiper 

 portion of the lead wall upon tlie lower part. 



"The original iron rods will now be at tlieir ujiper ends worked 

 into the inner and dry side of the lead wall, forming bond as we 



proceed. At tlie extreme base of the tower the water will now be 

 shut out by warm gutta percha, and then by hydraulic cement. 

 Lead will then be run in to make the base perfect; a core of 

 masonry perfectly bonded together is to occupy a great part of the 

 interior. The fixed crane post may then be built in, and thus be 

 converted into bond and load; even if it should in some degree 

 oxidise, the rust will do good rather than harm. 



^^ff^^.^'"^""" 



"Lead is no doubt a costly article as estimated below, but its 

 durability in salt water; the facilities which it affoi'ds for making 

 ]ierfect bond; its inertia; and its not being susceptible of vibra- 

 tion, point it out as the best material under the circumstances. 



"The lead walls are to be carried up about '-il feet above the 

 water-line, and upon them and the core of masonry (the latter in 

 many places bonded together with lead), there will be a super- 

 structure of cast-iron bonded together with wrought-iron floors 

 and fastened down to the rock by many \ery strong wrought-iron 

 ties; which latter will not (where there is moisture) be allowed 

 to come in contact witli the lead. The junction of the iron shell 

 and the lead wall must have special attention. In the event of 

 oxidation of the iron to any considerable extent, such plates can 

 be removed; and if the oxidation be small, we can repair the 

 damage by scraping away the oxidised iron and running in more 

 lead in its place. The superstructure would be much the same as 

 that of my other lighthouses. Its cost may be estimated at 17,000/. 



" Ale.xa.ndkb Gobdo.n. ' 



