ROOF. 



From fixed pointe of the trufs ; the weight of the covering 

 being always uniform, or nearly fo. 



Fig. 3, N c 1, a defign for a roof, with lanterns in feveral 

 ftages, diminifhing in the form of fome Chinefe buildings. 

 Thefe towers may be carried to any height, at pleafure, by 

 always trufling in the plane of the diagonal between every 

 two ftages from the lower, to fupport that immediately 

 above. There are fome excellent fpecimensto be feen in the 

 buildings of Deptford, belonging to the victualling office, 

 called ufually the Red-houfe. N° 2. is a plan for the feat of 

 the upper ft age 



Fig. 4. is the defign of a conic roof for a rotunda or 

 circular building, fupporting a lantern at the top. If the 

 extent of the building is very large, the rafters would re- 

 quire to be fupported in the middle ; for this purpofe the 

 bottom piece may be continued, as is exprefted by the 

 dotted lines. 



Fig. 5. is a defign for a pent roof, fupporting a lantern 

 of an oftagonal form. N° 1. is the form of the trufs ; N° 2, 

 the lower plan, with the feat of the polts ; N J 3, the plan 

 of the upper part ; N° 4, the trufs of the two tranfverfe 

 part6. 



Having defcribed the form of roofs fupporting lanterns, 

 and the trufies in which they may be executed with fafety, 

 it will now be proper to give a few examples of domes, and 

 (hew how they may be conftruCted under various circum- 

 ftances. If the dome to be conltru&ed be on a circular 

 plan, and have no lantern above, the ribs may be built in 

 the following manner, with planks of convenient lengths in 

 three thicknefles. Having afcertained the length of the 

 ribs, and the number of pieces in that length, and having 

 properly (haped all the pieces to the curve, the middle piece 

 at the bottom may be one of thefe lengths ; to each fide 

 may be joined two other pieces, one reaching to a third of 

 the middle piece, and the other to two-thirds from the bot- 

 tom; fo that by continuing with planks of the whole length 

 to the other extremity of the rib, the middle thicknefs will 

 always be covered two-thirds from the bottom on one fide, 

 and one-third on the other ; the deficiency at the top mult 

 be filled up with pieces, one of a third and the other of 

 two-thirds, as at the bottom ; the whole, being well bolted 

 together and ftrapped acrols the joints, will be nearly as 

 ftrong as a folid rib. Plate LIX. Jig. I (hews the manner 

 of confirming this kind of dome ; N J I being the femi-plan, 

 N° 2 the elevation, N° 3 the manner of building the rib. 

 In domes of this kind it may fometimes be ncceflary to dif- 

 continue the ribs, in order to divide the fpacs more equally 

 for the horizontal ribs. It is evident that a dome built in 

 this manner may be carried almoft to any degree of extent, 

 provided that it have a fllfficient number of horizontal ribs. 

 Of this conflni&ion is the Halle du Bled at Paris, of 200 

 feet in diameter, the invention ol a judicious carpenter, the 

 Sieur Molineau, a man of little education in the point of 

 fcience, but of coniiderablc al experience, from 



which he formed his theory. Being convinced that a very 

 thin (hell of timber might not only be fo (haped as to be 

 nearly in equilibrio, but that, if well connected with hori- 

 zontal ribs, it would have- all the (lifFnefa that was neccflary ; 

 he accordingly prelented his fcheme to the magistracy of 

 Paris : the grandeur of the idea pleafed them, but t hey doubted 

 of the pofhbility of its being put in practice. Being a great 

 public work, they prevailed on the Academy of Sciences to 

 confider it. The member!;, who were competent judges, 

 were (truck with the juftnefs of Mr. Molim-aii's principles, 

 and attonifhed that a thing fo plain had not become familiar 

 to every houfe-carpenter. It quickly became an univerfal 

 topic of converfation, difpute, and cabal in the polite circles 



12 



of Paris. But the Academy having given a favourable re- 

 port of their opinion, the projeft was immediately carried 

 into execution and foon completed, and now (lauds as one 

 of the greateft exhibitions of Paris. The circular ribs 

 which compofe this dome confilt of planks 9 feet long, 

 13 inches broad, and 3 inches thick, made in three thick- 

 nefles, as in that already defcribed. At various dillances 

 thefe ribs were connected horizontally by purlins and iron 

 ftraps, which made fo many hoops to the whole. When the 

 work had reached fuch a height, that the diltance of the 

 ribs was two-thirds of the original diftance, every fecond 

 (now confiding of two ribs, very near each other,) was in 

 like manner difcontinued, and the void glazed. A little 

 above this the heads of the ribs are framed into a circular 

 ring of timber, which forms a wide opening in the middle, 

 over which is a glazed canopy or umbrella, with an opening 

 between it and the dome, for allowing the heated air to get 

 out. All who have feen this dome fay, that it is the molt 

 beautiful and magnificent objetl they ever beheld. 



The only difficulty which occurs in the onllru&ion of 

 wooden domes, is when they are loaded in the upper part by 

 a heavy lantern or cupola. Such a dome as has now been de- 

 fcribed would be in danger of being crullied at the top in- 

 wardly ; the molt effectual method of preventing which is by 

 making the ribs in the form of trufies, as in Plate lAX.Jig. 2, 

 where the itraight pieces connecting the two extremities of 

 the exterior fide, forming as it were the bafe of a trufs in a 

 common roof, aft contrary to the nature of a tie-beam ; 

 they refill the vertical preflure of the lantern, without hav- 

 ing any tendency to burft out the fides, by acling entirely 

 longitudinally on the wall-plate. In order to fecure the lan- 

 tern, horizontal braces are fixed from the bottom of the 

 lantern to the middle of the principal braces under the jog- 

 gles, fo that the whole is refolved into triangles, which are 

 all immoveable at the angles. The wall-plates fhould be 

 framed as the ribs of a dome, conllrufted as in the lalt 

 example. 



When a dome is to fupport a heavy cupola of ilone, fuch 

 a confirmation as that of the cathedral of St. Paul's, Lon- 

 don, may be employed. Fig. 3. N° 1. exhibits the trufs of 

 this dome, taken from accurate meafurement. A. a a A, a 

 dome of brick, two bricks thick, which, as it rifes every 

 live feet, has a courfe of ftrong bricks 18 inches long, bond- 

 ing through the whole thicknefs. B b b B is a cone, built 

 with bricks one foot fix inches in thicknefs, for fupporting 

 the heavy cupola above, of Portland (tone, which is 2 1 feet 

 diameter, and near 61 feet high, and alfo the timber-work 

 of the dome. The horizontal or hammer-beams, C,C, &c. 

 being curioufly tied to the corbels D, D, D, &c. with iron 

 cramps, which are bedded into the corbels with lead, and 

 bolted to the hammer-beams. N° 2. (hews more particularly 

 the manner of tying the hammer-beams to the corbels. 



This dome is boarded from the bafe upwards, and the 

 ribs are therefore fixed horizontally, having their fides in 

 planes tending to the centre of the dome. The contour of 

 the dome is formed of two circular fegments, which meet in 

 the axis like a pointed arch. The fcanllings of the curve 

 rib of the trufs are 10 inches by 1 1! .11 the bottom, and 

 6 inches by 6 at top. It has a very ftrong double iron chain, 

 linked together at the bottom of the cone, and feveral other 

 lefs ones between that and the cupola, which may be feen 

 in that beautiful fcdlion of St. Paul's, engraved by Rooker. 

 This dome was turned upon a centre, which was fupported 

 without any flandards from below. As every (lory of the 

 fcantling was circular, and the ends of the ledgers meeting 

 like fo many rings, and truly wrought, it fupported itfelt ; 

 and as it was both centering and fcallolding it remained for 



the 



