540 



CARPENTRY. 



Carpentry. 



Construe- terior, or lower dome, consists of 18 inches brick- 

 W0 rk, and several courses of whole brick, bonding 

 ^ ent: j re thickness of the spheric arch, at five feet 

 distance. This dome, as we are informed, was turn- 

 ed upon a centre which supported itself without any 

 standards from below ; and this centering remained 

 for the use of the painter. The interval between it 

 and the dome was twelve feet. This dome is hoop- 

 ed with a double chain of iron at the base, to pre- 

 vent any risk 'by its lateral pressure. A cone of 

 eighteen inches brick-work supports the exterior 

 dome, the beajns being tied to stone corbels by iron 

 cramps, which are well bedded in lead, and bolted 

 to the beams. The stairs which lead to the golden 

 gallery at the top of the dome, are carried up be- 

 tween the trusses, or between the brick cone and the 

 external dome. The boards are bent from the base 

 upwards to the bottom of the lantern, having their 

 joints in planes tending to the axis. The repandous 

 sides of the exterior dome are framed of segments 

 of circles, which would meet, if produced ia a point- 

 ed arch at the top. The brick cone supports a lan- 

 tern of Portland stone, of 21 feet diameter, and near 

 64 feet high. The diameter of the dome is 104 feet, 

 and there are 32 trusses in its circumference. 



Fig. 8. shews the elevation of two opposite trus- 

 ses, the sections of the brick dome and cone : 



AB One side of the section of the interior dome. 

 CD One side of the section of the brick cone. 

 EF Repandous side of the exterior dome. 

 The hammer-beams are marked by the small let- 

 ters a, b, c, &c. ; and the posts by the numbers 1, 

 2, 3, &c. 



The struts are all five inches by eight : the scant- 

 lings of the other timbers, denoted by the small let- 

 ters and numbers, are as follow : 



PLATE 

 CXXIX. 



Fig. 9- 



Scantlings. 



Hammer beams,- 



Posts 



The curve ribs of the dome are 10 by 11^ inches 

 at the bottom, and 6 inches square at the top : the 

 purlins which support the boarding are 4 inches 

 square. 



Fig. 9. shews part of the brick cone, one of the 

 stone corbels, with part of a hammer-beam connected 

 therewith by means of an iron strap bedded into the 

 stone with lead, and bolted to the hammer-beam. 



The construction of this dome and truss is, upon 

 the whole, very judicious, and the timbers are well 

 disposed. 



Centerings to Arches of Bridges. 

 Center; in carpentry, is a frame of timber con- 



structed for the purpose of supporting the stones or Comtru*. 

 bricks of an arch or vault during the erection. tive 



The center, therefore, serves as a foundation to ^P cn ^Jj 

 build the arch, and is taken down, or struck, at the ^ "^Y^"" 

 completion of the work. 



The centre of a large vault, as that of a bridge, is 

 constructed of trusses disposed equidistantly in ver- 

 tical parallel planes, and boarded over, so that the con- 

 vexity of the boarding may coincide with the internal 

 intended concavity of the arch. The distances of the 

 ribs may be disposed at from 3 to 8 feet, according 

 to the strength of the boarding and the ponderosity of 

 the arch. In very large works, a bridging is laid for 

 every course of arch-stones, with blockings between, 

 to keep the ring-stones at regular distances; but the 

 ring-stones do not always rest upon these bridgings ; 

 planks are sometimes put between, that they may be 

 cut away afterwards, and separate the centre and the 

 intrados from each other, in order to ascertain whe- 

 ther there are any settlements in the arch, and to 

 repair the damages and put the arch in a state of 

 equilibrium. 



The principle of trussing having already been given 

 under the department of Roofing, little more requires 

 to be said than a few observations upon the forms of 

 trusses applicable to centering. 



Where the river is not navigable, the trusses may be 

 constructed with a beam at the bottom. In this case 

 there is no difficulty; the forms tor the trusses of 

 roofs may form the grand or principal part of the 

 truss for the centre : but when the river is navigable, 

 the centre requires as large an opening as is consistent 

 with its strength, in order that the vessels may pass 

 under it, and the disposition of the timbers will re- 

 quire much greater skill of the carpenter. 



The general principle of construction is a series of 

 triangles, of which every two is connected by a com- 

 mon side. 



Let ABCDEFG, Fig. 10. be the curve of the arch PLATB ^ 

 which requires a centre. Let the points A, B, C, ^ 

 &c. be connected so as to form the equilateral poly '*>' 

 gon ABCDEFG, and join AC, CE, and EG ; the 

 timbers thus disposed will form three triangles, which 

 may be looked upon as so many solids revolvable 

 about the angular points A, C, E, G ; suppose now 

 these to be in equilibrium, the smallest force on either 

 side would throw it down, and therefore without 

 other connecting timbers, it would be unfit for the 

 purpose of a centre. 



Let ABCDEFG, Fig. 11. be the curve of anarch Fig. 11. 

 which requires a centre ; first form the equilateral 

 polygon ABCDEFG, with the timbers AB, BC, 

 CD, &c. and fix the timbers AC, CE, EG, as be- 

 fore, which will form three triangles, moveable round 

 A, C, E, G. Let the timbers BD and DF be fas- 

 tened, and thus the whole will be immutable, so that, 

 if supported at the points A and G, and a force be 

 applied at any other of the angles B, C, D, or F, the 

 limbers will be all in a state ot tension, or in a state 

 of compression, and the whole may be looked upon 

 as a solid body. For since the sides and angular points 

 of a triangle are fixed, when the triangle is supported 

 at two of the angles, and a torce applied to the other, 

 let us suppose the triangle ABC to be supported at 

 the points A and B, and the point C, and the other 



