C A R P E N T 11 V . 



PtATF. 



CXX1X. 



5. 



this two thirds on the middle piece, so that 

 nv;ht hand piece will hip one third upon tl 



the ends of this third screw, 



..- or p.ii the three boa- r ; tin- middle 



ill want a third, the rijHit hand board two 

 thirds to make it complete at the bottom. These 

 tig supplied and fixed, lay another board at 

 the hij;her end of the right hand board, the end ot 

 another to abut upon the higher end of the middle 

 b -aid, and the end of a third board to abut upon the 

 upper ends of the left hand board ; then there will be 

 three plies of boards, which must be fastened toge- 

 ther between each pair of heading joints, which are 

 three in number. Proceed in like manner with every 

 succeeding three boards as with the last three, until 

 you arise to the top, and the deficiency must be sup- 

 plied as at the bottom. In this manner, every rib in 

 succession must be constructed until they are all 

 finished. Each rib ought to be fitted to the curva- 

 ture of the axal section of the dome drawn on a floor, 

 and the three thicknesses fixed together throughout 

 the whole length before it is removed. If, in addition 

 to the fixing, the joints be strapped, it will add con- 

 siderably to the strength, and will not be much in- 

 ferior to the strength of a solid piece. In large 

 domes of this construction, it becomes necessary to 

 discontinue the ribs, otherwise an unnecessary quan- 

 tity of timber would be employed ; and it should 

 be observed, that the greatest intervals must be so 

 regulated in their dimensions, as not to be greater 

 than what would make the horizontal ribs for the 

 boarding, when fixed, sufficiently strong. 



Fig. 5. is a dome of this construction. No. 1. the 

 elevation ; No. 2. an ichnographical projection. In 

 this design, the ribs are only once discontinued, but 

 in large works they may be twice or several times 

 discontinued. The wall plate and kirb at the top, 

 are framed in the same manner as the ribs. 



As all domes are best boarded with their joints in 

 vertical planes tending to the axis, horizontal pieces 

 must, in this case, be strutted between the ribs, and 

 their outer sides formed with the spherical surface. 

 A dome constructed in this manner, might also be 

 made to support a heavy lantern, provided the strut- 

 ting pieces were supported together. 



In the above manner, was the timber dome of the 

 Halle du Bled at Paris, constructed by Moulineau, 

 which is supposed to be the first of the kind. 



If the boarding of the dome is required to be bent 

 with the joints in horizontal planes, and the dome 

 have no lantern, a very good method is to construct 

 it with several horizontal ribs, disposed at equal 

 angles round the axis, as their common vertex. Ac- 

 cording to the above method, between every pair of 

 such ribs place other ribs, the curvature of which 

 will be portions of less circles of the sphere, unless 

 one stand in each interval, and its plane bisect the 

 ine! i, on ot the vertical planes of the two adjacent 

 principal ribs : dispose of these ribs in equidistant 

 parallel planes, and fit their upper ends upon the sides 

 of the principal ribs. This disposition of the ribs 

 will be a considerable saving of timber, besides what 

 it w >uld havt been had all the planes of all the ribs 

 tended to the axis. 



Fig. G. in a design for a dome constructed in this Contiruc- 

 manner : No. 1. the elevation, and No. 2. the ichno- 



i.ical projection. 



. 



If the dome is required to have greater height p L4T e 

 on the exterior side of the building than can be given CXXIX. 

 by employing ribs built as above* the curved part of Pig. 6. 



rib may be supported upon a vertical truss, and 

 the inter-curves of the ribs fixed to the under sides 

 of the truss ; and if a lantern is required, the better 

 method is to place the lantern in its situation, and 

 then to place the ribs ; the curved parts which form 

 the exterior and interior domes, will be fixed more 

 accurately after the ribs are put up. In each of these 

 trusses it will give much less stress^to the timbers, if 

 one of them inclines upwards its whole length from 

 the wall-plate to the lantern, as these timbers in every 

 truss will support the lantern of themselves, being 

 only compressed in a direction of their length, and it 

 should be observed, that the abutting parts on the lan- 

 tern should never be on the middle of the sides, but 

 upon the angles of the posts, and that small beams or 

 interstices ought to be framed between the posts op- 

 posite the abutting places. These inclined timbers 

 having a tendency to bend by the weight of the lan- 

 tern, will hardly require any other trussing than what 

 is sufficient to keep them from sagging by their own 

 weight ; and it is evident that they can only support 

 the lantern when it is properly balanced. The lan- 

 tern will therefore not be proof against the variable 

 force of the atmosphere ; a gust of wind would soon 

 put it out of its place. If the trusses consist of two 

 triangles, a piece of timber may be made to reach from 

 the lower end of the lantern to the middle division 

 of each truss ; and these braces will render the lan- 

 tern secure from atmospheric power. It is evident 

 that the disposition of these pieces and the upper 

 trussing timbers must be made to accommodate the 

 curvature of the exterior and interior domes. The 

 trussing might have been done without, provided the 

 slanting timbers which support the lantern were strut- 

 ted in horizontal equidistant rows, and the exterior 

 curve might have been supported by radiating pieces. ' 



Fig. 7. shows the lantern, and one of the trusses pig. 7. 

 on each side of it. The pieces AB, AB, are two 

 of the inclined pieces supporting the lantern ; and 

 the pieces CD, CD, secure the equilibrium against 

 wind, or sudden impulse. 



Though any elevation might be given to a dome 

 by this last method, and the timbers might even be 

 made so secure as to support a lantern of stone of 

 considerable ponderosity, yet, as this material is lia- 

 ble to decay, and would therefore be subjected to 

 frequent reparations, a vaulting of stone may be car- 

 ned from the cylindric walls upwards, in the form 

 of the frustum of a cone, with its axis vertical, and 

 the lantern built upon the top of this vaulting. If 

 the exterior dome is to be formed of timber-work, 

 the ribs may be constructed in three thicknesses, as 

 in the Halle du Bled, or in single pieces scarfed to- 

 gether, and supported by posts and beams, which 

 mutually support the extremities and bearings of 

 each other. 



The dome of St Paul's cathedral, designed by Sir 

 Christopher Wren, is of this construction. The in- 



