Circular roofs may be executed without ties, or without any precaution of trussing. as in rectangular buildings, but the wall-plate ought to be one continued mass. There are two methods of covering circular roofs with boards: one is, to bend the boards with their joints in horizontal planes ; and the other is, to bend them in planes passing through the axis. As that species of circular roofs called (lollies lays considerable claim to our attention, it will here he proper to say something on their construction.
It' the dome he spherical, and have 110 lantern to support, the ribs may he Constructed of hoards in two or three thkk nesses, with the longitudinal joints of the hoards tending to the axis of the dome, and intersecting the spherical edges, and the butting joints intersecting the sides, If the ribs, which tend to the said axis.
Let us now suppose the thickness of a rib to consist of three boards, and suppose the circular pieces which are to compose the ribs, to be all prepared of equal lengths and breadths. Take One of the lengths, suppose for the left-hand piece at the bottom, and lap the next higher length, which is the middle piece, two-thirds upon the lower piece; take another length tier the right-hand piece, next higher, and lap this two-thirds on the middle piece ; so that the right hand piece will lap one-third upon the left-hand piece ; between the ends of this third, bolt or pin the three pieces together; the middle hoard will want a third, the right-hand board two-thirds, to make it complete at the bottom ; these parts being supplied and fixed, lay :mother board at the higher end of the right-hand board, the end of another to abut upon the higher end of the middle board, and the end of a third board to alma upon the upper end of the left-hand hoard. then there will le three piles of boards. which must be tbstened together between each pair or heading joints, which are three in 1111111 Proeeed in like manner with every succeeding three Inards, as with the last three, until you arrive at the top, and the deficiency must be supplied as ant the bottom. In this manner, ever' rib in succession must be eonstructed, until they are all finished. Each rib ought to be fitted to the curvature of the axal section of the dome, drawn on it floor, and the three thicknesses fixed together throughout the whole length, before it is removed. lf, in addition to the fixing. the joints be strapped, it will add
considerably to the strength, and will not be much inferior to that of it solid piece. in large domes of this construction, it becomes necessary to discontinue the ribs, otherwise an unnecessary quantity of timber would be employed ; and it must be observed, that the greatest intervals must be so regulated in their dimensi4ms. as to be greater than what would make the horizontal ribs for the boarding, when fixed, sufficiently strong.
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 strutting-picees were strapped together. In the above manner was the tither dome of the IIalle du Bled, at Paris, constructed by Moulineau, supposed to be the first of the kind.
1• 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 Vertical ribs, their planes being disposed at equal angles round the axis as their common vertex. and constructed according to the above method ; between every pair of such ribs, place other ribs, the curvature of which will be of less circles of the sphere, unless one stand in cavil interval. and its plane bisect the inclination of the vertical planes of the two adjacent principal ribs: dispose of these ribs in equidis tant 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 would have hero, had the planes of all the ribs tended to the axis.
In the construction of plaster groins, two methods are employed in the disposition and fixing of the ribs. By both methods. ribs arc made to answer the intersections at the angles; by one method, ribs are formed to the transverse sections of the vault, and disposed in vertical phones accord ingly ; but by the other, the ribs are prepared straight, and fixed parallel to the axis of each vault.
The lathing for plaster is sustained upon walls. by it number of parallel posts of very small scantlings, called balleniny, and ranged according to the figure they are intended to