Um e r ae gree of curvature from their weight, which inSa 9ginn UC, r S tbem to a concavity on the upper side, calledany i u ^‘ -*- 0 . prevent this disagreeable consequence, withoutin th e f rme ^* ate support from the floor below, a strong truss,bnajeg rm a low roof, is introduced between two equal(iXtreraij 0 88 to ma ^ e the whole discharge the weight at each8 ^ r ink.in e^° P revent the bad effects resulting from theStru cteq£ • the timber, the truss-posts are generally con-a firmer °v, lr ° n ’ screwe< 4 and nutted at the ends ; and to givefiiteti... a .utfflent, the braces are let into a groove in each

‘tch (is either

atl< l is ' bbe abutment at each end is also made of iron,

a ess of b^u Screwe d> nutted, and bolted through the thick-^’‘Uensio halves, that the braces may abut the whole

ate madg 118 tbe * r section ; or otherwise the two abutments

s ei'ew> ec j 1[ \^he form of an inverted wedge, where they are' v itli on e 8 j nut ted. These modes may either be constructed''hole l en ri J ss 'holt in the middle, or with two, dividing theP^ced i n ln . to three equal parts : a straining-piece beingat a higher 16 shortens the braces, and elevates them

!’ e sist; u ® ce r . an gle, so that the truss may give a more powerfuliR coi\st ri ] ? \ be su perincumbent weight. The braces mayl|at Ure c ea of oak, or of cast-iron ; the bolts, from theirA, i’ r ^ Ust b e of wrought-iron.

.'gihle f 0 r !v Su hj ect to contraction and expansion, it is lessl? an y deo- r tle h*'aces than wood, which is almost invariableUle Reno?. ?, e temperature, as to heat or cold ; oak is there-

fpP clll V pmv.1_ 1

s >st

i’JUrc 3 J ® m Pl°yed for this

re ^ ___ A - ./ ..- purpose.

JJig of t w "r tongitudinal section of a truss-girder, conded '^ Ure 4 ° » aces ’ tueeting the truss-bolt in the middle.

bra

"Ho three ^ ^ on g*tudinal section of a truss-girder, divi-ti'if fi' ece in ’ con sisting of two braces, wit!' ,i strain-

ViV h!o. 9 ^.f'hldle. No. 1. A longitudinal vertical sec-

to 7 llt;n the'hi r Up ? er side ‘

Con. 8 dee P, to aUn ® ' s vel 'J great, the truss would requirej>' U(: tion rv, en ?hle it to resist with greater efficacy : thisa be a8 hi Figure 5.

r'lr

iin^-e 8 ;

[i'JUrn ma y he as in Figu

£gur e 7'_-hhddle bolt.

Abutment, as shown in Figures 3 and 4.

„ ^ ushers, to prevent a partial sinking of the

'Jv ( l ! ler raftlrs^h 11 * plain r aftering for a floor, by joiningfeet; S P ace , so tl >a ^ °^ lerw * se would not extend across thethe ’ J ; and *** tcpop ‘ extent of the area may be per-flhol e , to estimate, by calculation, the pressure upon' t'he Cr w t >on the parts separately.

1 the ra flering.— Plate II. Let

fitte \ ap lc the ] 16 8 I uare area, any side of which is about

1'hc '*ito each en °f' 1 *" be longest rafter: the rafters are soWan la hers tlint . * er as rRC 'procally to support themselves.*V ’i ai ' e cut the ai j e dovR tailed into the beams laid on theW ali .° v ctai[; t w hole depth of the timber, into the tenon ofis ft,. ea m. 'pi' 6 Inoi 'tise for this tenon is therefore in the

. toil . 4 he other

imp ‘ ea into the^ otiler ends of tliese rafters, eacli of which"''refo' 6 de Pth of^ 8 ) ^ eams ’ ’ s hirmed into a tenon of about

Soti^iu- llle rafter; the mortise for this tenon is!l8 Vli lbe u PDer 6r ra ^ ter ’ notched to a correspondent depth,• a ' s o hir, P' uts °f both may be flush with each other,il C, 18 hut h 8 ?^" to the lower

t half T ° tlle l° vver sides. And because the-ici, is 8U . le df'Pth of the wood, the rafter to which'fitted- 18 the m *!! 01 ' te< l hy half the depth of the rafter in> as t! ' ae -’ ever y faster carrying the one that is’fil | >Ur ai 'ea, they 1 ' 8 18 ^ le case throughout the whole extent^aw. 80 !' the n. n ' USt necp ssarily support each other over

0lll y r. 8j ^'lere ft 6 ''’ and the parts towards the middle oftl. e j lfc ar i s ‘ ®> ttom the :

Vo^inr of the fl ‘'P pi ' e hcnded, lest, by the weight above,00r should sink down, have this disadvan-3 M

natural flexibility of the timber, the

tage provided against, by the excavations not extending pre-cisely to the middle of the timber, but being a little deficienttowards the middle. For, as by this method the rafteringwill rise progressively with each joint from the exterior tothe centre of the area, any small depression that, from theweight above, might take place, will not be sufficient toreduce it below the fair level throughout; the curvature pro-duced on the whole will compensate the weight, and preventany hollow taking place.

“ Figure 2, exhibits the side face of one of the longer tim-bers ; where the end tenons and mortises, at one-third andtwo-thirds of the whole lengths, are sufficiently well exhi-bited : and Figure 3, shows one of the shorter timbers in thatview which offers the end tenons and the single mortise.The upper face of all the rafters appears very plainly fromFigure 2.

“ But since they are disposed in so compact an order inthis last-named Figure, the process may be much more easily ;understood from a close investigation of the diagrams than itcan be from any explanation.

“ It is very obvious, that the four great beams laid uponthe wall are the first with which we ought to begin ; for wewould naturally proceed from these principal beams to thesecondaries or rafters. Now, if we examine these principal jbeams, we shall find, that into each there are dovetailed five :rafters or secondaries, and tliese have their other ends sup- Iported by other rafters parallel to the wall-beams; and those jwhich carry the rafters dovetailed into the wall-beams, arethemselves supported by others in a transverse direction ; andso on, till they arrive at the opposite wall. 1

“ For example, the rafter Sf s has one end dovetailed intothe wall-beam, hut the other end is supported by the rafterfy z ; this last rafter, § z, has one end dovetailed into anotherwall-beam, but the, other end, z, is supported by the rafter |p Q ; and the rafter u Y lias one end dovetailed into the wall- ibeam, but the other end, y, is carried by P Q in the same man- :ner as fy z : p Q is supported at p by 1 K, and at Q by R s: but j 'IK and r s do also support the rafter on, which in its turnsupports the ends f, m, of the rafters h mi, sup-

ports RX and v\v; then we have determined the support ofR, one end of the rafter sr; and v w supporting v, the endof the rafter v u, we come to v u supporting h g and 1 K :but the end, u, of the rafter v u, is supported by yu; Y issupported by p Q, and p <5 supports also Sf z, and is itself sup-ported by S R: but s R is supported by R x and s Sf : and thus |we trace round the exterior framing, and discover the aid !which these rafters reciprocally lend each other, to render the !whole secure and compact. For if we follow the concate- |nation, we shall discover, from the slightest inspection of tlie jdiagram, the principle upon which, in regular succession, the ;parts conduce to give strength to the whole. In the same jmanner, when we begin to trace, from the centre of the fra- ;ruing, every rafter, a b and CD are supported by A B and e f; !and also from these we trace lit, G H, and NO, reciprocally ;supported and giving their support; and following the others ■that are connected with those now named, we go on till we !arrive at those which terminate, and are supported by the ;principal beams on the wall ; as clearly appears from the ‘plan. | i

“This method enables us to construct a floor scantling of jthis description with many, or even with few rafters ; butin any other method, as in the case of an oblong differingbut little from a square, we shall have to employ the follow- i :ing conditions : !

“ For example, in this same case the whole wall can be ! ilaid out, so that the rafters r s, u v, and Sr s brace, and are j jthemselves braced; or, where the rafters <§■ z, ml, and pQ, I j