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is no other remedy than by giving the puncheons a scantling suf-ficient to withstand this transverse strain, or horizontal thrust,at the points G and O : however, the shape of the contour maybe pretty well secured by introducing two abutments, hi andm n, No. 6 ; these by being bolted through the two ends, willadd greatly to the stiffness of the rafters b c and d e : thebolts that go through the upper ends may also serve for thebraces i K and L m : the shape of the horizontal beam, o d,will likewise be very much preserved by the piece k l, boltedin three places, one at each end, into the braces i K and l m,and another in the middle : the contour of a roof, thus sup-ported, would be quite unchangeable, if the rafters wereinflexible ; but, as this is not the case, and as they are actedupon transversely by the braces, the truss will, therefore, insome degree, be expanded at b and e, and consequently occasionlateral pressure on the walls ; it will, therefore, be unfit foran oblong building, without other precautions for this purpose.By inserting parabolic curves in the sides b c and d e, as inPlate IV. Figure 1, it will be effectually prevented.

In roofs of this description, joggle-pieces of wood shouldnever be used, as their shrinking would tend greatly to alterthe outline of the rafters.

Having laid down such principles as will enable the work-man to judge of the strength and strain of timbers in theframings of carpentry, it will now be necessary to proceedto show the mode of constructing roofs to answer variouspurposes ; to give some practical observations relative totheir strength, and to show the various modes of joining tim-bers, the forms of traps, &c.

As we have above stated, the simplest form of a perfecttrussed roof, consists of two principal rafters inclined to eachother, and meeting at the apex, the lower ends being tiedtogether by a horizontal tie-beam, to prevent their spreadingand thrusting out the walls upon which they are supported.As, however, the gravity of this tie-beam, especially if of anyconsiderable length, is apt to cause it to sag in the centre,between the bearings, it is neoessary to provide another bear-ing for it between the walls, and this is effected by suspend-ing it in the centre by means of what is termed a king-post,which, in its turn, is held up between the principal raftersat the apex of the roof. To assist in effecting the sameobject, the tie-beam is sometimes cambered or arched, butthis plan can scarcely be recommended, for if the timberssettle, as in all probability they will do, the cambered beambeing extended into a straight one is liable to push out thewalls, and cause the mischief which it was intended to pre-vent. The better plan is, to camber the beam only on theupper side, the lower side being horizontal, so as to havethe greater scantling in the centre ; this plan, however, isnot without objection, for by it you have the greater weightat the centre, where it is most effective to cause the beam tosag. To afford the same assistance to the principals, andto prevent their sagging by their own weight and that of thecovering which they have to support, short struts are carriedfrom their centre to the foot of the king-post on either sideof it, and on this they have their support. In a truss of thiskind we have the tie-beam and king-post in a state of tension,while the principals and struts are subject to compression.Eoofs of this class are very common, and are adapted forbuildings where the span is not above 30 or 35 feet.

"When the span gets beyond this, it is necessary to suspendthe tie-beam at more points, and first of all, at two wherethe extreme bearings are not much more distant. In thiscase two suspenders, termed queenposts, are employed, hold-ing up the tie-beam at two points equidistant from each otherand from the walls of the building; the principals are notextended to meet each other at the apex, but terminate at the J

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queen-posts against which they abut, and the cjueen*P°retained at the required distance from each other 7 ^ tl' eof a straining-beam, which also abuts against the h ea 'plii 3queen-posts, and by this means they are suspende g pos®description of truss is something like the first, if "’® r j z0 nt a *the king-post separated into two halves, with a . l0 j g frob 1strut placed between them, to prevent the prindp 8 o>being pressed together; struts are also placed atthe queen-posts, to support the principals. snea^ 1 ''’

A truss for a still greater span is formed by sl | i jj 1 g-p o9tthe tie-beam at three intermediate points, having a prii*'between two queen-posts, without a straining-beam- ^ - 0cipals being carried to meet each other at the apex> p O 0t a*first description of roof; struts are carried from * '^ oJO th ethe king-post to the head of the queen-posts, an caS e9 8foot of the queen-posts to the principals. I n nlil .' t diff® 1 "'roof with the same number of suspenders is somew ^ 0 f aently constructed, and this consists, in the fi rst , ^ j ia vi n ^’roof precisely similar to the second above describe ^. g( j uphowever, in addition, a king-post in the centre, ' nt °

through and above the straining-beam, which is j £ j, e r sidf'two lengths, and abuts against the king-post on e ^j jer piO rAbove and parallel to the principals is placed ^Jier®

of principals, which are continued to meet at the a P pri®'they abut against the king-post; these alone are e ‘ j n st "J®cipals, the other subordinate rafters which abut a f ailJC iU a1 ^queen - posts, being termed straining - braces, 0 tl‘ e "pP.®rafters. The principals are further supported a ^, 0 vepart by struts, which spring from the king-p 03 on mstraining-beam, and below by a continuous beat k £e j bystraining-braces, which, in their turn, are supp ^ f 0 r ^the struts from the foot of the queen-posts. , t i,e p‘ e ,

of roof is calculated for a span of 60 to 80 feet, a . ng£ea d 0vious one for a span of 50 or 60 feet. Sometimes, ^g-p os ’a single king-post, this kind of truss will have t 1 g 0 f t 1as it were, split in half and hung on either e %teo^° ttruss, from the heads of the upper principal 3 ’ c0 U&down to the tie-beam ; in this case the straini 4 * 3

beam is in one piece, and passes between the > £ j )e tf u _king-post. Another variation consists in split pet"’ 6 ^into two half-trusses, and keeping the k’ n ?"Pj; s kind ' v g[]them both, so as to form one mass. A roof ot ^re mconstructed over the Basilica of St. Paul at Kom _ ^four centuries ago. , u se °^'/ nS e

This expedient may be entirely obviated by t ^ie t0 tb ^king-posts, which, in all cases, would seem pro e ^ m®of wood, both for king or queen post, or, indee , ^ for .her of the truss which is subjected to tensl0 ”j en t, ob v ’ ia [J)3where its employment, to a very considerable e ^ ^ t j e ,be» ,the difficulty which is experienced by the sagg u jo .j. c omP.Vt,of timber. The number of suspenders, especm o r . jfeof iron-rods, may be still further extended to^s io

or, indeed, any number, the only limit bem 0 „.

length of the principals. eader 10 v i {

For the strength of different materials the ^

suit the article on that subject, but as a S eI ^ g j on is ^j^v'may be observed, that oak when exposed to e jj e ;pg- r9)than fir, and is therefore less adapted for for J ' 9 { jts

ever, less compressible, it is usually P r ® , ,, er ves> ( fo fstraining-pieces, and struts; hut Tredgold o . £ j e ss

greater tendency to warping in summer ren , g n ot

to

rafters and purlins than foreign fir. Cast u re qui reS 9 ridused, except in fire-proof roofs, and each P ,e sti' a P . 3 ar/well tested. Wrought-iron is very use u -= ne c _„<a

also for ties and trussing-posts ; but care isto guard against imperfections, which are n:unobserved than in wood. Wherever iron

morel^ e d,P r0

is apP 1 '