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Purlins.— The stress upon purlins is proportionable tothe distance they are apart, and the weight being uniformlydiffused, the stiffness is reciprocally as the cube of the length.Rule. —Multiply the cube of the length of the purlin in feet,by the distance the purlins are apart in feet ; and the fourthj root of the product for fir will give the deptli in inches ; ormultiplied by 1.04, will give the depth for oak ; and thej depth multiplied by the decimal 0.6, will give the breadth.

; Common Rafters. —Common rafters are uniformly loaded,i and the breadth need not be more than from 2 inches to 2£inches. The depth for Welsh slate may be found by the' following rule :

Rule. —Divide the length of bearing in feet, by the cuberoot of the breadth in inches ; and the quotient multipliedby 0.72 for fir, or 0.74 for oak, will give the depth ininches.

The largest roof ever executed was that of the Riding] House, built at Moscow , in 1790, by Paul I. Emperor ofRussia . The span was 235 feet, and the slope of the roofi about 19 degrees. The principal support of this immense! truss consisted in an arch or curved rib of timber, in threethicknesses, indented or notched together, and strapped and| bolted with iron. The principal rafters and the tie-beams,were supported by several vertical pieces notched to thecurved rib ; and the whole stiffened by diagonal braces. Thedisposition of the parts of this roof is extremely ingenious ;j but it was too slight for the immense extent of the span, and; it appears that it settled so much, that it was proposed toI add another curved rib to the original design. This exampleaffords an instance of the impropriety of adding material sonear to the neutral line of the framing ; a like want of atten-i tion to principle is sometimes found in a first design ; a roof| designed by Bettancourt for a riding-school, as given by■ Krafft, is an example. The external dimensions of the build-ing were 1,920 feet by310 feet; it was lighted from the top\ by a lantern; and there was a gallery round the inside of thej building for spectators. The method of notching the timberin the curved rib, is objectionable, on account of the dangerof the splitting of the timber under a considerable strain.This system of trussing is termed the bow suspension truss ,and has been much used of late for bridges for railway-works.

Of a somewhat similar construction are the roofs executedby Philibert de Lorme, in the sixteenth century, which areconstructed with a series of arched timber ribs in lieu oftrusses, these ribs being formed of planks in short lengths,

: placed edgewise, and bolted together in thicknesses, the! planks in one thickness breaking joint with those in the| adjoining thickness.

j A fine example of this class of roof was that constructedj over the Halle au Ble, at Paris , 120 feet in diameter, whichi has been destroyed by fire,

I A smaller example, but nearer home, is to be found over! the central compartment of the Pantheon, Oxford-street :

| the span measures 38 feet. The roof is circular, and is sup-; ported by nine semicircular ribs. Each rib is in three thick-nesses ; the middle thickness is of teak, the side thicknesses' or flitches are of fir ; the pieces of timber forming these' thicknesses are scribed on the under side to the curve of the; roof, (and ceiling,) but the top edge is left straight; thereforeat their abutting ends they are considerably broader than attheir middle. Each abutment of every piece, throughoutevery rib, has a shoe of cast-iron interposed. Iron bolts con-nect the three thicknesses together at each end, with a washerconnecting each pair of bolts, as explained on the section.The ribs rest alternately on the pillars, constructed of iron,and on the centre of the longitudinal plate extending the

whole length of the roof. In the former case the r*framed into strong upright pieces, which become s carry*,?post of a semi-truss. These serm-trusses, besides c a1the timber of the roof and ceiling over the gallcneSi. ra of,culated to receive the lateral thrust of the semicire recei^a large cast-iron shoe laid upon a stone templ J t e » g flU terthe end of the tie-beam of these semi-trusses next oS gibl®wall ; the object of this shoe being to prevent the o*decay of this end of the beam from affecting the sthe roof. gf en d

There is also a cast-iron shoe receiving the ot > ^ tothis beam, which is formed with sockets and flange 9 ^^ ojreceive and connect together the king-post and j 10 a ]iudedthe above-named semi-truss, and the longitudinal p ato above, and the head of the great iron upngn , g } a tei fl jThe chief use of this iron shoe is to prevent ^ nat urr

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compression of the timbers, and to preventshrinkage of the horizontal plate from letting downThe intermediate ribs (those which do not rest on ^pillars) rest on another iron shoe, which is fitted ■ohead of an upright piece that rests on the centre o ^tudinal plate ; but the bearing is thrown on to the 11 ^ jo«'

by stout braces framed into the said upright piecegitudinal plate. fthis r0 °]

It will thus be seen, that throughout the skeleton vith i»^ hthere is no case of any important timber resting ' ^ eaJ .upon the side of any other piece, but that they a aris*?an end-grain abutment, so that the cracks and fa* a ^ j a tei' afrom the necessary shrinkage, as well as from ^ ^ p 1-eca °gcompression of the timbers, are avoided. This w ‘ 1 ® 9 0 f tb®tion rendered necessary by the very large scant 1 jgfit 0and by the discharge of the who e „ u npo r

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timbers, and by

roof and ceiling being thrown onThe circular part of the roof is covered .. ,. rb

diagonal boarding, which is supported on ia 'flie r °°[,on to the great ribs, and running longitudina y- , gte rover the sides or galleries are slated, with a fla )ne lled>..ing ; the ceiling of the circular part is deep y en rl ° jmain ribs forming the core for the cradling- ad a P l

ments are of papier mache, a material that wa3 . (g iightn e ^for the large architectural ornaments, from crevV ed U Pand from the safety with which it could be sthe timbers. insists 0 ‘. „g

The longitudinal plate above alluded to . * e ° r diS'pieces of fir, each 12 X 6, bolted toget liel ' w hiel‘ 0 „between the iron pillars are reduced by bra^ ^ ga uiecharge the bearing on to the iron pillar, and a j^ aC [ 1 en jreceive the wood cradling for the spandre ^y a c °this plate is let into the wall, and is recei,, j do"' 11 -.. gf tof teak wood, on to which it is bolted and loc ^ ^ ph 1 ‘ er ,A considerable improvement upon the sy® nC h eng’de Lorme was effected by Colonel Emy, a ^ eI np*fin the early part of the present century, ny, , wee n t )i®of laminated ribs. The principal difference veJO en a .consists in this, that in Colonel Emy a 1 the c j g9 gdirection of the fibres of the wood coincides^ ^, e m uC g0 »9ture of the rib, and in consequence the join ^ agt jcity> ^in-frequent, and the rib possesses greater v j 0 ient ® rfl0 f,slightly to yield, rather than break, under < ^ p,orin eThe alternate thicknesses break joint, as m ^ 9 t

and all are securely bolted together. . f as lii° a t a a “

The earliest roof constructed after , g g5 * ola te j

Marac, near Bayonne , in 1825. 'H ie f* oU nd °n t0 q, i> a °the main ribs are formed of planks ben ^ , ron g

&

tne mam nos are tormea or pmu*=> , j r on P ‘". n ' c ip* 1 -to the proper curve, and kept together y p r 'also by the radiating struts which ® ,l PP 15n die fi*they are in pairs, notched out so as

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