385

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°H th e roof the entire weight of the roof is thrown

H * t ^ a ^ 3 at feet of the ribs, which are considerablyhie Wa u e Principals, so that the weight of the upper part ofhie l 0 3 8e rves to diminish the effect of any thrust against

. Dart of tl,« A .,.11

18 hi

- 2 % effected.

Part of the walls. A great saving of wall-material

I]ig .

■y bri(j l in:: 'Pl e has often been applied in the case of rail-

r °>im ~ &eS> an< ^ one has been erected upon it at the London snonj 0 ^ l ' ie Great Northern Eailway. In this instanceOf 6o arels are of cast-iron.

triltl: -eDf° meW ^ at tnilar construction was the roof over the° f rtiai n a" Crystal Palace , Hyde-park ; the principals,( r 'hs, of which were placed at intervals of 24 feet from,tl eh es . Ce ntre, and were made up of three planks, two of 2NuldV^kHess, and the middle one 4 inches thick, witht 'itch , P^ce on the under side 2 § inches thick, and twoto S 8 th er ^ an hs a t t he top, each one inch thick, and nailed®°r>n ec t e to form the gutter-board, the whole being firmly"''ts 0 (- together by wrought-iron bolts passing throughj n same material, running at top and bottom of each. pi 0r der to form the ribs with the proper curvature,NerT k > inches wide, was cut on one edge to its? a jled t Q Staental form, and the two complements cut off wereN&g n tle lower or straight edge of the flank; the wholetogether, ‘

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H

i h

Pren

so as to breach joint throughout, thee ach section so cut being about 9 feet 4 inches.

t ” °a tip 131 *- 011 ^ or tke rece Ptl° n of the ends of the purlins° f ° r lt a ' 8 W ! se : the two wide planks were cut across, so as

T rd Cavit y to admit the ends of the purlins, the partstl^ton a i 8 thus separated being connected together byj 1 ^Porafij 1ates< The openings thus made were filled in° ®t in ^ ' v *th wooden blocks, until the joiners were ready

a str 6 en< ^ 3 tke P ur ^ n3,

p Cc ordi nrr ® n Sth of every purlin had been duly calculated,° * t3 relative position in the arch ; thus, the threein' 1 ® 134 . • P Ur hns, having the greatest strain on them, were

a inr.hoo 1_ a i - 1 m_ -I. n

pches in depth by 4^ in width ; while the four•Hin' 1 ^ °n each side, having gradually less strain toregularly in depth to 9 inches, all having thet 0 e nd l s j, a3 those at the crown ; whereas, the lowest oneP 8 Cantr 0 ^ le ar ch, being nearly horizontal, was increasedo f °f 8 inches by 6 j inches.

88 Caii e j a ?^ P a * r raa ' n 1 - ‘b s were two intermediate ribs,^ P by g . *n ordinary roofing, common rafters, 4 inchesbb^ 8 ’ >n 1 / lc ® es wide. On the top of these were gutter-Spj 8 ' ’J?l le Wo thicknesses, as those described for the mainof.^d gjj Sutters, each 5 inches wide, were formed by twoski- ®<>a r j t8 ’ ' et into and nailed securely to the upper side

C^v 8 - ... ■

fhese fillets also served as abutments for the

for ,v rS > ^hich were rather larger in section and size than

nan 8k yhghts in other parts of the buildin,11 ton. Ue d at tVio cU n i

Each

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the rin~ at tke iower end to the fillet, and at the upper

l UQ " v “'-* iu»»w cum iv mo unci; auu at mo u^oi,

H tk®° Ve a a S e ~piec e , which was formed of three pieces of fir,W a ‘ok ; tf . 01 . > the lower section being 4 inches wide by°>ia! r Piece g , ln *ddle piece, 3 inches by Iftlis inch; and the

: inches by l|ths inch, the latter having a groove’ ‘ The three thicknesses were

to suit the curvature of the arch. The

low^^ry 1 ^ to rec eive the glass,

0,1 tI en ®of ° r< ^ er to su ’t th e _ _ _

SC. > 8a d ridge-piece was carried down to, and rested8 tit ’ 83 in n flat> Condensation-gutters were formed in thisIsUtt'^hod e toofs of the aisles and avenues, but by a differ-0 || 0 8 r 'bo ilr( j ^ sloping fillet was nailed on to each side of theHillside ’ ail< l continued from the springing of the archOf 3 ^ to r ’ *? ihe corresponding point on the other. WithK -’j„ In ilte ridges in their places, wrought-iron rodset ®r, extended from the purlins to the under sideIn order thoroughly to carry off the rain-water,5 F

from every part of the roof, all the skylight bars were fixeddiagonally from the ridge to the gutter, and the water col-lected in each curved gutter, was carried into a sloping troughat bottom, and thus discharged on the lead flat, which wassloped towards the water-heads at tops of the hollow columns.Looking at the roof from the lead flats, the whole had aherring-bone appearance.

Temporary ladders, fixed to suit the arched roof, were usedfor fixing the ridge-pieces, skylight bars, &c.; but, in order ;to facilitate and expedite the glazing of this roof, travelling iscaffolds were used, which could be raised and lowered atpleasure, by means of ropes and pulleys, and by the power iof four men working a crab engine, placed on the lead flatcontiguous. Each travelling scaffold was formed with sides jand ends, and had boxes, in convenient positions, for the glass,putty, and tools ; the whole running on small friction rollers,suited to the tops of the ridge-pieces, which served as rails;without such a contrivance, the glazing of the roof of the •transept would have been an almost endless job.

The vertical supports for the roof consisted of cast-ironcolumns placed one above another in three tiers, sixteen in :each tier on either side. Above the top pier were fixed thetrussed girders, 3 feet in depth, spanning from column to 1column, which were each 24 feet from centre to centre ; and 1across the intersecting lines of the middle arch, were two \double trussed girders, 6 feet in depth, and corresponding jin length with three spaces of 24 feet each; except under Ithe columns of the south-east angle of the transept, the wholewere placed on broad base plates, which rest on concrete foun- 1dations. In the exceptional case, the foundation consisted ofa solid brick pier, built in cement. At the top of each columnwhich supported the roof, was a cast-iron socket, 4 feet 4 inchesin height. The use of these sockets was to receive what maybe termed the vertical legs or supports of the ribs, and which jformed parts of the ribs as framed together on the ground. In ;order to resist the lateral thrust of the roof, a strong gangway, j24 feet wide, was formed on each side of the transept, which 1 >may be considered as an abutment, being constructed of jstrongly-framed and braced flooring, supported by trussed igirders. The materials of the building in Hyde-park, were ■afterwards used in constructing the Crystal Palace atSydenham.

The following is a description of a system of constructionin roofs, invented by M. Laver, architect to the late king ofHanover. It is also applicable to bridges, and similar works.

This new principle consists in a combination of the twoprincipal forces of materials—that of resistance to compression, j 1and resistance to tension. The first of these forces has beenused from the remotest periods in the construction of bridges, |and arches in general; the second has been more lately 'employed—at least in Europe —for the construction of sus-pension bridges by the application of chains. The firstrequires great masses of materials and strong abutments; the j

second requires less materials than the first, but secure fasten- |

ings, for the chains are frequently obtained with difficulty. !Very sensible vibrations and undulations are experiencedwhere this last mode of construction is employed. ; ;

The disposition of the principal parts of this truss is that ; 1of two segments of a circle, placed with the concave sidesopposite each other, and tied together at the extremities, beingfurther connected together, or rather kept apart from each ;other, by vertical and diagonal struts. Imagine two bows sodisposed ; it will be perceived, the chain a k g, fastenedat the extremities of the upper bow, acts with the positiveforce of tension, which the strongest materials possess, !varying from 10 to 20,000 lbs. for every square inch ofthe transverse section of the several kinds of wood employed 1