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fr ith ft

th(, 8 fi Ile Purlin® already referred to. The curved ribs area fi„| , rm 'y drawn together and attached to one another, andCor^ lB framework is formed, upon which the covering ofjv' ron an( l glass is laid.

b*atn q ? r ^ nc ’P a l girder is composed of a wrought-iron deck-in c i,. inches in depth, with a plate 10 inches wide and \be aill • Uc ‘ < ' riveted on the top. The upper flange of the deck-ing. 18 inches wide and inch thick ; the lower flange 3'fhi s ' Vide and . one thick : the web is about § inch thick.ot l'er CUrVed r *i J i® formed of seven pieces connected with eachby ^ at tl*e points where the radiating struts are attached,6 f eg ans °f plates riveted on both sides: these plates areal So ° n S> T inches broad, and ^ inch thick. The beam isfr 0tn pugthened at the haunches, for a distance of 27 feetf»st en l e s Pringing, by plates 7 inches broad and § inch thick,b>y rivets.

ffo m e ^ e are 6 radiating struts in each rib, varying in length8f>rin„- fr et to 12 feet, the lengths increasing from thei ,0 t ^ le centre. They are similar in section to theth^ * >a S> ^ ut are only 7 inches in depth, being attached toPlat e8 and to tie-rods, by means of wrought-iron linking-®f th e ' The top of the strut is made to touch the under sidePlate, Pri " ci P al > in which position it is clasped by linking-P|ie a ^ d secured by a bolt l£ inch in diameter,toils tle - rod s in each rib are composed of three lines ofthese 6t ' Veen the two extreme radiating struts, and fromhvo lin truts to t ^ le extremities of the principals they are inS q u es > the sectional area in each case being the same, viz.te inches. The ends of the tie-rods, which are pre-Wtty e ' Vlt ' 1 e yes to receive the bolts, are placed side by sideh th en n Unking plates, attached to the struts, and a bolt"Hy el 0 i )ass *d through them : it will thus be evident, that if’•pon n S a tion takes place in the ties, the struts will be acted

Tli e

*° tl le e t ^’ a o on al braces extend from the bottom of each strut8 hu tg P of one next towards the springing ; they hold theNs t tj, ^ U P against the principal, and at the same time°^fou n( j e . tle ' rod3 in their duty. These braces are formed^"sirit? ;, lron > If inei 1 in diameter, secured at top by bolts

So

as

nt viiuim/uoij cvmui vu uv UUHO

V»»gh fr'e linking-plates, and at bottom by wedges,

i re( j 2 lv e an opportunity of tightening them up if

^ach

8r >d tj , C0In partment of principals is thus separately trussedI'Ssjjq ’ and the whole made fast at the extremities by■ ir> a St . ra P or stirrup iron round the back of a metalJ boi ted w l'ich each end of the girder rests, and to which itl °th e at the side; the jaws of this stirrup-iron are attachedf Tl le Xtr ernities of the rods by wedges.e8 bti» 6rida °f the principals are fixed in cast-iron chainsg a ll, Q r 0tl on e side of a metal pillar, and on the other on theJToti th e Upon box-beam : those upon the pillars are cast8s t Up 0n Up P er ca P> and those upon the wall and box-girderP°n a tvv ° rollers, which can travel a space of 3 inchesplate, so as to admit of expansion and

til are eac ^ P° rmed by a combination of three

f f ^ f Win f Cen tre T iron running straight from principalhi? tQ e ach * and those at the sides branching off at 5 feetOq 'as a r „ end ’ 80 as t° strut the girder in three points.

, iv; Oil Ul/ ino giAUQi lil Ull CO J^UI1119< The

l(l .® secured to the deck-beam by angle plates, fixed" > *be o',v es ’ one bmb being fixed to the blade of the purlin,ff a ddit' to ^e deck-beam.

t^ th ege '° n to the lateral trussing which the ribs receivejj 0 tOff e P u rlins, diagonal braces are fixed between eacha *i<l P° n< ling struts, connected at the top with the pur-a a c bottom with the linking-plates, by bars of their’°t. jj ln S i thus the ribs are all braced together, and5 G

a firm and rigid mass of framing, formed to carry thecovering.

The box-beam, upon which a portion of one side of theroof is supported, is 63 feet 4 inches long; 3 feet 2 inchesdeep at the ends; and 2 feet 6 inches at the centre, theversed sine being 8 inches. The upper chamber is 20 incheswide, and 8 inches deep, and the body is 13| inches wide,by 1 foot 10 inches deep. The bottom, 19f inches wide, isformed of two rows of plates, -ftths inch thick in the middle,and ftths at the ends. The thickness of all the otherplates is -feths inch. On the opposite side of the roof, 17 castiron columns, 21 feet 6 inches apart, serve to support the ribs.The columns are 19 feet high from the base to the cap, and4 feet 3 inches from the cap to the metal chair. At thebottom they are fastened into stones of 5 tons weight each,at 3 feet below the base. The roof is covered with galva-nized corrugated wrought-iron and rough plate-glass. Thetotal cost was £15,000.

A patent has been taken out by that eminent engineer, Mr.Nasmyth, for improvements in the construction of fire-proofflooring and roofing.

These improvements consist in constructing floors and roofsof iron-plates, which are bent into the form of a segment ofa circle, or into a conical, polygonal, or other shape, by theordinary plate-bending machinery, or by any other suitablemeans. These bent plates are supported on chord plates, ortension-bars, which have their ends bent upwards, wherebythe plates are retained in their curved position, when sub-jected to pressure. The ends of the chords rest upon flangesof cast or wrought-iron girders, above which are cast orriveted knee-pieces, which prevent the bent ends of thechords from springing ; or, instead of iron-plates, angle,or T iron, bent into the required shape, and supported uponchords resting upon the flanges of girders, may be employed.Over these curved ribs, iron plates are bent, with their endsplaced underneath the bent-up ends of the chords. Thespaces above the iron-plates are filled up, to form the floor-ing with Portland cement, mixed with broken bricks andother suitable materials. The improved girders are formedby bolting iron plates to the. sides and tops of stone archesand chords, combined as before. The side-plates are madewith flanges to support the arches and chords which formthe joists, and have also knee-pieces bolted to them to pre-vent the chords from springing, when the arch is subjectedto pressure. The arches and chords may be made of onepiece each, or may be made of several pieces, and bolted orriveted together.

Allusion has been made in a previous column, to the variousmaterials used for the covering of roofs, with reference tothe different degrees of inclination suitable for them. Thatchedroofs have been considered by some to maintain the mostequable temperature in the buildings covered by them ;keeping out alike the extreme heat of summer, and cold ofwinter. They are objectionable on account of their harbour-ing vermin, being easily damaged by wind, and dangerouslycombustible. The frequent repairs required, make thatchalso an expensive material. Besides straw, reeds and heathare sometimes used for thatching, and possess the advantageof greater durability. Tiles admit heat and moisture morethan good slates. Pantiles having no holes for nailingthrough, are simply hung by ledges, upon laths nailed to therafters. Plain tiles, laid in mortar, and overlapping so asto be double thickness everywhere, make a very good, thoughheavy covering. Tiles of a peculiar form, called hip-tiles, areused for covering salient angles ; and gutter-tiles, which aresimilar to them, but placed with the concave side upwards,in the valleys or reoeding angles. Slates are laid in various