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diameter, are fixed small ribs of wood, which being connected with the main and minor ribs bv means of sash bars, become available as ridges. The space between
them and the ribs is glazed and finished on the same system as that adopted in the flat roof of the building, the sash bars being set at an oblique angle, or "herring - bone" fashion, in order to assist the conduction of the water, and prevent its lodging against the lower putty bed of each pane of glass over which it trickles. Along the summit of this semi-cylindrical vault runs, from north to south, a narrow lead path, in order to afford access to the apex of the roof, and to provide a means of lowering down workmen to repair any damage that may possibly happen to it. As the quantity of water discharged from this vault must necessarily be considerable, it was deemed advisable that the 24-feet aisles on each side of it should be covered with lead flats, instead of the ordinary glass roofing. These lead flats presented also the further advantage of being so completely connected, as to form solid abutments, steadying the feet of the ribs. In order to convey any pressure in the way of thrust, that the ribs might exert, to those
Section of Transept Rib, andlprinpng of ditto. points best capable of resisting
strain, horizontal trusses of wrought-iron were constructed beneath the lead flats, consisting of bars capable of being keyed up at any time from the lead flats, and thus any tendency to movement, on the part of any ot the ribs, would be immediately transferred either to the extreme north and south ends, where their feet are securely tied together by the cast-iron girders which cross the transept at those points; or to the angles where the transept intersects the nave, and where the whole force of the vis inertia of the nave roof would serve as abutment.
The general effect produced by this semi-cylindrical roof covering the large
elms beneath, is shown in Plate I.
The external enclosures or walls, as seen from within, on the two upper
stories, consist of glass in wooden sash frames, inserted between columns 8 feet from centre to centre, and louvre frames for ventilation, surmounting the sash frames. On the ground story, boarding takes the place of the glass; and the height being 24 feet instead of 20 feet, an additional tier of ventilators is inserted. The columns at 24 feet apart being of iron, the intermediate ones, dividing that width into 8 feet compartments, are of wood.
As no less than 1,500 sash frames have been used, they may justify a few The »»u-framc». words of description. To quote again from the Transactions of the Institution of Civil Engineers, we learn that " the sash frames are 2$ inches thick, with seven "bars in their width; the bars being 2$ inches deep, double grooved for glass. "Wrought-iron bolts, tr inch diameter, pass completely through the sash bars and "sash frames, at the points where they are attached to the columns; and thus a "chain tie is kept up all round the building, in order to prevent displacement of "the sashes either bodily or in portions, by the pressure of the wind. To further "guard against the same action, timber bridges, 3^ inches byl$ inch in the centre, "are fixed across the middle of the length of the sash; and at the internal "angles, where the wind will exert its greatest force, iron rods, half an inch in "diameter, are fastened from column to column, pressing against the wooden "bridge, and converting it into a continuous strut, bearing up against any force "applied to the exterior of the sash. In order to glaze the sashes, the glass "is slipped down between the bars, and provision is made for mending, by "causing one groove to be cut deeper than the other, so that the glass may be "slipped in from one side, and puttied into its exact place. A similar pro"vision is made for mending the roof glass."
As whatever lateral force the wind may exert upon the building will beThe»r.uonor principally received by these sashes, it may not be inappropriate, in considering£!SonUwTu'iidthem, to advert briefly to the general question of the action of wind upon the ing' building.
On the 15th of January, 1851, a meeting was held at the Society of Arts, at which Mr. Fox, one of the contractors for the building attended, to afford the members of that Society an opportunity of asking any questions as to the general points of stability and durability, on which they might desire information. One of the questions proposed was, " What would be the effects of the wind Howdweribedby "on such an extensive surface as the building presented, and what means were "taken to counteract them?" and as Mr. Fox's reply embraced with remarkable clearness the principal facts connected with the argument, we shall give it at length. Mr. Fox replied " that the building rested on 1,060 columns on the *' ground floor, and the most likely direction for the wind to have any injurious "effect on the building, must of course be in the direction of its greatest width, "which was 1,800 feet as compared with 400 in the opposite direction. These "columns rested on cast-iron plates based upon concrete; and there was no possi"bility of their rocking about without the base-plates being broken. Above "these plates were sleepers, that carry the floor. They were 13 inches in depth, "and fitted accurately up against the two sides of the column, and running "transversely from one side of the building to the other; so that it would be "very difficult to conceive that one of these columns could be possibly upset "until it was actually broken in two. And again, at the top the columns are "united together by cast-iron girders 3 feet deep, and four columns are framed "together, very much as they would frame a table. Now to break the column,
"they must exert a force equal to that of twice the transverse strength of the "column. According to the experiments it was found that six tons was the "bearing weight, and 12 tons the breaking weight of the columns in the centre. "Now, 1,060 columns multiplied by six tons, the bearing weight, was equal to "6,360 tons; so that it would be necessary to exert a force equal to 6,360 tons, "at a height of 24 feet from the ground, before they would be able to blow down "the building, and he was now treating of the building independently of its "bracings. The greatest force of wind ever known had been computed at 22 lbs. "to the superficial foot. Taking 28 lbs. as the force, and assuming that they "could have a gust of wind which would strike the whole side of the building "from top to bottom at the same moment, the total force which could be brought "against it would be from 1,400 to 1,500 tons. Now, they had got power to "resist it of 6,360 tons, not taking into account the bracings and the other con"structions and offices which were within the building, and which must of "course add to its strength. The building had been tested in the late gale, when "Colonel Reid ascertained that the force of the wind was 19 J lbs., and it did "no harm whatever; and that was at a time when the roof was not on, and the "building was quite exposed." n.e ventilator.. The ventilators, to which allusion has been made, and which are shown in fig. 15, fulfil an important office in the building, acting as the organs of respiration
to the whole body. The total surface of ventilation is nearly 50,000 superficial feet, and the whole has been so arranged, that, by the application of one man's strength, at about 90 different points, the whole may be opened, closed, or set and secured at any desired angle, simultaneously. The ventilators themselves consist of galvanized iron blades of an S form, 6j inches wide, fixed on pivots at 6 inches from centre to centre. Of these there are eight in the wooden frame inserted between the columns and the sill on the ground floor, and six in those which surmount the sash-frames on the two upper stories. The section of the blade is of a novel form, and is calculated, when open, to afford the minimum interruption to the passage of the air, compatible with being weather-tight at all times. To each blade, in the centre of its length, are attached small iron brackets furnished with eyes, through which pins are inserted; which pins are secured in a species of wooden rack. These racks are connected with cranks attached to iron rods, to which a movement of torsion is conveyed by screws and powerful levers. A moderate exertion of the strength of one man applied to one of these levers, suffices to regulate, with facility, no less than 600 feet superficial of ventilation. The decoration. Hitherto the building has been considered only in a structural point of view. The vivacity of any impression it may convey to the casual visitor will, however, probably depend more strongly on the system of decoration which it has received from Mr. Owen Jones than on any of the constructional details we have been describing. That gentleman, whose studies in Egypt and in the East generally,