xxviii

EXHIBITION OF 1851.

levels already given. In the lower tier these columns are 19 feet long and in thetwo tipper 17 leet, between each of which are inserted short pieces, each 3 feetlong, of such a form that they serve to support girders in horizontal tiers at threedifferent levels; the bases of the columns are also separate pieces, and vary inlength to suit the different levels of the site. No less than 3,300 columns wererequired altogether. The girders, part of which are of cast and part of wroughtiron, arc all of the same depth, 3 l’ect (with the exception of four, which are 6 feetdeep, and which occur in the roof at the intersection of the Nave and Transept),thus producing continuous horizontal lines through every part of the Building.The girders are all similar in appearance, forming a kind of lattice-work, by whicharrangement great strength is combined with an appearance of lightness suitable tothe slender proportions of the columns. All the 24 feet girders are of cast iron, andof these there are 2,150 ; roof trusses of greater length, 372 in number, are con-structed principally in wrought iron, the general lines being the same as in thecast iron girders.

The lower tier of girders in parts of the Building more than one story in heightforms the support for the floor of the galleries, which are 24 feet wide, and extendthe whole length of the Building in four parallel lines (two on cither side of' thecentre avenue), interrupted only by the transept, round the ends of which they arecontinued. Numerous cross galleries connect the longitudinal lines, and the totaladditional area thus obtained is 217,100 square feet. The floor of the galleries con-sists of cross beams, undertrussed so as to distribute the whole weight that may bebrought upon the floor pretty equally upon the eight points at which the ends of thebeams rest upon the cast-iron girders; upon this construction are fixed the ordinaryfloor-joists and floor. The galleries arc readied by 10 double staircases with flights8 feet wide, so arranged as to communicate equally readily with either of the twolines of gallery between which they arc placed. In those parts of the Buildingwhich arc more than two stories in height there is a second horizontal tier of girdersat a height of 20 feet above the gallery ; these do not support another gallery, butserve to give stiffness to the columns between which they arc fixed. The upper tierof girders and trusses in all cases supports the roof, which is the most novel andinteresting portion of the whole structure. In its general form the roof is flat, butconsists in detail of a series of ridges and furrows, the rise and fall of which isbut small. The roof girders or trusses being 24 feet apart longitudinally are madeto carry on their upper edge the main gutters in the transverse direction of theBuilding. The space between these is spanned by light wood beams or raftersingeniously contrived, so as to support the glass roof, and at the same time to carryinto the main gutters both the rain water falling on the surface of the roof, andthe condensed vapour forming under it. The total length of the gutters used isabout 24 miles. Between these rafters, the glass roof is supported by light woodensash bars sloping upwards at an inclination of 2^ to 1. The advantage of this formof roofing for large areas is its lightness and economy. The glass of the roof i sinserted into the sash bars, which are grooved to receive it. About 200 miles of