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of these trusses is shown in fig. 8. In order to give additional support to them, four extra columns have been attached to those situated at the intersection of the nave and transept.
The direction of the 72-feet trusses of the roof of the nave being of course the roof trum*, transverse to its length, and the trusses occurring at intervals of 24 feet, it became iongitudin»ny. necessary, in order to perfectly steady them, that timbers (shown in section and elevation at BB, fig. 7) should be thrown across from one to the other, in
the direction of from east to west. These timbers are introduced at intervals of eight feet, being placed directly over the vertical standards of the roof trusses.
Between the top of the truss and the end of these timbers is placed ll box (shown in section and elevation at C C C, fig. 7), running along the whole length, and attached to the upper flange of the truss, forming a gutter of considerable capacity, for the purpose of carrying the water away from the roofs to the hollow columns, through which it ultimately descends to the drains.
The peculiar form of the timbers, spanning from truss to truss, and the offices they are called upon to perform, together with the fact that no less than 20 miles of them are required for the construction of the roofing, render them worthy of a detailed description. They are now known as the Paxton gutters. The form of their section is clearly shown in fig. 9.
They consist of pieces of timber 24 feet in length, five inches wide by six inches deep. On the upper surface, a semicircular groove (C, fig. 9), If th inch radius, is cut, in order to receive the external water from the roofs. On each of the two vertical sides of the timbers an oblique groove (BB, fig. 9) is cut, in order to receive the condensed water, which, trickling down the inner surface of the glass, finds its way to these small channels, and is carried along them to their ends, where oblique cuts connect them with the box-gutters.
As the length of 24 feet would be too great for the gutter to carry itself without bending, or, as it is technically termed, "sagging," the alternative presented itself of either very much increasing the scantling, or contriving some system of trussing. The former was rejected on account of its heaviness and unsightly appearance; it was therefore determined that a rod of iron (shown at A AAA, figs. 9,10, and 11) should be passed beneath the Paxton gutter, should be secured to its two ends by cast-iron shoes (B, fig. 10), and should press up, at eight feet intervals in its length, two cast-iron standards (C, fig. 11), so as to effect a camber or rise in its whole length of 2 J inches. So trussed, the gutter is capable of supporting no less a weight than li tons. A semicircular cut is given through the depth of the gutter at both ends, so that when two are placed end to end, the water may flow down to the box-gutters through a circular cavity. Twenty-seven notches are marked by a template, and cut on each side of the upper edge; a few of these are shown at D D D, figs. 10 and 11. The Paxton gutter, thus complete, is attached to a flanged iron plate, bolted on to the edges of the box-gutters, so that the parallel lines of gutter may form a continuous tie from the eastern or western ends of the Building to the transept, and so that, should the wood shrink in length, it may not pull apart and distort the sides of the box-gutters.
Three of the notches referred to as being cut on each side of the gutters, are Their connection
n in-ill- l J with the roofing;
larger than the others, and on them bars of wood, 2 inches by li inch, grooved for glass on both sides, are notched down. These bars (shown at A, fig. 12) form principal rafters, and, being set at a pitch of 24- to 1, are fixed to a ridge 3 inches by 3 inches, grooved for glass on both sides. The angle at which these are fixed being the one which Mr. Paxton, in numerous experiments, has found to be best adapted to the construction of similar roofs.
One of the principal reasons which led to the adoption of eight feet and not i<enon« for fining
. n n 1 1 JT their pitch at 8
more, as the pitch of these gutters from centre to centre, was, that the distance feet
The method of construction of a 24-feet bay of this system of roofing is shown A 2jieet bay of in fig. 13, and from it the reader may be enabled to follow the description we are about to give of the construction of a length of roofing.
The gutters, principal rafters or main sash-bars, and ridge being fixed in How constructed. place, the long edge of a sheet of glass, 4 feet 1 inch, by 10 inches, is inserted into the groove of the principal rafter, and a sash-bar (shown in section at B, fig. 12), 1 inch by I5 inch, also double grooved, is then put on to the other
the advantages of Mr. Paxton's roofing.
long edge of the glass. The sash bar is then brought down and secured at the top to the ridge, and at the bottom to the edge of the gutter; the lower edge of the glass bedding on putty about three-quarters of an inch wide. A slight blow to the lower end brings the upper edge of the glass home into the groove in the ridge. The glass being then pressed down, the putty is made good in the grooves externally, and by the repetition of this simple operation, the essentials of this system of roofing are constructed. «y es one of One among the many advantages of Mr. Paxton's roofing is its extreme lightness. In the instance of that of the Exhibition Building, the whole of the roofing (the weight of the trusses that support it being of course deducted), weighs only 3i lbs. per foot superficial upon the average.
In order to mitigate the intensity of the light, and at the same time to assist in^wTM"^" In keeping the building cool, a canvas covering has been provided, extending over the entire area of the flat roof. The canvas is attached to the ridges, and allowed to hang down between them in a festoon. As one width of canvas is insufficient to reach from ridge to ridge, two are sewn together, the seam occurring in the centre, immediately over the Paxton gutter. The rain descending falls on the canvas, and clings to it by capillary attraction, creeping down until it arrives at the scam, where it passes through the canvas, and falls into the Paxton gutter; thus the danger of the passage of water which might take place through broken panes or imperfect putty-joints, is obviated, and the chances of leakage are consequently materially diminished.
In order to trace continuously the course of the vertical supports from the ground to the roof, we have not interrupted our description by detailing the nature of the flooring; but as that is one of Mr. Paxton's ingenious contrivances, it would not be right to allow it to pass unnoticed. In a paper, read by Mr. Paxton at the Society of Arts, on the 13th of November, 1850, that gentleman narrated the experiments which led to the origination of the present design, and stated that he had tried many methods, in order to find out the most suitable floors for the pathways of horticultural structures. After enumerating the objections to the use of stone and close boarding, he mentioned, that "he had ultimately been led to the adoption of trelliced wooden pathways, "with spaces between each board, through which, on sweeping, the dust at once "disappears, and falls into the vacuity below." He thus describes his application of these experiments to the Exhibition Building:—
"Whilst the accomplishment of this point" (the speedy removal of dust) "was most important in plant-houses, I consider it doubly so with respect to the "Industrial Building, where there will be such an accumulation of articles of "delicate workmanship. Before sweeping the floors of the Great Building, the "whole will be sprinkled with water from a movable hand-engine, which will be "immediately followed by a sweeping-machine, consisting of many brooms, fixed "to an apparatus on light wheels, and drawn by a shaft. By this means a large "portion of ground will be passed over in a very short space of time." The boards for the floor are l£ inch thick, laid half an inch apart, upon joists 7 inches by 2b inches, which rest upon large timbers or sleepers, 13 inches by 3£ inches, at intervals of 8 feet apart. Through the interstices left between the boards the dust passes, and the merits of this system of flooring are thus summed up by Mr. Paxton :—" It is very economical, dry, clean, pleasant to walk upon, admits "of the dust falling through the spaces, and even when it requires to be
as described by Mr. Paxton.
"thoroughly-washed, the water at once disappears between the openings, and the conclusion of "boards become almost immediately fit for visitors." «mitn.«ion of
Having now endeavoured to furnish the reader with sufficient detail of a small portion of the building, to enable him to use it as a scale, whereby to estimate the quantity of labour represented by a structure of the general dimensions we General dimers are about to enumerate, it may be stated that the total area of the ground floor is S""1,'""' 772,784 square feet, and that of the galleries 217,100 square feet. The galleries extend nearly a mile in length. The total cubic contents of the building arc about 33,000,000 feet; there are nearly 2,300 cast-iron girders, and 358 wroughtnron trusses for supporting the galleries and roof, 30 miles of gutters for carrying water to the columns, 202 miles of sash bars, and 900,000 superficial feet of glass. The width of the nave is, within 10 feet, double that of St. Paul's Cathedral, whilst its length is more than four times as great.
With a general knowledge of the construction of the nave, we may imagine the visitor, returning to the transept, better qualified to enter into the mccha- The tnuuept. nical details, and the amount of difficulties presented to his notice, by that great feature of the building. The arrangement of the vertical shafts, galleries, &c, is similar to that of the nave; the main points of difference commencing at the level of the flat roof. It will be remembered that the spaces to be covered at a height of iu mof. 64 feet from the ground, are, firstly, a main avenue, 408 feet long by 72 feet wide; and secondly, two aisles, each 408 feet long by 24 feet wide. It was determined that a semi-cylindrical vault should span the larger of these areas, and for that purpose semicircular ribs (see Plate I.) extend from side to side, their ends being inserted into the hollow columns, whilst they are steadied by the insertion between, and at right angles to them, of stout timbers, 9 feet 2 inches from one another, acting as purlins.
The structure of the ribs is shown in fig. 14. To quote from a paper descrip- Conitrurtion of tive of the building, read at the Institution of Civil Engineers, on the 14th of tilLept^oof!" January, 1851, "they are made in three thicknesses of timber, cut into segments, nwription "9 feet 6 inches long, of a circle of 74 feet extreme diameter, the centre thickness "TnuiMcti'o'ni of "being 4 inches by 134 inches, and the outer, or flitches, breaking joint with the civil ungin'eeTM." "centre, being 2 inches by 134 inches. The Bitches are nailed to the centre "thickness, and iths inch bolts, about 4 feet apart, on the segment, traverse and "bind together the three thicknesses. On the extrados, or outer circum"ference of the wooden arch thus formed, two planks serving as a gutter board "11 inches by 1 inch, and a bar of iron 2 inches by iths inch, are bent to the "curve; and on the intrados, or inner circumference, a piece of timber, 7 inches "by 2 inches, moulded to correspond with the form of the columns, and a bar of "iron, 3i inches by -|ths inch, are also bent to the curve. Bolts, at intervals "of 2 feet from centre to centre, passed through the depth of the rib, unite these "additions to each other, and to the main rib, which, thus increased in scantling, "measures, complete, 1 foot 6 inches by 8 inches." In order to perfectly con- Thetranteptro«r,
nect these ribs, so that any force exerted, by wind or other causes, tending to o """""
the displacement of any one of them, may be distributed over the whole mass, iron rods have been set diagonally, forming a complete reticulation over the whole inner surface of the roof. The main ribs are fixed spanning the transept, at intervals of 24 feet from centre to centre. Each of these 24 feet widths is divided into three parts, and at 8 feet from one another, and from the main ribs, minor ribs are introduced. Between them again, but being semicircles of larger