Chapter III: Part 3

In this manner forty-two lengths of solid gutter, each twenty-four feet and a fraction long, were completed in a day of ten hours; and as the machine was worked double time, a length of more than 2,000 feet was turned out daily ready for use: this, it has been calculated, would have required the labour of about three hundred men to be employed for the same length of time. The absolute necessity for such rapid production will be evident when it is known that no less than 110,000 feet, or about twenty miles length, of such gutters were required--very nearly the distance from Buckingham Palace to Windsor Castle.

Finished as described above, the Paxton's gutters arrived at the building, where the first operation they underwent was that of cutting them to the exact length requisite. This was a nice operation, as the smallest deviation would have caused a difficulty in fitting them into their place, and to perform it a framework was constructed by which the solid gutter could be bent to the same curve it would have when fixed; a precaution that was necessary in order that the ends might be cut off quite vertically so as to fit together when in their place. At one end of this frame-work was placed a circular saw, twenty inches diameter, hung with a pulley and balance weight, so as to be moved up and down by means of a lever. The gutter being fixed in the frame by means of hinged guage-plates, one end was cut by the circular saw being brought down upon it; and at the same time another operation was performed: two cutters, placed in the centre of the circular saw, were so arranged that when brought down upon the end of the solid gutter they cut out a semi-circular notch, so that when the ends of two gutters were afterwards placed together there was a circular hole left, through which the water passed down into the main gutter. When these operations were completed at one end of the gutter, the guage-plates were taken off, and the timber was swung round on a pivot or crutch in the centre, and the same process gone through as before; the whole scarcely occupying two minutes. We shall presently have to return to this piece of machinery, as it was also used in finishing the ridge rafters.

The solid gutter was now transferred to the hands of the carpenter, who fixed at each end, on the under-side, a small cast-iron shoe; and two struts, nine inches long, were placed so as to divide the whole length into three equal parts--the struts spread out at the top in order to present a large surface of pressure against the under-side of the gutter; and tenons projected upwards, which were fitted into mortices cut into the timber. The lower end of the struts were formed so as to give them a firm hold upon a wrought-iron rod, thirteen-sixteenths of an inch diameter, which was passed under them and through the shoes, where it was screwed up with nuts; and the struts pressing up against the timber produced the requisite bend or camber. Twenty-seven notches, to receive the sash bars, were marked with a templet and cut out on each edge of the upper-side of the gutter; and a small cast-iron plate having been fitted on the under-side at each end, the Paxton's gutter was complete and ready for fixing. The under-trussing of the rafters increased their strength considerably, so that a weight of one-and-a-half tons was required to break one which was experimented upon.

The Sash-bars.

We will next consider the sash-bars which support the ridge of the roof and receive the glass. The total length which was required of these amounts to about two hundred miles; it will, therefore, be easily understood that mechanical contrivance for cutting them out became an absolute necessity; this Mr. Paxton appears to have discovered in his works at Chatsworth, as he mentions in his lecture.

The sash-bars are one inch thick and one-and-a-half inches deep, and are grooved on each side, besides having all the four edges bevelled or chamfered; all which was done in one passage through the machine. The plank which was to form the sash-bars was passed in at one end of the machine, between pressure-rollers; it then passed between cutters placed both above and below it, which made about twelve hundred revolutions per minute, and hollowed out the different grooves; and, lastly, it passed between circular saws which divided it into separate sash-bars, after which they had only to be cut into their proper lengths.[6] The exact length of each sash-bar when finished is four feet one inch.

In this state the skylight bars were sent to the building, where they underwent several finishing operations, necessary to make the ends fit down into the notches prepared in the ridges and gutters. Thirty of the bars were first placed together in a horizontal traversing-frame on a saw-table, on each side of which circular saws were fixed at the distance of the required length of the sash-bar; the frame was then moved forward against the saws, so that both ends of the whole set of bars were cut off simultaneously, and at the same time a cut was made at one end half-way through the bar, in order to form the shoulder against the gutter. They were then removed to another bench, where the end of the bar was bevelled and the shoulder formed by means of a small instrument having a handle with two projecting jaws fitting into the ends of the glass grooves of the bars; between these there was a small blade which, being pressed down, cut out the shoulder which had been sawn through in the other direction, and another blade was placed at the proper angle to remove the bevelled piece at the end of the bar.

One more process made the sash-bars complete for fixing--this was the drilling a hole at each end to nail them down on the gutter and ridge; and this was also done by machinery, to insure all the holes being drilled at the same angle. On one side of a horizontal bench were placed a set of four-inch driving pulleys (_a a_), with as many horizontal drills projecting towards the other side of the bench; a wooden traversing-plate (_c_) opposite each drill, and working towards it, received one end of the sash-bar, while the other rested in an inclined position against a wooden rail (_b_) placed longitudinally above the pulleys, having as many sinkings thereon as there were drills. The traversing-plate being then pushed forward, the sash-bar was perforated by the drill; the plate was then drawn back, and the same operation repeated with the other end of the bar, which left it ready for fixing.

The action of the traversing-plate (_c_) is shown more distinctly in the second engraving.[7] One out of every nine of the sash-bars of the roof is stronger than the rest, to serve for fixing the ridge previous to glazing. These extra-strong bars are two inches wide and one inch and a half deep, and were formed by the same machinery already described, by an adjustment of the different cutters and saws.

The Ridges.

The total length of these required was about sixteen miles. They are cut out of timber three inches square, in section, and are of the form shown in the diagram, with a groove on each side to receive the glass. This was also done by machinery which, with about five-horse power, turned out one hundred lengths of twenty-four feet in a day of ten hours, allowing the time for the necessary stoppages. After they had been delivered at the building, these ridge-pieces were cut to the exact lengths by means of the same apparatus used for the solid gutters which has already been described. At each end of the ridge-piece two holes were also drilled to receive dowells to connect it with the adjoining length. By no other than mechanical means could the immense number of holes thus drilled have been placed so exactly that those in the opposite ends of any two ridge-pieces should correspond precisely.

The different essential component parts of the roof having thus been described, we propose to take the different members of the construction in succession downwards.

The Glass.

But first it may be mentioned here that the glass used throughout the building is sheet, on an average about one-sixteenth of an inch thick, and weighing one pound per foot superficial. This gives an aggregate weight of about four hundred tons for the whole of the work, the greater part of which was supplied by Messrs. Chance and Co., of Birmingham. Each square is forty-nine inches long and ten wide, the greatest length of sheet glass that has ever been made in this country. The manufacture of this kind of glass is of comparatively recent introduction into England, though practised for some time on the Continent; and the rapid progress made by the manufacturers alluded to must be in a great measure attributed to the wise removal of the fiscal burden on the article, made by the late Sir Robert Peel. That lamented statesman, with his usual foresight, doubtless contemplated that great social benefits would follow from that enactment; and it is, perhaps, not too much to say that, but for Sir Robert's enlightened measure, this "huge pile of transparency" would never have been reared.

The Box Gutters.

It has been mentioned that the triple gutters deliver the water into main gutters running in the transverse direction of the building; these are formed of wood, with a bottom piece, into which are grooved two upright sides, they are firmly bolted down upon the upper flange of the roof-girders, and where these are quite horizontal the fall in the gutter is given by a false bottom laid to a slope. Of these gutters there is a length of about five-and-a-half miles in the building, which, added to the aggregate length of the Paxton's gutters, makes a total of about twenty-five-and-a-half miles of gutter.

Roof Girders.

These are of cast-iron, where not more than twenty-four feet long, and the rest of wrought-iron. The cast-iron ones are precisely the same in appearance as those used for the galleries, but lighter in metal; a separate description of them is not, therefore, necessary. The weight of each of these girders is twelve cwt., and each was proved to nine tons previously to being used; but it is calculated that the greatest weight they may have to bear will not exceed five tons: the total number required was about 470.

The wrought-iron girders, or trusses, are partly forty-eight and partly seventy-two feet long, to span the avenues of those respective widths; the principle of the construction is the same in each. The top rail (if it may be so called) of the truss is formed with two pieces of [L section] iron placed back to back [double L sections], and the bottom rail with two flat bars [parallel flat bars], the total depth being three feet; at the ends these bars are riveted on to cast-iron standards, and the intermediate distance is divided into eight-feet lengths by other cast-iron standards, to which the bars are also riveted, and thus a framework of rectangles is formed. In the trusses forty-eight feet span there are, therefore, six such divisions in the length, and nine in those of seventy-two feet span. These are then divided in the direction of ONE of the diagonals by a flat bar passing between and riveted to those forming the top and bottom rails. This completes the constructional part of the truss; but to render the appearance more uniform with that of the cast-iron girders, a flat bar of wood (shown by the dotted lines) is made to form the other diagonal of the rectangles.

The trusses for a span of seventy-two feet are cambered or bent upwards about ten inches, which both adds to their strength and improves the appearance. The form and arrangement of these roof-trusses may be clearly traced in several of the views of the interior which are presented to the reader. The weight, when completed, of each of the trusses of seventy-two feet span is about thirty-five cwt., and of those of forty-eight feet span about thirteen cwt.

It has been already mentioned that four of the roof-trusses vary from the rest on account of the greater load they have to sustain. The depth of these exceptional trusses is six feet, and their length seventy-two feet, or the width of the main avenue, which they bridge over. The principle of their construction is similar to that employed in the lighter trusses; but the arrangement of the parts is somewhat modified. The top rail consists of two pieces of [L section] iron, placed, as before, back to back; but they are further connected on the top by a flat piece [double L sections with flat]. The lower rail is formed by two flat bars placed upright [parallel flat bars], and these are riveted at the ends to standards of cast-iron, which, however, are considerably heavier in construction than those before described; and they have also in the centre, at (_a_) two slots, or sinkings, into which the ends of two of the diagonal bars are riveted. The whole length is then divided into three equal parts, each 24 feet long, by strong CAST-iron standards at (_b_) the ends of which are riveted between the rails, and these spaces are again subdivided into three eight-feet lengths by WROUGHT-iron standards at (_c c_). The top of each standard is next connected with the foot of the next but one to it by diagonal flat bars, which, together with the short pieces fastened into the slots at (_a_), complete the figure of the whole, forming a kind of trellis-work, two diamonds in depth. In the diagram only half the length of the girder is shown.

The dimensions of the different bars of iron in this piece of construction are proportional to the amount of strain they have to bear. The two heavier out of the four trusses just described weighed when completed eight tons each, and the other two, which are of rather lighter construction, six tons each.

The riveting together of the wrought-iron trusses was performed on horizontal supports, on which the curve that they were to be made to was marked out. The bars having been previously cut to the requisite lengths, and punched and drilled with holes for the rivets, were laid out on the stages in the proper forms with the cast-iron standards, which were temporarily kept in place by bolts passed through some of the rivet-holes. The whole framework was then riveted up with red-hot rivets supplied from small portable furnaces, several sets of men being employed upon each truss, by which means as many as sixteen were completed in one day. The whole of the trusses, three hundred and seventy-two in number, required for the building were put together on the ground, and several ingenious mechanical contrivances were made use of to facilitate and hasten the work. To form some idea of the amount of labour that had to be performed, it may be mentioned that each of the trusses forty-eight feet in length, or the smallest, is held together by more than fifty rivets, requiring more than twice that number of holes to be made in bars of iron varying in thickness from a quarter of an inch upwards. About 25,000 rivets were thus required for the whole of the work.

Iron Drilling Machine.

The holes for the rivets were made partly by drilling and partly by punching. In the machine used for the former the bar to be bored was laid upon a flat surface forming part of the solid cast-iron stand of the machinery; the drilling-point worked vertically, and could be moved in that direction to suit the different thicknesses of iron brought under its operation. It was suspended at one end of a lever, with a counterpoise at the other. This lever was also connected by a rod and crank, with another near the ground, one end of which was formed into a tread to be worked by the foot. The workman, when he had arranged the iron in the right position under the drill, pressed his foot upon the tread; thus raising the counterpoise end of the upper lever, and pressing the point of the drill, which was of a spear-head form, down upon the iron. Underneath the iron to be drilled was placed a piece of wood to protect the point of the drill when it had passed through the iron. It was also necessary to moisten the iron during the operation, in order to keep the drill-point cool. Three men were required to attend to this work, which was not so rapid as the other method of making the holes by punching.

The Punching Machine.

The enormous power exerted by this piece of machinery renders it necessary that the stand containing the punch, &c., should be exceedingly solid, and it is formed accordingly by a heavy mass of cast-iron, in which there are two indentations, as seen by the engraving. In the lower of these the punching operation is performed, and in the upper there are shears for cutting off the ends of the bars when required. The motion is communicated to each of these by means of a cogged wheel at the back; but both the punch and the shears work in a vertical direction, slowly moving up and down with irresistible force. There is no sudden blow or jerk, which makes the effect the more striking, as the unpractised eye has no means of discovering the amount of the force which is being put in operation. It is, however, so great that, although the punching of a hole scarcely occupies two or three seconds, the iron becomes quite hot from the effect of the pressure. In using this machine, the workman arranges the iron bar on a solid rest, placing it so that when the punch descends it makes the hole in the position required. As soon as the punch has passed through the bar, the action of the machinery is reversed, and the instrument ascends again; during which time the bar is re-arranged, and the operation is thus continually repeated. This piece of machinery also requires three men to work it, if the bars to be punched are of considerable length, so as to require the ends to be held up; otherwise, one alone is sufficient; and in the course of a ten-hours day about three thousand holes can be punched out--the number, of course, varying according to the thickness of the bars.

Neither of the mechanical contrivances just described are novel inventions, though they are thus, perhaps, brought for the first time under the notice of many of our readers, to whom they may be so far rendered interesting from their being connected with the execution of THE building of the day.

The Adzing and Planing Machine.

At the Chelsea Saw-mills, where the reader has already seen the Paxton's gutters shaped out, another interesting piece of machinery was in use for these works, for the purpose of finishing planks to a certain size and thickness, called the adzing and planing machine. An adze is a tool used by carpenters to remove any unevenness in the surface of a board in a particular spot. In this piece of machinery two cutters are fixed to a revolving arm, under which the plank is made to pass; and as it does so the cutters remove a certain thickness from the whole of the surface. The arrangement of these cutters is very plainly shown in the annexed engraving. On the under-side of the same bench to which this apparatus is fixed, three planes are set, each at an angle of about 5 degrees, by which the under-side of the plank is brought to an even face, while the upper surface is operated on by the adzing-cutters, and in this manner the plank is reduced to an even thickness throughout. As it passes on it is brought between two circular saws, which are adjusted to the width which it is desired to give to the plank. It is dragged forward towards the planes and cutters by means of an endless chain, composed of open links; which chain passes over a wheel provided with projecting pegs, so arranged as to fit into the links. The plank is kept down upon the planes, and otherwise held in position, by pressure-rollers.

The Columns and Connecting Pieces.

The columns in the building perform three important offices. They support the roof and the galleries, and serve as pipes to convey the rain-water from the roofs. Their form, which is beautiful, both mechanically and artistically, was suggested by Mr. Barry; it is a ring, eight inches in diameter externally, the thickness varying in the different columns, according to the weights they have to support respectively. Four flat faces, about three inches wide, are added on the outside of this ring, so that when the column is in its place, they face nearly north, south, east, and west. The column may therefore be considered as a hollow tube, of the section just described, and of the same form at each end, having at its extremities horizontally projecting rings called SNUGS, through which the bolts are passed, to fasten the columns to the connecting-pieces and base-pieces. That the hollow form adopted for the columns is that best suited to obtain the greatest strength with the least amount of material has been abundantly shown by experiments, as even two straws placed in an upright position will bear a very considerable weight; it is that also seen in the structure of the bones of animals. Of these columns there are 3,300 in the whole building.

Those portions of the height of the columns which correspond with the depth and position of the girders form separate lengths, which are called connecting-pieces, as they unite the lengths of columns of the different storeys. These connecting-pieces have the same sectional form as the columns themselves, and, like them, are the same at each end, where there are projections cast on, which serve to support the girders, and which are provided with holes through which the bolts pass to connect them with the columns. These holes alternate with the projections to receive the girders, which projections are so formed that they clip others cast on to the ends of the girders, which will be hereafter described. In the centre of each projection there is formed a small notch which receives the key or wedge for fixing the girders.

The meeting faces of the columns and connecting-pieces were all turned in a lathe, in order that, when set up, they might fit so precisely as not to require any packing to adjust them in an upright position; and only in the cases of those columns which serve as water-pipes is any such packing introduced. In those a piece of canvass, with white lead, is put into the joint. An enormous amount of additional labour was involved by this proceeding, as no less than twelve hundred of such faces had to be operated on; but this did not deter the enterprising contractors, who were fully alive to the importance of the object to be attained. When fixed, the projecting "snugs," with the bolts passing through them, were covered by ornamental caps and bases of cast-iron, fixed after the rest of the work was completed.

The Base Pieces.

The lower storey of columns in every case stands upon base-pieces of which the upright portion is a continuation of the column, with "snugs" at the top, to correspond with those of the column, and standing on a horizontal bed-plate, from which "shoulders" rise to strengthen the upright portion. These bed-plates vary in size from three feet by two feet to one foot six inches by one foot, in proportion to the weight which the several superincumbent columns have to sustain. The longest dimension of the bed-plate is in the transverse direction of the building, in which the greatest overturning strain might be expected to act upon the columns. From the vertical portion of the base-pieces, sockets six inches in diameter project, in the direction of the length of the building, into which are fitted the cast-iron drain-pipes, which convey away the water brought down by the columns from the roof. The height of the base-pieces varies to suit the different levels at which the floor is supported above the ground. These levels had therefore to be determined in every individual instance previous to the castings being made. It was done, however, with such precision that, when they came to be used, they were all found to be of the exact length required for their situation. Of these base-pieces, 1,074 were required for the building.

Cast-iron Girders.

It has been mentioned that the columns supported girders at three different heights, dividing the greatest altitude of the building into three storeys; and that the lower tier of girders, where the building consisted of more than one storey, served to support a gallery.

These gallery girders are all twenty-four feet long and three feet deep, the upper and lower "flanges" or rails having a [T section] formed section with standards at the ends of similar section. The rectangular space between them is then divided into three equal parts, by uprights having a [+ section] form of section, and the three smaller spaces thus obtained have diagonal "struts" in each direction. The girder thus described forms a double truss, in which the diagonal braces are subjected both to the strain of compression and tension. At the top and bottom of the end-standards small projections are cast on, by which the connecting-pieces hold the girders; and at each end of the flat portion of the top and bottom rails small sinkings are cast, by means of which the girder is keyed up to its position. The flat portion of the upper and lower "flanges" of the girder is swelled out in width from the ends towards the centre, in order to increase the quantity of metal in that part where the strain is greatest.

The description just given of the gallery girders will apply to all the cast-iron girders throughout the building, of which there are 2,150; the only difference between them being, that those for the roofs or other internal portions, where no gallery is to be supported, are cast with a less amount of metal. The form of girder just described, which is unusual, was the result of several experiments performed under the superintendence of Messrs. W. Cubitt, C. H. Wild, C. Fox, and other gentlemen, previous to the commencement of the building; and the thickness of metal for the different parts of these, as well as for all the other cast-iron work in the building, was minutely calculated and determined by Mr. C. H. Wild and Mr. C. Fox, under the supervision of Mr. Cubitt, the President of the Institution of Civil Engineers, to whom the Royal Commission had intrusted the responsible duty of the chief superintendence of the whole of the work.

The Galleries.

To proceed to the gallery itself, supported by the girders just described. The timbers supporting the floor are so arranged that the weight of each bay of twenty-four feet square is distributed equally to the four girders inclosing it, and in such a manner as to bear upon them at the points immediately over the vertical standards.

In the transverse direction of the building two pairs of joists, eight feet apart in each bay, are formed into trusses by tie-rods, 1-3/8 inches diameter, passed through a cast-iron shoe at each end, and pressing up two "struts," which are made to bear against the under-side of binding-timbers running longitudinally, or crossing the joists, and immediately under them. The cast-iron shoes for the trusses are bolted down to the girders, and serve at the same time to receive the standard supports of the gallery railing. The ends of the binding-timbers are secured by bolts and oak suspension-pieces to the other two girders inclosing the square. Joists about two feet six inches apart bear from girder to girder parallel to the trusses, and resting on the binding-timbers. On these is laid the floor, 1¼ inches thick, grooved and iron-tongued. A light cast-iron railing, forming a kind of trellis-work, is fixed between the columns, and is capped with a round mahogany hand-rail. From the view at page 60 the arrangement of the galleries will be readily understood.

Testing the Cast-iron Girders.

From the very important office which the girders perform throughout the building, but more particularly those supporting the galleries, it was of the utmost importance that, previously to their being fixed in their places, the soundness of the casting should be proved; for it could hardly be expected that so large a number of girders could be produced without some of them being defective. The ordinary means of testing girders, by loading them with weights, would have occupied far too much time; and therefore an ingenious apparatus was devised by Mr. C. H. Wild for this purpose, by the use of which the testing of a girder occupied but a few minutes.

It consisted of a very strong cast-iron frame rather longer than the girder, the bottom of which was formed by two fixed beams placed eight inches apart, and supported a few inches above the ground. At each end of these a cast-iron standard was firmly bolted between them and rose to a height rather greater than the depth of the girder to be tested; on the inner faces of these standards two "shoulders" were formed, which received the projections cast on the ends of the girder, as before mentioned. Between the fixed beams below, at two points dividing the whole length into three equal parts, were placed strong cylinders, with rising pistons connected with a forcing-pump, together with which they formed a Bramah's hydraulic press. A girder being placed in this frame, in an inverted position, the force applied by means of the pistons rising from the cylinders acted upon it precisely at those points, and in the same manner, as the load from the gallery or the roof would do when afterwards fixed in its place.

The essential parts of the Bramah's press may be thus briefly described. It consists of two cylinders, the diameter of one being considerably larger than that of the other. The smaller cylinder is fitted with a solid plunger or piston, by means of which water may be forced from it into the larger; this being also fitted with a rising piston, the force is communicated by it to the weight which it is desired to raise. The power obtained by means of this apparatus arises from the distributive power of fluids and the practical incompressibility of water, and it is proportioned to the difference of the diameters of the two cylinders; so that if a pressure of one pound per square inch be applied on the surface of the piston in the smaller cylinder, and the piston in the larger cylinder present a surface ten times greater, the power is multiplied by that number; whilst, in addition, the lever power used in applying the pressure to the smaller piston is obtained. The cylinders are fitted with valves, so arranged as to prevent the return of the water from the larger to the smaller, while the apparatus is in action, and thus the power is accumulated in the former.

In the instance before us, the two 3-inch cylinders already alluded to in the proving-frame took the place of the larger cylinder of the ordinary apparatus; and they were connected with the forcing-pump by a strong metal tube. When a girder had been fixed in the frame for proving, the force-pump was worked till the pistons underneath the girder carried it off its lower bearings and pressed it upwards against the "shoulders," by which it was firmly held, and the pressure was then continued until the amount previously fixed upon as necessary for proof had been obtained. This was ascertained by means of a self-adjusting apparatus attached to the hydraulic press.

An iron cylinder 1½ inches diameter was placed in communication with the pipe connecting the pump and the press, so that the pressure obtained in it was, in proportion to its diameter, the same as that in the large cylinder; and it was fitted with a piston-rod, working in a vertical direction. This piston-rod was connected with a lever, from the end of which a scale-pan was suspended, at a distance from the fulcrum ten times greater than that of the point of attachment of the piston from the same. The weight of the scale-pan and lever were balanced by a large mass of iron at the other end. In the scale-pan a certain weight was placed, proportioned to the proof desired to be obtained; and the action of the pump was continued until the water, rising in the iron cylinder just described, forced up the lever, and with it the weight attached; and thus indicated that the pressure to which it was desired to subject the girder had been reached. The weight to be placed in the scale-pan was thus determined: the diameter of the lever cylinder being 1½ inches, and that of each of those in the proving-frame three inches, the pistons or "rams" in the latter presented together eight times the surface of that in the lever cylinder; which being multiplied by the difference of length of the two parts of the lever, determines the weight for the scale-pan to be one-eightieth of that to which it was desired to prove the girder.

The ordinary gallery girders were tested with a pressure equivalent to a weight of fifteen tons; but it was calculated that, when fixed, the greatest weight they would have to sustain would be seven-and-a-half tons. In one instance, for the sake of experiment, the pressure was continued beyond the proof weight of fifteen tons, to see what amount of strain the girders would bear without fracture, and it was found that a strain of thirty tons produced no injurious effect; but the girder broke with an additional weight of half a ton.

Roof of Transept.

We will now return to describe that portion of the roof which varies in form and arrangement from the rest, namely, the semicircular covering of the transept. This is supported by arched ribs, placed twenty-four feet apart, and constructed of Memel timber, in three thicknesses; the centre-piece four inches thick, with a 2-inch piece on each side of it. They are formed in lengths of about nine feet, placed so as to break joint; that is, the joints of the outer pieces fall upon the centre of the inner one. The thicknesses are fastened together by bolts passing through them about two feet six inches apart, besides being nailed at other points. On the inner circumference of the rib thus constructed there is then placed a piece of timber moulded to correspond with the form of the columns; and on the outer circumference two boards, each one inch thick, are bent round and attached to the rib with strong nails. On both the outer and inner circumference a flat bar of iron is secured by bolts passing through the whole depth of the rib, which, thus finished, measures eighteen inches in depth by eight inches in thickness. The ends of the ribs are fitted into sockets, formed by the upward continuation of the columns, to which they are attached by iron straps.

The ribs, which are supported by the trusses over the main avenue, have their ends bolted down upon a piece of timber secured on the upper portion of the truss; and they are further fixed in their places by oak brackets, forming a spreading foot on each side upon the same piece of timber.

Between these large ribs horizontal timbers, called "purlins," are fixed about nine feet apart, by means of cast-iron shoes, bolted both to them and to the ribs. These serve to support the minor or intermediate ribs, occurring at distances of eight feet apart; which consist of a single square piece of timber, having the two thicknesses of 1-inch board bent round their outer circumference, as on the main ribs. The boards form the gutters or furrows between which rise the ridges, in the same manner as in that portion of the roof which is horizontal.

The ridges, in this case, instead of being cut out of solid pieces, are formed in three thicknesses, bent round to the requisite curve, and so retained by small bolts tying them down to the "purlins." The sash-bars which receive the glass form, as elsewhere, the sloping rafters or supports of the ridge.

The space below the first "purlin" or plate at the springing of the arch, down to the level of the lead-flat beneath it, is fitted with louvre-frames for ventilation. The diagonal bracing between the main ribs has been already alluded to. Each set consists of four wrought-iron rods three quarters of an inch in diameter, having eyes at one end, by means of which they are secured with bolts, passing through the thickness of the ribs; in the centre they meet in a cast-iron ring, on the inner side of which the ends are screwed up with nuts.

The semicircular ends of the transept are filled in with tracery, formed by radiating timbers, strutted apart with short pieces placed in concentric rings. The circular heads of the openings are formed by iron castings screwed into their places, and the eye from which the radiating lines of the tracery proceed is also formed by solid iron castings bolted together. On the outer face the ribs of the tracery are moulded, and on the inner side glazed sashes are fixed, filling in the openings.

The lead-flat, twenty-four feet wide, extending the whole length of the transept, on either side of the semicircular roof, is constructed in a similar manner to the floor of the galleries, by under-trussing two pairs of joists in each bay. In the width of the lead-flat roof a horizontal truss is formed by flat bars of iron fixed in the direction of the diagonal of the 24-feet square bays, to resist any possible thrust or tendency of the ends of the ribs to open outwards at the springing.

The Facework.

The external inclosures of the building, on the levels of the different storeys, require but little description in detail beyond that already given. The sash-bars dividing the sashes of the upper tiers are grooved for glass similarly to those used in the roof, and were cut out by the same machinery. The glass was put in after they were framed together, so that it was necessary to arrange the ends of the bars that it could be slipped in at one end. As the bars of these sashes were of slight dimensions and considerable length, they were strengthened by wrought-iron rods passed through the sash-frame and the bars, and screwed up at the ends, causing the whole to work together. The sashes are held in their position by small cast-iron clips, which are bolted on to the columns; and as the surface presented to the wind by the upright sides of the building is of such considerable extent, wooden bridges are fixed against the sashes on the inside, by small cast-iron shoes bolted to the columns; and at the internal angles, where the wind would exert its greatest force, these bridges are further strengthened by wrought-iron rods half an inch in diameter, pressing against the back of them, which is grooved for the purpose, and screwed up at each end in the cast-iron shoes. In this manner a connected chain of resistance to any external pressure is established round the whole circuit of the building.

The louvre-frames, which form part of the face-work in all the different storeys, consist of a deal frame in which bent louvre-blades are hung on pivots at each end. These blades are of galvanised iron of an [S section] form. On the back of each blade is fixed a loop of thin iron, to which a rack is fitted; and by these means all the blades in each frame are moved simultaneously. A considerable number of these racks may also be connected, so that a large area of ventilation may be regulated at once.

The Diagonal Bracing.

From the total absence in this building of any internal division-walls, which in ordinary structures considerably add to their stability, it was thought desirable to introduce into the construction something to compensate for this deficiency. At several points in the length of the building, where a continuous connexion could be established transversely, the squares formed by the columns and girders on the different storeys have their four corners connected by diagonal rods, seven-eighths of an inch in diameter, having eyes at the ends, by which they are secured to the bolts connecting the different parts of the columns. In the centre of the square the four rods meet in a cast-iron ring, and are screwed up with nuts; ornamental faces are fitted into the rings, so that this addition to the construction is by no means detrimental to the general effect.

In a similar manner this diagonal bracing is introduced in a horizontal direction immediately under the floor of some portions of the galleries; of these there are twenty-two sets, and of those placed vertically there are, altogether, 220 sets in the building, and the manner of their introduction will be readily understood from the views of the interior.

The Staircases.

The double staircases, of which it has been mentioned there are eight in the building, consist each of four flights, about eight feet wide; two parallel ones, leading from the ground-floor to a landing, at the half-height, and the other two branching in opposite directions from the landing to the two galleries. The treads of the steps are made of a species of mahogany called sabicu, which is much harder than oak, and therefore peculiarly suited to the purpose for which it is here employed. The risers, or faces of the steps, are of deal. The stairs are supported by cast-iron girders, following the slope, the lower ones being fixed at the foot to stout timbers under the flooring, and the upper ends bolted to the cast-iron columns which support the landing. These columns are of the same pattern as the rest throughout the building, but only five inches in diameter. They are supported on concrete, and eight of them are required for each staircase. The floor of the landing is carried by lesser cast-iron girders, with flooring-joists.

The girders carrying the upper flights spring from the landing girders, and have their upper ends bolted on to the main girders supporting the galleries, which are varied in pattern for this purpose. The railing of the staircase is formed in separate cast-iron standards, one to each step, which are bolted on to the top flange of the girders; and the foot of the standard is so continued that the ends of the treads are fitted into it, and are thus supported. The pattern of these standards is assimilated to that of the gallery railing.

The hand-rail is formed of Honduras mahogany, with carved ends. On each side of the upper flight, which occupies the centre of a 24-feet space, connecting-galleries about eight feet wide are carried, establishing a communication between the two lines of gallery without descending to the level of the landing and then re-ascending. The landing is sufficiently high above the ground-floor to give ample headway for passing underneath it; so that the space occupied by the staircases on the ground-floor is but small.

The Floor and Foundations.

It now only remains to mention briefly the construction of the floor of the building, and the foundations for the base-pieces. The substratum of the site consists of gravel of an excellent quality, and sufficiently dense to have sustained, perhaps without any preparation, the load brought upon it by the bases of the columns. A thickness of concrete, proportioned in all cases to the amount of the weight to be borne by the superincumbent columns, and of such a size as to be two feet in each direction larger than the bed-plates, was placed upon the gravel, and the upper surface was finished with a bed of fine mortar to receive the bed-plates. In this manner it was calculated that in no case would a greater weight than two-and-a-half tons be borne by each foot superficial of the gravel--previous experiments having shown that a considerably larger weight could be placed upon it without any injurious effect.

The timbers supporting the joists for the floor are also placed upon small blocks of concrete, about one foot cube, at a distance of eight feet apart. On these are fixed the flooring-joists, and a deal floor an inch and a half thick is laid on them, as has been already mentioned, with intervals of about half an inch between the boards.

In order to carry off the water brought down from the roof by every alternate longitudinal row of columns, 6-inch cast-iron pipes are fitted into the sockets described in the base-pieces, and are carried in the lines of those columns through the whole length of the building, with discharges into the larger drains at the centre and at each end; the natural slope of the ground gives a sufficient fall to the pipes.

Having thus described in detail all the different portions of the construction of the building, we must proceed to give some account of its actual erection, which will enable us to mention many very ingenious mechanical contrivances which were employed in the course of its progress.

The First Operations on the Ground.

From the great extent of the area required for the building, it was not to be expected that any site would be found of the necessary size, perfectly level. On the ground occupied by the building there is a difference of level between the two extreme ends of about eight feet. In consequence of this fall of the natural surface from west to east, and in order to avoid having a considerable flight of steps at one end of the building to compensate for it, it was determined to arrange the floor with an inclination following nearly that of the ground, such fall being at the rate of one inch in twenty-four feet. All the lines of the building which would be called horizontal in fact follow this line of the floor, and those which are supposed to be upright are placed at right angles to the floor, and therefore slightly inclined from the perpendicular towards the east. The deviation, however, is so exceedingly small as to be perfectly imperceptible even to those who are aware of the fact; and no one who was not previously informed of it would be able to detect it.

It has been mentioned that Messrs. Fox and Henderson's tender for the building was verbally accepted on the 16th of July, 1850, and on the 30th of that month they obtained possession of the site from the Commissioners of Woods and Forests.

The first proceeding was to inclose the whole area (including a considerable space at each end more than would be covered by the building) with a hoarding about eight feet high, put together in a very simple manner, so that the boards were afterwards available for the flooring. The supports for the hoarding consisted of pieces of timber fixed in the ground in pairs, at intervals of the length of the boards, leaving a narrow space between them, into which the boards were dropped, and thus held in their place without any nails. Temporary offices were then erected in a convenient portion of the site, and were covered with a roofing which was a specimen of that to be used in the building itself. Considerable ranges of carpenters' sheds were also put up, and even stables for twenty or thirty horses, which were required in the progress of the works.

Setting out the Ground.

The first thing to be done towards the building itself was to set out accurately all the points where the columns would stand, as well as the general outline of the building. It will be readily understood that this was an exceedingly important part of the work, as upon its accuracy depended the fitting together of the various parts that had afterwards to be put in place.

This part of the work was executed with great precision by Mr. W. G. Brounger. He commenced by determining the four extreme angles of the building, and the centre lines of the main avenues. These formed fixed points from which were determined the whole of the centres for the columns.

Our readers will recollect that the dimension of twenty-four feet occurs horizontally throughout the building, either in multiples or sub-multiples. In order to measure off the different distances, rods of American pine were made, into which, near the ends, pieces of metal were fixed, having corresponding notches at the exact distance of twenty-four feet apart. By these means the lengths were measured off with great accuracy, as the wood used is not liable to alteration in the length of its fibre; and by means of the metal notches the rods were sure to be placed correctly together. It was necessary to make these sockets or notches of metal, from the great amount of work the rods had to perform.

In determining the length of the rods, the standard of the Astronomical Society was used; and this was referred to in all important measurements for the castings and other parts of the building, to insure their precise eventual agreement in length. This will hardly be considered to have been unnecessary when it is remembered that, from the great length of the building, a very minute error in any of the parts would have been so multiplied as sensibly to throw out the ends.

To those who are unacquainted with the fact, it may be well to mention that the standard of length referred to is obtained from a pendulum, which oscillates seconds, in the latitude of London, in a vacuum, at the level of the sea, at a certain fixed temperature. The length of this pendulum is then divided into a certain registered number of feet and inches.

The rods above described were carried along the centre lines of the columns, and the position of each column was marked by a small stake driven into the ground; and in order still more accurately to fix the centre, a long nail was driven into the head of the stake. In this manner the position of every column throughout the building was determined.

The level at which the floor was to be fixed was the next point determined by the ordinary method of levelling, and stakes, with a [T section] piece at the top, called boning-sticks, were fixed in different parts of the building; by the aid of which the tops of the base-pieces for the columns were all afterwards fixed in one plane of the required slope.

Fixing the Base Plates.

The next proceeding was to excavate the holes for the concrete, on which the base-pieces were to stand. To do this, the stakes marking the centres of the columns had to be removed, and it was therefore necessary to adopt some method of finding those centres again with precision. For this purpose a large carpenter's square, as it is called, was made. This instrument forms a right-angled triangle, and in this instance was used in the following manner:--The centre of its longest side, or hypothenuse, was marked by a line, which, if continued, would pass through the right angle of the triangle, and at an equal distance along each of the other sides of the triangle from the right angle an upright saw-cut or notch was made. The square was then placed horizontally, so that the line marked on the hypothenuse coincided with that of the centres of a row of columns, and so that the right-angled corner of the square touched the nail marking the exact site of a column. Two small stakes were then driven under the notches in the short arms of the square, and nails were driven into them through the notches. It will be seen that by these means the site of the first stake could easily be again ascertained after its removal. The holes for the concrete were then dug of an oval form and of the various sizes and depths required, and the concrete filled in to the proper height. The gravel used for the concrete was raised in a pit at one end of the ground.