Part 8

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.

Next to the setting out of the positions of the columns, perhaps the operation of fixing the base-pieces was that in which the greatest accuracy was required; for as there were in some parts three storeys of columns to be fixed over them, any inaccuracy as to their level or position would be very much increased at the top of the building. To fix the base-pieces over the centres that had been determined for the columns, another carpenter's square was made use of, like that already described, but having the right-angled corner cut out to the form of the section of a column. This square being placed with the notches in its short sides over the two stakes already described, the upright portion of the base-piece was fitted into the notch at the angle; and as the reader will at once see, if he has followed us in the description of the various processes, its correct position was thus exactly found.

In order to determine the level of the top of the base-pieces, boning-sticks were placed in the lines of the columns, and when the base-piece had been approximately fixed, a piece of wood was placed on it edgeways, the top of which was to range with the top of the boning-sticks. This was easily arranged by looking along them; and the workmen drove down the base-piece with a wooden mallet till the desired level was obtained.

From what has been previously stated, it may be gathered that the base-pieces had to be fixed truly upright in one direction, but slightly inclined in the other; and to effect this a plumb-rule was made, on which the deviation from the perpendicular line was marked; and this, when applied to those faces of the base-pieces which were to incline, served to show when the proper inclination was arrived at, whilst an ordinary plumb-rule applied to the other upright faces tested their vertical position.

The first column was raised on the ground on the 26th of September, but little more than two months after the tender had been accepted. In the meantime, many of the different castings had already arrived on the ground, and a considerable advance had been made in the carpenter's work for the gutters and other parts. The semi-circular ribs for the transept roof were also being put together, and stacked in such a manner as not to stand in the way of the other works.

We may mention here that every casting, as it came on to the ground, was weighed and registered, and every girder proved, as already described; in doing which considerable assistance was derived from one of Mr. Henderson's patent Derrick cranes, which was erected near the proving-apparatus. By its means a girder was raised from the waggon in which it arrived, placed on the weighing-machine, weighed, removed to the proving-press, tested, raised again, and deposited on the ground in a stack, in less than four minutes.

Henderson's Derrick Crane.

A brief description of this useful engine may not be out of place here. It consists of an upright mast (E), steadied when the crane is in use by two sloping stays (F F). These stays are fixed into horizontal timbers (G) on the ground, connected with the foundation-plate (H) on which the mast turns. At the foot of the mast is fixed a combination of wheels and working handles for raising the weight, technically called a crab. A beam (A) working at the bottom in a socket (B, Fig. 3) fixed to the foot of the mast, but hanging out from it in a sloping direction, is called the DERRICK, and forms the principal peculiarity of the crane, as it can be raised more to the upright line, or lowered to slope more outwards, as may be desired, by means of the chain (C). The advantage of this is obvious; for a weight may thus be raised from or deposited at any point within a circle of a certain radius, depending on the length of the derrick; whereas, in an ordinary crane, the weight can only be placed at points upon the circumference of that circle. The whole engine revolves on a pivot (H, Fig. 2) at the foot of the mast. Cranes of this description are made varying in power from one to forty tons, and with derricks ranging from twenty to sixty feet radius.

Raising and Fixing the Columns and Girders.

Many of the persons who visited the building during the progress of its erection were heard to inquire "where was the scaffolding;" and others even imagined that the skeleton framework they saw was, in fact, only the scaffolding for the building, and not parts of its actual construction. This leads us to point out one of the most interesting peculiarities of the structure; namely, that it formed, as it were, the scaffolding for its own erection. In order to raise the columns upon the base-pieces, two poles were placed upright, connected by a horizontal piece, forming what is called shear-legs; the whole being steadied in its position by ropes from the summit fixed to the ground in various directions. A rope with pulleys fixed to the horizontal piece served to hoist the column, and sustain it in a vertical position until the bolts were passed through the projecting rings at the bottom of the column and the corresponding ones at the top of the base-piece, and screwed up. When two columns had been thus fixed, a connecting-piece was attached to each end of a girder, and the whole raised by the same apparatus, and fixed on the top of the columns; bolts being passed through the holes in the projections of the connecting-pieces, corresponding with those on the top of the columns. The shear-legs were then moved on twenty-four feet to perform the same duties to another pair of columns; and two sides of a 24-feet bay were thus formed. To complete the square, two more girders were raised in a similar manner, and fixed between the connecting-pieces over the columns. The square bay then became a firm structure, requiring no further support; and by repeating these operations all the smaller avenues of the building were erected, of the different heights of one, two, or three storeys. The greatest number of columns thus fixed in one week was 310.

Hoisting the Roof Trusses.

The wrought-iron roof-trusses over the 48-feet avenues were raised in a similar manner to the columns and girders; and in all cases horses were employed to run out the end of the fall-rope, which was passed through a pulley or catch-block at the foot of the shear-legs, in order to change its direction from vertical to horizontal.

For raising the roof-trusses of seventy-two feet span over the main avenue a somewhat different method was employed. A single mast or derrick, more than seventy feet high, was placed in the centre of the avenue, and steadied in an upright position by guide-ropes spreading from the top in various directions. Near its summit the hoisting-tackle was firmly lashed on. The trusses to be hoisted were brought from the places where they had been put together, and placed across the main avenue at the points where they were to be fixed. Two ends of a stout chain were passed round the upper portion of the truss, at points dividing its length into about three equal parts. To this chain the hoisting-tackle was attached, guide-ropes being further fastened to each end of the truss to steady it in its ascent. In order to stiffen the truss horizontally, struts were attached at the centre projecting on each side, and held in their place by tie-rods attached to the upper part of the truss, and forming a triangle on each side. Before the truss, therefore, could bend in a horizontal direction, the attachment of these tie-rods must have given way. Six horses drew out the end of the fall-rope, and in the course of a very few minutes the truss was hoisted to its giddy height, and each end slipped in between the projections made in the connecting-pieces to receive it.

The animated scene presented by these operations was highly interesting from the number of men employed, both on the ground and for fixing the trusses in their position aloft, and from the rapid progress so many hands made. Each gang of men was managed by a foreman, who was obliged to issue his orders through a speaking-trumpet, to enable his voice to be heard in the din caused by the other works going on around. Besides the two large gangs of men engaged in the hoisting of the trusses, other smaller gangs were at work at different points getting up the columns and girders. In one part, the roofing of which was completed as early as practicable, a crowd of carpenters were preparing the Paxton's gutters and other portions of the work. In another place, as soon as a sufficient space could be roofed over and a temporary floor laid, various parts of the machinery we have already described were fitted up and worked by portable steam-engines. Of these there were three in different parts: one drove the machinery for finishing the sash-bars, gutters, ridges, &c.; another worked the drilling, punching, and other machinery connected with the iron-work; and a third was used for working circular saws.

Of the number of trusses that were hoisted as above described, in only one instance (and that the first) was the result otherwise than perfectly successful. The first truss was raised by its ends, instead of from the centre; but that method was afterwards abandoned, from the difficulty of maintaining the truss in an upright position during its ascent; which was important, as, if it turned on its side, its lateral strength was not sufficient to prevent it from bending, which would have destroyed the joints of the work.

One of the tall masts was worked on each side of the transept, from the centre to the ends of the building, being maintained constantly in an upright position, while traversing from point to point, by alternate slackening and hauling up of the ropes which steadied it; and it was curious to witness the motion of these tall giants, as they slowly progressed from one point to another, in the performance of their important office. Stout planks were laid along the ground, upon which the foot of the mast was forced forward by crowbars and levers; the planks served also to distribute the weight, which would otherwise have sunk the end into the ground. As many as seven trusses were hoisted in one day by each derrick, which had therefore to travel a distance of 168 feet.

So careful were the men, under the direction of the manager (to whom was intrusted the active superintendence of the whole erection of the building), that no accident of importance occurred in these difficult operations.

Provision for Expansion of Girders.

In connexion with the fixing of the girders, it may be desirable to mention the provision that was made for the expansion and contraction of the iron, which in so great a length as that of the building might have otherwise produced results prejudicial to its stability.

Between the projections cast on to the connecting-pieces and those projecting from the ends of the girders which they were made to clip, sufficient space was left for the introduction of oak keys, by driving in which the girder was fixed in its place, whilst the compressibility of the wood left sufficient play for the expansion of the metal. In describing the girders, it was mentioned that in the upper and lower flat flanges small sinkings were cast near the ends. Corresponding with these sinkings, a notch was left in the projection which came out from the connecting-piece; and when the girder was put into its place, iron wedges were driven in between the notch and the sinking, by which means any lateral motion of the girder was prevented. It was a great advantage to have the means of fixing the girders of so simple a nature, as any arrangement presenting the least complication, or requiring great nicety, would have materially retarded the progress of the work.

The wrought-iron trusses were held by the connecting-pieces in a similar manner to the cast-iron girders; but, as an additional security, bolts were passed through holes provided in the standards at the ends, and through the connecting-pieces, where they were screwed up with nuts.

The raising and fixing of the extra-strong roof-trusses crossing the main avenue near the side of the transept required particular care, from their great weight; the heaviest being, as we have before mentioned, no less than eight tons. These trusses were the first that were fixed across the central avenue, and about 150 men were engaged in the hoisting of each one. They are secured to the columns by four strong bolts passing through the end-standards.

In order to provide additional support for the great weight brought upon the last-mentioned trusses by the transept roof, extra columns were introduced underneath them. These were built up in storeys corresponding with those of the other columns, with which they were connected, at the levels of the girders, by bolts and straps. A cast-iron shoe, fixed on the top of the columns, provided a bearing for the ends of the truss. The columns just described project slightly into the main avenue from the line of the other columns; and this is the only instance in the interior of the building of the iron columns occurring at a less distance than twenty-four feet apart.

Glazing the Roof.

We have now traced the erection of the building up to the level of the roof, in which it will be readily conceived the operation of glazing was one of extreme difficulty, there being no scaffolding to aid the workmen in conducting their operations. When the glazing was first commenced a light scaffolding was suspended from the rafters; but this was found to be too tedious and troublesome a method of proceeding for so large an extent of roofing. It was, moreover, of great importance that some means should be devised for completing this part of the construction independently of the weather; a matter of some moment, when it is remembered that the work had to be done in the winter, when in our climate such operations are liable to be very much impeded by heavy rain. The arrangements made to meet this difficulty, as well as some others for carrying on the works, are very clearly described in a paper by Mr. Digby Wyatt, read at the Institution of Civil Engineers, on the 14th January, 1851, from which we quote some passages, by permission, for the benefit of our readers.

With reference to the means employed for glazing the roof he says: "To effect this purpose, a travelling stage was devised by Mr. Fox, which superseded the necessity of any scaffolding for glazing, and by means of seventy-six of these machines nearly the whole of the work has been executed. The stage was about eight feet square, and rested on four small wheels travelling in the Paxton's gutters. It thus embraced a width of one bay of eight feet of the roof, with one ridge and two sloping sides. Each bay in width required, therefore, a separate stage."

"Each stage was occupied by two workmen, and was covered by an awning of canvass stretched over hoops, to protect them in bad weather, and was further provided with a box on each side to contain a supply of glass. The sash-bars and other materials were piled upon the stage itself, the centre of the platform being left open for the convenience of hoisting up materials, for which purpose there was a small iron arm with a single block pulley."

"Whilst working, the men sat at one end of the platform (the ridge having been previously fixed in position by means of the extra-strong sash-bars), and they fixed the glass in front of them, pushing the stage backwards as they completed each pane. On coming to the strong sash-bars previously fixed, they temporarily removed them to allow the stage to pass. In this manner each stage travelled, uninterruptedly, from the transept to the east and west ends of the building, and the glaziers were enabled to follow up the previously-fixed work very closely. The average amount of glazing done by one man per day was fifty-eight squares, or about 200 superficial feet; and the largest amount done by any one man in a working-day was 108 squares, or 367 superficial feet."

The mode of fixing the squares of glass was this: a sash-bar having been nailed down between the ridge and the gutter, the workman inserted one long edge of a square of glass into the groove in the sash-bar, he then placed a loose bar against the other long edge of the glass and brought the whole down to bear upon the ridge and gutter, the second sash-bar fitting into the notches prepared for it; the glass was then pressed up a little, in order to insert its upper edge into the groove in the ridge, and the workman then filled in the grooves on the outside of the glass with putty, the lower edge of the glass having been also bedded on putty where it bears on the edge of the gutter. The ends of each sash-bar were fixed with a nail driven into the holes previously drilled.

Stage for Repairing Glass.

As it might naturally be expected that out of the thousands of panes of glass employed, particularly in the flat roof of the building, many would be broken in the course of the works, subsequently to their being fixed, it was necessary that a ready means should be devised for repairing any such damage, as the glazing-waggons used for the first execution of the work would not be available for that purpose. A light stage was therefore constructed, travelling with wooden wheels upon the ridges instead of in the gutters; and from this the men were able to perform their work without walking along the narrow gutters, which would have been attended with much risk. This stage was also used for fixing the canvass on the outside of the roofing, where it is nailed along the ridges, and allowed to bag down slightly between them. The object of the canvass, which covers externally the whole of the roof except the transept, is twofold: it preserves the glass from damage, and also protects the objects exhibited from the direct rays of the sun, which would, of course, in many instances, be very prejudicial; for the latter purpose the upright sashes on the south side are also covered with canvass on the inside.

Hoisting the Ribs for Transept Roof.

One of the most interesting operations which attracted the attention of the numerous visitors to the works was the raising the ribs for the semicircular roof of the transept, the description of which we give from Mr. Wyatt's paper:--

"The operation about which most anxiety had been felt was the hoisting of the arched ribs of the transept. These ribs were constructed on the ground horizontally, and when completed with all their bolts, two of them were reared on end, and maintained in a vertical position, at a distance of twenty-four feet from each other, by guy-ropes. As the ribs singly possessed little lateral stiffness, they were framed together in pairs with the purlins, intermediate small ribs and diagonal tie-rods, forming a complete bay of the roof twenty-four feet long; two complete sets of temporary ties were also introduced to provide for the strains incident to the variations in position of the ribs during the hoisting. The feet of the ribs were bolted on to a stout piece of timber, and the lower purlins strutted up from the same." In this state the framework is shown in the engraving.

"The whole framework was then moved on rollers to the centre of the square formed by the intersection of the transept and the main avenue, where it was afterwards hoisted. All the ribs were landed over this square, and were afterwards moved on a tramway formed of a half baulk of timber constructed over the columns on either side of the transept, at a height of about four feet above the lead-flat. The hoisting-tackle consisted of four crabs, each one being placed on the side of the transept opposite to the part of the ribs to be lifted by it, so that the men at the crabs might watch the effect of their exertions with greater convenience."

"The hoisting-shears were placed on the lead-flat immediately over the deep trusses of seventy-two feet span; each set consisted of three stout scaffold-poles, lashed together at the top, and footed on planks laid across the flat, and secured by the necessary guy-ropes. The hoisting-rope passed from each of the crabs across the transept horizontally, to a leading block attached to the foot of the opposite angle column of the square; it then passed up to a treble block fastened to the shears on the flat, and from thence down to a double block secured by chains to the bottom part of the ribs."

"There was a peculiar difficulty to be overcome in this operation, which arose from the circumstance that the width of the framework was greater than that of the transept, the extreme width of the framework to be hoisted being seventy-four feet, and the clear width apart of the trusses above which it had to be hoisted being only seventy-one feet four inches. It was therefore necessary to raise one side to a height of thirty-five feet before raising the other, so as to diminish the horizontal width of the whole, the diameter of the semicircle being maintained at this angle; the whole was then hoisted, until the highest end could clear the tramway."

This accounts for the slanting position in which the ribs are shown in the view given.