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.
[Illustration: Interior View of the Central Avenue towards the West.]
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.
[Illustration: THE LOUVRE-FRAME.]
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.
[Illustration: VIEW OF STAIRCASE.]
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.
[Illustration: FIXING CAST-IRON DRAIN-PIPE.]
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.
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.
[Illustration: VIEW OF CRANE AND PROVING-PRESS.]
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.
[Illustration: (FIG. 1.) HENDERSON'S DERRICK CRANE.]
[Illustration: (FIG. 2.) PART OF HENDERSON'S DERRICK CRANE.]
[Illustration: (FIG. 3.) PART OF 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.
[Illustration: FIXING THE GIRDERS.]
[Illustration: General View of the Works in Progress.]
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.