:

111 sleepers; the number of trussed rods and king-posts which support the suspended road is altogether 444.

Iron frames to which the suspension chains are made fast. Three oblique circular cavities were blasted in the solid rock on the Anglesea side, 6 feet in diameter, care being taken to leave a considerable portion of rock between the openings; through these the suspension chains pass down an inclined plane of not less than 60 feet in length.

At the lower ends of these inclined tunnels is another at right angles, which connects them together, through which the workmen passed to fix the iron plates into the natural rock, and a passage is left on the south-west side of the bridge, which communicates with these subterraneous chambers.

On the Caernarvonshire side there is the same arrangement, but the depth of earth was considerable before the native rock could be arrived at, which occasions a different length to be given to the catenary on this side to that on the other.

The flat cast-iron plates were let into the natural rock, and so firmly secured that they

are perfectly immoveable, and, unless the entire mass of rock were to give way, would bear any stress that might be given them.

The saddles and ties which carry the main chains through the front of the Caernarvonshire toll-house are formed of cast-iron plates; they are 3 feet 5 inches wide; the holes through which the chains pass are 2 feet 4 inches in width; the four plates, with the passage for the sixteen chains, are admirably put together, two being in the middle, and one at each extremity; the cast-iron blocks or saddles are 8 feet 7 inches in width across the bottom, in the direction of the chains, and 7 feet in height, the spaces between the four bars which hold the chains being each 7 inches These saddles were all cast with holes, in order that the tackle might be adjusted and attached by which they could be hoisted. The frame for raising these castings was made of timber; the platform at bottom was 30 feet in extent, and its total height was 74 feet; an additional timber was fixed to the sides to a height of 26 feet; the blocks were secured by ropes to the top, and passed as shown in the figure, which represents the action of elevating a part of the iron work. At the side is a section of the frame-work, to the top of which is attached two guy-ropes. There was also an ingenious machine or clam made use of for holding the ropes of the hoisting tackle whilst fleeting them on the capstan, and which was found of the most essential service. The plan of the saddles shows their position; the parallelogram, upon which the four are placed, measures 37 feet 6 inches in length, and 14 feet in width.

The

The plan of the great pyramid exhibits the planking of the two carriage-ways. mass of masonry in the middle is 6 feet in width; the space for the traffic on each side 9 feet, and the thickness of the exterior walls at the greatest 9 feet, and in the less 6 feet. The pavement, as it is laid over the planking, is partly shown in the figure. The trans

Fig. 520

SECTIONS.

METHOD OF SECURING MAIN CHAINS TO THE ROCK.

I LAN.

Fig. 521.

PLAN OF THE SADDLES.

Fig. 522.

PLAN OF GREAT PYRAMID.

verse section through the main chains, exhibiting a little more than one half of the entire width; the middle footway and one passage for carriages, also represents the position" of the several lines of chains where they cross or pass over the saddles; the sections and elevation of the main pier show the position of the main chains in the other direction, or in a line at right angles with the passage across the stream.

The suspension chains were first firmly secured to the flat cast-iron plates; the chain bars, each 10 feet in length, were then laid down by placing the five together, which constituted one breadth; and the consecutive lengths were thus carried on, united by flat iron plates and bolts, until the apex of the suspension piers was arrived at, the whole chain being supported by a timber framework placed underneath.

On the apex of each of the suspension piers was a cast-iron saddle, in which were wrought-iron rollers and brass bushes, which as the temperature varied allowed some play to the chains, and regulated any contraction or expansion of the iron; as the rollers were self-acting, no derangement could very well take place from this cause.

Mr. Davies Gilbert furnished Mr. Telford at various times with information highly useful for the completion of this bridge; that gentleman calculated the dimensions to be given to the several bars of a catenary arch formed in iron or steel upon the suspension principle. He assumed the tenacity of iron to be 50,000 for a square inch, and its specific gravity 7-8, and Mr. James Jardine formed the following table for the construction of the chains. The first column shows the distance between the points of support; the second, the length of curve, or chain, between the points of support; the third, the axis of curve, or versed sine, of the chain; the fourth, the angle, nearly, between horizontal line and curve, at the point of support; the fifth, the tension, or strain, on each chain by its own weight, at either point of support; the sixth, the tension or strain on the sixteen chains by their own weight, at either point of support; the seventh, the weight of one chain; and the eighth column the weight of the sixteen chains.

The transverse section through the main chains and roadway shows the positions of the several parts, and the cast-iron pipes 3 inches in diameter, strutted or braced with diagonal bars; within the pipes is a wrought-iron bolt, 1 inch in diameter, which connects together the four lines of suspending chains. The footpath in the middle of the transverse section is 4 feet in width, and the two other divisions are each 12 feet: between the oak guards of each carriage-way is a width of 7 feet 6 inches; the iron truss-rods and king-posts introduced beneath the roadway bars add very considerably to the strength of the platforms. In the arrangement of the several parts of this bridge the greatest attention was paid to the manner in which they were united together, and also to allow of any portion being taken out and reinstated, in case of accidental fracture or injury of any kind; in this stupendous undertaking the constant stress upon particular parts, where destructive in its effects, could be easily remedied without endangering the entire work.

The chains were supported on the scaffolding, whilst the lengthening took place on the Caernarvonshire side, until the chain bars, suspended from the apex in a perpendicular direction, nearly reached the level of high water mark.

The most difficult part of the whole operation then commenced, that of conveying the first suspension chain across the Strait, and making a junction; this was performed on the 26th of April, 1825.

Fig. 523.

FRAME FOR RAISING CASTINGS ON TO THE PYRAMIDS.

At half flood tide, a raft prepared for the purpose, which carried the portion of the chain intended to be drawn over, was moved from Trebroth Mill, on the Caernarvonshire side,

by four boats assisted by the tide, to the centre of the river, and between the two main piers, where the raft was made fast to several buoys, anchored for the purpose.

One

Fig. 524.

CLAM FOR HOLDING THE ROPE OF THE HOISTING TACKLE WHILST FLEETING IT ON THE CAPSTAN.

end of the chain was then made fast to that which hung perpendicularly from the apex of the pier, on the Caernarvonshire side. The other end of the chain to be suspended was

fastened to two blocks of great power, the tension of the chain at this time being equal to

After the blocks were made fast to the chain, two capstans and two preventive capstans, each worked by 32 men, commenced raising, and within two hours and twenty minutes from the movement of the raft from the

shore the last bolt was fixed, which completed the entire length of the first chain, three of the men employed passing along it. And by the 9th of July the other fifteen chains were secured, and the entire line of suspension completed.

Vertical rods. - These are placed at equal distances of 5 feet, and are fastened to the sixteen suspension chains. The iron sleepers or transverse roadway bars are bolted to the lower ends, and to each of these 111 sleepers four vertical rods are attached transversely, making altogether 444.

Timber roadway.-The first tier of planks was laid across the iron sleepers on the 24th of September, 1825, there being three altogether, the lowest 3 inches thick, the

middle and upper each 2 inches; the last is laid transversely to the width of 8 feet, with side guides to prevent any injury from the carriage wheels. The planks are all spiked together, and between each layer was a coating of Borrodaile's patent felt saturated with boiled tar.