Wiebiking states that a rise of 1 in 24 is a convenient ascent for a bridge. In timber bridges the settlement is 1 in 72 : that is, if a timber bridge of 144 feet span rise one foot in the middle when first framed, it will settle so as to be horizon tal; so that when it is intended for the bridge to have an ascent of 1 in 24 when finished, it must be framed so as to have a rise of 1 in 18: for 4, .*-1- h. The Britannia Tubular Bridge is one of the most remarkable structures in the world, the design of the celebrated Ar chitect, Sir R. Stephenson. This bridge is on the line of the Chester and Holyhead Railway, crossing the Menai Straits, within sight of Telford's Chain Suspen sion Bridge. It is made of cast iron of a tubular form, in the tube of which the railway passes. Four of these span the Strait, and are supported by piles of ma sonry; that on the Anglesey side is 143 feet 6 inches high, and from the front to the end of the wing walls is 173 feet. These wing walls terminate in pedestals, on which repose colossal lions of'Egyptian character. The Anglesey pier is 196 feet high, 55 feet wide, and 82 feet long. In the middle of the Strait is the Britannic Rock, from which the bridge derives its name ; on this the Britannia pier is raised.
It 1 'A pi-111,111v fligtnnt. from the Anglesey and Caernar von piers, being 460 feet in the clear from each, and sustains the four ends of the four long tubes, which span the distance from shore to shore. There are two pair of short and two of long tubes, the lengths of these pairs being 250 feet and 470 respectively. The Egyptian lions are 25 feet 6 inches long, 12 feet 6 inches high, 8 feet wide, and weigh 80 tons. Two thousand cubic feet of stone were required for each lion. The total quantity of stone in the bridge is cubic feet. The weight of malleable iron in the tubes is 10,000 tons, of cast iron, 1400 tons. The whole length of the en ti.) • re bridge, measuring from the extreme front of the wing walls, is 1,833 feet, and its greatest elevation at Britannia pier, 240 feet above low-water mark. The total cost of the structure is £601,865. The following observations refer to the arrangement of parts in ordinary bridg ing: In the building of bridges, where piers are required in the stream for the support of the arches, it is important to place them as nearly as possible at right angles to the stream or current ; and the piers should be made convex towards the stream, for their better resistance to floods. The position of a bridge, more over, should not be in a narrow part, nor one liable to swell with tides or floods, inasmuch as the contraction of the water way increases the depth and velocity of the current, and may thus endanger the navigation as well as the bridge itself. It
is usual to construct bridges with an odd number of arches, and this on several ac counts : among which arc, that the stream being usually most powerful in the mid dle, an egress through that part is best provided for by having a central arch ; and again, if the bridge be not perfectly horizontal, symmetry is gained by rising from the sides to the centre, and the whole roadway may be made one con tinued curve. When a bridge is equally high throughout, much saving of cen tring is made, because the arches being all equal, two sets ofeentres will be sufficient. If, however, the bridge be higher in the middle than at the extremities, the arches on each side the centre one must dimin ish similarly, so as to be respectively symmetrical ; and by this arrangement beauty is gained, and the centring for one side equally suits the other. It is de sirable to construct a bridge with as few arches as circumstances will allow, that there may be a free passage for the water, aa well as for the vessels the have to pass np and down, not to mention the saving of materials and labor where there are fewer piers and centres. When a single arch can be compassed, no more should be permitted. The piers should be of sufficient solidity to resist the thrust or push of the arch, independent of other arches, so that the centring of an arch may be struck without danger of over turning the pier left naked ; and the piers be spread as much as possible on the bases, and diminish gradually up wards from their foundations. The me thod of laying the foundations in a river is now usually by means of coffer-dams, which are large inclosures, made by pil ing round the space to be occupied by the pier, so as to render it water-tight, and then pumping out the water, and keeping the space dry till the pier is built up to the ordinary level of the water ; but lithe ground about be loose, this method can not be well practised, and recourse is had to caissons, which are a species of flat bottomed boat, in which the pier is built up to a certain height,. and then sunk over the place where it is intended to re main, the bed of the river being dredged out to receive it, or piles driven on which it may lodge when the sides of the chest or caisson are knocked away. In con structing the centres, great care must bo taken to make them incapable of bending or swerving while the arch is being turned, the form of the arch will be crippled.