The sides of the tube (Fig. 188) between the cellular top and bottom are formed of sheets g, 2 ft. wide; the lengths of which are so arranged that there are alternately three and four plates in each panel, the sheets of each panel abutting end to end, and forming a continuous vertical joint between the adjacent panels. These vertical joints are secured by strips of iron, h and i, of the T cross-section, placed over each side of the joint, and clamping the sheets of the adjacent panels between them. The T irons within and without are firmly riveted together with 1-inch rivets, placed at 3 in. between their centres. Over the joints, between the ends of the sheets in each panel, pieces of sheet iron are placed on each side, and connected by rivets. The sheets of the panels at the centre of the tube are ths of an inch thick; they increase to 18ths to within about 10 ft. of the piers, where their thickness is again increased: and the T irons are here also increased in thickness, being composed of a strip of thick sheet iron, clamped between strips of angle iron which extend from the top to the bottom of the joints. The object of this increase of thickness, in the panels and T irons at the piers, is to give sufficient rigidity and strength to resist the crushing strain at these points.

16

The Tirons on the interior are bent at top and bottom, and extended as far as the third cell from the sides at top, and to the second at bottom. The projecting rib of each in the angles is clamped between two pieces, n, of sheet iron, to which it is secured by rivets, to give greater stiffness at the angles of the tube.

The arrangement of the ordinary T irons and sheets of the panels is shown in cross-section by D, Fig. 189; and that of the like parts near the piers by E, same Fig.

For the purpose of giving greater stiffness to the bottom, and to secure fastenings for the wooden cross sleepers that support the longitudinal beams on which the rails lie, cross plates of sheet iron, half an inch thick, and 10 in. in depth, are laid on the bottom of the tube, from side to side, at every fourth rib of the T iron, or 6 ft. apart. These cross plates are secured to the bottom by angle iron, and are riveted also to the T iron.

The tube is firmly fixed to the central pier, but at the intermediate piers and the abutments it rests upon saddles supported on rollers and balls, to allow of the play from contraction and expansion by changes of temperature.

The following tabular statements give the details of the dimensions, weights, etc., of the Britannia Bridge.

Representing by A, the total area in inches of the cross

section of the metal.

d, the total depth in inches of the tube. 7, the length in inches between the points of support.

"C, a constant to be determined by experiment.

"W, the breaking weight in tons.

Then the relations between these elements, in tubes of cylindrical, elliptical and rectangular cross-section, will be expressed by

cAd

W-C

The mean value for C for cylindrical tubes, deduced from several experiments, was found to be 13.03; that for elliptical tubes, 15.3; and that for rectangular tubes, 21.5.

647. Victoria Bridge. This bridge is located near Montreal. It is a tubular bridge, a cross-section of which is shown in Fig. 190. It is the largest bridge of its kind in existence. It consists of twenty-four openings of 242 feet each, and a central span of 330 feet, and the total length of the tube, including the width of the abutments, is 6,538. The embankment forming the approach at the Montreal end is 1,200 feet long, and at the south end it is 800 feet, making a total length, including the approaches, of nearly 8,000 feet.

The centre span is level, but each side of the centre the bridge falls on a grade of 40 feet per mile.

Each tube covers two openings, being fixed in the centre, and free to expand or contract on the adjacent piers. They are 16 feet wide and 19 feet deep at their ends, and gradually

L

Sec. on D.D.

Fig. 190 B-Is a section on the line D D of Fig. 190.

KK is a vertical side plate.

LL are angle irons which are riveted to the side plates.

E

Fig. 190 C-Section of the bottom plates E of Fig. 190. There are three continuous plates and four joint plates.

increase in depth to the middle, where they are 16 feet wide by 21 feet 8 inches deep. The total length of each of these double tubes is,

On the centre pier...
Two openings in the clear.
Resting on the east pier..
Resting on the west pier...

Total.....

The weight of each tube of 516 feet is about 644 tons. At each end are seven expansion rollers, each 6 inches in diameter, upon which the tubes rest. The rollers which are turned rest on planed cast-iron bed plates.

The centre pier is 24 feet wide, the remaining ones each 16 feet wide at the top.

The work of laying the foundation was begun in 1854, and the centre tube was put in place in March, 1859.

The scaffolding for the centre tube rested on the ice in the river, which began to move the day after the tube was put in place. From a record which had been kept of the breaking up of the ice, it was presumed that it would remain sound several days longer than it did.

The foundations were made on the solid rock by means of coffer-dams. Two kinds were used, one a floating dam, and the other a permanent crib-work; and each possessed certain advantages over the other which was, peculiar to itself and to the objects which were to be accomplished.

VII.

SUSPENSION BRIDGES.

648. The use of flexible materials, as cordage and the like, to form a roadway over chasms and narrow water-courses, dates from a very early period; and structures of this character were probably among the first rude attempts of ingenuity, before the arts of the carpenter and mason were sufficiently advanced to be made subservient to the same ends. The idea of a suspended roadway, in its simplest form, is one that would naturally present itself to the mind, and its consequent construction would demand only obvious means and but little mechanical contrivance; but the step from this stage to the one in which such structures are now found, supposes a very advanced state both of science and of its application to the industrial arts, and we accordingly find that bridge architecture, under every other guise, was brought to a high degree of perfection before the suspension bridge, as this structure is now understood, was attempted.

With the exception of some isolated cases which, but in the material employed, differed little from the first rude structures, no recorded attempt had been made to reduce to systematic rules the means of suspending a roadway now in use, until about the year 1801, when a patent was taken out in this country for the purpose, by Mr. Finlay, in which the manner of hanging the chain supports, and suspending the roadway from it, are specifically laid down, differing, in no very material point, from the practice of the present day in this branch of bridge architecture. Since then, a number of structures of this character have been erected both in the United States and in Europe, and, in some instances, valleys and water-courses have been spanned by them under circumstances which would have baffled the engineer's art in the employment of any other means.

A suspension bridge consists of the supports, termed piers, from which the suspension chains are hung; of the anchoring masses, termed the abutments, to which the ends of the suspension chains are attached; of the suspension chains, termed the main chains, from which the roadway is suspended; of the vertical rods, or chains, termed the suspending-chains, etc., which connect the roadway with the main chains; and of the roadway.

649. Bays. The natural water-way may be divided into