purposes, until an extraordinary case occurred which seemed about to baffle all the means hitherto employed. The occasion arose when it became a question to throw a bridge of rigid material, for a railroad, across the Menai Straits; suspension systems, from their flexibility, and some actual failures, being, in the opinion of the ablest European engineers, unsuitable for this kind of communication.

Robert Stephenson, who for some years held the highest rank among English engineers, appears, from undisputed testimony, to have been the first to entertain the novel and bold idea of spanning the Straits by a tube of sheet iron, supported on piers, of sufficient dimensions for the passage within it of the usual trains of railroads. The preliminary experiments for testing the practicability of this conception, and the working out of the details of its execution, were left chiefly in the hands of Mr. William Fairbairn, to whom the profession owes many valuable papers and facts on professional topics. This gentleman, who, to a thorough acquaintance with the mode of conducting such experiments, united great zeal and judgment, carried through the task committed to him; proceeding step by step, until conviction so firm took the place of apprehension, that he rejected all suggestions for the use of any auxiliary means, and urged, from his crowning experiment, reliance upon the tube alone as equal to the end to be attained.

Numerous experiments were made by him upon tubes of circular, elliptical, and rectangular cross-section. The object chiefly kept in view in these experiments was to determine the form of cross-section which, when the tube was submitted to a cross strain, would present an equality of resistance in the parts brought into compression and extension. It was shown, at an early stage of the operations, that the circular and elliptical forms were too weak in the parts submitted to compres sion, but that the elliptical was the stronger of the two; and that, whatever form might be adopted, extraordinary means would be requisite to prevent the parts submitted to compres sion from yielding, by "puckering" and doubling. To meet this last difficulty, the fortunate expedient was hit upon of making the part of the main tube, upon which the strain of compression was brought, of a series of smaller tubes, or cells of a curved or a rectangular cross-section. The latter form of section was adopted definitively for the main tube, as having yielded the most satisfactory results as to resistance; and also for the smaller tubes, or cells, as most easy of construction and repair.

As a detail of each of these experiments would occupy more space than can be given in this work, that alone of the tube which gave results that led to the forms and dimensions adopted for the tubular bridges subsequently constructed, will be given in this place.

644. Model Tube. The total length of the tube was 78 feet. The distance, or bearing between the points of support on which it was placed to test its strength, was 75 ft. Total depth of the tube at the middle, 4 ft. 6 in. Depth at each extremity, 4 ft. Breadth, 2 ft. 8 in.

The top of the tube was composed of a top and bottom plate, formed of pieces of sheet iron, abutting end to end, and connected by narrow strips riveted to them over the joints. These plates were 2 ft. 11 in. wide. They were 6 in. apart, and connected by two vertical side plates and five interior division plates, with which they were strongly joined by angle irons, riveted to the division plates, and to the top and bottom plates where they joined. Each cell, between two division plates and the top and bottom plates, was nearly 6 in. wide. The sides of the tube were made of plates of sheet iron similarly connected; their depth was 3 ft. 6 in. A strip of angle iron, bent to a curved shape, and running from the bottom of each end of the tube to the top just below the cellular part, was riveted to each side to give it stiffness. Besides this, precautions were finally taken to stiffen the tube by diagonal braces within it. The bottom of the tube was formed of sheets, abutting end to end, and secured to each other like the top plates; a continuous joint, running the entire length of the tube along the centre line of the bottom, was secured by a continuous strip of iron on the under side, riveted to the plates on each side of the joint. The entire width of the bottom was 2 ft. 11 in.

The sheet iron composing the top cellular portion was 0.147 in. thick; that of the sides 0.099 in. thick. The bottom of the tube at the final experiments, to a distance of 20 ft. on each side of the centre, was composed of two thicknesses of sheet iron, each 0.25 in. thick, the joints being secured by strips above and below them, riveted to the sheets; the remainder, to the end of the tube, was formed of sheets 0.156 in. thick.

The total area of sheets composing the top cellular portion was 24.024 in., that of the bottom plates at the centre portion,

22.450 in.

The general dimensions of the tube were one sixth those of the proposed structure. Its weight at the final experiment, 13,020 lbs.

The experiments, as already stated, were conducted with a view to obtain an equality between the resistances of the parts strained by compression and those extended; with this object, at the end of each experiment, the parts torn asunder at the bottom were replaced by additional pieces of increased strength.

The following table exhibits the results of the final experi

The tube broke with the weight in the 25th experiment; the cellular top yielding by puckering at about 2 feet from the point where the weight was applied. The bottom and sides remained uninjured.

The ultimate deflection was 4.89 in.

645. Britannia Tubular Bridge. Nothing further than a succinct description of this marvel of engineering will be attempted here, and only with a view of showing the arrangement of the parts for the attainment of the proposed end.

It differs in its general structure from the model tube, chiefly in having the bottom formed like the top, of rectangular cells, and in the means taken for giving stiffness to the sides.

The total distance spanned by the bridge is 1,489 ft. This is divided into four bays, the two in the centre being each 460 ft., and the one at each end 230 ft. each.

The tube is 1,524 ft. long. Its bearing on the centre pier is 45 ft.; that on the two intermediate 32 ft.; and that on each abutment 17 ft. 6 in. The height of the tube at the centre pier is 30 ft.; at the intermediate piers 27 ft.; and at the ends 23 ft. This gives to the top of the tube the shape of a parabolic curve.

Fig. 188-Represents a vertical cross-section of the Britannia Bridge.

A, interior of bridge.

B, cells of top cellular beam.

C, cells of bottom cellular beam.

a, top plates of top and bottom beams.

b, bottom plates of top and bottom beams.

c, division plates of top and bottom beams.

d and e, strips riveted over the joints of top and bottom plates.

o, angle irons riveted to a, b, and c.

g, plates of sides of the tube 4.

A, exterior T irons riveted over vertical joints of g.

interior T irons riveted over vertical joints of g, and bent at the angles of A, and extending beyond the second cell of the top beam, and beyond the first of the bottom beam.

n, triangular pieces on each side of i, and riveted to them.

The cellular top (Fig. 188) is divided into eight cells B, by division plates c, connected with the top a, and bottom b, by angle irons o, riveted to the plates connected. The different sheets composing the plates a and b abut end to end lengthwise the tube; and the joints are secured by the strips d and e, riveted to the sheets by rivets that pass through the interior angle irons.

The sheets of which this portion is composed are each 6 ft. long, and 1 ft. 9 in. wide; those at the centre of the tube are

ths of an inch thick: they decrease in thickness towards the piers, where they are 18ths of an inch thick. The division plates are of the same thickness at the centre, and decrease in the same manner towards the piers. The rivets are 1 inch thick, and are placed 3 in. apart from centre to centre. The cells are 1 ft. 9 in. by 1 ft. 9 in., so as to admit a ınan for painting and repairs.

The cellular bottom is divided into six cells C, each of which is 2 ft. 4 in. wide by 1 ft. 9 in. in height. To diminish, as far as practicable, the number of joints, the sheets for the sides of the cells were made 12 ft. long. To give sufficient strength to resist the great tensile strain, the top and bottom plates of this part are composed of two thicknesses of sheet iron, the one layer breaking joint with the other. The joints over the division plates are secured by angle irons o, in the same manner as in the cellular top. The joints between the sheets are secured by sheets 2 ft. 8 in. long placed over them, which are fastened by rivets that pass through the triple thickness of sheets at these points. The rivets, for attaining greater strength at these points, are in lines lengthwise of the cell. The sheets forming the top and bottom plates of the cells are ths of an inch at the centre of the tube, and decrease to ths at the ends. The division plates are ths in the middle, andths at the ends of the tube. The rivets of the top and bottom plates are 1 in. in diameter.

E

fores

Fig. 189-Represents a horizontal cross-section of the T irons and side plates.

D, cross-section near centre of bridge.

E, cross-section near the piers.

g, plates of the sides.

h, exterior Tirons.

, interior Tirons.