It was this that led to the grand and untried design of the present rigid tvrought-iron tubular which Mr. Robert Stephenson, afier great and labour, assisted by Mr. Fairbairn, Mr. 1 lodgkinson, of the I2oya] S'oeiety, Mr. Edwin Clarke. the engineer of the works—gentlemen well known fir their mathematical and scientific attainments—matured. The entire length of the stupendous structure is I.S41 feet front cod to end, consisting of four large sections, the two being each of them 230 feet long, and the two middle ones 460 feet t tick When originally proposed before the committee of the llouse of Commons, the plan was received with ge•eral incredulity. The word •` tube." it may here be observed, is not one of the best epithets that could be used to describe the structure, seeing that the bridge, instead of being round, is a perfect square. Though almost a misnomer, the name arises from the circumstance of the experiments that were to decide the form of the bridge, having been made with c\ lindri•al. elliptical, and rectangular tubes ; but in reality, the structure, as it now rests on the banks of the Total, the site of its construction, is one immoise closed-in iron corridor, a horizontal iron gallery or passage. within which the rails for the trains are to be placed, and 450 feet in length. It is hollow from end to end, and would, if filled with shops, and lighted by sky lights, loake a Burlington Areade.
A structure of this kind, though on a rude and miniature scale, appears to have existed finr years on the Cambridge line of the Eastern Counties Railway : and Mr. Stephenson, the originator of it, upon this, designed the pre sent, tube. A long series of experiments, by engineers and mechanics fully conversant with such researches, were made, directed to the aseertaininp--divested of all preconceived ideas—the strongest form for a sheet-iron tubular bridge; and the inquiry, in addition to the more immediate object it had in view, has been of immense public service in deter mining the strength of the materials used in the formation of railways. These experiments have been extremely laborious, and very costly. In the Coarse Of them. the remarkable fact has been disclosed, that the power of wrought-iron to resist compression is much less than its power to resist tension, or exactly the reverse of that which holds with cast-iron ; and the important filet has also been arrived at, that rlijidity and strength are best obtained by throwing the greatest thickness of material on the upper side. While the cylindrical tube, with a given weight. was ruptured by tearing asunder at the bottom, the elliptical showed weakness at the top both were consequently discarded ; and the rectangular tube, which indiacted strength of a higher order. and greater
rigidity, ryas adopted. The result of every recent experiment on this species of structure, on a small settle, over the Conway, are very interesting, as confirming the aecuraey of the origi u.11 calculations. Measured Ivy a cord-line in the inside of the tube. formed by the axis of a powerful telescope fixed to its side, the deflections have liven made with a weight of '2 tons, In t6 inch; 112 tons, 0.116; 173 tons, 1-30 ; 235 tolls, 1.47 ; and on the removal of these loads, the tube has reco vered its rigidity in ten minutes. The tube is eonstrueted to bear, in addition to its own weight. 2,000 tons, it load ten times greater than it will ever he called upon to sustain. The deflection caused by trains and locomotives passing at full speed is very slight. A weight of 300 tons has produced a deflection of :; inch A very remarkable phenomenon is connected with this lingo mass of iron of 1.000 tons, caused by changes of temperature in the w eathers which affect it like a thermometer. A little sunshine raises the centre an inch, and produces a horizontal deflection of an inch and a half.
Its great length, and the nature of its material, so sensitive to temperature in the peculiar litrm that it takes, causing it tc. expand .0001 of its length, or an inch fur each inerease of 50' of Fahrenheit, and contracting in the same ratio, is the assigned cause of its such a delicate thermometer. Alternate sunshine and showers of rain cause these tubes to expand and contract ; and one of them, if placed on end in St. Paul's churchyard, would be 101 feet higher than the top of the cross. It is calculated that the wind, at a velocity of till miles an hour—the rate of a hurricane—would Only give a total pressure of 126 tons, distributed over the whole side of these tubes.
have now to describe the means by which this gigantic structure was raised to its present lofty height, and the enor mous towers by which it is supported there.
The spot selected by Mr. Stephenson for the site of the bridge, was at that part of the Menai straits where the Bri tannia, rock rises from the bed of the stream nearly in its cen tre. On this rock is erected a tower of masonry ; at the limit of the water-way on either side of it, similar towers ; and on the land, on each shore of the strait, beyond the towers, continuous abutments of masonry. The bridge is therefore supported by the Britannia rock in the centre ; at 400 feet from it, on each side, by a tower built in the sane manner as that on the rock ; and at 230 feet from each of these towers, by the land abutments.
The following description of these masses of masonry is taken from a very interesting little work published by Mr. Weak," Treatise on Tubular and other Iron Girder Bridges," by Mr. Dempsey, C. E.