cross the Swilly Rocks, he proposed to consist of three castiron arches, 350 feet span each, and 150 at the crown, above an ordinary spring-tide, and to connect these arches on the Carnarvon side by smaller stone arches to the extent of 200 yards, and on the Anglesea side by land arches to the extent of 434 yards, besides embankments, thus making a total length of 1076 yards: the expense of this design (which he strongly recommended to be adopted in preference to the other) he estimated at £290,417. It is much to be regretted that neither of these designs was adopted, which the expense alone, however, prevented, and the present chain or suspension bridge, by Mr. Telford, was adopted instead, as it was supposed that it could have been completed for £70,000; but if the ultimate costs could have been foreseen, it is more than probable that the fixed cast-iron bridge would have been carried into effect. With reference, however, to carrying the railway across the straits, some similar plan of a bridge ought to be adopted; and, taking into consideration the magnitude of the work and the difficulties of the situation, I do not think that it would be prudent to estimate the cost at a less sum than stated by Mr. Rennie, viz. £290,417. The time also for completing such a work, considering its extent and difficulty, and the numerous contingencies to which it would necessarily be exposed, could not be taken at less than from five to seven years; indeed, the present suspension bridge occupied above seven years, and the late Mr. Telford considered that the site of the Swilly Rocks would be attended with greater difficulties."

In his designs for carrying the Chester and Holyhead Railway over the straits, Mr. Robert Stephenson had thus to determine the two fundamental points of site and construction of his proposed work. The site which, after careful examination, he selected, although not one which had received the approval of former engineers, offers one peculiar advantage, which Mr. Stephenson duly remarked, and determined to avail himself of in the situation of his bridge. This consists

in a mass of rock, occupying the centre of the stream, of suitable dimensions to serve as the foundation of a central pier, and standing considerably above the level of low water. The distance of this rock, and of the bridge now being built over it, from the suspension bridge of Telford, is one mile lower down the straits, or in a southern direction. Upon the other great question, viz. the construction of the bridge, Mr. Stephenson brought some of his own experience to bear, which proved far more conclusively than any theoretical inquiries, that the suspension principle is utterly inapplicable for sustaining railway traffic. The following extract from his Report, presented to the Directors of the Railway, in February, 1846, gives the results of this experience:

"The injurious consequences attending the ordinary mode. of employing chains in suspension bridges were brought under my observation in a very striking manner, on the Stockton and Darlington Railway, where I was called upon to erect a new bridge for carrying the railway across the river Tees, in lieu of an ordinary suspension bridge, which had proved an entire failure. Immediately on opening the suspension bridge for railway traffic, the undulations into. which the roadway was thrown, by the inevitable unequal distribution of the weights of the train upon it, were such as to threaten the instant downfall of the whole structure. These dangerous undulations were most materially aggravated by the chain itself, for this obvious reason,-that the platform or roadway, which was constructed with ordinary trussing, for the purpose of rendering it comparatively rigid, was euspended to the chain, which was perfectly flexible, all the parts of the latter being in equilibrium. The structure was, therefore, composed of two parts, the stability of the one being totally incompatible with that of the other; for example, the moment an unequal distribution of weights upon the roadway took place, by the passage of a train, the curve of the chain altered, one portion descending at the point immediately above the greatest weights, and consequently causing

some other portion to ascend in a corresponding degree, which necessarily raised the platform with it, and augmented the undulation. So seriously was this defect found to operate, that immediate steps were taken to support the platform underneath by ordinary trussing; in short, by the erection of a complete wooden bridge, which took off a large portion of the strain upon the chains. If the chains had been wholly removed, the substructure would have been more effective; but as they were allowed to remain, with the view of assisting, they still partake of those changes in the form of the curve consequent upon the unequal distribution of the weight, and eventually destroyed all the connections of the wooden framework underneath the platform, and even loosened and suspended many of the piles upon which the frame-work rested, and to which it was attached. The study of these and other circumstances connected with the Stockton bridge leads me to reject all idea of deriving aid from chains employed in the ordinary manner." A fixed and rigid structure being thus indispensable to sustain railway traffic, Mr. Stephenson proposed to cross the straits with a cast-iron arched bridge in two spans of 450 feet each, and prepared his designs accordingly, the height of the arches being 100 feet from the level of the water to the crown of the arch, and the springing 50 feet from the same level. As it was necessary that the waterway should not be interrupted by scaffolding or centering, such as is usually employed in erecting arched bridges, Mr. Stephenson designed to fix the half-arches on each side of the central pier in portions simultaneously, and connect them with tie-rods, so that the weight on either side should balance that on the other.

The Commissioners of the Admiralty, however, who constitute the final authority in these matters, insisted upon one condition which rendered this design inapplicable; viz., that the clear height of water-way under the lowest part of the arches or their springing should not be less than 100 feet. To have retained the same general design, it would therefore

have become necessary to elevate the whole structure 50 feet above the proposed position, an alteration involving immense additional cost in the piers and abutments of the bridge, besides being irreconcileable with the adjoining levels of the railway. Under these circumstances the indomitable engineer determined to abandon the arched form altogether, and to seek a horizontal form of construction which should possess all the strength and inflexibility required for the support of its destined loads over spaces of 450 feet, and be at the same time susceptible of erection without obstructing the navigation of the straits.

Here was a problem of nearly unexampled difficulty, demanding for its solution the union of original bold conception, careful scientific experiment, and practical art and skill, rarely required and rarely to be commanded even on the most momentous occasions of engineering expedient. The first of these essentials was early supplied by Mr. Stephenson, who, in the month of February, 1845, announced his suggestion of wrought iron as the best material for the bridge over the straits, and the form of a hollow girder or tube as the shape in which this material should be combined for the purpose. To obviate the difficulty respecting scaffolding, it was determined that each of the tubes should be constructed at some unoccupied place contiguous to its permanent position, and raised and deposited in that position en masse. These decisions, which comprised the leading outlines of the plan, were wisely followed up by an elaborate series of experiments to determine, first, the peculiar sectional form which should be given to the tubes, and secondly, the distribution and dimensions of the material which would ensure the required strength and stiffness of the entire structure.

For these detail purposes, it was determined that a high authority in the theoretical and practical departments connected with the strength of the proposed material, and the best methods of its combination, should be enlisted in completing the design; and the authority selected was Mr. William

Fairbairn, who, after conducting a series of experiments to ascertain the strongest form for the tube, called in the aid of Mr. Eaton Hodgkinson in reducing the results and evolving practical formula for determining the details of the work. These gentlemen proceeded with their inquiries, and presented Reports embodying the results to Mr. Stephenson, who appended them to his own Report, presented to the Directors of the Railway Company at their meeting in February, 1846. The importance of these summary Reports renders it necessary to quote the results which they exhibit: this we propose to do in the following Section, after stating the general principles which distinguish all beam or girder bridges, whether tubular or solid, from those whose strength depends upon their arched form.

SECTION VI.

General Principles which distinguish Girder Bridges from Arched Bridges --Mr. Fairbairn's Experiments and Report on Tubular Girders-Mr. Hodgkinson's Experiments and Report-Mr. Stephenson's Report. THE earliest philosophers who essayed to develop the laws which regulate the resistance of bodies to transverse strains, viz. Galileo and Leibnitz, assumed a fundamental principle which the celebrated James Bernouilli seems to have been the first to expose. This radical error was, that all the particles of a beam submitted to an excessive transverse pressure are in a state of tension, and that the separation of them by the overcoming of their tensile power is the only action exerted by the weight which breaks the beam. James Bernouilli, however, detected the fallacy of this assumption, and showed that the particles of which a beam so loaded is composed, exert a different kind of force on that side which receives the pressure of the load from that which they exert on the opposite side. The sensible indication of this fact is afforded by the form which the beam assumes, the loaded side or surface becoming concave, while the opposite side becomes convex. On the concave side the particles are thus compressed towards each