No sooner had the formation of railways commenced for carrying passengers in long trains of carriages drawn by heavy locomotive engines, than the want was experienced of some different kind of bridge from any then existing for crossing rivers, roads, and valleys. The train could not be turned sharply round a curve to cross a road at right angles; and to make the requisite bend to enable it to do so would have taken the railway considerably out of its direct course. To overcome this difficulty "skew bridges" were designed, that crossed roads and canals in slanting directions. Iron girder bridges were also constructed, and thus the railway trains were carried across roads and narrow rivers at any required inclination, supported on flat beams of iron. Suspension bridges were found to be unfitted, on account of their oscillation, for the passage of locomotive engines; therefore, when it became necessary to carry railways across arms of the sea, or wide navigable rivers, at heights sufficient to allow the largest ships to pass underneath, neither girder bridges nor suspension bridges were suited for the purpose. Then arose the necessity of contriving some form of bridge of extensive span that would be sufficiently strong and rigid for railway trains to pass over them in safety. The Britannia Bridge, across the Menai Straits, was a triumphant response to the call for a new kind of suspended roadway adapted to the requirements of railways. The tubular principle of construction, designed by Mr. Robert Stephenson, was practically tested by Mr. Fairbairn; and the result of numerous experiments on the strength of iron, in different forms and combinations, established the soundness of that principle. The rigidity and strength of the Britannia Bridge depend on cellular cavities at the top and bottom, which, acting as so many tubes, give stability to the riveted plates of iron, and enable the bridge to bear the immense pressure and vibration of a heavy railway train without deflecting more than half an inch. It was Mr. Stephenson's original intention to make a circular or oval tube, suspended by chains, for the trains to run through; but Mr. Fairbairn's experiments proved that a rectangular shape is stronger, provided the top and bottom, which bear the greatest part of the strain, are made rigid, either by additional plates of iron, or by tubes. The notion of a circular tube was, therefore, abandoned, and the rectangular form, with cells at the top and bottom, was adopted; first for the railway bridge at Conway, and afterwards for the much greater work across the Menai Straits.