He took every imaginable precaution against accident or failure; and well was it that he did so, for an accident happened which, but for the careful building under the tube in the towers as it was raised, would have been most calamitous. The accident occurred while Mr. Stephenson was absent in London. One day, suddenly, while the machinery was at work raising the tube, the bottom burst from one of the hydraulic presses, and down fell the tube on to the bed provided for it. Though the fall was but nine inches, tons weight of metal castings were crushed, and the mighty tube itself was strained and slightly bent. But it was serviceable still, and the fact that it stood the strain so well showed its great strength. It weighed some five thousand tons, and for such an immense weight to fall even three-quarters of a foot was a very severe test. But for Stephenson’s wise precaution in lifting it slowly, and building underneath it as it was raised, the tube would have crashed to the bottom of the water. As it was, the accident cost £5000; but the tube was soon being hauled upward again. In due course the others followed, and on the 5th of March, 1850, Robert Stephenson inserted the final rivet in the last tube, and the bridge was complete. He crossed over with about a thousand persons, three locomotives whirling them along. The tubes of the bridge are made of iron plates, and at the top and bottom are a number of small cells or tubes—instead of thick iron plating—which assist in giving strength to the whole gigantic tube. Thus it may be said the floor and roof are tubular, as well as the body. These hollow cells appear to have been Fairbairn’s invention. The size of the tube grows slightly larger at the middle by the Britannia tower, where externally the tubes are 30 feet high, and 26 internally, while they are 22¾ feet and 18¾ feet at the abutments. The width is 14 feet, 8 inches externally, and 13 feet 5 inches inside. At the Britannia tower the tubes are placed solidly on their bed, but at the abutments, and at the land towers, the tubes rest on roller-beds. This arrangement was adopted to permit of expansion and contraction. Iron, of course, solid and unyielding as it appears, is yet very susceptible to warmth, and the effect of the sun’s rays on this massive iron structure is very marked. A rise of temperature causes it to expand in a comparatively short time, and it is said that the tubes occasionally move two and a-half inches as the sun gleams upon them. Mr. Edwin Clark observed the effect of the sun on the iron, which appears in a small degree to be always moving as the temperature varies. Well, therefore, that the able engineer planned an arrangement allowing for this constant expansion and contraction of the iron mass. [Illustration: THE BRITANNIA TUBULAR BRIDGE.]