Engineers and their triumphs / the story of the locomotive, the steamship, bridge building, tunnel making

by F. M. Holmes

ROBERT STEPHENSON.

“Often at night,” he declared, “I would lie tossing about, seeking sleep in vain. The tubes filled my head. I went to bed with them, and got up with them. In the gray of the morning, when I looked across Gloucester Square, it seemed an immense distance across to the houses on the opposite side. It was nearly the same length as the span of my tubular bridge.”

The principle adopted was to construct the shorter tubes on scaffolds in the places which they were to occupy. This could be done, for such scaffolding would not impede navigation. But scaffolding could not be built for the large tubes across the great spans of water. What then was to be done?

It was decided to build them on platforms on the shore quite close to the water, and float them when ready on pontoons to their places between the piers, raising them to their position by hydraulic power. Such a task would be hazardous enough. It was first tried at Conway, where a similar bridge was being built by Robert Stephenson, being indeed part of the same railway. The Britannia was, however, a much greater enterprise, though the span of the Conway is 400 feet. The Conway bridge, indeed, is but of one span, and contains two tubes.

The experience at Conway was of great benefit to the gigantic undertaking at the Menai Strait. The floating of the first tube was to take place on the 19th of June, 1849, in the evening; but owing to some of the machinery having given way, the great event was put off to the next night. The shores were crowded with spectators. When the tube was finished it could be transferred to the pontoons; for the tubes had been built at high-water mark. When the pontoons were fairly afloat on this fateful evening, they were held and guided by leading strings of mighty strength. Stephenson himself directed in person, from a point of vantage at the roof of the tube. Thence he gave the signals which had been agreed upon, whilst a crew of sailors, directed by Captain Claxton, manned the strange barque.

A pontoon is a light, buoyant boat, and the tube was supported on sets of these, their speed increasing terribly as they approached their place by the towers. The idea was, as related by Mr. Edwin Clark, Stephenson’s assistant, that they should strike a “butt” properly, underneath the Anglesey Tower, “on which, as upon a centre, the tube was to be veered round into its position across the opening. This position was determined by a twelve-inch line, which was to be paid out to a fixed mark from the Llanfair capstan. The coils of the rope unfortunately over-rode each other upon this capstan, so that it could not be paid out.”

Destruction seemed imminent. The capstan was actually dragged from the platform, and the tube seemed likely to be swept away. Then Mr. Rolfe, the captain of the capstan, shouted to the spectators, and threw out a spare twelve-inch rope. Seizing this, the crowd, with right good-will, rushed it up the field, and clung tightly to it, checking the voyage of the mighty tube. It was brought to the “butt,” and duly turned round.

A recess had been left in the masonry of the tower, and the end near the Britannia pier was drawn into it by means of a chain. The Anglesey end followed. Then the tide gradually sank, the pontoons sank with it, and the tube subsided also to a shelf which had been made at either end. The first stage was accomplished; the mighty tube was in position to be raised.

Shouts of rejoicing burst from the sympathetic crowds, and the boom of cannon joined its congratulatory note at the grand success. But the further stages remained. At midnight the pontoons were all cleared away, and the huge, hollow beam hung silent over the surging water. It rested on the shelves or beds prepared for it at either end. The second great operation, of course, was to haul it up the towers to its permanent position. This was to be performed by hydraulic machinery of great power, and Mr. Stephenson’s instructions were to raise it a short distance at a time, and then build under it.

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.