CHAP. XVIII.] FALL OF THE MAIN TUBE! 455 and carefully under-built, before being raised to a farther height. When it had been got up by successive stages of this kind to about 24 feet, an extraordinary accident occurred, during Mr. Stephenson's absence in London, which he afterward described to the author in as nearly as possible the following words: “In a work of such novelty and magnitude, you may readily imagine how anxious I was that every possible contingency should be provided for. Where one chain or rope was required, I provided two. I was not satisfied with 'enough' I must have absolute security, so far as that was possible. I knew the consequences of failure would be most disastrous to the company, and that the wisest economy was to provide for all contingencies, at whatever cost. When the first tube at the Britannia had been successfully floated between the piers, ready for being raised, my young engineers were very much elated; and when the hoisting apparatus had been fixed, they wrote to me, saying, 'We are now all ready for raising her: we could do it in a day, or in two at the most.' But my reply was, No; you must only raise the tube inch by inch, and you must build up under it as you rise. Every inch must be made good. Nothing must be left to chance or good luck. And fortunate it was that I insisted upon this cautious course being pursued; for, one day, while the hydraulic presses were at work, the bottom of one of them burst clean away! The crosshead and the chains, weighing more than 50 tons, descended with a fearful crash upon the press, and the tube itself fell down upon the packing beneath. Though the fall of the tube was not more than nine inches, it crunched solid castings, weighing tons, as if they had been nuts. The tube itself was slightly strained and deflected, though it still remained sufficiently serviceable. But it was a tremendous test to which it was put, for a weight of upward of 5000 tons falling even a few inches must be admitted to be a very serious matter. That it stood so well was extraordinary. Clark immediately wrote me an account of the circumstance, in which he said, 'Thank God you have been so obstinate; for if this accident had occurred without a bed for the end of the tube to fall on, the whole would now have been lying across the bottom of the Straits.' Five thousand pounds extra expense was caused by this accident, slight though it might seem. But careful provision was made against future failure; a new and im 456 STEPHENSON CAIRN.-HYDRAULIC PRESSES. [PART II proved cylinder was provided; and the work was very soon advancing satisfactorily toward completion."* When the queen first visited the Britannia Bridge, on her return from the North in 1852, Robert Stephenson accompanied her majesty and Prince Albert over the works, explaining the principles on which the bridge had been built, and the difficulties which had attended its erection. He conducted the royal party to near the margin of the sea, and, after describing to them the incident of the fall of the tube, and the reason of its preservation, he pointed with pardonable pride to a pile of stones which the workmen had there raised to commemorate the event. While nearly all the other marks of the work during its progress had been obliterated, that cairn had been left standing in commemoration of the caution and foresight of their chief. The floating and raising of the remaining tubes need not be described in detail. The second was floated on the 3d of December, and set in its permanent place on the 7th of January, 1850. The others† were floated and raised in due course; on *The hydraulic presses were of an extraordinary character. The cylinders of those first constructed were of wrought iron (cast iron being found altogether useless), not less than 8 inches thick. They were tested by being subjected to an internal pressure of 3 or 3 tons to the circular inch. The pressure was such that it squeezed the fibres of the iron together; so that, after a few tests of this character, the piston, which at first fitted it quite closely, was found considerably too small. "A new piston," says Mr. Clark, was then made to suit the enlarged cylinder; and a farther enlargement occurring again and again with subsequent use, the new pistons became as formidable an obstacle as the cylinders. The wrought-iron cylinder was on the point of being abandoned, when Mr. Amos (the iron manufacturer), having carefully gauged the cylinder inside and out, found to his surprise that, although the internal diameter had increased considerably, the external diameter had retained precisely its original dimensions. He consequently persevered in the construction of new pistons, and ultimately found that the cylinder enlarged no longer, and to this day it continues in constant use. Layer after layer having attained additional permanent set, sufficient material was at length brought into play, with sufficient tenacity to withstand the pressure; and thus an obstacle, apparently insurmountable, and which threatened at one time to render much valuable machinery useless, was entirely overThe workman may be excused for calling the stretched cylinder stronger than the new one, though it is only stronger as regards the amount of its yielding to a given force."-Clark, vol. i., p. 306. The hydraulic presses used in raising the tubes of the Britannia Bridge, it may be remembered, were afterward used in starting the Great Eastern from her berth on the shore at Milwall, where she had been built. + While the preparations were in progress for floating the third tube, Mr. Stephenson received a pressing invitation to a public railway celebration at Darlington, in honor of his old friend, Edward Pease. His reply, dated the 15th of May, 1850, was come. MENAI BRIDGE. [By Percival Skelton, after his original Drawing.] the 5th of March Mr. Stephenson put the last rivet in the tube, and passed through the completed bridge, accompanied by about as follows: "I am prevented having the pleasure of a visit to Darlington on the 22d, owing to that or the following day having been fixed upon for floating the next tube at the Menai Straits; and as this movement depends on the tide, it is, of course, impossible for me to alter the arrangements. I sincerely regret this circumstance, for every early association connected with my profession would have tended to render my visit a gratifying one. It would, moreover, have given me an opportunity of saying publicly how much the wonderful progress of railways was dependent upon the successful issue of the first great experiment, and how much that issue was influenced by your great discernment, and your confidence in my late revered father. In my remembrance you stand among the foremost of his patrons and early advisers; and I know that throughout his life he regarded you as one of his very best friends. One of the things in which he took especial delight was in frequently and very graphically describing his first visit to Darlington, on foot, to confer with you on the subject of the Stockton and Darlington Railway." 458 THE BRITANNIA BRIDGE A MONUMENT [PART II. a thousand persons, drawn by three locomotives. The bridge was found almost entirely rigid, scarcely showing the slightest deflection. When, in the course of the day, a train of 200 tons of coal was allowed to rest with all its weight, for two hours, in the centre of the eastern land tube, the deflection was only four tenths of an inch, or less than that produced upon the structure by half an hour's sunshine ;* while the whole bridge might with safety, and without injury to itself, be deflected to the extent of 13 inches. The bridge was opened for public traffic on the 18th of March. The cost of the whole work was £234,450. The Britannia Bridge is one of the most remarkable monnments of the enterprise and skill of the present century. Robert Stephenson was the master spirit of the undertaking. To him belongs the merit of first seizing the ideal conception of the structure best adapted to meet the necessities of the case, and of selecting the best men to work out his idea, himself watching, controlling, and testing every result by independent check and counter-check. And, finally, he organized and directed, through his assistants, the vast band of skilled workmen and laborers who were for so many years occupied in carrying his magnificent original conception to a successful practical issue. But it was not accomplished without the greatest anxiety and mental pressure. Mr. Clark has well observed that few persons who merely witness the results of the engineer's labors can form any conception of the real difficulties overcome, and the intense anxiety involved in their elaboration. "If the stranger," he says, "who contemplates the finished reality, requires so much thought *The effect of sunshine in deflecting the bridge is very curious. When the first main tube was tested, ballast-wagons loaded with iron were drawn into the centre and left standing there. The first 20 tons increased the deflection an eighth of an inch, and with 50 tons the deflection was 9 inches. After standing all night, the deflection in the morning was found to be only 84 inches. How was this to be accounted for? Mr. Clark says: "This was attributed at the time to an error made in the reading; but this, and many other anomalies in the deflection, were afterward fully accounted for by local changes of temperature. A gleam of sunshine on the top of the tube raised it on one occasion nearly an inch in half an hour with 200 tons at the centre, the top plates being expanded by increase of temperature, while the lower plates remained constant from radiation to the water immediately beneath them. In a similar manner, the tube was drawn sidewise to the extent of an inch from the sun shining on one side, and returned immediately as clouds passed over the sun, being, in fact, a most delicate thermometer in constant motion, both vertically and laterally." CHAP. XVIII.] OF SKILL AND INDUSTRY. 459 to appreciate its principles and comprehend its detail, what weary hours must he have undergone who first conceived its bold proportions-who, combating, almost alone, every prejudice that assailed him, and with untiring labor discussing every objection, listening to every opinion, and embodying every inquiry, at length matured, step by step, this noble monument?" On the occasion of raising the last tube into its place, Mr. Stephenson declared, in reply to the felicitations of a large company who had witnessed the proceedings with intense interest, that not all the triumph which attended this great work, and the solution of the difficult problem of carrying a rigid roadway across an arm of the sea at such a height as to allow the largest vessels to pass with all their sails set beneath it, could repay him for the anxieties he had gone through, the friendships he had compromised, and the unworthy motives which had been attributed to him; and that, were an other work of the same magnitude offered to him with like consequences, he would not for worlds undertake it! The Britannia Bridge was indeed the result of a vast combination of skill and industry. But for the perfection of our tools, and the ability of our mechanics to use them to the greatest advantage-but for the matured powers of the steam-engine-but for the improvements in the iron manufacture, which enabled blooms to be puddled of sizes before deemed impracticable, and plates and bars of immense size to be rolled and forged-but for these, the Britannia Bridge would have been designed in vain. Thus it was not the product of the genius of the railway engineer alone, but of the collective mechanical genius of the English nation. |