[94] "Account of the Construction of the Britannia and Conway Tubular Bridges." By W. Fairbairn, C.E., London, 1849.
[95] Mr. Stephenson continued to hold that the elliptical tube was the right idea, and that sufficient justice had not been done to it. A year or two before his death, Mr. Stephenson remarked to the author that, had the same arrangement for stiffening been adopted to which the oblong rectangular tubes owe a great part of their strength, a very different result would have been obtained.
[96] "Mr. Fairbairn's Account," p. 22.
[97] The following passage occurs in Robert Stephenson's report to the directors of the Chester and Holyhead Railway, dated the 9th of February, 1846: "You will observe in Mr. Fairbairn's remarks that he contemplates the feasibility of stripping the tube entirely of all the chains that may be required in the erection of the bridge; whereas, on the other hand, Mr. Hodgkinson thinks the chains will be an essential, or, at all events, a useful auxiliary, to give the tube the requisite strength and rigidity. This, however, will be determined by the proposed additional experiments, and does not interfere with the construction of the masonry, which is designed so as to admit of the tube, with or without chains. The application of chains as an auxiliary has occupied much of my attention, and I am satisfied that the ordinary mode of applying them to suspension bridges is wholly inadmissible in the present instance; if, therefore, it be hereafter found necessary or desirable to employ them in conjunction with the tube, another mode of employing them must be devised, as it is absolutely essential to attach them in such a manner as to preclude the possibility of the smallest oscillation."
[98] In a letter of Mr. Fairbairn to Mr. Stephenson, dated July 18th, 1846, he says: "To get rid of the chains will be a desideratum; and I have made the tube of such strength, and intend putting it together upon such a principle, as will insure its carrying a dead weight, equally distributed over its hollow surface, of 4000 tons. With a bridge of such powers, what have we to fear? and why, in the name of truth and in the face of conclusive facts, should we hesitate to adopt measures calculated not only to establish the principle as a triumph of art, but, what is of infinitely more importance to the shareholders, the saving of a large sum of money, nearly equal to half the cost of the bridge? I have been ably assisted by Mr. Clark in all these contrivances; but in a matter of such importance we must have your sanction and support."—"Mr. Fairbairn's Account," p. 93.
[99] "The Britannia and Conway Tubular Bridges." By Edwin Clark. Vol. ii., p. 683-4.
[100] No. 34 Gloucester Square, Hyde Park, where he lived.
[101] 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 3-1/2 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 overcome. The 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.
[102] 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 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."
[103] 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 8-3/8 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."