THE BRITANNIA TUBULAR BRIDGE. Menai Strait, Monday, Oct. 1849.

New Zealand Spectator and Cook's Strait Guardian, Volume VI, Issue 486, 30 March 1850, Page 4

THE BRITANNIA TUBULAR BRIDGE. Menai Strait, Monday, Oct. 1849.

This afternoon the final lift of the Britannia tube took place at 3 o'clock, the permanent level of 100 feet above high water mark having been successfully attained on Saturday, when its ultimate elevation was enthusiastically hailed from all parts of the works with cheers and a succession of salutes. This additional hoist of three feet had to be made, ard was indispensably important to enable the engineers engaged in the operation to join the I great tube on to the end, or land lubes, before laying it down on its permanent bed plates and rollers, which will have to be placed beneath it. - This process of adjustment will occupy three weeks. It will be a matter of surprise, however, to be made acquainted with the fact, , that in the successful raising of the one tube to its permanent level one-eighth part only of these colossal labours was accomplished. There are three similar lengths of tube to be floated and lifted, besides four of what are called land tubes, before the stupendous fabric is completed. The one set of tubes, giving a length of 1,849 feet, will only form the passage for one line of rails across the Strait, and alongside of these another set of tubes, exactly similar and parallel, will have to be raised to furnish a double line of rails for the up and down line. The great object at present in view is to get one completed, which, uatil the second set is ready, will be used both for the up and down trains, the passage of which will be regulated accordingly. To shew what has yet to be done, as well as to illustrate the arduousness of the operations, a statement of the approximate dimensions, and of the gross amount of the material to be employed in the entire fabric," will be interesting. The total length, of these continuous tubes will be 1,849 feetV^the total length of the tubes for both lines of railway, 3,698 feet ; the greatest span in the clear, 460 feet; height at intermediate piers, 27 feet ; height at each end, 23 feet ; extreme width, 14 feet 8 inches.

i The number of rivets in the entire bridge is ! 1,764,000; the amount of iron calculated ; to be used is in tbe first land tube, 274 feet long, 450 tons of plates ; in tbe other three ! of the same length, 1,350 tons ; in the first tube of 472 feet long, 965 tons of plates ; in the three others of the same length, 2,895 tons of plates ; making a total with pier tubes of 5,778 tons. Tbe total computed weight of angle iron to be employed in them - is 1,240 tons of T iron, 856 tont of rivet iron, 686 tons of cast iron, in the frames and beams 2,000 tons ; making a total dead weight of 10,750 tons. The total length of tbe tubes, if placed in a line, would be about three-fifths of a mile. The strength of this great rigid structuie consists in what is called the cellular system, found so effective in resisting lateral and vertical pressure. The plan, as in the tube now laised, consists of a series of cells or pipes at the top and bottom of the tube, ranged in hollow compartments, covered above and below by iron plates riveted together, and having a parallel direction to the long axes of the tubes. These cells are sufficiently large to admit of tbe entrance of a man for the purpose of being cleaned and repaired. It was originally proposed to have a double tier of these cells at the top, more effectually to resist the crushing force, but the construction of a singe tier was subsequently resolved on as sufficient. These cells have the effect of placing the resistances of compression and expansion nearly in equilibrium with each other. The utmost tenacity and stiffness is obtained, with tbe least possible weight of material, and it is believed that a tube of 1,000 feet span might be constructed with safety upon this cellular system of rigidity and strength. Trains have passed through the smaller scale of tube at Con way for months, without any injury or effect whatever. No motion can be detected by the eye during the passage of a train, and instrumental observations detect a deflection of only the eighth of an inch. It has been found that the effect of a very severe gale from the north-west on the side of tbe Conway tube, at an angle of about 50 degrees, produced an oscillating motion, which being carefully measured gave a lateral deflection of onlyO'23 inch. It is found, moreover, that the enormous weight of the tubes themselves, opposed to the impulse of tbe wind, is a security against continuous oscillation. Tbe effect of temperature constantly produced, but in no way injuriously, on these large surfaces of iron, is remarkable. In the afternoon, the sun shining on the top, and on one side, tbe tube is bent in that direction 096 inch, or nearly 1 inch. On comparing tbe deflections of the morning with those of mid-day, a decrease of 7*lo of an inch was observable, or a rising in the middle of the tube had taken place to that extent. The result of a change of temper, ture is an increased convexity in curvature of the top and bottom sides, to the extent of raising the entire tube, o*7l inch in the middle. A large number of distinguished visitors have been to inspect tbe tubes and stapendous works luring the last few weeks, amongst them their Royal Highnesses the Duke and Duchess of Cambridge, who spent a considerable time over the tubes, Prince George of Cambridge, Lord Vivian, several of the nobility, and almost every engineer in the kingdom. It is intended by the Directors of the Chester and Hoi j head Railway to give come cheap excursion trains periodically to the Straits, to enable the middle and humbler classes to visit these noblf works of art. Two additional cylinders have been cast for the lifcing of the next tube, as a reserve, in the event of the one in work failing. All the preparations are complete for the floating of the next tube, which will take place about the middle of November. The day at present fixed on, should no unforeseen contingency occur, for the completion of one highway and the transit of ;he first train over the Straits is tbe first of March next.