120

FORCE OF THE WIND UPON THE TUBES.

ton at a time, till it attained a deflection of 2^ inches with 12 tons. Atthis stage, it was deemed expedient to discontinue the experiments; andalthough I am satisfied it would have carried about 21 tons, it was never-theless desirable not to damage the tube, but to reserve it, with all theparts unimpaired, for the final experiment, which is now in progress.

If we adhere to our original calculations of 50 lbs. on the square footfor the lateral pressure of the wind, we shall find that the model tube, ifused as a girder, would have to resist a force of 16,950 lbs. or 7 \ tons.Now assuming the limit of fracture to be 20 tons (which I have no doubtthe tube would have carried), we shall then have a resistance equal to40 tons over the whole surface, which is about 5| times greater than theforces to which it ever can, at any time, be subjected. If we extend thisreasoning to one of the Britannia tubes, and again assuming the lateralstrength to be 750 tons at a depth of 30 feet in the middle, the resistingpowers will then be 750—500 (half the weight of the tube) =250 tonsin the middle, or 500 tons equally distributed over its surface. This isirrespective of its own weight; and supposing the tube to be in its ver-tical position, the lateral power of resistance would then be half its ownweight 500 + 500=1000 tons, as an antagonist force to the action of thewind.

Again, comparing this force with the actual strength of the wind at50 lbs. to the square foot, we have an excess of strength in the ratio of1000 to 280, which I apprehend is more than sufficient to render eventhe single tube secure. I am aware, that much stress is laid upon theeffect of pulsations produced by sudden gusts. All this must be guardedagainst by looking at the resisting powers of the model, and the verylimited deflection produced by heavy weights. My mind is quite re-leased from all fears on this score, and indeed much more so than it wason any former occasion. Viewing altogether the many corroborativefacts that have been brought to bear upon this subject, and the import-ant results that have been obtained, it appears to me that we have the mostconvincing and satisfactory proofs of the accuracy, as well as the import-ance, of the first and earliest conception of this structure.

It is true that we have it asserted, and from no mean authority, thatwe are verging upon the utmost limit to which a tubular bridge couldbe made; but to this doctrine I do not subscribe, as I firmly believe thatthe present span of the Britannia Bridge may be doubled with our presentmeans; and even then we may be at some distance from the practical