3

E.—2

The facts as to the breaking of the bridge are described in the accompanying Report, Appendix A, from the District Engineer. As the accident occurred at 4 o'clock in the morning, these facts are necessarily meagre. All that can be positively known must be derived from the subsequent condition of the materials. The anchors on the south side broke; which broke first cannot now be ascertained ; the other followed immediately. Both broke in the same relative places, about a foot from the outside bearing relatively to the line of the bridge; that is to say, the eastern anchor about a foot from the eastern end, and the western anchor the same distance from the western end of the anchor. This is not theoretically the weakest point of the beam; but it is apparent that in both, the fracture occurred at the point at which two holes are most nearly opposite one another. This is clearly shown in the plan attached to Mr. O'Connor's report. The only question remains to assign a cause for the accident. It appears from the calculation of the Engineer-in-Chief (Appendix B) that the anchor ought to have borne a strain of 396 tons, if equally distributed, equivalent to 198 tons at the middle. Prom the reports of the superintendents of the two principal iron-foundries in Wellington, who have examined the fractured anchors, and whose reports are appended (Appendix C), each anchor is estimated to bear a breaking strain, according to one of these gentlemen, of 139 tons 16 lbs., and by the other, of 141 tons 18 lbs., a very nearly similar result. A variety of formulae are used by engineers for calculating the breaking weight which cast-iron beams Avill bear. We have tested the calculations by several of these formulse, and arc satisfied that with ordinarily good iron, such as alone should have been used for the purpose, the estimated strain of 140 tons is within that which each anchor should have borne, applied at the middle. It appears, hoAvever, from the calculations furnished by the Engineer-in-Chief (Appendix B), and from Mr. O'Connor's report, that the actual strain on each anchor could not at the moment of fracture have exceeded 75 tons distributed, equal to a strain of 56 tons in the middle; and in this estimate no notice is taken of the fact that the trusses of the bridge rested at each side on the solid ground; and although they were not quite equal to support themselves for the whole length of the bridge unaided, still, to whatever extent they may be regarded as rigid, they must to that extent have relieved the chains of some part of the weight of the bridge. The extent of this relief cannot be readily calculated. Mr. Carruthers states that he has roughly calculated that as much as 50 tons of the whole weight of the bridge, which was about 82 tons (without the chains), must have been borne by the trusses. No such calculation could be relied on as exact. It is sufficient, however, to show that a very appreciable part of the weight was not on the chains, and the result is that the real strain on each chain cannot have exceeded 50 tons on the middle of the anchor-plate, giving a factor of safety of 36 according to Mr, Carruthers' calculation, and more than 2*B according to the estimate of Mr. Robertson and Mr. Seager. We attribute the accident to two causes : First, tne misunderstanding as to the filling in of the concrete in front of the anchor-plates as soon as the chains had been brought to an even strain. Secondly, the defect in the anchor-plates. We do not give much importance to the faults stated to exist in the castings. Mr. Dobson detected a considerable fault, but his examination was rather hurried, as he was just leaving for Melbourne; and when the rust was removed from the surface of the fracture by washing, it appeared that he was deceived in the appearance of an air bubble. Mr. O'Connor speaks of a crack of some depth, but was unable to say how far it extended. On the whole, it may be admitted, the casting was a tolerably fair one; but the iron used was of weak quality, inferior to that which ought to have been put into a beam to be submitted to a high strain. Again, the mould was in a wrong position, the top of the casting being the part to be brought under tension, and the bottom under compression. A casual inspection of the two fragments submitted herewith will show the difference in the texture of the iron at the top and bottom of the casting. The part in tension should have been at the bottom. The casting was also undoubtedly cooled too quickly, and was in natural tension before any weight was brought upon it.