Mr. CUDWORTH said, he had but little to add to the Paper, in which, although condensed as much as possible, it did not occur to him that anything material had been omitted. He was glad to have had the opportunity of bringing under the notice of the Institution, a viaduct in which economy of materials had been much studied, and in which this appeared to him to have been carried almost as far as was consistent with due regard to safety. He had brought into comparison with this viaduct one constructed of iron; and as both structures were designed by the same Engineer, it might be inferred that similar ideas, as to economy of materials and of fitness, prevailed in both cases.

Mr. G. P. BIDDER, Past-President, said, every Engineer must be guided in the selection of the particular material, or mode of construction, by the locality, the facility of obtaining different materials, and the purposes which the work was intended to serve. No doubt viaducts, like all other works, were now constructed at less cost than was the case in the early days of railways, when the stereotyped price of a viaduct was 6s. 8d. per cubic yard. It appeared from the Paper, that the Hownes Gill Viaduct had cost 58. 1d. per cubic yard. This was a reduction of nearly 20 per cent. ; but he did not think that was more than had been effected upon other works, as the result of better appliances, and the application of larger capital. The admirable spirit in which this Paper was written was highly commendable; and if it had been prepared by Mr. Bouch himself, the comparison between the different works could not have been more fairly stated. This evinced such a desire to elicit real truth, that the Author was deserving of great praise. With regard to the use of either iron, or wood, or brick for structures of this description, the decision must entirely rest upon the judgment of the Engineer. There were circumstances in which timber structures were obliged to be erected, as ancillary to other designs. In Norway, viaducts of 90 feet and 100 feet in height had been constructed of that material, in the readiest manner, and with great economy, where they were afterwards to be replaced by embankments. It would appear from the statement of the Author, that under the particular conditions with which Mr. Bouch had to deal, iron was the most suitable material for those works, which were simple, substantial, and free from undue motion; and looking at the cost, it showed that economy of material had been carefully considered. The Paper brought out facts of great practical value, and gave data for the construction of extensive viaducts. This information could not fail to be useful to Engineers, when they were called upon to design works of this description.

Mr. BOUCH said, the subject had been dealt with in so fair and comprehensive a manner, as to leave but little room for remark.

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With regard to the comparison which had been made between the Hownes Gill viaduct and the iron structure, he thought that there were no points of analogy between the two works. He quite agreed that the brick structure described in the Paper was a proper one in that particular situation; but in the other case the circumstances were so changed, that neither brick nor stone could have been used. The Hownes Gill viaduct was designed for a railway which had already been many years in operation. There were thus facilities for the conveyance of the materials, which were procured in the district, on the most favourable terms, as the viaduct was to be built for the owners of the railway. Therefore, if he had again to design that bridge, he should adhere to the plan that had been adopted. But in the case of the two iron viaducts on the South Durham and Lancashire line, the circumstances were entirely different. At Deepdale, there was a cutting of 200,000 cubic yards, which could not be commenced till the viaduct was completed, as the whole of the material had to be carried across the valley, to form an embankment on the other side; while at Beelah, there was a cutting containing 50,000 cubic yards of rock, which also had to be taken across the bridge, and could not on that account be commenced until the viaduct was built. Those were circumstances which weighed with him, in estimating time as money, and as railways must be treated as commercial enterprises, the delay of a few years in their construction might be sufficient to condemn an undertaking. It was therefore necessary, without reference to cost, to adopt some form of structure which should enable the line to be completed within a reasonable time. Although in the immediate vicinity of both works there was a vast quantity of stone, yet it was only on one side of the Gills, for on the other side, especially at Beelah, he scarcely knew any district in which stone was so scarce. In point of fact, looking at the quantity of stone required for the foundations of the Beelah viaduct, it cost nearly as much as the whole of the ironwork. On the South Durham line, there were a great number of viaducts, many upwards of 100 feet in height, which were built of rubble, after the plan first adopted by Mr. Hawkshaw. Those were the cheapest on the line, and he had built two viaducts upon that plan in Scotland, 80 feet in height, at a cost of £20 per lineal yard for a single line. In all cases he had used stone, where time allowed him to do so, and the material was at hand. In other cases, he had adopted stone piers, with trussed iron girders; but then the iron was merely used to save time, bearing out the remark, that in works of this description no fixed rule could be laid down. He might mention that the sixteen openings of the Beelah bridge were completed in four months, or at the rate of one span per week, including the erection of the piers. In