some instances the difficulties with regard to the foundations were the greatest he had ever encountered. The Westmoreland Gills, as they were termed, were nothing more than great cracks in the mountains. There might be rock at the sides of the Gill, giving the appearance of great solidity; but as soon as it was disturbed, it was found to be only débris from the sides, easily set in motion, and with considerable tendency to slip. In fact, there was one large slip close to the viaduct, which carried away a cottage and a smithy, and for a time placed the viaduet in jeopardy. As an instance of the difficulties of the foundations, he stated that, after the railway had been for some months in operation, it was found that one of the columns of the piers, instead of resting upon the masonry foundation to which it was bolted, had the masonry hanging to it. Although express and heavy mineral trains had been running over the line perhaps for months before this condition was discovered, still the work had withstood this test. It was speedily remedied, and he had no fear that it would occur again. It was but fair to the contractors to add, both as regarded the Hownes Gill viaduct, and the Deepdale and the Beelah iron viaducts, that as examples of good workmanship he believed they could not be surpassed.

Mr. SHEILDS thought the comparison between the cost of a brick viaduct and of an iron one was hardly fairly put. In his opinion, in almost all cases, except under extraordinary circumstances, an iron viaduct of the dimensions referred to, could be erected more economically than one of masonry. He did not mean to say that either in durability, or in solidity, an iron viaduct was superior to one of masonry, provided the foundations were good; as a more rigid structure could be formed of masonry than of iron girders, which always involved a greater amount of deflection and vibration, and probably therefore of comparative instability; but for various commercial and practical considerations, he thought iron was generally superior to masonry for this class of structures. For instance, in the comparison made by the Author, the brick structure in question was estimated to cost £20,681, whereas it was calculated that an iron one could be erected in a similar situation for £16,249, or with a saving of one-fifth. Moreover, in considering the prices on which these estimates were founded, it must be noticed that the bricks were obtained on favourable terms, both as regarded first cost and delivery, whilst the ironwork was taken at £22 10s. per ton, which, in the present day, was certainly a high rate. Under these circumstances, he believed the saving in first cost by the use of iron would be considerably more than had been stated. The fact that large quantities of bridge ironwork were sent to India, Spain, Russia, and the most distant parts of the world, even where masonry work was cheap,

was an argument in favour of the superior advantages of iron structures for large bridges, as it was reasonable to suppose that cost was an important element in these cases.

Mr. BIDDER could not allow the observation to pass unnoticed, that the large extent to which ironwork was sent to India and other places was a proof of its superiority to masonry. That was a mistake; and indeed it might just as well be said, that iron girders were used in the Charing Cross Bridge, because they were cheaper than brickwork, whereas it was well known that that plan of construction was adopted, in order to avoid increasing the height of the railway. Ironwork was sent to India, not on account of its economy over brickwork, but to serve a particular object—in other words, to keep down the level of the railway over the rivers where brickwork could not be applied without raising the railway to an undue height.

Mr. SHEILDS said he had referred to cases in his own practice, where the cost was calculated, and where iron was selected solely for the purposes of economy. There was more scope for economically designing a large viaduct in iron than in masonry; as the spans could be made much wider in the former material. Of course the greater the openings, the fewer would be the piers, and vice versâ ; and when the piers were of great height, it was important to diminish their number by enlarging the spans; which could be done more easily and cheaply with iron girders than with arches of masonry. Keeping this in view, it would be seen, that when the cost of a pier, and the weight of ironwork for girders of various-sized openings, were known beforehand, it would be easy, by a trial and error process, to arrive at a minimum of cost for the piers and girders together forming the whole bridge, and he repeated that his own experience had proved to him that ironwork, except under peculiar circumstances, could be erected at a cheaper cost than masonry, in a viaduct such as that referred to in the Paper.

Professor POLE said, that having sent out large quantities of material for Indian railways, he could state that one great reason for the large use of iron upon works of construction in that country, was the saving of time. On the greater part of the East Indian line, for example, brickwork was the natural material of the country; both bricks and labour were very cheap, and the work could be well executed by the native workmen at a cost much lower than in Europe. But there was found to be a limit to the quantity of bricks which could be made in a given time, and as the nature of the country required bridges and flood-works of enormous extent, it soon became evident that if all these were to be constructed of brickwork, the completion of the line must be delayed to an almost indefinite period. For this reason, it was the interest of the com

pany to send out large quantities of ironwork, at an expense much greater than need have been incurred, if the normal material of the country could have been used.

Mr. W. H. BARLOW concurred in the remark, that no rule could be laid down, as to the material of which viaducts should be constructed. He was at present constructing a line of railway upon which there were several viaducts of considerable extent; and in all ordinary cases he had found simple rubble to be the cheapest and the best. But for skew bridges, and under some other circumstances, iron could be used with advantage. He did not think that, at present, either iron or stone could be said to be the cheaper material for viaducts; the choice must be governed by the locality.

Mr. JOHN FOWLER, V.P., said, it was difficult usefully to compare ironwork with masonry, unless all the circumstances were stated in great detail. He had seen the Hownes Gill viaduct, and could pronounce it to be a most substantial work. The dimensions were of the ordinary kind, the material was good, and the foundations. were carried down according to the nature of the ground. There was, however, one peculiarity upon which further information was desirable; that was the remarkable spread of the piers, by means of the buttresses. This was carried to a greater extent than he had ever known in a viaduct of similar dimensions. The piers had been extended, laterally, to nearly three times their length at the springing of the arches. He should be glad to know whether the object was to give greater base to the foundations (which he presumed must be the case), and for architectural effect, or whether it had been done with reference to the vertical stability of the piers, which he could scarcely think necessary. The buttresses were certainly not, primâ facie, a necessity for the stability of the work, or for its appearance, and they must have involved considerable additional expense. As to recesses, or openings, in piers, the question, whether desirable or otherwise, must always depend upon the nature and value of the materials used.

Mr. J. SCOTT RUSSELL said, in the bridge aqueduct of Roquefavour, on the line of the canal of Marseilles, there was the peculiarity of the buttresses spreading very much at right angles to the centre line; but the piers were perfectly parallel and vertical on their inner faces. In the Hownes Gill viaduct there was an increase in the dimensions both ways, which required to be accounted for.

Mr. BOUCH remarked, that the lateral spread of the piers was to secure a greater area for the foundations. As the piers were only 12 feet in length at the springing of the arches, and the height of

1 Vide Minutes of Proceedings Inst. C.E., vol. xiv., p. 190.

the loftiest ranged from 110 feet to 114 feet, he thought buttresses were necessary to give steadiness to the structure. As a proof that this view was agreed in by other Engineers, it was only necessary to refer to the fact, that inverts had been added to the original design, in order to give a larger base.

Mr. CUDWORTH said, with regard to the remark, that the comparison between the two constructions was not a fair one, he had furnished data from which every one might form their own conclusions, and thus be enabled to judge how far he had been justified in the observations he had made.

No. 1,051.-"On the Use of the Timber of the Palmyra Palm in the Construction of Bridges." By HENRY BYRNE, M. Inst. C.E. THE well-known fact, that all trees of the palm tribe contain a mass of pithy fibre in the centre of the stem, (the woody and useful portion being thus reduced to the condition of a mere pipe or tube,) would seem to preclude the possibility of employing such timber in a structure pretending to any degree of permanence, and in which beams are a prominent feature, and framing a matter of necessity. But there are extensive districts in Southern India and in Ceylon, and doubtless in other tropical regions, where no other description of timber grows in any quantity, and where consequently bridges, jetties, &c., must either be constructed with such imperfect material, or dispensed with, if the cost of square timber, and still more of stone or brick, be an object of serious consideration.

The tract of country at the northern extremity of Ceylon, known as the Jaffna peninsula, a district of the province of which the Author had charge till recently, as Resident or Assistant Civil Engineer, is thus circumstanced: Intersected in various directions by tidal inlets, its wants as to means of communication, and especially of bridges across these inlets, (which are of considerable width, and fordable only by foot-passengers and not always without risk,) far exceeded the pecuniary means that could immediately be devoted to it out of the colonial treasury, when the Author took charge of the province. These wants could only be met gradually, by adopting the least expensive plans, consistent with efficiency, that could be devised. Obviously, then, timber piled bridges, of the most simple form, were the best adapted to all the circumstances; and the only question to be decided was, what description of timber should be used. The Jaffna district, embracing an area of about 400 square miles, does not produce, in any quantity, such timber as admits of being squared, and the little that there is, can only be procured in short lengths, and of small scantling fit, merely, in fact, for verandah-posts, wall-plates, door-frames, and similar purposes; while imported timber of large dimensions is as costly as it is in England. But, on the other hand, it abounds with cocoa-nuts and Palmyra palms, the first of which is seldom to be found straight for any considerable length, and is in all other respects inferior as a material for building to the Palmyra. This latter timber was accordingly selected, as the only fitting material which the circumstances admitted of being used.