Mr. Johnson. tar for coating rails in tunnels. He had been making some small experiments with it, and had requested that a considerable quantity should be reserved at the Great Northern Railway Company's gasworks at Holloway, in order that it might be tried on an important bridge near King's Cross, where the deterioration on the underside of the bridge was serious. He thought the preparation was one which, if the ingredients were properly selected, would be most useful in the protection of steel against corrosion. Mr. Hawksley. Mr. CHARLES HAWKSLEY, Past-President, asked whether, in lieu of covering the steel or iron with cement, the Author had tried asphalt, because if that would adhere properly to the iron it would probably obviate the difficulty referred to by Mr. Cudworth, namely, liability to crack and leave a space between the iron and the cement, in which moisture might accumulate. On railway-bridges the ballast often rested against the ironwork, which appeared to him to be very undesirable, because naturally the ballast retained moisture. It could easily be prevented by putting in an iron fender to keep the ballast away from the ironwork, leaving sufficient room for painting between the two. Sir Benjamin Sir BENJAMIN BAKER, K.C.B., Past-President, in reference to Mr. Baker. Hawksley's question, mentioned that he had tried asphalt, tar his experience showed that some sort of bituminous mixture Baker. should be used. There were as many fashions in bituminous Sir Benjamin mixtures as there were in pills; he did not think it made much difference which was used. In a place like the Underground Railway he thought nothing short of actually casing the girders in concrete or cement would answer, and even that casing would have to be renewed from time to time. A good deal was heard at the present time as to reinforced concrete obviating many of the difficulties referred to in the Paper, and the Americans were putting in the whole of the floors of some railway-bridges without cross girders at all, using simply slabs of concrete with steel rods buried in them. He had never adopted that plan, and he certainly would not live long enough to have sufficient experience to do so, because he would not trust anything of the sort which had not been tested for 20 years. Engineers were told that steel rods would not rust if they were cased in concrete, and sometimes rods were put in only about 1 inch from the surface of the concrete. He very much doubted whether these latter would not rust in time. Quite recently he had seen at Alexandria hundreds of what might be called concrete joists; they looked about the size of ordinary wooden joists, 12 inches by 4 inches, but were really made of concrete with 1-inch rods put in. They had been up for about 2 years, but at the bearing end they were quite destroyed. The sea-air had penetrated through the rather weak concrete, made of Portland cement with a matrix of rather weak limestone, which was evidently neither air-tight nor damp-proof. The air had got in and rusted the rods so that at the bearings of the joists the concrete had burst out and the rods were exposed. Homely timber joists and wooden props had been put underneath, which was not a good testimony to the efficacy of a coating of concrete as a preventive of rust. He had tried protecting steel girders also with asphalt, and had found the result was not always better than concrete. It was an old practice of shipbuilders to put concrete between the frames of vessels, because it was found that, notwithstanding the racking and strain going on there, it was the only thing which would keep the rivet-heads from rusting off. When the floor of the Battersea suspension-bridge was renewed he saw the state of the buckle-plates. There was about 3 inches of concrete and some wooden pavement which had been down for 20 years; and when the concrete was taken up the paint was still on the plates, which were as sound as ever: so that no general rule could be laid down. The engineers of an earlier day used reinforced masonry, but they called it "hoop-iron bond," and there were many experiences of injurious effects from oxidization, so that Baker. Sir Benjamin special care must be taken to exclude both water and air in order to ensure durability. He thought the Paper did not lend itself so much to discussion as to the contribution of written communications illustrated by sketches. He therefore hoped there would be numerous written communications on the subject, which was of the utmost interest not merely to the Institution but to American engineers, who were quite as keen about the subject as English engineers. The Author. The AUTHOR, in reply, remarked that it was through noting shipbuilders' practice on the Tyne many years ago that he first thought of using concrete on certain bridges which had given trouble through rusting. Contrary to his expectation he had not found any difficulty from vibration. In one bridge the concrete on the top flange of a girder had not been put on thickly enough, and consequently it had peeled off; but underneath the structure had remained perfectly good. In no other case had he had any failure. He generally coated the concrete with tar, so that if a slight crack developed through vibration, the heat on a hot summer's day allowed the tar to run down the crack, and the water did not get in. On taking down some bridges built by him 20 years ago, he had found that the concrete had preserved the iron perfectly. He had recently heard from Mr. L. G. Mouchel that on breaking open some stumps of ferro-concrete, which, after being cut from the tops of ferro-concrete piles in course of construction at Woolston, near Southampton, had lain on the foreshore exposed to the tide for 7 years, the metal bars inside the concrete had been found to be as blue as when they left the maker's mill. There was much difference of opinion with regard to the relative durability of iron and steel, as had been amply evidenced by the discussion. He had not read Mr. Wood's excellent book, "Rustless Coatings," until after he had written his Paper, otherwise it might have been thought he had copied from it. On many points Mr. Wood held the same opinions as were advanced in the Paper with regard to the rusting of steel. Like other members, the Author could only give the result of his own experience. He had had iron and steel bridges under his observation side by side, and he had had iron bridges patched with steel; so that he had had some opportunity of comparing the behaviour of the two materials. In his opinion maintenance was not a remedy for corrosion he was acquainted with one bridge at the seaside which, after being carefully scraped and painted by the trusted men of his own staff, with the best materials, had been almost as bad as ever within 6 months. In an interesting letter Mr. John Wilson, M. Inst. C.E., had communicated to the Author The Author. his views on the question of iron versus steel. Mr. Wilson infinitely preferred iron to steel for girders, as giving a more durable and reliable structure; and although he had recently had to agree to the substitution of steel owing to the difficulty sometimes experienced by makers in obtaining the necessary iron for pressing work, he had held this opinion ever since the use of steel girders became recognized, and he had found no reason to alter it. The question of steel for boilers was also an important one, although it was not referred to in the Paper. The Author had both iron and steel boilers under his charge, and while some of the iron boilers needed renewing only after 20 years of life, the steel boilers often needed renewal after 6 or 7 years. Some years ago he coated the inside of certain steel boilers, with which trouble had been experienced, with a coating of cement. Of course it took a considerable time to obtain experimental results which were of any value, and he was still awaiting the result of this experiment; but he expected to find the insides of those boilers in a much better condition than that of an uncoated boiler. He had found nothing better for an inside coating than tarvarnish—which, he presumed, might be considered to be a kind of bituminous coating-if it was made properly, so that it dried quickly. Girders over important public roads could not be coated with tar-varnish, but he believed that, properly applied in fairly warm weather, it was as good a coating as could be obtained. Asphalt cracked in the hot weather and let the water through, and moreover it needed special gangs of men to lay it. His former chief, Mr. Richard Johnson, M. Inst. C.E., had told him many years ago that in all his experience he had not found anything better than red-lead. He believed that if red-lead was properly mixed with pure linseed-oil, and two coats were applied, nothing better could be obtained. There were many nostrums in the market-most of them being only tar-varnish under another name, for which the public had the privilege of paying about four times their proper value compared with a good tar-varnish made from selected tar. He was informed that the tar should contain very little uncombined carbon (some tars having as much as 50 per cent.), so that the pitch which was one of the chief constituents of tar-varnish should have little or no free carbon in it; otherwise the varnish would not present a glazed face. It should also be made with the lighter oils, so as to dry in about hour. There was a great difference between the rusting of iron and steel bridges, and although it was practically impossible to distinguish a piece of It was a The Author. iron from a piece of steel by the mere appearance and without looking at the fracture, yet it was comparatively easy to tell of what material a bridge was built if there was any rust. frequent occurrence to find a bridge-member heavily painted and in perfect order with the exception of certain places where the rust had forced off the protective coating, leaving quite a deep pitting; and it was this which rendered the proper maintenance of steel much more expensive than that of iron. With regard to the application of concrete, neat or nearly neat cement should be placed next the girder, to which it would adhere strongly. No slag or cinders should be used in the concrete. The President. The PRESIDENT observed that the subject of the Paper was a very interesting one, and he was sorry the time at disposal did not allow of further discussion, as this was the last Ordinary Meeting of the session. He hoped, however, that the subject would be brought up again next session, and he would be glad if some members would be public-minded enough to prepare during the recess one or two Papers on the subject, with an even more extended scope. Traffic-managers were always crying out for an increase of speed and load; the locomotive-superintendent was forced to larger and heavier engines; while the permanent-way and works engineer found that his rails and girders were insufficient to meet these ever-increasing demands. Mr. Archbutt. Correspondence. Mr. L. ARCHBUTT, of Derby, considered that in the painting of ironwork the result depended more upon the way in which the work was done than upon the kind of paint that was used. Of all the paints which he had tried, under conditions which gave each a fair chance, he had found nothing better than red-lead and boiled linseed-oil. As the Author pointed out, the iron or steel must in the first instance be perfectly dry and free from rust or oxide, and the red-lead must be ground with the oil in a mill, and not simply stirred up with it in a bucket. Mr. Archbutt had found it a good plan to grind the red-lead with about 10 per cent. of raw linseed-oil and to thin this mixture with boiled linseedoil just sufficiently to run from the stirrer. The paint must be applied as thinly as possible and be well rubbed in with a brush, |