Discussion. The Craigellachie Viaduct and the Grand River Viaduct, Mauritius Railways.

Abstract

248RAILWAY VIADUCTS.Mr. J. A. LONGRIDGE thought the Paper was so complete, that there was scarcely anything left for him to add. The total cost of this viaduct was upwards of ~30,000, or about $50 per foot forward. The cast-iron columns which were sent from England, Cost,, in erecting, about 28s. per ton. The expense of launching the girders was considerable, owing t,o the disasters that were, in the first instance, met with ; it amounted to nearly Z5. 10s. per ton, including the riveting up of' the sections of the girders, plant, cutting out the gullet, and labour. Had the plan that was subsequently adopted been proceeded with in the first instance, the launching would probably not have cost more than &3. 10s. per ton, as was shown by the fact that one complete span was launched, with the whole weight moved, in about seven hours, by the labour of not more than 100 men. H e felt such confidence in the system, that he should not hesitate to adopt it on almost any scale. There were great difficulties in carrying out such a work in the Mauritius from the want of sufficiently strong tackle, but, in a country where all the necessary appliances were at hand, he considered it the cheapest and best way of launching the girders. The cast-iron columns delivered cost ZS per ton, and the cost of transport to the place was about 21 per ton, which added to 28s. per ton for erection, made nearly Z10.10~. per ton ill place. The wrought-iron work delivered in the Mauritius cost $16 per ton, &l per ton transport, and X23 per ton in place. The land carriage was about 4 miles, over some very steep hills. H e would add, that in putting in the concrete in No. 2 pier, the tarpaulin system, 'which had been originally introduced by Mr. Smeaton, was most successful. There were springs of water from the bottom of the excavation, such as he had seldom seen; but by planking the whole dam in the first instance, and laying down a tarpaulin at the bottom and by pumping, in a few minutes the water did not show a vestige of cement, and when the place was drained for No. 2 block, the cement had set so hard that picks had to be employed to get in the joint. The coral lime, which was the kind used, had no hydraulic properties ; but when kept dry, it set extremely hard, and was one of the best he had ever met with. Mr. G. W. HEMANS remarked, that the Grand River bridgewas an experiment of a striking nature, and one which at first sight almost seemed to threaten failure, but such had not happened. From the height of the columns, the thrust of such long girders upon the rollers must have been very great, and it seemed astonishing that it could have been withstood by the piers witllout rocking. H e thought he should have taken the precaution of tying back the tops of each of the columns, to the feet of the adjoining ones ; but that did not seem to have been necessary