212 



SCIENTIFIC NEWS. 



[Aug. 31, ri 



Bridge," by Mr. F. E. Cooper, M.I.C.E., resident engineer. 

 After a reference to the Forth Bridge as a most important 

 link in railway communication, he said — The total length 

 of the viaduct will be 8,296 ft., or nearly if mile. There 

 are two spans of 1,710 ft., two of 680 ft., and fifteen of 

 168 ft. girders, four of 57 ft, and three masonry arches 

 of 25 ft. The clear headway for navigation will be not 

 less than 150 ft. for 500 ft. in the centre of the 1,710 ft. 

 spans. The extreme height of the structure is 361 ft. 

 above, and the extreme depth of foundations 91 ft below 

 the level of high water. There will be about 53,000 

 tons of steel in the superstructure of the viaduct, and 

 about 140,000 cubic yards of masonry and concrete in 

 the foundations and piers. The main piers, three in 

 number, consist each of a group of four masonry columns, 

 faced with granite, 49 ft. in diameter at the top and 36 ft. 

 high, which rest either on the solid rock or on concrete 

 carried down in most cases by means of caissons of a 

 maximum diameter of 70 ft. to the rock or boulder clay, 

 which is of almost equal solidity. The stresses to be 

 provided for are those arising from the weight of the 

 structure itself, the rolling load, and wind, as well as 

 from change of temperature. The rolling load has been 

 taken as one ton per foot run on each line of rails over 

 the whole structure, or a train on each line, consisting of 

 60 short coal trucks of 15 tons each, headed by two 

 locomotives and tenders,- weighing in the aggregate 142 

 tons. The wind pressure provided for is a pressure of 

 55 lbs. per square foot, striking the whole or any part of 

 the exposed surface of the bridge at any angle with the 

 horizon, the total amount on the main spans being 

 estimated at nearly 8,000 tons. The material used 

 throughout is open-hearth, or Siemens-Martin steel. 

 That used for parts subject to tension is specified to 

 withstand a tensile stress of 30 to 33 tons to the square 

 inch, with an elongation in 8 in. of not less than 20 per 

 cent. ; that subject to compression only a tensile stress of 

 34 to 37 tons per square inch, with an elongation in 

 8 in. of not less than 17 per cent. Strips of each class 

 i\ in. wide are to bend cold round a bar, the diameter 

 of which is double the thickness of the strip. The 

 tensile strength of the rivet steel is 26 to 30 tons per 

 square inch. The superstructure of the main spans is 

 made up of three forms of double cantilevers resting on 

 the three piers before mentioned. Those on the shore 

 sides are 1,505 ft , and that on Inch Garvie (an island 

 fortuitously dividing the deep water space into two 

 channels of nearly equal width) is 1,620 ft. in length. 

 The effective depth over the piers is 330 ft. and at the 

 ends 35 ft. The centre portions of the two 1,710 

 ft. spans on each side of Inch Garvie are formed by 

 two lattice girders 350 ft. in length and 50 ft. deep 

 in the centre, and 37 ft. deep at the ends. 



Mr. T. G. Clark, of New York, said the work was of 

 such a novel nature, so different from anything which 

 had ever been done before, that one felt extreme modesty 

 in giving an opinion at all. The point, he thought, which 

 would strike the visitor for the first time would be that 

 the bridge was not only a very strong bridge, but that it 

 looked very strong. It carried to the eye that appearance 

 of stability which every bridge ought to have. When 

 they looked at the bridge, they would see that it was a 

 great object lesson in the mode of calculating the strains 

 upon frame structures. Everything was done in a plain 

 and common-sense manner, and the great difference 

 between the Forth bridge and all other bridges of 

 British construction was the fact of the great concen- 



tration of material along the lines of strain, the object 

 being to expose as little surface as possible to the enor- 

 mous force of the wind. It would not be possible to 

 appreciate the architectural features of the bridge until 

 the scaffolding was removed, but when that was done he 

 believed it would be said that the structure was one of 

 the finest pieces of architecture in the world. 



Mr. Wrightson (Stockton-on-Tees) said he thought one 

 of the features which ought to be recognised in this great 

 engineering work was the immense leap forward which 

 it represented in building practice. There was another 

 bridge crossing the East River in New York, which 

 approximated to the span of the Forth Bridge, but that 

 bridge, wonderful though it might be, was merely an ex- 

 tension of the suspension principle so well known in 

 bridge work, whereas this was, as he had said, a leap 

 forward of a very remarkable character. He did not 

 think there was any bridge of cantilever structure which 

 approached even half the span of the Forth Bridge. It 

 was always a very interesting thing to watch any sudden 

 leap forward in engineering practice, but, as a rule, they 

 found that the most successful works were carried out by 

 a gradual movement forward, and not by these sudden 

 leaps. 



A paper giving a description of a new pyrometer by 

 Professor J. Wiborgh, School of Mines, Stockholm, was 

 read by the secretary. It was stated that compared with 

 measures of temperature of the same sort that had pre- 

 viously been used, this new air pyrometer had several 

 important advantages, as it was of a simpler construction 

 and could be handled by a common workman. 



A paper by Professor Crum Brown (of Edinburgh) on 

 the chemical processes involved in the rusting of iron 

 was read by Dr. Gibson. The writer of the paper said 

 his attention was first called to the subject by observing 

 what happened when a drop of rain fell on a clean, 

 bright surface of iron. At first for a short time the drop 

 remained clear, and the bright surface of the iron was 

 seen through it ; but soon a greenish precipitate formed 

 in the drop, and this rapidly became reddish-brown. 

 The brown precipitate did not adhere to the iron, but 

 was suspended in the water, and became a loosely 

 adhering coating only when the water had evaporated. 

 In speaking of rusting he meant the formation of rust on 

 the surface of metallic iron exposed to ordinary atmo- 

 spheric conditions. It had been conclusively shown that 

 the necessary conditions for the production of rust are — 

 1st, metallic iron ; 2nd, liquid water ; 3rd, oxygen ; and 

 4th, carbonic acid ; both the latter being dissolved in the 

 the liquid water. Iron remained quite free from ru=t in 

 an atmosphere containing oxygen, carbonic acid, and 

 water vapour, so long as the water vapour did not con- 

 dense as liquid water on the surface of the iron. He 

 then considered the action on iron of the three substances 

 — liquid water, oxygen, and carbonic acid. The con- 

 tinuation of the process of rusting was not dependent on 

 new carbonic acid absorbed from the air, but the original 

 carbonic acid, if not removed, could carry on the process 

 indefinitely as long as liquid water was present and 

 oxygen was supplied from the air. Once the process 

 was started, it went on more rapidly, because the 

 porous rust not only did not protect the iron, but 

 favoured by its hygroscopic character the condensation 

 of water vapour from the air as liquid water. A piece 

 of iron, therefore, which had begun to rust would con- 

 tinue rusting in an atmosphere not saturated with water 

 vapour, an atmosphere in which a piece of clean iron 



