428 REPORT— 1882. 



struts with tlie tabular bottom members offered some little diflBculty at 

 first in designing, but this being surmounted, the manufacture of the 

 junction lengths becomes, as it were, a piece of ship-builder's work, except 

 that the ship-builder has to deal with far more complex curvatures than 

 will be found in the junction lengths. 



Before dismissing the question of steel in compression, the author 

 would wish to mention the results of some experiments made by him on 

 steel which had been previously subjected to end pressure in a tube, so 

 that flexure could not occur. The rods were 1 inch diameter by 30 inches 

 long. With the mildest steel, the resistance to flexure was 14"'5 tons per 

 square inch in the uncompressed rods, and 22 tons, or say 50 per cent, 

 more, in the rods which had previously been subjected to an end pressure 

 of 36 tons per square inch. With somewhat harder steel, the corre- 

 sponding figures were 16 tons in the uncompressed, and from 26 to 29 

 tons in the previously compressed bars — a gain of from 60 to 80 per cent. 

 The bars, when tested in tension, showed no loss in strength or elonga- 

 tion from the previous compression. As 30.ton steel, after the end pres- 

 sure had been applied, bore as much load without flexure as 54-ton steel 

 which had not been so treated, it is clear that the adoption of mild steel 

 in railway bridges would be much accelerated if some simple practical 

 method could be devised for bringing about the molecular change effected 

 in the above instances by an end pressure. 



It has been stated that the maximum wind-pressure upon the 1,700- 

 feet span has been assumed to be equivalent to a pressure of 56 lbs. per 

 square foot upon double the supci'ficial area of the girder. It is to be 

 regretted that this assumption necessarily involves many matters of piire 

 conjecture. For example, though a wind-pressure of 56 lbs. has un- 

 doubtedly been registered by anemometers exposing a surface of a couple 

 of square feet, it has never been proved to prevail instantaneously over 

 so gi'eat a width as the 1,700-feet span. Again, the relative resistances 

 offered by the windward and leeward girders of a bridge have not been 

 measured, and still less has any experimental approximation been ob- 

 tained to the resistance of an entire bridge with its floor and cross- 

 bracing. Probably much may be done by models, and the author intends 

 to so ascertain, if possible, the probable resistance of the Forth Bridge 

 expressed in square feet of flat surface. Exjjeriments with relation to 

 wind-pressure have been commenced at the site of the bridge — a pressure 

 board, 20 feet long and 15 feet wide, having been fixed on the top of a 

 tower on the island of Inchgarvie, where the central pier of the bridge 

 will be placed. The apparatus is adapted to test the relative resistances 

 offered by different surfaces, and during the progress of the works it is 

 hoped many now open questions may be settled. The experiments will 

 probably not affect in any way the design of the bridge, because the lead- 

 ing features of the design and the working stresses and wind-pressure 

 to be provided for have long since been settled with the Board of Trade. 

 Until the experiments are complete, however, it will be impossible to 

 state with precision what factor of safety will belong to the woi-k, though 

 it is possible to state, without reservation of any kind, that in any event 

 the Forth Bridge, as designed, will be relatively stronger than any other 

 bridge yet constructed. Owing to the large dead weight and the spread 

 of the girders at the piers, a wind-pressure equivalent to 2 cwt. per 

 square foot upon the surface of one girder would be required to overturn 

 the bridge, assuming it not to be held down by bolts. The holding-down 



