May 26, 1887] 



NATURE 



83 



chamber nine remained imprisoned. Of these, two 

 managed to get their heads into the shaft of access, and 

 were taken out alive after twenty-eight hours, and the re- 

 maining seven were smothered in the mud. It was nearly 

 a year before sinking was renewed. Again, in 1877, one 

 of the air-locks suddenly gave way, and of the men then 

 in the chamber, three escaped uninjured, nine were blown 

 out by the rush of air, and, falling into the water and on 

 craft, were mortally injured, whilst twenty were smothered 

 in the caisson. It was thirteen months before the 

 chamber was accessible, and then the vitiated atmosphere 

 in the charnel-house below rendered it very difficult to 

 work. Happily we had no such experiences at the 

 Forth. 



With one of our caissons we unfortunately had an 

 accident and loss of life, which, although it had nothing 

 to do with the sinking of the caisson, as in the Neva 

 Bridge, was indirectly due to the same cause, viz. the 

 softness of the mud bottom. On New Year's Day, 1885, 

 the south-west Queensferry caisson, which had been 

 towed into position, and weighted with about 4000 tons 

 of concrete, stuck in the mud, and, instead of rising with 

 the tide, remained fixed so that the water flowing over 

 the edge filled the interior. The 4000 tons of water 

 caused the caisson to sink further in the mud, especially 

 at the outer edge, and to slide forward and tilt. The 

 contractors determined to raise the skin of the caisson 

 until it came above water-level, and then pump out and 

 float the caisson back into position. About three months 

 were occupied in doing this, but when pumping had pro- 

 ceeded a certain extent the caisson collapsed, owing to 

 the heavy external pressure of the water, and two men 

 were killed. It was necessary then to consider very 

 carefully what had better be done, as the torn caisson was 

 difficult to deal with. Finally it was determined to case 

 it in " tubbing •'■ of whole balks of timber strutted with 

 ring girders and rakers This was a very tedious work, 

 as every balk had to be fitted water-tight to its neighbours 

 by divers. Finally, on October 19, 1885, or between nine 

 and tea months after the first accident, the caisson, to 

 the relief of everyone, was floated into position and the 

 sinking proceeded without further difficulty, this, the last 

 of the main piers, being completed in March 1886, or 

 almost exactly two years after the first caisson was floated 

 out. No doubt some of my hearers have passed through 

 air-locks and experienced the physiological effects of com- 

 pressed air, one of the first of which is a painful pressure 

 on the drums of the ears. It is necessary to restrict the 

 hours of work, and even then most men suffer more or 

 less inconvenience. Pains in the limbs are generally 

 relieved by galvanism ; a long continuance often leads to 

 paralysis if the depth is great. At the St. Louis Bridge 

 in America, for example, out of 600 workmen who worked 

 in the compressed air, 119 were attacked, 16 died, and 2 

 were crippled. We had no deaths directly attributable 

 to the air-pressure. Personally I felt no inconvenience 

 whatever, ^ Photographs were taken in the caisson, a 

 total lighting power of 6000 candles and an exposure of 

 as much as 15 minutes in some cases being given. Owing 

 to the fog formed when the air blew under the edge the 

 results were not so good as could be wished, the eyes 

 especially coming out in glaring spectral fashion. 



Superstructure.— I must now say a few words 

 respecting the design, manufacture, and erection of the 

 superstructure. 



Design. — I have already illustrated the principle of the 

 cantilever bridge, and need only deal with the details. At 

 the Forth, owing to the unprecedented span and the weight 

 of the structure itself, the dead load is far in excess of any 

 number of railway trains which could be brought upon 

 it. Thus the weight of one of the 1700-feet spans is about 

 16,000 tons, and the heaviest rolling load would not be 

 more than a couple of coal trains weighing say 800 tons 

 together, or only 5 per cent, of the dead weight. It is 



hardly necessary therefore to say that the bridge will be 

 as stiff as a rock under the passage of a train. Wind, 

 even, is a more important element than train weight, as 

 with the assumed pressure of 56 lbs. per square foot the 

 estimated lateral pressure on each 1700-feet span is 2000 

 tons, or two and a half times as much as the roUing load. 

 To resist wind the structure is " straddle-legged,'' that is, 

 the lofty columns over the piers are 120 feet apart at the 

 base and 33 feet at the top. Similarly, the cantilever 

 bottom members widen out at the piers. All of the main 

 compression members are tubes, because that is the form 

 which with the least weight gives the greatest strength. 

 The tube of the cantilever is, at the piers, 12 feet in dia- 

 meter and \\ inch thick, and it is subject to an end 

 pressure of 2282 tons from the dead load, 1022 tons from 

 the trains, and 2920 tons from the wind ; total, 6224 tons, 

 which is the weight of one of the largest Transatlantic 

 steamers with all her cargo on board. The vertical tube 

 is 343 feet high, 12 feet in diameter, and about | inch 

 thick, and is liable to a load of 3279 tons. The tension 

 members are of lattice construction, and the heaviest- 

 stressed one is subject to a pull of 3794 tons. All of the 

 structure is thoroughly braced together by " wind bracing " 

 of lattice girders, so that a hurricane or cyclone storm 

 may blow in any direction up or down the Forth without 

 affecting the stability of the bridge. Indeed, even if a 

 hurricane were blowing up one side of the Forth and 

 down the other, tending to rotate the cantilevers on the 

 piers, the bridge has the strength to resist such a contin- 

 gency. We have had wind-gauges on Inch Garvie since 

 the commencement of the works, and know, therefore, the 

 character of the storms the bridge will encounter. The 

 two heaviest gales were on December 12, 1883, and 

 January 26, 1884. On the latter occasion much damage 

 was done throughout the country. At Inch Garvie the 

 small fixed gauge was reported to have registered 65 lbs. 

 per square foot, but I found on inspection that the pointer 

 could not travel further, or it might have indicated even 

 higher. I did not believe this result, and attributed it to 

 the joint action of the momentum of the instrument, and 

 a high local pressure of wind too instantaneous in duration 

 to take effect upon a structure of any size or weight. The 

 great board of 300 square feet area on the same occasion 

 indicated only 35 lbs. per square foot, and I doubt much if 

 the pressure would have averaged more than 20 lbs. on so 

 large a surface as the bridge. 



Manufacture. — The bent plates required for the tubes 

 of the Forth Bridge would, if placed end to end, stretch 

 42 miles. Special plant had to be devised for preparing 

 these plates. Long furnaces, heated in some instances 

 by gas-producers, and in others by coal, first heated the 

 plates, which were then hauled between the dies of an 

 800-ton hydraulic press, and bent to the proper radius. 

 When cool, the edges were planed all round, and the 

 plates built up into the form of a tube in the drilling- 

 yard. Here they were dealt with by eight great travelling 

 machines, having ten traversing drills radiating to the 

 centre of the tube, and drilling through as much as 

 4 inches of solid steel in places. A length of 8 feet was 

 drilled in a day of twenty-four hours. When complete, 

 the tubes were taken down, the plates cleaned and oiled, 

 and stacked ready for erection. 



The tension members and lattice girders generally are 

 of angle bars, sawn to length when cold, and of plates 

 planed all round. Multiple drills tear through immense 

 thickness of steel at an astonishing rate. The larger 

 machines have ten drills, which, going as they do, day 

 and night, at 180 revolutions per minute, perform work 

 equivalent to boring an inch hole through 280 feet thick- 

 ness of solid steel every twenty-four hours. About 4 per 

 cent, of the whole weight of steel delivered at the works 

 leaves it again in the form of shavings from planing- 

 machines and drills. The material used throughout is 

 Siemens's steel of the finest quality, made at the Steel 



