1887.] on Bridging the Firth of Forth. 145 



there was still less difficulty in removing the sand bags temporarily 

 used to form a level bed. The interior rock was excavated as easily 

 as on dry land, the whole of the 70 ft. diameter by 7 ft. high 

 chamber being thoroughly lighted by electricity. Access was 

 obtained through a vertical tube with an air lock at the top, and 

 many visitors ventured to pass through this lock into the lighted 

 chamber below, where the pressure at times was as high as 35 lbs. 

 per square inch. 



At Queensferry all four piers were founded on caissons identical 

 in principle with those used for the deep Garvie piers. The deepest 

 was 89 ft. below high water. Instead of a sloping surface of rock 

 the bed of the Forth was of soft mud to a considerable depth, 

 through which the caissons had to be sunk into the hard boulder 

 clay. The process of sinking was as follows : The caisson being 

 seated on the soft mud which, of course, practically filled the 

 working chamber, air was blown in and a few men descended the 

 shaft or tube of access to the working chamber in order to clear away 

 the mud. This was done by diluting it to the necessary extent by 

 water brought down a pipe under pressure, and by blowing it out in 

 this liquid state through another pij)e by means of the j^ressure of 

 air in the chamber. It was found that the mud sealed the caisson, 

 so that a pressure of air considerably in excess of that of the water 

 outside could be kept up, and it was unnecessary to vary the pressure 

 according to the height of the tide. In working through this soft 

 mud both intelligence and courage wove called for on the part of the 

 men, and it is a pleasure and duty for me to say that the Italians and 

 Belgians engaged on the work were never found wanting in those 

 qualifications. There was always a chance of the caisson sinking 

 suddenly or irregularly, and imprisoning some of the men, and 

 indeed on one occasion a few men were buried up to their chins in 

 the mud, and on another the caisson gave a sudden drop of 7 ft. 



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

 of the mud bottom. On new year's day, 1885, the S.W. 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 proceeded a certain 

 extent the caisson collapsed owing to the heavy external pressure 

 of the water, and two men were killed. 



Vol, XII, (No. 81.) 



