150 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



Each of the lifting bars was proved to a tensile strain of 

 150 tons, or nearly 8 tons per square inch. The aggregate proof 

 strain of the four bars was therefore 600 tons, and as their 

 calculated breaking strain was about 1600 tons, the coefficient 

 of safety was 4'5. 



The foundation was prepared first by dredging, and the 

 surface afterwards completed and very carefully levelled by 

 means of a diving-bell. This bell (Figs. 127, 128) was of special 

 construction, and consisted of a chamber 20 feet square at the 

 bottom, 16 feet square at the roof, and 6 feet 6 inches high 

 inside. A vertical tube 3 feet in diameter rose from the centre 

 of the roof to a height of 37 feet 6 inches, and in the upper 

 end was formed the air lock, 6 feet 6 inches high, with suitable 

 fittings for admitting compressed air into the bell. 



The bell weighed about 80^ tons, and was suspended from 

 shear legs projecting over the stern of the barge by two If -inch 

 close-linked chains rove through two sets of pulley blocks. Two 

 other sets of blocks were attached to the ends of a cross-head 

 of plate and angle iron riveted outside the funnel, and connected 

 by strong iron stays to the chamber of the bell. Inside the 

 bell, two large sheet-iron trays were suspended by chains from 

 the roof. Each tray was 11 feet long, 4 feet wide, and 18 

 inches deep, and were for the purpose of removing the excavated 

 material. They were hung on gimbals, and were so adjusted as 

 to readily tilt and to discharge the load. 



The barge for carrying the bell was rectangular (Figs. 127, 

 128) in form' 80 feet long, 30 feet wide, by 8 feet deep, and 

 was fitted with suitable shears from which the bell was sus- 

 pended over the end. The barge also carried the engine and 

 boilers, air-pumps and winches, and all other gear necessary 

 for working the bell. A quantity of concrete at one end 

 balanced the weight of the gear at the other, and ensured 

 a safe free-board at the stern when the bell was out of the 

 water. 



The operations of preparing a section of foundation were 

 as follows : The site having been roughly dredged, the bell 

 was placed in position and rested on the bottom. The excavated 

 material was then loaded into the trays ; when filled, the proper 

 signal was given, the bell raised from the bottom, and removed 

 to the proper position for discharging. On a signal from the 

 deck, the hanging trays were canted by the men in the bell, and 



