472 REPORT—1899. 
under the regular water level, and the left abutment to a depth of 
750 metres (244 feet) according to indications from soundings. . 
The rear part of each one was left hollow and filled with sand. 
Foundations of such a size are quite beyond the proportions com- 
monly used even for bridges over very large rivers ; but the unfavourable 
conditions have been so great, namely, width of the bridge, flatness of 
arches, and character of subsoil, that the foundations of this modest 350 feet 
single span come into comparison for size with those of its older brothers, 
the Brooklyn Bridge, the Forth Bridge, and the magnificent Tower 
Bridge. 
Surface of Foundations at the Bottom. 
sq. m. sq. ft. 
Brooklyn Bridge (single caisson) é ; : : . 1,716 = 18,450 
St. Louis Bridge (single caisson) 5 : : j . 620 = 5,590 
Forth Bridge (largest caissons) . “ 5 - - about 350 = 3,760 
Tower Bridge (each pier) . é : : : . about 1,510 = 16,200 
Pont Alexandre III. (each abutment, single caisson) . 1,474 = 15,850 
Each abutment was put in place by means of a single pneumatic 
caisson. The frame of the caisson, entirely made of mild steel, consisted 
of a vertical water-tight wall 3°68 metres (12 feet) in height, encircling the 
whole surface of the foundation, and of a horizontal equally water-tight 
partition at about 1:90 metres (6 feet 3 inches) above the lowest level of 
the wall. 
This partition was the roof of the working space. As it was designed 
to sustain a heavy load of masonry while the caisson was sinking, the © 
steel sheets (5 millimetres thick) which form the water-tight ceiling of the 
caisson were reinforced by two systems of girders perpendicular in 
direction. 
Below the ceiling four partitions or supporting girders divided the 
working space into five sections or rooms, communicating with each other 
through the central latticed parts of the girders. 
These four partitions and the walls were fitted with solid edge cutters 
at the bottom, and with angle brackets. 
Above the roof a set of twenty-seven latticed beams 1°60 metres (5 feet 
6 inches) in height were laid at right angles to the girders just described. 
These beams were arranged to support the load of concrete while the 
space between them was being filled, and to contribute to the rigidity of 
the roof after the concrete had turned into a solid mass. 
Each of the five rooms of the working space was furnished with two 
entrance wells, provided with air locks and engines for lifting excavated 
materials. 
The contractors, in order to save time, erected these wells or shafts at 
the start with the total length which would be required at the end of the 
sinking, and so they were forced to erect a wooden scaffolding twenty- 
five feet in height over the whole surface of the caisson. 
The caissons were rapidly and successfully sunk. The contractors 
received orders to commence work in April 1897 ; the sinking was begun 
on the first day of August, and finished at the beginning of November on 
the right bank ; begun in January 1898 and finished in the middle of 
March on the left bank. The work with compressed air lasted seventy- 
nine days on the right bank, and seventy-one on the left one. 
It would be fruitless to deny that in spite of the great abilities of the 
