le row, and sunk through the bed of the river into the hard chalk.
This plan is the reverse of Pott's mode of sinking cylinders, in which the air is withdrawn from the interior so as to utilize the pressure of the atmosphere in forcing the pile downward, while the material, of a somewhat soft nature, ascended into the cylinder.
The contents were from time to time scooped out, and the air-exhausting process repeated.
This plan was adopted with a bridge which crosses the Thames near Richmond, England.
Fig. 1022 is a section of the movable iron caisson used in building the piers of a bridge at Copenhagen, Denmark.
It comprises an upper chamber communicating with the air, an intermediate or airchamber, both equal and cylindrical in section, and a lower working-chamber of larger section than the foregoing, and adapted to the shape of the pier; the whole raised or lowered by suspension-chains, and ballasted with iron and water contained in two annular chambers A and B, surroundin