Interior of working chamber, Inchgarvie caisson, around 1885.
Caissons are watertight structures used to work on foundations under water.
For the Forth Bridge, each cylinder was floated into position, then weighted with cement until it sat on the bed of the Forth estuary.
Sinking the caissons was hazardous.
At one stage a caisson ruptured while water was being pumped out, and two men drowned when the damaged structure flooded.
Effects of air pressure
Air locks were needed to maintain the air pressure in the chamber while allowing men and materials through. Limits were placed on the amount of time men spent working in the bottom of the caissons, but some still suffered ill-effects.
See also 'Constructing the Forth Bridge'.
This item featured in our display, 'The Forth Bridge: Building an icon', at the Library from 1 April to 21 June 2015.