Triger's caisson. 19th-century artwork of workers using the pneumatic caisson invented in 1839 by the French engineer Triger. This was the first use of pressurised air to dig underwater shafts, a technique that is still used today. The lowest section is pressurised to keep the water out. The middle section is an airlock, through which the soil was passed to the upper section which was at atmospheric pressure. Triger's caisson used sheet metal for the walls of the caisson, and was first used during mining in the Basse-Loire coalfield. Artwork published in Mines and Miners (L. Simonin, 1868).