Although Henshaw, an English clergyman, constructed a pressurized chamber — or ‘domicilium’, as he termed it — in 1662, the technology which allowed the construction of large vessels capable of holding greater pressure developed in the early to mid nineteenth century, and was associated with the construction of bridges and tunnels. Water ingress was a major problem in such workings, and in 1830 Admiral Lord Thomas Cochrane patented the technique of using compressed air in tunnels and caissons to exclude water. In 1839 Triger used the technique during the sinking of a shaft through quicksand to a seam of coal at Châlons in France. The technique was successful, was applied elsewhere, and its use was soon followed by reports of decompression illness in the workers. In 1854 Pol and Watelle recorded the relief obtained by workers so afflicted who went back into compressed air, and the workers, finding this cure out themselves, voluntarily went back to the pressurized caisson to obtain relief when they got the ‘bends’. These observations were strongly supported by Paul Bert’s demonstration of the success of recompression as a treatment in animals, but it was only after large projects, such as the New York tunnels under the Hudson and East rivers in 1889 and 1893, which caused more than 5000 instances of the disease, that the benefit of systematic recompression therapy for decompression illness was fully established. In order to avoid the inconvenience and danger of carrying patients back into the workings, special hyperbaric chambers were used at tunnelling sites. These were called medical locks, and the early ones were nothing more than boilers mounted horizontally with an airtight door at one end. The chamber formed could be divided by a bulkhead incorporating a door, and this formed a lock whereby the inner chamber could be entered without lowering its pressure. The chamber was equipped with electric light, bunks, and the necessary compressed air connections and controls. Following their development for compressed air workers, hyperbaric chambers were also widely used to treat divers and, later on, aviators with decompression illness.