A diving bell, also known as a wet bell, is a cable-suspended airtight chamber, open at the bottom like a moon pool structure, that is lowered underwater to operate as a base or a means of transport for a small number of divers. The pressure of the water keeps the air trapped inside the bell. They were the first type of diving chamber. Unlike a submarine the diving bell is not designed to move under the control of its occupants, nor to operate independently of its tether.
Encyclopaedia Britannica (11th ed., vol. 8) (New York, NY: The Encyclopaedia Britannica Company, 1910)