small diving apparatus that is used to transport divers between the seafloor or lower depths and the surface. Early bells consisted of a container open only at the bottom, usually provided with a source of compressed air. Though the diving bell in rudimentary form is mentioned by Aristotle, the device was not fully practicable until the end of the 18th century, when the British engineer John Smeaton (Smeaton, John) fitted an air pump to the bell. Regardless of the depth to which a diving bell is lowered, in principle at least, fresh air fills the available vital space. Its pressure is automatically regulated by the pump and by the water pressure; surplus air escapes through the edges of the container. As the bell descends, the water level tends to rise inside the bell. As it surfaces, the decreasing water pressure lowers the level inside the bell. Thus, the pressure inside the bell remains the same as that outside. Some bells, however, are kept at working-depth pressure and are used to commute to and from an outfitted surface decompression chamber and the work site, thus eliminating the need for decompression between dives on a mission. Modern bells may accommodate up to four divers and have been used at depths of more than 1,000 feet (300 m).