moderately negative buoyancy, and either the ability to walk or swim, or the use of finely controllable thrusters. Both walking and thruster propulsion have been applied with some success. Swimming has not been effective. The dexterity to perform useful work is limited by joint mobility and geometry, inertia, and friction, and has been one of the more difficult engineering challenges. Haptic perception through manipulators is a major limitation on finer control, as the friction of the joints and seals greatly reduces the sensitivity available. Operator visual input is relatively easy to provide directly by using transparent viewports. A wide field of view can be achieved simply and structurally effectively by using a transparent partial dome over the diver's head. Close-up views of the manipulators are limited by joint flexibility and geometry of the suit's arms. External sound and temperature perception are greatly attenuated, and there is no sense of touch through the suit. Communications must be provided by technology, as there is normally no-one else in the immediate vicinity. Design constraints The main environmental factors affecting design are the ambient hydrostatic pressure of the maximum operating depth, and ergonomic considerations regarding the potential range of operators. The structure and mechanics of the suit must reliably withstand the external pressure, without collapsing or deforming sufficiently to cause seals to leak or joints to experience excessive friction, and the full range of movement must not change the internal or external displaced volume. ambient pressure, structural stability, constant volume geometry, buoyancy, insulation, mass, bulk. ergonomic considerations - strength of the user, operating forces on joints, field of vision, general underwater conditions Hazards and failure modes structural: leaks, implosion, lockup of joints, loss of buoyancy. systems failures: loss of power, communications, propulsion, life-support: breathing air, temperature control History Early designs In 1715, British inventor John Lethbridge constructed a "diving suit". Essentially a wooden barrel about in length with two holes for the diver's arms sealed with leather cuffs, and a viewport of thick glass. It was reportedly used to dive as deep as , and was used to salvage substantial quantities of silver from the wreck of the East Indiaman , which sank in 1719 off the Cape Verde islands. The first armored suit with real joints, designed as leather pieces with rings in the shape of a spring (also known as accordion joints), was designed by Englishman W. H. Taylor in 1838. The