The bathyscaphe has two main sections – a large float and a spherical cabin slung beneath it. The float, which is lighter than water and filled with gasoline, gives the craft positive BUOYANCY. It also contains iron shot, held in hoppers by electromagnets. When on the surface, some of the compartments in the float are filled with air. For diving, these compartments are filled with water.
The gasoline-filled compartments are left open at the bottom. As the bathyscaphe dives deeper and the pressure increases, some water finds its way into the gasoline-filled compartments, equalizing the pressure inside and outside the float. This allows the float to be constructed from much lighter gauge materials than the crew sphere.
Ballast is dumped when the vessel reaches the desired depth. Ballast dumping ceases when the vessel achieves neutral buoyancy. A guide chain, an ingenious device for automatically adjusting the quantity of ballast when a submersible is near the bottom, is often used. A length of heavy-duty chain is hung from beneath the vessel. When the chain hits the bottom the ballast is effectively lightened because the weight of some of the chain is being supported by the sea bed. The vessel will continue descending until sufficient chain is in contact with the sea bed for it to achieve neutral ballast.
Returning to the surface is simple. The electromagnets keeping the remaining shot in place are turned off. The ballast falls away, leaving the vessel to float to the surface, buoyed by the gasoline-filled compartments in the float.
The cabin is made from steel and plexiglass and must be designed to withstand great pressures – at 10,000 ft (3050 m) the pressure is about 8 tons per square inch (1250 bar). The plexiglass windows are carefully seated into the surrounding metal to give a perfect seal – at a great depth even a small leak could prove disastrous.
The portholes are constructed like a cone with the point sliced off and the base facing the outside world. The plexiglass portholes are glued into place. Electric conduits connecting the bathyscaphe to the surface are sealed using epoxy adhesive. Submersibles are normally equipped with external grabs to assist with research or salvage operations.