The principal use to which the submarine vessel has thus far been turned has been that of a naval weapon, for scouting and for firing explosive automobile torpedoes, either for defensive or offensive purposes. Its full capacity has by no means been realized up to the present time. All submarines, regardless of their design, have certain essential features which will be described in the order of their importance. =The Hull.=--This must be watertight and capable of withstanding a pressure corresponding to the depth at which the vessel is designed to operate. The hull in most submarines is circular in cross-section; the circular form is best adapted for withstanding pressure. In some cases this circular hull is surrounded by another hull or is fitted with other appendages which will both increase the stability and seaworthiness of the submarine and add to its speed. =Superstructure.=--Most of the early military submarines built for the French, Spanish, United States, and English governments were circular in cross-section and of cigar-or spindle-shaped form in their longitudinal profile view. It is difficult, in vessels of this form, to secure sufficient stability to make them seaworthy. They are apt to roll like a barrel when light, due to a diminishing water plane, and when under way the water is forced up over their bows, making a large "bow wave" which absorbs power and causes such vessels to dive at times when least expected. In some instances this tendency to dive has caused loss of the vessel, and, in some cases, of the lives of the crew as well. They are also very wet for surface navigation, as the seas break over their inclined sides like breakers on a beach. These difficulties led to the invention of the buoyant superstructure, first used on the _Argonaut_. This is a watertight structure built of light-weight plating--in some cases it has been built of wood--with valves which admit free water to the interior of the superstructure before submerging. By the admission of the water, danger of collapse is prevented. By this expedient the pressure upon these light plates is equalized when the vessel is submerged. This combination of a circular pressure-resisting inner structure, surmounted by a non-pressure-resisting outer structure of ship-shaped form, is now common to all modern submarines of all navies of the world. This superstructure adds to the seaworthiness and habitability of submarine vessels and increases their speed, both in the light and submerged conditions, as it admits of better stream lines.