=Stability.=--The stability of a vessel refers to its ability to keep upright and on a level keel. It is desirable to have great stability in a submarine in order that it may not assume excessive angles when submerged. The measure of stability is expressed in inches of metacentric height. The metacentric height of a vessel when submerged is the distance between the centre of buoyancy--or submerged volume--of the vessel and the centre of all the weights of hull, machinery, stores, and equipment contained within the vessel. This distance between the centre of buoyancy and the centre of gravity must be determined very accurately in order to obtain conditions of ideal stability in a submarine. The metacentric height of a vessel is a term used in naval architecture to express the stability of the ship. In surface ships the term may be used to express either the longitudinal or transverse stability of the vessel, and varies according to the load line and trim or heel of the ship. On the other hand, in submarine boats _when submerged_ the metacentric height is constant and expresses the distance between the centre of gravity and the centre of buoyancy of the vessel, and is the same either in the transverse or longitudinal plane of the vessel. In other words, the centre of buoyancy of the vessel when submerged must be directly over the centre of gravity of the vessel to cause her to submerge on a level keel. We then get the effect of a pendulum, the length of the pendulum arm being the distance between the two points, and the weight of the pendulum equalling the weight of the ship. Therefore, if a submarine has a submerged displacement of five hundred tons, with a metacentric height of twelve inches, her stability, or ability to remain upright, is equal to a pendulum of five hundred tons hung by an arm twelve inches long, and it would require the same force to incline the ship as it would to incline the pendulum. Therefore it is evident that the greater the metacentric height the more stable the ship, and the less likely she is to make eccentric dives to the bottom or "broach" to the surface. =Ballast Tanks.=--All submarines are fitted with tanks which may be filled with water so that the vessel will submerge; these are called ballast tanks. When the vessel is navigating on the surface she has what is called "reserve of buoyancy," the same as any surface vessel. It is this reserve of buoyancy which causes the vessel to rise with the seas in rough weather. It means the volume of the watertight portion of the vessel above the water line. In surface cruising a vessel with great buoyancy will rise to the seas, while if the "reserve" is small the vessel is termed "loggy" and will not rise to the sea. In the latter case the seas will break over the vessel just as they break over a partially submerged rock in a storm. On such a vessel the men cannot go on deck in a storm; in a sea-going submarine a large reserve of buoyancy is therefore essential.