Other things being equal, the protection of a ship against sinking is exactly proportionate to the number of separate watertight compartments into which the interior of her hull is subdivided. If she contains no watertight partitions whatsoever, her sinking, due to damage below the water-line, is a mere matter of time. If the inflow exceeds the capacity of the pumps, water will flow into the ship until all buoyancy is lost. Protection against sinking is obtained by dividing the interior of the hull into a number of compartments by means of strong, watertight partitions, or bulkheads. Usually, these are placed transversely to the ship, extending from side to side and from the bottom to a height of one or two decks above the water-line. They are built of steel plates, stiffened by vertical I-beams, angle-bars, or other suitable members. The bulkheads are strongly riveted to the bottom, sides, and decks of the ship, and the joints are carefully caulked, so as to secure a perfectly tight connection. In the standard construction for merchant ships, as used in the _Titanic_, the bulkheads are placed transversely to the length of the ship, and the number of separate compartments is just one more than the number of bulkheads, ten such bulkheads giving eleven compartments, fifteen, as in the _Titanic_, giving sixteen compartments, and so on. In the case of a few high-class merchant steamers, built to meet special requirements as to safety, bulkheads are run lengthwise through the ship. These longitudinal bulkheads, intersecting the transverse bulkheads, greatly increase the factor of safety due to subdivision; for it is evident that one such, running the full length of the ship, would double, two would treble, and three would quadruple the number of separate compartments. [Illustration: HYDRAULICALLY-OPERATED, WATERTIGHT DOOR IN AN ENGINE-ROOM BULKHEAD] The bulkhead subdivision above described is all done in vertical planes. Its object is to restrict the water to such compartments as (through collision or grounding) may have been opened to the sea. As the water enters, the ship, because of the loss of buoyancy, will sink until the buoyancy of the undamaged compartments restores equilibrium and the ship assumes a new position, with the water in the damaged compartments at the same level as the sea outside. This position is shown in Fig. 2, page 57. It must be carefully noted, however, that this condition can exist only if the bulkheads are carried high enough to prevent the water in the damaged compartments from rising above them and flowing over the tops of the bulkheads into adjoining compartments.