[Illustration: THE ROTOR, OR ROTATING ELEMENT, OF ONE OF THE LOW-PRESSURE TURBINES OF THE IMPERATOR. DIAMETER OVER TIPS OF BLADES IS 18 FEET] The purpose of the present chapter is to show how successfully the methods of underwater protection employed in naval ships may be applied to passenger ships of the first class; and the _Mauretania_ is given first consideration, for the reason that she is the best example afloat to-day of a merchant ship fully protected against sinking by collision. The protective elements may be summed up as consisting of multiple subdivision, associated with a complete inner skin and a watertight steel deck, answering to the heavy protective deck at the water-line of the warship. By reference to the hold plan on page 129 it will be noticed that she is subdivided by 22 transverse bulkheads, 12 of which extend entirely across the ship and 10 from the side inboard to the longitudinal bulkheads. The space devoted to the turbine engines is subdivided by two lines of longitudinal bulkheading, and the compartment aft of the engine-room spaces is divided by a longitudinal bulkhead placed upon the axis of the ship. Altogether there are 34 separate watertight compartments below the water-line. The most important feature of the subdivision is the two lines of longitudinal bulkheads, which extend each side of the boiler-rooms and serve the double purpose of providing watertight bunker compartments and protecting the large boiler-room compartments from being flooded, in the event of damage to the outer skin of the ship. The main engine-room, containing the low-pressure turbines, is similarly protected against flooding. Now, all of these bulkheads are carried up to a watertight connection with the upper deck, which, amidships, is over two decks, or say about 20 feet above the water-line, the exception being the first or collision bulkhead, which extends to the shelter deck. A most important feature of the protection, borrowed from warship practice, is that the lower deck, which, amidships, is located at about the water-line, is built of extra heavy plating, and is furnished with strong watertight hatches. It thus serves the purpose of a protective deck, and water, which flooded any compartment lying below the water-line, would be restrained by this deck from finding its way through to the decks above. The _Mauretania_, therefore, could sustain an enormous amount of damage below the water-line without foundering. It is our belief that she would have survived the disaster which sank the _Titanic_. The first three compartments would have been flooded, it is true, but the water would have been restrained from her large forward boiler-compartment by the "inner skin" of the starboard bunkers. Furthermore, the watertight hatches of her lower, or protective, deck would have prevented that upward flow of water on to the decks above, which proved so fatal to the _Titanic_.