Calculations being made of the volume of the vessel’s hull to intermediate distances between the keel and the maximum load line, it is thus possible to construct a “curve of displacement” from which the actual amount of displacement at any intermediate draught can be obtained. From this curve a set of scales—usually set up alongside a vertical scale of feet and inches, representing the vessel’s draught-marks—are constructed, showing—1st, the tons “displacement” at any draught; 2nd, the tons of “dead-weight” capability—_i.e._, the tons displacement due to the weight of cargo, coal, ballast, stores, fresh water, spare gear, &c.—at any draught above the vessel’s light-draught: “light-draught” being that at which the vessel floats with holds clean-swept, bilges dry, water in boilers, and with such spare gear on board as is required by Board of Trade; and 3rd, the amount of “freeboard”—_i.e._, the distance in feet and inches from any particular draught line to the top of the deck amidships. BUOYANCY AND STABILITY. A ship floating upright and at rest in still water must fulfil two conditions—1st, as stated above, she must displace, a weight of water equal to her own weight; 2nd, her centre of gravity must lie in the same vertical line with the centre of gravity of the volume of displacement or “centre of buoyancy.” The whole weight of the ship may be supposed to be concentrated at her centre of gravity, and to act vertically downwards, and the resultant vertical pressure of the surrounding water in the same way to act upwards through the centre of buoyancy. When the ship has been inclined from the upright position, by any force, the downward and the upward forces—weight and buoyancy respectively—act through two separate but parallel vertical lines, and form what is technically known as a “couple.” The perpendicular distance between the vertical lines usually varies with the inclination, and is called the “arm” of the couple. This arm measures the leverage with which the weight and buoyancy of the ship tend either to force her back into the upright position, or to incline her still further, and, it may be, to capsize her. The former effect would be the result of what is known as a “righting couple,” the latter the result of an “upsetting couple.” [Illustration: FIG. 14. FIG. 15.]