BAL'LAST (tirst part of uncertain origin; perhaps Eng. bare, mere -I- last, burden. weight). Weight carried by a ship or boat to secure proper stability, both to avoid risk of capsizing and to secure the greatest effectiveness of the propelling power. Modern steamers have tanks fitted for water ballast, and rarely carry any other kind. The tanks are located forward. aft. and amidships. in order that the vessel may not only be immersed to a safe extent, to avoid danger of capsizing, but also so that the trim, or inclination of the keel to the surface of the water. may be adjusted. When light (i.e. with little or no eargol, steamers require trimming by the stern (i.e. to have the stern more deeply immersed •than usual), in order to submerge the screw or screws. Sailing vessels require greater stiffness (i.e. power to resist capsizing) than steamers. in order to stand up under the force of wind against their sails, as well as to resist the motion imparted by the waves. They usually carry a certain amount of permanent ballast in the form of pig iron, if additional ballast is necessary, any convenient heavy ma terial. sub as gravel, stones. etc., is used. The amount of ballast needed depends not only upon the depth of immersion, but upon the shape of the hull, some forms of which require more ballast than others. Under average cir•um stances, a ship is said to be well ballasted when the water CO MPS up to about her extreme breadth amidships. In ballasting or stowing cargo, the ballast and cargo are considered to gether, the quantity and distribution of the former being dependent on the latter. The
combined effect of both should not. place the centre of gravity too low or too high. If too low, the ship will be unduly stiff, and will roll violently and jerkily, though perhaps not deep ly; and she will sail sluggishly, except in a comparatively smooth sea. If the centre of gravity is too high. the vessel will be crank, and will he dangerous from lack of power to right herself when heeled over by the wind pressing on her sails or the action of the waves: she may roll deeply, but her rolls will be slow; ^rankness interferes with speed tinder sail, be .fitise it reduces the amount of sail which can be safely carried; furthermore, the lack of righting power. or stability, which causes •rank ness, prevents the vessel from responding quick ly to the action of the waves. A vessel is said to be in ballast when she has ballast only, and no cargo, on hoard.
The term ballast is applied to bags of sand placed in the ear of a balloon to steady it and enable the aeronaut to lighten the balloon, when necessary to effect a rise, by throwing part of the sand out. Civil engineers use the term bal last to signify the sand or gravelly material which is laid as a foundation fur roads, con crete floors, etc., or as packing between railway sleepers. in order to give them solidity. No modern railway is considered to he complete or safe for transit until it is dressed and finished by ballasting. The possibility of procuring bal last at a cheap rate considerably affects the course of railway undertakings.