BALLAST is a heavy substance employed to give a ship sufficient hold of the water, to insure her safe sailing with spread canyas,when her cargo and equipment are too light. The amount of B. required by a ship depends not. only on her size and cargo, but also on her build; some forms of construction requiring more B. than others. It is not merely the quantity of B. which a skillful mariner has to consider; he is required also to take into account its distribution. If a heavy mass of B. be deposited within a small compass near the keel. it places the center of gravity very low down; the ship will sail sluggishly, and is said to be "stiff." If, on the other hand, the B. be massed too high up, the ship becomes "crank," and cannot carry much sail without danger of being upset. Under average circumstances it is considered that a ship is well ballasted when thewater comes up to about the extreme breadth amid-ships.
In ballasting a ship, the cargo and B. are considered together, the quantity and dis tribution of the latter being made dependent on the former. In a ship of war, the B. is made subservient to the requirements of the necessary stores and war materiel.; iu a mer chant or passenger vessel, to the convenience of the passengers and the careful stowage of the cargo. During the last great European war, the- ships of the British navy were supplied with a certain conventional weight of B.. according to size and armament. Thus, a 100-gun ship had 550 tons of B.; an 80-gun, 440 tons; a 50-gun, 235 tons; a 36 gun, 225 tons; a 20-gun, 110 tons, etc. The recent revolution in the sizes and shape of
war-ships, however, and the introduction of steam-propulsion, have rendered all such fixity of ratio inapplicable.
The substances used as B. are various—chiefly iron, stone, gravel, sand, mud, and water. Iron is now superseding the next three varieties in ships of any importance ; and water-ballast is gradually being introduced in the collier-ships of the Tyne, Wear, and Tees. Water-ballast is employed in four different ways. Bag-crater B. is contained in water-proof bags laid on the floor of the vessel, and filled or emptied by means of a pump and a hose. Bottom-water B. is confined beneath a false bottom in the vessel. Hold-water B., first employed in screw-steamer colliers constructed, by Mr. Scott Russell, is con tained in a large receptacIC, ,which may be filled with the cargo when theship is not in B.
Tank-water 13. is contained in two fore-and-aft tanks, which can easily be filled and emptied. The customs' laws relieve merchant-ships from certain formalities and pay ments when leaving a port in ballast.
The term 13. .is employed by civil engineers to signify the sand or gravelly material which is laid as a packing between railway-sleepers, in order to give .
them solidity. No English railway is considered to be complete or safe for transit until it dressed and finrstied by ballasting. The possibility of procuring B. at a cheap rate, considerably affects the cost of railway undertakings.