Ballast, in Civil Enyineering, a term applied to the covering of roads generally, laid for the purpose of keeping them dry, and for giving strength. Ballast is mostly composed of gravel, broken stone, or broken cinders. It should be pervious to water, and slightly elastic. On ordinary roads it is laid to a depth of six to twelve inches; on railroads a thickness of two feet is the rule.
In Marine Engineering the term denotes the material taken into a ship when emptied of its cargo, to bring its displacement (q.v.) back to the normal amount. For a vessel to sail uniformly well its total weight should be of constant amount, and should be properly distributed. The cargo, therefore, requires proper placing, and when removed, ballast is required instead. If placed too near the bottom of the vessel, heavy rolling results; if too high, there is a tendency to top-heaviness. The material used is generally stone, gravel, iron, or water. In the case of water ballast, which has many advantages over the others, and is much adopted now, vessels are built with double bottoms, the space between being divided into separate compartments. Into some or all of these compartments water may be admitted when required, the trim of the vessel allowing adjustment by selection of the compartments to be filled. They are usually emptied by steam-pumps.