Ballast
2 of the 6 encyclopedias on this shelf carry an entry for Ballast. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
any weight carried in the hold of a ship to keep it right side up. It is necessary, especially in the case of a sailing ship, to load the hold heavily, or the immense expanse of canvas spread to the wind may cause the ship to tip over. In the case of an empty ship going after cargo, or departing from port with a light load, sailors take on board any heavy material, as bags of sand, gravel, stones, or earth. The best of all ballast is, of course, bars of iron, or better yet, lead, that lie low and occupy little space. Harbor rules enforced by the authorities of the various ports prescribe the places where the ballast of incoming ships may be thrown overboard, so as not to obstruct the channel. Very frequently it is piled up on the shore, so as to be available for the next ship that needs it. Steam vessels showing little canvas require less ballast. Most modern ships use water, which is pumped into or out of tanks arranged for the purpose. In the case of war ships with a heavy weight of guns on deck, heavy ballasting of lead is run into the lowest part of the hold to balance the metal above. The term is also applied to the bags of sand carried in balloon ascensions, and to the earth packed between the sleepers on a railroad track. An unstable person is said to lack ballast.
Collier's New Encyclopedia (1921)
a term applied (1) to heavy matter, as stone, sand, iron, or water placed in the bottom of a ship or other vessel, to sink it in the water to such a depth as to enable it to carry sufficient sail without oversetting. (2) The sand placed in bags in the car of a balloon to steady it and to enable the aeronaut to lighten the balloon by throwing part of it out. (3) The material used to fill up the space between the rails on a railway in order to make it firm and solid.
What Has Changed in the Last Century
Aiton's 1910 entry treats sand, gravel and pig-iron as normal ballast and notes almost in passing that "most modern ships use water"; that footnote is now the whole story, since virtually every steel ship carries ballast in dedicated tanks and pumps it in and out at the berth. What neither account could foresee is that this water is alive: ships loading in one port and discharging in another have moved organisms worldwide, including the zebra mussels that reached the Great Lakes in the late 1980s and now foul intakes and pipes. The International Maritime Organization's Ballast Water Management Convention, in force since September 2017, requires ships to exchange or treat ballast water so it does not seed new coasts. The 1921 senses for balloon sandbags and railway track ballast still stand, though the balloon usage is now confined to sport and the earth-and-stone track ballast is more likely to be crushed rock.
Written for this edition, 2026.