What does "ballast" mean?

Additional senses

1. 2.Gravel, broken stone, etc., laid in the bed of a railroad to make it firm and solid.
2. 3.The larger solids, as broken stone or gravel, used in making concrete.
3. 4.Fig.: That which gives, or helps to maintain, uprightness, steadiness, and security. It [piety] is the right ballast of prosperity. Barrow. Ballast engine, a steam engine used in excavating and for digging and raising stones and gravel for ballast. -- Ship in ballast, a ship carring only ballast.
4. 5.To steady, as a vessel, by putting heavy substances in the hold.
5. 6.To fill in, as the bed of a railroad, with gravel, stone, etc., in order to make it firm and solid.
6. 7.To keep steady; to steady, morally. 'T is charity must ballast the heart. Hammond.