ballast (for ships, submarines, balloons)
Used for weight added to stabilize a vessel or aircraft, such as water in submarine tanks or sandbags in a hot-air balloon.
The submarine submerges by taking water into its ballast tanks.
The hot-air balloon ascended by dropping ballast.
ballast (for road or railway foundations)
Refers to the layer of gravel or crushed stone placed under railway tracks or road surfaces to provide stability and drainage.
The ballast on the railway track needs to be replaced periodically.