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ballast (noun) heavy material put in a ship to enhance stability; also a similar substance put in a hot air balloon to make it heavy enough to keep it steadily afloat.
If a ship dips to a side for lack of balance, a heavy material is put on the other side to make it heavy. This material is ballast. As hot air balloons are too light, they are likely to drift. To prevent the drift and to keep it floating steadily in the air, a heavy material is placed in the balloon. This is ballast. Ballast is also the strong layer of stone laid to form the base of a road, or a railroad. This prevents the road from sinking.
- Finally, when the crew shifted the ballast, and when the balance restored, the ship floated on an even keel.
- The balloon floated steadily in the air after being adrift for an hour, when after being grounded, some more ballast was placed in it and flown.
- The ballast laid under the railroad long ago had to be strengthened to support the movement of faster and heavier trains of the modern times.