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Webster 1913 Edition
Water hammer
Wa′ter ham′mer
.(Physics)
1.
A vessel partly filled with water, exhausted of air, and hermetically sealed. When reversed or shaken, the water being unimpeded by air, strikes the sides in solid mass with a sound like that of a hammer.
2.
A concussion, or blow, made by water in striking, as against the sides of a pipe or vessel containing it.