Measurement of Depth of Water
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Question is: Measurement of Depth of Water
and the answer is: FATHOM
Word Meaning:
FATHOM — (chiefly nautical, historical, US) A man's armspan, generally reckoned to be six feet (about 1.8 metres). Later used to measure the depth of water, but now generally replaced by the metre outside American usage.
Definitions from Wiktionary.