An instrument that measures the depth of water, especially in oceans and seas.
Greek bathos 'depth' + -meter 'measure'. The term emerged in the 18th-19th centuries as oceanography developed and scientists needed tools to map underwater topography.
Before GPS and sonar, bathometers using weighted lines were the only way to measure ocean depth—sailors would literally drop a rope overboard and count how much played out before hitting bottom!
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