The term originates from Greek βαθύς (bathys), "deep" and μέτρον (métron), "measure".
The earliest idea for a bathometer is due to Leon Battista Alberti (1404–1472) who sunk a hollow sphere attached to some ballast with a hook.
[6] The pressure of the mercury acting under the force of gravity pushed down on, and deformed, a thin steel sheet.
The theory of the instrument was that the greater the depth of water under the ship, the lower the gravitational force would be.
Previously, depth was determined by taking soundings with a lead line, a time-consuming and difficult method.