Deck mounted depth sounders like this one were used to determine the depth of water below a ship at sea. A line would be wrapped around the rotating drum with a weight tied to the end. When dropped the gauge displays the depth in fathoms.

The top handle is a lock which stops the rotation of the drum when upright and allows it to move when down. The two rotating arms which move the drum can be locked into the holes in the base when not in use.

Marked on the gauge Market Forge Co., Everett, Mass. Gauges like this one were overseen by the US Navy Bureau of Ships. Comes with a 4lb. lead weight.