The Harpoon sounding machine is a brass depth log that works by counting the revolutions of a rotor as the log is lowered to the seabed. There are two dials, one reading from 0 to 30 fathoms, the other from 0 to 150 fathoms. The rotor is free to rotate as the log is lowered, but when the line is drawn up again a locking piece drops down and holds it in place. The log has a piece of rope attached to it and is held in its original wooden box, which has a set of printed instructions inside the lid.

Being able to determine the depth of water beneath a ship has always been vital in navigation. The lead and line was used for this until the invention of sounding machines like this one in the 19th century, and it was then superseded by echo sounding in the 20th century.