The sounding lead and line is probably one of the oldest of all navigational instruments and was still being used in the 20th century to determine the depth of water beneath a vessel. This was done by lowering it over the side and letting the line out until the lead hit the bottom. By about 1600, the line was marked in fathoms (6ft) with pieces of material.

As a further help to navigators, the bottom of the lead was hollowed out and would have some tallow inserted in order to take a sample of the sea-bed. This could help in determining a ship's position.