One of the sounding leads
from the Mary Rose

When the lead hit the seabed, bits of stone, mud or small shells would stick to the tallow. This would help the pilot tell where he was, because the seabed varies from place to place.

A sailor stood on the bow (front) of the ship and swung the lead out in front of it.

When the line was vertical he would measure the depth of water by counting bits of cloth, rope and leather fixed to the line. This let the sailor tell the depth of water by sight, or at night by touch.

The depth was measured in fathoms, a fathom is 6 feet, or just under 2 metres.