The Lead is a lead weight at the end of a line (thin rope) is used to determine the depth of the water. Small pieces of cloth various colours and/or shapes are attached at various distances along the line to tell the depth in fathoms (6 feet). The bottom of the lead has a concave end filled with tallow so that some of the bottom sticks to it so the crew can see the character of the bottom, sandy, gravel, etc.
The leadsman stands in the mainchains and swings the lead in a vertical circle, then throws it forward so that it hits the water. By the time the line comes even with him, it should be on the bottom, he then calls out the depth according to a predetermined standard call (i.e. by the deep ten, or a half seven, meaning 7½ fathoms, etc.).
- Depth sounding at Wikipedia