Lead and reel, circa 1890

A reel with lead.
To take soundings while under way, the leadsman would take his place at the bow of a small boat, or at the forward chain plates on a large ship, secured from falling overboard by a "breast band", a wide strip of canvas used like a seat belt tied between two shrouds. The leadsman could then lean forward against the band to swing his lead in the clear. He would then swing the lead round and throw it as far forward as possible, so that the lead would be resting on the bottom and the line tight, when the vessel is directly over the lead.
If the lead is hove properly, so that the line pays out with a little tension as it passes through the hands, it is easy to tell when it has reached the bottom by the sudden slack felt in the line. When sailing in shoal waters, soundings can be taken much quicker with a pole or boathook than with a lead.
There is a hollow at the base, or "heel" of the lead which can be filled, or "armed", with tallow; a specimen of the bottom (mud, sand, or shingle) is brought up with the lead, and this, by referring to the chart, which generally marks the nature of the bottom, may help find your position precisely.