The Lead-Line Depth Scale
A depth scale you could read in the dark, by touch alone.
Origin
In use for centuries and standardised by the Royal Navy, remaining in service well into the twentieth century.
The catch
Only certain depths were marked. The rest were called deeps, and had to be estimated between the marks. That is where the phrase deep six comes from: six fathoms, an unmarked depth, deep enough that nothing came back.
Leather with two strips. Samuel Clemens took his pen name from this call on the Mississippi.
Leather with three strips.
White calico, identifiable by feel at night.
Unmarked. Estimated between the five and seven marks, and the origin of the phrase.
Red bunting.
Leather with a hole punched through it.
Blue serge.
White calico again.
Red bunting again.
Cord with two knots. Beyond this the deep-sea lead was used instead.
Each mark used a different material so that the leadsman could identify the depth by touch in darkness or fog, calling it aloud to the bridge. The lead itself was hollowed at the base and filled with tallow, so that it brought up a sample of the seabed: sand, shingle, mud or shell, each of which told a navigator where he was as surely as the depth did.
On the evidence
The marks and the calling of deeps are well documented. The link between the leadsman’s “deep six” and the modern phrase meaning to throw something away is popular but unproven: that sense is first recorded in twentieth-century American slang, and some trace it instead to burial at sea at six fathoms. Treat the connection as plausible rather than established.