The captain of every ship must know the depth of the water below his ship. In the days before automatic depth sensors, a sailor in the bow of the ship cast a line with a weight, and could tell when the weight hit bottom, and report the depth. Perhaps not all of us know that Samuel Clemens chose the pen name "Mark Twain" because he had been a pilot on the Mississippi before the Civil War - the equivalent of an astronaut in those days. Depth was measured in fathoms, six feet. So the leadman's cry of "Mark Twain" meant twelve feet of water, plenty for the shallow draft river steamers.

The difference is between "mark" and "deep". A sounding-line had a number of knots or artefacts attached to it a fathom apart which could be readily distinguished by feel, even when the leadsman's hands were cold. Because there are limits to the weight you can use in such a line, no more than 6 fathoms, or six "marks" had the distinguishing artefacts. So, "by the mark, five" or "by the mark, twain" called out by the leadsman told the skipper that the line had gone down that far. "By the deep, six" meant "deep or deeper than six fathoms", because beyond that, the depth really didn't matter, since even the first-rate sailing ships of the day drew no more than about 20 feet.. The soundings were carried out by a reliable, or "leading" hand. It was important to get it right, obviously, to prevent the ship from running aground. But in the navies of the world, losing a ship is a court-martial affair regardless of circumstance. Losing a ship through running aground was just about the worst thing a captain could do, so the ship - and the captain's fate - were very much in the hands of the leading hand.

HTH!

There was one other thing that the leadman could do. I have read in sea stories that when bad weather had made navigational instruements useless, the leadsman with tallow on concave weight could bring up samples from the bottom which were often reliable indications of the position of the ship.

On a barely related note, I've always loved the word "fathom," both as noun and verb.

That is all.

OE word meaning outstretched or encircling arms, embrace, grasp. Also, by extension, protection : interior, bosom, lap, breast, womb.

Dear Faldage: I just got a surprise. The next word on spelling bee list is "patulous", which I first encountered in an obstetrical record of a woman who had had fourteen children, and as result had "a patulous cervix" - meaning that instead of admitting a catheter with moderate force, it would readily admit a couple fingers. The surprise came when the defintion told me to look up "fathom".
fathom
n.
5ME fadme < OE f+thm (akin to OFris fethm, OS pl. fathmbs), the two arms outstretched (to embrace, measure), akin to Ger faden, thread < IE base *pet3, to stretch out > L patere, to stretch out6 a length of 6 feet, used as a unit of measure for the depth of water or the length of a rope or cable
vt.
1 to measure the depth of; sound
2 to get to the bottom of; understand thoroughly
fath4om[a[ble
adj.