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.