Learn where the term "mark twain" originated, and what it means.
Mark twain is a riverboat term meaning 2 fathoms (a depth of 12 feet or 3.65 meters). A hand lead is used for determining the depth of water where there is less than 20 fathoms. The lead consists of a lead weight of 7 to 14 pounds (3.17 to 6.35 kilograms) and a line of hemp or braided cotton, 25 fathoms (150 feet or 46 meters) in length. The line is marked at 2, 3, 5, 7, 10, 15, 17, and 20 fathoms. The soundings are taken by a leadsman who calls out the depths while standing on a platform projecting from the side of the ship, called "the chains." The number of fathoms always forms the last part of the call. When the depth corresponds to any mark on the lead line, it is reported as "By the mark 7," "By the mark 10," etc. When the depth corresponds to a fathom between the marks on the line, it is reported as "By the deep 6," etc. When the line is a fraction greater than a mark, it is reported as "And a half 7," "And a quarter 5"; a fraction less than a mark is "Half less 7," "Quarter less 10," etc. If bottom is not reached, the call is "No bottom at 20 fathoms."
"Mark Twain" was also the pseudonym chosen by American humorist Samuel L. Clemens. Supposedly, he chose the name because of its suggestive meaning, since it was a riverman's term for water that was just barely safe for navigation. One implication of this "barely safe water" meaning was, as his character Huck Finn would later remark, "Mr. Mark Twain ... he told the truth, mostly." Another implication was that "barely safe water" usually made people nervous, or at least uncomfortable.