30-10-2008, 11:59pm
Samuel Clemens's pseudonym "Mark Twain" was the nickname of a riverboat pilot about whom Clemens wrote a needless nasty satirical piece. Apparently, Clemens felt guilt later and adopted the name as a nom de plume as some sort of expiation. The phrase does not mean measuring the depth of the river; it means a specific depth, to wit, two fathoms (twelve feet.)