Dead reckoning was born in the early days of oceanic sailing vessels and has been bringing mariners home for at least six centuries. Every hour or two during the voyage the log [a quadrant shaped piece of wood weighted to float upright with an attached knotted log-line] was heaved over the stern of a vessel under way and the vessel's speed was reckoned from the amount of line paid out over a particular period of time.In 1637 an English navigator, Richard Norwood, calculated that the spacing between knots should be 47.25 feet with a 28 second sand glass being used as the timer. If you do the calculation, using the then estimated 6075 feet to the nautical mile, you will see that the number of knots that passed over the stern rail during the 28 second period equals the ship's speed in nautical miles per hour - hence knots. The log was presumed to be 'dead in the water' i.e not affected by tide or current. Each reading was marked on a log-slate and, during each watch, the course, speed and distance reckonings - adjusted for tide and current - were entered in the logbook. Dead reckoning nowadays covers basic manual methods such as chart plotting to the highly complex machine methods such as inertial navigation systems. Non-nautical people reckon that 'dead reckoning' is a diminutive of 'deduced' reckoning, but we reckon their reckoning is wrong. According to the Oxford English Dictionary the term 'dead reckoning' first appeared in print in 1613 in a work titled 'Magnetic Bodies' written by one M.Ridley.