Private Pilot Flight Training and Instruction Flight Training Article Library | Back To 4VFR.COM It's About Time A sailing ship's speed over a nautical a mile was, historically, measured by means of a knotted (knots) rope tied to a log. A sand filled timing glass would be used to measure the time from leaving the log dead (much as a dead man might appear) in the water (dead reckoning) and the number of evenly spaced knots passed along the rope. All of this would be recorded in the logbook. Since the chronometer was yet to be invented, sailors had no way to determine longitude except by this dead reckoning. Within crude limits, speed and compass indications could be used to determine estimated distance and estimated longitude. Magellan in 1519 had access to charts, globe, theodolites, quadrants, compasses, magnetic needles, hourglasses, and timepieces. He was unable to determine exact longitude. An 18th Century a chronometer (weighed over 36 pounds) was first used to get longitude. A chronometer differs from a clock or watch because it has a temperature adjustment for greater accuracy. Captain Cook in 1768 had three different clocks for his voyage. In 1779 he sailed with 4 chronometers and a nautical almanac which enabled him to determine longitude. The very first effort to make a calculator was financed by the British to make the making of the nautical almanac easier. The effort was stopped when the mechanical calculator was only a year from being completed. The original design was completed in 1991 and found to work accurately. Interesting to speculate where the world would be had it been completed in the 1700s. The complete story of the chronometers and the failure of the British to follow up with a 'computer' is in a small book "Longitude" now available as a pocket book. 30 years ago I knew a pharmacist who spent his evenings at an all-night pharmacy working out prime numbers on rolls of butcher paper with a pencil. Did we miss a 300-year head start on computers by so little? _______________________________________________________________________ Revolutions per minute - rpm First counted by paddle wheel ship captains._____________________ Written by Gene Whitt Flight Training Article Library | Back To 4VFR.COM