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ตอบ คห.85 สมัยก่อนใช้ Chip log ประมาณความเร็วของเรือ ครับ
Origin
Until the mid-19th century vessel speed at sea was measured using a chip log. This consisted of a wooden panel, weighted on one edge to float upright, and thus present substantial resistance to moving with respect to the water around it, attached by line to a reel. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another sailor used a 30 second sand-glass (28 second sand-glass is the current accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%
http://en.wikipedia.org/wiki/Knot_(unit)#Origin
http://en.wikipedia.org/wiki/Chip_log
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Sgt.Oat  
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8 ม.ค. 55 12:47:48
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