<P> Until the mid-19th century, vessel speed at sea was measured using a chip log . This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it . 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 8 fathoms - 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30 - second sand - glass (28 - second sand - glass is the currently 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 6999514350000000000 ♠ 1851. 66 m / h . The difference from the modern definition is less than 0.02% . </P> <P> Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because standard nautical charts are on the Mercator projection and the scale varies with latitude . On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland . A single graphic scale, of the sort on many maps, would therefore be useless on such a chart . Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart . Recent British Admiralty charts have a latitude scale down the middle to make this even easier . </P> <P> Speed is sometimes incorrectly expressed as "knots per hour", which is in fact a measure of acceleration . </P> <P> Prior to 1969, airworthiness standards for civil aircraft in the United States Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour . In 1969, these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots . </P>

Knot is a unit of which of the following