Device used in the past on sailing vessels for measuring the rate of speed of the vessel. A quarter circle quadrant of wood, or "chip," fastened to a line, was allowed to run out over the stern, and the amount of line run was measured in time with a half minute sand glass. The sand glass replaced counting the seconds. The line was knotted at specific intervals of 47 feet 3 inches, and each interval was divided into fifths. The length of the knot was derived from the proportion that one hour (3600 seconds) is to 28 seconds as one mile (nautical mile of 6080 feet) is to the length of a knot (47 feet 3 inches). These slightly odd proportions are the result of standardizing on a nautical mile of 6080 degrees and that a 30 second sand glass allowed for turning over the glass. This is the origin of the term "knots" for the speed of a vessel.