Knot (unit) fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG) (ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots. 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
Apparent wind of the true wind" minus the "velocity of the object". In sailing, "apparent wind" is the speed and direction of wind indicated by a wind instrument (anemometer) on a "moving" craft (on water, land or ice) in undisturbed air. It is composed of the "combined" speeds and directions of the craft and wind observed by a "stationary" wind instrument—the "true wind". A true wind coming from the bow increases the apparent wind induced by the speed of the craft, coming from the stern it decreases apparent wind, and coming from the side the apparent wind angle and speed change according