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
War Wind have to be promoted at a specific facility. Additionally, units could be individually upgraded in a variety of areas such as speed, strength, resilience, stealth, and vision. The purpose for these micro upgrades was to encourage more strategic and smaller conflicts instead making players simply building up units and "steam rolling" the opposition. Another unique aspect of war wind was stealth, there are four levels of stealth in the game. Stealth is always broken, regardless of level, when a unit attacks or is attacked. It will not be restored unless the unit remains out of combat for a period of