The term "waterline" generally refers to the line where the hull of a ship meets the water surface. It is also the name of a special marking, also known as the national Load Line or Plimsoll Line, to be positioned amidships, that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures. Temperature affects the level because warm water provides less buoyancy, being less dense than cold water. The salinity of the water also affects the level, fresh water being less dense than salty seawater. This marking was invented in the 1870s by Samuel Plimsoll.