is rated for a maximum of , which is approximately 14 w.g. Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used. Boiling point of liquids Pure water boils at at earth's standard atmospheric pressure. The boiling point is the temperature at which the vapour pressure is equal to the atmospheric pressure around the liquid. Because of this, the boiling point of liquids is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes or pressure cooking. A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers. Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in distillation, the atmospheric pressure may be lowered by using a vacuum pump, as in a rotary evaporator. Measurement and maps An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains, thanks to reliable pressure measurement devices. In 1774, Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion mountain in Scotland, and he needed to measure elevations on the mountain's sides accurately. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations, the agreement being to be within one meter (3.28 feet). This method became and continues to be useful for survey work and map making. See also – physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid. Collapsing can – an aluminium can is crushed by the atmospheric pressure surrounding it , a tabulation of typical variations of principal thermodynamic variables of the atmosphere (pressure, density, temperature, etc.) with altitude, at middle latitudes. , an empirical, global reference atmospheric model of the Earth from ground to space References External links 1976 Standard Atmosphere from NASA Source code and equations for the 1976 Standard Atmosphere A mathematical model of the 1976 U.S. Standard Atmosphere Calculator using multiple units and properties for the 1976 Standard Atmosphere Calculator giving standard air pressure at a specified altitude, or altitude at which a pressure would be standard Current map of global mean sea-level pressure Calculate pressure from altitude and vice versa Experiments Movies on