The weight or pressure of the atmosphere is ascertained by the mercurial barometer, the aneroid, or from the temperature of the boiling point of water. The height of a hill is measured barometrically, from observations made simultaneously at its base and top, and the application of certain well-known formulæ. The height of a place at no great distance from another place whose height is known, and at which observations are made about the same time, may similarly be ascertained with a close approximation to the truth.

But, with regard to places far from any place of known elevation, or from any place at which meteorological observations are made, it is plain that the height can only be computed by assuming a certain pressure as the sea-level pressure at that place.