The readings of the mercurial barometer are subjected in nice observations to several corrections:
1st. To 32° F. allowance being made for expansion of both mercury and scale for all observations above that temperature. A barometric pressure of thirty inches at 32° would be indicated by a height of 30⅒ inches at 70°.
2d. For decrease of gravitation at stations above the level of the sea, acting on both the mercury and the air.
3d. For increase of gravity with increase of latitude.
4th. For temperature of air; the density decreasing as temperature rises.
5th. For humidity of the air which also influences its density.
6th. For capillary attraction of the tube.
CHAPTER II.
BAROMETRIC MEASUREMENTS
OF ALTITUDES.
The text books in physics present formulas for computing heights from barometric observations, based on physical laws which we will briefly give.
If the density of the air were constant throughout, the measurement of heights would be a problem of the simplest character; for as mercury weighs 10,500 times as much as air at the sea level, the mercurial column would fall one inch for every 10,500 inches of ascent above the sea. But air is compressible, and, in accordance with Boyle’s law, its density varies with the pressure to which it is subjected.