Some barometers give the atmospheric pressure in millibars (one millibar = 100 Pascals, or one hectoPascal).
Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.
A variation of this type of barometer can be easily constructed.
Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers.
The first barometer of this type was devised in 1643 by Evangelista Torricelli.
Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere.
Fitzroy barometers combine the standard mercury thermometer with a thermometer, as well as a guide of how to interpret pressure changes.
A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base.
An aneroid barometer uses a small, flexible metal box called an aneroid cell.
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column.
To create a vacuum with mercury takes less than three feet, which makes its use more practical than a water barometer.