The Kew Pattern Barometer is the standard type of barometer used to measure atmospheric pressure.
The barometer consists of a column of mercury enclosed in a vertical glass tube. The lower end of the column ends in a cistern containing a reservoir of mercury. The space above the mercury in the tube is evacuated and sealed, while the surface of the mercury in the cistern is exposed to atmospheric pressure. An increase in atmospheric pressure causes the mercury level in the cistern to fall and that in the tube to rise, while falling pressure has the opposite effect. The mercury level in the tube can then be read off against a fixed scale on the transparent body of the instrument.