Kew Pattern Barometer  
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.

  Aspirated Hygrometer  
The aspirated hygrometer is used to measure wet and ambient (dry) temperatures, which in turn are used to determine humidity.
As air flows though the instrument, a fan draws air past two thermometer bulbs. One bulb is kept moist by a close fitting sleeve of water-absorbent material while the other is left uncovered and dry. As the water evaporates, the wet bulb temperature is lowered to an equilibrium point.
  The wet and ambient temperature readings are then taken and used to calculate (using a slide rule) the humidity.  

Copyright 2002, Meteorological Service, Jamaica . 65 3/4 Half Way Tree Road, Kingston 10, JAMAICA