Creative Science Centre
Creative Science Centre: Air Pressure, Manometers, Barometers and Altimeters
Explains what manometers, barometers, and altimeters are and how they are used to measure air pressure.
University of Illinois
University of Illinois Extension: Ww2010: Atmospheric Pressure
This site from the University of Illinois describes what air pressure is and how it is measured. Explains high and low pressure areas, isobars, and how pressure changes with height.