Inch (in or ") [2] : One of several traditional units of pressure. Air pressure is measured traditionally in inches of mercury (next entry) and water pressure in inches of water column (following entry)
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}