SM001      Fortin Barometer      circa 1950     Hydrostatics

   The normal pressure of air at sea level is about 14.7 pounds per square inch (1.01x10⁵ Pascals).   This is the direct result of the weight of the atmosphere bearing down on the surface of the earth.   Because of this, different air pressures exist at different heights and vary with time and place.  

  Torricelli demonstrated in the year 1643 that a tube about a yard (1 metre) long will hold a column of mercury of about 30 inches (760mm) in height with a vacuum at the closed end and atmospheric pressure on the reservoir at the bottom.

  The Fortin Barometer is based on the above principle but it includes a vernier scale for accurately measuring small variations in the height of the mercury.