Measuring Atmospheric Pressure With a Barometer

Like any gas, air exerts a pressure. Atmospheric pressure is about  
\[10^5 M/m^2\]
  and 'standard atmospheric pressure at sea level' is defined as  
\[1.01325 \times 10^5 N/m^2\]
We can measure atmospherc pressure using a barometer.

The atmospher pushes doen on the mercury surface, but there is gas at the top of the tube to push the mercury down, so the mercury is forced up the tube until the pressure at the bottom of the mercury column equals atmospheric pressure. The equation for atmospheric pressure using this apparatus is  
\[P_{Atmospheric}= \rho gh\]
The density of mercury is 13600 kg/m3, g=9.8 m/s2, h=760mm=0.76m, then  
\[P_{Atmospheric}=13600 \times 9.8 \times 0.760=1.012928 \times 10^5N/m^2\]
Atmospheric pressure is due to the earth pulling down the air towards the ground, and is equivalent to ten medium sized family cars to every square metre. You do no feel this force because atmospheric pressure acts in all directions - down, upwards and sideways, and almost cancel each other out.