Atmospheric Pressure at a surface is defined as the force per unit area exerted by the weight of the atmosphere normally on that surface.
On the earth’s surface, this pressure is approximately 105 N per square-m area. Assuming the surface area of an average-sized man to be 1.5 m2, the total thrust acting on his body by the atmosphere is approximately 1.5 x 105 N. So, it can’t be said that atmospheric pressure is negligible. But living beings are quite acquainted with this tremendous pressure.
An Italian scientist Toricell devised for the first time a method for the first a method for measuring atmospheric pressure. A glass tube of uniform diameter closed at one end having a length of 1 m is filled with mercury. This device is known as a mercury barometer.
The pressure p above the mercury column in the tube is so small that it may be neglected. So, the atmospheric pressure, p0 = hydrostatic pressure exerted by the vertical mercury column in the tube.
where h = vertical height of the mercury column
ρ = density of mercury
g = acceleration due to gravity in that place