Atmospheric Pressure
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Atmospheric pressure at a point : It equals the weight of a vertical air column of unit c.s. area from this point to the end of the atmosphere. it decreases as the altitude increases.

Torricilian Mercury Barometer experiments to measure Atmospheric Pressure .

A thin glass tube open at one end about 1 m long is filled with Mercury , shacked to get rid of air bubbles for accurate results .

The open end is closed by the finger and the tube is inverted to immerse its open end in a trough containing mercury .

when the finger is removed , mercury falls down and stops when pressure at the base of the tube Px becomes equal to atoms. pressure Pa
Pa = Px = rhg        PASCAL's 

NB :

1- tilting the tube has no effect on the vertical height of Mercury columns .


2- The space in the tube above Mercury level is called " Torricelli's space" it's not a true space ( vacuum ) because it contains Mercury vapor at nearly zero pressure 
3- Atmospheric  pressure at a point acts equally in all directions .