Torricelli was the first do devise an experiment for measuring atmospheric pressure . He took calibrated hard glass tube , 1 m in lengt and of uniform cross section , closed at one end . He filled the whole tube with dry mercury taking care than no air or water droplets remain inside the tube , closed the opposite end of the tube tightly with thumb and inverted it . He put this inverted mercury tube into a mercury through , taking care that the end of the tube remains inside the mercury through , An interesting thing was noticed . Mercury in the tube fell down at first and then stopped at a particular position . The height was 76 cm above the free surface of mercury in the through . When the given tube was inclined or lowered in the mercury trough , the vertical haight of mercury level in the tube was always found constant .
Torricelli explained this by saying that the free surface of mrcury in the trough . Hence , the hydrostatic pressure exerted by the trough measures the atmospheric pressure .
If a small hole is made at point P in the barometer tube , then
