Explanation: The Above animation will show you that....

A tube fully filled with mercury is inverted in a trough also filled with mercury.Gradually the mercury level in the tube drops, such that the weight of mercury in the tube (from the top of the trough to height 'h') balances the force due to atmospheric pressure on the surface of mercury in the trough. Balancing pressures-
P_v + ρ_mercury * g * h = P_atmospheric     
Where, P_v is the vapour pressure of mercury (Torricellian Vacuum), can be assumed to be negligible.
ρ_mercury is the density of mercury
g is acceleration due to gravity

So, ρ_mercury * g * h = P_atmospheric

h= 76 cm (at sea level)

Hence, P _atmospheric = 76 cm of mercury (at sea level)