In order to derive Torricelli's formula the first point with no index is taken at the liquid's surface, and the second just outside the opening. Since the liquid is assumed to be incompressible, ρ ρ 1 is equal to ρ ρ 2 and; both can be represented by one symbol ρ ρ . The pressure p 1 and p 2 are typically both atmospheric pressure, so p 1 = p 2 ⇒ ⇒ p 1 − − p 2 = 0 . Furthermore y 1 − − y 2 is equal to the height h of the liquid's surface over the opening: