Evangelista Torricelli's original derivation can be found in the second book 'De motu aquarum' of his 'Opera Geometrica' (see ): He starts a tube AB (Figure (a)) filled up with water to the level A. Then a narrow opening is drilled at the level of B and connected to a second vertical tube BC. Due to the hydrostatic principle of communicating vessels the water lifts up to the same filling level AC in both tubes (Figure (b)). When finally the tube BC is removed (Figure (c)) the water should again lift up to this height, which is named AD in Figure (c). The reason for that behavior is the fact that a droplet's falling velocity from a height A to B is equal to the initial velocity that is needed to lift up a droplet from B to A.