But
p1/G1γ = p2/G2γ ∴ p2/G2 = (p1/G1) (p2/p1)(γ−1)/γ
v22/2g = (p1/G1) {γ/(γ − 1)} {1 − (p2/p1)(γ−1)/γ};
(1)
or
v22/2g = {γ/(γ − 1)} {(p1/G1) − (p2/G2)};
an equation commonly ascribed to L. J. Weisbach (Civilingenieur, 1856), though it appears to have been given earlier by A. J. C. Barre de Saint Venant and L. Wantzel.
It has already (§ 9, eq. 4a) been seen that
p1/G1 = (p0/G0) (τ1/τ0)
where for air p0 = 2116.8, G0 = .08075 and τ0 = 492.6.