Therefore, the momentum part of the Euler equations for a steady flow is found to have a simple form: { u ∂ ∂ u ∂ ∂ s = − − 1 ρ ρ ∂ ∂ p ∂ ∂ s , u 2 R = − − 1 ρ ρ ∂ ∂ p ∂ ∂ n ( ∂ ∂ / ∂ ∂ n ≡ ≡ e n ⋅ ⋅ ∇ ∇ ) , 0 = − − 1 ρ ρ ∂ ∂ p ∂ ∂ b ( ∂ ∂ / ∂ ∂ b ≡ ≡ e b ⋅ ⋅ ∇ ∇ ) .