  It is shown, that the introduction in Saint-Venant -Wantzel formula radicand
of the free component in the form of quadrate of velocity of the incident gas
flow on inlet of flowing element that was offered in the end of the XIX century
and is remained till now it is physically inadequate. It is shown that this way
is doubtful for hydromechanics and it absolutely is not reasonably for
gasdynamics. Physically exact form of writing of static head law for flowing
element and relevant to this, the modern form of Saint-Venant - Wantzel formula
are submitted ad-hoc. The obtained expressions allow physically correctly to
take into account both combined and isolated influence of pressure drop
applying to the flowing element and velocity of incident gas flow on flowing
element, on quantity of the outflow velocity of gas stream out of this flowing
element, system. The obtained expressions are valid for a subsonic velocity of
the incident gas flow. The particular expressions are obtained for liquid.

Fluid Dynamics