where [omega] is the area of the orifice, c[omega] the area of the
contracted section of the jet, and h the effective head measured to
the centre of the orifice. If h and [omega] are taken in feet, Q is in
cubic feet per second.
It is obvious, however, that this formula assumes that all the
filaments have sensibly the same velocity. That will be true for
horizontal orifices, and very approximately true in other cases, if
the dimensions of the orifice are not large compared with the head h.
In large orifices in say a vertical surface, the value of h is
different for different filaments, and then the velocity of different
filaments is not sensibly the same.