When 
constant and 
is given (from experiments), the above equations have 4 equations with 4
unknowns, u, v, w and P. When 
constant, and is given ( from experiments) the above
set of equations have 4 equations with 5 unknowns u, v, w, P and 
. This is not good since there is
one less equation. We need one addirtional equation to specify the function of density