The mathematical description of the state of a moving fluid is effected by means of functions which give the distribution of the fluid velocityv=v(x, y, z, t) and of any two thermodynamic quantities pertaining of the fluid. These two quantities can be taken to be the pressurep(x, y, z, t) and the densityq(x, y, z, t). The above quantitiesv, pandqare functions of the coordinatesx, y, zand of the timet.