Similarly, the Lagrangian continuity equation can be derived by analysing the motion of a small, mobile, rectangular Lagrangian (material) volume of initial dimensions x0, y0 and z0 (Fig. 1.6(b)). In contrast to the steady Eulerian volume, the amount of mass m in the moving Lagrangian volume remains constant (since this volume is always related to the same material points),but the dimensions of the volume may change due to internal expansion/contraction processes. The initial average fluid density (ρ0), within the volume is given by