Figure 167. Fluid flow across a small parallelepiped

The mass of the fluid entering through the face during a short time interval at a point is given by

as is the mass crossing over per unit area, in the positive direction of the axis, in unit time. The mass of the fluid leaving in the direction of the -axis, across the face is given at a point by

Thus, the net change in mass in the direction of the -axis is given by