Equations (3.9) and (3.10) are generally adequate for the analysis of flows in conduits with solid boundaries, but for the consideration of flow in space, as that of air around an airplane, for example, it is desirable to express the continuity equation in another form, as indicated in Sec. 5.1 Or, for the case of unsteady flow of a liquid in a canal (fig. 3.4), the principle of conservation of mass