We now shift our attention from rotating waves to simple, non-rotating one-dimensional flows.We begin the next phase of our journey in the following chapter with an orientation to one-dimensional flows. Following this orientation, the appropriate forms of the continuity and momentum equations that governing these flows are developed in § 19.1. These equations are applied to steady and un-steady flows in a horizontal channel in Chapters 20 and 21, respectively. In these preliminary studies the channel is horizontal, so gravity has no effect (other than to keep the fluid in the channel). In Chapter 22 we allow the channel to slope downward and investigate how this affects the flow of simple materials. Virtually all natural flows of water and air are turbulent; we develop a simple model of turbulent flows in Chapter 23 and then conclude this part by applying this model to some simple flows in Chapter 24.