According to Landau and Lifshitz's (1975) eloquent discussion of the broad subject of Fluid Mechanics^{1}, the state of a moving fluid is determined by five quantities: the three components of the velocity v and, for example, the pressure P and the density r. For our discussions of astrophysical fluid systems throughout this Hypertext Book [H_Book], we will add to this the gravitational potential F. Accordingly, a complete system of equations of fluid dynamics should be six in number. For an ideal fluid these are: