The pressure at the bottom of an imaginary box is greater than the pressure at the top of the box. Furthermore, the box is just an imaginary construct, so the real meaning of Equation 11-7 is as follows: At \(\textit{any}\) two points in the fluid with vertical separation \(d\), the pressure at the lower point is greater than the pressure at the upper point by an amount \(\rho{gd}\). In particular, let \(p_0\) be the pressure at the upper point (Figure 11-6). Then the pressure \(p\) at a point a distance \(d\) below the upper point is:
Equation 11-8 must be satisfied for a fluid to remain at rest, so this is also called the \(\textbf{equation of hydrostatic equilibrium}\).