negative gradient.

We have modified the [[constant pressure]{}]{} option in version 08.00 of [Cloudy]{} to account for the gravitational term $\Delta P_{\rm grav}$. Source code is available upon request, and a brief user guide is provided in Appendix \[secUserGuide\]. The required modifications are relatively minor, and they are not expected to incur in any side effect on the functionality of the algorithm. [[They are currently being implemented in the development version of the code and will be available in the next official release[^2].]{}]{}

First, new variables are created to store the values of $\Delta P_{\rm grav}$ (initialized to 0 at the illuminated face), the symmetry of the problem (spherical or plane-parallel) and the mass (0 by default) of an additional component located at the centre.

As [Cloudy]{} moves away from the illuminated face, $\Delta P_{\rm grav}$ is updated according to $$\Delta P_{i+1} = \Delta P_i + \rho_i\, g_i\, h_i$$ where $\Delta P_i$ and $\Delta P_{i+1}$ denote the old and new values, while $\rho_i$, $g_i$, and $h_i$ are the density, gravitational acceleration, and thickness of the last zone, respectively. The gravitational acceleration is computed according to expression (\[eqGrav\_sphere\]) or (\[eqGrav\_plane\]) depending on the geometry. [Cloudy]{} keeps track of the cumulative mass 