The structure of the z-distribution of mass and pressure in the solar neighborhood are explored. Three gravitational potentials were tried, differing in their content of dark matter in the Galactic plane. The weight integrals, combined with the observed contributions to the total midplane pressure yield a quite restrictive range P(0) ≈ (3.9 ± 0.6) × 10 -12 dyn cm -2, with roughly equal contributions from magnetic field, cosmic-ray, and kinetic terms (of which the actual thermal pressure is quite small). This boundary condition restricts both the magnitude of gravity and the high z-pressure. It favors lower gravity and higher values for cosmic-ray, magnetic field, and (probably) kinetic pressures than have been popular in the past, consistent with recent investigations of all four. Inclusion of the warm H+ distribution, inferred from pulsar dispersion measures, carries a significant mass component into the z ∼ 1 kpc regime, a region which several lines of investigation have previously implied was occupied by cosmic rays and magnetic fields. We have found this to be consistent with hydrostatics when magnetic tension is included and that the distribution may satisfy even the most restrictive stability criterion, criterion. We are encouraged that, within the uncertainties, acceptable distributions of material and pressure components exist that satisfy all known constraints. An essential feature is that cosmic rays, magnetic fields, and a significant component of the interstellar mass all have high z-distributions characterized by scale heights ≥ 1 kpc. Very little further progress on the diffuse ISM is likely to be made without consideration for the effects of its great extent. An additional feature not often appreciated is that at high z, the Galactic magnetic field probably has a significant vertical component. This should be immediately obvious when rotation measures to pulsars in globular clusters have been surveyed.
