  A break in the distribution function of Ly$\alpha$ clouds (at a typical
redshift of $2.5$) has been reported by Petitjean et al. (1993). This feature
is what would be expected from a transition between pressure confinement and
gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The
column density at which the feature occurs has been used to determine the
external confining pressure, $\sim 10 {\rm cm}^{-3} {\rm K}$, which could be
due to a hot, intergalactic medium. For models that provide a good fit to the
data, the contribution of the gas in clouds to $\Omega$ is small. The specific
shape of the distribution function at the transition (predicted by models to
have a non-monotonic slope) can serve as a diagnostic of the distribution of
dark matter around Ly$\alpha$ forest clouds, and the present data already
eliminate certain models.
