  We study critical point finite-size effects on the behavior of susceptibility
of a film placed in the Earth's gravitational field. The fluid-fluid and
substrate-fluid interactions are characterized by van der Waals-type power law
tails, and the boundary conditions are consistent with bounding surfaces that
strongly prefer the liquid phase of the system. Specific predictions are made
with respect to the behavior of $^3$He and $^4$He films in the vicinity of
their respective liquid-gas critical points. We find that for all film
thicknesses of current experimental interest the combination of van der Waals
interactions and gravity leads to substantial deviations from the behavior
predicted by models in which all interatomic forces are very short ranged and
gravity is absent. In the case of a completely short-ranged system exact
mean-field analytical expressions are derived, within the continuum approach,
for the behavior of both the local and the total susceptibilities.
