Fig. 1.1 When a region of high pressure is
separated from a region of low pressure by
a movable wall, the wall will be pushed into
one region or the other, as in (a) and (c).
However, if the two pressures are identical,
the wall will not move (b). The latter
condition is one of mechanical equilibrium
between the two regions.
gas will be in mechanical equilibrium with another gas with which it shares a movable
wall.