Consider a
fluid which is subject to gravity and at rest. The pressure in such a
fluid will not be constant but increases with depth in the fluid.
To appreciate this we mentally isolate a small
cylindrical volume of height (vertical) 
and with a horizontal
cross sectional area, A.
The forces acting on this volume of fluid are gravity and the pressure
from fluid outside the volume which we are considering. If the fluid
is at rest there can be no net force acting on the fluid in the volume
of interest. Horizontal forces trivially cancel out. For the vertical
component we get :
That is the pressure must be higher on the bottom end to counteract
the gravitational force. This equation holds for gases as well as
liquids. For gases the density varies considerably with pressure so
we would need to use the differential form of this expression
for a liquid we can often neglect the ability if the liquid to compress
(vary its density) and in that case
then 
and then we are back at
