The pressure of a point in a fluid is the sum of the external pressure and $pgh$. However, when the fluid is not placed perpendicular to the ground, then what is $h$ here? For example, a cup of water can be tilt an angle from the vertical axis.
In particular, can you explain clearly about this: a tube containing mercury and air. When the tube tilts at an angle 30○ from the vertical, then the length of the column of air will change too. But in order to calculate the change in its length, we have to know the pressure exerted by the mercury. But each point of the mercury in contact with air will exert a different amount of pressure so how can we do it?