Here is a little exercise which illustrates the idea of a pressure in a
fluid. I have a tube and a water filled beaker. Knowing the density of
water I would like to determine the density of this motor oil. To do
this I fill the tube with oil and immerse in the water. I find that the
oil stands higher than the water level. The pressure in the oil must
be the same as the pressure in the water at the interface between the two. I also know that the pressure (atmospheric pressure)
above the two liquids match. Thus the pressure difference
from the surface of the liquids to the interface
between the liquids must be the same:
measuring the lengths we find
