"Grandpa" wrote in message

There's also a reason why oil is used instead of water in
hydraulics. Oil isn't as compressible as water.

While it can be compressed, it will also boil at the operating temperature
of many hydraulic systems, as well as corrode the internal metal parts. Oil
is much better that way.


http://www.physlink.com/Education/AskExperts/ae15.cfm
Question

Can you compress a liquid (water)?

Asked by: Guy Matthews

Answer

The answer is yes, You can compress water, or almost any material. However,
it requires a great deal of pressure to accomplish a little compression. For
that reason, liquids and solids are sometimes referred to as being
incompressible.

To understand what happens, remember that all matter is composed of a
collection of atoms. Even though matter seems to be very solid, in
actuality, the atoms are relative far apart, and matter is mostly empty
space. However, due to the forces between the molecules, they strongly
resist being pressed closer together, but they can be. You probably have
experienced compressing something as hard as steel. Have you ever bounced a
steel ball bearing off a sidewalk? When you do that, the 'bounce' is due to
compressing the steel ball, just a tiny little spot that comes into contact
with the sidewalk. It compresses and then springs back, causing the bounce.

The water at the bottom of the ocean is compressed by the weight of the
water above it all the way to the surface, and is more dense than the water
at the surface.

A consequence of compressing a fluid is that the viscosity, that is the
resistance of the fluid to flow, also increases as the density increases.
This is because the atoms are forced closer together, and thus cannot slip
by each other as easily as they can when the fluid is at atmospheric
pressure.

Answered by: David L. Alexander