Leigh
Whether Bernoulli knew about molecules or not, we do. Since the pressue
changes are an observable fact we should be able to understand them in terms
of molecules. I still maintain that pressure results from collisions of
molecules with the pipe for either a liquid or a gas. How else can you get the
pressure? If the fluid is incompressible you can't change the number of
molecules hitting the pipe so you have to change the speed or angle with
which they hit the pipe. How else can you get a pressure change?