1) Chapter 6, Passage III, page 78

If a more viscous fluid is used with the same mass density, what would be observed?

Well, Poiseuille's eq. says that flow rate is proportional to delta P and inversely proportional to viscosity. If a more viscous solution was used, one would predict that the overall flow rate would decrease throughout the system, given that the pressures have not changed. So at point D, velocity should decrease. However the solutions state that the answer is C, and that the fluid heights in the columns are unchanged. How can this be? If the fluid velocity is slower at the outlet, it should be slower everywhere, due to this increased viscosity. No pressures have increased, so the pressure differential, the driving force of flow, is the same as before. Therefore, to me, the fluid heights should be higher in the individual pipes at the same time t after opening the valve. The pressure difference is the same, but there is a greater resistance to flow.

I had another question but already figured out my mistake, so this fluid flow one is the only one bothering me at the moment.