Flypaper & Bruce, You are both absolutely correct about the column of water. As soon as you open the valve at one end the fluid will start flowing at the other end... BUT, if you change the amount of fluid flowing through that column you do have to wait for change to happen at the far end. This is what I am referring to. I will clarify in saying that they delay isn't very long (a second if that much) but there is a perceptible delay when you make a change.

Ken