  In the selective withdrawal experiment fluid is withdrawn through a tube with
its tip suspended a distance S above an unperturbed two-fluid interface. At low
withdrawal rates, Q, the interface forms a steady state hump and only the upper
fluid is withdrawn. When Q is increased (or S decreased), the interface
undergoes a transition so that the lower fluid is entrained with the upper one,
forming a thin steady-state spout. Near this discontinuous transition the hump
curvature becomes very large and displays power-law scaling behavior. This
scaling is used to show that steady-state profiles for humps at different flow
rates and tube heights can all be scaled onto a single similarity profile.
