Laminar flow of a viscous fluid through a horizontal capillary tube is described by Poiseuille's equation,³ Q = (P₁ – P₂)r⁴/8L, where Q is the volume flow per unit time, r is the radius of the tube of length L, (P₁ – P₂) is the pressure difference between the two ends of the tube, and is the coefficient of viscosity of the fluid. When one end of the tube is placed at depth h below the surface of the open vertical column of fluid of density cross section A, the pressure difference is simply gh when the other end of the tube is open to atmosphere. By applying the continuity equation where the volume flow rate at the top of the vertical column is expressed as Av = Adh/dt, where v is the instantaneous speed, it is readily found that the height of the column is given by h = h₀exp(–t), where h₀ is the column height above the tube at a reference time of t = 0 and