Water flows from a little cistern or reservoir R through a wide vertical tube S, about two metres long, thence through a horizontal capillary tube C', 50 or 100 centimetres long; thence through a wide horizontal metal tube T, 20 or 30 centimetres long; thence through a second horizontal capillary C; and lastly, out by a little constant-level overflow cup L. A vertical glass manometric tube M, a metre and a half long, standing up above the end of T next C to measure the pressure in T by a water column; and a means of giving any uniform temperature to the outsides of S and C, and any other uniform temperature, to the outsides of T and C; complete the instrument.