On the resistance to the motion of a thread of mercury in a glass tube

The number of physical measurements which depend on the observation of a short thread of mercury in a glass tube is considerable. Rankine's recent determinations of the viscosities of gases, by driving the gases through a tube by the pressure of such a thread, may be mentioned as an example. Notwithstanding the frequant use of the mercury thread, there does not appear to be any definite knowledge as to the connection between the motion of the thread and the forces which must act in it to produce that motion. It is the object of the present work to investigate the point. It is comparatively well known that the upper surface of a drop of clean mercury which is slowly rising in a vertical glass tube, is more convex than when the drop is stationary, whilst the lower surface is less convex. The whole of the resistance the drop offers to motion has been attributed to this change of the angle of contact of mercury and glass. This is correct for wide bore tubes, but in narrow bore tubes viscous forces come into play.