The only surface of revolution having this property is the **catenoid** formed by the revolution of a catenary about its directrix.

This **catenoid**, however, is in stable equilibrium only when the portion considered is such that the tangents to the catenary at its extremities intersect before they reach the directrix.

Hence a **catenoid** whose directrix coincides with the axis of revolution has at every point its principal radii of curvature equal and opposite, so that the mean curvature of the surface is zero.

Hence if a film in the form of the **catenoid** which is nearest the axis is ever so slightly displaced from the axis it will move farther from the axis till it reaches the other **catenoid**.

Hence if a film in the form of the **catenoid** which is nearest the axis be displaced towards the axis, it will tend to move farther towards the axis and will collapse.

Hence the film in the form of the **catenoid** which is nearest the axis is in unstable equilibrium under the condition that it is exposed to equal pressures within and without.

If, however, the circular ends of the **catenoid** are closed with solid disks, so that the volume of air contained between these disks and the film is determinate, the film will be in stable equilibrium however large a portion of the catenary it may consist of.

The criterion as to whether any given **catenoid** is stable or not may be obtained as follows: Let Pabq and ApqB (fig.

The condition of stability of a **catenoid** is therefore that the tangents at the extremities of its generating catenary must intersect before they reach the directrix.