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I called one of the bubble forms, if you remember, by its name, catenoid; this is produced when there is no pressure.

There was only one parabola, and that was formed when the side of the cone was parallel to the plane of section, that is when the falling grease just touched the edge of the candlestick; there is only one bubble with no pressure, the catenoid, and this is drawn by rolling the parabola.

Between the cylinder and the catenoid.

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.