On the last occasion on which I had the honour of reading a paper before this (Society 1 took as my subject the shape of stability, of bodies falling through the air. 1 endeavoured to show that if you wish to make an apparatus which will descend in perfect equilibrium you must do something more than give it lifting surfaces with the usual dihedral angle; that it is, in fact, necessary to extend this plan and form them with an upward angle in all directions from an approximately central point, thus making them take a form like the surface of an inverted cone.

To-night I want to ask you to look at stability from another point of view. A flying body, to be of practical value, must have, in addition to equilibrium of buoyancy, another equilibrium, viz., that of progression, i.e., it must have a strong tendency to proceed with the same point of its framework always to the fore.