I had satisfied myself that only one thing needful was as yet wholly beyond the reach and even the proximate hopes of science.

Human invention could furnish as yet no motive power that could fulfil the main requirement of the problem--uniform or constantly increasing motion _in vacuo_--motion through a region affording no resisting medium.

This must be a _repulsive_ energy capable of acting through an utter void.

Man, animals, birds, fishes move by repulsion applied at every moment.

In air or water, paddles, oars, sails, fins, wings act by repulsion exerted on the fluid element in which they work.