may be able to draw an unlimited supply of power from gravity by simply
changing the position of the crystal; for example, by raising it when
lighter, and letting it fall when heavier. This form of 'perpetual
motion' might be equally obtainable if Dr. Preston's[3] theory of an
ether as the cause of gravity be true. Indeed, Professor Poynting is now
engaged in searching for such a crystal, which, if discovered, will
upset the second law of thermo-dynamics. I merely mention this to show
that science is on the track of concealed motive powers derived from
the ether, and we cannot now tell what the engines of the future will be
like. For ought we know, the time is coming when there will be a regular
mail service between the earth and Mars or Venus, cheap trips to
Mercury, and exploring expeditions to Jupiter, Saturn, or Uranus."

[Footnote 3: _Philosophical Magazine_, February, 1895.]




CHAPTER III.

A NEW FORCE.


"SIR,

"I have read your article on the possibility of travelling to the
other members of the Solar system with much interest. It is a
problem at which I, myself, have been working for a great many
years, and I believe that I have now discovered a means of solving
it in a practical manner. If you would care to see my experiments,