surface." _G_. "What puzzles me is how you are to get back to the earth." _I_. "Whoever goes must take the risk; but if, as appears likely, both Mars and Venus are inhabited by intelligent beings, we should probably be able to construct another cannon and return the way we came." _G_. (_smiling_). "Well, I confess the project does not look so impracticable as it did. After all, travelling in a vacuum seems rather pleasant. One of these days, I suppose, we astronomers will be packed in bullets and fired into the ether to observe eclipses and comets' tails." _I_. "In all that has been said we have confined ourselves to ways and means already known; but science is young, and we shall probably discover new sources of energy. It may even be possible to dispense with the gun, and travel in a locomotive car. Lord Kelvin has shown that if Lessage's hypothesis of gravitation be correct, a crystal or other body may be found which is lighter along one axis than another, and thus we 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, and will do me the honour of coming here, I shall be glad to show them in confidence any time you may appoint.--Yours truly, "NASMYTH CARMICHAEL."