is so slight at the distance of Uranus, that a
stone flung out of the car would have no perceptible motion, as it
would only fall towards the sun a mere fraction of an inch per second,
or some 355 feet an hour; hence, as Dr. Preston has calculated, one
ounce of matter ejected from the car towards the sun every five minutes,
with a velocity of 880 feet a second, would suffice to keep a car of one
and a half tons at rest on the orbit of the planet. Indeed, the vitiated
air, escaping from the car through a small hole by its own pressure,
would probably serve the purpose. Just before the planet came up, and in
the nick of time we could fire some rockets, and give the car a velocity
of two or three miles a second in the direction of the planet's motion,
so that he would overtake us, with a speed not over great to ensure a
safe descent. Our parachutes would be out, and at the first contact with
the atmosphere, the car would probably be blown away; but it would soon
acquire the velocity of the planet, and gradually sink downwards to the
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
upse