working model that I can reverse
the force of gravity acting upon the model, and make it sail away into
space. I will show you this whenever you like. It is so arranged that
the polarizing action ceases in three minutes, after which the positive
current controls, and the model falls to the Earth again."

"But have you ever attempted a trip yet?" I inquired.

"Oh, no. The model was not built to carry me, but it has demonstrated
all the important facts, and I now need ten thousand dollars to build
one large enough to carry several persons, and to equip it with
everything necessary to make a trip to one of the planets. With a man
inside to control the currents, it will be far more easily managed than
the experimental model has been."

"Suppose you had the projectile built, and everything was ready for a
start," I said, "what would be the method of working it?"

"I should enter the forward compartment," began the doctor.

"But would you make the trial trip yourself?"

"I certainly would not trust the secret of operating the currents to any
one else," he remarked, with emphasis. "And will you accompany me in the
rear compartment?"

"No, indeed; unless you will promise to return in time for the following
day's market," I replied.

"Then I shall engage some adventurous fellow as assistant. First, we
must set the rudder, which is both horizontal and vertical, so that the
projectile can be steered up, down, or to either side. Having fixed it
so as to be directed a little upward, I begin with the currents.
Suppose the projectile weighs a ton, I gradually neutralize the positive
current, which we are acquainted with as gravity. When it is exactly
neutralized, the projectile weighs nothing, and the pressure of the air
is enough to make it rise more rapidly than a balloon. When I have
created a negative current, the projectile acquires a buoyancy equal to
its previous weight. That is, it will now _fall up_ as rapidly as it
would previously have fallen down. It will not do to put on the full
negative current at once, for we should acquire a velocity that would
simply burn us up by friction with the atmosphere. However, the air is
soon passed; if in the ether beyond there is very little friction, or
none at all, we shall go at full speed, which will be the constantly
increasing velocity of a falling body.

"Somewhere between the Earth and the nearest planet," he continued,
"there is a place where the attraction of one is jus