y the rapidity with which
the closer suns and their planets were passed gave any indication of the
frightful speed at which they were being hurtled along by the
inconceivable power of that disintegrating copper bar.

When the vessel was nearly half-way to "X," the bar was reversed in
order to change the sign of their acceleration, and the hollow sphere
spun through an angle of one hundred and eighty degrees around the
motionless cage which housed the enormous gyroscopes. Still apparently
motionless and exactly as she had been before, the _Skylark_ was now
actually traveling in a direction which seemed "down" and with a
velocity which was being constantly decreased by the amount of their
negative acceleration.

A few days after the bar had been reversed Seaton announced that the
mechanical educator was complete, and brought it into the control room.

In appearance it was not unlike a large radio set, but it was infinitely
more complex. It possessed numerous tubes, kino-lamps, and
photo-electric cells, as well as many coils of peculiar design--there
were dozens of dials and knobs, and a multiple set of head-harnesses.

"How can a thing like that possibly work as it does?" asked Crane. "I
know that it does work, but I could scarcely believe it, even after it
had educated me."

"That is nothing like the one Dunark used, Dick," objected Dorothy. "How
come?"

"I'll answer you first, Dot. This is an improved model--it has quite a
few gadgets of my own in it. Now, Mart, as to how it works--it isn't so
funny after you understand it--it's a lot like radio in that respect. It
operates on a band of frequencies lying between the longest light and
heat waves and the shortest radio waves. This thing here is the
generator of those waves and a very heavy power amplifier. The headsets
are stereoscopic transmitters, taking or receiving a three-dimensional
view. Nearly all matter is transparent to those waves; for instance
bones, hair, and so on. However, cerebin, a cerebroside peculiar to the
thinking structure of the brain, is opaque to them. Dunark, not knowing
chemistry, didn't know why the educator worked or what it worked on--he
found out by experiment that it did work; just as we found out about
electricity. This three-dimensional model, or view, or whatever you want
to call it, is converted into electricity in the headsets, and the
resulting modulated wave goes back to the educator. There it is
heterodyned with another wave-