es, each about
three feet high and a foot wide, connected by a central shaft that
caused series of little vanes in each of them to revolve at lightning
speed.

Around the apparatus moved numerous small attendants, oiling, wiping,
adjusting its many delicate parts.

"Well, what do you think now?" asked Cor.

Kendrick made no reply, though he was thinking plenty.

"You see, it is your invention, my dear Professor," the dwarf went on
in his taunting voice, "only anteceded by a thousand years--and rather
more perfected, you must admit."

* * * * *

He walked now to the center of the laboratory, where stood a huge dial
of white crystal, ranked with many levers and switches, all capped
with the same material.

"Behold!" he said, throwing over one.

Instantly there came again that peculiar low humming that had so
puzzled him a few minutes before--and the entire room, its engines,
its attendants, Cor himself, leapt into invisibility. Only Kendrick
remained, facing the faintly visible crystal dial.

Then he saw a switch move, as though automatically. But no, for the
dwarf's hand was on it now. Visibility had returned. The vibration
ceased.

"That is the central control," said Cor. "Our city and all its
inhabitants become invisible when that switch is thrown. Only the dial
remains, for the guidance of the operator, and even that cannot be
seen at a distance of more than fifty feet. But now behold!"

He raised his hand, touched a watch-like device strapped to his
wrist--and was instantly invisible. But the laboratory and every
machine and person in it remained in plain view. Nor was there any
vibration now.

* * * * *

The next moment, having touched that curious little device again, Cor
reappeared.

"That is the local control," he said. "Every one of our inhabitants,
except those under discipline, has one of these little mechanisms. It
enables us to make ourselves invisible at will. A convenience at
times, you must admit."

"Decidedly," Kendrick agreed. "And the principle?"

"Quite simple. One of those, in fact, that lies behind your
researches. Doubtless you would have hit upon it yourself in time.
Your own scientist, Faraday, you may recall, held the opinion that the
various forms under which the forces of matter manifest themselves
have a common origin. We of the disc, thanks to our great Ravv, have
found that common origin."

It was the ori