le in the open mouth. Then the not-rat scuttled backwards faster
than any rat could have moved.

For a second, the real rat hesitated, and it may be that the realization
penetrated into its dim brain that rats did not fight this way. Then, as
the tiny needle dissolved in its bloodstream, it closed its eyes and
collapsed, rolling limply off the rail.

The rat might come to before it was found and devoured by its fellows--or
it might not. The not-rat moved on, not caring either way. The human
intelligence that looked out from the eyes of the not-rat was only
concerned with getting to the Nipe.

* * * * *

"That's how we found the Nipe," Colonel Mannheim said, "and that's how we
keep tabs on him now. We have over seven hundred of these
remote-controlled robots hidden in strategic spots in those tunnels now,
but it took time to get everything set up this way. Now, we can follow the
Nipe wherever he goes, so long as he stays in the tunnels. If he went out
through an open air exit, we could have him followed by bird-robots but--"
He shrugged wryly. "I'm afraid the underwater problem still has us
stumped. We can't get the carrier wave for the remote-control impulses to
go far underwater."

"How do you get your carrier wave underground to those tunnels?" Stanton
asked.

The colonel grinned widely. "One of the boys dreamed up a real cute
gimmick. The rails themselves act as antenna for the broadcaster, and the
rat's tail is the pickup antenna. As long as the rat is crawling right on
the rail, only a microscopic amount of power is needed for control, not
enough for the Nipe to pick up with his instruments. Each rat carries its
own battery for motive power, and there are old copper power cables down
there that we can send direct current through to recharge the batteries.
And, when we need them, the copper cables can be used as antennas. It took
us quite a while to work the system out."

Stanton rubbed his head thoughtfully. _Damn these gaps in my memory!_ he
thought. It was sometimes embarrassing to ask questions that any schoolboy
should know.

"Aren't there ways of detecting objects underwater?" he asked after a
moment.

"Yes," said the colonel, "But they all require beamed energy of some kind
to be reflected from the object, and we don't dare use anything like
that." He sat down on one corner of the table, his bright blue eyes
looking up at Stanton.

"That's been our problem all along