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 to the rotted wooden tie beneath.
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-control robots hidden in strategic spots throughout those tunnels now, and we can put more in whenever we want, 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 those tunnels. If he went out through the one open-air exit up in the northern part of the island, 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 very far underwater."
"How do you get your carrier wave underground to those tunnels?" Stanton asked. "And how do you keep the Nipe from picking up the radiation?"
The colonel grinned widely. "One of the boys dreamed up a real cute gimmick. Those old steel rails themselves act as antennas 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, but it's running smoothly now."
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 the answers to.
"Aren't there ways of detecting objects underwater?" he asked after a moment.
"Yes," said the colonel, "several of them. But they all require beamed energy of some kind to be reflected from the object we want to look at, 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 big problem all along," he said seriously. "We have to keep the Nipe from knowing he's being watched. In the tunnels themselves, we've only used equipment that was already there, adding only what we absolutely had to—small things. A few strands of wire, a tiny relay, things that can be hidden in out-of-the-way places and can be made to look as though they were a part of the original old equipment. After all, he has his own alarm system in that maze of tunnels, and we have deliberately kept away from his detecting devices. He knows about the rats and ignores them. They're part of the environment. But we don't dare use anything that would tip him off to our knowledge of his whereabouts. One slip like that, and hundreds of human beings will have died in vain."