listen. They'll dig, and they'll find what I say is there. You wait and see."

"They'll lock you up, darling," Dotty said. "They won't believe you."

The guide was calling everyone back to the bus. I watched Herb scowl fiercely at the stone marker that he believed to have been his, open his mouth to say something, then turn away so that his lips were out of sight. Regretfully I put the mirroscope away and went back to the bus.

I knew where we were going next, and I was uneasy about it. Herb and Dotty managed to sit together and I got a place right behind them where I could eavesdrop. But they sat in silence.

The bus had left the ancient city behind, to head out over the desert toward one of the few structures on Mars which had withstood the ravages of time without crumbling. An immense dome of solid concrete reinforced with pure copper rods harder than steel. The Martians had known what Earth civilization didn't learn until around the year three thousand: that copper can't be tempered, but pure copper becomes tempered of itself in a thousand years.

That immense dome was a honeycomb of passageways and rooms, some of which were not open to tourists. It would be a natural for Herb.

The bus stopped. The people were piling out and staring curiously at the smooth surface of the dome. Especially at places where the reinforcement rods were protruding and glittering like tarnished gold.

Two of the permanent guards had come out to take charge of the tour. I caught the eye of one of them and nodded toward Herb. The guard caught my meaning, edged over to his partner, and soon both men were warned that Herb was to be closely watched.

I felt better, knowing that a couple of others knew about him. Maybe it would have been smarter to have taken him in custody right then. But it would have meant a scene.

The procedure of the tour was for the guide to do all the talking, leading the procession through the roped off parts of the dome, while the two guards followed along behind to make sure no stragglers got left.

I let three or four people move in front of me so Herb wouldn't get suspicious. Dotty was sticking close to him, plainly worried. And he was more excited than he had been at any of the other spots. He fairly quivered, his eyes caressing the walls with a fevered look.

Dotty didn't miss his increased agitation. Especially after he whispered in her ear a couple of times.

The guide took the usual path. Straight into the dome, pausing at half a dozen small rooms with carved walls, to arrive at a bank of elevators installed in the exact center; then straight up to the roof and the observation platform from which miles and miles of desert and ruins could be seen. Then back down to the second level, a zig-zag course through other rooms, and finally down a flight of steps to where the tour started.

I kept my eyes on the back of Herb's head. You can tell a lot by doing that. At first his head turned this way and that, indicating he was full of curiosity. I was waiting for that telltale sudden tensing, with the head directed at some spot, that would tell of a sudden "memory" stirring in the man's mind.

I almost missed it when it came, because it was between two passages—a blank wall. The briefest pause, then Herb was going on again as though nothing had happened.

But now his head had stopped its curiosity-motivated pivotings. It was the head of a man who was no longer curious—who has made up his mind about something. I didn't like it.

And when the group emerged into open air once more without Herb having tried anything I knew as certainly as I had ever known anything that he intended coming back here, and soon.

In the comfort station before boarding the bus I scrawled a hasty note to the guards to investigate the spot halfway between passageways 14 and 15 on the first level, and slipped it to one of them as I passed him to get on the bus.

We visited four other spots on the tour. When Herb showed no