And deep inside him some part of his mind said, "This is a madness you cannot escape. The city is evil, an evil like you have never known," and a fear as old as time coursed through his frame. He seized the copper hat from his head, and beat on the lotus carvings of the great door, crying, "Let me in! Please, take me back into the city." And as he beat the city changed. It became dull and sordid and evil, a city of disgust, with every part offensive to the eye. The spires and minarets were gargoyles of hatred, twisted and misshapen, and the sound of the city was a macabre song of hate. He stared, and his back was chill with superstitions as old as the beginning of man. The city flickered, changing before his eyes until it was beautiful again. He stood, amazed, and put the metal hat back on his head. With the motion the shift took place again, and beauty was ugliness. Amazed, he stared at the illusion, and the thought came to him that the metal hat had not entirely failed him after all. He turned and began to walk away from the city, and when it began to call he took the hat off his head and found peace for a time. Then when it began again he replaced the hat, and revulsion sped his footsteps. And so, hat on, hat off, he made his way down the dusty floor of the canal, and up the rocky sides until he stood on the Martian desert, and the canal was a thin line behind him. He breathed easily then, for he was beyond the range of the illusions. And now that his mind was his own again he began to study the problem, and to understand something of the nature of the forces against which he had been pitted. The helmet contained an electrical circuit, designed as a shield against electrical waves tuned to affect his brain. But the hat had failed because the city, whatever it was, had adjusted to this revised pattern as he had approached it. Hence, the helmet had been no defense against illusion. However, when he had jerked the helmet off suddenly to beat on the door, his mental pattern had changed, too suddenly, and the machine caught up only after he had glimpsed another image. Then as the illusion adjusted replacing the helmet threw it off again. He grinned wryly. He would have liked to know more about the city, whatever it was. He would have liked to know more about the people he had seen, whether they were real or part of the illusion, and if they were as ugly as the second city had been. Yet the danger was too great. He would go back to his ship and make the arrangements to destroy the city. The ship was armed, and to deliver indirect fire over the edge of the canal would be simple enough. Garve North, his brother, waited back at the ship. If he knew of the city he would have to go there. Eric must not take a chance on that. After they had blasted whatever it was that lay in the canal floor, then it would be time enough to tell Garve, and go down to see what was left.