Startled, Rutledge sent the cap flying into the darkness.

"Here, now!" the constable exclaimed as he got closer and took in the two men, a knife lying some two yards away. From his vantage point, Rutledge appeared to be the aggressor, and Rutledge's attacker took swift advantage of it.

He screamed, "Don't let him hurt me-he's trying to kill me. Help me-"

The constable was there, catching at Rutledge's shoulder, hauling him away from his victim, and for the first time Rutledge glimpsed the flushed and frightened face of a boy who looked eighteen or nineteen but for all his size must be no more than sixteen.

And then as the constable's fist closed over Rutledge's bleeding arm, his fingers just as quickly opened again.

"What's this, then?" the constable demanded, stepping back. He was thin and middle-aged, an imposing figure with the light reflecting from the crown of his helmet, giving the impression he was taller than he was. "Is that your knife, or his?" he asked the boy.

In that split second of hesitation, Billy wriggled free of Rutledge's grip and set off over the bridge, his feet flying. The constable looked from him to Rutledge, and Rutledge said rapidly, "I'm Scotland Yard. Rutledge, Inspector. Go after him, man."

But it was too late. By the time the constable had collected himself and pelted after the suspect, he had turned at the bridge abutment and was lost in the darkness on the far side of the river.

The constable came back, breathing hard, to meet Rutledge halfway. "I'm sorry, sir-"

"So am I. His next victim might not be as lucky." He gave the constable a description of the boy, including the false name, and added, "He's frightened enough to be dangerous."

"I didn't get a close look at him," the constable admitted. "But I'll see word is passed on." He gestured to Rutledge's arm. "You'd best have that seen to, sir."