"Vaguely," Manchek said, suppressing a yawn. In fact, the audio screen was a system he had developed three years before. In simplest terms, it was a computerized way to find a needle in a haystack- a machine program that listened to apparently garbled, random sound and picked out certain irregularities. For example, the hubbub of conversation at an embassy cocktail party could be recorded and fed through the computer, which would pick out a single voice and separate it from the rest.

It had several intelligence applications.

"Well," Comroe said, "after the transmission ended, we got nothing but the static you hear now. We put it through the audio screen, to see if the computer could pick up a pattern. And we ran it through the oscilloscope in the corner."

Across the room, the green face of the scope displayed a jagged dancing white line- the summated sound of static.

"Then," Comroe said, "we cut in the computer. Like so."

He punched a button on his desk console. The oscilloscope line changed character abruptly. It suddenly became quieter, more regular, with a pattern of beating, thumping impulses.

"I see," Manchek said. He had, in fact, already identified the pattern and assessed its meaning. His mind was drifting elsewhere, considering other possibilities, wider ramifications.

"Here's the audio," Comroe said. He pressed another button and the audio version of the signal filled the room. It was a steady mechanical grinding with a repetitive metallic click.

Manchek nodded. "An engine. With a knock."

"Yes sir. We believe the van radio is still broadcasting, and that the engine is still running. That's what we're hearing now, with the static screened away."

"All right," Manchek said.

His pipe went out. He sucked on it for a moment, then lit it again, removed it from his mouth, and plucked a bit of tobacco from his tongue.