BLACKOWL STOOD ON THE DOCK and watched as a team of FBI agents swarmed over the Eagle. They were an efficient lot, he observed. Each man was a specialist in his particular field of scientific detection.

They went about their job of scrutinizing the yacht from bilge to radio mast with a minimum of conversation.

A constant parade of them crossed the dock to vans parked along the shore, removing furniture, carpeting, anything that wasn't screwed down and a considerable amount that was. Each item was carefully wrapped in a plastic covering and inventoried.

More agents arrived, expanding the search for a mile around the first President's estate, examining every square inch of ground, the trees and shrubbery. In the water beside the yacht, divers scoured the muddy bottom.

The agent in charge noticed Blackowl rubbernecking beside the loading ramp and came over. "You got permission to be in the area?" he asked.

Blackowl showed his in without answering.

"What brings the Secret Service to Mount Vernon on a weekend?"

"Practice mission," Blackowl replied conversationally. "How about the FBI?"

"Same thing. The Director must have thought we were getting lazy, so he dreamed up a top-priority exercise."

"Looking for anything in particular?" Blackowl asked, feigning indifferent interest.

"Whatever we can determine about the last people who were onboard-identification through fingerprints, where they came from. You know."

Before Blackowl could reply, Ed McGrath stepped onto the dock from the gravel path. His forehead was glistening in sweat and his face was flushed. Blackowl guessed he had been running.

"Excuse me, George," he panted between intakes of breath. "You got a minute?"

"Sure." Blackowl waved to the FBI agent. "Nice talking with YOU."

"Same to you."

As soon as they were out of earshot, Blackowl asked softly, "What's going down, Ed?"

"The FBI guys found something you should see."

"Where?"

"About a hundred and fifty yards upriver, hidden away in trees.

I'll show you."

McGrath led him along a path that bordered the river. When it curved toward the outer estate buildings, they stayed in a straight line across a manicured lawn. Then they climbed a rail fence into the unkempt undergrowth on the other side. Working their way into a dense thicket, they suddenly came upon two FBI investigators who were hunkered down studying two large tanks connected to what looked like electrical generators.

"What in hell are these things?" Blackowl demanded without a greeting.

One of the men looked up. "They're foggers."

Blackowl stared, puzzled. Then his eyes winened. "Foggers!" he blurted out. "Machines that make fog!"

"Yeah, that's right. Fog generators. The Navy used to mount them on destroyers during World War Two for making smokescreens."

"Christ!" Blackowl gasped. "So that's how it was done!" OFFICIAL WASHINGTON TURNS INTO A GHOST TOWN over the weekends. The machinery of government grinds to a halt at five o'clock Frinay evening and hibernates until Monday morning) when it fires to life again with the obstinacy of a cold engine. Once the cleaning crews have come and gone, the huge buildings are as dead as mausoleums.