expressing his gratitude for Barney's generous help, and his relief
that because of it the work on the Tube now could be brought to an
end.

"Just one thing about that still bothers me a little, doctor," Barney
said candidly.

McAllen looked concerned. "What's that, Mr. Chard?"

"Well ... you're in good health, I'd say." Barney smiled. "But suppose
something did happen to you before you succeeded in shutting the
McAllen Tube down." He inclined his head toward the locked door.

"That thing would still be around waiting for somebody to open it and
step through...."

McAllen's expression of concern vanished. He dug a forefinger
cheerfully into Barney's ribs. "Young man, you needn't worry. I've
been aware of the possibility, of course, and believe me I'm keeping
_very_ careful notes and instructions. Safe deposit boxes ... we'll
talk about that tomorrow, eh? Somewhere else? Had a man in mind, as a
matter of fact, but we can make better arrangements now. You see, it's
really so ridiculously _easy_ at this stage."

Barney cleared his throat. "Some other physicist--?"

"_Any_ capable physicist," McAllen said decidedly. "Just a matter, you
see of how reliable he is." He winked at Barney. "Talk about that
tomorrow too--or one of these days."

Barney stood looking down, with a kind of detached surprise, at a man
who had just pronounced sentence of death casually on himself, and on
an old friend. For the first time in Barney's career, the question of
deliberate murder not only entered an operation, but had become in an
instant an unavoidable part of it. Frank Elby, ambitious and
money-hungry, could take over where McAllen left off. Elby was highly
capable, and Elby could be controlled. McAllen could not. He could
only be tricked; and, if necessary, killed.

It was necessary, of course. If McAllen lived until he knew how to
shut the Tube down safely, he simply would shut it down, destroy the
device and his notes on it. A man who had gone to such extreme
lengths to safeguard the secret was not going to be talked out of his
conviction that the McAllen Tube was a menace to the world.
Fredericks, the morose eavesdropper, had to be silenced with his
employer to assure Barney of his undisputed possession of the Tube.

Could he still let the thing go, let McAllen live? He couldn't, Barney
decided. He'd dealt himself a hand in a new game, and a big one--a
fantastic, staggering game when one considered the possibilities in
the