"I can't see anything,” said Sual Av. “Your lamp must have burned out accidentally."
"Wait,” said Thorn tensely.
He tore a bit of cloth from his jacket, and cautiously pushed it forward until it was over one of the row of holes. Instantly the cloth burst into flame and vanished in fine ashes.
John Thorn felt cold sweat stand out on his brow. He knew now the invisible death he had nearly, blundered into.
"There's a web of heat-beams here across the drain,” he said hoarsely. “A little trap fixed up by Haskell Trask's guards for anyone who might try to enter the citadel this way."
The nature of the diabolical trap was clear. Buried somewhere near the cement drain was a generator of heat beams — those “focused” rays of radiant heat which were produced in a mirrored inertrum chamber by transformation of atomic energy into vibratory force in the proper octaves. Such beams had an effective range of only a few feet, but were deadly within that distance.
"The beams are projected through three holes in the floor and disappear through the holes in the roof of the drain, to be dissipated above,” Thorn said. “It's a fiendishly clever idea. Anyone crawling up this drain would never see anything until he blundered into those beams that would sear through. and kill him instantly."
"Hell, we can't pass this until we find some way to shut off these beams!” swore Gunner Welk from behind.
Thorn frowned tensely. “We can't get at the generator of them,” he muttered. “That must be located outside the drain. It would take lots of tools and time to dig down to it."
"Inertrum is proof against high heat,” Sual Av said hopefully. “If we had some inertrum plugs to stop those holes the beams come up through—"