For an hour, Barney tinkered with first one screw and then the other one. He found a point where the minimum current was really low; the two screws were interdependent and only by adjusting them alternately was he able to reach the proper point on each. Then he smiled and thrust the power on full. The current remained at a sane value. "Now what?" asked Barney. "I don't know. Anything coming out of the business end?" "Heat." "Yeah, and it's about as lethal as a sun lamp. D'ye suppose the Martians used to artificially assist their crops by synthetic sunshine?" Barney applied his eye to a spectroscope. It was one of the newer designs that encompassed everything from short ultraviolet to long infrared by means of fluorescent screens at the invisible wave lengths. He turned the instrument across the spectrum and shook his head. "Might be good for a chest cold," he said, "but you wouldn't get a sunburn off of it. It's all in the infra. Drops off like a cliff just below the deep red. Nothing at all in the visible or above. Gee," he said with a queer smile, "you don't suppose that they died off because of a pernicious epidemic of colds and they tried chest-cooking _en masse_?" "I'd believe anything if this darned gadget were found in a populated district," said Jim. "But we know that the desert was here when the Martians were here, and that it was just as arid as it is now. They wouldn't try farming in a place where iron oxide abounds." "Spinach?" "You don't know a lot about farming, do you?" asked Jim. "I saw a cow once." "That does not qualify you as an expert on farming." "I know one about the farmer's daughter, and--" "Not even an expert on dirt farming," continued Jim. "Nope, Barney, we aren't even close." Barney checked the book once more and scratched his nose. "How about that eighty-four cycle supply," asked Jim. "It's eighty-four, all right. From the Martian habit of using twelve as a base, I've calculated the number to be eighty-four." "Diddle that, too," suggested Jim. "O.K.," said Barney. "It doesn't take a lot to crank that one around from zero to about fifteen thousand c.p.s. Here she goes!" * * * * * Barney took the main dial of the beat-frequency oscillator and began to crank it around the scale. He went up from eighty-four to the top of the dial and then returned. No effect. Then he passed through eighty-four and started down toward zero. He hit sixty cycles and the jackpot at the same time! At exactly sixty cycles, a light near the wall dimmed visibly. The wallpaper scorched and burst into a smoldering flame on a wall opposite the dimmed light. Barney removed the BFO from the vicinity of sixty cycles and Jim extinguished the burning wallpaper. "Now we're getting somewhere," said Barney. "This is definitely some sort of weapon," said Jim. "She's not very efficient right now, but we can find out why and then we'll have something hot."