Since he was familiar with the unit, he had no difficulty. "Solar disturbances as usual," he muttered, "but no radio signals or undiscovered mass formations—wait a second. Maybe I'm wrong. The indicator won't remain on the zero line. Ah! There is a disturbance caused by the presence of matter. It's center—let me calculate roughly—just as I thought— about seventeen degrees to the left of the planet Neptune."
"Well?" Carruthers' voice had a touch of impatience.
Vignot peered at a map of star constellations on the nearest wall. "You tell me, Aaron. There's nothing but bleak emptiness in that part of the sky. It's a place where time seems to stand still, where distances from one body to another are fixed at millions of miles. It's a vast immensity where there is no light, no heat, no sound, and nothing more substantial than occasional streamers of dark, gaseous clouds."
He turned to Carruthers and spread his hands, palms upward. "The disturbance is caused by a comet. Any astronomer could have told you that much. It's that simple."
"Not quite," said Carruthers. "I thought of comets. On the table beside the Thermo-cell unit you'll find charts. The top one was made in 1967, and based on figures and negatives furnished me by the Palomar Observatory. Plotted on this chart are the paths of various wanderers of the sky—meteors, asteroids and comets. None of them are to be found in the sky area on which the unit's detector beam is centered.
"On the second chart you'll find the periodic comets and their paths across the heavens. Biela's comet, first observed in 1772, returns every seven years. It isn't due again for five years. Rule that one out."
Vignot shrugged. "Go on," he urged.
"Following it is one discovered by Encke. Its period of visibility at a fixed point in the sky occurs every three years. Then Halley's comet comes along with a period of seventy-six years, followed by Donati's which appears at intervals several thousand years apart. None are due this year—or now."
George Vignot tugged thoughtfully at his beard. "I see," he nodded. "But all this talk about comets must mean something. What?"
Carruthers watched both men seat themselves in comfortable chairs but made no motion to follow their example. Instead he began to pace the floor. "I didn't say anything about comets. You brought them into our talk yourself. The thing that is causing the disturbance on the sensitive plates of the Thermo-cell unit might be a planet or a star, or a globe like our own inhabitated with human beings.
"Or it may be nothing more than a sphere of black gas with a metallic core because it isn't yet visible. And it's out there in that bleak emptiness as, you call it, beyond the gravitational pull of Neptune. It's still impossible to correctly determine its size or structure. But if the Thermo-cell unit is accurate to within one tenth of a degree, that invisible body is headed toward our earth at a tremendous speed which will accelerate to an even greater velocity as its expanding gases drive it onward. And unless it meets with some other mass in the sky, it should be hurling itself in a mighty cataclysm against our earth—"
"Godd Lord," breathed Vignot. "When does all this take place?"
"That's the problem in arithmetic you so caustically referred to. We have its location in the sky. We have its speed—"
"Speed?" Vignot looked doubtful.
"That can be determined by examining the strength of the first disturbance signals on the cell plate recording tape. Each day they have grown stronger. By comparing this difference from day to day—"
