Groating nodded. "Our problem now is to sift the billions of factors stored in the Prog Building and discover which of them is that tiny snowball."

The controller, who had been silent in a state of wild suppression all the while, suddenly spoke up. "I tell you it's impossible, Mr. Groating. How can you dig the one significant factor out of all those billions?"

Groating said: "It will have to be done."

"But there's an easier way," the controller cried. "I've been suggesting it all along. Let's attempt the trial and error method. We instigate a series of changes at once and see whether or not the future line is shifted. Sooner or later we're bound to strike something."

"Impossible," Groating said. "You're suggesting the end of Stability. No civilization is worth saving if it must buy salvation at the price of its principles."

I said: "Sir, I'd like to make a suggestion."

They looked at me. The C-S nodded.

"It seems to me that you're both on the wrong track. You're searching for a factor from the present. You ought to start in the future."

"How's that?"

"It's like if I said old maids were responsible for more clover. You'd start investigating the old maids. You ought to start with the clover and work backwards."

"Just what are you trying to say, Mr. Carmichael?"

"I'm talking about a posteriori reasoning. Look, sir, a fella by the name of Darwin was trying to explain the balance of nature. He wanted to show the chain of cause and effect. He said in so many words that the number of old maids in a town governed the growth of clover, but if you want to find out how, you've got to work it out a posteriori; from effect to cause. Like this: Only bumblebees can fertilize clover. The more bumblebees, the more clover. Field mice attack bumblebee nests, so the more field mice, the less clover. Cats attack mice. The more cats, the more clover. Old maids keep cats. The more old maids ... the more clover. Q. E. D."

"And now," Groating laughed, "construe."

"Seems to me you ought to start with the catastrophe and follow the chain of causation, link by link, back to the source. Why not use the Prognosticator backwards until you locate the moment when the snowball first started rolling?"

There was a very long silence while they thought it over. The controller looked slightly bewildered and he kept muttering: Cats—clover—old maids—But I could see the C-S was really hit. He went to the window and stood looking out, as motionless as a statue. I remember staring past his square shoulder and watching the shadows of the helios flicking noiselessly across the façade of the Judiciary Building opposite us.

It was all so unreal—this frantic desperation over an event a thousand years in the future; but that's Stability. It's strictly the long view. Old Cyrus Brennerhaven of the Morning Globe had a sign over his desk that read: If you take care of the tomorrows, the todays will take care of themselves.

Finally Groating said: "Mr. Carmichael, I think we'd better go back to the Prog Building—"

Sure I felt proud. We left the office and went down the hall toward the pneumatics and I kept thinking: "I've given an idea to the Chief Stabilizer. He's taken a suggestion from me!" A couple of secretaries had rushed down the hall ahead of us when they saw us come out, and when we got to the tubes, three capsules were waiting for us. What's more, the C-S and the controller stood around and waited for me while I contacted my city editor and gave him the official release. The editor was a little sore about my disappearance, but I had a perfect alibi. I was still looking for Hogan. That, my friends, was emphatically that.

At the Prog Building we hustled through the main offices and back up the curved stairs. On the way the C-S said he didn't think we ought to tell Yarr, the little old coot I'd hood-winked, the real truth. It would be just as well, he said, to let Yarr go on thinking I was a confidential secretary.

So we came again to that fantastic clockwork room with its myriad whirling cams and the revolving crystal and the hypnotic bam-bam of the motors. Yarr met us at the door and escorted us to the viewing desk with his peculiar absent-minded subservience. The room was darkened again, and once more we watched the cloud of blackness seep across the face of the Universe. The sight chilled me more than ever, now that I knew what it meant.

Groating turned to me and said: "Well, Mr. Carmichael, any suggestions?"

I said: "The first thing we ought to find out is just what that spaceship has to do with the black cloud ... don't you think so?"

"Why yes, I do." Groating turned to Yarr and said: "Give us a close-up of the spaceship and switch in sound. Give us the integration at normal speed."

Yarr said: "It would take a week to run the whole thing off. Any special moment you want, sir?"

I had a hunch. "Give us the moment when the auxiliary ship arr............