you know. On paper. With print arranged in columns."
"Oh yes. I can't seem to separate literature and film somehow... Well, we have a first approximation of another sort now. We must look for a half-column advertisement which will, practically at a glance, give evidence that the man who placed it came from another Century (in the upwhen direction, of course) and yet which is so normal an advertisement that no man of that Century would see anything suspicious in it."
Harlan said, "What if I don't find it?"
"You will. Eternity exists, doesn't it. As long as it does, we're on the right track. Tell me, can you recall such an advertisement in your work with Cooper? Anything which struck you, even momentarily, as odd, queer, unusual, subtly wrong."
"No."
"I don't want an answer so quickly. Take five minutes and think."
"No point. At the time I was going over the news magazines with Cooper, he hadn't been in the 20th."
"Please, boy. Use your head. Sending Cooper to the 20th has introduced an alteration. There's no Change; it isn't an irreversible alteration. But there have been some changes with a small 'c,' or micro-changes, as it is usually referred to in Computation. At the instant Cooper was sent to the 20th, the advertisement appeared in the appropriate issue of the magazine. Your own Reality has micro-changed in the sense that you may have looked at the page with that advertisement on it rather than one without that advertisement as you did in the previous Reality. Do you understand?"
Harlan was again bewildered, almost as much at the ease with which Twissell picked his way through the jungle of temporal logic, as at the "paradoxes" of Time. He shook his head, "I remember nothing of the sort."
"Well, then, where do you keep the files of that periodical?"
"I had a special library built on Level Two, using the Cooper priority."
"Good enough," said Twissell. "Let's go there. Now!"
Harlan watched Twissell stare curiously at the old, bound volumes in the library and then take one down. They were so old that the fragile paper had to be preserved by special methods and they creaked under Twissell's insufficiently gentle handling.
Harlan winced. In better times he would have ordered Twissell away from the books, Senior Computer though he was.
The old man peered through the crinkling pages and silently mouthed the archaic words. "This is the English the linguists are always talking about, isn't it?" he asked, tapping a page.
"Yes. English," muttered Harlan.
Twissell put the volume back. "Heavy and clumsy."
Harlan shrugged. To be sure, most of the Centuries of Eternity were film eras. A respectable minority were molecular-recording eras. Still, print and paper were not unheard of.
He said, "Books don't require the investment in technology that films do."
Twissell rubbed his chin. "Quite. Shall we get started?"
He took another volume down from the shelf, opening it at random and staring at the page with odd intentness.
Harlan thought: Does the man think he's going to hit the solution by a lucky stab?
The thought might have been correct, for Twissell, meeting Harlan's appraising eyes, reddened and put the book back.
Harlan took the first volume of the 19.25th Centicentury and began turning the pages regularly. Only his right hand and his eyes moved. The rest of his body remained at rigid attention.
At what seemed aeonic intervals to himself Harlan rose, grunting, for a new volume. On those occasions there would be the coffee break or the sandwich break or the other breaks.
Harlan said heavily, "It's useless your staying."
Twissell said, "Do I bother you?"
"No."
"Then I'll stay," muttered Twissell. Throughout he wandered occasionally to the bookshelves, staring helplessly at the bindings. The sparks of his furious cigarettes burned his finger ends at times, but he disregarded them.
A physioday ended.
Sleep was poor and sparse. Midmorning, between two volumes, Twissell lingered over his last sip of coffee and said, "I wonder sometimes why I didn't throw up my Computership after the matter of my-- You know."
Harlan nodded.
"I felt like it," the old man went on. "I felt like it. For physiomonths, I hoped desperately that no Changes would come my way. I got morbid about it. I began to wonder if Changes were right. Funny, the tricks emotions will play on you.
"You know Primitive history, Harlan. You know what it was like. Its Reality flowed blindly along the line of maximum probability. If that maximum probability involved a pandemic, or ten Centuries of slave economy, a breakdown of technology, or even a-a-let's see, what's really bad-even an atomic war if