He ignored her. Sarquist's identifying code-it had to be here somewhere.

"I tell you, we should get out of here!" she whined. "Shut up."

Mitch occasionally plucked a paper from the stack and laid it aside while the girl watched.

"What are those?" she asked.

"Messages he typed into the unit at various times." "What good are they?"

He showed her one of the slips of yellowed paper. It said: Unit 67-BJ is retired for repairs. A number was scrawled in one corner: 5.00326.

"So?"

"That number. It was his identifying code at the time." "You mean it's different every day?"

"More likely, it's different every minute. The code is probably based on an equation whose independent variable is time and whose dependent variable is the code number."

"How silly!"

"Not at all. It's just sort of a combination lock whose combination is continuously changing. All I've got to do is find the equation that describes the change. Then I can get to Central Coordinator."

She paced restlessly while he continued the search. Half an hour later he put his head in his hands and gazed despondently at the desk top. The key to the code was not there.

"What's the matter?" she asked.

"Sarquist. I figured he'd have to write it down somewhere. Evidently he memorized it. Or else his secretary did. I didn't figure a politician even had sense enough to substitute numbers in a simple equation."

The girl walked to the bookshelf and picked out a volume. She brought it to him silently. The title was Higher Mathematics for Engineers and Physicists.

"So I was wrong," he grunted. "Now what?"

He shuffled the slips of paper idly while he thought about it. "I've got eleven code numbers here, and the corresponding times when they were good. I might be able to find it empirically."

"I don't understand."

"Find an equation that gives the same eleven answers for the same eleven times, and use it to predict the code number for now."

"Will it work?"

He grinned. "There are an infinite number of equations that would give the same eleven answers for the same eleven substitutions. But it might work, if I assume that the code equation was of a simple form."

She paced restlessly while he worked at making a graph with time as the abscissa and the code numbers for ordinates. But the points were scattered across the page, and there was no connecting them with any simple sort of curve. "It almost has to be some kind of repeating function," he muttered, "something that Central could check by means of an irregular cam. The normal way for setting a code into a machine is to turn a cam by clock motor, and the height of the cam's rider is the code number for that instant."

He tried it on polar coordinates, hoping to get the shape of such a cam, but the resulting shape was too irregular to be possible, and he had no way of knowing the period of the repeating function.

"That's the craziest clock I ever saw," the girl murmured. "What?" He looked up quickly.

"That electric wall clock. Five minutes ahead of the electric clock in the living room. But when we first came it was twenty minutes ahead."

"It's stopped, maybe."

"Look at the second hand."

The red sweep was running. Mitch stared at it for a moment, then rose slowly to his feet and walked to her side. He took the small clock down from its hook and turned it over in his hands. Then he traced the cord to the wall outlet. The plug was held in place by a bracket so that it could not be removed.

The sweep hand moved slowly, it seemed. Silently he removed the screws from the case and stared inside at the works.

Then he grunted surprise. "First clock I ever saw with elliptical gears!^"

"What?"

"Look at these two gears in the train. Ellipses, mounted at the foci. That's the story. For a while the clock will run faster than the other one. Then it'll run slower." He handled it with growing excitement. "That's it, Marta-the key. Central must have another clock just like this one. The amount of lead or lag-in minutes-is probably the code!"

He moved quickly to the direct-contact unit. "Tell me the time on the other clock!"

She hurried into the living room and called back, "Ten-seventeen and forty seconds…forty-five…fifty-"

The other clock was leading by five and one-quarter minutes. He typed 5.250 on the keyboard. Nothing happened. "You sure that's right?" he called.

"It's now ten-eighteen-ten… fifteen…twenty."

The clock was still slowing down. He tried 5.230, but again nothing happened. The unit refused to respond. He arose with an angry grunt and began prowling around the library. "There's something else," he muttered. "There must be a modifying factor. That clock's too obvious anyway. But what else could they be measuring together except time?"

"Is that another clock on his desk?"

"No, it's a barometer. It doesn't-"

He paused to grin. "Could be! The barometric pressure difference from the mean could easily be mechanically added or subtracted from the reading of that wacky clock. Visualize this, inside of Central: The two clock motors mounted on the same shaft, with the distance between their indicator needles as the code number. Except that the distance is modified by having a barometer rigged up to shift one of the clocks one way or the other on its axis when the pressure varies. It's simple enough."

She shook her head. Mitch took the barometer with him to the unit. The dial was calibrated in atmospheres, and the pressure was now 1.03. Surely, he thought, for simplicity's sake, there would be no other factor involved in the code. This way, Sarquist could have glanced at his watch and the wall clock and the barometer and could have known the code number with only a little mental arithmetic. The wall time minus the wrist time plus the barometer's reading.

He called to the girl again, and the lag was now a little over four minutes. He typed again. There was a sharp click as the relays worked. The screen came alive, fluttered with momentary phosphorescence, then revealed the numbers in glowing type.

"We've got it!" he yelled to Marta.

She came to sit down on the rug. "I still don't see what we've got."

"Watch!" He began typing hurriedly, and the message flashed neatly upon the screen.

CENTRAL FROM SARQUIST. CLEAR YOUR TANKS OF ALL ORDINANCE DATA, EXCEPT ORDINANCES PERTAINING TO RECORDING OF INFORMATION IN YOUR TANKS. PREPARE TO RECORD NEW DATA.

He pressed the answer button and the screen went blank, but the reply was slow to come.

"It won't work!" Marta snorted. "It knows you aren't Sarquist. The subunits in the street have seen us."

"What do you mean by 'know,' and what do you mean by 'see'? Central isn't human."

"It knows and it sees."

He nodded. "Provided you mean those words in a mechanical sense. Provided you don't imply that she cares what she knows and sees, except where she's required to 'care' by enforced behavior patterns-ordinances."

Then the reply began crawling across the screen. SARQUIST FROM CENTRAL. INCONSISTENT INSTRUCTIONS. ORDINANCE 36-J, PERTAINING TO THE RECORDING OF INFORMATION, STATES THAT ORDINANCE DATA MAY NOT BE TOTALLY VOIDED BY YOU EXCEPT DURING RED ALERT AIR WARNING.

"See?" the girl hissed.

DEFINE THE LIMITS OF MY AUTHORITY IN PRESENT CONDITIONS, he typed. MAY I TEMPORARILY SUSPEND SPECIFIC ORDINANCES?

YOU MAY SUSPEND SPECIFIC ORDINANCES FOR CAUSE, BUT THE CAUSE MUST BE RECORDED WITH THE ORDER OF SUSPENSION.

Mitch put on a gloating grin. READ ME THE SERIES NUMBERS OF ALL LAWS IN CRIMINAL AND TRAFFIC CODES.

The reaction was immediate. Numbers began flashing on the screen in rapid sequence. "Write these down!" he called to the girl.

A few moments later, the flashing numbers paused. WAIT, EMERGENCY INTERRUPTION, said the screen.