But net theory had never been able to determine what those critical moments were or how much slippage any given drop might produce. The parameter checks gave probabilities, but Gilchrist hadn't done any. Kivrin's drop might have been off by two weeks or a month. For all Gilchrist knew, she might have come through in April, in her fur-lined coat and winter kirtle.

But Badri had said minimal slippage. That meant Kivrin was off by no more than a few days, with plenty of time to find out the date and make the rendezvous.

"Mr. Gilchrist?" Mary was saying. "Can I get you a brandy?"

"No, thank you," he said.

Mary rummaged for another crumpled note and went over to the bar.

"Your tech seems to have done a passable job," Gilchrist said, turning to Dunworthy. "Mediaeval would like to arrange to borrow him for our next drop. We'll be sending Ms. Engle to 1355 to observe the effects of the Black Death. Contemporary accounts are completely unreliable, particularly in the area of mortality rates. The accepted figure of fifty million deaths is clearly inaccurate, and estimates that it killed one-third to one-half of Europe are obvious exaggerations. I'm eager to have Ms. Engle make trained observations."

"Aren't you being rather premature?" Dunworthy said. "Perhaps you should wait to see if Kivrin manages to survive this drop or at the very least gets through to 1320 safely."

Gilchrist's face took on its pinched look. "It strikes me as somewhat unjust that you constantly assume Mediaeval is incapable of carrying out a successful practicum," he said. "I assure you we have carefully thought out its every aspect. The method of Kivrin's arrival has been researched in every detail.

"Probability puts the frequency of travellers on the Oxford- Bath road as one every 1.6 hours, and it indicates a 92 per cent chance of her story of an assault being believed due to the frequency of such assaults. A wayfarer in Oxfordshire had a 42.5 per cent chance of being robbed in winter, 58.6 per cent in summer. That's an average, of course. The chances were greatly increased in parts of Otmoor and the Wychwood and on the smaller roads."

Dunworthy wondered how on earth Probability had arrived at those figures. The Domesday Book didn't list thieves, with the possible exception of the king's censustakers, who sometimes took more than the census, and the cutthroats of the time surely hadn't kept records of whom they had robbed and murdered, the locations marked neatly on a map. Proofs of deaths away from home had been entirely de facto: the person had failed to come back. And how many bodies had lain in the woods, undiscovered and unmarked by anyone?

"I assure you we have taken every precaution possible to protect Kivrin," Gilchrist said.

"Like parameter checks?" Dunworthy said. "And unmanneds and symmetry tests?"

Mary came back. "Here we are, Mr. Latimer," she said, putting a glass of brandy down in front of him. She hooked Latimer's wet umbrella over the back of the settle and sat down beside him.

"I was just assuring Mr. Dunworthy that every aspect of this drop was exhaustively researched," Gilchrist said. He picked up the plastic figurine of a wise man carrying a gilt box. "The brass-bound casket in her equipage is an exact reproduction of a jewel casket in the Ashmolean." He set the wise man down. "Even her name was painstakingly researched. Isabel is the woman's name listed most frequently in the Assize Rolls and the Regista Regum for 1295 through 1320.

"It is actually a corrupted form of Elizabeth," Latimer said, as if it were one of his lectures. "Its widespread use in England from the twelfth century is thought to trace its origin to Isavel of Angoulème, wife of King John."

"Kivrin told me she'd been given an actual identity, that Isabel de Beauvrier was one of the daughters of a Yorkshire nobleman," Dunworthy said.

"She was," Gilchrist said. "Gilbert de Beauvrier had four daughters in the appropriate age range, but their Christian names were not listed in the rolls. That was a common practice. Women were frequently listed only by surname and relationship, even in parish registers and on tombstones."

Mary put a restraining hand on Dunworthy's arm. "Why did you choose Yorkshire?" she asked quickly. "Won't that put her a long way from home?"

She's seven hundred years from home, Dunworthy thought, in a century that didn't value women enough to even list their names when they died.

"Ms. Engle was the one who suggested that," Gilchrist said. "She felt having the estate so distant would ensure that no attempt would be made to contact the family."

Or to cart her back to them, miles from the drop. Kivrin had suggested it. She had probably suggested the whole thing, searching through exchequer rolls and church registers for a family with a daughter the right age and no court connections, a family far enough up into the East Riding that the snow and the impassable roads would make it impossible for a messenger to ride and tell the family a missing daughter had been found.

"Mediaeval has given the same careful attention to every detail of this drop," Gilchrist said, "even to the pretext for her journey, her brother's illness. We were careful to ascertain that there had been an outbreak of influenza in that section of Gloucestershire in 1319, even though illness was abundant during the Middle Ages, and he could just as easily have contracted cholera or blood poisoning."

"James," Mary said warningly.

"Ms. Engle's costume was hand-sewn. The blue cloth for her dress was hand-dyed with woad using a mediaeval recipe. And Ms. Montoya has exhaustively researched the village of Skendgate where Kivrin will spend the two weeks."

"If she makes it there," Dunworthy said.

"James," Mary said.

"What precautions have you taken to ensure that the friendly traveller who happens along every 1.6 hours doesn't decide to cart her off to the convent at Godstow or a brothel in London or see her come through and decide she's a witch ? What precautions have you taken to ensure that the friendly traveller is in fact friendly and not one of the cutthroats who waylay 42.5 per cent of all passersby?"

"Probability indicated there was no more than a 0.04 per cent chance of someone being at the location at the time of the drop."

"Oh, look, here's Badri already," Mary said, standing up and putting herself between Dunworthy and Gilchrist. "That was quick work, Badri. Did you get the fix all right?"

Badri had come away without his coat. His lab uniform was wet and his face was pinched with cold. "You look half-frozen," Mary said. "Come and sit down." She motioned to the empty place on the settle next to Latimer. "I'll fetch you a brandy."

"Did you get the fix?" Dunworthy said.

He was not only wet, he was drenched. "Yes," he said, and his teeth started to chatter.

"Good man," Gilchrist said, standing up and clapping him on the shoulder. "I thought you said it would take an hour. This calls for a toast. Have you any champagne?" he called out to the barman, clapped Badri on the shoulder again, and went over to the bar.

Badri stood looking after him, rubbing his arms and shivering. He seemed inattentive, almost dazed.

"You definitely got the fix?" Dunworthy asked.

"Yes," he said, still looking after Gilchrist.

Mary came back to the table, carrying the brandy. "This should warm you up a bit," she said, handing it to him. "There. Drink it down. Doctor's orders."

He frowned at the glass as if he didn't know what it was. His teeth were still chattering.

"What is it?" Dunworthy said. "Kivrin's all right, isn't she?"

"Kivrin," he said, still staring at the glass, and then seemed suddenly to come to himself. He set the glass down. "I need you to come," he said, and started to push his way back through the tables to the door.