"And what about the transit room? What's going on there?"

"I'll show you."

The rubber floor of the transit site had been cleared of debris and cleaned. In the places where acid had eaten through the rubber, the flooring was being replaced by workmen on their hands and knees. Two of the glass shields were in place, and one was being inspected closely by a man wearing thick goggles and carrying an odd hooded light. But Stern was looking upward as the next big glass panels were swung in on overhead cranes from the second transit site, still being built.

"It's lucky we had that other transit site under construction," Gordon said to him. "Otherwise, it'd take us a week to get these glass panels down here. But panels were already here. All we have to do is move them over. Very lucky."

Stern still stared upward. He hadn't realized how large the shielding panels were. Suspended above him, the curved glass panels were easily ten feet high and fifteen feet wide, and almost two feet deep. They were carried in padded slings toward special mounting brackets in the floor below. "But," Gordon said, "we have no spares. We just have one full set."

"So?"

Gordon walked over to one of the glass panels, already standing in place. "Basically, you can think of these things as big glass hip flasks," Gordon said. "They're curved containers that fill from a hole at the top. And once we fill them with water, they're very heavy. About five tons each. The curve actually improves the strength. But it's the strength I'm worried about."

"Why?" Stern said.

"Come closer." Gordon ran his fingers over the surface of the glass. "See these little pits? These little grayish spots? They're small, so you'd never notice them unless you looked carefully. But they're flaws that weren't there before. I think the explosion blew tiny drops of hydrofluoric acid into the other room."

"And now the glass has been etched."

"Yes. Slightly. But if these pits have weakened the glass, then the shields may crack when they are filled with water and the glass is put under pressure. Or worse, the entire glass shield may shatter."

"And if it does?"

"Then we won't have full shielding around the site," Gordon said, looking directly at Stern. "In which case, we can't safely bring your friends back. They'd risk too many transcription errors."

Stern frowned. "Do you have a way to test the panels? See if they'll hold up?"

"Not really, no. We could stress-test one, if we were willing to risk breaking it, but since we have no spare panels, I won't do that. Instead, I'm doing a microscopic polarization visual inspect." He pointed to the technician in the corner, wearing goggles, going over the glass. "That test can pick up preexisting stress lines  -  which always exist in glass  -  and give us a rough idea of whether they'll break. And he's got a digital camera that is feeding the data points directly into the computer."

"You going to do a computer simulation?" Stern said.

"It'll be very crude," Gordon said. "Probably not worth doing, it's so crude. But I'll do it anyway."

"So what's the decision?"

"When to fill the panels."

"I don't understand."

"If we fill them now, and they hold up, then everything is probably fine. But you can't be sure. Because one of the tanks may have a weakness that will break only after a period of pressure. So that's an argument to fill all the tanks at the last minute."

"How fast can you fill them?"

"Pretty fast. We have a fire hose down here. But to minimize stress, you probably want to fill them slowly. In which case, it would take almost two hours to fill all nine shields."

"But don't you get field bucks starting two hours before?"

"Yes  -  if the control room is working right. But the control room equipment has been shut down for ten hours. Acid fumes have gotten up there. It may have affected the electronics. We don't know if it is working properly or not."

"I understand now," Stern said. "And each of the tanks is different."

"Right. Each one is different."

It was, Stern thought, a classic real-world scientific problem. Weighing risks, weighing uncertainties. Most people never understood that the majority of scientific problems took this form. Acid rain, global warming, environmental cleanup, cancer risks  -  these complex questions were always a balancing act, a judgment call. How good was the research data? How trustworthy were the scientists who had done the work? How reliable was the computer simulation? How significant were the future projections? These questions arose again and again. Certainly the media never bothered with the complexities, since they made bad headlines. As a result, people thought science was cut and dried, in a way that it never was. Even the most established concepts  -  like the idea that germs cause disease  -  were not as thoroughly proven as people believed.

And in this particular instance, a case directly involving the safety of his friends, Stern was faced with layers of uncertainty. It was uncertain whether the tanks were safe. It was uncertain whether the control room would give adequate warning. It was uncertain whether they should fill the tanks slowly now, or quickly later. They were going to have to make a judgment call. And lives depended on that call.

Gordon was staring at him. Waiting.

"Are any of the tanks unpitted?" Stern said.

"Yes. Four."

"Then let's fill those tanks now," Stern said. "And wait for the polarization analysis and the computer sim before filling the others."

Gordon nodded slowly. "Exactly what I think," he said.

Stern said, "What's your best guess? Are the other tanks okay, or not?"

"My best guess," Gordon said, "is that they are. But we'll know more in a couple of hours."

06:40:22

"Good Sir Andre, I pray you come this way," Guy de Malegant said with a gracious bow and a wave of his hand.

Marek tried to conceal his astonishment. When he had galloped into La Roque, he fully expected that Guy and his men would kill him at once. Instead, they were treating him deferentially, almost as an honored guest. He was now deep in the castle, in the innermost court, where he saw the great hall, already lit inside.

Malegant led him past the great hall and into a peculiar stone structure to the right. This building had windows fitted not only with wooden shutters but with windowpanes made of translucent pig bladders. There were candles in the windows, but they were outside the pig bladders, instead of inside the room itself.

He knew why even before he stepped into the building, which consisted of a single large room. Against the walls, gray fist-size cloth sacks stood heaped high on raised wooden platforms above the floor. In one corner, iron shot was piled in dark pyramids. The room had a distinctive smell  -  a sharp, dry odor  -  and Marek knew exactly where he was.

The arsenal.

Malegant said, "Well, Magister, we found one assistant to help you."

"I thank you for that." In the center of the room, Professor Edward Johnston sat cross-legged on the floor. Two stone basins containing mixtures of powder were set to one side. He held a third basin between his knees, and with a stone mortar, he was grinding a gray powder with a steady, circular motion. Johnston did not stop when he saw Marek. He did not register surprise at all.

"Hello, Andre," he said.

"Hello, Professor."

Still grinding: "You all right?"

"Yes, I'm okay. Hurt my leg a little." In fact, Marek's leg was throbbing, but the wound was clean; the river had washed it thoroughly, and he expected it to heal in a few days.

The Professor continued to grind, patiently, ceaselessly. "That's good, Andre," he said in the same calm voice. "Where are the others?"

"I don't know about Chris," Marek said. He was thinking of how Chris had been covered with blood. "But Kate is okay, and she is going to find the - "

"That's fine," the Professor said quietly, his eyes flicking up to Sir Guy. Changing the subject, he nodded to the bowl. "You know what I'm doing, of course?"

"Incorporating," Marek said. "Is the stuff any good?"

"It's not bad, all things considered. It's willow charcoal, which is ideal. The sulfur's fairly pure, and the nitrate's organic."

"Guano?"

"That's right."

"So, it's about what you'd expect," Marek said. One of the first things Marek had studied was the technology of gunpowder, a substance that first became widely employed in Europe in the fourteenth century. Gunpowder was one of those inventions, like the mill wheel or the automobile, that could not be identified with any particular person or place. The original recipe  -  one part charcoal, one part sulfur, six parts saltpeter  -  had come from China. But the details of how it had arrived in Europe were in dispute, as were the earliest uses of gunpowder, when it was employed less as an explosive than as an incendiary. Gunpowder was originally used in weapons when firearms meant "arms that make use of fire," and not the modern meaning of explosive projectile devices such as rifles and cannon.

This was because the earliest gunpowders were not very explosive, because the chemistry of the powder was not understood, and because the art hadn't been developed yet. Gunpowder exploded when charcoal and sulfur burned extremely rapidly, the combustion enabled by a rich source of oxygen  -  namely nitrate salts, later called saltpeter. The most common source of nitrates was bat droppings from caves. In the early years, this guano was not refined at all, simply added to the mixture.

But the great discovery of the fourteenth century was that gunpowder exploded better when it was ground extremely fine. This process was called "incorporation," and if properly done, it yielded gunpowder with the consistency of talcum powder. What happened during the endless hours of grinding was that small particles of saltpeter and sulfur were forced into microscopic pores in the charcoal. That was why certain woods, like willow, were preferred; their charcoal was more porous.

Marek said, "I don't see a sieve. Are you going to corn it?"

"No." Johnston smiled. "Corning's not discovered yet, remember?"

Corning was the process of adding water to the gunpowder mixture, making a paste that was then dried. Corned powder was much more powerful than dry-mixed powder. Chemically, what happened was that the water partially dissolved the saltpeter, allowing it to coat the inside of the charcoal micropores, and in the process, it carried the insoluble sulfur particles inside, too. The resulting powder was not only more powerful but also more stable and long-lasting. But Johnston was right; corning was only discovered around 1400  -  roughly forty years from now.

"Should I take over?" Marek said. Incorporating was a lengthy process; sometimes the grinding went on for six or eight hours.

"No. I'm finished now." The Professor got to his feet, then said to Sir Guy, "Tell my Lord Oliver that we are ready for his demonstration."

"Of Greek Fire?"

"Not precisely," Johnston said.

In the late afternoon sun, Lord Oliver paced impatiently along the massive wall of the outer perimeter. The battlement was more than fifteen feet wide here, dwarfing the row of cannon nearby. Sir Guy was with him, as well as a sullen Robert de Kere; they all looked up expectantly when they saw the Professor. "Well? Are you at last prepared, Magister?"

"My Lord, I am," the Professor said, walking with two of his bowls, one under each arm. Marek carried a third bowl, in which the fine gray powder had been mixed with a thick oil that smelled strongly of resin. Johnston had told him not to touch this mixture on any account, and he needed no reminding. It was a disagreeable, reeking goo. He also carried a bowl of sand.

"Greek Fire? Is it Greek Fire?"

"No, my Lord. Better. The fire of Athenaios of Naukratis, which is called 'automatic fire.' "

"Is that so?" Lord Oliver said. His eyes narrowed. "Show me."

Beyond the cannon was the broad eastern plain, where the trebuchets were being assembled in a line. They were just out of shot range, two hundred yards away. Johnston set his bowls on the ground between the first two cannon. The first cannon he loaded with a sack from the armory. He then placed a thick metal arrow with metal vanes into the cannon. "This is your powder, and your arrow."

Turning to the second cannon, he carefully poured his finely ground gunpowder into a sack, which he stuffed into the cannon mouth. Then he said, "Andre, the sand, please." Marek came forward and set the basin of sand at the Professor's feet.

"What is that sand for?" Oliver asked.

"A precaution, my Lord, against error." Johnston picked up a second metal arrow, handling it gingerly, holding it only at each end and gently inserting it into the cannon. The tip of the arrow was grooved, the grooves filled with thick brown acrid paste.

"This is my powder, and my arrow."

The gunner handed the Professor a thin stick of wood, glowing red at one end. Johnston touched the first cannon.

There was a modest explosion: a puff of black smoke, and the arrow flew onto the field, landing a hundred yards short of the nearest trebuchet.

"Now my powder, and my arrow."

The Professor touched the second cannon.

There was a loud explosion and a blast of dense smoke. The arrow landed alongside a trebuchet, missing it by ten feet. It lay in the grass.

Oliver snorted. "Is that all? You will forgive me if I have - "

Just then, the arrow burst into a circle of fire, spitting blobs of flame in all directions. The trebuchet immediately caught fire, and men on the field ran forward, carrying the horses' water bags to put it out.

"I see . . . ," Lord Oliver said.

But water seemed to spread the fire, not quench it. With each new dousing, the flames leapt higher. The men stepped back, confused. In the end, they watched helplessly as the trebuchet burned before them. In a few moments, it was a mass of charred, smoking timbers.

"By God, Edward and Saint George," Oliver said.

Johnston gave a small bow, smiled.

"You have twice the range and an arrow that alights itself  -  how?"

"The powder is ground fine and so explodes more fiercely. The arrows are filled with oil, sulfur and quicklime, mixed with tow. Touching any water makes them catch fire  -  here it's the dampness of the grass. That is why I have a basin of sand, should the slightest bit of the mixture be upon my fingers and start to burn from the moisture of my hands. It is a most delicate weapon, my Lord, and delicate to handle."

He turned to the third basin, near Marek.

"Now, my Lord," Johnston said, picking up a wooden stick, "I pray you observe what follows." He dipped the stick into the third bowl, coating the tip with the oily, foul-smelling mixture. He held the stick in the air. "As you see, there is no change. And there shall be no change for hours, or days, until . . ." With the theatricality of a magician, he splashed the stick with a small cup of water.

The stick made a hissing sound, began to smoke, and then burst into flames as the Professor held it. The flame was a hot-orange color.

"Ah," Oliver said, sighing with pleasure. "I must have a quantity of this. How many men do you require to grind and make your substance?"

"My Lord, twenty will do. Fifty is better."

"You shall have fifty, or more as you will," Oliver said, rubbing his hands. "How quickly can you make it?"

"The preparation is not lengthy, my Lord," Johnston said, "but it cannot be done in haste, for it is dangerous work. And once made, the substance is a hazard within your castle, for Arnaut is certain to attack you with flaming devices."

Oliver snorted. "I care nothing for that, Magister. Make it now, and I shall put it to use this very night."

Back in the arsenal, Marek watched as Johnston arranged the soldiers in rows of ten, with a grinding bowl in front of each man. Johnston walked down the rows, pausing now and again to give instructions. The soldiers were grumbling about what they called "kitchen work," but Johnston told them that these were, in his words, the herbs of war.

It was several minutes later when the Professor came over to sit in the corner with him. Watching the soldiers work, Marek said, "Did Doniger give you that speech, about how we can't change history?"

"Yes. Why?"

"It seems like we're giving Oliver a lot of help to defend his castle against Arnaut. Those arrows are going to force Arnaut to push his siege engines back  -  too far back to be effective. No siege engines, no assault on the fortress. And Arnaut won't play a waiting game. His men want quick scores  -  all the free companies do. If they can't take a castle right away, they move on."

"Yes, that's true. . . ."

"But according to history, this castle falls to Arnaut."

"Yes," Johnston said. "But not because of a siege. Because a traitor lets Arnaut's men in."

"I've been thinking about that, too," Marek said. "It doesn't make sense. There are too many gates in this castle to open. How could a traitor possibly do it? I don't think he could."

Johnston smiled. "You think we might be helping Oliver keep his castle, and so we're changing history."

"Well. I'm just wondering."

Marek was thinking that whether or not a castle fell was actually a very significant event, in terms of the future. The history of the Hundred Years War could be seen as a series of key sieges and captures. For instance, a few years from now, brigands would capture the town of Moins, at the mouth of the Seine. In itself, a minor conquest  -  but it would give them control of the Seine, allowing them to capture castles all the way back to Paris itself. Then there was the matter of who lived and who died. Because more often than not, when a castle fell, its inhabitants were massacred. There were several hundred people inside La Roque. If they all survived, their thousands of descendants could easily make a different future.

"We may never know," Johnston said. "How many hours have we got left?"

Marek looked at his bracelet. The counter said 05:50:29. He bit his lip. He had forgotten that the clock was ticking. When he had last looked, there were almost nine hours; there had seemed to be plenty of time. Six hours didn't sound quite so good.

"Not quite six hours," Marek said.

"And Kate has the marker?"

"Yes."

"And where is she?"

"She went to find the passage." Marek was thinking that it was now late afternoon; if she found the passage, she could easily make her way inside the castle in two or three hours.

"Where did she go to find the passage?"

"The green chapel."

Johnston sighed. "Is that where Marcel's key said that it was?"

"Yes."

"And she went alone?"

"Yes."

Johnston shook his head. "No one goes there."

"Why?"

"Supposedly, the green chapel is guarded by an insane knight. They say his true love died there and that he lost his mind with grief. He's imprisoned his wife's sister in a nearby castle, and now he kills anybody who comes near the castle, or the chapel."

"Do you think all that's true?" Marek said.

Johnston shrugged. "No one knows," he said. "Because no one has ever come back alive."

05:19:55

Her eyes squeezed tightly shut, Kate waited for the ax to fall. The knight above her was snorting and grunting, his breath coming faster, more and more excited before he delivered the killing blow -

Then he was silent.

She felt the foot in the middle of her back twist.

He was looking around.

The ax thunked down on the block, inches from her face. But he was resting it, leaning on it while he looked at something behind him. He started grunting again, and now he sounded angry.

Kate tried to see what he was looking at, but the flat blade of the ax blocked her view.

She heard footsteps behind her.

There was someone else here.

The ax was raised again, but now the foot came off her back. Hastily, she rolled off the block and turned to see Chris standing a few yards away, holding the sword that she had dropped.

"Chris!"

Chris smiled through clenched teeth. She could see he was terrified. He kept his eyes on the green knight. With a growl, the knight spun, his ax hissing as he swung it. Chris held up his sword to parry. Sparks flew from clanging metal. The men circled each other. The knight swung again, and Chris ducked, stumbled backward, and got hastily to his feet again as the ax thunked into the grass. Kate fumbled in her pouch and found the gas cylinder. This foreign object from another time seemed absurdly small and light now, but it was all they had.

"Chris!"

Standing behind the green knight, she held up the cylinder, so he could see it. He nodded vaguely, continuing to dodge and back away. She saw he was tiring fast, losing ground, the green knight advancing on him.

Kate had no choice: she ran forward, leapt into the air, and landed on the green knight's back. He grunted in surprise at the weight. She clung to him, brought the canister around to the front of his helmet, and fired gas through the slit. The knight coughed and shivered. She squeezed again, and the knight began to stagger. She dropped back to the ground.

She said, "Do it!"

Chris was on one knee, gasping. The green knight was still on his feet, but weaving. Chris came slowly forward and stabbed the sword into the knight's side, between the armor plates. He gave a roar of fury and fell onto his back.

Chris was on him immediately, cutting the laces of his helmet, kicking it away with his foot. She glimpsed tangled hair, matted beard, and wild eyes as he swung the sword down, and severed the knight's head.

It didn't work.

The blade came down, crunched into bone, and stuck there, only partway through his neck. The knight was still alive, looking at Chris in fury, his mouth moving.

Chris tried to pull the sword out, but it was caught in the knight's throat. As he struggled, the knight's left hand came up and grabbed his shoulder. The knight was immensely strong  -  demonically strong  -  and pulled him down until his face was inches away. His eyes were bloodshot. His teeth were cracked and rotten. Lice crawled in his beard, among bits of discolored food. He stank of decay.

Chris was revolted. He felt his hot, reeking breath. Struggling, he managed to put his foot on the knight's face, and he stood up, forcing himself free of the grip. The sword came free in the same moment, and he lifted it to swing down.

But the knight's eyes rolled upward and his jaw went slack. He was already dead. Flies began to buzz over his face.

Chris collapsed, sitting on the wet ground, trying to catch his breath. Revulsion swept over him like a wave, and he started to shiver uncontrollably. He hugged himself, trying to stop it. His teeth were chattering.

Kate put her hand on his shoulder. She said, "My hero." He hardly heard her. He didn't say anything. But eventually he stopped shivering and got to his feet again.

"I was glad to see you," she said.

He nodded and smiled. "I took the easy way down."

Chris had managed to stop his slide in the mud. He had spent many difficult minutes working his way back up the slope, and then he took the other path down. It turned out to be an easy walk to the base of the waterfall, where he found Kate about to be beheaded.

"You know the rest," he said. He got to his feet, leaned on the sword. He looked up at the sky. It was starting to get dark. "How much time do you think is left?"

"I don't know. Four, five hours."

"Then we better get started."

The ceiling of the green chapel had fallen in at several places, and the interior was in ruins. There was a small altar, Gothic frames around broken windows, pools of stagnant water on the floor. It was hard to see that this chapel had once been a jewel, its doorways and arches elaborately carved. Now slimy mold dripped from the carvings, which were eroded beyond recognition.

A black snake slithered away as Chris went down spiral stairs to the crypts below ground. Kate followed more slowly. Here it was darker, the only light coming from cracks in the floor above. There was the constant sound of dripping water. In the center of the room they saw a single intact sarcophagus, carved of black stone, and the broken fragments of several others. The intact sarcophagus had a knight in armor carved on the lid. Kate peered at the knight's face, but the stone had been eroded by the omnipresent mold, and the features were gone.

"What was the key again?" Chris said. "Something about the giant's feet?"

"That's right, so many paces from the giant's feet. Or gigantic feet."

"From the giant's feet," Chris repeated. He pointed to the sarcophagus, where the feet of the carved knight were two rounded stumps. "Do you suppose it means these feet?"

Kate frowned. "Not exactly giant."

"No. . . ."

"Let's try it," she said. She stood at the foot of the sarcophagus, turned right, and went five paces. Then she turned left, and went four paces. She turned right again, and took three paces before she came up against the wall.

"Guess not," Chris said.

They both turned away and began to search in earnest. Almost immediately, Kate made an encouraging discovery: half a dozen torches, stacked in a corner, where they would stay dry. The torches were crudely made, but serviceable enough.

"The passage has to be here somewhere," she said. "It has to be."

Chris didn't answer. They searched in silence for the next half hour, wiping mold off the walls and floor, looking at the corroded carvings, trying to decide if one or another might represent a giant's feet.

Finally, Chris said, "Did the thing say the feet were inside the chapel, or at the chapel?"

"I don't know," Kate said. "Andre read it to me. He translated the text."

"Because maybe we should be looking outside."

"The torches were in here."

"True."

Chris turned, frustrated, looking.

"If Marcel made a key that took off from a landmark," Kate said, "he wouldn't use a coffin or sarcophagus, because that could be moved. He would use something fixed. Something on the walls."

"Or the floor."

"Yes, or the floor."

She was standing by the far wall, which had a little niche cut into the stone. At first she thought these were little altars, but they were too small, and she saw bits of wax; evidently, they had been made to hold a candle. She saw several of these candle niches in the walls of the crypt. The inner surfaces of this niche were beautifully carved, she noticed, with a symmetrical design of bird's wings going up each side. And the carving had not been touched, perhaps because the heat of the candles had suppressed the growth of mold.

She thought, Symmetrical.

Excited, she went quickly to the next candle niche. The carvings depicted two leafy vines. The next niche: two hands clasped in prayer. She went all around the room in this way, checking each one.

None had feet.

Chris was sweeping his toe in big arcs across the floor, scraping away the mold from the underlying stone. He was muttering, "Big feet, big feet."

She looked over at Chris and said, "I feel really stupid."

"Why?"

She pointed to the doorway behind him  -  the doorway that they had passed through when they first came down the stairs. The doorway that had once been elaborately carved but was now eroded.

It was possible to see, even now, what the original design of the carving had been. On both left and right, the doorway had been carved into a series of lumps. Five lumps, with the largest at the top of the door and the smallest at the bottom. The large lump had a sort of flat indentation on its surface, leaving no doubt what all the lumps were meant to represent.

Five toes, on either side of the door.

"Oh my God," Chris said. "It's the whole damned door."

She nodded. "Giant feet."

"Why would they do that?"

She shrugged. "Sometimes they put hideous and demonic images at entrances and exits. To symbolize the flight or banishment of evil spirits."

They went quickly to the door, and then Kate paced off five steps, then four, then nine. She was now facing a rusty iron ring mounted on the wall. They were both excited by this discovery, but when they tugged at it, the ring broke loose in their hands, crumbling in red fragments.

"We must have done something wrong."

"Pace it again."

She went back and tried smaller steps. Right, left, right again. She was now facing a different section of wall. But it was just wall, featureless stone. She sighed.

"I don't know, Chris," she said. "We must be doing something wrong. But I don't know what." Discouraged, she put her hand out, leaned against the wall.

"Maybe the paces are still too large," Chris said.

"Or too small."

Chris went over, stood next to her by the wall. "Come on, we'll figure it out."

"Do you think?"

"Yeah, I do."

They stepped away from the wall and had started back to the doorway when they heard a low rumbling sound behind them. A large stone in the floor, right where they had been standing, had now slid away. They saw stone steps leading downward. They heard the distant rush of a river. The opening gaped black and ominous.

"Bingo," he said.

03:10:12

In the windowless control room above the transit pad, Gordon and Stern stared at the monitor screen. It showed an image of five panels, representing the five glass containers that had been etched. As they watched, small white dots appeared on the panels.

"That's the position of the etch points," Gordon said.

Each point was accompanied by a cluster of numbers, but they were too small to read.

"That's the size and depth of each etching," Gordon said.

Stern said nothing. The simulation continued. The panels began to fill with water, represented by a rising horizontal blue line. Superimposed on each panel were two large numbers: the total weight of the water and the pressure per square inch on the glass surface, at the bottom of each panel, where the pressure was greatest.

Even though the simulation was highly stylized, Stern found himself holding his breath. The waterline went higher, and higher.

One tank began to leak: a flashing red spot.

"One leaking," Gordon said.

A second tank began to leak, and as the water continued to rise, a jagged lightning streak crossed the panel, and it vanished from the screen.

"One shattered."

Stern was shaking his head. "How rough do you think this simulation is?"

"Pretty fast and dirty."

On the screen, a second tank shattered. The final two filled to the top without incident.

"So," Gordon said. "The computer's telling us three out of five panels can't be filled."

"If you believe it. Do you?"

"Personally, I don't," Gordon said. "The input data's just not good enough, and the computer is making all kinds of stress assumptions that are pretty hypothetical. But I think we better fill those tanks at the last minute."

Stern said, "It's too bad there isn't a way to strengthen the tanks."

Gordon looked up quickly. "Like what?" he said. "You have an idea?"

"I don't know. Maybe we could fill the etchings with plastic, or some kind of putty. Or maybe we could - "

Gordon was shaking his head. "Whatever you do, it has to be uniform. You'd have to cover the entire surface of the tank evenly. Perfectly evenly."

"I can't see any way to do that," Stern said.

"Not in three hours," Gordon said. "And that's what we have left."

Stern sat down in a chair, frowning. For some reason, he was thinking of racing cars. A succession of images flashed through his mind. Ferraris. Steve McQueen. Formula One. The Michelin Man with his rubber tube body. The yellow Shell sign. Big truck tires, hissing in rain. B. F. Goodrich.

He thought, I don't even like cars. Back in New Haven, he owned an ancient VW Bug. Clearly, his racing mind was trying to avoid an unpleasant reality  -  something he didn't want to face up to.

The risk.

"So we just fill the panels at the last minute, and pray?" Stern said.

"Exactly," Gordon said. "That's exactly what we do. It's a little hairy. But I think it'll work."

"And the alternative?" Stern said.

Gordon shook his head. "Block their return. Don't let your friends come back. Get brand new glass panels down here, panels that don't have imperfections, and set up again."

"And that takes how long?"

"Two weeks."

"No," Stern said. "We can't do that. We have to go for it."

"That's right," Gordon said. "We do."

02:55:14

Marek and Johnston climbed the circular stairs. At the top, they met de Kere, who was looking smugly satisfied. They stood once more on the wide battlements of La Roque. Oliver was there, pacing, red-faced and angry.

"Do you smell it?" he cried, pointing off toward the field, where Arnaut's troops continued to mass.

It was now early evening; the sun was down, and Marek guessed it must be about six o'clock. But in the fading light, they saw that Arnaut's forces now had a full dozen trebuchets assembled and set out in staggered rows on the field below. After the example of the first incendiary arrow, they had moved their engines farther apart, so that any fire would not spread beyond one engine.

Beyond the trebuchets, there was a staging area, with troops huddled around smoking fires. And at the very rear, the hundreds of tents of the soldiers nestled back against the dark line of the forest.

It looked, Marek thought, perfectly ordinary. The start of a siege. He couldn't imagine what Oliver was upset about.

A distinct burning odor drifted toward them from the smoking fires. It reminded Marek of the smell that roofers made. And with good reason: it was the same substance. "I do, my Lord," Johnston said. "It is pitch."

Johnston's blank expression conveyed that he, too, did not know why Oliver was so upset. It was standard practice in siege warfare to lob burning pitch over the castle walls.

"Yes, yes," Oliver said, "it is pitch. Of course it is pitch. But that is not all. Do you not smell it? They are mixing something with the pitch."

Marek sniffed the air, thinking Oliver was almost certainly right. When burning, pure pitch had a tendency to go out. Thus pitch was usually combined with other substances  -  oil, tow or sulfur  -  to make a more robustly burning mixture.

"Yes, my Lord," Johnston said. "I smell it."

"And what is it?" Oliver said in an accusing tone.

"Ceraunia, I believe."

"Also called the 'thunderbolt stone'?"

"Yes, my Lord."

"And do we also employ this thunderbolt stone?"

"No, my Lord - " Johnston began.

"Ah! I thought as much."

Oliver was now nodding to de Kere, as if their suspicions were confirmed. Clearly, de Kere was behind all this.

"My Lord," Johnston said, "we have no need of the thunderbolt stone. We have better stone. We use pure sulfure."

"But sulfure is not the same." Another glance at de Kere.

"My Lord, it is. The thunderbolt stone is pyrite kerdonienne. When ground fine, it makes sulfure."

Oliver snorted. He paced. He glowered.

"And how," he said finally, "does Arnaut come to have this thunderbolt stone?"

"I cannot say," Johnston said, "but the thunderbolt stone is well known to soldiers. It is even mentioned in Pliny."

"You evade me with tricks, Magister. I speak not of Pliny. I speak of Arnaut. The man is an illiterate pig. He knows nothing of ceraunia, or the thunderbolt stone."

"My Lord - "

"Unless he is aided," Oliver said darkly. "Where are your assistants now?"

"My assistants?"

"Come, come, Magister, evade me no further."

"One is here," Johnston said, gesturing to Marek. "I am given that the second is dead, and I have no word of the third."

"And I believe," Oliver said, "that you know very well where they are. They are both working in the camp of Arnaut, even as we speak. That is how he comes to possess this arcane stone."

Marek listened to this with a growing sense of unease. Oliver had never seemed mentally stable, even in better times. Now, faced with impending attack, he was becoming openly paranoid  -  goaded by de Kere. Oliver seemed unpredictable, and dangerous.

"My Lord - " Johnston began.

"And further, I believe what I suspected from the first! You are the creature of Arnaut, for you have passed three days in Sainte-M��re, and the Abbot is the creature of Arnaut."

"My Lord, if you will hear - "

"I will not! You shall hear. I believe you work against me, that you, or your assistants, know the secret entrance to my castle, despite all your protestations, and that you plan to escape at the earliest moment  -  perhaps even tonight, under cover of Arnaut's attack."

Marek was carefully expressionless. That was, of course, exactly what they intended, if Kate ever found the entrance to the passage.

"Aha!" Oliver said, pointing at Marek. "You see? His jaw clenches. He knows what I say is true."

Marek started to speak, but Johnston put a restraining hand on his arm. The Professor said nothing himself, just shook his head.

"What? Will you stop his confession?"

"No, my Lord, for your surmises are not true."

Oliver glowered, paced. "Then bring me the weapons I bade you make earlier."

"My Lord, they are not ready."

"Ha!" Another nod to de Kere.

"My Lord, the grinding of the powder takes many hours."

"In many hours, it will be too late."

"My Lord, it will be in good time."

"You lie, you lie, you lie!" Oliver spun, stamped his foot, stared off at the siege engines. "Look to the plain. See how they make ready. Now answer me, Magister. Where is he?"

There was a pause. "Where is who, my Lord?"

"Arnaut! Where is Arnaut? His troops mass for attack. He always leads them. But now he is not there. Where is he?"

"My Lord, I cannot say. . . ."

"The witch of Eltham is there  -  see her, standing by the engines? You see? She watches us. The damnable woman."

Marek turned quickly to look. Claire was indeed down among the soldiers, walking with Sir Daniel at her side. Marek felt his heart beat faster, just to see her, though he was not sure why she would walk so near the siege lines. She was looking up at the walls. And suddenly she stopped abruptly. And he thought, with a kind of certainty, that she had seen him. He had an almost irresistible impulse to wave, but of course he did not. Not with Oliver snorting and puffing beside him. But he thought, I'm going to miss her when I go back.

"The Lady Claire," Oliver growled, "is a spy of Arnaut and was so from the beginning. She let his men into Castelgard. All arranged, no doubt, with that scheming Abbot. But where is the villain himself? Where is the pig Arnaut? Nowhere to be seen."

There was an awkward silence. Oliver smiled grimly.

"My Lord," Johnston began, "I understand your concer - "

"You do not!" He stamped his foot and glared at them. Then, "Both of you. Come with me."

The surface of the water was black and oily, and even looking down from thirty feet above, it stank. They were standing beside a circular pit, located deep in the bowels of the castle. All around them, the walls were dark and damp, barely illuminated by flickering torches.

At Oliver's signal, a soldier beside the pit started to crank an iron winch. Clattering, a thick chain began to rise from the depths of the water.

"They call this Milady's Bath," Oliver said. "It was made by Fran?ois le Gros, who had a taste for these things. They say Henri de Renaud was kept here for ten years before he died. They threw live rats down to him, which he killed and ate raw. For ten years."

The water rippled, and a heavy metal cage broke the surface and began to rise, dripping, into the air. The bars were black and filthy. The stench was overpowering.

Watching it rise, Oliver said, "In Castelgard I promised you, Magister, that if you deceived me, I would kill you. You shall bathe in Milady's Bath."

He looked at them intently, his eyes wild.

"Confess now."

"My Lord, there is nothing to confess."

"Then you have nothing to fear. But hear this, Magister. If I discover that you, or your assistants, know the entrance to this castle, I shall lock you away in this place, from which you will never escape, never in your life, and I will leave you here, in darkness, to starve and rot forever."

Holding a torch in the corner, Robert de Kere allowed himself a smile.

02:22:13

The steps led steeply downward, into darkness. Kate went first, holding the torch. Chris followed. They went through a narrow passage, almost a tunnel, that seemed to be man-made, and then came out into a much larger chamber. This was a natural cave. Somewhere high up and off to the left, they saw the pale glimmer of natural light; there had to be a cave entrance up there.

The ground before them still sloped down. Ahead, she saw a large pool of black water and heard the rush of a river. The interior smelled strongly of a sweet-sour odor, like urine. She scrambled over the boulders until she reached the black pool. There was a little sandy margin around the edge of the water.

And in the sand, she saw a footprint.

Several footprints.

"Not recent," Chris said.

"Where's the path?" she said. Her voice echoed. Then she saw it, off to the left, a protruding section of rock wall that had been artificially cut back, making an indentation that allowed you to skirt around the pool and to pass by.

She started forward.

Caves didn't bother her. She'd been in several in Colorado and New Mexico with her rock-climbing friends. Kate followed the path, seeing footprints here and there, and pale streaks in the rock that might have been scratches from weapons.

"You know," she said, "this cave can't be all that long if people used it to carry water to the castle during a siege."

"But they didn't," Chris said. "The castle has another supply of water. They would have been bringing food, or other supplies."

"Even so. How far could they go?"

"In the fourteenth century," Chris said, "peasants didn't think anything about walking twenty miles a day, and sometimes more. Even pilgrims walked twelve or fifteen miles in a day, and those groups included women and old people."

"Oh," she said.

"This passage could be ten miles," he said. And then he added, "But I hope it's not."

Once past the protruding rock, they saw a cut passage leading away from the dark lake. The passage was about five feet high and three feet wide. But at the edge of the dark pool, a wooden boat was tied up. A small boat, like a rowboat. It thunked softly against the rocks.

Kate turned. "What do you think? Walk, or take the boat?"

"Take the boat," Chris said.

They climbed in. There were oars. She held the torch and he rowed, and they moved surprisingly fast, because there was a current. They were on the underground river.

Kate was worried about the time. She guessed they might have only two hours left. That meant they had to get to the castle, reunite with the Professor and Marek, and get themselves into an open space so they could call the machine  -  all within two hours.

She was glad for the current, for the speed with which they glided deeper into the cavern. The torch in her hand hissed and crackled. Then they heard a rustling sound, like papers ruffled in the wind. The sound grew louder. They heard a squeaking, like mice.

It was coming from somewhere deeper in the cave.

She looked at Chris questioningly.

"It's evening," Chris said, and then she began to see them  -  just a few at first, and then a hazy cloud, then a torrent of bats flying out of the cave, a brown river in the air above their boat. She felt a breeze from hundreds of flapping wings.

The bats continued for several minutes, and then it was silent again, except for the crackle of the torch.

They glided onward, down the dark river.

Her torch sputtered, and began to go out. She quickly lit one of the others that Chris had carried from the chapel. He had brought four torches, and now they had three left. Would three more torches see them to the surface again? What would they do if the final torch went out and they still had farther  -  perhaps miles  -  to go? Would they crawl forward in darkness, feeling their way along, perhaps for days? Would they ever make it, or would they die here, in darkness?

"Stop it," Chris said.

"Stop what?"

"Thinking about it."

"Thinking about what?"

Chris smiled at her. "We're doing okay. We'll make it."

She didn't ask him how he knew. But she was comforted by what he said, even though it was just bluster.

They had been passing through a twisting passageway, very low, but now the cave opened out into a huge chamber, a full-blown cave, with stalactites hanging down from the roof, in some places reaching to the ground, and even into the water. Everywhere the flickering light of the torch faded into blackness. She did, however, see a footpath along one dark shore. Apparently there was a path running the entire length of the cave.

The river was narrower, and moved faster, threading its way among the stalactites. It reminded her of a Louisiana swamp, except it was all underground. Anyway, they were making good time; she began to feel more confident. At this rate, they would cover even ten miles in a few minutes. They might make the two-hour deadline after all. In fact, they might make it easily.

The accident happened so fast, she hardly realized what had occurred. Chris said, "Kate!" and she turned in time to see a stalactite just by her ear, and her head struck the stone hard, and her torch hit it as well  -  and the burning cloth tip shook free from the stick it was tied to, and in a kind of ghastly slow motion, she watched it fall from her torch onto the surface of the water, joining its reflection. It sputtered, hissed and went out.

They were in total blackness.

She gasped.

She had never been in such darkness before. There was absolutely no light at all. She heard the dripping of the water, felt the slight cold breeze, the hugeness of the space around her. The boat was still moving; they were banging against stalactites, seemingly at random. She heard a grunt, the boat rocked wildly, and she heard a loud splash from the stern.

"Chris?"

She fought panic.

"Chris?" she said. "Chris, what do we do now?"

Her voice echoed.

01:33:00

It was now early night, the sky deepening from blue to black, the stars appearing in greater numbers. Lord Oliver, his threats and boasts finished for the moment, had gone with de Kere into the great hall to dine. From the hall, they heard shouts and carousing; Oliver's knights were drinking before the battle.

Marek walked with Johnston back to the arsenal. He glanced at his counter. It said 01:32:14. The Professor didn't ask him how much time was left, and Marek didn't volunteer. That was when he heard a whooshing sound. Men on the ramparts yelled as a huge fiery mass arced over the walls, tumbling in the air, and descended toward them in the inner courtyard.

"It's starting," the Professor said calmly.

Twenty yards away from them, the fire smashed onto the ground. Marek saw that it was a dead horse, the legs protruding stiffly from the flames. He smelled burning hair and flesh. The fat popped and sputtered.

"Jesus," Marek said.

"Dead for a long time," Johnston said, pointing to the stiff legs. "They like to fling old carcasses over the walls. We'll see worse than that before the night is over."

Soldiers ran with water to put the fire out. Johnston went back into the powder room. The fifty men were still there, grinding the powder. One of them was mixing a large, wide basin of resin and quicklime, producing a quantity of the brown goo.

Marek watched them work, and he heard another whoosh from outside. Something heavy thunked on the roof; all the candles in the windows shook. He heard men shouting, running onto the roof.

The Professor sighed. "They hit it on the second try," he said. "This is just what I was afraid of."

"What?"

"Arnaut knows there is an armory, and he knows roughly where it is  -  you can see it if you climb the hill. Arnaut knows this room will be full of powder. If he can hit it with an incendiary, he knows he'll cause great damage."

"It'll explode," Marek said, looking around at the stacked bags of powder. Although most medieval powder wouldn't explode, they had already demonstrated that Oliver's would detonate a cannon.

"Yes, it will explode," Johnston said. "And many people inside the castle will die; there will be confusion, and a huge fire left burning in the center courtyard. That means men will have to come off the walls to fight the fire. And if you take men off the walls during a siege . . ."

"Arnaut will scale."

"Immediately, yes."

Marek said, "But can Arnaut really get an incendiary into this room? These stone walls must be two feet thick."

"He won't go through the walls. The roof."

"But how . . ."

"He has cannon," the Professor said. "And iron balls. He will heat his cannonballs red-hot, then fire them over the walls, hoping to hit this arsenal. A fifty-pound ball will tear right through the roof and come down inside. When that happens, we don't want to be here." He gave a wry smile. "Where the hell is Kate?"

01:22:12

She was lost in infinite darkness. It was a nightmare, she thought, as she crouched in the boat, feeling it drift in the current and bump from stalactite to stalactite. Despite the cool air, she had begun to sweat. Her heart was pounding. Her breathing was shallow; she felt like she couldn't get a full breath.

She was terrified. She shifted her weight, and the boat rocked alarmingly. She put both hands out to steady it. She said, "Chris?"

She heard a splashing from far off in the darkness. Like someone swimming.

"Chris?"

From a great distance: "Yeah."

"Where are you?"

"I fell off."

He sounded so far away. Wherever Chris was, she was drifting farther and farther from him every minute. She was alone. She had to get light. Somehow, she had to get light. She began to crawl back toward the stern of the boat, groping with her hands, hoping her fingers would close on a wooden pole that meant one of the remaining torches. The boat rocked again.

Shit.

She paused, waiting for it to steady beneath her.

Where were the damn torches? She thought they were in the center of the boat. But she didn't feel them anywhere. She felt the oars. She felt the planking. But she didn't feel torches.

Had they fallen off the boat with Chris?

Get light. She had to get light.

She fumbled at her waist for her pouch, managed to get it open by feel, but then could not tell what was in there. There were pills . . . the canister . . . her fingers closed over a cube, like a sugar cube. It was one of the red cubes! She took it out and put it between her teeth.

Then she took her dagger and cut the sleeve of her tunic, tearing off a section about a foot long. She wrapped this cloth around the red cube and pulled the string.

She waited.

Nothing happened.

Maybe the cube had gotten soaked when she went in the river at the mill. The cubes were supposed to be waterproof, but she'd been in the river a long time. Or maybe this one was just defective. She ought to try another one. She had one more. She had started to reach into her pouch again, when the cloth in her hand burst into flame.

"Yow!" she cried. Her hand was burning. She hadn't thought this through very well. But she refused to drop it; gritting her teeth, she held it high above her head, and immediately she saw the torches to her right, pushed up against the side of the boat. She grabbed one torch, held it against the burning rag, and the torch caught fire. She dropped the rag in the river and plunged her hand under the water.

Her hand really hurt. She looked at it closely; the skin was red, but otherwise did not appear too bad. She ignored the pain. She'd deal with it later.

She swung the torch. She was surrounded by pale white stalactites hanging down into the river. It was like being in the half-open mouth of some gigantic fish, moving between its teeth. The boat banged from one to another.

"Chris?"

Far away: "Yeah."

"Can you see my light?"

"Yeah."

She grabbed a stalactite with her hand, feeling the slippery, chalky texture. She managed to stop the boat. But she couldn't row back to Chris, because she had to hold the torch.