The moon trog led the way through the tangle of stems, swinging his way easily and skilfully, as naturally as a man walking the streets of a Tharian city. The Tharians got into all kinds of difficulties, though. Floating off in all directions and tumbling helplessly through the air until they managed to grab hold of a stem, tendril or one of their friends. After some experimentation, though, they got the idea and managed to make some progress in the right direction, although awkwardly and clumsily, and the tunnel entrance was soon out of sight behind them. After a few minutes the foliage thinned ahead of them, the sunlight grew brighter and they were able to see the base of the dome itself.

     Tallium turned out to be the moon trog name for the silvery grey moon metal they’d puzzled over earlier. The metal framework was worryingly thin, with struts two inches by six in cross section holding huge, ten foot wide, triangular plates of transparent crystal. No, three plates, Thomas saw as he looked closer. Three layers of crystal. The two innermost about half an inch thick, the outermost a full inch thick, all three separated by a two inch gap from its neighbours but with round pegs of the same transparent crystal providing support every ten inches or so. The outermost pane was pitted and pockmarked on its outer surface by micrometeorite impacts, obscuring their vision a little, but they could still clearly see the stark, desolate surface of the tiny moon.

     Above them, the stars shone with impossible clarity and brilliance, moving slowly across the sky as Kronos moved in its orbit around Tharia, but the yellow sun was out of sight, on the other side of the dome. It was the surface that caught their attention, though. It was markedly different from what they'd seen through the windows of the observatory. There, the surface had been all jumbled rocks, with such a stark contrast between brilliantly sunlit rock and inky black shadows that it had hurt their eyes. Here, though, the surface was completely covered by a vast number of shiny dark domes, each about six inches across and packed almost shoulder to shoulder. It looked as though some giant reptile had made its nest out there and laid thousands of large, dark eggs.

     “What are they?” asked Jerry in wonder.

     “Cupolas,” said the moon trog. “Domes of glass, each one covering the entrance to a light tunnel. The cupola collects pretty much all the light that falls on it and directs it down through optical fibres. We collect all the light that falls on this side of Kronos. We depend on sunlight for everything. It lights those caverns and tunnels that are close to the surface, it…”

     “Wait a minute, wait a minute,” said Thomas, staring in disbelief. “You collect all the light that falls on this side of Kronos? All of it?”

     “The side of Kronos facing away from Tharia, yes.”

     “But Kronos is twenty five miles across! I know that's small compared to the other two moons, but it's still twenty five miles across!”

     “Yes,” agreed the moon trog with a smile.

     “That's, what…?” Thomas stared ahead into nothing as he did the mental arithmetic.

     “A little under a thousand square miles,” said Ban Chin.

     Thomas stared at him. “How many cupolas is that?”

     “I've got no idea. A few million I would think. I can ask if you like.”

     “More than a few million! A few thousand million perhaps! And all the optical fibre…”

     “We haven't just been doing this since the fall of Agglemon. We were doing it for centuries before that. The Empire knew what we were up to. They helped us.”

     “Helped you how?”

     “Their wizards created some magical devices for us, in return for the iron we gave them. Our first ancestors chose to make their lives up here, you see. They married each other, raised children who also chose to remain up here. Several generations spent their entire lives up here. They had no idea that they were trapping themselves up here, that their descendants would never be able to return.”

     “Their bodies were adapting to the low gravity,” said Thomas.

     The moon trog nodded. “It took just a few generations. When some of them did try to return they found that their bones and muscles were too weak. Their lower limbs were adapted for grasping and holding; they were barely able to walk on them. They were forced to come back up here, and with every generation that’s passed since then the situation’s just got worse, until…” He displayed all four of his limbs for them and shrugged helplessly. “And there are other adaptations as well. Inside our bodies. Things you can't see.”

     “So you had no choice but to make the best possible lives for yourselves up here.”

     “Indeed. For that, though, we needed magical help, and like our trog ancestors we are unable to use any kind of magic. We needed the help of human wizards. Fortunately, we had a few bargaining chips. We had had several generations in which to figure out the best way to mine and refine the iron and other minerals hard to come by back on Tharia. We could threaten to withdraw our labour. If they had tried to replace us they would have had to start again from scratch. They decided it would be simpler to just give us what we wanted. Devices that turn rock into the things we need in vast numbers. Panes of glass for the farm domes. Cupolas and optical fibre for the light tunnels. Other things. They kept similar devices for their own use, of course. We knew we were giving up our monopoly on those substances, but we needed more of them than we could create by our own labour, as you surmised, and our alchemists still had a monopoly on plenty of other things. Glowoil, glass ceramic armour, stuff like that.”

     Vast numbers, thought Thomas, his brain feeling numb with shock. Vast numbers indeed! He shook his head to try to clear it, to try to keep the huge numbers from overwhelming him. Don't think about it, he told himself. Don't think about just how many cupolas there must be out there. He glanced out over the surface of Kronos again. The small glass domes were still there, stretching away from him in ranks and files, getting smaller and smaller as they approached the horizon until they merged into a uniform dark smoothness that blended into the black sky. He turned his back on them and tried to think of something else.

     The others noticed his distress, and Jerry and Lirenna shared a worried glance. They were both very smart. They had to be to be wizards, but they didn't have the same spark of genius that Thomas possessed. They could calculate the numbers in their heads, but they were just numbers to them.  They lacked Thomas’s ability to see the reality behind the numbers. They could see that the facts and figures Ban Chin had given them were threatening to overwhelm Thomas’s mind, but they didn't quite know why.

     “So, you've got all the sunlight you could ever need,” said Jerry, just for something to say.

     “Indeed,” the moon trog replied. “It lights our tunnels, grows all our food. It drives all our alchemy and heats all our furnaces.”

     “Heats your furnaces?” said Jerry curiously. “How?”

     “Have you ever focused light with a lens to burn your name into a dry leaf or a piece of wood?” asked the moon trog. “We do the same thing with mirrors, but on a larger scale. Our furnaces are located down near the centre of Kronos."

     "But that's over ten miles away!" cried the tiny nome in disbelief. "The thicker a pane of glass is, the less light can go through it. How can light pass through ten miles of glass?"

     "Our optical fibres are supertransparent," explained the moon trog. "We and our ancestors down on Tharia have spent thousands of years experimenting with different chemical compositions and different processing procedures, and we've developed a form of glass that transmits light with no absorption whatsoever. All the light that goes in one end comes out the other. It also means that virtually all the light that hits the cupolas ends up going down the light tunnels. Very little light is reflected away. Some is, of course. Rays of sunlight that hit the cupolas at the wrong angle. That light is lost to us and wasted, but we've managed to cut it down to only five percent or so."

     “And sunlight can heat the ore hot enough to melt it?” declared Thomas in wonder, coming back into the conversation.

     “Oh yes!” replied Ban-Chin, delighted at the human’s reaction. “We can easily reach temperatures as high as a thousand degrees. If it needs to be hotter, we just focus it more.”

     “But why doesn’t it melt the optical fibres?” asked the wizard in bafflement.

     “There's no absorption!" repeated the moon trog. "It can't heat the fibres because it's not absorbed by them. And besides, it’s not focused too much until it gets down to the furnaces, in case the fibre breaks somewhere along its length. There it’s focused to its industrial concentration by special ceramic mirrors that are specially designed to tolerate high temperatures.”

     “Incredible,” breathed the awestruck wizard. “Rocks and metals melted by sunlight!”

     “It’s the only way we can do it,” explained Ban-Chin. “Our ancestors back on Tharia used, sorry, they probably still use, wood and charcoal in their furnaces, maybe even coal if they come across a seam, but we have none of these things up here. Sunlight is all we have.”

     Thomas looked down at the ground, at the base of the dome, and marveled at the contrast between moist, living soil on one side and the field of dull, round cupolas on the other, separated by only four inches of moon metal. Then he looked up at the huge inner surface of the dome, curving gently inwards, the triangles of transparent crystal of which it was composed getting smaller and smaller with height. “Incredible,” he breathed. “Unbelievable.”

     “Where’s Tharia?” asked Matthew, eager for another sight of their homeworld.

     “You can’t see it from here,” said Ban-Chin. “All our farm domes, and all the light tunnels, are on the side of Kronos facing away from Tharia. The original reason for that was so that Tharia would never block out the light of the sun, reducing the amount of sunlight the domes received, but now we’re glad of it for another reason. If the side of Kronos facing Tharia were covered with cupolas, it would be sooty dark and people down on your world would notice. They would remember that it had once been bright and they would wonder why it had changed. They might deduce our presence. If civilization were ever to rise again down there, we’d prefer them not to know about us. We just want to be left alone.”

     A cold chill settled over Thomas’s heart, and a quick glance at the others told him that they were thinking the same thing. They were looking for a way to get back home, but if they actually found a way, if they found the key to the teleportation chamber for instance, then the moon trogs might not let them go, in case they told everyone on Tharia about them.

     They'd stupidly told Ban-Chin that their arrival on Kronos was an accident, and that no-one back on Tharia knew they were there, that no-one back on Tharia had any idea there were people living in the smallest moon. Now he cursed himself bitterly for speaking so freely. If they'd told the moon trogs that everyone back on Tharia knew about them, they'd have seen no reason not to let them go free, but now the moon trogs had a solid gold reason to keep them prisoner. Such duplicity simply went against their natures, though. Being honest and open with people came so naturally to them that they did it without thinking.

     “But what about the bore worms?” said Jerry. “You said you’d welcome help from the Tharian wizards in dealing with them. You said they were the greatest threat to your continued existence here.”

     “I spoke without thinking,” replied the moon trog. “Renewed mining activity would be an even greater threat to us. The promise of cheap iron would be a temptation that humans would be unable to resist and they would flood up here in their thousands, destroying everything we’ve created with their tunnels and their foundries. We would become merely their hired workforce, as we were in the past. We would prefer things to remain just the way they are, thank you very much. We have a civilization of our own now, and humans are no part of it.”

     Oh great! thought Thomas wearily. Just when we thought we’d finally found friends. Are we fated to be taken prisoner by everyone we come across up here? All of a sudden he was looking forward to their meeting with the Dallak with considerable apprehension.

     He tapped Lirenna on the shoulder to get her attention and motioned for her to come with him a short distance away from the others. “Have you still got Lord Basil’s ring?” he asked.

     “The Ring of ESP?” she asked. She searched among her pockets and pulled it out. “Why?”

     Thomas indicated Ban-Chin, still chatting happily with the others. “I want you to read his mind.”

     “I don’t know how,” protested the demi shae. “It might require a word of command or something.”

     “I don’t think so,” replied Thomas. “I never heard Lord Basil use a word of command. I think it just requires an act of will. Try it and see.”

     “Okay. I’ll try.” She slipped it onto a finger, and the magic ring adjusted its size to fit perfectly. She then looked at the moon trog and concentrated. “It works!” she said after a moment. “He’s delighted by our reaction to the dome, full of pride and joy. Mainly pride.”

     “Can you tell what his intentions for us are?”

     “No. I can only read what he’s actually thinking at the moment. You’ll have to talk to him and guide him onto the right subject.”

     “Okay. I want you to wear that ring all the time from now on, and read the minds of everyone we meet. Okay?”

     Lirenna looked doubtful, however. “I don’t know,” she said. “It’s a terrible invasion of privacy. Everyone’s entitled to the privacy of their own thoughts.”

     “You’ve only got to take a quick glance, just enough to see whether or not they mean us any harm,” said Thomas. “You only have to look deeper if you don’t like what you see on the first glance.” He took her by the shoulders. “Please, Lenny. This is important. Our liberty, maybe our lives, may depend on it.”

     The demi shae agreed reluctantly, but then she clapped her hands and gave a little cry of joy. “Oh! Oh how wonderful!” she cried happily.

     “What?” asked Thomas in puzzlement.

     “I was still reading his mind without realising it,” she explained. “He’s talking to the others about the dome and an image just came into his head, their dream of the future. They plan, one day, to colonize the other moons, and other moons that move around the yellow sun. They plan to build ships to sail the voids between the planets. Tiny worlds with sails dozens of miles wide to ride the winds that blow from the two suns. Oh Tom, it’s so beautiful! I just can’t believe that people with such a beautiful dream could mean us any harm!”

     “Well, read their minds anyway, just to be on the safe side. Okay?”

     Lirenna nodded, and the two returned to the others. Ban-Chin was telling the others about the strange moon metal that the dome’s frame was made of, an alloy of aluminium and silicon. “Steel is too heavy for this purpose,” he was saying. “Our priests of Caratheodory told us that a dome with a steel frame would collapse under its own weight, even in Kronos’s low gravity. We tried to build one anyway, and it collapsed, just as they said it would. The trouble with tallium, though, is that the metal of which it is principally composed binds tightly with the other elements of Kronos’s rocks, so that even though it makes up nearly half of Kronos’s total mass, we can only produce enough of it to make one new dome every ten years or so.”

     The Tharians stared back at him with blank looks on their faces, even Thomas. “That’s very interesting,” lied Shaun, wishing he hadn’t asked the original question. He yawned, in fatigue, not boredom. “I’m sorry,” he said, “but it’s been a long day. We’ve been through a lot since we last rested.”

     “Oh I’m sorry, how thoughtless of me!” apologised the moon trog in genuine regret, seeing for the first time how tired they all were. “I’ll take you to the farmers’ rest rooms. You can stay there and go see the Dallak when you're fully rested.”

     “Could we stay here please?” begged Lirenna. “It’s been so long since we last slept under the stars!”

     “I don’t see why not,” replied Ban-Chin with an indulgent smile. “I’ll tell the farmers to stay away from this area for a while, so as not to disturb you.” He then vanished among the dense greenery, leaving the Tharians alone at the edge of the farm dome.

     They had a supper of Konnen trail rations before settling down for the ‘night’, and Lirenna had the best night’s sleep she’d had since entering the caves of Shanathin and leaving the surface of Tharia, so many months before. Even the rising of the yellow sun failed to awaken them, the tinted crystal protecting them from the worst of its harsh light, and they continued to sleep peacefully as it crept with unseemly haste across the sky and disappeared once more behind the thick, tangled masses of greenery that filled the dome.