‘What’s the matter?’ He squatted beside her. She did not answer and he had to give her his hand to sniff before she would rouse. Whenever she was distressed, the smell of him seemed to comfort her. He did not understand that either.

‘Tired,’ she whispered. ‘Feet hurt.’

She had thrown off her boots and socks, and her feet were resting on a patch of snow. The little toes, as small as a child’s, were red and one heel had a large blister. He clicked his tongue in vexation.

‘I’m sorry, Nish,’ she wailed. ‘I tried really hard.’

Ullii never lied or exaggerated, and was so sensitive that walking in those boots must have been agony. There was no possibility of her wearing them again. Nor could she go any distance in bare feet. It was too cold.

‘Climb onto my shoulders, Ullii. I’ll carry you.’ She probably would not like that either but there was no choice.

She did so willingly enough, and once up there she smiled. ‘I can smell you, Nish.’ Lifting the blindfold, she peered down the front of his shirt.

‘Whatever makes you happy,’ he muttered. She was no heavier than a ten-year-old but even that was a hefty burden to carry down the mountain.

By the time they reached the balloon, whose basket was wedged between two boulders, he was drenched in sweat and Ullii’s smile was broader than ever. Setting her down in the weak sun, he lay beside her.

‘I love you, Nish,’ she said.

Had Nish been standing up, he would have fallen over. All he could do was gape. Ullii never made remarks like that. What did she expect of him? He could hardly reciprocate. He liked Ullii, cared for her, and many a night had lain awake burning with desire for her sweet little body, but he could never, except perhaps to get that desire fulfilled, have said that he loved her.

Taking her hand, he drew it to his lips. She shivered and her eyelashes fluttered. He could have screamed with frustration. Why now, when he could do nothing about it? To hide his confusion, he climbed up to look at the balloon, ignoring her little whimper. Tonight, he thought. When everything is prepared.

The gasbag was flaccid, though being formed around a series of struts and stretched wires, it maintained its shape. The air inside had gone cold and he would have to burn the brazier for at least half a day to lift off. First he must gather fuel, for all he had was a large flask of distilled tar spirits. It was useful for burning wet wood but could not be used by itself in the brazier, or the explosion would have blown balloon and boulders back up to Tirthrax.

There was little fuel here, just scrubby heath and a few patches of grass. If he filled the basket with the stuff, it would barely lift the balloon. No time to waste. He headed for the nearest patch of vegetation.

By the middle of the afternoon, Nish had gathered a great mound of shrubbery. As he’d expected, it burned quickly, generating plenty of ash but little heat. After an hour the balloon was almost as flaccid as when he had started. Already he had exhausted the closest supplies of fuel. What if the witch-woman (as he thought of the Matah) was already on her way?

Forcing down panic, he considered other options. The rocks were hung with feathery strands of lichen. Perhaps if he tied that into bales and soaked it in tar spirits? Nish began collecting the material but soon gave the idea away. It took an hour to gather a small bag of lichen and it weighed nothing. There could be no heat in it either.

By then the sun was going down. The sky was clear; the night would be cold and they would need a fire; more precious fuel wasted. He trudged off for another armload of scrub.

On his return Nish could not find Ullii anywhere. He felt like screaming, but did the sensible thing and lit the fire before he went looking for her. She was not far away, just down the slope at their original campsite. Ullii had discarded her mask in the evening and was drawing on a slab of sand-coloured rock with a black lump of pencil-stone.

‘I wish you’d told me where you were going,’ he said irritably.

For once she did not cringe. ‘I knew where you were.’ She gave him such a sweet smile that it was impossible to be angry with her.

‘Come up. It’s time for dinner.’

He followed, admiring her figure. Nish prepared dinner, a gruel made of mashed and boiled grains for her, since she could not bear any kind of strong flavour, and much the same for him but with hot spices and smoked meat added.

Nish ate his dinner moodily. If he began the instant it was light, he might just manage to collect enough fuel by darkness, and that was not good enough. The witch-woman might have discovered that the crystal was gone. She could stop him with a single flaming arrow, for the tar-sealed silk would burn like a torch.

By the time he had cleaned up, Ullii was asleep and Nish knew better than to disturb her. He spent a frustrated, agonising night, punctuated by trips to replenish the brazier, and before dawn gave up hope of sleep.

The day crawled by. Nish set Ullii to keep lookout for Tiaan and the Matah. Each time he returned with his burden of fuel, the brazier was out. By lunchtime the balloon had begun to fill but it was a long way from lifting off. Ullii sat beneath the boulder, still scribbling with her pencil-stone. The patterns made no sense at all. He was gnawing on a lump of smoked meat when the seeker gave a whimper and curled up.

‘Ullii?’ he whispered. ‘What’s the matter? Is it the witch-woman?’

She did not answer, which meant it was a major distress. He felt for his knife, though it was useless against the likes of the Matah. Climbing the rope ladder to the brazier, Nish scanned the surroundings. He saw nothing in any direction. Nothing moved but a white eagle soaring on the updraught above the icefall. Its beak was bright yellow.

When he reached the ground, Ullii had partly unfolded. He tried to discover what had scared her but she was unable to articulate it. ‘Hooks and claws,’ she said over and over again, referring to something seen in her lattice. He tried to put it out of mind.

Nish was about to go for another load when he noticed the lump of pencil-stone in her hand. The manufactory sometimes burned it in the furnaces. ‘Where did you get that, Ullii?’

‘Up mountain,’ she said in a barely audible voice, still suffering.

He took her hand. ‘Is it far? Can you show me?’

‘Not far.’

After a short climb they reached a steep face where the dark and light rock stood on end, dipping back into the mountain like layers in a cake. At head height the soft rock had weathered away, leaving an elongated cavity the width of Nish’s hips. Several lumps of black, shiny pencil-stone were stuck to the overlying slate. Inside, the cavity was half full of chunks the size of his fist.

Nish climbed in and began to scoop them into his bag. To his amazement, Ullii joined in with the work, and soon the bag was bulging. ‘Beautiful fuel,’ he said, laughing for joy.

Back at the balloon, he stuffed the brazier, packed lichen all around and carefully poured in half a cup of tar spirits. The pencil-stone would need a hot fire to burn. He flew down the ladder, afraid he had used too much spirits. Nothing happened for a couple of minutes, then with a whoomph the fuel went up and flames roared out the top of the flue.

‘More!’ They raced up the slope, filling another bag each. The balloon was starting to swell visibly as they returned, though they would need more fuel to take them any distance.

He had come back with a third load and was topping up the brazier when Ullii choked and dropped her bag, spilling pencil-stone across the ground. ‘What is it?’ he called.

The little seeker looked as if she was having a fit. Her teeth were bared, her eyes staring. She tried to tell him something but managed only incoherent squeaks.