Uzzia tended to these victims. ‘I’ve seen such symptoms before. Mountain sickness. If you’re used to the high ground it’s not so bad, and best of all is to have been born up here. But if you’re born a lowlander you can suffer. Some of these fellows have probably never come up here before, never dreamed they’d have to, before the advance of one of your master’s glaciers forced them out of their farms, to come labour for us like your mule.’

‘He’s not my mule. He’s his own, I think.’

She laughed. ‘And if he’s a lowlander he’s not showing it.’

Pyxeas had an explanation for everything. He said the sickness was caused by the thinness of the air at altitude, and a lack of a particular part of the air he called ‘the vital air’, necessary for life, and indeed for the sustenance of fires, as experiments had shown. As for Pyxeas himself, he actually seemed to thrive in the thin air. He even took to walking a spell each day, rather than riding. ‘Do you know,’ he said, as he trotted alongside his companions one morning, ‘the worst blight of old age is the constant pain. In your head, in your back, your joints, your bowels – somewhere. Constant, nagging, continual pain. Nobody talks about it, or if they do nobody younger ever listens, but it’s there all the same. Up here, though, my own various aches, my faithful companions for years, are lessened – some of them gone altogether. Make a note, Avatak.’

‘Yes, scholar.’

Still they climbed.

It grew colder. They woke to morning ground frost, and patches of old ice in the shadows of rocks and ridges. Pyxeas was fascinated by this, but fretted. ‘When we try to return, in a year, two years, we might not be able to come this way again.’

Uzzia growled, ‘If we make it to Cathay, we’ll find another way back. But in the meantime, sage – watch your step!’

And there was the silence. Avatak was increasingly aware of it, behind the small noises of the people, their morning coughs, their soft voices, the clink of pots hanging from the mule’s back – a huge silence that stretched to the mountains all around them. It wasn’t just the absence of people. It was the absence of any sound at all. Not even the sound of birds, he realised, not even their caws and cries and songs. Maybe the birds could not fly in this thin air. Pyxeas might be interested in the observation, but he would only make Avatak dig out his journal and write it down, so he kept the thought to himself.