‘A daily hand-out from the Khan. And they send grain from the city stores to the provinces too. Apparently they have been feeding twenty thousand people from the city alone this way. But the granaries are emptying and every day the dole is cut down a little more. Of course the soldiers are always well fed. We need to get back inside the city.’

‘Why?’

‘A messenger found me. There’s been an explosion at the palace.’

‘There’s been a what?’

She grinned. ‘That rascal Pyxeas. You can’t leave him alone for a heartbeat, can you?’

They were brought to another corner of the palace, a smaller laboratory. Here another elaborate experimental apparatus had been set up, on a lesser scale – stands and tubes and flasks and pipes, mirrors, small oil heaters. But this equipment had been scattered around the room. The carpet was scorched, the paint on the wall blistered. One servant seemed to have been injured, a weeping woman, and a doctor was tending to her burned arm. Soldiers stood around, looking shocked, dismayed, as well they might, Avatak supposed. The explosion, deep inside the home of the Khan himself, must have made them fear assassins, and, worse, the punishment they would receive if any harm came to the Khan or his family.

And here was Pyxeas, a blissful smile on a soot-stained face, the fringe of white hair around his scalp vertical, his robe smeared black. Bolghai stood behind him, equally begrimed, rather more shamefaced. ‘Avatak!’ Pyxeas cried. ‘I hope you enjoyed your walk. You missed a bit of fun.’

‘I can see that. What were you making here – eruptors?’

‘Nothing of the sort! Though I can see you might have difficulty working it all out given that it’s lying in pieces everywhere . . . Where is the gas tube, Bolghai?’

‘Over there,’ said the Mongol. ‘And there. Oh, and there’s a bit stuck in the ceiling, I think.’

As the philosophers and their servants gathered the fragments of the broken apparatus, gradually Avatak pieced together what Bolghai had been attempting here.

‘What controls our weather?’ Pyxeas asked. ‘The sun, whose position in the sky as determined by astronomical considerations fixes the amount of heat delivered to the world – and the air, through which that heat must pass. But how does the sun’s heat pass through the air? How much of it is blocked – and how much trapped, as the thinnest linen blanket will trap some of the warmth of the body? That’s what this apparatus seeks to determine.’

The core of it had been a long brass tube, held horizontally. One end of this was heated, either by reflected sunlight or an oil lamp. At the far end of the tube was a thin, upright glass flask containing oil; by seeing how the oil expanded and climbed up its flask you could tell how much heat was passed through the tube – or rather, through whatever was trapped inside.