Pyxeas grunted. ‘It would take a brave king to put his people at such risk for the sake of stocking up a few hungry sailors. Well, the spread of plagues is to be expected – I wrote it down in my notes some years ago, you can check it. When people are stirred up and on the move, and animal populations too, plagues are carried from their natural reservoirs, even carried between continents. It’s about time al-Quds sacrificed a few of those prize cattle down in the holds to feed us. I must have a word with him, I must . . .’ He gazed at the biscuit, picked it up, then set it down again, as if puzzled by its very presence. Then he glanced at Avatak. ‘The numbers, boy – it’s all in the numbers. It always was.’

‘What numbers?’

Distracted, the scholar cast about, shifting in his chair, and brought together lists of numbers, either in the hand of a scribe or his own spidery writing, some set out neatly on scrolls or books, some scrawled on scraps of paper and parchment. ‘Thank the mothers Bolghai was good enough to have his results translated into the Northlander system. Here now – can you see?’ He pointed to two lists of numbers.

‘See what? I’m sorry, scholar.’

‘Of course you won’t see it, of course not, it’s been staring me in the face for years, it still is, and I can make no firm conclusion, not yet . . . Look! What drives the weather, I mean the grand changes like the coming of a longwinter?’

‘The sun in the sky,’ Avatak said promptly; he had absorbed that much.

‘Yes. Good. The world bobs about like a duck on the Khan’s ornamental ponds in Daidu. The higher the sun is at midsummer, the warmer the world is that year. But how high, how warm? We Northlanders have been keeping records of the weather for millennia. And in those records I, Pyxeas, have found a good measure of that changing warmth.

‘Look here – this is a list of years, and this is a list of solar elevations at Etxelur at midday on midsummer day on each of those years. And these dates, Avatak, record the last spring frost of each year and the first frost of winter. Pieces of information easily and unambiguously recorded, and though they vary with circumstance the overall trend is clearly related to the warmth of the year. Can you see the correlation between the two? Oh, it takes a trained eye. Avatak, these tables show conclusively that the elevation of the sun drives our climate, as indicated by the span of the frost. But.’

‘But, scholar?’

‘But the tidy patterns break down! Centuries, millennia back, you can see it. The world should have got colder, quicker. The longwinter should already be here! I had long suspected this, from historical accounts, anecdotes. Now, after intensive study, I have assembled quantitative proof.’