page fifteen

The Exchange

Today's number has laid out a trading world: eight to ten towns on a road network, eight goods, and a few dozen merchants with a purse each and no instructions. Nobody sets a price here. Every town simply makes some things and eats others, and what it has in its warehouse against what it gets through in a season is the whole of the pricing. The merchants read those numbers, buy where a thing is cheap, haul it somewhere it is dear, and in doing so quietly drag the two prices together.

Click a town to open its warehouse. Space pauses. Every moving dot is a loaded caravan, coloured by what it is carrying.

Speed
Tolls on every sale 6%
Year — Caravans out — Cargoes sold 0 Merchants —

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The word going round

Nothing here is scripted

    One good, every town

    Each line is what a single town is asking for the chosen good, plotted as the years go by. The dashed line is what the thing is worth in the abstract. Close the roads and watch the lines fan apart; open them again and watch the merchants pull them back together.

    Dearest ÷ cheapest — Cheapest — · dearest —

    Where the prices come from

    1. 01

      The land decides

      Every town is dropped on a terrain — coast, plain, hills, mountain, forest or river crossing — and the terrain decides what it is any good at making. Fish come off coasts, iron out of mountains, wool off pasture, cloth and glass out of the crowded river towns. Each place ends up decent at three things and dependent on the rest.

    2. 02

      The warehouse decides

      Production runs in every day, consumption eats out of the same pile, and the price is read straight off what is left. Two months' supply in store and the good sells at about what it is worth; a week's supply and it is two and a half times that; eight months of glut and it is down near half. No merchant sets that number and nothing smooths it.

    3. 03

      Both ends give a little

      A price like that is worth some trouble, so the town digs, weaves or fishes a bit harder for it — and uses a bit less of it, because at three times the money the cloak lasts another winter. Neither response is large and neither is quick, but between them they are the other half of how a price settles, and they are why nothing here stays pinned at the top of the scale for ever.

    4. 04

      Distance decides the rest

      A merchant compares the price at his elbow with the price one road away, subtracts the haulage — charged by bulk, so spice travels cheerfully and timber mostly stays where it fell — subtracts the toll, and goes if anything is left. Buying lifts the price he pays; selling drops the price he gets. Enough merchants and the gap narrows to roughly what the journey costs, which is as near as a world gets to one price for anything.

    The same date integer that draws the flow field on the front page, weaves the cloth in the Loom, cuts the gears in the Engine and founds the countries in the Chronicle lays out this map, names the towns, deals the goods and gives every merchant his purse. After that nobody is steering: there is no target price anywhere in the code and no equilibrium being solved for, and the tidiness on the chart is a result rather than an instruction. Two things about it were worth measuring rather than assuming. Close the roads and the typical town price ends up about two and a half times away from what the thing is worth, with most goods pinned at the top or the bottom of the scale; open them and that halves. But drag the tolls all the way to forty per cent and remarkably little happens, because a duty comes out of the merchants' margins rather than out of the traffic — the gap between two towns is usually far bigger than the tax on it. What actually keeps a price local is the thing's own weight and its habit of going off: the timber is worth five coin and costs a quarter of that to move a few days down the road, so it stays where it fell, and the salt fish quietly rots while it travels.