N ear the midpoint of London Bridge, a bit closer to the City than to Southwark, was a firebreak – a short gap in the row of buildings, like a missing tooth in a crowded jawbone. If you were drifting down-river in a boat, so that you could see all nineteen of the squat piers that held the bridge up, and all twenty of the ragstone arches and wooden drawbridges that let the water through, you'd be able to see that this open space – "the square,' it was called – stood directly above an arch that was wider than any of the others – thirty-four feet, at its widest.