The Præsidium stood on four pillars and for most of its height was square in cross-section. Not far above the dials, however, the corners of the square floor-plan were cleaved off, making it into an octagon, and not far above that, the octagon became a sixteen-sided polygon, and above that it became round. The roof of the Præsidium was a disk, or rather a lens, as it bulged up slightly in the middle to shed rainwater. It supported the megaliths, domes, penthouses, and turrets of the starhenge, which drove, and was driven by, the same clock-works that ran the dials.
( Neal Stephenson )
[ Anathem ]
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