A prayer book finished by April 1229, probably in Jerusalem, was made the ordinary way: seven older manuscripts taken apart, scraped back, split down the middle, turned ninety degrees and written across. One of the seven was Archimedes. Scraping, it turns out, is not erasing — the mathematics stayed on the page beneath the liturgy, and imaging has since pulled out of it two treatises that exist in no other copy in any language, plus the only Greek text of a third.