In May 1902 someone in an Athens museum looked at a lump of corroded bronze from a shipwreck and saw a gear wheel in it. It was the Antikythera mechanism, about two thousand years old, a hand-cranked model of the sky whose gearing even made the Moon’s pointer speed up and slow down the way the Moon does. Nothing that intricate turns up again until the astronomical clocks of the 14th century, and it is usually told as a mystery, a machine with no ancestors. But nobody’s first attempt is that good, so there were probably others, and bronze was worth money: when a thing like this wore out or went out of fashion, it could go in the pot. That may be part of the gap. Some of the long silence could be people melting the evidence, one useful scrap at a time, and the one we have is the one nobody could get to.