Deptford to Port Royal, November 1761

The problem is old and has killed a great many people. Latitude is easy; the sun tells you. Longitude requires knowing what time it is somewhere else at the same moment, and no clock will keep time on a ship — the roll, the damp, the temperature, the ordinary indignities of the sea take a pendulum apart in a week.

John Harrison, a joiner from Lincolnshire with no formal training, spends forty years on it. The first three machines are the size of furniture. The fourth is a watch, five inches across, and it goes to Jamaica in the pocket of his son and comes back having lost about five seconds in eighty-one days.

The Board of Longitude does not want to believe it and takes another decade to pay.

What the watch actually did was not navigational. It made every point on the ocean addressable. Before H4 the sea is a passage between known coasts, described by soundings and dead reckoning and the memory of pilots, and its middle is genuinely unmapped in the sense that there is nothing there to have a name. After H4 the sea is a grid, and every part of it has coordinates, and the coordinates exist whether or not any ship has been there.

The grid was drawn first. Then the ships went to the places the grid said were places. Then the places were places. The order is not an accident of history; it is the order in which this always happens, and Harrison did it to two thirds of the surface of the planet with a watch.