In the 18th century, Europe's scientific community was torn between two opposing theories: Descartes' argument that the Earth was spherical, and Newton's contention that it was flattened at the poles. Recognizing that the answer was the key to securely navigating the earth's oceans, France and Spain organized a joint expedition to colonial Peru. Their goal was to measure a degree of latitude at the Equator; by comparing this measurement to one taken back in Europe, they would be able to determine the planet's shape and put an end to the debate. But what seemed a straightforward scientific...