One summer in the early 1740s, on the last day of his life, a young man from Paris became the first modern cartographer to see the mountain called ‘Le Gerbier de Jone.’ This weird volcanic cone juts out of an empty landscape of pastures and ravines, blasted by the freezing wind called the ‘burle.’ Three hundred and fifty miles south of Paris, at a point on the map diametrically opposed to the capital, it stands on the watershed that divides the Atlantic from the Mediterranean. On its western slope, at a wooden trough where animals once came to drink, the river Loire begins its six hundred and forty mile journey, flowing north then west in a wild are through the mudflats of Toraine to the borders of Brittany and the Atlantic Ocean. Thirty miles to the east, the busy river Rhone carried passengers and cargo down to the Mediterranean ports, but it would have taken more than three days to reach it across a sparsely populated chaos of ancient lava-flows and gorges.

The traveler in question (his name has not survived) belonged to an expedition that was to lay the groundwork for the first complete and reliable map of France. A team of young geometers had been assembled by the astronomer Jacques Cassini, instructed in the new instruments. Cassini’s father had studied the rings of Saturn and measured the size of the solar system. His map of the Moon was more precise than many maps of France, which still contained several uncharged regions. Now, for the first time, France would be revealed in all its detail as if from a greater height above the earth.