In June 1792, the erudite and cosmopolitan Jean-Baptiste-Joseph Delambre and the cautious and scrupulous Pierre-Francois-Andre Mechain set out from Paris -- one north to Dunkirk, the other south to Barcelona to calculate the length of the meter. In the face of death threats from village revolutionary councils, superstitious peasants, and civil war, they had only their wits and their letters to each other for support. Their findings would be used to create what we now know as the metric system. Despite their painstaking and Herculean efforts, Mechain made a mistake in his calculations that he covered up. The guilty knowledge of his error drove him to the brink of madness, and in the end, he died in an attempt to correct himself. Only then was his mistake discovered. Delambre decided to seal all evidence of the error in a vault at the Paris Observatory. Two hundred year later, historian Ken Alder discovered the truth. With scintillating prose and wry wit, Alder uses these previously overlooked letters, diaries, and journals to bring to life a remarkable time when everything was open to question and the light of reason made every dream seem possible.