 Parennin then induced him to consent to a map of the Great Wall of China.Fathers Bouvet, Régis and Pierre Jartoux measured their route to the eastern extremity of the famous rampart by means of regularly divided cords, keeping track of directions with the assistance of a compass, and frequently observing the meridian of the sun in order to calculate latitudes.In four days they reached the Gulf of Zhili (8 June 1708) and began operations on the great Wall.