Second paragraph of third chapter:

Zhu Di and his father, Hong Wu, had encouraged the development of every aspect of navigation. A handbook titled Notebook on Sea Bottom Currents, found in Quanzhou, states that, after announcing the ascension of the Yongle emperor (Zhu Di) to the throne, Zheng He and his admirals were instructed to search for navigation charts, collecting all the information about currents, islands, mountains, straits, and the positions of stars. They used this data to revise their navigation charts, including compass points and the cross-references of stars.

I was warnedThe Story of English in 100 Words, by David Crystal.