John Harrison was the subject of a 1995 Dava Sobel book with this title (subtitled: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time) that likely helped get Harrison named as the 39th greatest Briton in a 2002 BBC poll. Harrison, a clockmaker, is famous for inventing the H4 which was the first successful device for helping (indirectly) compute what quantitative measure which was the title of Sobel's book?
Longitude