For many years, Tony Mercer, grandson of the most enduring English maker, has studied and collected a huge amount of information about marine chronometers, instruments which enabled explorers and the Royal Navy to map the world, the Navy to police it, and merchant venturers to sail in relative safety. The purpose of the chronometer is reviewed largely as a navigational aid for ocean-going vessels, but also for survey, medical, and other activities calling for precise time measurement. Representative examples of chronometers and deck watches by international makers--from the earliest known to contemporary instruments--are illustrated in both color and black and white. A comprehensive list of makers and craftsmen brings together details of men who worked in the industry, their places of work, and dates and serial numbers for their instruments.