This book centres on the lesser-known work of one of the giants in the 18th century longitude story, the maverick clockmaker John Harrison (1693-1776). It is set out in three parts. The first section gives some context to the project: Harrison’s background, methodology, and thinking are analysed alongside a brief account of the progression of the standard observatory astronomical clock or precision regulator. The second section deals with the contemporary story of researching, making, adjusting and trialling of Clock B. The final section of the book offers a physical analysis of the clock system, how, and why it works so well. The Clock B project has generated lively debate and discussion and some of the most pertinent points raised will be discussed in an end word.