Earth's Magnetism in the Age of Sail primarily focuses on the historical investigation of geomagnetism, and on geomagnetism and navigation, from 1500 to 1800. This well-written and well-documented book is highly enjoyable. Once the reader has cleared the first chapter, the remainder of the book is a historical treasure.
The book is divided into two parts. Part I, which consists of four chapters, deals with the efforts to understand the Earth's magnetic field during the period under investigation. Part II, which has three chapters, describes the intersection of properties of the magnetic field with advances that were being made in navigational techniques in the period.