This book is a nearly ideal choice for summer reading. It is small and short, it tells the fascinating, true story of John Harrison, who may have contributed more than any other individual to the establishment of the British Empire. Working alone, the self-taught Mr. Harrison built clocks that were capable of keeping time to within a few seconds per month, despite temperature changes and the rolling and pitching of a ship. By knowing the accurate time back in London and local time from observations of the sun, her Majesty's navigators could easily determine how far around the earth they had traveled. Harrison's tale is not solely a story of technology; equally interesting are the scientific alternatives to his amazing clocks, most of which involved astronomy - either observations of our moon or of Jupiter's moons. The pro-astronomy prejudices of the Board of Longitude, established by act of Parliament in 1714 to bestow a prize of 20,000 pounds on the first person to establish a practical means of establishing longitude, delayed Harrison's award for more than four decades. "Longitude" tends to slight the important technological details of his contributions. Interested readers might want to consult "Revolution in Time" by David S. Landes (Belnap/Harvard Press 1983) or "It's About Time", by Paul M. Chamberlain (Holland Press/Richard R. Smith Co. 1941 and 1964). A fine article about the modern way of navigation is "The Global Positioning System" by Thomas A. Herring in Scientific American, February 1996, p. 44.