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This book describes the John Harrison marine Chronometers H1 thru H4. The author also describes other key technology that enabled mechanical timepieces to keep accurate time in the harsh conditions of sailing ships. Includes drawings of key parts. Without accurate time keeping, enabling the determination of longitude, positioning at sea used to be very inaccurate resulting in many lost lives and ships. Book has been reprinted a few times. The story of the authors restoration of the Harrison clocks was described in book & film "Longitude".