It's present everywhere but occupies no space. We can spend it, but we can't destroy it or even change it, and there's never any more or less of it. Everyone knows what it is and uses it every day, but no one has been able to define it. It's time, of course, an elusive concept that has helped unravel some of nature's greatest mysteries, yet remains a mystery itself. This volume, newly revised and updated, offers an unusually clear and accessible introduction to time -- its measurement, historic methods of timekeeping, the uses of time information, and the role of time in science and technology. Beginning with a discussion of the nature of time, natural clocks, the relation of time and frequency, and the role of time in navigation, the authors then proceed to a fascinating treatment of man-made clocks and watches -- from the sundials and waterclocks of ancient Egypt to today's amazingly precise atomic clocks -- accurate to within one second every 370,000 years Subsequent chapters offer detailed, yet accessible explanations of the continuing search for more uniform time; the application of time to energy, communication, and transportation; and the role of the world's official timekeepers. Finally, the authors look at time in the context of theoretical science and technology, showing how time has long been a crucial element in theories of the fundamental laws of nature and in astronomy, while improvements in the measurement of time have fostered major developments in the realm of physics. More than 300 drawings and cartoons enliven the pages of this fascinating, coherent, and comprehensive treatment of the age-old enigma of time.