In the early days of humanity, only three divisions of time existed: days (the interval between successive sunrises), months (the interval between complete lunar orbits), and years (the interval between the start of one planting season and the start of the next). The first artificial division of time was the hour, probably established by the Egyptians during the fourth millennium b.c. Beginning at dawn and dusk, twelve hours each were given to night and day. Unfortunately, since the changing seasons cause the length of night and day to vary by several hours, the Egyptian hour was not really a fixed unit. In winter, for example, because nights are longer than days, twelve night "hours" would last longer than twelve day "hours." What was needed was a device that could measure time in regular, unvarying amounts. Toward this end, early scientists began the evolution...