Evidence that the stones of Stonehenge (and some 900 other circles of stones found in Britain) were used to clock the seasons and later, the tides, is convincingly brought forth in this scholarly and articulate summary. The author, Deputy Director of the Admiralty Surface Weapons Establishment and a member of the Royal Archaeological Institute, clearly combines mathematical knowledge with the archaeological findings to build a case for the geometrical and scientific prowess of Neolithic and early Bronze Age peoples, He shows how simple empirical methods could have been used to construct circles, ellipses, or combinations thereof, and how land could be leveled with considerable accuracy. Particular stones were positioned to catch the sun or moon at first rising or last descent. Sometimes the stones would be aligned with a horizon landmark--a notch or slope in a hill--thus achieving greater accuracy. The sites would have religious significance, and may even have been presided over by an elite priestly class. But the henges served the practical purposes of early farmers and seagoers. All these ideas were scandal and heresy in the Sixties. Gradually the work of innovators, especially that of Alexander Thom, an engineer equipped with a variety of statistical arguments, lent credibility Fred Hoyle contributed further support with the hypothesis that certain holes at Stonehenge could be used to predict eclipses. Today most authorities accept that the henges and other megalithic sites were used lo calculate summer or winter solstices or spring and fall equinoxes. The more avant-garde suggest that successive groups around the third millennium B.C. devised monthly calendars, used a standard of measurement (the megalithic yard--0.83 meter), and could predict eclipses and tides. These are exhilarating ideas and in harmony with a growing climate of opinion that while human knowledge has accumulated, human intelligence has not altered Ancient man had the brains and modern man is beginning to figure out what he did with them. Hear! Hear!--and high marks to Dr. Wood.