and by the Librarians and Staffs of the University and the Public Libraries at Southampton. Finally, we wish to thank Mrs H. G. Jerrard and Miss A. J. Tutte for typing the manuscript. Department of Physics H. G. JERRARD D. B. McNEILL University of Southampton 1963 Preface to the fifth ed ition Since the publication of the fourth edition in 1980 advances in technology have led to more precise values of the fundamental physical constants and a movement towards definitions of the fundamental units of mass, length and time based on atomic parameters. More precise definitions of some other units such as the candela have been approved by the international committees. These changes, together with the definitions of several new units have been included in this edition, the text of which has been revised and which now contains over 850 units and dimensionless numbers. The authors wish to thank all those who have helped in this latest compilation by suggestion and kindly criticism and Margaret Wainwright who has had the difficult and tedious task oftyping, retyping and copying the fragmented parts that arise from a text revision. At the time of going to press we believe this book to provide the most complete and up-to-date information of its kind available.