end of Chapter 11 (geophysical aspects). Once again we decided that, except for a brief mention, convection in unsaturated media had to be beyond the scope of this book. D. A. N.againenjoyedthehospitalityoftheDepartmentofMechanicalEngine- ing and Materials Science at Duke University while on Research and Study Leave from the University of Auckland, and both of those institutions again provided ?nancial support. We are grateful for comments from Graham Weir and Roger Young on a draft of Section 11.9, a topic on which we had much to learn. We also are grateful to a large number of people who provided us with preprints of their papers prior to publication. Other colleagues have improved our understanding of the subject of this book in ways too numerous to mention here. Once again we relied on the expertise and hard work of Linda Hayes for the preparation of the electronic version of our manuscript, and again the staff at the Engineering Library of Duke University made our search of the literature an enjoyable experience.