The study of stability aims at understanding the abrupt changes which are observed in fluid motions as the external parameters are varied. It is a demanding study, far from full grown, whose most interesting conclusions are recent. I have written a detailed account of those parts of the recent theory which I regard as established. Acknowledgements I started writing this book in 1967 at the invitation of Clifford Truesdell. It was to be a short work on the energy theory of stability and if I had stuck to that I would have finished the writing many years ago. The theory of stability has developed so rapidly since 1967 that the book I might then have written would now have a much too limited scope. I am grateful to Truesdell, not so much for the invitation to spend endless hours of writing and erasing, but for the generous way he has supported my efforts and encouraged me to higher standards of good work. I have tried to follow Truesdell's advice to write this work in a clear and uncomplicated style. This is not easy advice for a former sociologist to follow; if I have failed it is not due to a lack of urging by him or trying by me. My research during the years 1969-1970 was supported in part by a grant from the Guggenheim foundation to study in London.

St. Alphonsus writes: "a single bad book will be sufficient to cause the destruction of a monastery." Pope Pius XII wrote in 1947 at the beatification of Blessed Maria Goretti: "There rises to Our lips the cry of the Saviour: 'Woe to the world because of scandals ' (Matthew 18:7). Woe to those who consciously and deliberately spread corruption-in novels, newspapers, magazines, theaters, films, in a world of immodesty " We at St. Pius X Press are calling for a crusade of good books. We want to restore 1,000 old Catholic books to the market. We ask for your assistance and prayers. This book is a photographic reprint of the original The original has been inspected and many imperfections in the existing copy have been corrected. At Saint Pius X Press our goal is to remain faithful to the original in both photographic reproductions and in textual reproductions that are reprinted. Photographic reproductions are given a page by page inspection, whereas textual reproductions are proofread to correct any errors in reproduction.

This is a reproduction of a book published before 1923. This book may have occasional imperfections such as missing or blurred pages, poor pictures, errant marks, etc. that were either part of the original artifact, or were introduced by the scanning process. We believe this work is culturally important, and despite the imperfections, have elected to bring it back into print as part of our continuing commitment to the preservation of printed works worldwide. We appreciate your understanding of the imperfections in the preservation process, and hope you enjoy this valuable book.

St. Alphonsus writes: "a single bad book will be sufficient to cause the destruction of a monastery." Pope Pius XII wrote in 1947 at the beatification of Blessed Maria Goretti: "There rises to Our lips the cry of the Saviour: 'Woe to the world because of scandals ' (Matthew 18:7). Woe to those who consciously and deliberately spread corruption-in novels, newspapers, magazines, theaters, films, in a world of immodesty " We at St. Pius X Press are calling for a crusade of good books. We want to restore 1,000 old Catholic books to the market. We ask for your assistance and prayers. This book is a photographic reprint of the original The original has been inspected and many imperfections in the existing copy have been corrected. At Saint Pius X Press our goal is to remain faithful to the original in both photographic reproductions and in textual reproductions that are reprinted. Photographic reproductions are given a page by page inspection, whereas textual reproductions are proofread to correct any errors in reproduction.