Apart from the provision of an index of names, the main changes in this edition arc in the Notes at the end of each ehaptcr. These have been revised to include references to results published since the third edition went to press and to correct omissions. There are simpler proofs of Theorems 234, 352. and 357 and a new Theorem 272. The Postscript to the third edition now takes its proper place as part of Chapter XX. I am indebted to several correspondents who suggested improvements and corrcctiona.