PREFACE volume completes the collection of my Father's published papers. The two last papers (JSTos. 445 and 446) were left ready for the press but were not sent to any channel of publication until after the Author's death. Mr W. F. Seclgwick, late Scholar of Trinity College, Cambridge, who had done valuable service in sending corrections of my Father's writings during his lifetime, kindly consented to examine the proofs of the later papers of this volume [No. 399 onwards] which had not been printed off at the time of the Author's death. He has done this very thoroughly, checking the numerical calculations other than those embodied in tables, and supplying footnotes to elucidate doubtful or obscure points in the text. These notes are enclosed in square brackets [ ] and signed W. F. S. It has not been thought necessary to notice minor corrections. EAYLEIGH. Sept. 1920.