vi PREFACE 



The care with which Professor Tait preserved the letters he received from 

 his scientific correspondents has enabled me greatly to enrich the pages of 

 the Memoir by the inclusion of letters from Sir William Rowan Hamilton, 

 Professor Cayley, Lord Kelvin, and Professor Clerk Maxwell. Introduced as 

 far as possible in its immediate setting, the correspondence brings out interesting 

 points of history, and shows how heartily all these great men helped one 

 another in their scientific investigations. It is much to be regretted that 

 Professor Tail's own letters to Clerk Maxwell are not now available. 



Professor Tail's foreign correspondence was carefully arranged and 

 annotated by Dr J. S. Mackay, to whom I am greally indebted for thus 

 enabling me rapidly to choose what was serviceable for the purposes of 

 the Memoir. 



Several of the old students having suggested that the controversy 

 between Professor Tait and Mr Herbert Spencer would prove interesling, 

 I have given ihe delails at some length. It seemed advisable to bring 

 the real points at issue clearly before the reader's mind, more especially 

 as Mr Spencer had given his own views at great length in a published 

 pamphlet and in the appendix to subsequent editions of his First Principles. 

 On looking into the matter I found myself forced to begin with what 

 preceded Professor Tail's share in the controversy ; and in ihis conneclion 

 I wish lo lhank Lord Justice Fletcher Moulton for his help in presenling 

 an accurate accounl of ihe stages of a lively debate which had its origin 

 in his review of Mr Spencer's Work. 



The original photograph of Professor Tait writing a note in his retiring- 

 room is the property of ihe Rev. L. O. Crilchley, M.A., who most willingly 

 granted the inclusion of the portrait in the present volume. To him also 

 special thanks are due. 



I wish also to record my thanks to ihe Editors of Nature, of the 

 Philosophical Magazine, and of ihe Badminton Magazine, for permission lo 

 reprinl articles contribuled by Professor Tait ; and to ihe Council of ihe 

 Royal Sociely of Edinburgh for certain diagrams and figures which have 

 been reproduced. 



CARGILL GILSTON KNOTT 



EDINBURGH UNIVERSITY 

 February 1911 



