xii PREFACE TO THE FOURTH EDITION 



FOURTH EDITION 



In the four years which have elapsed since the 

 publication of the third edition of this book, 

 physicists have developed further the subjects 

 with which it deals, but no striking new branches 

 of knowledge have appeared. Hence it is possible 

 to re-issue the book, with some additions, but with 

 no fundamental changes of plan. 



\Zth January 1909. 



FIFTH EDITION 



The fifteen years which have passed since the 

 last edition of this book appeared have seen 

 great advances in many of the subjects described 

 therein, and two new discoveries of fundamental 

 importance — the Principle of Relativity and the 

 Quantum Theory. 



It has been necessary, therefore, completely 

 to revise the book ; many sections have been 

 re-written, and much new substance has been 

 added, in the attempt to give a fair account of 

 the latest development of physical science. 



22nd JU7tt 1924. 



