PREFACE TO THE SIXTH EDITION 



THE alterations in this edition are unimportant, and 

 mostly verbal. I have been tempted to replace some 

 of the simple and well-tried methods for some more 

 recent ones. I do not think, however, that the latter 

 give results which are so much better than the old 

 ones as to justify our dropping them, if by so doing 

 we remove the examinations in question from those 

 which the keen practitioner may hope to carry out for 

 himself. 



December, 1920. 



