viii PREFACE 



and ME. F. J. TANNEK, for suggestions and criticisms, and 

 the last-named gentleman for the aid he has freely given 

 me in the revision of the proof-sheets. I am also 

 indebted, indirectly, in many ways to my colleagues, 

 DR. MACFADYEN and MR. FOULERTON. My thanks are due 

 to MR. J. BARNARD and to MR. FRANK STRATTON respec- 

 tively for the photo-micrographs and original drawings, 

 while for the eight borrowed illustrations blocks have 

 been kindly lent by Messrs. BAIRD and TATLOCK, and 

 Messrs. SWIFT and SON. 



May 1898. 



