PREFACE. Vll 



Central Technical College, London, and Miss Patterson, of 

 the Manchester High School, for many suggestions and for 

 reading the proofs, and also one of his assistants, Mr. W. Hyde, 

 for preparing the figures and drawings. 



It is to be hoped that no important error has escaped notice. 

 Should, however, any reader detect any errors, the author will be 

 glad if he will be good enough to point them out. 



W. G. KHODES. 



TOWER CHAMBEBS, 



BROWN STREET, 



MANCHESTER, 



1902. 



