PREFACE TO THE SIXTH EDITION. 



THIS edition has been carefully revised, and many of the 

 figures have been redrawn and improved. I owe special 

 thanks to Professor W. Le Conte Stevens of the Packer 

 Collegiate Institute, Brooklyn, N. Y.; and to Professor 

 Theodore Sterling of Kenyon College,, Ohio, for calling 

 my attention to errors printed in earlier editions: and I 

 repeat the statement I made in the preface to the fifth 

 edition, that a "text-book is a co-operative work, and sure 

 to be better the more that teachers and students co-operate 

 with the writer, or compiler, to make it what it should be." 



H. NEWELL MARTIN. 

 June 11, 1890. 



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