PREFACE TO THE THIRD EDITION. 



IT is a satisfaction to both author and publisher to 

 find that our book continues to meet with such appro- 

 bation that its second edition has been exhausted and 

 a third edition made necessary. 



We hope that it may be even better liked in its im- 

 proved form, which, though not in any way departing 

 from its original plan and purpose, has received care- 

 ful correction and such small additions as may make it 

 consistent with the best scientific thought of the day. 



JOSEPH MCFARLAND. 



PHILADELPHIA, PA., 

 November, 1916. 



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