PREFACE TO THE FOURTH EDITION 



The present revision is made in the interest of accuracy of 

 details and to record and interpret such progress in the Science 

 as bears most directly upon the instruction of beginners. In 

 attempting these ends the author is greatly indebted to the 

 painstaking work of the co-reviser, Professor Paul S. Welch, 

 who is entirely innocent of any responsibility for the general 

 plan of the book, which is unchanged from former editions. 



T. W. Galloway. 

 New York City. 



