VI PREFACE TO FOURTH EDITION. 



been added, bringing into the volume subjects which were wholly 

 unknown at the writing of the last edition. We have endeavored to 

 present to our readers everything of importance done in the lines 

 treated of down to the close of the year 1901. If this volume meets 

 with the kind reception extended to its predecessors, its authors will 

 feel themselves amply repaid for the labor that they have placed 

 upon it. 



University op AIichigan, June 1, 1902. 



