PREFACE. 



If there prove to be anything of permanent value in these investigations, I 

 desire that they may be remembered in connection with the name of the late 

 William Thaw, whose generosity provided the principal means for them. 



I have to thank the board of direction of the Bache fund of the National 

 Academy of Sciences for their aid, and also the trustees of the Western Uni- 

 versity of Pennsylvania for their permission to use the means of the observatory 

 under their charge in contributing to the same end, and I desire to acknowledge 

 especially the constant and valued help of Mr. Frank W. Very, who has assisted 

 me in all these experiments, and my further obligation to Mr. George E. Curtis, 

 who has most efficiently aided me in the final computations and reductions. 



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