Obituary. 257 



of no error of sufficient magnitude to sensibly affect the result. 

 It seems also impossible to attribute this result to erroneous 

 observation. 



In order to reduce the density of this mountain to ordinary 

 rock density, which is considerably above that generally obtained 

 for volcanic mountains, it would be necessary to assume an error 

 in the period of the vibrating pendulum of ] : 20,000th part of 

 the whole. Now with our new pendulums and new methods of 

 determining the period of vibration we expect, even with the 

 swinging of but a single hour, to get a result which shall not be 

 out of the way — that is, as far as the mere period independent of 

 the clock error is concerned — more than one part in two millions 

 or two and a half millions. That the difficulty can hardly be 

 with the pendulums themselves is evident from the facts. 



First. That the results do not depend upon the gravitation 

 determinations made here in Washington but are relative merely, 

 swings having been made at the level of the sea in the neighbor- 

 hood of the mountain. 



Second. These pendulums, which had been very carefully vi- 

 brated before leaving for this expedition, were again swung at 

 our base station here on their return, and although more than a 

 year had elapsed, there was no sensible difference in the period of 

 the pendulums. 



I am bound to admit that I have great confidence in the obser- 

 vations themselves. Work which Mr. Preston did on this expe- 

 dition with these pendulums check very closely with my own 

 work at the same stations, and his previous determination in 

 which the long Peirce pendulums were used are also in very close 

 agreement with the later ones by the short pendulums. 



I do not, therefore, see how we can avoid the conclusion that 

 these results are real and that the mountain has an extraordinary 

 density. I should be very glad to hear anything you have to say 

 upon this problem and pai'ticularly any suggestion that you have 

 to make as to methods of testing the veracity of the conclusions 

 which we seem to have reached in this work. 



Obituary. 



Frederick Augustus Genth, the veteran mineralogist, died 

 at Philadelphia on the 2d of February in his seventy-third year. 

 Dr. Genth was born in Waechtersbach, Hesse-Cassel, on May 17, 

 1820. He early studied at Heidelberg, later under Leibig at 

 Giessen and under Bunsen at Marburg, where he received the 

 degree of Ph.D. in 1846. For three years he acted as assistant 

 to Professor Bunsen, and not long after he came to the United 

 States, where he has since resided. In 1872 he became Professor 

 of Chemistry and Mineralogy in the University of Pennsylvania. 

 He also held the office of Chemist to the Geological Survey of 

 Pennsylvania and to the Board of Agriculture. Dr. Genth was 

 an excellent chemist, a man of great industry and enthusiasm and 



