PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 768th meeting was held on February 19, 1916, at the Cosmos 

 Club; President Briggs in the chair, 41 persons present. The minutes 

 of the 767th meeting were read in abstract and approved. 



Mr. William Bowie presented an illustrated paper on Determi- 

 nation of the intensity of gravity on land in the United States. Numerous 

 attempts have been made to determine accurately the absolute gravity 

 by the reversible pendulum, but the results were not very satisfactory 

 for there were a number of errors which entered into the determination. 

 The principal ones were probably in the determination of the distances 

 between the two knife edges on the pendulum, the flexure of the pen- 

 dulum support, and the temperature changes. Later attempts were 

 made by using the invariable pendulums which were swung only in 

 the direct position, the period being determined at the base station and 

 then at the new station. Since the length of the pendulum is invariable, 

 the difference in gravity at the two stations could be obtained by a 

 simple formula involving only the two periods. Because of the great 

 length of the second pendulum used at first, great accuracy was not 

 obtained from the observations. A great step forward was made when 

 Baron von Sterneck of Vienna designed and constructed the half- 

 second invariable pendulum which he swung in a closed case from which 

 the air had been almost entirely exhausted. These pendulums give a 

 high degree of accuracy in the determination of the relative intensity of 

 gravity at any two stations. All the gravity determinations in the United 

 States during the last 25 years have been made with the Mendenhall 

 pendulum, which is a modification of the von Sterneck pendulum, 

 and the results have been most satisfactory. The value of the intensity 

 of gravity at each station in the United States has been corrected for 

 topography and isostatic compensation, as well as for elevation above 

 sea level. The resulting anomalies (the differences between the ob- 

 served and computed values) are small in comparison with the anomalies 

 obtained by the older methods which are not based upon the theory of 

 isostacy. 



Discussion: Mr. Swann asked whether the proposed use of invar 

 in the construction of the pendulum would not require correction for 

 magnetization. Mr. Humphreys stated that some invars were practi- 

 cally non-magnetic. Mr. Abbot asked how much time was required to 

 make a satisfactory determination. Mr. C. A. Briggs thought that 



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