188 proceedings: philosophical society 



the transfer of the knife edges from the pendulum to the support 

 would introduce inaccuracies. Mr. Bowie stated that, on the average, 

 4 stations were occupied per month; the transfer of knife edges from 

 the pendulum to the support causes no sensible error. 



Vice-President Humphreys took the chair and Mr. L. J. Briggs then 

 presented an illustrated communication on Measurement of the acceler- 

 ation of gravity at sea. The different Methods which have been proposed 

 and used in the measurement of the acceleration of gravity were first 

 discussed. Two other methods, the first based on the current-balance 

 and the second on the viscosimeter, were suggested. The speaker 

 then presented a series of gravity measurements at sea from New York 

 to San Francisco via Panama with instruments of the type already 

 used in trans-Pacific measurements; in this type the pressure of an in- 

 closed mass of gas is balanced by a column of mercury of variable 

 height. The apparatus is so designed that the volume of gas is con- 

 stant at the time of making the observations and, since the temperature 

 is maintained constant by a bath of melting ice, the heights of the 

 mercury column at two stations are theoretically inversely proportional 

 to g at the two stations. The average probable error of the mean of 

 the readings of 3 instruments at base stations on the voyage from New 

 York to San Francisco was 1 part in 60,000. Apparent anomalies 

 were observed at sea on both sides of the Isthmus, off the coast of 

 Lower California, and off the California coast near San Francisco. 



Discussion: Mr. Swann stated that one should expect systematic 

 differences for observations during rough weather because of the effect 

 of centrifugal action on the mercury column and called attention to a 

 possible method for compensation. Mr. Bowie thought that the 

 results presented indicated a decided improvement in the accuracy of 

 determination at sea over previous work ; along the shallow waters 

 near the coast the errors doubtless would be greater than those due to 

 topography, but over the deep-sea areas the data obtained should be 

 valuable. Mr. Abbot asked how much time was required for ob- 

 servations at base stations. Mr. Curtis referred to the difficulties 

 experienced with reference to designation of units for gravity work. 

 Mr. White referred to possible improvements in details of contraction 

 of the apparatus. Mr. Briggs stated that he thought further improve- 

 ment could be effected by exercising greater precaution with reference 

 to the cleaning of the mercury and the apparatus before sealing. 



The 769th meeting was held on March 4, 1916, at the Cosmos Club: 

 President Briggs in the chair, 68 persons present. 



The evening was devoted to the address of the retiring President, 

 Mr. W. S. Eichelberger, The distances of the heavenly bodies. (This 

 Journal, pp. 161-175). 



J. A. Fleming, Secretary. 



