556 proceedings: philosophical society 



stated that some redeterminations of elevations for old Coast and 

 Geodetic Survey marks by the New York City survey indicated no differ- 

 ences with time for stations set on solid rock; for one at Sandy Hook 

 a subsidence of 0.35 foot is indicated; there is probably no error on 

 account temperature changes because of constant conditions at slight 

 depth and rapidity with which work is done. 



The 755th meeting was held at the Cosmos Club, March 27, 1915, 

 President Eichelberger in the chair; 62 persons present. 



Mr. C. G. Abbot spoke on Recent progress in astronomy . The speaker 

 summarized briefly results of recent discussion placing total number 

 of stars between 1 and 2 million, and determinations of proper motions 

 and stellar drifts, reviewing particularly the work of Boss. Attention 

 was called to work on star spectra by Pickering and the types of spectra; 

 the excellent agreement for position of apex of solar motion by measure- 

 ments from displacement of spectrum lines by Pickering and from 

 Boss' discussion of star groups was pointed out. A brief review of our 

 knowledge of star distances and the methods for determining them 

 was given. 



Mr. L. J. Briggs then presented an illustrated paper on A new method 

 for measuring gravity at sea. The necessit}^ of measuring the boiling- 

 point of water in connection with barometer readings in determining 

 gravity at sea can be eliminated by the use of an apparatus in principle 

 similar to a closed barometer, maintained at constant temperature. 

 This was the method employed, the apparatus in addition being so de- 

 signed that the enclosed mass of gas supporting the mercury column 

 always occupied the same volume at the time of making the observa- 

 tions. This avoids the necessity of measuring volume changes, and 

 doubles the sensitiveness of the apparatus. The apparatus was con- 

 structed of glass and consisted of a capillary mercurial column to reduce 

 pumping, which expanded at the top into an evacuated spherical bulb, 

 while the lower end of the column opened below a mercury surface in 

 the pressure chamber. The evacuated bulb contained at its center a 

 fixed reference point. Above the air chamljer the capillary was bent into 

 a zigzag spring which allowed a small vertical adjustment of the ol)- 

 serving bulb, controlled by a micrometer screw. In making an observa- 

 tion the bulb was so adjusted that the mercury surface was barely in 

 contact with the fixed point. This confined the gas always to the same 

 volume. The height of column was then read from the micrometer 

 head. This arrangement avoids the necessity of any measurement of 

 the position of the lower mercury surface. The whole apparatus can 

 thus be kept surrounded by melting ice, which was used to maintain 

 constant temperature. On board ship, the apparatus was swung on 

 gimbals to secure verticalitj^, and suspended from spiral springs to 

 reduce vibration. Observations were made daily between Tahiti and 

 San Francisco. The results indicate a slight excess of mass in this part 

 of the ocean, particularly in latitude 16° to 24° north, and longitude 

 130° to 135° west. The paper was discussed by Messrs. Abbot, Mar- 



