Febeuary 18, 1910] 



/SCIENCE 



279 



progress of triangulation, variation of latitude, 

 deflections of the zenith, gravity observations and 

 mean sea level determinations and leveling. 



Some of tlie interesting topics discussed were 

 mentioned, among which was the great progress 

 made in the Cape to Cairo triangulation; base 

 measurements with tapes, the use of which was 

 so ably defended by the Americans as against tlie 

 use of wire; the variation of latitude observations, 

 especial attention being given to the method for- 

 mulated by Dr. Ross and submitted by tlie speaker, 

 for observing latitude photographically at Gaitli- 

 ersburg, Md. Dr. Hecker reported briefly on the 

 gravity measures made in the Black Sea, with 

 special reference to getting data and of testing 

 the agreement between the theoretical effect of 

 the velocity of the ship when going in an easterly 

 or westerly direction on the observed intensity of 

 gravity. Baron Eotvos gave an account of in- 

 vestigations with his torsion balance or gravity 

 variometer, for determining the curvature of equi- 

 potential surfaces of the geoid. 



One of the most important papers read at the 

 conference was by Hayford on the reduction of 

 gravity observations, the main feature of his 

 method being that isostasy is taken into account, 

 and the topographic correction is applied for the 

 whole earth's surface. The methods and results 

 contained in this paper elicited the following 

 flattering comment from Dr. Helmert that " the 

 Americans were w be congratulated on having 

 introduced a new epoch in geodesy." 



Many courtesies were extended to the delegates 

 through Sir George Darwin acting as the repre- 

 sentative of Great Britain and the University of 

 Cambridge, and as a distinguished and hospitable 

 citizen. 



Some Apparent Variations of the Vertical Ob- 

 served at the Cheltenham Magnetic Observa- 

 tory: Mr. J. E. BUBBANK, of the Coast and 

 Geodetic Survey. 



The paper discussed some changes of level of 

 the piers on which the Omori seismograph has 

 been mounted at the Cheltenham Magnetic Ob- 

 servatory. Tlie instrument was first operated in 

 the variation observatory where there is no di- 

 urnal range of temperature and the annual range 

 is only about 2° to 3° C. 



With falling external temperature, as in cold 

 waves, the top of the W.-E. pier moved toward 

 the east and the top of the N.-S. pier toward the 

 south; with rising temperature there was reverse 

 movement. 



The W.-E. pier showed a distinct diurnal oscil- 



lation of level on all clear days. Shortly after 

 sunrise it began to tip toward the east, reaching 

 its maximum east deviation about 10 A.M., then 

 it tipped toward the west reaching its maximum 

 west deviation about 4 p.m., and then slowly re- 

 turned to its normal position. This diurnal oscil- 

 lation was superposed on the changes of level, due 

 to external temperature changes. This tilting of 

 the pier began at an earlier hour in summer than 

 in winter and the range varied greatly on different 

 days, depending apparently on the intensity of 

 the solar radiation. The range of motion was 

 greatest in winter and had a yearly average of 

 about one second of arc. The N.-S. pier did not 

 show any appreciable diurnal variation of level. 



These results are in good accord with similar 

 observations made at Potsdam and Wilhelrashaven, 

 Germany, both as regards the nature and magni- 

 tude of the diurnal oscillation of the level. 



In October, 1907, the seismograph was moved 

 to a new location on a massive concrete pier in a 

 small house about one hundred yards southeast of 

 its former location. When the external tempera- 

 ture rises this pier tips towards the southeast, 

 oppositely to the pier in the variation house. 

 There is a diurnal variation of level in the W.-E. 

 direction but no appreciable change in the N.-S. 

 direction. 



This oscillation begins as a tilt towards the 

 east about 10 to 11 a.m., and reaches a maximum 

 east deviation about 4 to 5 P.M., and then returns 

 slowly to the normal position; it appears only on 

 clear days. The range is about the same as in the 

 former location, but during clear winter weather 

 when the nights are very cold the combined effect 

 of the solar radiation and the external tempera- 

 ture changes may give an apparent oscillation as 

 great as three seconds of arc. In mild summer 

 weather when the temperature changes are small 

 the oscillation rarely exceeds a half second of are. 



Sudden heavy downpours of rain cause this con- 

 crete pier to tip towards the northeast by an 

 amount in some oases as great as three or four 

 seconds of arc. When the ground is very dry 

 before the rain, the pier receives a semi-permanent 

 set and does not recover its former position for 

 several days, if at all. When the ground is 

 already partly saturated the pier recovers its for- 

 mer position more rapidly. 



This tilting of the pier is undoubtedly local, as 

 it did not appear on the records obtained in the 

 variation house. 



R. L. Faeis, 

 Secretary 



