48 



SCIENCE. 



[N.S. Vol. XXI. No. 524. 



ceiving special attention. In the last- 

 named countries the junction and correla- 

 tion of the triangulation, which was for- 

 merly disjointed, makes it possible to take 

 up the study. Similarly in this country 

 the completion of the transcontinental arc 

 and its connection with the lake survey tri- 

 angulation furnished the opportunity and 

 occasion for adopting a standard datum of 

 geographical coordinates for the whole 

 country. This in turn furnished the de- 

 flections of the vertical referred to a com- 

 mon origin of coordinates on the same 

 spheroid and made it possible to begin the 

 study of the form of the geoid in this coun- 

 try over a very extended area. 



The investigation has so far been extend- 

 ed over the eastern part of the United 

 States. Here as elsewhere it was found 

 that the curves of elevation of the geoid 

 above the spheroid reflect perceptibly the 

 visible topographic features. 



A preliminary statement of the scope of 

 these investigations was recently given be- 

 fore the International Geographic Congress 

 by Mr. Ilayford, the chief of the computing 

 division of the Coast and Geodetic Survey. 

 From it I quote as follows: 



The conclusion that for the eastern lialf of the 

 United States and the adjacent portion of the 

 Atlantic the theory of isostasy is true to a con- 

 siderable extent is reasonably safe. The conclu- 

 sion that the depth within which the isostatic 

 comjjcnsation takes place is 205 miles is one 

 which may be modified considerably as the inves- 

 tigation ])roceeds. 



The investigation thus far leaves the signs of 

 the corrections to the constants of the Clarke 

 spheroid of 18GG uncertain. 



Mr. Ilayford will give before this meet- 

 ing an account of the method devised by 

 him of computing the topographic correc- 

 tion. The task of computing this correc- 

 tion to a distance of 4,000 kilometers for 

 each of say 500 stations has been rendered 

 possible by this method, which is, therefore, 

 referred to by me as a distinct advance in 

 peode<:y. 



It is hoped that the completion of the 

 study of the data now available in regard 

 to the deflections will serve as a guide to 

 the most effective use in the future of the 

 pendulum, and it is on this account largely 

 that pendulum observations have been for 

 the present deferred by the coast survey. 

 They are, however, being actively made by 

 other nations. 



A new impetus was given to relative 

 gravity observations by the adoption of 

 short and light pendulums in place of the 

 heavy seconds' pendulum. Aside from 

 their portability, their lightness insures 

 greater invariability for the knife edges, 

 simplifies the task of securing uniformity 

 of temperature and pressure in the metal 

 cases in which they are swung, and the ease 

 with which a low and constant pressure 

 can be maintained in the case insures the 

 continuance of the swing through so long 

 a period that the errors of the chronometer 

 or other timepiece are eliminated. Thanks 

 to the efi'orts of the International Geodetic 

 Association, the widely scattered base sta- 

 tions have been connected with the central 

 station of the as.sociation at Potsdam, where 

 a long series of absolute gravity determina- 

 tions were brought to a successful conclu- 

 sion two years ago. The association has 

 available now the data from nearly 1,800 

 stations scattered over various parts of the 

 globe. A most interesting and valuable 

 extension of relative gravity measures to 

 the surface of the ocean was made two 

 years ago. The principle upon which the 

 new method depends is that if the atmos- 

 pheric pressure is determined at the same 

 time and place by means of a mercurial 

 barometer on the one hand and by the 

 temperature of the boiling point of water 

 on the other, the observed height of the ba- 

 rometer will be affected by gravity at the 

 place, while the result by the hypsometi'v 

 will be independent of it. According to 

 Dr. IT(>e]<er, who carried out the laboratory 



