July 7, 1916] 



SCIENCE 



9 



made by Canada, the last link in the tele- 

 graphic longitude circuit of the globe was 

 completed, and thus nearly all of the longi- 

 tude observations made in the world are 

 united into one great single system, accu- 

 rately correlated through this circuit. 



THE TRANSIT MICROMETER 



Among improvements made by the Sur- 

 vey to the instrumental equipment used in 

 astronomic work only one will be men- 

 tioned. This is the transit micrometer used 

 in the determination of time by stars at 

 meridian passage. Although the transit 

 micrometer had been in use at fixed ob- 

 servatories, it was not until the investiga- 

 tions made at the Coast and Geodetic Sur- 

 vey, in 1904, that its adaptability to por- 

 table transits was thoroughly proved. The 

 many tests it has had in actual field work 

 have shown for it many features of excel- 

 lence. With its use, the relative personal 

 equation between two observers is so small 

 as to be masked by the accidental errors of 

 observation and is certainly not more than 

 one tenth as large as the average using the 

 key. No interchange of observers is neces- 

 sary, and the time of the determination of 

 a difference of longitude is about one half 

 the time taken by the older method. 



THE FIGURE OP THE EARTH 



The very important problem of deter- 

 mining the shape and size of the earth is 

 probably the climax, from the scientific 

 point of view, in the geodetic work of the 

 Survey. 



Reference has already been made to the 

 use of the arcs of triangulation in deter- 

 mining the figure of the earth. When many 

 arcs, both meridional and latitudinal, are 

 all joined together on the same trigonomet- 

 ric and astronomic basis, the area method, 

 developed in the Coast and Geodetic Sur- 

 vey since about 1901, is, without doubt, far 



superior to the arc method. In it are all 

 of the features of the arc method, to which 

 many important new features are added. 

 Using the great system of triangulation in 

 the United States to furnish the area factor 

 and the many astronomical measures con- 

 nected with the system to furnish the curva- 

 ture factors, a value for the figure of the 

 earth was derived which is of a very high 

 degree of accuracy. The investigations and 

 results obtained in this work are noteworthy- 

 contributions to geodesy. Some of the 

 prominent features of this investigation are 

 shown in the wide area treated, the large 

 number of astronomic observations in- 

 volved, and the unusual methods of com- 

 putation used. Topographic irregularities 

 within 4,000 kilometers of each astronomic 

 station were considered, and account was 

 taken of possible distribution of density be- 

 neath the surface of the earth. These fea- 

 tures, together with the actual results ob- 

 tained, make this a monumental work. 



By a study of the station errors, or deflec- 

 tions in the verticals, which were developed 

 when the astronomical and geodetic meas- 

 ures were compared, evidence was brought 

 forth which established the fact that the 

 condition of isostasy exists in the earth — a 

 fact which is of interest and value to geod- 

 esy and geology. 



These studies of the figure of the earth 

 and isostasy have attracted the attention 

 of the scientific world. Dr. Woodward, the 

 distinguished geodesist, is authority for the 

 statement that the work done by the Coast 

 and Geodetic Survey on isostasy is the 

 greatest contribution to geodesy since the 

 time of Bessel and Gauss. 



GRAVITY MEASURES 



Another method of attacking this impor- 

 tant problem of the earth's shape and size 

 is by the use of the pendulum in the deter- 

 mination of gravity. The contribution of 



