182 G. P. BECKER ISOSTASY AND RADIOACTIVITY 



isostasy at 120 kilometers and yet produce a very considerable anomaly 

 at the surface. If this batholith were 4^4 miles in radius, as in a previous 

 example, and only just buried, it would increase the attraction by 0.094 

 dynes, which is almost exactly the largest anomaly detected in the United 

 States, the mean anomaly being only 0.01 T dyne. 



Hayford and Bowie compare their anomalies with what they call the 

 Bouguer and free-air anomalies. Of these the former corresponds to 

 solution B and implies that there is no compensation, or that the earth 

 is infinitely rigid. The free-air anomalies answer to solution A and 

 imply that topographic forms exert no attraction. Comparison shows 

 that the new method anomalies are only a fourth as great as the Bouguer 

 anomalies if all stations are considered, whereas if only mountainous 

 regions (whei'e the Bouguer correction is of great moment) are included 

 this is twelve times as large as the Hayford and Bowie anomaly. As 

 has long been known, total neglect of the attraction of mountains gives 

 better results than their consideration by Bouguer's methods; but in 

 mountainous areas the new method anomalies are only a third as great 

 as the free-air anomalies. 



Hayford's investigations on deflection were confined to the United 

 States. Hayford and Bowie's study of the intensity of gravity included 

 89 stations in the United States, besides 16 selected stations outside of 

 that area, and scattered all over the world, seven of them being at sea 

 and having been occupied by Mr. 0. Hecker, of whose work more will be 

 said presently. In Switzerland the method developed by Hayford and 

 Bowie has been applied to 13 stations. This is too small a number to 

 give strong evidence, but, so far as it has gone, the investigation indicates 

 an isostatic condition in that country of mountains and a mean anomaly 

 scarcely differing from that in the United States.^^ Mr. Bowie has also 

 discussed 14 gravity stations in British India. ^- The number is again 

 very small, but the results tend strongly to confirm those obtained in this 

 country. 



That eminent veteran in geodesy, Mr. F. R. Helmert, has not only 

 expressed the warmest interest in the American investigations on isostasy, 

 but has made very important contributions of his own to the subject. 

 While Hayford determined the level of isostatic compensation from de- 

 flections of the vertical alone, Helmert devised a method of finding this 

 level from observations on the intensity of gravity without reference to 

 deflections, the numerical result being substantially the same. This ac- 

 cordance is most gratifying. A conclusion i-eached in only one way can 



2^ Hayford and Bowie, p. 122. 



32 Jour. Wash. Acad., vol. 4, 1911, p. 245. 



