Apeil 13, 1917] 



SCIENCE 



353 



have negative anomalies, that is, for the inten- 

 sity of gravity to be below normal at these 

 stations. On the other hand, for stations in 

 the United States on pre-Cambrian forma- 

 tions there is a decided tendency toward posi- 

 tive anomalies, that is, for gravity to be in 

 excess. The evidence of a tendency toward 

 defective gravity in regions in which the sur- 

 face geologic formations are comparatively 

 recent, and toward excessive gravity where 

 old geologic formations occur at the surface, 

 is full of inconsistencies, and is weak except 

 for the two extremes, the Cenozoic and the 

 pre-Cambrian. It is very suggestive that as 

 the number of stations considered was in- 

 creased the evidence has become more clear 

 and convincing. The writer believes that the 

 apparent relation cited is real and that it is 

 important to determine, if possible, the true 

 reason why the relation exists. At this, as at 

 many other points, Mr. Bowie is properly 

 cautious in interpretation, though vigorous 

 in presenting the facts and in calling attention 

 to the evidence of relations between the facts. 

 Mr. Bowie examined fourteen pairs of grav- 

 ity stations in which the two stations of each 

 pair are at very different elevations, although 

 they are separated by a comparatively short 

 distance in a horizontal sense. In one typical 

 pair, one station, Yavapai, Arizona, is on the 

 edge of the Grand Canyon of the Colorado, 

 and the other station of the pair, although 

 only 2,600 meters away in the horizontal sense, 

 is 1,330 meters lower, in the bottom of the 

 canyon. It is found that in thirteen of the 

 fourteen cases the anomalies remaining on the 

 Coast and Geodetic Survey basis of computa- 

 tion are such that if the anomaly at the lower 

 station is subtracted from the anomaly at the 

 higher station the remainder is positive. The 

 average of the fourteen differences is -\- .014 

 dyne. This seemed to indicate that there is 

 some error in the formula used to compute 

 the correction to gravity for elevation. But 

 " a careful study of the matter showed no 

 error in the formula, but it seemed to indicate 

 that the difference in the anomalies could re- 

 sult from the combination of several causes 



no one of which could alone make the differ- 

 ence." 



Mr. Bowie has made an extensive, ingenious 

 and careful study to determine from gravity 

 observations the most probable depth of com- 

 pensation. The methods of study and the re- 

 sults are fully set forth in the publication. 

 Much difficulty was encountered arising from 

 the intimate manner in which this unknown 

 quantity is unavoidably entangled with other 

 unknowns in the computations, and from the 

 non-sensitiveness of the computed results to 

 an assumed change of depth. There is also 

 some evidence of systematic errors aifecting 

 the computed depth. ISTevertheless Mr. Bowie 

 has succeeded in getting from the gravity ob- 

 servations an independent determination of 

 the depth of compensation. His conclusion is 

 that the most probable value of the depth of 

 compensation is 96 kilometers and that future 

 values " derived from much more extensive 

 data will fall between 80 and 130 kilometers." 

 Having a keen realization of the difficulties of 

 determining the depth of compensation, the 

 writer recognizes this as a welcome confirma- 

 tion of the independent and stronger deter- 

 mination of the depth by means of observed 

 deflections of the vertical. The conclusion 

 from the deflections of the vertical in 1910 

 was that the most probable value of the depth 

 of compensation is 122 kilometers and that it 

 is practically certain that it lies between 100 

 and 140 kilometers.^ 



Both these computations of the limiting 

 depth of compensation are based upon the as- 

 STmiption that the compensation is uniformly 

 distributed with respect to depth. In each 

 case also the value is believed to be the aver- 

 age depth, it being conceded that the depth 

 may possibly be different in different regions. 



Mr. Bowie has computed the reciprocal of 

 the flattening of the earth from gravity ob- 

 servations at 348 stations, of which 216 are 

 in the United States and the remainder in 

 Canada, India and Europe, and finds it to be 



3 ' ' Supplementary Investigation in 1909 of the 

 Figure of the Earth and Isostasy, ' ' by John F. 

 Hayford, published by the Coast and Geodetic 

 Survey, p. 77. 



