184 G. F. BECKER ISOSTASY AND RADIOACTIVITY 



accuracy^ but yet accurate enough to establish a fact of great importance, 

 namely, that over the widely extended oceans of nearly uniform depth the 

 intensity of gravity is substantially the same as on continental plains. 

 Till Hecker's determinations were made, there was no assurance that this 

 was the case. Tlis results shoAv that the greater volume of continents is 

 compensated by their smaller density, and therefore that isostasy prevails 

 under the ocean as well as on those continental areas within wliich it ha.s 

 been tested. That improved methods of determining gravity at sea will 

 be evolved is scarcely to be doubted, but the most vital point at issue 

 seems to me to have been settled by Mr. Hecker's observations.^'' 



From his studies on deflection in the United States, Hayford got a 

 value of the ellipticity of the meridian which depends on the depth of 

 the level of isostatic compensation. If this is 120.9 kilometers, then the 

 reciprocal of the ellipticity so.found is 297.0 ± 0.5. One of the questions 

 still to be solved is whether the same value of the flattening will result 

 from similar surveys in other countries; or, in other words, whether the 

 depth of the level of compensation will be found constant. There seems 

 a possibility that it may vary with latitude,^^ and the data for the deter- 

 mination of this point already exist in the records of the geodetic surveys 

 of northern Europe, as Mr. 0. H. Tittmann informs me. Unfortunately 

 the subject of compensation has not there been methodically investigated. 

 This particular point is of the more interest since Mr. E. W. Brown's 

 recent researches on the moon^^ give for the flattening of the earth very 

 accordant values of about 1/294. Some means of reconciling so large a 

 difference must be found, and possibly it may be discovered in a variation 

 of the depth of compensation with latitude. 



Hayford, Bowie, and Helmert all regard gravity anomalies as repre- 

 senting real loads, positive or negative. For this view there seem to be 

 two good reasons: this assumption strains to the utmost the Fratt-Hay- 

 ford hypothesis and it also lends itself readily to computation. Of 

 course, they do not deny that the anomalies might be due wholly or in 

 part to irregular distributions of density; but this explanation does not 

 appeal to them as it does to Mr. G. K. Gilbert*" and, as T shall explain 

 presently, to me also. The geodesists are at least ' on safe groimd. 

 Messrs. Hayford and Bowie conclude that the average excess or deficiency 



^ See Helmert's luminous discusf5ion. Sltzungsber. k. Preuss. Akad. der Wlss., 1912, 

 Jan. to June, especially p. 309. 



^ To my thinking, it would be very strange if the depth at the pole should be the 

 same as at the equator, for it is difficult to conceive that the physical conditions to 

 which compensation is due can have lieen tho same at the axis of rolalinn and the 

 extreme periphery. 



•"""Science, vol. 40. 1914, p. .389. Vice-presidential address, B. A. A. S. 



«U. S. Geol. Surv. Prof. Paper, 8.5-C, 1913. 



