May 1, 1914] 



SCIENCE 



645 



professors have been advanced to the grade 

 of associate professor : Carroll W. Doten, in 

 economics; A. A. Blancliard, in inorganic 

 chemistry; S. M. Gunn, in sanitary biology 

 and public health; A. T. Robinson, in English; 

 A. G. Woodman, in food analysis. The fol- 

 lowing instructors have been advanced to the 

 grade of assistant professors in the depart- 

 ments indicated: Charles W. Green, in elec- 

 trical engineering; Henry H. W. Keith, in 

 naval architecture; John F. Norton, in chem- 

 istry of sanitation; Joseph W. Phelan, in 

 inorganic chemistry; George W. Swett, in 

 machine design, Frederick H. Lahee in geol- 

 ogy. Assistants advanced to the grade of in- 

 structors are: Ralph G. Adams, in mechanical 

 engineering; Arthur E. Bellis and Charles L. 

 Burdick in theoretical chemistry; Edward A. 

 Ingham in biology; Norman Osann in elec- 

 trical engineering, and DeWitt M. Taylor, in 

 mechanical engineering. 



DISCUSSION AND COBBESPONDENCE 



INTERPRETATIONS OF THE ANOMALIES OF GRAVITY 



Under this title Mr. G. K. Gilbert discusses^ 

 the investigations of Messrs. Hayford and 

 Bowie^ (of the Coast and Geodetic Survey) 

 relating to terrestrial gravity, and its appli- 

 cation to observed earth movements by J. W. 

 Spencer.^ Any consideration of such impor- 

 tant new problems should be welcomed as they 

 tend to confirm previous results, or show their 

 weakness or the lack of information. 



Hayford and Bowie have stated that, for the 

 purpose of making computations, the earth's 



1 TJ. S. Geological Survey, Professional Paper 

 85-C, pp. 29-37. 



2 Hayford, J. F., and Bowie, "William, ' ' The 

 Effect of Topography and Isostatic Compensation, 

 upon the Intensity of Gravity," U. S. Coast and 

 Geodetic Survey Special Pub. No. 10, Washing- 

 ton, 1912; Bowie, William, "Effect of Topog- 

 raphy and Isostatic Compensation upon the Inten- 

 sity of Gravity," id.. Special Pub. No. 12, Wash- 

 ington, 1912. 



3 Spencer, J. W., ' ' Relationship between Ter- 

 restrial Gravity and Observed Earth Movements 

 of Eastern America," Am. Jour. Sci., 4th ser., 

 Vol. 35, pp. 561-573, 1913. 



crust is assumed to be in a state of perfect 

 isostacy. They show, contrary to Gilbert's 

 ideas on the subject, that while this is true for 

 the whole area of the United States, there are 

 large areas where the anomalies depart slightly 

 from the perfect balance and smaller areas 

 where the anomalies are considerable. This 

 last is the special feature of Spencer's applica- 

 tion of the anomalies of gravity to earth move- 

 ments. Thus, at Washington, the excess of 

 gravity is equal to 1,200 feet, while the defi- 

 ciency at Virginia Beach (160 miles distant) is 

 equivalent to a thickness of 1,600 feet of rock. 



Hayford and Bowie have found that the 

 topography is all compensated within a depth 

 of 122.2 kilometers below sea-level (although 

 they used 113.7 km. in their gravity computa- 

 tions) ; that is, the condition of stress at and 

 below the depth of compensation is isostatic, or 

 in other words " any element of the mass is 

 subject to equal pressures from all directions 

 as if it were a perfect fluid." Gilbert has mis- 

 construed their conception of this, for he 

 states " immobility at all depths below that of 

 compensation is either explicitly or implicitly 

 assumed by Hayford and Bowie." He also ap- 

 pears to take the view that even very small 

 areas are completely compensated, and that 

 much of the compensation in the vicinity of 

 the stations, with decided or large anomalies, is 

 located in the nucleus. This view is untenable 

 as shown by such anomalies as those of Wash- 

 ington and Virginia Beach, or still greater 

 ones between Olympia and Seattle. This com- 

 pensation, located in the nucleus, presupposes 

 very high rigidity, which is contrary to the 

 idea of complete local isostacy, which on the 

 other hand presupposes great plasticity. 



Gilbert discusses the causes of the anomalies, 

 favoring the one based upon the local varia- 

 tion of density of the column and heterogeneity 

 of the nucleus with a sub-crustal mobile layer, 

 which accounts for the isostacy. But all 

 materials of the earth's crust are mobile under 

 long-continued stress differences, yet there is 

 sufficient rigidity in the crust to sustain local 

 anomalies. 



In his discussion, Gilbert assumes that the 

 nucleus is composed of the same materials as 



