220 



JOSEPH BARRELL 



duced at the station by each cylinder respectively?^ The results 

 are as follows: 



TABLE XI 



Vertical Attr.\ction in Dynes on One Gram at Station by Cylinder 22.8 Km. 



Thick, Density 0.00357, Equivalent in Mass to Thickness 



OF 100 Ft. at Density 2.67 



The results for radius 58.8km. show that masses of this size 

 situated near the bottom of the zone of compensation exert but a 

 fraction of the influence given by equivalent masses near the sur- 

 face. A balancing of light and heavy masses in a column of this 

 radius would give isostasy at the base and yet produce notable 

 anomalies. For radius 166.7 km. the importance of depth is 

 much diminished. For radius 1,190 km. it practically disappears. 

 This means that a wide regional variation in depth with plus and 

 minus departures from the uniform density, the light and heavy 

 layers balancing, would not produce anomalies provided, as stated, 

 there was isostatic equilibrium at the base. 



To give a somewhat extreme illustration; suppose that the 

 upper cyHnder, I, is 2 per cent lighter than the mean density of 



' The formula for making these computations was kindly worked out for me by 

 Professor H. S. Uhler, checking it as given by B. O. Pierce, Neii'tonian Potential Func- 

 tion, p. 8. It is as follows: 



in which 



F = force in dynes per gram. 



p = density, in this case =0.003, 57. 



7=constant of gravitation =0.000,000,066,58. 



a = radius of cylinder. 



c = distance on axis from station to top of cylinder. 



/!=depth of cylinder; in this case 22 .8 km. 

 For radii of 58.8 and 166.7 km. no correction need be made for curvature of the earth's surface. 

 For = 1190 km. an empirical correction was obtained by comparing the results with Hayford's 

 computations. 



The writer overlooked until later the fact that Hayford and Bowie also give this 

 formula with a different notation on p. 17 of their work. 



