Chap. 8] 



MAGNETIC METHOD 



393 



rotation with varying proportions of axes and depths. They have also 

 given tabulations for determination of depths and dimensions from relative 

 amplitudes, from distances of positive and negative anomalies, and from 

 distances between the half-, quarter-, and tenth-value points. The for- 

 mulas for ellipsoids of revolutions are fairly involved and will not be 

 given here. 



2. Relation between magnetic and gravitational anomalies. Calculations 

 of anomalies due to induction in magnetic bodies have benefited greatly 

 from the fact that a relation exists between the magnetic and gravitational 

 attractions of such bodies. This makes it possible to apply torsion balance 



(b) (0 



Fig. 8-51. Influence of magnetic latitude on magnetic anomalies (after H. Haalck). 



interpretation formulas in connection with a simple conversion. Strictly 

 speaking, this procedure is applicable only to bodies of uniform magnetiza- 

 tion. Its use in the calculation of the anomalies of faults, dikes, slabs, 

 plates, and the like, involves approximations, neglects the influence of edges 

 and corners, and disregards the demagnetization. 



A theorem of Poisson states that the magnetic potential is proportional 

 to the gravity component in the direction of magnetization: 



kh di ' 



(8-60a) 



where V is the magnetic potential, d the intensity of magnetization, k the 

 gravitational constant (66-10"^ C.G.S.), 5 difference in density, U the 



