Chap. 8] 



MAGNETIC METHOD 



379 



of forces is virtually identical with the method just discussed. Under- 

 ground measurements are made by stretching a cord between two points, 

 A and B. The line AB may make the angle ^ with the normal meridian. 

 At each point the actual magnetic meridian referred to AB, and the 

 actual horizontal intensity are measured by using the Dahlblom sine arm. 

 This gives H and 7 and, thus, R (Fig. 8-43c). 



(a) 



(C) 



, -*(^) 



Af 



(9) 



Fig. 8-43. (a) Horizontal anomalous vector from X and Y. (6) Horizontal 

 anomalous vector from Ho , 5, and H. (c) Tiberg's "arrow" method, (d) Vertical 

 anomalous vector from Hp and Z. (e) Vertical anomalous vector from To , lo , T, 

 and I. (/) Total anomalous vector in isometric projection, {g) Total anomalous 

 vector in two-dimensional representation (Jenny). (A) Total anomalous vector in 

 two-dimensional representation. 



Vertical vectors may be plotted from horizontal and vertical intensity 

 observations with Schmidt balances or from inclination and total intensity 

 observations with the Hotchkiss superdip. If AH has been reduced to 

 the direction of the profile, the vertical disturbance vector, Sp = 

 V (AHp) + (AZ)^ is obtained from the construction indicated in Fig. 

 8-43c?. A positive AHp is plotted toward north, a negative toward south. 

 Inclination I and total intensity T observed with the Hotchkiss superdip 



