78 DONALD C. BARTON 
The observed variation of Z,, the vertical component of the earth’s 
magnetic field, in this report gives the horizontal variation of Z cor- 
rected for latitudinal variation and as of the time when the first 
observation was made at the primary base station. The observed 
variation of Z, is the effect of the variation of magnetic permeability 
in the earth’s crust, and mostly of the upper 10 kilometers. 
The numerical values of Ag and Az of this paper are not absolute 
values, but are relative to some convenient arbitrarily chosen value 
at some base station. 
The absolute value of gravity along the torsion-balance traverses 
can be calculated from the observed values of relative gravity of this 
paper (Fig. 5) by re-introducing the latitudinal effect and by tying 
those recorrected values to the known value of absolute gravity at 
Munich. To obtain the value of gravity at the surface, a correction 
for elevation would also have to be applied. 
The absolute value of Z along the magnetic traverses can be cal- 
culated from the observed values of this paper (Fig. 3) by re-intro- 
ducing the latitudinal effect and by tying those recorrected values to 
the known value of absolute Z, wherever known within the area. 
REGIONAL VARIATION OF Z, AND OF GRAVITY (g,) 
The regional variation of the vertical component of Z,, the vertical 
component of the earth’s magnetic field, is shown in Figure 4. The 
observed magnetic isogams are shown in Figure 3. The complex ir- 
regularity of their winding obscures their regional pattern. In Figure 
4, the 25 isogams have been smoothed out without regard to local 
anomalies. The smoothing is not difficult over most of the area; but 
from Ingolstadt westward along the Danube, the large local anomalies 
obscure the regional picture; and for the area from Ingolstadt west, 
it is a question whether the northern ends of the regional isogams 
should not show a curvature westward. 
The following characteristics of those isogams should be noticed: 
(z) their trend in general is north-northwest and south-southeast, ap- 
proximately parallel with the southwest edge of the Bohemian massif; 
(2) they curve toward the southwest in the southeastern part of the 
area, in front of Salzburg; (3) the spacing between the 575, 600, and 
625 isogams in the north half of their course is closer than normal; (4) 
the isogams in the southwest part of the area tend to ray into a west- 
northwest trend; (5) the failure of the northern ends of the 325-525 
isogams to begin to curve westward is not necessarily significant, 
as the large local anomalies and the lack of data on the north obscure 
the pattern of the regional isogams. 
600 
