262 
MR. A. W. RUCKER AND DR, T. E. THORPE ON A MAGNETIC 
rotated about A in the limiting cases when they coincide with AE or AN, the 
distorted curves will assume the forms shown in Figures 3, and 4, or 5. 
-Terrestrial Isogonals running ai^proximately north and south. 
-Ti'ue Isogonals produced by (Fig. 4) a weak, and (Fig. 5) a strong centre of attraction at A. 
Fig. 6. 
-Terrestrial Isogonals approximately parallel. 
- True Isogonals produced by a centre of attraction at A. 
It is, of course, quite possible that a single closed curve may sometimes be formed. 
An underground attracting surface, rising up steeply on the eastern side and falling 
away very gently to the west, might cause a maximum declination on the east 
without any corresponding minimum on the west; but in general the normal comple¬ 
ment of a closed isogonal is another of the same type. 
The curves shown in figs. 3 to 6 will not be realised in practice, but their main 
