C.--ON A MACJNliTrC DISTU KBANCE IN THK SOlTH KAST 

 OF CAPE COLONY. 



By .1. C. Hkattik, D.8c. 



Fi'Kin tlu' ubservatiiins<,)f the \aiiuus magiietif t'leinents in ditt'ereiit 

 parts of tlu' Cape Colony carried out during the last ten years it has 

 been possible to detennine in many districts the effect of the local 

 circumstances on the magnetic state of the country. The method 

 emi>loved tt» determine the local effect is that first used by Thorpe and 

 Kiicker in their survey of the British Isles. It consists in first deter- 

 mining the mean effect for the district, and then calculating to what 

 extent the actual results obtained by observation at any station in the 

 district differ from this mean. In certain places the magnet points 

 more to the east, or to the west, than the same magnet woidd point 

 were it under the influence of the mean magnetic effect of the district 

 alone : oi- such a magnet may dip more or less than the mean value 

 would lead one to expect : or, again, the horizontal intensity of the 

 field may be greater or less than the mean effect. 



When such local effects are present the}- may be indicated in 

 other ways. For examj^le, it is customary to show the magnetic state of 

 a country by means of magnetic maps ; on these the places which ha\ e 



all the same value of a certain element are connected by a continuous 

 line which is called an isomagnetic line. When a disturbance is pre- 

 sent in a given district the isomagnetic lines are modified accordingly. 

 In the Eastern Province the elements which are most disturbed 

 are the horizontal intensity and the dip. The effect of the disturbance 

 is seen in the two maps appended. In the first the lines of equal 



