MAGNETIC SHOAL, WESTERN AUSTRALIA. 417 



Within the above — 



1. A line of maximum easterly repulsion over which 



the north-seeking end of the needle is violently 

 repelled to the east, in one place as much as 56°. 



2. A line of maximum westerly repulsion, over which 



the north-seeking- end of the needle is repelled to 

 the west, but only to the extent of one-half the 

 easterly repulsion. 



3. Between these two lines, which are from one to 



three cables apart, a line of no repulsion 2| 

 miles long, over which the needle points to the 

 true north, and the directive force is very small. 

 This is called the " axis " or " hue of vanished 

 repulsion." 



4. A point on this line about one mile from the south- 



west end of the magnetic shoal, where the 

 intensity is greatest, which is called the focus. 



The axis, or line of vanished repulsion, is inclined to the 

 true meridian at an angle of 56° in the neighbourhood of 

 the focus. This angle coincides with the amount of maxi- 

 mum easterly repulsion. 



A vessel passing in a straight line across the magnetic 

 shoal at the focus, on a north-westerly course, would find the 

 north-seeking end of the needle behave in the following 

 manner: — 



When about one mile from the focus a slight disturbance 

 would be observed, the north-seeking end of the needle 

 being repelled to the east ; but this disturbance would not 

 amount to more than half a point until she had run to 

 within 2i cables of the focus. The needle would then be 

 more ancl more repelled until 300 feet from the focus, when 

 it would be deflected as much as 56° from the true north. 

 It would then quickly resume its correct position, and over 

 the focus — for a hardly ajipreciable distance, say 10 feet — 

 would point to the true north. After passhig the focus it 

 would be repelled to the west ; and at 200 feet from the 

 focus would be deflected as much as 26° from the true north. 

 It would now begin to return again to its correct position, 

 and at three cables from the focus on the N.W. side would 

 not deviate from the normal more than half a point. At 

 one mile from the focus all signs of disturbance would 

 disappear. 



Crossing the shoal rectangularly elsewhere than at the 

 focus, similar but less powerful repulsion would be observed. 



