1886.] 



On Local Magnetic Disturbance in Islands. 



87 



line of the diagram, the red ends of the declination and dipping 

 needles being attracted towards this focus with a force varying 

 according to the place of observation. 



Thns in the case of the declination at all stations situated to the 

 north-east of the focus, the westerly declinations are seen to be in 

 excess, and those to the south-west in defect of the normal value. 



In the inclination and vertical force a great increase of value may 

 be seen in passing from the seaward side of the islands towards the 

 area enclosed by the dotted line of the diagram. 



A portion of the disturbance just noticed may possibly be due to 

 the ferruginous nature of the soil at Bermuda;* but this does not 

 detract from the evidence just adduced of the existence of a strong 

 focus of blue magnetism about the position assigned to it in the 

 diagram. 



In the eastern extremity of the group there are also evidences of 

 local magnetic disturbance at the observing stations of Hen Island, 

 Button Island, and Fort Cunningham. 



Madeira. 





Declination. 



Inclination. 



Total force. 



Observer. 





16° 49' W. 



17 8 



18 25 



19 35 



20 33 

 20 33 



56° 14' N. 



55 12 



56 36 



8- 784 



9- 184 



9-49 



H.M.S. 

 " Challenger," 

 1873. 





Cliff west of Loo Bock . 





The above observations of declination, made near Funchal, on the 

 south side of the island, differ considerably (with one exception) from 

 the normal, 3° 44' being the greatest difference. 



There is also much disturbance in the inclination. In one position 

 visited by the officers of the " Challenger," and at 1 foot above the 

 ground, a value of 48° 46' was observed, and at the usual height of 

 the observing stand — 3 J feet — over the same spot, 56° 18'. At two 

 other positions 20 yards apart, the inclination differed 40'. 



The greatest difference in the values of the total force is a decrease 

 of 0'71 below the normal. 



These results point to the importance of adopting an uniform height 

 for the observing stand if comparable results are to be obtained. 



* See " Remarks on the Chemical Analyses of Samples of Soil from Bermuda." 

 By Genera] Sir J. H. Lefroy. Bermuda, 1873. 



