92, 



On Local Magnetic Disturbance in Islands. [Jan. 28, 



The above observations, like those at St. Helena and Ascension, 

 show the existence of an excess of red magnetism above the normal 

 from the increased values of the inclination and total force. 



The general results of the observations just discussed is to show 

 that in islands far from a continent and north of the magnetic 

 equator, the local disturbances of the three magnetic elements are 

 caused by an excess of blue magnetism above the normal values due to 

 the position of the islands on the earth considered as a magnet. 

 South of the magnetic equator red magnetism is in like manner pre- 

 dominant. 



Considering, however, that the observations were made with the 

 instruments between 3 and 4 feet above the ground, the disturbances 

 are not large. 



As an instance of large disturbance the results obtained at the 

 bluff, Bluff Harbour, in the South Island, New Zealand, may be 

 mentioned. In 1857, during the land survey by the local government 

 officials, the following values of the declination were observed.* 



On the summit of the bluff 6° 54' E. 



30 feet north of the same position 9 36 W. 



„ west „ , 5 04 E. 



„ east „ 46 44 E. 



Normal from sea observations i6 20 E. 



On the summit of the bluff there was thus shown to be a strong 

 focus of red magnetism. 



During the survey of the South Island by the officers of H.M.S. 

 "Acheron," it was found necessary to give up the use of compass- 

 bearings at this place, and adopt the plan of observing nothing but 

 true bearings. 



The evidences of local magnetic disturbance form a great difficulty 

 in estimating the values of the secular change in these islands for 

 past years. For example, Madeira may be mentioned, where it has 

 been seen that a change in position of a few feet gave very different 

 results of the inclination, and even at the same position the height at 

 the observing instrument above the ground must be taken into con- 

 sideration if comparable results are to be obtained. 



Before concluding this paper, I desire to draw attention to the 

 following remarks on this subject of local magnetic disturbance. 



Firstly, reasons have been given for believing " that terrestrial 

 magnetism is not produced in any important degree by magnetic 

 forces external to the earth. "f 



* " Transactions New Zealand Institute," 1873, vol. vi, p. 7. 

 f See "Treatise on Magnetism," p. 100, par. 43, by Sir Gr. B. Airy, K.C.B., 

 Ast. Royal, 1870. 



