1886.] 



On Local Magnetic Disturbance in Islands. 



83 



January 28, 1886. 



Professor STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



L " On Local Magnetic Disturbance in Islands situated far 

 from a Continent" By Staff-Commander E. W. Creak, 

 R.N., F.R.S., of the Admiralty Compass Department. 

 Received January 11, 1886. 



[Plate 1.] 



It has been known for many years past that in the islands of 

 St. Helena and Ascension observations of the three magnefic elements 

 made at different stations gave remarkably divergent results, caused 

 by some undefined local magnetic attraction. 



Thus in the observations at St. Helena,* carried out under the 

 auspices of the British Government in 1840-46, it was found that the 

 observed inclination and intensity differed considerably at the two 

 stations of Longwood and Sister's Walk:, 2 \ miles apart, and these 

 again differed from those made on board ships in the anchorage. 



This question of local magnetic disturbance is one which has 

 engaged the attention of observers on continents and islands adjacent 

 to them, in making magnetic surveys, when sometimes large areas 

 have been found affected, and in others but very small ones. In such 

 cases the normal values of the three magnetic elements have been 

 obtained either by calculation from observations made in supposed 

 undisturbed districts, or by graphic methods. 



Thus in discussing the results of the magnetic survey of Scotland, 

 made by the late Mr. Welsh in 1857-58,f Professor Balfour Stewart 

 obtained the values of the local disturbances in the islands of Skye 

 and Mull by calculating the normal lines of equal values of the 

 magnetic elements for the mainland of Scotland, and extending 

 beyond the adjacent islands on the west coast. 



* See pp. 60, 61, vol. i of Observations made at the Magnetical and Meteoro- 

 logical Observatory at St. Helena. Published under the superintendence of 

 Lt.-Colonel Sabine, 1847. 



t Magnetic Survey of Scotland. See Eeport of British Association, 1859. 



G 2 



