84 



Staff-Commander E. W. Creak. [Jan. 28, 



As an instance of unusual local disturbance in that part of the 

 British Islands, the Compass Hill in the island of Canna,* near Skye, 

 may be mentioned. Here the disturbance was sufficient to render a 

 compass useless, and every small movement of the observing instru- 

 ments gave different results. 



For Ascension and St. Helena, and some other islands situated far 

 from a continent, normal values of the magnetic elements have been 

 obtained from observations at sea, and the object of this paper is to 

 show how this may be done, and the resulting amount and nature of 

 the disturbances in the islands visited. 



The following values of the three magnetic elements observed at 

 eleven islands represent the data collected for this purpose. 



An inspection of them shows that they consist of a series of obser- 

 vations made on land, and which when made at different stations on 

 the same island give divergent results caused by some local magnetic 

 disturbance. 



In order to obtain undisturbed or normal values, observations made 

 on board ships, in which the amount of magnetic disturbance is 

 known, have been adopted. 



These normal values obtained on board ship are from observations 

 made with the ship's head placed on eight or sixteen azimuths 

 equally distributed round the horizon in the process called " swinging," 

 all effects of the iron of the ship being eliminated by the methods set 

 forth in the "Magnetical Instructions for H.M.S. ' Challenger.' " 



It is important to note that these swingings took place in the neigh- 

 bourhood of the island to be magnetically examined, and some small 

 corrections had to be applied for differences of geographical position 

 from the land stations. An example is given below of the method of 

 deriving the normal values of the magnetic elements for a position on 

 the green outside the Dockyard, Bermuda, f 



The observations have been divided into two sections — first, those 

 made on islands situated north of the magnetic equator, and in which 

 the local disturbances have been generally found to be due to an 

 excess of bluej magnetism above the normal ; secondly, those made on 

 islands situated south of the magnetic equator, in which the disturb- 

 ances are generally caused by an excess of red magnetism. 



Section I. — Islands situated North of the Magnetic Equator. 

 Bermuda Islands. 

 The Bermuda group is that on which a more complete series of 



* Topographically and magnetically examined by the late Captain Evans, R.N. 



f This position has been selected as the declination is there almost undisturbed. 

 It appears to be a suitable place for future observations. 



+ In a freely suspended magnet the north-seeking end has red magnetism, the 

 other end blue magnetism. 



