Coast Lines and Magnetic Declination. 1^5 



march over Europe ; — the effect of the intersection of the 

 land areas by the basin of the Mediterranean and other 

 great inland depressions, extending more or less parallel to 

 the equator over 6o° of longitude. The polarising effect 

 of the arctic coast line appears in the small amount of the 

 declination at St Petersburg, which has not varied more 

 than 8° during the last 150 years. The polarisation of the 

 coast lines is again seen on the chart at the shoulder of the 

 South Amerian continent at Pernambuco, where the lines 

 bend upwards towards the polar axis, and resume their 

 westerly direction in the Carribean Sea and in the basin of 

 the North Atlantic. Strong polarising effects, to diminish 

 and retard the declination, are also produced by the 

 longitudinal coast lines of the Gulf of Mexico, the West 

 Indian Islands, the north and south coasts of Australia, the 

 great coast lines within the Antarctic circle, the Malayan 

 Archipelago and the southern coasts of India and China. 

 So great is the polarity of the West India Islands, that 

 the secular change of the declination at Jamaica and Cuba 

 has not amounted to more than 3 during the last 200 

 years. From a comparison of the zero lines of declination 

 of the internal electro-dynamic sphere in relation to the 

 earth's axis and to the zero lines on the terrestrial surface, 

 it will be seen that the appellation of magnetic poles and 

 zeros of declination applies with strictness only to the poles 

 of the earth's axis, and to the poles and meridians of the 

 internal electro-dynamic sphere, as the zeros of declination 

 on the earth's surface, for the present epoch, are generally 

 the resultants of the changing electro-dynamic and 

 permanent magnetic forces acting through and upon the 

 outer crust of the terrestrial globe. 



An interesting instance in confirmation of my views and 

 experiments on the polarising action of longitudinal coast 

 lines which I desire to bring before the Society, on account 

 of its importance to practical navigation, was brought to 



