REPORT ON THE MAGNETISM OF THE EARTH. 115 



result at the corresponding places on the earth's surface, on 

 the supposition that such electric currents as I have supposed 

 are excited on contrary sides of the equator, in consequence of 

 different parts on the earth's surface becoming successively the 

 places of greatest heat, during its revolution upon its axis. 

 Should there be found in these results that accordance w^hich I 

 have reason to expect, there will, I think, be no doubt that the 

 diurnal deviation of the needle is due to electric currents excited 

 by the heat of the sun. 



I have already adverted to the hypotheses of two or more 

 poles, by means of which attempts have been made to explain 

 the phaenomena of terrestrial magnetism, and I may now re- 

 mark, that if we admit the existence of such poles, we must be 

 careful not to consider the magnetic meridians as great circles : 

 they are unquestionably curves of double curvature. Nor must 

 we consider these poles to be, like the poles of a magnet, cen- 

 tres of force not far removed from the surface. If such centres 

 of force exist for the whole surface of the earth, the experi- 

 mental determinations of the magnetic force at different places, 

 to which I shall shortly advert, at least show that they cannot 

 be far removed from the centre of figure. 



In the delineation of magnetic charts, more attention has 

 hitherto been paid to the Halleyan lines, or lines of equal varia- 

 tion, than to any others ; and I am not disposed to undervalue 

 charts where such lines alone are exhibited : to the navigator they 

 are of the greatest value ; but they throw little light on the phae- 

 nomena in general. If the meridians wei'e correctly represented, 

 they would at least indicate clearly their points of convergence, if 

 such in all cases exist ; but the lines that would be most likely to 

 guide us to a true theory of terrestrial magnetism, are the nor- 

 mals to the direction of the needle. If, as is highly probable, 

 the direction of the needle is due to electric currents circulating 

 either in the interior or near the surface of the earth, these 

 normals would represent the intersection of the planes of the 

 currents with the surface of the earth ; and, by their delineation, 

 we should have exhibited in one view the course of the currents 

 and the physical features by which that course may be modified, 

 so that any striking correspondences which may exist, would 

 be immediately seized, and lead to important conclusions. 

 Changes of temperature I consider to be the principal cause of 

 the diurnal changes in the direction of the needle : and if any 

 connexion exist between these electric currents and climate, 

 we are to expect that the curvature of these normal lines will 

 be influenced by the forms, the extent and direction of the con- 

 tinents or seas over which they pass, and also by the height, 



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