122 THIRD RKPOKT — lS3o. 



have not yet compared with these results of theory the numer- 

 ous observations made by Captain Foster, both in the northern 

 and in the southern hemispheres ; but it is my intention to do 

 this as soon as I can determine what correction ought to be 

 made for the differences of temperature at the several stations: 

 I do not, however, anticipate any very close accordance. 



In Captain Sabine's observations, the observed intensities, 

 compared with those deduced from the preceding formulas, are 

 in excess near the equator, and in defect near the pole ; and it 

 is not improbable that, as Mr. Barlow has suggested, this in- 

 crease of magnetic action near the equator above that which 

 the theory gives, is due to the higher temperature in the equa- 

 torial regions *. I am, however, disposed to assign even a more 

 powerful influence than this to difference of temperature ; for I 

 think it very possible, and indeed not improbable, that this may 

 be the primary cavise of the polarity of the earth, although its 

 influence may be much modified by other circumstances. At 

 the conclusion of the paper on the diurnal variation f, to which 

 I have already referred, I have suggested an experiment which 

 I think might throw much light on this subject. I have pro- 

 posed that a large copper sphere, of uniform thickness, should 

 be filled with bismuth, the two metals being in perfect contact 

 throvighout, and that experiments should be made with it simi- 

 lar to those which I had made with one of smaller dimensions, 

 but from which I was unable to obtain any very definite results, 

 in consequence of the want of uniformity in the thickness of the 

 copper and in the contact of the two metals. On heating the 

 equator of such a sphere, the parts round the poles being cooled 

 by caps of ice — which might not unaptly represent the polar ices, 

 — we may expect that currents of electricity would be excited ; 

 in which case the direction of those currents would decide whe- 

 ther the experiment wei'e illustrative of the principal phaenome- 

 non of terrestrial magnetism, or not. Should these currents of 

 electricity be in the direction of the meridians, — which is impro- 

 bable, since in this case opposing currents would meet at the 

 poles, and there would be no means of discharge for them, — I 

 think we might then conclude that the magnetism of the earth 

 cannot be due to the difference in the temperature of its polar 

 and equatorial regions ; but if, on the contrary, the currents 

 should be in a direction parallel to the equator,^ — in which case 

 their action upon a magnetized needle would be to urge it in 

 the direction of the meridians, — I should then say that, in order 

 to account for the terrestrial magnetic forces, and the diurnal 



* Edinburgh New Philosophical Journal, July 1 827. 

 f Philosophical Transaclions, 1827, p. 354. 



