REPORT ON THE MAGNETISM OF THE EARTH. 127 



Whetliei- the direction of the needle may be influenced by 

 the electrical state of the clouds, is much more doubtful than 

 the influence of the aurora. I am not aware of any extended 

 series of observations made with a view to determine this point. 

 Having adjusted, in a particular manner, a needle between two 

 magnets, so that the directive force was considerably diminished, 

 I found that the changes in the positions of electric clouds was 

 accompanied by changes in the direction of the needle ; but, 

 although the observations indicate that the needle was thus 

 aflTected, they are of too limited a nature to draw any general 

 conclusion from*. Some observations of Captain Sir Everard 

 Home, however, indicate the same kind of influence. In a con- 

 versation which I had with him last year, having referred to the 

 effect I had observed to be produced by the sun's rays, of bring- 

 ing a vibrating needle to rest, it brought to his mind a similar 

 effect which he observed dui-ing a thunder-storm. He has fa- 

 voured me with his observations, and from these it appears 

 that, in two instances, a needle came sooner to rest during a 

 thunder-storm than it had previous or subsequent to it. The 

 arc at which the vibrations ceased to be counted is not recorded, 

 but the number of vibrations was reduced in one case from 100 

 to 40, and in another from £00 to 120. I have, in consequence 

 of these observations, requested Lieutenant Barnett of the Royal 

 Navy, who is engaged in the survey of the southern coast of the 

 Gulf of Mexico, to make similar observations, should he have 

 an opportunity; and as thunder-storms are so frequent, and of 

 such intensity on that coast, I think he may obtain some im- 

 portant results as connected with the influence of the electric 

 state of the atmosphere upon the vibrations and direction of 

 the needle. 



Upon a review of all the phaenomena of terrestrial magnetism, 

 and considering the intimate relation which has been established 

 between magnetism and electricity, by which it appears that, if 

 not identical, they are only different modifications of the same 

 J)rinciple, there can, I think, be little doubt that they are due 

 to electric currents circulating round the earth. How these 

 currents are excited, whether by heat, by the action of another 

 body, or in consequence of rotation, we are not at present able 

 to determine ; but however excited, they must, though not 

 wholly dependent upon them, be greatly modified by the phy- 

 sical constitution of the earth's surface. We are, therefore, not 

 to expect that symmetry in their course which would be the 



• Philosophical Transaction.t, 1823, p. 364. The arrangements which I have 

 just described for determining the influence of the aurora borealis are well 

 adapted for deciding this point. 



