Theory of Terrestrial Magnetism. 409 



observations with his electrometer, that all the phenomena 

 brought to light by atmospheric electricity, on a fine day, 

 could be produced by the sole agency of the earth having 

 a negative charge and without any charge in the air itself. 

 The "negative charge, therefore, required for our explanation 

 of the cause of terrestrial magnetism is sufficient to account 

 for all the ordinary phenomena of atmospheric electricity. 



In the preceding investigation we have supposed the electric 

 charge to be uniformly distributed over the earth, and so have 

 arrived at a law of magnetic intensity merely varying with the 

 latitude. But the sun and other members of the solar system 

 may very likely have potentials so different from that of the 

 earth that we can hardly conceive the amounts ; consequently 

 we should expect the static electric distribution of the earth 

 would undergo periodic changes corresponding in time with 

 those of the ocean-tides. But alteration in the static distribution 

 of electricity on the earth's surface means, as we have shown, 

 alteration in the law of magnetic intensity ; consequently we 

 should expect that this magnetic intensity would vary somewhat 

 as do the ocean-tides ; and this is known to be the case. But it 

 is also evident that, besides these regular changes, every time a 

 great mass of vapour is suddenly formed and condensed on the 

 earth, and whenever great changes are occurring in the solar 

 atmosphere whereby the lines of electrostatic induction from the 

 sun to the earth are altered, we should find corresponding 

 changes in terrestrial magnetism such as we now know as 

 magnetic storms. And not only this, but as the planets are 

 charged bodies, their motions relatively to the sun ought to 

 cause motions in the sun's atmosphere such that, for instance, 

 the allineation of a number of planets and the sun, or the near 

 approach of any planets, if the alligned or approached planets 

 have potentials nearer that of the sun than many of the other 

 bodies of the solar system, ought to diminish the storms in the 

 solar envelope, and ought to alter the electrostatic distribution 

 on the earth. But it has been shown that the near approach 

 of a planet to the sun both affects the sun's spots and terres- 

 trial magnetism. 



And, lastly, since the iron in the earth may, from its great 

 pressure, possess great coercive force, we should expect (as 

 we know to be the case) that magnetic changes would lag 

 behind the astronomical influences accompanying them. 



Addition, April 17th. — Since the reading of this paper be- 

 fore the Physical Society, several criticisms have appeared of 

 this proposed explanation of terrestrial magnetism. Some of 



