Source of Terrestrial Magnetism. 19 



since there is a preponderance of land in the northern hemi- 

 sphere, and the currents are always towards the positions of 

 greatest atom-density, that they will flow out of the northern 

 parts and, after circulating, enter into the southern parts. This 

 is uniformly taken to be the direction of the earth's currents in 

 my hydrodynamical theory of magnetism. 



Accordingly the positions of greatest efflux and influx, although 

 they cannot be far distant from the earth's poles, will be partly 

 dependent on the distribution of land and water. It is thus 

 consistent with the theory to find that in the northern hemi- 

 sphere there are two positions of maximum intensity — one, 

 which at present appears to be the principal one, in a 'high lati- 

 tude of the North- American continent, and the other a little 

 north of the Asiatic continent. There is not the same geogra- 

 phical reason for two positions of maximum influx in the south- 

 ern hemisphere; and, in fact, observation has hitherto only de- 

 termined that, if there be two, they cannot be far apart. Hence 

 the theory at once accounts for the circumstance that, of all po- 

 sitions on the earth's surface, the magnetic intensity is greatest 

 in the neighbourhood of the south pole. For the total influx is 

 necessarily equal to the total efflux ; and consequently the influx 

 is intensified in that quarter, either because there is but one 

 position of maximum intensity, or, if there be two, because they 

 are near each other. The foregoing comparisons of the theory 

 with facts of observation are confirmatory of the view taken of 

 the nature and source of terrestrial magnetism. 



It will be seen that this theory attributes the generation 

 of terrestrial magnetic streams altogether to impulses given to 

 the aether by the earth's atoms in motion, and that it would be 

 much corroborated by any independent evidence of the actuality 

 of this action between atoms and the aether. Now it happens 

 that such evidence is obtainable by reference to the explanation 

 of certain phenomena of aberration, which I have given in the 

 Number of the Philosophical Magazine for April 1872, inasmuch 

 as that explanation depends on an impulsive atomic action of the 

 very same kind as that which has been under consideration, as 

 will appear from the following argument. 



The aberration of light is simply due to the circumstance that 

 whereas the pointing of a telescope, as instrument ally determined, 

 is in the direction of the straight line joining the optical centre 

 of the object-glass, or object-mirror, and a certain fixed point in 

 the field of view, the actual course of a ray from the first point 

 to the other deviates from that line by reason of the earth's 

 movement in the interval occupied by the passage of the ray. The 

 deviation is in the direction towards which the earth is moving ; 

 and its angular amount is measured bv the ratio of the earth's 



C2 



