Source of Terrestrial Magnetism. ] 5 



My first researches relative to a theory of magnetic force were 

 published in a communication consisting of Parts I. and II. 

 contained in the Numbers of the Philosophical Magazine for 

 January and February 1861. The account of the source of 

 terrestrial magnetism given in articles 27-30 of that commu- 

 nication agrees for the most part with the views I hold at 

 the present time, especially as regards the explanation in art. 

 27 of the generation of magnetic streams of the aether by the 

 impulses it receives from the component atoms of the earth in 

 their rotation about the earth's axis. Also I consider it to be 

 correctly asserted in art. 33 that no recognizable magnetic cur- 

 rent results either from the motion of the earth in its orbit, or 

 from that due to the motion of translation of the solar sys- 

 tem in space. This assertion, however, is partly dependent on 

 facts of observation, no independent theoretical reason being 

 there given for it. 



Some discussion of the essential principles of the hydrodyna- 

 mical theories of magnetic force, and of other physical forces, 

 will be found in the Philosophical Magazine for April 1861 

 (p. 250). 



Two papers in the Numbers of the Philosophical Magazine 

 for February and March 1863 treat (1) of the zodiacal light; 

 (2) of the analogous phenomenon of a uniform luminous arch 

 seen at night across the sky, and symmetrically disposed about 

 the ecliptic ; (3) of diffused nocturnal luminosity. These phe- 

 nomena are considered to give ocular evidence of the existence 

 of solar and terrestrial magnetic streams. 



In my work ' On the Principles of Mathematics and Physics ' 

 (published in 1869), the Theory of Magnetic Force occupies 

 pages 604-676, including, in pages 61 3-6 J 8, a theory of ter- 

 restrial magnetism. In discussing the source of the earth's 

 magnetism I have there maintained, in contradiction to an un- 

 sustained assertion in art. 28 of the before-cited communica- 

 tion, that the earth by its motion does not carry with it any sen- 

 sible amount of the aether. It is, however, to be said that no 

 distinct account is given in that work of the respective effects of 

 the orbital and rotatory motions of the earth in generating 

 magnetic streams. 



With the view of making the hydrodynamical theory of mag- 

 netism more complete, I subsequently went through a new dis- 

 cussion of its mathematical principles, which is published in the 

 Philosophical Magazine for June 1872. Lastly, in a work en- 

 titled "An Essay on the Mathematical Principles of Physics," 

 produced in the present year, I have brought under review the 

 whole of the hydrodynamical theory of magnetism, and in such 

 manner that as little use as possible is made of analytical symbols. 



