XV. ON THE DIEECT MAGNETIC INFLUENCE OF A DIS- 

 TANT LUMINARY UPON THE DIURNAL VARIATIONS 

 OF THE MAGNETIC FORCE AT THE EARTH'S SUR- 

 FACE. 



Philosophical Magazine, March, 1858. 



IT has been usual to ascribe the ordinary diurnal variations of the 

 terrestrial magnetic force to solar heat, whether operating directly 

 upon the magnetism of the earth, or generating thermo-electric 

 currents in its crust. The credit of these hypotheses has been 

 somewhat weakened by the discovery of a variation which is cer- 

 tainly independent of any such cause, namely, the lunar varia- 

 tion of the magnetic elements ; while at the same time new laws 

 of the solar diurnal change have been established, which are 

 deemed to be incompatible with the supposition of a thermic agency. 

 There has been, accordingly, a tendency of late to recur to the 

 hypothesis, that the sun and moon are themselves endued with 

 magnetism, whether inherent or induced ; and it is therefore of 

 some importance to determine the effects which such bodies would 

 produce at the earth's surface, and to compare them with those 

 actually observed. 



I have endeavoured, in what follows, to solve this question, on 

 the assumption that the supposed magnetism of these luminaries 

 is inherent. The result will show the insufficiency of the hypo- 

 thesis to explain the phenomena ; and will therefore bring us one 

 step nearer to their explanation, by the removal of 'one of their 



Let x, y, z be the coordinates of any point of a fixed magnet, 

 referred to three rectangular axes passing through its middle 

 point ; cr, b, c those of a distant magnetic element, m ; and e their 



