193 Dr. Lloyd on the Influence of a Distant Luminary upon the 



cannot be distinguished in appearance or properties from oxin- 

 dicanine or oxindicasine. 



I did not enter into a more minute examination of these 

 bodies, since their formation from indican is the only point of 

 interest in their history. 



XXV. On the direct Magnetic Influence of a distant Luminary 

 upon the Diurnal Variations of the Magnetic Force at the 

 Earth's Surface. By the Rev. H. Lloyd, D.D., D.C.L. ^,-c.* 



IT has been usual to ascribe the ordinary diurnal variations of 

 the terrestrial magnetic force to solar heat, either 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 certainly independent of any such cause, namely, the 

 lunar variation of the three magnetic elements; while at the 

 same time new law^s of the solar diurnal change have been esta- 

 blished, which are deemed to be incompatible with the suppo- 

 sition of a thermic agency. There has been, accordingly, a ten- 

 dency of late to recur to the hypothesis that the sun and moon 

 are themselves endued with magnetism, Avh ether inherent or in- 

 duced ; 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 phsenomena ; and will therefore bring us 

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

 their supposed causes. 



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

 referred to three rectangular axes passing through its middle 

 point; a, h, c those of a distant magnetic element m; and e 

 their mutual distance. Then, if yu. denote the quantity of free 

 magnetism contained in the element ds of the magnet at the 

 point [x, y, z), the force exerted by /* on m is_ 

 mfjuds 



and its resolved portions in the directions of the three axes of 

 coordinates are 



m{a—x)iMds m[b—y)ixds m{c~z)/xds 



* Commimicatecl by the Author. 



