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XXXVIII. On the Amount of the direct Magnetic Effect of the 

 Sun or Moon on Instruments at the Earth's Surface. By G. 

 Johnstone Stoney, M.A., F.R.S., Secretary to the Queen's 

 University in Ireland*. 



IN the Philosophical Magazine for March 1858, Dr. Lloyd 

 showed that the observed disturbances of the magnetic 

 needle, depending on the hours of solar or lunar time, follow 

 laws inconsistent with their being due to the direct magnetic 

 attraction of the sun or moon. Hence it might be too hastily 

 concluded, from the absence of observed effects following the 

 proper laws, that these luminaries are not magnetic. An inquiry 

 into the amount of this influence, however, shows that, though 

 the sun or moon were as highly magnetized as the earth, their 

 direct effects would be so small as to be masked by the more 

 powerful unknown perturbating causes which the observations 

 prove to be at work. 



In fact let and / be the centres of a distant magnet and of 

 a needle acted on. Let x, y, z be the coordinates oidm, a mole- 

 cule of the distant magnet referred to as origin, and rectan- 

 gular coordinates so taken that the axis of x may pass through 

 0'. Let also x', y', z' be the coordinates of dm', a molecule of the 

 needle acted upon, referred to parallel coordinates passing 

 through 0'. Then using p for the distance betwen dm and dm', 

 and D for the distance between the centres of the magnets, the 

 components of the action of dm on dm' will be 

 7V dm dm' D + x'— x 



P* P 



dY _ dm dm' _ y'-y 

 p* p 



,„ dm dm' z'—z 



dl = 2~ ' 



P P 



Therefore the elementary moments turning dm' round 0' will be 



-r, dm dm! . . . 

 d?= —^- • {yz'-zy'), 



ir v dm dm' . , . ,„, 

 dQ= — 3 - ■ (zx'-xz'+z'B), 

 P 



d^^-ixy'-yx'-y'B). 

 Butp 2 =(D + X 1 - xf + {y'-yY+ {z'-zf. Therefore, expanding 



* Communicated by the Author. An abstract of this paper was read at 

 the recent Manchester Meeting of the British Association. 



