384 



the effect announced was supposed to outweigh the positive 

 evidence adduced in proof of it, it has so happened that none 

 of the numerous staff of magnetical observers, now scattered 

 over the globe, have since resumed the question. 



"I confess myself to have been one of those who doubted 

 the conclusion announced on both the grounds alluded to. I 

 did not think that variations so small as those inferred could 

 be fairly construed into a physical law, unless they were 

 found systematically consistent with themselves for a greater 

 number of periods than those hitherto examined ; and the an- 

 tecedent improbability of the action combined with the na- 

 ture of the evidence to deter me from the labour of the inquiry. 

 The periodical laws in terrestrial magnetism, hitherto disco- 

 vered, point to the sun as their physical cause ; and many cir- 

 cumstances appear to indicate that the sun in this case acts 

 mainly, if not entirely, through the medium of changes of 

 temperature. Thus I have shown that a very remarkable re- 

 lation exists between the diurnal ranges of the declination and 

 horizontal intensity, and the diurnal ranges of temperature ; 

 and the annual variations of the same elements present a simi- 

 lar correspondence.* Now, the thermal effect of the moon is 

 so small as to be incapable of being detected by the most de- 

 licate instrumental means ; and I inferred that its thermo- 

 magnetic properties must be likewise insensible. But, having 

 now satisfied myself of the fact of the lunar action, it is plain 

 that there was an error somewhere in this reasoning ; and I 

 believe the erroneous premiss to be, the assumption that the 

 lunar action, if it existed, must be analogous to the solar. 



" The most hopeful mode of inquiry into the fact of the lu- 

 nar magnetic action appears to be an analysis of the diurnal 

 range of the magnetic declination in reference to the moon's 



* Results of Observations made at the Magnetical Observatory of Dub- 

 lin, Transactions Royal Irish Academy, vol. xxii. pp. 85 and 91 ; and Pro- 

 ceedings, vol. iv. p, 379. 



