5H POPULAR LECTURES AND ADDRESSES. 



sun in the course of a year ; while the latter 

 would give rise to a daily variation, the same as 

 would be observed if the red ends of terrestrial 

 magnetic needles were attracted towards an ideal 

 star of declination o and right ascension 270. 

 Hence, to discover the disturbances of terrestrial 

 magnetism, if any there are, which are due to the 

 direct action of the sun as a magnet, the 

 photographic curves of the three magnetic 

 elements given by each observatory should be 

 analysed for the simple harmonic constituent of 

 annual period and the simple harmonic consti- 

 tuent of period equal to the sidereal day. We 

 thus have two very simple problems, each of which 

 may be treated with great ease separately by a 

 much simplified application of the principles on 

 which Schuster has treated his much more com- 

 plex subject, according to Gauss' theory as to the 

 external or internal origin of the disturbance, 

 and Professor Horace Lamb's investigation of 

 electric currents induced in the interior of a globe 

 by a varying external magnet. The sidereal 

 diurnal constituent which forms the subject of the 



