XIII. ON EARTH-CURRENTS IN CONNEXION WITH MAG- 

 NETIC DISTUBBANCES. 



Proceedings of the Royal Irish Academy, 1862. 



IN a Paper recently communicated to the Academy, the author 

 showed that the regular diurnal changes of the horizontal com- 

 ponent of the earth's magnetic force were probably due to electric 

 currents traversing the earth's crust, these currents operating as 

 disturbing forces, which cause the magnets to deviate from their 

 mean positions according to known laws. This relation being 

 established, the diurnal laws of the earth-currents may be inferred 

 from their effects. It was thus concluded that the azimuth and 

 the intensity of the currents varied throughout the day, according 

 to fixed laws depending upon the hour-angle of the sun. At 

 different parts of the globe these laws were found to exhibit certain 

 well-marked features in common; while their differences were 

 probably accounted for by the geographical and physical characters 

 of the region in which they occur. The author now proceeds 

 to extend the same inquiry to the currents which produce the 

 magnetic disturbances. 



It has been shown, by the labours of Kreil, Sabine, and others, 

 that the disturbances of the magnetic elements are subject to 

 periodical laws, depending upon the hour, which are constant for 

 a given place, and for a given season of the year. The sums of 

 the changes produced by these disturbances, at each hour of 

 observation, have been calculated by General Sabine for three of 

 the British Colonial Observatories. The corresponding quantities 

 have been deduced by Dr. Lamont, for Munich ; by Mr. Broun, 

 for Makerstoun, in Scotland ; and by the author, for Dublin. We 

 possess, in addition to the foregoing, similar results nt Lako 

 Athabasca, in British North America, deduced by Colonel Lrfroy 



