ON MAGNETIC DISTURBANCES AND EARTK-CDREENTS. 4G3 



This represents a fall of summer temperature more than three times as 

 great as would be required to glaciate Norway. 

 At the latitude of Dablin we have 



Dablin .... 68= F. _ 



Nain (Labrador) . . 48° ^'^- " " ^ ''^ ' 



This fall of temperature in July woald probably be sufficient to 

 glaciate the whole of the British Islands down to the sea-level. 



On Magnetic Disturbances and Earth-currents. 

 By Professor William Grylls Adams, F.R.S. 



[Plates YIIL-XIII.] 



[A communication ordered by the General Committee to be printed in extcnso 



among the Reports.] 



In considering the changes or disturbances produced in the three magnetic 

 elements, viz., the declination, the horizontal force, and the vertical force, 

 Ave must distinguish between the regular changes, which depend on the 

 apparent motions of the sun or the moon, and those more sudden and some- 

 times violent changes which are especially termed magnetic disturbances. 



Among the regular changes are daily and yearly changes, which de- 

 pend on the time of the day and the season of the year, showing that 

 the change of position and the apparent motion of the sun with respect 

 to the place of observation produce res:ular magnetic changes. These 

 regular daily changes are accompanied by and have very generally been 

 supposed to be due to electric currents or electric waves traversing the 

 earth's crust ; and a discussion by Dr. Lloyd of the observations made by 

 Mr. Barlow, in 1847, of currents on telegraph wires, showed a very close 

 relationship between the two-hourly changes of the declination needle 

 and the changes of intensity and direction of earth-currents on tele- 

 graph linos. 



Both Dr. Lamont and Dr. Lloyd conclude, from their comparisons of 

 earth-currents and magnetic changes, that the changes of the declination 

 needles cannot be due to the direct action of the electric current travers- 

 ing the earth's crust, but that these currents or waves, extending to a 

 considerable depth, alter by induction the magnetism of the earth itself, 

 and this change of magnetism causes the observed changes iu the decli- 

 nation needle. Thus th.e magnetic changes are the indirect effects of 

 —not the earth-current in its immediate neighbourhood but of — a change 

 in the magnetism of the earth itself, which may be due to an electric 

 wave extending over a considerable area of the earth's surface. 



The point towards which the total earth-current is directed follows 

 the sun, and seems to lag two or three hours behind, but not the same 

 distance behind at different places. 



These earth-cnrr^nts have been ascribed to different causes ; thus 

 Dr. Lamont regards them as the results of electric force emanating from 

 the sun ; De Saussure regards them as developed by evaporation, the 

 vapour being positively charged, and the water being negative ; Dr. Lloyd 

 regards them as effects of solar heat ; whilst M. de la Rive ascribes them 

 to chemical action going on in the interior of the solid crust of the earth, 

 the electricity being transp6rted into the atmosphere by evaporation. 



