VARIATIONS OF THE MAGNETIC NEEDLE. 273 



Mr. Barlow was thus led to examine whether any relation 

 subsisted between these movements and the daily changes of the 

 horizontal magnetic needle. And having made, for this purpose, 

 a series of simultaneous observations with a delicate declinometer, 

 he came to the conclusion that, although generally the currents 

 flow southwards during that part of the day in which the variation 

 of the horizontal needle is tcesterly, and northwards when the vari- 

 ation is easterly, " yet simultaneous observations showed no simi- 

 larity in the paths described by the magnetic needle and the 

 galvanometer." 



An examination of the galvanometrio observations above re- 

 ferred to led me, some time since, to an opposite conclusion ; and 

 at the last meeting of the British Association I stated my convic- 

 tion, founded on these observations, that the earth-currents, whose 

 continuous flow Mr. Barlow has the merit of establishing, would 

 eventually explain the variations of terrestrial magnetism, both 

 periodic and irregular. I now proceed to state some of the grounds 

 of this conviction ; and in the present paper I hope to show that 

 the diurnal changes of the earth-currents correspond with those of 

 the horizontal component of the earth's magnetic force. 



4. Before proceeding to the detailed examination of this .corre- 

 spondence, it is necessary to advert to a difference between the things 

 compared, which will account for the method employed in the com- 

 parison. The cause of this difference will be adverted to hereafter. 



When we examine the curves,* in which Mr. Barlow has re- 

 presented the course of the galvanometric deflections caused by the 

 earth-currents, we observe that the regularity of that course is con- 

 tinually interrupted by rapid reciprocating movements, in which 

 the needle oscillates from one side to the other of the zero, alter- 

 nately. These movements are very different in magnitude at 

 different times: thus, May 29 was a day of considerable movement ; 

 May 25, one of comparative rest. These movements are similar 

 to those of the magnetometers, with which we are familiar ; but 

 they are much more rapid, and bear a larger proportion to the 

 regular changes. Among the days of observation whose results 

 are given in this diagram, it fortunately happens that one, viz., 

 the day commencing May 28, 10 p. M. (Gdttingen time), was a 

 " term-day " of the magnetic system, during which the magneto- 

 meters were observed every five minutes at the Greenwich Obser- 



* Philowphical Transaction*, 1848, Diagram, No. 1. 

 T 



