1SS1.| an MtKjnrfi,- DisfurlKiiicr, Aiironr, aii<l E(trlh ( ^nrrrnfs. C)!')U 



it, tlieii tlie direction of the axis of the coil is tlio direetion of tlio 

 dipping needle, i.e. of tlie magnetic lines of force. 



We will suppose now that for some point of time, say Juno Ist at 

 twelve o'clock mid-day, the three magnetic elements, i. c. the declina- 

 tion, the horizontal force, and tlie vertical force, have been determined, 

 wo have now to consider the changes or disturbances produced in tliese 

 magnetic elements, and the connection of these changes witli other 

 phenomena, and especially the connection between auroras, earth 

 currents, and the larger and more irregular magnetic disturbances. 



I have already drawn attention to the declination needle and tho 

 balance magnetometer for measuring the changes of declination and 

 of the vertical force. 



For measurement of the changes in the horizontal force a special 

 instrument is employed, called a bifilar magnetometer, in which a 

 magnet is sus2)ended by two threads which are so placed that by 

 their torsion acting against the horizontal magnetic force of tho 

 earth, the magnet is kept at rest in a horizontal position in a direction 

 at right angles to the magnetic meridian. This comi)letes the list 

 of instruments for our magnetic observatory. Any change or dis- 

 turbance of the horizontal force pulls this magnet round more or less 

 in the horizontal plane, and its change of position is observed as in 

 the other instruments. The results I have to bring before you this 

 evening have been derived from the photograpliic registrations of 

 similar instruments in different parts of the world, so tliat the motion 

 of the needle has recorded its own tale on the prepared paj^er which 

 is wrapped on a cylinder driven by clockwork, and so placed as to 

 receive the spot of light reflected by the moving magnetic needle. 



First, there are regular daily and yearly changes, showing that the 

 sun produces regular changes in the three magnetic elements, which 

 depend on the time of the day and the season of the year, so that the 

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

 the place of observation produce regular magnetic changes. These 

 regular daily changes are accompanied by and have very generally 

 been su2)posed to be due to electric currents or electric waves 

 traversing the earth's crust, and a discussion by Dr. Lloyd of tho 

 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 of the needle and the changes of intensity 

 and direction of earth currents on telegraj)!! lines. 



Both Dr. Lamont and Dr. Lloyd conclude, from their comparisong 

 of earth currents and magnetic changes, tluit the changes of the 

 declination needle cannot be due to the direct action of the electric 

 current traversing the earth's crust, but that these currents or waves, 

 extending to a considerable depth, alter by induction the niagnetism 

 of the earth itself, and this change of magnetism causes the observed 

 changes in the declination needle. Thus the 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, 

 Vol. IX. (No. 7-i.) 2 z 



