REPORT ON THE MAGNETISM OF THE EARTH. 125 



tliem, and have pointed out circumstances which may, in this 

 case, have rendered the eflfect of the aurora upon the horizon- 

 tal needle less sensible than might have been expected. That 

 changes in the direction and intensity of the terrestrial forces 

 are simultaneous with the aurora borealis I feel no doubt, for I 

 have seen the changes in the direction of the needle to accord 

 so perfectly with the occurrence of this meteor, and to such an 

 extent, that in my mind the connexion of the phaenomena be- 

 came unquestionable*. As, however, the magnetic influence 

 of the aurora borealis has been doubted, I shall here point out 

 the manner in which I consider the effects may be best ob- 

 served. 



If the magnetic forces brought into action during an aurora 

 are in the direction of the magnetic meridian, they will affect a 

 dipping needle adjusted to the plane of that meridian, but the 

 direction of an horizontal needle will remain unchanged : on 

 the other hand, if the resultant of these forces makes an angle 

 with the meridian, the direction of the horizontal needle will be 

 changed, but the dipping needle may not be affected. In order 

 to determine correctly the magnetic influence of the aurora by 

 means of an horizontal needle, it is therefore necessary not only 

 to have regard to those forces which influence its direction, but 

 likewise to those which affect the horizontal intensity. The 

 effects of the former are the objects of direct observation, but 

 those of the latter are not so immediately observable. As, du- 

 ring an aurora, the intensity may vary at every instant, — and it is 

 these changes which are to be detected, — the method of deter- 

 mining the intensity by the time of vibration of the needle can- 

 not here be applied, and other means must be adopted. The 

 best method -appears to me to be that which I employed for 

 determining the diurnal variation of the horizontal intensity, 

 * the needle being retained nearly at right angles to the meridian 

 by the repulsive force of a magnet, or by the torsion of a fine 

 wire or thread of glass. For the purpose, then, of detecting 

 in all cases the magnetic influence of the aurora, I consider that 

 two horizontal needles should be employed ; one, adjusted in 

 the meridian, for determining the changes which may take place 

 in the direction of the horizontal force, and the other at right 

 angles to the meridian, to determine the changes in the inten- 

 sity of that force, arising principally from new forces in the 

 plane of the meridian, and which would affect the direction of 

 the dipping needle alone. Both these needles should be deli- 



* For the observations to which I here particularly refer, see the Journal of 

 the Roijal Institution, vol. ii. p. 272. 



