Mr. Christie on the Aurora Borealis. 277 



Visible on three days, was 2 14-25'; that for April 2 52-44, 

 and for May 3 44-39', during which latter months the aurora 

 was not seen on any occasion. So that there appears clearly to 

 have been a progressive increase of the variation, quite inde- 

 pendent of the aurora ; and to this progressive increase we 

 ought to attribute the difference above noticed, more especially 

 as, during the months in which the aurora was visible, the 

 effect appears to have been to increase, instead of diminishing, 

 the variation during an aurora. No one can set a higher value 

 than I do on Captain Foster's observations, but I consider that 

 conclusions have been drawn from them which they do not 

 warrant, 



I have already stated, that although magnetic effects may in 

 all cases be simultaneous with the aurora, yet the direction of 

 the horizontal needle may not invariably be affected. This 

 will be evident, if we consider the effects that may be produced 

 on a magnetic needle, freely suspended by its centre of gravity, 

 and refer the direction of this needle to the horizontal plane. 

 If the forces developed are wholly in the vertical plane passing 

 through this needle, it is evident that, although the inclination 

 of the needle may be increased or diminished, yet no change 

 will take place in its horizontal direction, and consequently no 

 changes, in such case, would be observable in the horizontal 

 needle. That this may frequently be the case is evident from 

 the circumstance that the most brilliant beams of the aurora 

 generally affect the magnetic north. I am not aware of the 

 observations which may have been made on the direction of 

 the horizontal needle, during the aurora of the 19th of April 

 last, but as I have before stated, the greatest deviation of the 

 needle which I observed, took place at the time when a strong 

 stream of light issued from the magnetic north. Now, although 

 the effect was here so sensible, owing to the peculiar adjust- 

 ment of the needle, yet this effect may not have been observ- 

 able on a horizontal needle, under the influence of terrestrial 

 magnetism alone, however delicately that needle may have 

 been suspended. 



If observations were made on a dipping needle, the effects 

 of the forces developed in the plane of the meridian during an 



