124 THIRD REPORT^ — 183S. 



Lussac, by its having been counteracted by the increase of in- 

 tensity, arising from the diminution of temperature. Mv. Hen- 

 wood informs me that he has made corresponding observations, 

 consisting of two series, each of 3900 vibrations at each place ; 

 on Cairn Brea Hill, 710 feet above the level of the sea; at the 

 surface of Dolcoath mine, 370 feet above the sea ; and at a depth 

 of 1320 feet beneath the surface in Dolcoath mine, or 950 feet 

 below the level of the sea ; and that, after clearing the results 

 from the effects of temperature, the differences are so minute 

 that he cannot yet venture to say he has detected any difference 

 in the magnetic intensity at these stations. If, notwithstanding 

 these results, we are to admit the correctness of M. KupfFer's 

 conclusions, I think we must infer that the diminution of hori- 

 zontal intensity at his higher station was due to an increase in 

 the dip, which element would not probably be so much affected 

 bv a change of elevation in a comparatively level country, like 

 Cornwall, as on the flank of such a mountain mass as Elbours. 



Before dismissing the subject of the terrestrial intensity, I 

 should mention that attempts have been made to delineate on 

 charts the course of isodynamic lines. Professor Hansteen has 

 published a chart in which this is done for the year 1824. Of 

 all observations, however, requisite for graphic exhibitions con- 

 nected with terrestrial magnetism, those on the authority of 

 which such lines must be drawn are fewest in number and least 

 satisfactory in their results ; we should, therefore, be very cau- 

 tious in drawing conclusions from such delineations. 



Hitherto I have only referred to such changes in the direction 

 of the magnetic force, and in its intensity, as appear to depend 

 upon general causes ; but, besides these, sudden and sometimes 

 considerable irregular changes occur. These have very gene- 

 rally been attributed to the influence of the aurora borealis, 

 whether visible or not at the place of observation ; and I think 

 it not improbable that some may be due to a peculiar electrical 

 state of the atmosphere, independent of that meteor. The inr 

 fluence of the aurora borealis on the magnetic needle has, how- 

 ever, been denied by some, principally because, during the 

 occurrence of that meteor at Port Bowen, Captain Foster did 

 not observe peculiar changes in the direction of the needle, al- 

 though, from his proximity to the magnetic pole, the diurnal 

 change sometimes amounted to 4° or 5° ; and, under such cir^ 

 cumstances, it was considered that these changes ought to have 

 been particularly conspicuous. In a paper inserted in the se- 

 cond volume of the Journal of the Royal Institution, I have, 

 however, shown that Captain Foster's Port Bowen observations 

 do not warrant the conclusions which have been drawn from 



