124 



Disturbance of the JVeedle during an Aurora. 



crease followed by a very sudden decrease, and a second increase, 

 which is followed by as sudden a diminution. With such irregulari- 

 ties before one, it would be ill judged to attempt any conclusion from 

 the apparently regular portions of the changes in dip : further obser- 

 vations may throw light upon the causes of the discrepancies, and I 

 have recorded them here, principally with that view. For future ob- 

 servations I hope to have another instrument in which the defects of 

 the present one will be remedied ; the results then obtained may 

 give a confidence which I do not now feel in those furnished by the 

 present instrument. 



The meteorological changes which took place about the time of the 

 occurrence of this aurora, are worthy of record. On the morning of 

 the 9th of July there was an irregular rain, with the wind at west, 

 terminated by a shower at noon, and succeeded by a clear afternoon, 

 with the wind at west and north west. On the morning of the 10th, 

 the wind was still from north west, but in the evening had hauled 

 round to the east and south east. 



I subjoin an extract from the meteorological diary kept by James 

 P. Espy, Esq., which shows a sudden diminution in the amount of 

 vapor in the air, between the 9th and 10th of July. The remarks in 

 relation to the winds agree with those already given. 



" On the 9th the wind was from the N.W., and on the morning of 

 the 10th from the N. N. W., and at noon the upper current from the 

 W. and the lower from the S. E. ; on the 11th the upper current was 

 all day from the W., and the lower from the S. S. W." 



To return to the observations on the magnetic needle. To deter- 

 mine from them the absolute effect attending the aurora, it would be 

 necessary to make two corrections; the first for the effect of a change 

 of temperature upon the needles, the second for the regular diurnal 

 variation. The first of these is not for any considerable range of tem- 

 perature amounting, during the few hours between 9, P. M. of the 

 10th, and 1, A. M. of the 11th, to 6^° Fah. for the needle out of 

 doors, and to 2° Fah. for the needle within. An idea of the general 



