114 Effect of an Aurora Borealis on the Magnetic Needle. 



lieved was nearly south west; below, the direction of the clouds was 

 not observed. 



"The dew point had risen, since the preceding day, twelve de- 

 grees Fah. It is highly probable that an upper current (not the up- 

 permost) of air, was moving in the direction in which the arch moved, 

 as the air had been moving in that direction a ievi hours before, and 

 I have frequently observed, when the wind changes, the lower strata 

 next the earth change first. From the 10th until the afternoon of 

 the 15th of May, the wind had constantly been by night and day, al- 

 most exactly south, with a high dew point, carrying an immense quan- 

 tity of vapor to the north ; on the evening of the 15tb, and until the 

 night of the 16th, the wind was NE. with rain, and on the morning 

 of the 17th the wind was north." 



On returning home at eleven o'clock, on the evening just referred 

 to, and observing the different magnetic needles which I have arrang- 

 ed for observations on the diurnal variation, a considerable disturb- 

 ance was indicated. The journal of the hourly observations, kept 

 during my absence in the evening, confirmed my opinion that what I 

 had witnessed was but a part of the disturbance which had actually 

 taken place, and which seems to have affected the horizontal needle 

 especially. 



The needles to which I have referred are three in number, two 

 long horizontal needles, of which one is within doors, and the other* is 

 under cover in the yard attached to my residence, and a long dipping 

 needle with a knife edge suspension, contained in a small observato- 

 ry, constructed for the purpose, and also in the yard of my dwelling 

 house. The observations of the horizontal needle, within doors, 

 were made very regularly, and also of the dipping needle out of doors, 

 but the observer not being avi^are of the appearance of the aurora, 

 did not take the corresponding hourly observations of the horizontal 

 needle out of doors, throughout the whole of the evening. 



In order to convey a better idea of the variation on the evening in 

 question, I precede the observations by those made on the following 

 day and night, on which the changes of variation and dip were nearly 

 the regular mean diurnal changes at this time of the year. The va- 

 riation is referred to the mean variation for the day, or to a point near- 

 ly corresponding to this, the sign + being prefixed to the positions 

 loest of this line of mean variation, and the sign — to those east of 



* A complete description of this needle is given in a paper read before the Ameri- 

 ican Philosophical Society, in November last, and which will appear in the first part 

 of the fifth volume of their transactions. 



