64 WRITINGS OF JOSEPH HENRY. [1832 



In this formula it is evident that F may remain constant, 

 although H is caused to vary by a change in the value of 

 cosin d. The fact therefore of a variation in the absolute 

 intensity, can only be determined by combining the obser- 

 vations of the vibrations of the horizontal needle with simul- 

 taneous observations on the dipping needle. 



If we suppose F constant during the change of horizontal 

 intensity as observed at Albany, we may, by means of the 

 above formula, calculate the change in declination or dip 

 required to produce the observed difference in the horizontal 

 intensity. Assumitig d = 75°, (the dip at Albany nearly,) 

 and 11= to the horizontal intensity observed at 6 o'clock, 

 we can readily find the value of F ; and since this value is 

 supposed constant, by substituting it in the expression 



cosin o =-— 

 F 



in which H' represented the intensity observed at 10 o'clock, 

 we shall have the value of 5 (the dip) corresponding to the 

 latter intensity. In this way, the change observed in the 

 horizontal intensity at the time of the aurora, gives 28' 48" 

 as the deviation of the needle in the plane of the dip. 



The aurora which appeared in connection with this mag- 

 netic disturbance, was probably one of the most interesting 

 ever observed in this country, particularly from the circum- 

 stance of the actual formation of a corona, which was seen 

 in several parts of this State. My friend Prof. Joslin, of 

 Union College, who happened to be in New York at the 

 time, has furnished me with the following account: 



" The aurora borealis of 19th April, as it appeared in the 

 city of New York at 9 p. m., was peculiarly interesting, on 

 account of the meeting of the luminous columns in the mag- 

 netic meridian, at the point in the direction of the dipping 

 needle towards which they usually tend. The luminous 

 matter occupied the whole northern half of the visible ce- 

 lestial hemisphere, and was very much condensed near 

 the point of convergence. Some of the eastern coruscations 

 were at times transiently curved, as though their middle 

 parts (as was probably the case) were driven eastward by the 



