624 PROFESSOR CHALLISS OBSERVATIONS OF THE 



V - Vo = 0,142518X + 0,159548^ 

 D - Do= 0,0277i3\ + 0,513523Z. 



These formulce give T- F„ and D-Da in minutes, X being expressed in seconds of time, and I 

 in minutes. 



For the Cambridge Observatory, V - ¥„= + 3',7, and D - Z>o = + 22',0. 



In order to make the proposed comparison of the position of the Corona with the Magnetic Zenith, 

 it is now only necessary to obtain the Magnetic Declination and Dip at the respective times of obser- 

 vation. These I have derived from observations made at the Greenwich Observatory during the 

 prevalence of the Aurora, which, on my preferring a request, were promptly forwarded to me with 

 all the requisite data, by James Glaisher, Esq., who is at the head of the Magnetical Department in 

 that Institution, and which the Astronomer Royal has allowed me to publish with this communica- 

 tion. For this favour 1 beg here to express my thanks. The observations are given at length in 

 Tables I, II, and III, at the end of this Paper, as well because they are used in the calculations, as 

 because they present so striking an instance of great magnetic disturbances occurring simultaneously 

 with an extraordinary display of the Aurora Borealis, that the connexion in some way of the two 

 kinds of phaenomena must be regarded as a physical fact. 



The Westerly Declinations at Cambridge at the times of observation were inferred from those at 

 Greenwich at the same times by merely applying the value of V- F„ already obtained, viz. + 3',7. 

 The latter were deduced from the declinations recorded in Table I. by simple interpolation, it being 

 understood that the motion of the magnet was uniform in the intervals between the times there given. 

 The Greenwich observations were made by the admirable photographic process, which has been 

 brought to so great perfection by C. Brooke, Esq., of St. John's College in tliis University. Between 

 9 . 25" and g*". 44", the disturbance was so great that the magnet passed the limits of the photo- 

 graphic paper. The same thing took place in the contrary direction between lo''. 10" and lo". 40". 

 As Mr. Glaisher states that the motions at these times were smooth and without checks, I have 

 ventured to deduce the maximum elongation between 9K 25" and 9^ 44" on the supposition that the 

 magnet continued to move after g*". 25" in the same manner as from 9". 20" to 9". 25", till it attained 

 the maximum, and then that it immediately returned by the same motion that it had from 9". 44" to 

 10''. 10". The maximum elongation between lo"". 10" and 10''. 40" was inferred on the same principle. 



Mr. Glaisher furnished me with the following values of the Dip at Greenwich : 



Dip. 



1848. Nov. 12. 21" 68". 54',0 



16". 3 (is . 56,3 



19. 21 68.53,7 



23. 3 68 . 55,5 



Hence it is inferred that the Dip, if undisturbed, would have been 68". 55',0 during the Aurora. 

 The disturbed Dip was calculated in the manner I am about to explain. In the Greenwich obser- 

 vations (Tables II. and III.), the readings for the horizontal force variations are given in terms of the 

 whole horizontal force ; but the vertical force readings are given in divisions of the scale, which 

 require to be converted into parts of the whole vertical force. The factor for this purpose is 0,00067, 

 which is the value of one division. 



The scale reading of the vertical force magnet at Nov. 17, O*", was 21'''''-,7, and at Nov. 18, 

 0", 21''"',5, at which times there appears to have been no disturbance. The undisturbed read- 

 ing is consequently assumed to be 21''", 6. 



The reading of the horizontal force magnet in parts of the whole horizontal force, was 0,1099 

 at November 17, 0^ and 0,1074, at November 18, o", the latter of which Mr. Glaisher states to be 

 somewhat below the average value for the season and time of day. The undisturbed reading 

 during the Aurora is assumed to be the mean between those two readings, viz. 0,1086. 



