On Hourbj Observations of the Maynetic Declination. 33 



cause is the same in both, the mode of operation is different in the 

 two cases. 



Another important distinction between the phenomena of the 

 solar diurnal variation and of the disturbance variation at Point Bar- 

 row and Toronto, is shown by the author to consist in the comparative 

 magnitude of their range. The increase in the range of the solar diurnal 

 variation between Toronto and Point Barrow is, as nearly as may be, 

 in the inverse ratio of the values of the horizontal force of the earth 

 at the two stations (which is the antagonistic force opposing all 

 magnetic variations) ; whilst on the other hand the increase in the 

 range of the disturbance variation is many times greater than it 

 would be according to the same proportion. It would appear there- 

 fore that the absolute disturbing force must be much greater at Point 

 Barrow than at Toronto ; suggesting the question, by what physical 

 or other conditions is the locality distinguished at which the disturb- 

 ing force is a maximum. 



In correspondence with the great amount of the absolute disturb- 

 ing force at Point Barrow is the frequency of the concomitant auro- 

 ral manifestations, which greatly exceed that of any previous record 

 known to the author. It was the custom at Point Barrow to attach 

 a distinguishing mark to all the hourl}^ magnetic observations which 

 were made when the Aurora was visible. Taking the mouths of 

 December, January, and February as those in which, in the latitude 

 of Point Barrow, there is nearly a constant absence of day-light, there 

 were, in those three months in 18.^2-1853, 1788 hourly observations, 

 at 461 of which the Aurora was visible; and, in the same three 

 months of 1853-1854, there were 1837 hourly observations, at 616 

 of which the Aurora was seen. There were therefore in the six 

 months 3625 hourly observations, at 1077 of which the Aurora was 

 seen contemporaneously, and at 2548 it was either not present or 

 obscured by clouds : the presence of the Aurora was thus recorded 

 at between one-third and one-fourth of the hours of observation. Or 

 possibly the frequency of its appearance may be more easily judged 

 of by stating, that during these six months the Aurora was seen on 

 six days out of every seven. 



The record thus furnished by Capt. ]\Iaguire has enabled the author 

 to treat the Aurora, for the first time, in the same way as the corre- 

 sponding and connected phenomena of the magnetic disturbances are 

 treated, viz. by distributing its recorded appearances into the several 

 hours of their occurrence. A table, in wliich the particulars of this 

 distribution are contained, shows that 1 a.m. is the hour of their most 

 frequent occurrence at Point Barrow, there having been 102 recorded 

 appearances in the six months at that hour. From 1 1 a.m. to 3 r.M, 

 is the epoch of minimum, not a single instance of Aurora at any of 

 those hours being recorded in the same six months. The increase 

 from the minimum to the maximum, and the decrease from the maxi. 

 mum to the minimum, are both continuous progressions, with only 

 such very slight occasional interruptions as might assuredly be eX' 

 pected to disappear in mean numbers taken from a longer interval of 

 time. 



