345 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 

 15 16 26 14 6 7 13 27 23 11 

 50 62 65 43 8 10 32 44 58 38 



The greatest number of aurorse were observed in March for the first 

 six months, and in October for the last six months of the year : 

 none were observed in June and July. When the six months of 

 1849 are included, the number for February is 26, and for March, 

 28. The law of visible frequency of the aurora is the same as that 

 deduced already for magnetic disturbance ; namely, maxima near the 

 equinoxes, and minima near the solstices, the minimum at the 

 summer solstice being the principal. As, however, the shortness of 

 night during the summer months must diminish the number of 

 visible aurorse, it is by no means certain from these numbers that a 

 minimum occurs at the summer solstice; the fact of the minimum 

 at the winter solstice is involved in no such difficulty. If we could 

 assume that the aurorse had the same diurnal law of frequency at 

 all seasons of the year, the existence of the summer minimum could 

 be satisfactorily determined, by comparing the numbers of times 

 which aurorse were seen at the five hours, 10*^ P.M.-2'' a.m., during 

 which (even in the months of August and May) there is little twi- 

 light to extinguish aurorse. The numbers are as follow, for these 

 five hours in each month of the years 1843—8 : — 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 

 15 24 38 31 8 9 14 16 18 12 



From these it is evident that the numbers in May and August are 

 certainly less than for April and September ; but it has been already 

 mentioned as probable that the diurnal law of frequency varies with 

 season, of which, indeed, a proof is to be found in the great excess 

 of the numbers above for the spring months, compared with those 

 for the autumn months, shewing the later epoch of the maximum 

 frequency in the former. An examination, however, of the table for 

 the disturbance of the magnetic declination (Table 18, Vol. xix., 

 Part 2), will shew that, though the maximum disturbance occurs after 

 midnight, in the months of May, June, and July; yet in August and 

 the two following months it occurs about lO** p.m., so that there can 

 be no doubt of the less number for August than for September and 

 October, if there should be a doubt in the case of May compared 

 with April. The difference, however, even in the latter case is too 

 great to be explained by any slight shift of the epoch of maximum 



