Feequency of the Aueoea Boeealis. 



Ixxxi 



171- It is probable that the numbers for midnight, and the hours thereafter, are too small, for the reason given, 

 No. 169. The greatest number of aurorje were seen at 9^ p.m. ; this result is independent of the effect of twi- 

 light, since 9^ p.m. is also the hour of maximum frequency for the winter months. This hour is nearly the 

 hour of maximum disturbance for the magnetic declination and dip ; as, however, the maximum disturbance of 

 the total magnetic force and a maximum of the magnetic dip appear to occur about 5'* p.m., this also may be 

 an epoch of maximum frequency or intensity, though this can only be determined in higher latitudes. It 

 should also be remarked, that, since the epoch of maximum disturbance varies with season, so, therefore, it is 

 probable \vill that of frequency of the aurora; some traces of this maybe deduced from the previous table. 

 In the winter quarter, November-January, four -fifths of the times at which aurorae were seen were for the hours 

 before \i)^ p.m., whereas in the spring quarter there were only three-fifths seen before 10^ p.m. (See No. 172). 



Table 71. — Numbei-s of Aurorse Boreales seen at Makerstoun in each Month of the Years 



1843-49. 



Years. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Sum. 



1843 



1 



1 



4 



2 



















3 



3 



3 



3 



20 



1844 



2 



3 



5 



3 



3 











2 







4 



6 



2 



30 



1845 



11 



6 



11 



4 



1 











2 



3 



4 



3 



2 



47 



1846 







2 



1 



2 















2 



4 



4 



1 



1 



17 



1847 



1 



1 



1 



1 















1 



2 



5 



5 



1 



18 



1848 







3 



4 



2 



2 















1 



7 



5 



2 



26 



1849 

 Sum, 



7 



10 



2 



2 















* * 



3 



2 



* * 



* * 



26 



22 



26 



28 



16 



6 











7 



16 



29 



23 



11 



184 



172. Annual Variation of Frequency of the Aurora BoreaUs. — The first line following contains the numbers 

 of aurorse observed in each month during the six complete years 1843-8, and the second line gives the numbers 

 of hours at which the aurorse were seen. 



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 aurorae was 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 in- 

 cluded, 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 vdnter 

 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 aurorae were seen at the five hours, lO^i p.m.-2^ a.m., during 



* It has been stated in the volume for 1844, p. 401, that this result was long ago obtained by Mairan ; this statement, made 

 chiefly on the authority of Kaemtz and Hansteen, is not quite accurate. It is true that Mairan's numbers give a rough indica- 

 tion of the law, as will be seen below ; but when it is remembered that his table includes all the observations (229) of which he 

 could find a record for upwards of 1000 years, it will be evident, that the conclusion that a greater number of aurorae occurred at 

 both equinoxes than at the winter solstice would have been hasty ; this conclusion, however, is not made by Mairan, and, though he 

 has combined the numbers of aurorae in a great variety of ways, he has made no combination exhibiting this fact. It did not enter 

 into the necessities of his theory (that aurorae are the product of the solar atmosphere) to shew that a greater number of aurorae hap- 

 pened in the northern hemisphere, at the vernal equinox than at the winter solstice; he shews, indeed, that the number for one equi- 

 nox is, and, in accordance with his theory, ought to be, greater than for the other. Some other philosopher has the merit of first 

 . pointing out this fact. 



The following are the numbers of aurorae by Mairan (Traite Physique et Historique de I'Aurore Boreale, par M. de Mairan, 



