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 aurorse were seen at 9^ p.m. ; tliis 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 higlier latitudes. It 
should also be remarked, that, since the epoch of maximum disturbance varies with season, so, therefore, it is 
probable will that of frequency of the aurora; some traces of this may be deduced from the previous table. 
In the winter quarter, November-January, four -fifths of the times at which aurorse were seen were for the hours 
before 10^ p.m., whereas in the spring quarter there were only three-fifths seen before 10^ p.m. (See No. 172). 
Table 71. — Numbers of Aurprsa 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 . 
0 
0 
0 
0 
3 
3 
3 
3 
20 
1844 
2 
3 
5 
3 
3 
0 
0 
2 
0 
4 
6 
2 
30 
1845 
11 
6 
11 
4 
1 
0 
0 
2 
3 
4 
3 
2 
47 
1846 
0 
2 
1 
2 
0 
0 
0 
2 
4 
4 
1 
1 
17 
1847 
1 
1 
1 
1 
0 
0 
0 
1 
2 
5 
5 
1 
j 18 
1848 
0 
3 
4 
2 
2 
0 
0 
0 
1 
7 
5 
2 
1 26 
1849 
7 
10 
2 
2 
0 
0 
0 
3 
2 
* * 
* * 
i 26 
Sum, 
22 
26 
28 
16 
6 
0 
0 
7 
16 
29 
23 
U 
184 
172. Animal Variation of Frequency of the Aurora Borcalis. — 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 aurora were seen. 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
15 16 26 14 6 0 0 7 13 27 23 11 
50 62 65 43 8 0 0 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 winter 
solstice is involved in no such difficulty. If we could assume that the aurora 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'^ 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 aurora; occurred at 
both equinoxes than at the winter solstice would have been hasty ; this conclusion, however, is not made by jMairan, and, though he 
has combined the numbers of aurora; 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 auror<E 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 aurora by Mairan (Traite Physique et Historique de I'Aurore Boreale, par M. de jMairan, 
