FREQUENCY OF THE AURORA BOREALIS. Ixxxi 
171. Itis probable that the numbers for midnight, and the hours thereafter, are too small, for the reason given, 
No. 169. The greatest number of aurorz were seen at 95 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 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 aurorz were seen were for the hours 
before 10% p.m., whereas in the spring quarter there were only three-fifths seen before 102 p.m. (See No. 172). 
TABLE 71.—Numbers of Aurore Boreales seen at Makerstoun in each Month of the Years 
1843-49. 
Sept. Oct. Nov. 
4 
5 
oO 
4 
= 
=] 
Years. || Jan. Feb. | March. | April. | May. Aug. 
i=} 
8 
i 
r=] 
B 
Owe pe wb 
oooocoeoco 
ooocoeco 
wre be WwW O ow 
bos or BB oo 
4 NO bo bo oo 
29 
_ 
lor) 
for) 
i=) 
i) 
I 
—_ 
lor) 
172. Annual Variation of Frequency of the Aurora Borcalis—The‘first line following contains the numbers 
of aurore observed in each month during the six complete years 1843-8, and the second line gives the numbers 
of hours at which the aurorz 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 aurora: 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 aurore, 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 aurore 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 aurore were seen at the five hours, 104 p.m—2" a.m., during 
* Tt 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 Kemtz 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 aurore 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 aurore 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 aurore are the product of the solar atmosphere) to shew that a greater number of aurore 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 aurore by Mairan (Traité Physique et Historique de l’Aurore Boreale, par M. de Mairan, 
