Ixxxii 
General Results of the Makeestoun Observations. 
whicli (even in the months of August and May) there is little twilight 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. Not. Dec. 
15 24 38 31 8 0 0 9 14 16 18 12 
From these it is evident that the numbers in May and August are certainly less than for April and Sep- 
tember ; 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, com- 
pared with those for the autumn months, shewing the later epoch of the maximum frequency in the former. 
An examination of Table 18, however, 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 10^ 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 frequency in the two months. Upon the 
whole, it appears certain that a minimum of actual as well as of visible frequency occurs in summer ; a result 
quite in accordance with that for the amount of magnetic disturbance, which accordance is sufficiently close to 
permit us to complete it, by assuming that the number of aurorse is a principal minimum in summer. 
173. Variation of Frequency of the Aurora Borealis with the Moon^s Age. — This invet.+igation is evidently 
beset with considerable difficulty, since the moonlight existing nearly extinguishes the appearances of all the 
fainter class of aurorse, and it renders the faintest wholly invisible ; the careful watch, however, which was 
kept for auroral appearances at Makerstoun, probably renders Table 69 better fitted for such a question than 
any previous series of observations.* 
174. Combining the numbers of aurorse observed at each day of the moon's age into six groups of 5 days (the 
first group, 4^ days), we find the average number of aurorse for one day of the moon's age in each group as 
follows, from the 6-|- years' observations : — 
Moon's Age. 28f— 2'i S^— 8'^— 12'i IS-i— l"* IS^— 22<i 23'i— 27'^ 
Number. '5-8 5-2 3-6 5-0 10-2 6-6 ' 
Did aurorse occur indifferently at all ages of the moon, we should expect to see the greatest nitmber at conjunction, \ 
and the least number at opposition ; this however is not the case, the greatest number was seen about two days j 
before the end of the third quarter, and the least number about two days after the first quarter, or the visible maxi- | 
mum and minimum occurred at times equidistatit from the epoch of opposition. The frequency of aurorse, ■ ( 
therefore, is a function of the moon's age. In order to determine the actual law, we may consider the probable | 
effect of moonlight in obliterating the auroral appearances; remarking, first, that 9*^ p.m., is the epoch of i 
maximum frequency for the aurora, and that upwards of five-sixths are seen before midnight. "When the moon 
is about three days old, in the months from September to March, it begins to set sufficiently late, and to have sufti- i 
cient light to render the earlier of the faint aurorse invisible ; about the end of the first quarter, it does not set | 
till midnight, and thus shines throughout the period of the occurrence of five-sixths of the aurorse ; afterwards it i 
increases in brightness, and the maximum effect in extinguishing faint aurorse is evidently attained at opposition, I 
when the moon begins to rise late enough to allow the earlier aurorse to be visible ; towards the end of the | 
1733, p. 199) ; by Kaamtz (Complete Course of Meteorology, translation by Walker, p. ioS) ; and by Hansteen (Mem. de I'Acad. Roy. 
de Belgique, t. xx., p. 117). 
Jan. 
Feb. 
March. 
April. 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Not. 
Dec 
Smn. 
Mairan, 
21 
27 
22 
12 
1 
5 
7 
9 
34 
50 
26 
15 
iS9 
Kajmtz, 
229 
307 
440 
312 
184 
65 
87 
217 
405 
497 
285 
225 
3253 
Hansteen, 
29 
31 
47 
34 
2 
0 
0 
17 
35 
33 
34 
23 
285 
J. A. Broun, 
22 
26 
28 
16 
6 
0 
0 
7 
16 
29 
23 
11 
184 
Sum of last three, 280 
364 
615 
362 
192 
65 
87 
241 
456 
559 
342 
259 
3722 
Mairan's numbers are probably included by Kaamtz ; a few of the aurora\ included in 51. Hansteen's list, are identical with I 
those in my own. 
* It should be remarked, that the latitude of Afakerstoun, or perhaps even a lower latitude, is better fitted for this investiga- 
tion, than much higher latitudes; at least this is the ease as long as only frequency of visibility can be considered. The French 
Commission du Nord, during tlieir stay in Lapland, found aurora> existing, or probably existing, almost every night. In such places 
variation of frequency there is none, and variation of intensity alone remains for investigation. It is obvious, that till some better 
mode of measuring this intensity can be devised for these high latitudes, we are forced to perform this operation in a rude manner, 
l)y moving to lower latitudes, where the fainter aurora; become invisible, and where, therefore, frequency is a test of intensity be- 
yond a certain limit. 
