Ixxxil GENERAL RESULTS OF THE MAKERSTOUN OBSERVATIONS. 
which (even in the months of August and May) there is little twilight to extinguish aurore. The numbers 
are as follow, for these five hours in each month of the years 1843-8 :— 
Jan. Feb. March, April. May. June. July. Ang. Sept. Oct. Noy. 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 autums 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 aurora is a principal minimum in summer, 
173. Variation of Frequency of the Aurora Borealis with the Moon’s Age.—This investigation is evidently 
beset with considerable difficulty, since the moonlight existing nearly extinguishes the appearances of all the 
fainter class of aurorsz, 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 aurorz 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 aurore for one day of the moon’s age in each group as 
follows, from the 6} years’ observations :— 
Moon’s Age. 2ga_o4 3a—74 Sa—]2¢ 134174 1gi—220 234974 
Number. 5°8 5:2 36 5:0 10-2 6°6 
Did aurore ocewr indifferently at all ages of the moon, we should expect to see the greatest number at conjunction, 
and the least number at opposition; this however is not the case, the greatest number was seen about two days 
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 equidistant from the epoch of opposition. The frequency of aurore, 
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 
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 suffi- 
cient light to render the earlier of the faint aurore 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 aurore ; afterwards it 
inereases in brightness, and the maximum effect in extinguishing faint aurorz is evidently attamed at opposition, 
when the moon begins to rise late enough to allow the earlier aurore to be visible; towards the end of the 
1733, p. 199); by Kemtz (Complete Course of Meteorology, translation by Walker, p. 458); and by Hansteen (Mem. de l’Acad. Roy. 
de Belgique, t. xx., p. 117). 
Jan. Feb. March. April. May. June. July. Ang. Sept. Oct. Noy. Dec. Sum. 
Mairan, . 21 27 22 12 1 5 7 9 34 50 26 15 229 
Kemtz, 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 é 16 29 23 11 184 
Sum of last three, 280 364 515 362 192 65 87 241 456 559 342 259 3722 
Mairan’s numbers are probably included by Kemtz; a few of the aurore, included in M. Hansteen’s list, are identical with 
those in my own, 
* Tt should be remarked, that the latitude of Makerstoun, or perhaps even a lower latitude, is better fitted for this investiga- 
tion, than much higher latitudes; at least this is the case as long as only frequency of visibility can be considered. The French 
Commission du Nord, during their stay in Lapland, found aurore 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, 
by moving to lower latitudes, where the fainter aurore become invisible, and where, therefore, frequency is a test of intensity be- 
yond a certain limit. 
