Ixxxii Geneeal Results of the Makeestoun Observations. 



which (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. Jane. Jnly. Aug. Sept. Oct. Not. Dec. 



16 24 38 31 8 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 \nth season, of 

 which, indeed, a proof is to be found in the gTeat 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 diffei-ence, liowevcr, 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 sufticiently close to 

 permit us to complete it, by assuming that the number of auroras is a principal minimum in summer. 



173. Variation of Frequenci/ of the Aurora Borcalis ivlth the Mooti's Age. — This investigation 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 Q\ years' observations : — 



Moon's Age. 



28^ — S"* 



3d_7d 



gd— 12d 



13d— 174 



18d— 22<' 



23<i_27d 



Number. 



5-8 



5-2 



3-6 



5-0 



10-2 



6-6 



Did aurora? occur indifierently 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 auroras, 

 therefore, is a function of the moon's age. In order to determine the actual law, we may consider the probable 

 eft'ect 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 tlu-ee 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 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 ; afterMards it 

 increases in brightness, and the maximum effect in extinguishing faint aurorse is evidently attained at opposition, 

 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 Kaemtz (Complete Course of Meteorology, translation by Walker, p. 458) ; and by Hansteen (Mem. de I'Acad. Roy. 

 de Belgique, t. xx., p. 117). 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. Sum. 



Mairan, 



21 



27 



22 



12 



1 



5 



7 



9 



34 



50 



26 



15 



229 



Kaemtz, 



229 



307 



440 



312 



184 



65 



87 



217 



405 



497 



285 



225 



3253 



Hansteen, 



29 



31 



47 



34 



2 











17 



35 



33 



34 



23 



285 



J. A. Broun, 



22 



26 



28 



16 



6 











7 



16 



29 



23 



11 



184 



Sum oflast three, 280 



364 



515 



362 



192 



65 



87 



241 



456 



559 



342 



259 



3722 



Mairan's numbers are probably included by Kaemtz ; a few of the aurorae, included in M. Hansteen's list, are identical with 

 those in my own. 



* It 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 aurorse 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 tiU 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 aurorse become invisible, and where, therefore, frequency is a test of intensity be- 

 yond a certain limit. 



