m 
Professor Haksteek. 
thereof. Thus, I assumed for unity the intensity correspond- 
ing to the duration of 813", 6 (to 300 vibrations), taking this to 
be a minimum, because I found this intensity at an observation 
which I made whilst there was an aurora borealis. 
Let I be the intensity, and T the duration corresponding to 
it. 
Again, I' another intensity, and T' the duration correspond- 
ing. 
Then, I : I' = (T')* : ; or I = I' (^) ; and as I have 
assumed I' 1, and T 813",6, we have, I = ^ ^ ; 
consequently, in the above example, I z= 1,0079. 
In this way I have calculated for each a Table for the re- 
duction of the mean durations, and corresponding intensities, of 
which the following is an abstract : 
Mean 
Dura- 
tion. 
Inten- 
sity. 
Mean 
Dura- 
tion. 
Inten- 
sity. 
Mean 
Dura- 
tion. 
Inten- 
sity. 
Mean 
Dura- 
tion. 
Inten- 
sity. 
813,6 
1,0000 
811,0 
1,0064 
808 
1,0139 
805 
1,0215 
813,0 
1,0015 
810,0 
1,0089 
807 
1,0164 
804 
1,0240 
812,0 
1,0039 
809,0 
1,0114 
806 
1,0189 
803 
1,0265 
From the numerous observations which I have made in the 
course of the year, I have calculated the following Table, which 
will shew for each month a mean intensity corresponding to the 
stated hour of the day. 
Hour.] 8 j 10 I 12, | 
I 6 i 9 I 10 I Mean. 
1819, 
Dec. l,0,1931il, 01902 
1,01915 11,01966 
1,01929 
1,0173211,01912 
1820, 
Mar. 
I I I I I 
l,01095|i, 0101 0il,01023'l,01136|l,011471, 01113 
1,01142 1,01063 
1,01081 
Hour of the day. 
8 A.M. 
10| 
4 P. M. 
7 
104 
Mean. 
1820, April, 
1,0.0717 
1,00^25 
1,00879 
1,00966 
1,00903 
1,00818 
May, 
1,00582 
1,00548 
1,00819 
1,00844 
1,00740 
1,00713 
June, 
1,00407 
1,00397 
1,00647 
1,00700 
1,00665 
1,00563 
July, 
1,00277 
1,00235 
1,00461 
1,00500 
1,00548 
1,00404 
August, 
1,00339 
1,00335 
1,00543 
1,00570 
1,00555 
1,00468 
September, 
1,00560 
ls00508 
1,00708 
1,00711 
1,00715 
1,00640 
1 October, 
1,00886 
1,00800 
1,00909 
1,00953 
1,00953 
1,00900 
