348 
NATURE 
[August 13, 1885 
THE AURORA? 
II, 
WE next come to the II-year period. On this the 
following pertinent remarks are made :— 
“Tt will be perfectly clear that it is chiefly observations 
from the Temperate Zone which have constituted the 
material for demonstrating the eleven-year period But 
as regards the Polar regions, it has been assumed that 
either the Aurora Borealis also follows the same laws in 
these parts, or that it appears with the same force and 
same manner all the year round. Neither of these alter- 
natives seem, however, to be right, as a series of observa- 
tions prosecuted with great care during fifteen years at 
Godthaab, in Greenland, have brought me to the some- 
what remarkable conclusion that, as regards the varying 
frequency of the Aurora Borealis at Godthaab, the daw 
seems to be the reverse of thatruling in southern latitudes. 
“These researches, which were effected by Mr. S. 
Kleinschmidt, extend over a period from August 1865 to 
May 1880. The number of days with aurore in the year, 
reckoned from August to May, were :— 
1865-66 66-67 67-68 68-69 69-70 70-71 71-72 72°73, «73°74 
OF 1T2) | 63) 5 184. * | Abe Ol See eae 
1874-75 75°76 7677 77-78 78-79 79°80 
97. «97 104 69 100 75 
“This series cannot, however, be accepted as giving the 
exact view of the relation between the varying frequency 
of the auroree, because the state of the clouds would 
exercise a great influence on the visibility of aurora. If 
thus the clouds vary greatly from one year to another, 
this circumstance would greatly reduce the number of 
aurore. A closer study of the nebulous conditions at 
Godthaab, compared with the frequency of the aurore, has 
caused me to consider that the number of aurorz decrease 
in proportion as the clouds increase in quantity. The 
above-recorded auroral totals must, therefore, be reduced 
to the same cloud unit, ze. it must be calculated how 
great the number would have been had the nebuiosity 
been the same every year. By this we obtain the values 
given under N. Under S. is given the relation between 
the sun-spots in the same year (July to June). 
1865-66 66-67 67-68 70-71 71-72 72-73 
IN| (86°2'  91°3 (6774 56°5 32:0 46'0 
S. 23°5 61 183 1335 986 8094 
1873-74 7475 75-76 76-77 77-78 7879 79-80 
IN: 87854 (o7to! 950) TOzion 173Olo hee 0375 
De bine B22ie yc walses 68 Z2ee 053 
“Tf the two series be compared it will be found that 
the law of relation between the frequency of sun-spots and 
aurore ts reversed. This fact will be still more apparent 
from Fig. 4, where both series of auroree and sun-spots 
are shown graphically. 
“The few series of observations which we possess from 
other Polar regions, and which I have been able to analyse, 
indicate, though incomplete, similar conditions. 
“ As the greater part of the Godthaab observations were 
made in the morning, I have not only used the auroral 
days for my researches—reckoned from noon to noon— 
but also examined evening and morning aurore separately. 
The evening and morning aurore lead, as regards the 
eleven-year period, to the same conclusion as the auroral 
days, z.e. that the Aurora Borealis ts scarcest under sun- 
Spot maxima.” . . 
“If we consider the relation between zenith and 
southern aurore in the various years of the Godthaab 
researches, we obtain the interesting result that the per- 
centages of zenith aurore in the main follow those of the 
auroral frequencies, 2.c., that at the periods of maxima at 
1 “Under the Rays of the Aurora Borealis.” 
by Carl Siewers. (London: Sampson Low and Co., 
p. 276. 
By S. Tromholt. Edited 
8Bc 
1885.) Continued from | 
Godthaab, the aurore which fall in the zenith of this 
place or further north, are not only absolutely, but also 
relatively, more frequent than at the periods of minima. 
And what deduction may be drawn from this? The 
deduction ¢hat the auroral zone in the course of the 
eleven-year period makes a movement too, of such a nature 
that it lies further north when the sun-spots are in their 
minimum than in their maximum period. 
“Tf this result be compared with what I have already 
propounded as to the eleven-year period in the Arctic 
regions, the interesting explanation will be obtained of 
the phenomenon, that this period in Greenland and 
similarly-situated places shows a reverse course to that in 
more southern regions. Zhe auroral maximum, occurring 
in the temperate regions simultaneously with the sun-spot 
maximum, ts due to the auroral zone being then in its 
southernmost position, which again causes an auroral 
minimum in the polar regions, and, in a reverse manner, 
the auroral zone has tts northernmost position when the 
sun-spots are in the minimum, which then causes an 
auroral minimum in the temperate regions and a maxt- 
mum one in those around the Pole.” 
Fic. 4.—Comparison of auroral frequencies at Godthaab with sun-spots. 
N = Northern light frequencies ; S = Spot frequencies. 
These very beautiful results will show that there can be 
very little doubt about the movement of the auroral zone 
as a whole. 
The next point on which much light is thrown by Mr. 
Tromholt’s work is that the various appearances are in the 
main due to movements bringing auroral striz in different 
relation to the spectator :— 
“In one respect in particular my sojourn at Koutokeino 
was very instructive—viz. with regard to the understand- 
ing of the true shape and position, and the changes to 
which the aurora is apparently subjected when altering 
its elevation above the horizon. Partly through the 
frequency of the aurora, and partly by its appearance 
now in the north, now in the south, and now in the 
zenith, there were excellent opportunities of studying the 
modifications which the form suffered as it changed its 
position in relation to the observer. 
“From this I came to the conclusion that the great 
many different forms referred to might certainly be re- 
duced to a few fundamental ones. In most instances the 
aurora forms belts or zones, which stretch across the earth 
in the direction of the magnetic east-west, which zones are 
formed by a conglomeration of thin sheets of luminous 
matter ranged one behind the other, their direction being 
