THE OPTICAL PHENOMENA OP THE ATMOSPHERE. 219 
side ; and the seventh on the sauie circle, a short distance without the 
smaller circle on the N. side. A lunar halo, and a small portion of the 
great circle, were seen for a short time by my assistant at 7 o’clock. 
“ This phenomenon was formed in cirri, and owed its disappearance to 
these clouds becoming cirrocumuli. The temperature was 32'2°, and the 
wet bulb thermometer 30'8°. The wind, a gentle breeze from E., the 
clouds floating in a S.W. current. At 12 30 a.m. (13th) the wind veered 
through N. to N.W.”* 
The Aurora- Borealis is a luminous phenomenon seen in high 
latitudes in both hemispheres, and would with more propriety 
be termed the Aurora Polaris. It is, however, more commonly 
spoken of as the Aurora Borealis, or Australis, according to which 
hemisphere is referred to. The appearances presented, consist 
of streaks or rays of Light, of greater or less intensity, diver- 
ging in every direction from a central point, and which became 
visible in the northern horizon soon after sunset. Frequently 
a dull confused mass of Light, of a pinkish hue, appears, and it 
is from this that the phenomenon derives its name.f 
The precise physical cause of the Aurora "Borealis is not yet 
satisfactorily ascertained, but all the appearances connected 
with it are electrical ; and its form, direction, and position, 
though ever varying, always bear a marked relation to the 
Magnetic meridian and poles. Whatever, therefore, be its 
physical nature, it is evident that the theatre of its action is the 
atmosphere, and that the agent to which the development is 
due is Electricity, associated, in some unascertained manner, 
with Terrestrial Magnetism. During the prevalence of Auroral 
displays, Magnetic needles are very generally subject to much 
disturbance — a fact first noticed by Halley. A copious deposi- 
tion of dew, sudden thaws, and severe gales in the English 
channel, are also among the occurrences which seem to be con- 
nected with the appearance of this interesting phenomenon. 
Increased brilliancy in the light of the stars situated in the 
vicinity of the display has also been noticed. If the reader 
should be in possession of an electrical machine, he may obtain 
a very satisfactory representation of the Aurora by discharging 
an intermittent supply of electricity through a partially exhausted 
glass tube bent into the form of a semi-circle. 
The Aurora Borealis seems subject to periodical visitations ; 
sometimes it is seen very frequently, at other times years pass 
by without any apparitions being noticed. Thus we find that 
few were noted in the seventeenth century ; at the beginning 
of the eighteenth, they recommenced, and lasted till about the 
middle of that century, when there was a partial cessation. 
During the last few years, as is well known, some splendid 
* Letter in The Times, November 15, 1859. 
t Aurora, the dawn. 
