PHYSICAL ASTRONOMY. 145 
shines with an indescribable hue. It is only recently that astronomers have 
included the Aurora or polar light within the circle of their observations, 
and have found in almost every case, that simultaneously with the appear- 
ance of a northern light, there are certain phenomena about the south pole, 
sometimes consisting only of unusual disturbance of the magnetic needle. 
The eighteenth century was very prolific of northern lights, particularly the 
middle part of it; since 1820 they have also become much more frequent. 
Pl. 14, fig. 57, is a view of the remarkable Aurora Borealis which was seen 
at Christiania in Norway, at 6 o'clock in the evening of January 7, 1831. 
Fig. 58 represents an Aurora Australis or south polar light. 
The mock suns and mock moons, or parhelia and parselenia, are a result 
of a peculiar reflection of the light of the sun, of the moon, and even of the 
brighter planets and fixed stars, upon particles of condensed vapor or ice 
crystals. When these halos are very much complicated about the sun and 
moon, they appear as if composed of many circles intersecting each other, 
of which one is generally horizontal, encircling the whole heavens. It is in 
this case that they form mock suns and mock moons. Generally, only two 
or four are seen; Hevelius, however, in 1660, saw six at once; sometimes 
they have comet-like trains. PJ. 14, fig. 59, exhibits a view of two such 
mock suns. | 
The Shooting Stars. 
63. The shooting stars have been long and universally known, but it is 
only in recent times that their true character, their peculiarities, and the 
various circumstances under which they are perceived, have been matters 
of observation to astronomers and meteorologists. The periodical and 
abundant return of the shooting stars towards the middle of August and 
November, has in many places been diligently observed and investigated. 
We need only mention the efforts of Benzenberg, Brandes, Olbers, Bessel, 
Erman, Boguslawski, Quetelet, Feldt, Herrick, and Olmstead, to ascertain 
their direction, their height above the earth, and their velocity. The reasons 
for the now generally received hypothesis (of Alexander von Humboldt) are 
well known; his theory being that these luminous appearances are caused 
by innumerable small bodies revolving about the sun, which become visible 
by their combustion when entering the atmosphere of our earth. There 
are yet many difficulties in the way of the establishment of this theory, as 
also of the supposition of Biot, that these falling stars are the same bodies 
which, seen at a distance, form the zodiacal light. 
Benzenberg and Brandes divide all shooting stars into three classes: 1, of 
the first and second magnitudes, similar to balls of fire, in which may be dis- 
tinguished a ball with aluminous train; 2, of the first and second magnitudes, 
without the ball, and with a luminous path; 8, from the third to the sixth 
magnitudes, the last being telescopic, and only visible through a comet 
seeker. 
The number of shooting stars is incredibly great. Humb:! it and Bon- 
ICONOGRAPHIC ENCYCLOPADIA.—VOL, I. 10 145 
