236 PHYSICS. 
in winter and in the colder countries (especially near the poles). It is a 
little remarkable, on this hypothesis, that they should occur in summer ; but 
even then particles of ice may float at a great height in the air. Upward 
and downward currents of air are unquestionably of influence, since, after 
south winds have saturated the air with moisture, an ascending current 
carries this to such a height as immediately to freeze the moisture condensed 
by the cold; or a descending cold current may produce the same effect on 
the moisture of an inferior current. In this way we explain the very rare 
occurrence of halos in the torrid zone. 
Of the Zodiacal Light. 
About the time of the vernal or autumnal equinox, and shortly before the 
rising or after the setting of the sun, there is seen a whitish streak of light, 
tending in the direction of the ecliptic or of the zodiac (whence the name), 
and running out above into a point. This is called the zodiacal light 
( pl. 27, fig. 13), which in spring is seen more frequently in the western 
evening sky, and in autumn early in the morning of the eastern 
heavens. The light of this object is generally much feebler than that of the 
milky way, being brightest in the neighborhood of the sun, feebler towards 
the point, and never so bright as not to allow the lesser stars to shine through 
it. The shape of the zodiacal light is that of a pyramid or cone, whose base 
rests on the sun; more accurately, that of a very eccentric ellipse, whose 
variable major axis passes through the sun, and is at least five times as long 
as the minor. The apex seems sometimes to run out into two straight lines 
which form an angle of 10°—26°; the cone, therefore, appears truncated. 
The axis falls nearly in the plane of the sun’s equator. The length. 
measured from the sun, amounts generally to not over 43°; sometimes, 
however, to 100°, and in single cases in the torrid zone even to 120°. 
The greatest breadth in the vicinity of the horizon varies from 8° to 30°. 
In the torrid zone, where the zodiacal light stands almost perpendicularly 
to the horizon, it is not only more frequently seen, but is far more brilliant and 
remarkable than in higher latitudes. Even in the island of Guadaloupe it 
may be seen at any time of the year with a clear sky, while in our own 
latitude it is visible only in spring and autumn. Humboldt observed it on the 
Andes, and on the plains of Venezuela, as also at sea, more luminous than the 
milky way in the constellation Sagittarius, and the exhibition was more bril- 
liant when the vaporous exhalations were projected against the light. At sea, 
between 10° and 14° N. L., he observed it for several weeks in summer in 
ereat beauty, and presenting a splendid play of colors: becoming visible an 
hour after sunset, it became weaker towards ten o'clock, and vanished 
almost entirely about midnight. He was especially struck by the varying 
brightness of its lustre: after being at its brightest, it would in a few 
minutes after become remarkably enfeebled, and in particular cases he 
thought he observed a kind of flickering and waving of the light. Our figure 
( pl. 27, fig. 18) represents the zodiacal light as observed by Horner, in the 
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