METEOROLOGY. 239 
in the two kinds of halos, the lesser and greater, the principal difference 
being, according to Kaemtz, that while cumulus cloud gives rise to the 
former, cirrus or very light cirro-stratus produces the latter. The two are 
never seen in the same cloud; when they do appear conjointly, the lesser 
is formed in a secondary cloud of fog. 
Sometimes halos are accompanied by other phenomena, namely, by 
circles and ares which pass through the sun, or by ares intersecting each 
other, and exhibiting the so-called mock suns and mock moons, or sun and 
moon dogs, more scientifically known as parhelia (pl. 25, fig. 138), AB. 
The simplest case is where, the sun or the moon being low in the sky, a 
portion of a vertical circle in the shape of a column stands directly over the 
luminary (rarely under it). A white circle, moreover, frequently passes 
through the sun, often encircling the whole heavens, and parallel to the 
horizon, the breadth being nearly equal to the diameter of the sun. Should 
there be then a vertical arc above and below the sun, the latter will stand 
in the middle of a cross, a phenomenon but rarely observed. The mock 
sus appear most generally where the inner ring and the horizontal circle 
intersect each other, and are themselves frequently seen when the circles 
are invisible. They have the color of the inner ring, but are usually 
provided on the outside with a long shining horizontal train. The higher 
the sun, the more do the mock suns lie outside the intersection of the 
circles, and at a considerable elevation of the sun, two mock suns on each 
side are sometimes visible. Sun dogs sometimes occur at a distance of 
ninety degrees from the sun. Rarer phenomena are, tangent circles at the 
highest and lowest parts of the ring; tangent arcs at a distance of sixty 
degrees from the lowest part of the ring; a sun opposite to the true sun, 
and at an equal height with it; &c. (figs. 14-20). The phenomenon 
observed on June 29, 1790, by Lowitz in St. Petersburg, is to be considered 
as normal in its character. It lasted for five hours in an atmosphere filled 
with vapor ; but cases of this perfection rarely occur. ‘The most important 
individual parts of this celebrated illustration of parhelia were: Ist, a ring 
round the sun of about twenty-two degrees’ radius, red within and bluish 
without; 2d, a second colored ring about the sun of twice the radius of the 
preceding; 3d, a white horizontal circle passing through the sun and 
encompassing the heavens; 4th, five mock suns on this circle—two of them 
colored with long brilliant trains a little outside of the lesser circle, two 
white at a distance of ninety degrees from the sun, and one white and pale 
directly opposite to the sun; 5th, a lively lustre at the uppermost point of 
the inner ring, where at times a contact arc was seen; 6th, an arc convex 
to the sun at the lowest point of the ring just mentioned ; 7th, a colored are, 
also convex to the sun, at the highest point of the greater ring; 8th, two 
pale arcs passing through the parhelia and the upper part of the inner ring ; 
9th, two colored circles touching the outer ring, whose points of tangency 
were about sixty degrees from the lowest point of the former ring. 
All these phenomena are explicable very satisfactorily by the refraction 
of light passing through prismatic crystals of ice, or snow crystals floating 
in the air. In proof of this, the appearances in question are most numerous 
409 
