226 PHYSICS. 
atmosphere, this enfeebling of light is much less than at the surface of the 
earth, on account of the greater rarity of the strata of air. Watery vapor, 
and the smoke and dust carried aloft in various ways, exert great influence 
on the transparency of the air. Gaseous vapor appears to increase in a 
very high degree this transparency, as oil does that of paper. Nevertheless 
in opposition to this theory, the dry air in the interior of continents, as in 
Siberia, Persia, Africa, and Brazil, is remarkably transparent. Condensed 
vapor may diminish or even entirely destroy the clearness of the atmo 
sphere. 
The remarkable blue: of the sky, or rather of the atmosphere, is not of the 
same tint in all places and at all times. Inand about the zenith the heavens 
generally appear the darkest, decredsing thence to the horizon. This color 
is very dark blue when seen from high mountains. It is darker in the © 
torrid zone than in higher latitudes; in Italy and Greece more so than in 
Germany ; above the sea more than over the land in the same latitude. To 
compare the various shades of blue, and to determine them according to a 
given scale, Saussure invented the cyanometer. This consists of a circular 
plate, on whose periphery, white and black, and between the two, fifty-one 
different shades of blue are painted. The principle on which this scale is 
constructed depends upon the fact that the difference of color of two 
surfaces painted with different shades of the same color, vanishes entirely at 
a certain distance. Two blue surfaces differ, now, by one degree when the 
difference of their color is insensible at a given distance (according to 
Saussure, that at which a black circle of 12 lines in diameter in his diapha- 
nometer vanishes entirely). To determine the blue of the sky at a given 
place, the cyanometer is held between the eye and the spot until the color 
corresponding to the former is found on the latter. Saussure and others 
found the color of the zenith in Chamouny to be nineteen, in Geneva 
twenty-two and a half, on the Col de Géant thirty one, and on Mont Blane 
thirty-nine degrees. Parrot has proposed the use of colored disks to deter- 
mine these shades. Arago has suggested the use of polarized light for this 
purpose ; his method, however, is not likely to be much employed. The 
reason of the blue color of the sky depends, according to Newton, on blue 
being the color of the particles of air, these absorbing all the other rays and 
transmitting the blue. Jf this hypothesis were correct, then distant snow- 
clad mountains would appear blue, which is not the case. The reason of 
the varying blueness of the sky is to be ascribed to the influence of vesicles 
of vapor, which may be proved by the fact that on the sea-coast the sky is 
darker on the land than on the sea side of the zenith, at the same zenith 
distance. The maximum of blue appears generally to be presented some 
time after the culmination of the sun. 
Besides the blue color, the heavens frequently present at the rise or 
setting of the sun, a beautiful redness, the so-called morning and evening 
red, whose color changes into innumerable shades from yellow and bright 
red to dark red. The evening red (generally more abundant and brilliant 
than the morning)'is most beautiful when the sky is of a very deep blue. 
The sun then at his setting appears very luminous, although not very red; 
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