METEOROLOGY. Pa a | 
even before sunset the whole horizon appears reddish yellow, this increasing 
until just as the sun is going out of sight: a remarkable reddening simulta- 
neously presents itself in the eastern heavens, opposite to the sun, reaching 
its maximum as the sun goes down. Just after sunset, a light purple red is 
often seen shading the whole blue of the sky, and with the increasing 
descent of the great luminary the eastern heavens become dark, and there 
is often observed there a more or less evident segment of dark blue, whose 
highest point is directly opposite to the sun, and which is generally sharply 
defined (sometimes by a white or yellow border). ‘The lustre of the western 
skies passes at the same time from the golden, more into the red; this 
portion, however, not extending very high. The evening red appears most 
magnificent when, with a deep blue sky, there are some clouds in the west. 
Should these be cirro-stratus clouds, they appear before sunset as grey 
streaks with bright borders, the latter becoming afterwards golden yellow, 
and often fiery red. Clouds near to each other are often very differently 
tinted, some dark red, some yellow; this depending on a higher or lower 
position in the atmosphere. 
This redness of the evening and morning sky has been generally explained 
by saying that the air transmits more readily the yellow and red rays. while 
it reflects more readily the blue. The sun’s rays having to traverse a 
considerable space through the atmosphere at his rising and setting, the air 
has an opportunity of acting to most advantage on his light, and of decom- 
posing it, absorbing most of the blue. More recently, Forbes has attempted 
to show that this explanation cannot be correct, since the blue of the sky, 
strictly speaking, is not complementary to the evening red; the latter he 
explains by the watery vapor contained in the air, which, when perfectly 
gaseous, is quite transparent and colorless, but in its passage to its condition 
of condensation, transmits only the yellow and red rays. 
_ The evening red and the morning grey are generally considered to be 
signs of fine weather: that the morning red Is indicative of bad weather is 
by no means so universally correct. When the evening red is tinted with 
a soft purple red it certainly indicates continued fine weather, which is not 
the case with a whitish yellow, a very red, or a dull red evening sky. 
Of Twilight. 
Closely connected with the phenomena of the morning and evening red 
stands the twilight, that gradual passage from daylight to darkness, and the 
reverse. Although the term twilight applies to both cases, yet the latter 
only is meant when we speak of the dawn. When the sun shines on the 
upper strata of the air, after ceasing to illumine the surface of the earth 
with his beams, a certain portion of the light is reflected towards the earth. 
thus producing a considerable degree of light. The beginning of morning or 
the end of evening twilight is marked by the disappearance or appearance 
of stars of the sixth magnitude. The depth at which the sun stands below 
the horizon at the beginning or end of twilight is differently estimated, 
ICONOGRAPHIC ENCYCLOP£DIA.—VOL. I. 26 401 
