Fog in moderation, the thin vapor on the open sea, and so forth, simply gives a lightness to the blue tint, or more plentiful, an absolute whiteness to the atmosphere. Now let us see whether we are yet able to make out the philosophy of a fine autumn sunset. As the sun comes near the horizon, he and the air about him become red, because the light from that direction has been robbed of the blue rays in traversing horizontally so large a portion of the atmosphere. The sky in the zenith pales, for it has little but the absorbed or diffused light to exist upon. Presently, we see a redness in the east, quite opposite to the sun, and this redness increases till the sun sinks from our sight. In this case, the last rays of the sun that traverse the whole breadth of the atmosphere, reflected from the east, from vapors there, and more especially from clouds, come red to our eyes; no blue can be remaining in them. From the west, where the sun is setting, the rays come from the surrounding air, and from the clouds, variously colored; they lose their blue, but there remain the red, green, orange, yellow, and the purple rays; and some or all of these may make the tints that come to us, according to the state and nature of the clouds, the atmosphere, and other circumstances that may modify the process of refraction. The sun has set; it is immediately below the horizon, and its rays still dart through all our atmosphere, except that portion which is shielded from them by the intervening shadow of the earth. That shadow appears in the east, soon after sunset, in the shape of a calm blue arch, which rises gradually in the sky, immediately opposite to the part glorified by sunset colors. Over this arch the sky is red, with the rays not shut out by the round shadow of our ball. As the sun sinks, our shadow of course rises; and within it there can be only the diffused twilight, always blue. When this arch--this shadow of the earth--has risen almost to the zenith, and the sun is at some distance below the horizon, then the red color in the west becomes much more distinct and vivid; for the sun then shoots up thither its rays through a still larger quantity of intervening atmosphere; so that the redness grows as the sun sinks, until the shadow of the earth has covered all, and the stars--of which the brightest soon were visible--grow numerous upon the vault of heaven. When stars of the sixth magnitude are visible, then, astronomically speaking, twilight ends. The length of twilight will depend upon the number of rays of light that are reflected and dispersed, and that, again, will depend entirely on the atmosphere. Where there is much vapor, and the days are dull by reason of the quantity of kidnapped light, there compensation is made by the consequent increase of twilight. In the interior of Africa night follows immediately upon sunset. In summer the vapor rises to a great height, and pervades the atmosphere; the twilight then is longer than in winter,