explains the redness of the sunrise and
the sunset. It will now easily be understood,
also, why the blue colour of the sky is deepest
in the zenith, faintest when we look over the
horizon; why the blue is at noon deeper than
after mid-day; why it grows more intense as
we ascend to higher elevations. From what we
have already said, the reason of these things
will come out with a very little thought.
Again, in the example of our London fogs, &c.,
when in the upper portion of the dense mass
the blue rays have been all refracted, there
can penetrate only those other rays which
make the lurid sky, with which we are
familiar, or the genuine old yellow fog. Fog
in moderation, the thin vapour 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, arid 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 vapours
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 coloured:
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 colours. 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 colour 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 vapour, 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 vapour
rises to a great height, and pervades the
atmosphere; the twilight then is longer than
in winter, when the colder air contains less
vapour, and the vapour it contains lies
low.

Now, since the appearances at twilight
depend on the condition of the sky, it follows
that our weather-wisdom, drawn from such
appearances, is based upon a philosophical
foundation. When there is a blue sky, and
after sunset a slight purple in the west,
we have reason for expecting fine weather.
After rain, detached clouds, coloured red
and tolerably bright, may rejoice those who
anticipate a picnic party. If the twilight
show a partiality for whitish yellow in its
dress, we say that very likely there will be
some rain next day; the more that whitish
yellow spreads over the sky, the more the
chance of water out of it. When the sun is
brilliantly white, and sets in a white light, we
think of storms; especially so when light high
clouds that dull the whole sky become deeper
near the horizon. When the colour of the
twilight is a greyish red, with portions of deep
red passing into grey that hide the sun, then be
prepared, we say, for wind and rain. The morning
signs are different. When it is very red, we
expect rain; a grey dawn means fine weather.
The difference between a grey dawn and a
grey twilight is this—in the morning, greyness
depends usually upon low clouds, which
melt before the rising sun; but in the
evening greyness is caused by high clouds,
which continue to grow denser through the
night. But if in the morning there be so
much vapour as to make a red dawn, it is
most probable that thick clouds will be formed
out of it in the course of the operations of the
coming day.

Refraction of light has a good deal to do
also with the twinkling of the stars; though
there may go to the explanation of that
phenomenon other principles which do not
concern our present purpose. The air contains
layers of different density, shifting over each
other in currents. The fixed stars are, to our
eyes, brilliant points of light; their rays
broken in passing through these currents,
exhibit an agitation which is not shown by
the planets. The planets are not points to our
sight, not points to our telescopes; being
much nearer, although really smaller, they