is partly a reflection from invisible
moisture globules and partly due to floating particles of dust that may
have been driven up from the surface of the earth, or may be the ashes
of meteorites disintegrated by contact with the air.

Some years ago, commencing in August, 1883, there was a wonderful
exhibition of red skies at sunset that lasted for several hours after
twilight ordinarily disappears. This phenomenon ran through a period of
several weeks, gradually fading away. It was afterward determined that
these displays were occasioned by small particles of ashes or dust
floating high in the air, that were thrown off from the volcanic
eruption of Krakatoa in the Island of Java. By the general circulation
of the air the ashes were carried to all parts of the world, making a
circuit of the earth in from twelve to thirteen days--which showed a
velocity of over eighty miles an hour. This is an instance of the high
velocity of the air currents in the upper regions of the atmosphere. The
reason why the illumination extended so late in the night was because of
the great height that these particles of dust attained. The higher the
reflecting surfaces are in the air the longer they may be seen after
sunset. Ordinary twilight is caused by a reflection of sunlight from the
upper air; and from its duration as ordinarily observed it is estimated
that the reflection does not proceed from a point more than thirty-six
miles high. In the higher latitudes the twilight is long, from the fact
that the sun does not go directly down, and if we go far enough north
the whole night is twilight. In the tropical regions the twilight is
shorter than at any other point on the globe for reasons that are
obvious. The sun there goes directly down and is soon hidden behind the
earth.

There are other optical effects to be seen sometimes on the horizon
somewhat resembling twilight. The "aurora borealis" (northern lights),
which we describe in Vol. III., is seen in the northern skies at certain
times, and has very much the appearance of twilight in some of its
phases. It is constantly changing, however, and is easily distinguished
by anyone who has observed both. These appearances are undoubtedly
electrical. There is another phenomenon seen in the arctic regions that
causes a band of white light to appear on the horizon called "ice
blink," and it is caused by the reflections from the great icebergs that
abound in that region.

Curious optical effec