year. As a consequence of this, the pole
shifts its place through a circle of, roughly, a few yards in width
during the time in question. This movement is, perhaps, the combined
result of two causes. One of these is the change of place during the
year of large masses of material upon our earth; such as occurs, for
instance, when ice and snow melt, or when atmospheric and ocean
currents transport from place to place great bodies of air and water.
The other cause is supposed to be the fact that the earth is not
absolutely rigid, and so yields to certain strains upon it. In the
course of investigation of this latter point the interesting conclusion
has been reached by the famous American astronomer, Professor Simon
Newcomb, that our globe as a whole is _a little more rigid than steel_.

We will bring this chapter to a close by alluding briefly to two strange
appearances which are sometimes seen in our night skies. These are known
respectively as the Zodiacal Light and the Gegenschein.

The _Zodiacal Light_ is a faint cone-shaped illumination which is seen
to extend upwards from the western horizon after evening twilight has
ended, and from the eastern horizon before morning twilight has begun.
It appears to rise into the sky from about the position where the sun
would be at that time. The proper season of the year for observing it
during the evening is in the spring, while in autumn it is best seen in
the early morning. In our latitudes its light is not strong enough to
render it visible when the moon is full, but in the tropics it is
reported to be very bright, and easily seen in full moonlight. One
theory regards it as the reflection of light from swarms of meteors
revolving round the sun; another supposes it to be a very rarefied
extension of the corona.

The _Gegenschein_ (German for "counter-glow") is a faint oval patch of
light, seen in the sky exactly opposite to the place of the sun. It is
usually treated of in connection with the zodiacal light, and one theory
regards it similarly as of meteoric origin. Another theory,
however--that of Mr. Evershed--considers it a sort of _tail_ to the
earth (like a comet's tail) composed of hydrogen and helium--the two
_lightest_ gases we know--driven off from our planet in the direction
contrary to the sun.

[13] Every one knows the simple experiment in which a coin lying at the
bottom of an empty basin, and hidden from the eye by its side, becomes
visible when a certain q