corona, and has been most industriously studied and photographed during
nearly every total eclipse for thirty years. Thus we have learned much
about how it looks and what its shape is. It has a fibrous, woolly
structure, a little like the loose end of a much-worn hempen rope. A
certain resemblance has been seen between the form of these seeming
fibres and that of the lines in which iron filings arrange themselves
when sprinkled on paper over a magnet. It has hence been inferred that
the sun has magnetic properties, a conclusion which, in a general way,
is supported by many other facts. Yet the corona itself remains no less
an unexplained phenomenon.

[Illustration with caption: PHOTOGRAPH OF THE CORONA OF THE SUN, TAKEN
IN TRIPOLI DURING TOTAL ECLIPSE OF AUGUST 30, 1905]

A phenomenon almost as mysterious as the solar corona is the "zodiacal
light," which any one can see rising from the western horizon just
after the end of twilight on a clear winter or spring evening. The most
plausible explanation is that it is due to a cloud of small meteoric
bodies revolving round the sun. We should hardly doubt this explanation
were it not that this light has a yet more mysterious appendage,
commonly called the Gegenschein, or counter-glow. This is a patch of
light in the sky in a direction exactly opposite that of the sun. It is
so faint that it can be seen only by a practised eye under the most
favorable conditions. But it is always there. The latest suggestion is
that it is a tail of the earth, of the same kind as the tail of a comet!

We know that the motions of the heavenly bodies are predicted with
extraordinary exactness by the theory of gravitation. When one finds
that the exact path of the moon's shadow on the earth during a total
eclipse of the sun can be mapped out many years in advance, and that
the planets follow the predictions of the astronomer so closely that,
if you could see the predicted planet as a separate object, it would
look, even in a good telescope, as if it exactly fitted over the real
planet, one thinks that here at least is a branch of astronomy which is
simply perfect. And yet the worlds themselves show slight deviations in
their movements which the astronomer cannot always explain, and which
may be due to some hidden cause that, when brought to light, shall lead
to conclusions of the greatest importance to our race.

One of these deviations is in the rotation of the earth. Sometimes, for
several y