of the
planets.

[Illustration: _Royal Observatory, Cape of Good Hope._

A GREAT COMET.]

To get a good idea of a really fine comet, until we have the opportunity
of seeing one for ourselves, we cannot do better than look at this
picture of a comet photographed in 1901 at the Cape of Good Hope. It is
only comparatively recently that photography has been applied to comets.
When Halley's comet appeared last time such a thing was not thought
of, but when he comes again numbers of cameras, fitted up with all the
latest scientific appliances, will be waiting to get good impressions of
him.

CHAPTER IX

SHOOTING STARS AND FIERY BALLS

All the substances which we are accustomed to see and handle in our
daily lives belong to our world. There are vegetables which grow in the
earth, minerals which are dug out of it, and elementary things, such as
air and water, which have always made up a part of this planet since man
knew it. These are obvious, but there are other things not quite so
obvious which also help to form our world. Among these we may class all
the elements known to chemists, many of which have difficult names, such
as oxygen and hydrogen. These two are the elements which make up water,
and oxygen is an important element in air, which has nitrogen in it too.
There are numbers and numbers of other elements perfectly familiar to
chemists, of which many people never even hear the names. We live in the
midst of these things, and we take them for granted and pay little
attention to them; but when we begin to learn about other worlds we at
once want to know if these substances and elements which enter so
largely into our daily lives are to be found elsewhere in the universe
or are quite peculiar to our own world. This question might be answered
in several ways, but one of the most practical tests would be if we
could get hold of something which had not been always on the earth, but
had fallen upon it from space. Then, if this body were made up of
elements corresponding with those we find here, we might judge that
these elements are very generally diffused throughout the bodies in the
solar system.

It sounds in the highest degree improbable that anything should come
hurling through the air and alight on our little planet, which we know
is a mere speck in a great ocean of space; but we must not forget that
the power of gravity increases the chances greatly, for anything coming
within a certain range of the e