tions of different
wave-lengths. From crest to crest, or from trough to trough, is the
length of the wave.]

[Illustration: THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT

The electric current passing in the direction of the arrow round the
electric circuit generates in the surrounding space circular magnetic
circuits as shown in the diagram. It is this property which lies at the
base of the electro-magnet and of the electric dynamo.]

[Illustration: THE MAGNET

The illustration shows the lines of force between two magnets. The lines
of force proceed from the north pole of one magnet to the south pole of
the other. They also proceed from the north to the south poles of the
same magnet. These facts are shown clearly in the diagram. The north
pole of a magnet is that end of it which turns to the north when the
magnet is freely suspended.]

The Fate of the World

Professor Soddy has given an interesting picture of what might happen
when the sun's light and heat is no longer what it is. The human eye
"has adapted itself through the ages to the peculiarities of the sun's
light, so as to make the most of that wave-length of which there is
most.... Let us indulge for a moment in these gloomy prognostications,
as to the consequences to this earth of the cooling of the sun with the
lapse of ages, which used to be in vogue, but which radio-activity has
so rudely shaken. Picture the fate of the world when the sun has become
a dull red-hot ball, or even when it has cooled so far that it would no
longer emit light to us. That does not all mean that the world would be
in inky darkness, and that the sun would not emit light to the people
then inhabiting this world, if any had survived and could keep
themselves from freezing. To such, if the eye continued to adapt itself
to the changing conditions, our blues and violets would be ultra-violet
and invisible, but our dark heat would be light and hot bodies would be
luminous to them which would be dark to us."

Sec. 12

What the Blue "Sky" means

We saw in a previous chapter how the spectroscope splits up light-waves
into their colours. But nature is constantly splitting the light into
its different-lengthed waves, its colours. The rainbow, where dense
moisture in the air acts as a spectroscope, is the most familiar
example. A piece of mother-of-pearl, or even a film of oil on the street
or on water, has the same effect, owing to the fine inequalities in its
surface. The atmos