it keeps going for a while after
it leaves your hand.

176. Clothes keep you warm, especially woolen clothes.

177. The Leaning Tower of Pisa does not fall over.

178. It is almost impossible to climb a greased pole.

179. Heat goes up a poker that is held in a fire.

180. A child can make a bicycle go rapidly without making his
feet go any faster than if he were walking.

CHAPTER FIVE

RADIANT HEAT AND LIGHT

SECTION 21. _How heat gets here from the sun; why things glow when
they become very hot._

If we were to go back to our imaginary switchboard we should find a
switch, between the heat and the light switches, labeled RADIATION.
Suppose we turn it off:

Instantly the whole world becomes pitch dark; so does the sky. We
cannot see the sun or a star; no electric lights shine; and although
we can "light" a match, it gives no light. The air above the burning
match is hot, and we can burn our fingers in the invisible flame, but
we can see nothing whatever.

Yet the world does not get cold. If we leave the switch off for years,
while the earth remains in darkness and we all live like blind people,
it never gets cold. Winter and summer are alike, day and night are
just the same. Gradually, after many ages, the ice and snow in the
north and in the far south begin to melt as the warmth from the rest
of the world is conducted to the polar regions. And the heat from
the interior of the earth makes all the parts of the earth's surface
warmer. Winds almost stop blowing. Ocean currents stop flowing. The
land receives less rainfall, until finally everything turns to a
desert; almost the only rain is on the ocean. Animals die even before
the rivers dry up, for the flesh eaters are not able to see their
prey, and since, without light, all green things die, the animals that
live on plants soon starve. Men have to learn to live on mushrooms,
which grow in the dark. The world is plunged into an eternal warm,
pitch-black night.

[Illustration: FIG. 60. It is by radiation that we get all our heat
and light from the sun.]

Turning off the radiation would cause all these things to happen,
because it is by radiation that we get all our heat from the sun and
all our light from any source. And it is by radiation that the earth
loses heat into space in the night and loses still more heat into
space during the winter.

We do not get our heat from the sun by conduction; we cannot, because
t