Explain the following:

201. The fire in the open fireplace ventilates a room well by
making air go up the chimney.

202. A drop of water glistens in the sun.

203. Dust goes up to the ceiling and clings there.

204. When you look at a person under moving water, his face
seems distorted.

205. You sit in the sun to dry your hair.

206. Paste becomes hard and unfit for use when left open to
the air.

207. In laundries clothes are partly dried by whirling them in
perforated cylinders.

208. Circus balloons are filled by building a big fire under
them.

209. Unevenness in a window pane makes telephone wires seen
through it look crooked and bent.

210. You can see the image of a star even in a shallow puddle.

[Illustration: FIG. 69. When the light from one point goes through
the lens, it is bent and comes together at another point called the
focus.]

SECTION 24. _Focus._

How can you take pictures with a camera?

What causes the picture in the camera to be inverted?

Why is a magnifying glass able to set things on fire when you
let the sun shine through it?

In your eye, right back of the pupil, there is a flattened ball, as
clear as glass, called the _lens_. If the lens were left out of your
eye, you never could see anything except blurs of light and shadow.
If you looked at the sun it would dazzle you practically as much as it
does now. However, you would not see a round sun, but only a blaze of
light. You could tell night from day as well as any one, and you could
tell when you stepped into the shade. If some one stepped between you
and the light, you would know that some one was between you and the
light or that a cloud had passed over the sun,--you could not be quite
sure which. In short, you could tell all degrees of light and dark
apart nearly as well as you can now, but you could not see the form of
anything.

In the front of a camera there is a flattened glass ball called
the _lens_. If you were to remove it, the camera would not take any
pictures; it would take a blur of light and shade and nothing more.

[Illustration: FIG. 70. The light from each point of the candle flame
goes out in all directions.]

In front of a moving-picture machine there is a large lens, a piece of
glass rounded out toward the middle and thinner toward the edges.
If you were to take that lens off while the machine was throwing the