the air in other
ways. You can do this by opening a door or window. Do not let the cold
air blow upon any one, for it may help to make him catch cold, if the
air of the room is impure. If we lower a window from the top, warm
impure air may pass out above it without making a draft.

[Illustration: =Diagram of the natural ventilation of a room.=

The arrows show the direction of the air currents.]

You need fresh air at night as much as in the daytime. You need not be
afraid of the night air, for it is good and pure like the day air. You
ought to sleep with your window open a little. You ought to open the
windows wide every morning and air your bed well. At night you ought
to take off all your clothes and put on a night-dress. Then hang your
clothes up to air and dry.

=113. When to air a room.=--When you first enter a room full of bad
air it smells musty and unpleasant. But after you have been in the
room a while, you get used to it. If, however, you go out of doors a
minute and then come back, you will smell the bad air again. If the
air smells bad, open a door or window until it is sweet again.

=114. How to breathe.=--When you run hard, the cells of your body use
up all the air, and then you feel short of breath. While you run,
burning goes on faster, and you feel warmer. You can work harder and
longer if you can breathe in a great deal of air. You will also feel
better and stronger for it. Then if you are sick, you will be able to
get well more quickly. You ought to know how to breathe right.

_First_, you ought to breathe through your nose. Even when you run,
you ought to keep your mouth closed.

_Second_, you should try to breathe deeply. You should take a very
deep breath often, and hold it as long as you can. By practice you can
learn to hold it a full minute.

_Third_, you ought to run, or do some hard work, every day. When you
get short of breath, you will have to breathe more deeply. After a
while you may be able to run a half mile, or even a mile, without
getting out of breath. But do not get tired out in your run, for this
will harm you.

_Fourth_, you must sit and stand with your shoulders back, and your
chest thrown forward. A round-shouldered boy cannot have large lungs
or be long winded.

By breathing right, you can make your lungs very much larger and
stronger.

=115. The voice.=--We talk by means of the breath. At the upper part
of the windpipe is a small box. Its front corner can be felt