h the records in the daily papers.

The average height of the barometric column is 30 inches at sea-level.
Explain how you could estimate heights of mountains and balloons with a
barometer.

THE COMMON PUMP

This is a valuable application of air pressure. A glass model will prove
useful, but a model made by pupils will be much more so. (See
_Laboratory Exercises in Physics_ by Newman.)

The water rises in the pump because the sucker lifts the air from the
water inside, allowing the air outside to push the water up. A common
pump will not lift water more than about 30 feet. Why is this? Compare
the pump to a barometer. (See _The Ontario High School Physics_.)

EXPANSIVE FORCE OF AIR

Air and all other gases manifest a pressure in all directions not due to
their weight. The power of air to keep tires and footballs inflated and
that of steam in driving an engine are examples. It is this force that
prevents the pressure of air from crushing in, since there are many air
spaces distributed throughout the body.

COMPOSITION OF AIR

This subject and the three immediately following it have a special
bearing on hygiene.

1. Invert a sealing-jar over a lighted candle. Has the candle used up
_all_ the air when it goes out?

2. Place a very short candle on a thin piece of cork afloat on water in
a plate; light the candle, and again invert the jar over it. Note that
the candle goes out and the water rises only a short distance in the
jar; therefore _all_ the air has not been used up.

3. Slip the glass top of the jar under the open end and set the jar
mouth upward on the table without allowing any water to escape. Now
plunge a lighted splinter into the jar. The flame is extinguished.

Air, therefore, contains an active part that helps the candle to burn
and an inactive part that extinguishes flame. The names _oxygen_ and
_nitrogen_ may be given. These gases occur in air in the proportion of
about 1:4. (This method is not above criticism. Its advantage for young
pupils lies in its simplicity.)

OXYGEN

Make two or three jars of oxygen, using potassium chlorate and manganese
dioxide. (See any Chemistry text-book.) Let the pupils examine the
chemicals, learn their names, and know where to obtain them. Perform the
following experiments:

1. A glowing splinter relights and burns very brightly if plunged into
oxygen.

2. A piece of picture wire tipped with sulphur burns with great
brightness.

3. Burn phosphorus or