is on the balance, admit the air again and note
increase in weight.

Tie a piece of thin sheet rubber over the large end of a thistle tube;
suck the air out of the tube and note how the rubber is pushed in. This
is due to the weight or pressure of the air. Turn the tube in various
positions to show that the pressure comes from all directions. To show
that "suction" is not a force, let a pupil try to suck water out of a
flask when there is only one opening through the stopper. If two holes
are made, the water may be sucked up, that is, _pushed_ up by the weight
of the air.

Fill a pickle jar with water. Place a piece of writing paper on the top
and then, holding the paper with the palm of the hand, invert the jar.
The pressure of the air keeps the water in.

A cubic foot of air weighs nearly 1-1/4 oz. Find the weight of the air
in your school-room.

The atmosphere exerts about fifteen pounds pressure on every square inch
of the surface it rests against. Find the weight supported by the top of
a desk 18 inches by 24 inches. If the surface of the body is eight
square feet, what weight does it have to sustain? Why does this weight
not crush us?

THE BAROMETER

The experiments immediately preceding will have paved the way for a
study of the barometer.

1. Fill a jar with water and invert it, keeping its mouth below the
surface of the water in another vessel. If the pupils can be led to see
that the water is sustained in the jar by the air pressing on the water
in the vessel, they can understand the barometer.

2. Fill a tube about 30 inches long, and 1/4 inch inside diameter with
water, and invert it over water, as with the jar in the previous
experiment.

3. Use the same tube or one similar to that in 2 above, but fill with
mercury and allow the pupils to notice the great weight of the mercury.
Holding the mercury in with your finger, invert the tube over mercury.
This time the fluid falls some distance in the tube as soon as the
finger is removed. A tube of this size requires 1 lb. of mercury.

Lead the pupils to see that the mercury remaining in the tube is
sustained by the air pressure, and that any increase or decrease of the
atmospheric pressure will result in the rise or fall of the mercury
column. Leave the barometer (made as in 3 above) in the room for a few
days and note whether its weight changes. The use of the instrument in
predicting weather changes should be emphasized. Compare your barometer
wit