IV. For
method see Form III.

PHYSICAL SCIENCE PHASE OF NATURE STUDY

WATER PRESSURE

1. Grasp an empty tin can by the top and push it down into a pail of
water. Note the tendency of the can to rise. The water presses upward.
Its downward pressure is evident.

2. Tie a large stone to a string, hold it at arm's length, shut the
eyes, and lower the stone into water. _Note_ the decrease in weight.
This is also due to upward pressure, which we call buoyancy. The actual
decrease may be found by means of a spring balance.

3. Try Experiment 2, using a piece of iron the same weight as the stone.
Is the decrease in weight as evident? Ships made wholly of iron will
sink. Explain.

4. Put an egg into water; it slowly sinks. Add salt to the water; the
egg floats.

EXERCISES

1. Will the human body sink in water? In which is there less danger of
drowning, lake or sea water?

2. When in bathing, immerse nearly the whole body, then take a full
inspiration. Note the rise of the body.

3. Why does ice float? (See expansion of water by freezing.)

4. Balloons are bags filled with some light gas, generally hydrogen or
hot air. They are pushed up by the buoyancy of the air. The rise of
heated air or water (see Convection) is really due to the same force.
Clouds, feathers, and thistledown are kept in the air more by the action
of winds and small air currents than by buoyancy.

STUDY OF AIR

(Consult _Science of Common Life_, Chaps. VIII, IX, X.)

1. Air takes up space. Put a cork with one hole into the neck of a flask
or bottle. Insert the stem of a funnel and try to pour in water. Try
with two holes in the cork. When we call a bottle "empty" what is in it?

2. Air is all around us. Feel it; wave the hands through it; run through
it; note that the wind is air; inhale the air and watch the chest.

3. Air has weight. This is not easy to demonstrate without an air-pump
and a fairly delicate balance.

Fit a large glass flask with a tightly fitting rubber stopper having a
short glass tube passing through it. To the glass tube attach a short
rubber one and on this put a clamp. Open the clamp and suck out all the
air possible. Close the clamp and weigh the flask. When perfectly
balanced, open the clamp and let the air enter again. Note the increase
in weight.

If an air-pump is available, procure a glass globe provided with a
stop-cock (see Apparatus). Pump some of the air from the globe, then
weigh and, while it