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.)