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 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.