Take a glass tube thirty-two inches long and one-quarter or one-eighth inch in diameter, and fill it with mercury, care being used to get rid of all the air bubbles. The mercury should be poured in with an eye dropper, one end of the tube being sealed, until filled, and then the finger is placed over the open end. (See Fig. 7A). The tube is inverted and immersed in a reservoir of mercury and clamped to an upright stand. Immediately the mercury falls to about thirty inches. (See Fig. 7B). Ask yourself what held the mercury up in the tube. Again the answer is that the pressure of the air on the mercury in the reservoir causes it to rise and fall in the tube, as the pressure of the air changes. You will soon learn what causes these changes in the pressure of the air. [Illustration: Fig. 8 ] EXPERIMENT NO. 5 Have you ever asked yourself why it is that the wind blows? Why doesn’t it stand still? Put your hand over a lamp chimney under which the lamp is lighted. You will soon discover that the heat is rising. Four things in connection with this are of great importance: 1. Air has weight. 2. When heated, it rises. 3. Air expands when heated. 4. Warm air will gather and hold more moisture than cold air. EXPERIMENT NO. 6 Cut a piece of stiff cardboard in a spiral shape. Thread a piece of thread through a pinhole in the center point of the spiral and fasten this to a support so that it swings freely in the air. (See Fig. 8.) Under this put a little alcohol lamp, or put it over a gas jet or radiator. WHAT HAPPENS The cardboard will spin around rapidly. Ask yourself what causes this. It is the force of the hot air rising which caused the spiral cardboard to turn in such an attractive manner. EXPERIMENT NO. 7 [Illustration: Fig. 9 ] When you are in a warm room, find out which air is the hottest, that in the upper or that in the lower part of the room. This answer you can get by placing the thermometer low down in the room and then putting it up near the ceiling. This is another conclusive proof that hot air rises. Another experiment that is quite familiar to all of us is that of opening the windows of a heated room a few inches top and bottom, and holding a lighted match or smoke paper at the bottom, when you will find that it blows the flame or smoke inward. Then put it near the top of the window and it will be drawn out. The same answer is true; the cold air is rushing in from below to take the place of the hot air rising and going out at the top. (See Fig. 9.) [Illustration: Fig. 10 ] EXPERIMENT NO. 8 This experiment is even more important than the preceding one, and you should by all means do it, for it is going to prove more conclusively than anything else what causes the wind, and in miniature it is a real storm.