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 with the records in the daily papers. The average height of the barometric column is 30 inches at sea-level. Explain how you could estimate heights of mountains and balloons with a barometer. THE COMMON PUMP This is a valuable application of air pressure. A glass model will prove useful, but a model made by pupils will be much more so. (See _Laboratory Exercises in Physics_ by Newman.) The water rises in the pump because the sucker lifts the air from the water inside, allowing the air outside to push the water up. A common pump will not lift water more than about 30 feet. Why is this? Compare the pump to a barometer. (See _The Ontario High School Physics_.) EXPANSIVE FORCE OF AIR Air and all other gases manifest a pressure in all directions not due to their weight. The power of air to keep tires and footballs inflated and that of steam in driving an engine are examples. It is this force that prevents the pressure of air from crushing in, since there are many air spaces distributed throughout the body. COMPOSITION OF AIR This subject and the three immediately following it have a special bearing on hygiene. 1. Invert a sealing-jar over a lighted candle. Has the candle used up _all_ the air when it goes out?