_APPLICATION 2._ A. C. Wheeler of Chicago bought a little farm in Indiana, and had a windmill put up to supply the place with water. But at first he was not sure where he should put the tank into which the windmill was to pump the water and from which the water should flow into the kitchen, bathroom, and barn. The barn was on a knoll, so that its floor was almost as high as the roof of the house. Which would have been the best place for the tank: high up on the windmill (which stood on the knoll by the barn), or the basement of the house, or the attic of the house? [Illustration: FIG. 2. Where is the best location for the tank?] _APPLICATION 3._ A man was about to open a garage in San Francisco. He had a large oil tank and wanted a simple way of telling at a glance how full it was. One of his workmen suggested that he attach a long piece of glass tubing to the side of the tank, connecting it with an extra faucet near the bottom of the tank. A second workman said, "No, that won't do. Your tank holds ever so much more than the tube would hold, so the oil in the tank would force the oil up over the top of the tube, even when the tank was not full." Who was right? [Illustration: FIG. 3. When the tank is full, will the oil overflow the top of the tube?] SECTION 3. _The sea of compressed air in which we live: Air pressure._ Does a balloon explode if it goes high in the air? What is suction? Why does soda water run up a straw when you draw on the straw? Why will evaporated milk not flow freely out of a can in which there is only one hole? Why does water gurgle when you pour it out of a bottle? We are living in a sea of compressed air. Every square inch of our bodies has about 15 pounds of pressure against it. The only reason we are not crushed is that there is as strong pressure inside of our bodies pushing out as there is outside pushing in. There is compressed air in the blood and all through the body. If you were to lie down on the ground and have all the air pumped out from under you, the air above would crush you as flat as a pancake. You might as well let a dozen big farm horses trample on you, or let a huge elephant roll over you, as let the air press down on you if there were no air underneath and inside your body to resist the pressure from above. It is hard to believe that the air and liquids in our bodies are pressing out with a force great enough to resist this crushing weight of air. But if you were suddenly to go up above the earth's atmosphere, or if you were to stay down here and go into a room from which the air were to be pumped all at once, your body would explode like a torpedo.