The Fairy-Land of Science · Lecture III, part 2 · Part 3: Lecture III: The aerial ocean in which we live
8. Air is springy, and it presses on everything
Goal: Your child can explain that gravity holds the air to the Earth, that air is springy and gets thinner as you go higher, and that air presses hard on everything.
You'll need
- A balloon
- A suction cup (from a hook or a bath toy) and a smooth tile, window or fridge
- A one-inch square of paper
- Science notebook and pencils
What's in the book
Pages 56 to 62. Gravity holds the air down; air is thick near the ground and thin higher up, like a pile of papers pressed down; a pop-gun; balloonists who fainted in the thin air high up; meteors showing the air is over 100 miles deep; and the weight of the air, 15 pounds on every square inch, shown with wet paper and a leather sucker, and limpets on a rock.
The lesson, step by step
Hook question
SayWhy doesn't the air float away into space?
Let your child guess. Buckley's answer: our old friend gravity holds it down.
Read together
Read pages 56 to 58.
This force draws together the earth and the atoms of oxygen and nitrogen
The air near the ground is squeezed by all the air above it, like the bottom papers in a tall pile. High up there is less on top, so the air is thinner.
Try it: springy air
Blow up the balloon partway and tie it. Squeeze it gently: it pushes back. Let go: it springs back to its shape.
Read about the pop-gun on page 58.
Out it flies, and the atoms spread themselves out comfortably again in the air all around them.
SaySqueezed air always tries to spread out again. That's what makes air springy.
Thin air high up
Tell the story on page 58: in 1862 two men went up five and a half miles in a balloon, and Mr. Glaisher fainted because the air was so thin. Mr. Coxwell let gas out of the balloon so they sank to thicker air.
Read about meteors on pages 59 and 60: shooting stars light up when they rush into the top of the air, which shows the air is very deep.
Try it: the weight of the air
Show the one-inch square of paper.
The atmosphere does, in fact, weigh down upon land at the level of the sea as much as if a 15-pound weight were put upon every square inch of land.
Press the suction cup hard onto the smooth tile or window and try to pull it straight off. It's hard! You squeezed the air out from under it, so the air outside pushes it on.
Have you ever tried to pick limpets off a rock? If so, you know how tight they cling.
Tell it back and check
Your child tells back why the suction cup sticks. Then ask:
1. What holds the air to the Earth?
2. Where is the air thinnest: at the beach or on top of a high mountain?
3. Why aren't we squashed by all that air? (Buckley: the air and fluids inside us push back just as hard.)
What we know now
Air pressure at sea level is about 14.7 pounds on each square inch, very close to Buckley's 15. The air has no sharp top; it just gets thinner and thinner. Most shooting stars burn up about 50 to 75 miles above the ground.
Afterwards
Notice it this week: ears popping in a car going up a hill or in an elevator, a juice box that crumples when you suck the air out, or a suction hook in the bathroom.
Tip
If the suction cup won't stick, wet the rim slightly. Buckley's own leather sucker failed the first time until she fixed it, so trying again is part of science.
Answers to the book's problems
1. Gravity. 2. On top of a high mountain. 3. The air and fluids inside our bodies press outward as hard as the air presses in, so the pushes balance.
Worked out for this site from the scan. If a number in your copy differs, trust the book.