Handbook of Nature-Study · Lesson CCXXII · Topic 36: Water and weather
222. Experiments to show air pressure
Goal: Your child shows with a glass of water and a suction cup that air presses in every direction.
You'll need
- A drinking glass with a smooth rim
- A square of stiff paper or an index card
- A sink or a large bowl to work over
- A rubber suction cup (a hook or a toy one)
- A smooth tile or hardcover book
What's in the book
The lesson has a leading thought, two experiments (a paper over an upturned glass of water, and a leather "sucker"), and hints to the teacher explaining them. Before it comes Wilford M. Wilson's chapter "The Weather," on the history of weather study, the atmosphere, the winds of the world, storms and the Weather Bureau.
The lesson, step by step
A story from the weather chapter
Tell the story from the chapter before this lesson in your own words. Benjamin Franklin planned to watch an eclipse of the Moon, but a northeast storm hid it in Philadelphia. A friend in Boston saw the eclipse, and the storm reached Boston only the next morning. Franklin collected reports from many towns and found that storms travel across the land. Ask: How could knowing that help people?
The leading thought
The air presses equally in all directions.
The upside-down glass
Work over the sink.
Fill a tumbler which has an unbroken edge as full of water as possible. Take a piece of writing paper and cover the tumbler, pressing the paper down firmly upon the edge of the glass. Turn the glass bottom side up and ask why the water does not flow out.
Turn the glass filled with water and covered with paper sidewise; does the water flow out? If not, why?
Let a little air in
Allow a little air to enter; what happens? Why?
Lift one corner of the paper and watch.
The sucker
The book's boys made a "sucker" of wet leather and string. A rubber suction cup works the same way. Wet it, press it on the tile or book, and lift gently. Lift only light things a few inches above a table, not heavy stones.
If a little air is allowed to get between the sucker and the stone, what happens? Why?
The teacher's hint
Read the explanation from the book:
The reason why we cannot pull it off is that the air is pressing down upon it with the force of about fifteen pounds to the square inch.
Ask: How much does the air push on a square the size of a postage stamp? (About fifteen pounds.)
What we know now
The weather chapter says the air is about two hundred miles deep. Today we know it has no sharp top: it just gets thinner and thinner. Half of all the air lies below about three and a half miles, lower than the highest mountains, which is why climbers there need extra oxygen.
Nature notebook
Draw the upside-down glass with arrows showing the air pushing up on the paper. Write the leading thought under it.
Answers to the book's problems
The air pushes up on the paper harder than the water pushes down, so the water stays in, even sideways, because air presses in every direction. When air gets in above the water, it pushes down too, and the water falls out. The suction cup holds because there is almost no air under it, so the outside air presses it down; let air in and it lets go. At sea level air presses about fifteen (14.7) pounds on each square inch.
Worked out for this site from the scan. If a number in your copy differs, trust the book.