Handbook of Nature-Study · Lesson CCXXIII · Topic 36: Water and weather
223. The barometer
Goal: Your child can explain that a barometer measures the weight of the air, and builds a simple jar barometer to watch it change.
You'll need
- A wide-mouthed jar
- A balloon, cut open and stretched over the jar mouth, and a rubber band
- A drinking straw and tape
- An index card taped upright behind the straw
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
The lesson has a leading thought, a method for making a mercury barometer from a 36-inch glass tube, and three observations on how high the column stands, whether it changes from day to day, and how it would be on a mountain.
The lesson, step by step
Watch only: the mercury barometer
The book has the class pour mercury into a long glass tube and turn it upside down into a dish of mercury. Don't try this at home: mercury is poisonous, and its invisible vapor harms the brain and kidneys. Look up a video of a teacher demonstrating a Torricelli (mercury) barometer instead.
The leading thought
The weight of our atmosphere balances a column of mercury about thirty inches high, and is equal to about fifteen pounds to the square inch. This pressure varies from day to day, and becomes less as the height of the place increases.
What the book's barometer shows
Read the three questions and talk them through:
How high is the column of mercury in the tube?
What keeps the mercury in the tube?
Would the mercury column be as high in the tube if it were placed on top of a mountain as it would at the foot? Why?
Build a jar barometer
Stretch the cut balloon tightly over the jar mouth and hold it with the rubber band. Tape one end of the straw to the middle of the balloon so the straw lies flat and sticks out past the edge. Stand the card behind the straw's tip and mark where it points.
How it works
When the air outside presses harder, it pushes the balloon down and the straw's tip goes up. When the pressure falls, the tip goes down. Keep the jar indoors, out of the sun and away from heaters, because warming the jar also moves the straw.
Watch it for a week
Mark the straw's position each morning and evening, and write the weather beside each mark. Does falling pressure come before clouds and rain? Does rising pressure come before clearing?
Nature notebook
Draw your barometer and make a little chart: date, time, straw up or down, weather.
Afterwards
Read the jar barometer twice a day for a week and compare with the weather.
Answers to the book's problems
At sea level the mercury stands about 30 inches (29.92 on an average day). The weight of the air pressing on the mercury in the dish holds the column up. On a mountain the column is lower, because there is less air above pressing down (about an inch lower for every thousand feet of climb, near sea level). A water barometer would need a tube about 34 feet tall.
Worked out for this site from the scan. If a number in your copy differs, trust the book.