What you need:
• glass or jar
• bowl
• 3 coins
• pen
• water

What you do:
• Fill the glass about two-thirds full with water.
• Place the bowl upside down over the glass.
• Carefully turn the bowl and the glass over so that the glass sits upside down in the bowl.
• Some of the water will run out of the glass but most will stay inside it.
• Slide the coins under the rim of the glass and space them equally around the rim.
• With a pen mark the level of the water in the glass at the beginning of the activity.
• Take your barometer outside into the open air.
• Look for changes in the water level in the glass over time. (This may take several hours or even longer than a day.)

How it works: When the atmospheric pressure of the air rises, the water in the bowl will be forced downwards by the weight of the air on the water. This, in turn, will cause the water in the glass to rise. A barometer measures the weight of the amount of air between the surface of the earth (the water in the bowl) and the top of the atmosphere.