Obtain a large, clean, dry bottle, put the lid on it, and place it in a bowl of warm water for a few minutes. Be careful not to let any water enter the bottle.

Stretch a piece of rubber from a balloon over the top of the bottle and secure it tightly with a rubber band. Glue a soda straw to the rubber membrane in such a way that the straw
extends out horizontally from the center of the bottle.

Hold the straw firmly in place until the glue dries, then flatten the free end of the straw by pinching it. Set a white card behind the straw, and mark the card at the level where the straw touches it. Number this mark "0."

Now mark the card at 1/8-inch intervals above and below the "0." Starting at 0, number these marks "1", "2", and "3," so that the mark farthest from the 0 on either side is number "3."

Figure out the direction in which the straw will move when the air pressure increases or decreases. Since the bottle contains a closed volume of air under a certain pressure, any changes in the outside pressure will cause the rubber membrane to move in or out slightly, causing the straw to rise or dip. That can be seen on the scale.

Now set the bottle away from direct sunlight, radiators, or other heat sources so that heat will not influence the operation of the device.

This device is called an air barometer. It is for keeping records of the changes in air pressure.

If a water barometer is also constructed, comparisons of readings can be made for the two devices.