How do we know how high we are when we go up in a tall building or an airplane? Carrying a tape measure that long would be impractical because it would weigh about a million
pounds. Then, if the airplane were moving, somebody down on the ground would have to run along very fast, holding the other end of the tape. It could get caught on trees and stuff. It would be awful. Surely, there must be some kind of instrument that could measure altitude in another way.

We could make one, and call it an altimeter (say "al-TIM-it- ter"), and help the world by giving the idea to the elevator and airplane manufacturers. Let's try this:

Draw a line lengthwise across the middle of a white card, and mark off 1/4 inch spaces along the line.

Insert the short arm of an L-shaped glass tube through a one- hole stopper, and place a drop of colored water in the center of the long arm.

Hold your finger over the end of the long arm while you insert the stopper into the mouth of a 1-gallon plastic jug. Be sure the stopper is air tight (some modeling clay or petroleum jelly will seal the stopper).

Now tape the card to the long arm of the tube and tie a string around the neck of the bottle, leaving a loop so that the jug and tube can be carried without fingers touching the jug.

If your school has several floors, carry the altimeter to the highest floor and check to see whether there is any change in the water marker (the colored water in the tube). Variations
in air pressure on the jug will cause the water in the tube to move.

If possible, carry the altimeter up and down in a moving elevator to observe what happens.

Apparently, air pressure is different according to how far up the instrument is. Air pressure has weight, and it has force, which is what causes the marker to move as the instrument is taken up and down.

It is somewhat disappointing to learn that the airplane manufacturers already have altimeters, and they are apparently more sophisticated, and smaller, than the one we made from a gallon jug. We'll have to help the world by learning more about it.

Do you suppose the depth gauge used on submarines works on the same principle? How can we find out about that?