To make experiments with compressed air, you must put your wits together to make a reservoir. Air, you know, is a gas consisting of particles called atoms. These atoms are at a certain distance from one another. They can be pushed further from one another as when you heat them, or closer together by cold and compression. So compressed air only means air whose atoms are pressed more closely together than as the case with the air around us.

Now you have heard that a column of air on a square inch weighs fourteen and a half pounds. Also, you know that air in a receiver or any other vessel presses on the vessel inside and out with a force (or weight) of fourteen and a half pounds.

If now into the vessel you push another quantity of air, equal to the vessel’s capacity, you simply push the atoms of air closer together. In fact, they are now only half as far apart as the atoms of an open vessel. But the pressure is doubled and the compressed air, therefore, will press on the inside of the vessel with a force of twenty-nine pounds.