Part 2

Fig. 5 ]

CONCLUSION

It demonstrates that the air is exerting a pressure from below on the paper, which is more than enough to support the weight of the water. The tumbler may be placed in any position and yet the water will stay in. This air pressure is exerted alike from all directions, and this pressure, which is 14.7 pounds to the square inch, is weighed down by the air about it and may be likened very much to ordinary water in that it exerts pressure in all directions.

EXPERIMENT NO. 3

Take an ordinary rubber sucker, such as is used on the end of a dart, and attach it to a string. Force this down on a piece of glass. (See Fig. 5.) The glass can then by lifted by the pressure of the air that holds the rubber to it.

We are indebted to a German experimental philosopher named Otto Von Guericke for knowledge of atmospheric pressure. Guericke is distinguished by his original discoveries of the properties of the air. He was born at Magdeburg in Prussian Saxony, November 20, 1602. He became interested at an early age in the politics of his city, and in 1627 was elected alderman, and in 1646 Mayor of Magdeburg. While serving in the above capacities, he devoted his leisure to science, especially on the creation of a vacuum and the action of bodies in a vacuum. His first experiments were conducted with a pump on a barrel of water. After drawing off all the water, he still found that air permeated the wood of the barrel, so he substituted a globe of copper and pumped out air also. He thus became the inventor of the air pump and illustrated in a simple but effective way the force of atmospheric pressure.

Fig. 6 ]

By placing two hollow hemispheres of copper (see Fig. 6) together, and exhausting the air, he found that fifteen horses pulling one way and fifteen pulling the opposite were unable to pull the hemispheres apart.

He further demonstrated that in a vacuum all bodies fall equally fast, that animals cannot exist therein, or, in fact, living matter. He is also credited with being the inventor of the air balance and a type of weather cock, called the anemoscope. He was interested also in astronomy.

Fig. 7 ]

EXPERIMENT NO. 4

This experiment should interest you very much, because it is going to lead up to the subject of weather instruments, and is absolutely essential that you understand the fundamental principles in order to intelligently interpret these instruments. This experiment will explain one of the principles of the barometer.

Take a glass tube thirty-two inches long and one-quarter or one-eighth inch in diameter, and fill it with mercury, care being used to get rid of all the air bubbles. The mercury should be poured in with an eye dropper, one end of the tube being sealed, until filled, and then the finger is placed over the open end. (See Fig. 7A). The tube is inverted and immersed in a reservoir of mercury and clamped to an upright stand. Immediately the mercury falls to about thirty inches. (See Fig. 7B). Ask yourself what held the mercury up in the tube. Again the answer is that the pressure of the air on the mercury in the reservoir causes it to rise and fall in the tube, as the pressure of the air changes. You will soon learn what causes these changes in the pressure of the air.

Fig. 8 ]

EXPERIMENT NO. 5

Have you ever asked yourself why it is that the wind blows? Why doesn’t it stand still?

Put your hand over a lamp chimney under which the lamp is lighted. You will soon discover that the heat is rising. Four things in connection with this are of great importance:

1. Air has weight.

2. When heated, it rises.

3. Air expands when heated.

4. Warm air will gather and hold more moisture than cold air.

EXPERIMENT NO. 6

Cut a piece of stiff cardboard in a spiral shape. Thread a piece of thread through a pinhole in the center point of the spiral and fasten this to a support so that it swings freely in the air. (See Fig. 8.) Under this put a little alcohol lamp, or put it over a gas jet or radiator.

WHAT HAPPENS

The cardboard will spin around rapidly. Ask yourself what causes this. It is the force of the hot air rising which caused the spiral cardboard to turn in such an attractive manner.

EXPERIMENT NO. 7

Fig. 9 ]

When you are in a warm room, find out which air is the hottest, that in the upper or that in the lower part of the room. This answer you can get by placing the thermometer low down in the room and then putting it up near the ceiling. This is another conclusive proof that hot air rises.