 Fig.1. Earth is immersed in air. It is a wellknown fact that the Earth is immersed inside a bigger globe of air called the ‘Atmosphere’. This does not have a definite height as it gradually becomes thinner as we go up. Just like any other substance, this mass of air is also attracted toward the earth. So this causes a pressure on every one and everything. It is this pressure that sends air into our lungs. There are so many simple experiments to demonstrate the existence of atmospheric pressure. Demonstration 1 Dip a straw into a liquid as shown and draw the air out. The liquid will come to your mouth.
Fig.2 Why does the milk come up? Explanation 1. Nelly:- Could it be that when the air is taken out a liquid tries to fill it up.Explanation 2. Ali: - As the air is taken into the mouth, inside the straw air pressure decreases. But the outside air pressure remains the same . So this pressure pushes the liquid into the mouth. Who is right? An experiment to find out. Take a glass tube with the two ends open. Press one end on to your hand and fill about half of it with water. Then suck the air out and create a low pressure area. Does the water come up. Demonstration 2. PRECAUTION Keep a basin to collect any water that may spill
Take a plastic drinking cup and make a hole at the bottom. Closing the hole with your finger, fill it with water, up to the brim. Then take a piece of paper from a picture card and place it on the top. Press it slightly making sure that no air gets in. Now invert it as shown here, while keeping the finger tight. You will be surprised to see that the water does not come down. Then release the finger and watch what happens. Fig.3. Air supports water. Demonstration 3 Take an empty metal oil can. Add some boiling water to it. Wait for a few seconds for the steam to repel the air out. Then close the lid tightly. And sprinkle some cold water. What happens? The tin gets crushed as the steam condenses back to water. What is the Atmospheric pressure? Nelly and
Ali live in an up stair house. Just below the window of Ali’s room there is a
pond. They dipped a transparent plastic tube into the pond and filled it
completely with water. They closed one end of it until it was not leaking any
water. Then Ali tied a string to it and let Nelly pull it up through the
window. This is shown in the picture. Then they discovered that the water level
came down a bit.  There is a limit to which water can ascend. When the water pressure become equal to the atmospheric pressure the height of water column is equal to the atmospheric pressure. This would be about 10 m. at sea level. As you go up to hills will this increase or decrease? Barometers. These are the instruments that can measure atmospheric pressure. There are two main types. Liquid barometers and Aneroid barometers. Fig. 4 . Measuring atmospheric pressure. Some Alternative Units of Pressure
Fig. 5. Principle of Mercury barometer 
 Model Mercury barometer shown here is one of the simplest and if made properly it is very accrate and long lasting. The tube shown here has to be very long at least 80cm. Professional type barometers use a capillary tube fixed to a wooden stand. There is a device to keep the mercury level in reservoir constant. In order to read slight changes in level there is a vernier scale attached. The normal pressure at sea level will be 76 cm Hg. (Hg is Mercury) The mercury in the cup is exposed. So it is not easy to make this mobile. 
Aneroid Barometers do not use a liquid. This is a very convenient instrument that can be used in ships and some other vehicles.  Fig.7. This is how it works. It has a small metal capsule from which air has been removed. The diapragm on top is very thin and flexible. As the air pressure varies it moves up or down. This is magnified bythe lever and the chain attached to a wheel. Expressing the pressure in Pascals One pascal is a pressure caused by a force of 1 newton on How the Pressure affects the Weather.
Pressure - Hydraulics
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