A bourdon gauge is often used to show the pressure in gas cylinders and boilers. It works in the same way as the paper tubes you may have blown out straight at parties. Higher pressure makes the tube straighten out slightly, and this movement is used to turn a pointer.
A manometer is a U-tube containing some liquid, usually water. The pressure of the gas changes the levels of the ware in the manometer. The pressure can be found (in 'centimetres of water') by measuring the height.
3. Aneroid barometer
This kind of barometer uses a flexible matal can which has had air taken out of it. A strong spring stops the atmosphere from completely crushing the can. If the air pressure increases, the top of the can is squeezed down slightly. If the air pressure decreases, the spring pulls up the top of the can. This small movement is magnified by a long pointer.If a barometer is taken up a mountain; it shows a reduced pressure- because it is going nearer to the top of 'sea' of air. Mountaineers and aircraft pilots can measure their height by using an aneroid barometer as an altimeter.