A barometer is an instrument that measures air pressure. It uses mercury, which reacts to air pressure just as it reacts to temperature in a thermometer, by rising or falling in a glass tube.
Perhaps you’re wondering why it is important to know something about air pressure. The answer is that it is an important aid to scientists who need to predict changes in the weather. A falling barometer, one in which the mercury falls quickly, is a sign of an approaching storm, while one in which the mercury is rising is an indication of good weather.
In a barometer, the mercury is sealed in a glass tube with a bulb at one end to serve as a reservoir, much like a thermometer. As air presses on the reservoir, the mercury is forced up into the tube. When the air pressure drops, so does the mercury.