A barometer measures the atmospheric pressure against a reference pressure. You set the reference pressure in a sealed body and it equalises with the atmospheric pressure by displacing water. THe pressure displaces water due to relationship P = pgh (pressure = density x gravity x height) so changing the height of the water (usually in a column with scaled lines on the side to read a value) shows you the change in pressure. This can be connected to a needle on a guage.

The classic mercury barometer is typically a glass tube about 3 feet high with one end open and the other end sealed. The tube is filled with mercury. This glass tube sits upside down in a container, called the reservoir, which also contains mercury. The mercury level in the glass tube falls, creating a vacuum at the top. The first barometer of this type was devised by Evangelista Torricelli in 1643. The barometer works by balancing the weight of mercury in the glass tube against the atmospheric pressure just like a set of scales. If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises. If the weight of mercury is more than the atmospheric pressure, the mercury level falls. Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir etc

mama says the devil makes the barometer work..... and when an crocodile cry its because he cant brush his teeeth and theres so many of them

1) "Barometers are used to measure atmospheric pressure--the pressure of the air. There are three different types of barometers: water-based, mercury and aneroid. Changes in atmospheric pressure, measured by a barometer, indicate a change in weather." "A water-based barometer is a sealed glass container filled half-way with water. The container has a spout connected to it below the water line. The spout extends up above the water line and is open at the end. The water is usually colored so that its levels can be easily read. When the atmospheric pressure is higher than it was when the barometer was sealed, the water level in the spout will drop below the water level in the container. This is an indication that cooler temperatures are coming. Conversely, when the atmospheric pressure is lower than it was when the barometer was sealed, the water level in the spout will rise above the water level in the container. This is an indication that warmer temperatures, storms and rain are coming. You can also read the water level in the container. If the water in the container is higher than in the spout, the atmospheric pressure is high. If the water in the container is lower than in the spout, the atmospheric pressure is low. Mercury barometers consist of a reservoir and a 30-inch tube, sealed at the top, rising up out of the center of the reservoir. The reservoir is filled with mercury and is open to the atmosphere. This type of barometer works similarly to the water-based barometer. When atmospheric pressure is high, it presses down on the mercury in the reservoir, forcing mercury up into the tube. When atmospheric pressure is low, it allows mercury to flow out of the tube and back into the reservoir. An aneroid barometer uses alloy coils, made of beryllium and copper instead of a liquid, to measure changes in atmospheric pressure. The coils are attached to a dial. Changes in atmospheric pressure cause the coils to expand or contract. Higher atmospheric pressure causes the coils to compress. Lower atmospheric pressure allows the coils to expand." Source and further information: http://www.ehow.com/how-does_4564020_a-barometer-work.html Further information: http://en.wikipedia.org/wiki/Barometer