A barometer is an instrument used to measure atmospheric pressure.
A standard mercury barometer has a glass column about 76 cm (30 inches) long, closed at one end, with a mercury-filled reservoir. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure forces the mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column. The first mercury barometer was devised by Evangelista Torricelli, a student of Galileo, in 1644. Torricelli had set out to create a perfect vacuum, and an instrument to measure air pressure. He succeeded in creating a vacuum in the top of a tube of mercury. Torricelli also noticed that the level of the mercury in the tube changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight.
Another type of barometer, the aneroid barometer, uses a small, flexible metal box called an aneroid cell. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives a series of mechanical levers and other devises which are displayed on the face of the aneriod barometer. Even though there is no column of mercury inside, most aneroid barometer displays are still calibrated to read in units of inches of mercury.
A barometer is used for weather prediction as follows: Increasing pressure predicts fair weather. Decreasing pressure predicts rain, storms, etc.
The concept of "decreasing pressure means bad weather" is the basis for a primitive weather prediction device called a weather glass or thunder glass. It consists of a glass container with a spout. The container is filled with water up to about the middle of the spout. (Some air is left in the main body of the container.) The idea is this: When the air pressure decreases, the pressure of the air pocket inside the device will push some of the water up the spout. If the air pressure is low enough, some of the water may even drip out of the spout.