A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second makes use of a diaphragm whose size changes as a result of air pressure.Modifications to the mercury barometerThe barometer described above is adequate for making rough measurements of atmospheric pressure. When more accurate readings are needed, however, modifications in the basic design of the barometer must be made. The most important factor to be considered in making such modifications is changes that take place in the mercury reservoir at the bottom of the barometer as a result of changes in atmospheric pressure.