Barometer

a well known instrument for measuring the weight or downward pressure of the atmosphere. It was invented by Torricelli, an Italian physicist, in 1643. It consists of a hollow glass tube about thirty-four inches long, closed at one end, and filled with mercury. When full of mercury, the tube is inverted and the open end placed in an open cistern or small cup of mercury. The pressure of the atmosphere on the surface of the mercury in the cup prevents the mercury in the tube from running down and out. The greater the atmospheric pressure, the higher the mercury stands in the tube. A change in the height of the mercury, or barometric column, as it is termed, denotes a corresponding change in the weight of the atmosphere. By a grading or marking on the tube, these changes may be observed readily. The standard height of the barometric column is 76 centimeters or 29.922 inches, which is its average height at sea level in a latitude of 45 degree, with the temperature at 0 degree C. Barometers are used not only to detect changes of weather, but also to measure heights. The column falls at a very nearly uniform rate of one inch for every 900 feet of ascent. If the mercurial column is two inches shorter than the above standard, it is fair to argue that the observer is 1,800 feet above the sea. A more convenient instrument, but less trustworthy than the mercurial barometer, is the aneroid barometer, consisting essentially of a cylindrical box with a flexible top. The air is partially exhausted from within. The top, or diaphram, rises and falls with change of atmospheric pressure. These movements are indicated by a needle and a dial. See Air; Balloon; Altitude; Thermometer; Weather Bureau