Barometer
2 of the 6 encyclopedias on this shelf carry an entry for Barometer. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
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
Collier's New Encyclopedia (1921)
an instrument used for measuring the atmospheric pressure. The most common form of barometer is what is called a cistern barometer. It consists essentially of a straight glass tube about 33 inches long, filled with mercury, and dipping into a cistern of the same metal. It is affixed to a stand, on the upper part of which is a graduated scale to mark the height in inches at which the mercury stands. When complete, a thermometer stands side by side with it to note the temperature at which the pressure of the atmosphere is tested. Gay-Lussac's barometer is in the form of a siphon. It has two scales with a common zero point, and graduated in contrary directions. As the one branch, the shorter one, corresponds to the cistern, and the other or longer one to the tube, the difference between the two levels is the true height of the mercury. A barometer is popularly termed a weather glass. In order to adapt it for this purpose Hooke devised what is called the wheel barometer. It is a siphon barometer, having in its shorter leg a float, a string from which passes over a pulley, and is connected with a weight somewhat lighter than the float. To the pulley is affixed a needle, which moves round a circle graduated to represent the different variations in the weather. Speaking broadly, a barometer rises for good and falls for bad weather, but there are exceptions to this rule.
What Has Changed in the Last Century
The physics in both accounts is exactly right, and Torricelli's 1643 tube is still the definition of atmospheric pressure — but the instrument itself has almost vanished. Mercury barometers of the kind Aiton's describes in 1910 have been progressively withdrawn on toxicity grounds, with the European Union restricting their sale to the general public from around 2009 and the 2013 Minamata Convention pushing the same way worldwide; the aneroid box that Aiton's called "less trustworthy" won, and has in turn given way to electronic piezoresistive sensors small enough to sit inside a mobile phone. Pressure is now normally reported not in the inches of mercury both entries use but in hectopascals or millibars, standard sea-level pressure being 1013.25 hPa rather than 29.922 inches. And Collier's rule of 1921 — "a barometer rises for good and falls for bad weather, but there are exceptions" — has been absorbed into computer forecasting, where pressure readings from thousands of stations feed numerical models rather than a needle on a dial.
Written for this edition, 2026.