Barometer (from Gr. βάρος, pressure, and μέτρον, measure), an instrument by which the weight or pressure of the atmosphere is measured. The ordinary or mercurial barometer consists of a tube about 36 in. long, hermetically closed at the upper end and containing mercury. In the "cistern barometer" the tube is placed with its open end in a basin of mercury, and the atmospheric pressure is measured by the difference of the heights of the mercury in the tube and the cistern. In the "siphon barometer" the cistern is dispensed with, the tube being bent round upon itself at its lower end; the reading is taken of the difference in the levels of the mercury in the two limbs. The "aneroid" barometer (from the Gr. α- privative, and νηρός, wet) employs no liquid, but depends upon the changes in volume experienced by an exhausted metallic chamber under varying pressures. "Baroscopes" simply indicate variations in the atmospheric pressure, without supplying quantitative data. "Barographs" are barometers which automatically record any variations in pressure.

Philosophers prior to Galileo had endeavoured to explain the Historical. action of a suction pump by postulating a principle that "Nature abhorred a vacuum." When Galileo observed that a common suction pump could not raise water to a greater height than about 32 ft. he considered that the "abhorrence" was limited to 32 ft., and commended the matter to the attention of his pupil Evangelista Torricelli. Torricelli perceived a ready explanation of the observed phenomenon if only it could be proved that the atmosphere had weight, and the pressure which it exerted was equal to that of a 32-ft. column of water. He proved this to be the correct explanation by reasoning as follows: - If the atmosphere supports 32 feet of water, then it should also support a column of about 2½ ft. of mercury, for this liquid is about 13½ times heavier than water. This he proved in the following manner. He selected a glass tube about a quarter of an inch in diameter and 4 ft. long, and hermetically sealed one of its ends; he then filled it with mercury and, applying his finger to the open end, inverted it in a basin containing mercury.

The mercury instantly sank to nearly 30 in. above the surface of the mercury in the basin, leaving in the top of the tube an apparent vacuum, which is now called the Torricellian vacuum; this experiment is sometimes known as the Torricellian experiment. Torricelli's views rapidly gained ground, notwithstanding the objections of certain philosophers. Valuable confirmation was afforded by the variation of the barometric column at different elevations. René Descartes and Blaise Pascal predicted a fall in the height when the barometer was carried to the top of a mountain, since, the pressure of the atmosphere being diminished, it necessarily followed that the column of mercury sustained by the atmosphere would be diminished also. This was experimentally observed by Pascal's brother-in-law, Florin Périer (1605-1672), who measured the height of the mercury column at various altitudes on the Puy de Dôme. Pascal himself tried the experiment at several towers in Paris, - Notre Dame, St Jacques de la Boucherie, etc. The results of his researches were embodied in his treatises De l'équilibre des liqueurs and De la pesanteur de la masse d'air, which were written before 1651, but were not published till 1663 after his death.

Corroboration was also afforded by Marin Mersenne and Christiaan Huygens. It was not long before it was discovered that the height of the column varied at the same place, and that a rise or fall was accompanied by meteorological changes. The instrument thus came to be used as a means of predicting the weather, and it was frequently known as the weather-glass. The relation of the barometric pressure to the weather is mentioned by Robert Boyle, who expressed the opinion that it is exceedingly difficult to draw any correct conclusions. Edmund Halley, Leibnitz, Jean André Deluc (1727-1817) and many others investigated this subject, giving rules for predicting the weather and attempting explanations for the phenomena. Since the height of the barometric column varies with the elevation of the station at which it is observed, it follows that observations of the barometer afford a means for measuring altitudes. The early experiments of Pascal were developed by Edmund Halley, Edme Mariotte, J. Cassini, D. Bernoulli, and more especially by Deluc in his Recherches sur les modifications de l'atmosphère (1772), which contains a full account of the early history of the barometer and its applications.

More highly mathematical investigations have been given by Laplace, and also by Richard Ruhlmann (Barometrischen Hohenmessung., Leipzig, 1870). The modern aspects of the relation between atmospheric pressure and the weather and altitudes are treated in the article Meteorology.

Many attempts have been made by which the variation in the height of the mercury column could be magnified, and so more exact measurements taken. It is not possible to enumerate in this article the many devices which have been proposed; and the reader is referred to Charles Hutton's Mathematical and Philosophical Dictionary (1815), William Ellis's paper on the history of the barometer in the Quarterly Journal of the Royal Meteorological Society, vol. xii. (1886), and E. Gerland and F. Traumüller's Geschichte der physikalischen Experimentierkunst (1899). Descartes suggested a method which Huygens put into practice. The barometer tube was expanded into a cylindrical vessel at the top, and into this chamber a fine tube partly filled with water was inserted. A slight motion of the mercury occasioned a larger displacement of the water, and hence the changes in the barometric pressure were more readily detected and estimated. But the instrument failed as all water-barometers do, for the gases dissolved in the water coupled with its high vapour tension destroy its efficacy. The substitution of methyl salicylate for the water has been attended with success.