BAROMETER (Gk. gdpos, baron. weight + p(rpov, metion, measure). An instrument for measuring the elastic pressure of the atmosphere which is sometimes, but erroneously, ealled its weight. Plato and Aristotle maintained that the atmosphere has weight; Galileo and To• rieelli knew that pressing upon the in well forced a portion of it to rise up the bore of a pump as fast as the piston rose. The fact that water could not be made to rise more than 33 feet in this manner showed to Galileo's mind the limiting pressure of the atmosphere —namely, about 15 pounds to the square inch.
In 1643 his pupil and successor, Torricelli, de siring to find a convenient method of measuring the variations of atmospheric prewsure, asked to use a short glass tube closed at one end, filled with mercury. and inverted in a basin of that heavy liquid, as in Fig. b. As lie predicted, so it was found that a column of mercury 30 inches high, co...responding to the atmospheric pressure (15 pounds to the square inch). was held up by the air-pressure and the upper portion of the glass tube was empty. This vacant space was called the Torri •ellian Vacuum. According to Hellmann, this simple apparatus was first called a barometer by Boyle. With the modifications and auxiliary apparatus necessary to secure the accuracy in measurement demanded by modern science. the mercurial barometer is not- considered the stand ard instrument for the elastic pressure of gases. In the normal barometers constructed for the International Bureau of Weights and Measures, m a i n tained at Paris by the contributions of all nations for the purpose of Se •uring accuracy and uniformity in all measurements, provision is made for measuring the temperature of the top and bottom of the mercurial col umn, the tempera ture of the metal li• measuring-scale, the imperfection of the vacuum, if any, the capillary effect at the surface of the mercury in the basin and in the tube, the vertical ity of the measur ing-scale, the er rors of pointing and reading, the exact density of the mercury, and every other source of error that has been suggested. It is supposed that with all these pre cautions the height of the column of mercury supported by the atmospheric pressure may be determined within the one-thousandth part of an inch, or the thirty-thousandth part of the whole pressure; but of course this accuracy is not obtained in the instruments ordinarily used in physical laboratories and meteorological vatories. Among the important forms of the curial barometer may be mentioned the siphon barometer illustrated iu Fig. 3. If the diameter of the tube is perfectly uniform, the capillary effects at the upper and lower ends of the column tially neutralize each other, but not entirely so, because the mercury in the lower end is exposed to the air, whose gases very quickly change the character of the superficial film of mercury. The measurement is facilitated by having the upper and lower arms of the tube in the same vertical line as shown in the illustration. In some barometers the mercury of the open end is protected from air and dust by a convenient flexible covering, such as a film of oil, or by tying a loose india-rubber bag over the open end of the tube. In general, the barometric tube should be of large bore, perhaps even a whole inch. in order that capillarity. or surface tension, may have as little effect as possible.
The standard form of portablebarometer is that illustrated in Fig. 4, as first constructed by For tin, and now very generally copied by makers throughout the world. In this form the meas uring,-scale extends from an ivory point (p in Fig. 5) upward, usually as a part of a metal cylinder inclosing and protecting the whole tube. By means of the screw S the mer cury in the cistern may he raised or lowered until its surface just touches the point p. The
observer sights across the upper end of the column of mercury and the corresponding read ing on the scale is the height of the mercurial column above the point p. When this barome ter is to be carried about the screw S is turned until the mercury completely fills the cistern and is pushed up to the very top of the glass tube. The barometer is then inverted and ear ried or kept bottom up until wanted. It is quite common for the vaenum-ehamber to de teriorate apparently by the occasional introduc tion into it of small bubbles of air slipping up along the inner side of the tube. In order to prevent this. Bunsen invented a so-called air trap. which consists simply of introducing into the interior of the tube, below the top of the mercury. a stricture terminating below in a tube of fine-drawn glass. whose small end opens downward beneath the mercury. Any bubbles of air slipping up the tube are almost certain to be caught in this trap and will escape hack into the open air when the barometer is turned upside down for transportation. The most ac curate barometers and the newest patterns are now made by Fuess, of Berlin, Green. of New York. and Casella. or Negretti S. Zambra. of London. The normal barometers of Paris, Ber lin. and Saint Petersburg are read by means of reading microscopes so adjusted that some di vision of the standard scale is seen as reflected from the surface of the mercury of the barometer so that the slightest change of atmospheric pres sure alters the position of this reflection. The ordinary barometers of laboratories and me teorological stations are read by means of a vernier (q.v.) sliding up and down the scale, and by sighting across the edge of the vernier plate to the top of the mercurial column. Si phon barometers having a float with index and dial. by means of which the slight movements of the mercurial column are greatly magnified. are not sufficiently accurate to find any place in scientific work.
Barometric pressures are properly expressed o»ly in standard units of force or dynes. but it is more common to express them in inches or milli metres, meaning thereby the force or pressure corresponding to the weight of a mercurial col umn of that height: but this pressure depends. not merely upon the height of the column of mer cury. but also upon its density and on the force of gravity. The former varies with the tempera ture: the latter varies with latitude and slight ly with height above sea-level. Moreover, the surface tension or so-called capillarity of the mercury in the glass tube and the cistern affect the height of the column of mercury. Correc tions for these sources of error must therefore he made, and. by vote of the International Con gress. meteorologists now follow the example of the physicists in reducing all barometric meas urements to the standard density of pure mer cury at zero degrees. Centigrade, and to standard gravity at the latitude of forty-five degrees and sea-level. The barometer has many important uses and applications in science, and is con stantly employed in hypsometry, or the meas urement of heights, meteorology, and investiga tions in physics. (See METEOROLOGY and HY P som ) The mercurial barometer was made to register its own indications by Samuel More land as early as 1670 or 16S0, and many im proved forms have since been devised, register ing by the help of photography, electrically. mechanically, or by gravity. The best of these appears to he the sliding-weight barograph, as perfected by Sprung in Germany. and Marvin in the United States. As a substitute for the mercurial barometer. a form called the aneroid (q.v.) was devised by Vidi, and a better form. sometimes called the pressure gauge (see MANo mETER) , was devised by Bourdon.