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 ojien 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 portable barometer 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 be 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 car- ried or kept bottom up until wanted. It is quite common for the vacuum-chamber 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 back 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 & 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 only 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. Tle former varies with the tempera- ture: the latter varies with latitude and slight- ly with height above sea-level, iloreover, 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 be 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 Hypsometry.) The mercurial barometer was made to register its own indications by Samuel More- land as early as 1670 or 1680, 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 be 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 Manometer), was devised by Bourdon.
Bibliography. For many details as to the history and construction of the instrument and corrections for sources of error, consult Abbe, Treatise on Meteorological Apparatus and Methods (Washington. 1887). For an interesting account of the invention and development of the barometer, with illustrations showing early instruments, consult Gerland and Traumüller, Geschichte der physikalischen Experimentirkunst (Leipzig, 1899). The ordinary student manuals of practical physics contain descriptions of barometers and their use, while further information can be obtained from the circulars and bulletins of the United States Weather Bureau.
BAROMETER, Water. A barometer in
which water is used instead of a column of
mercury. As water has about one-fourteenth of
the specific gravity of mercury, the pressure of
the atmosphere will support a column nearly
fourteen times as high, consequently the move-
ment caused by variations in atmospheric pres-
sure will be over a more extended scale. The
chief disadvantage of this arrangement is due
to the water vapor in the upper part of the
tube exerting a pressure on the liquid below,
and causing a shortening of the column. The
tension of the aqueous vapor depends upon the
temperature, and changes in the reading may
be produced independent of variations in the
atmospheric pressure. Glycerine and sulphuric
acid have also been used in similar barome-
ters with much smaller errors.
BARO'MET'RIC LIGHT. A faint electric light produced in the vacuum of a mercurial barometer by swinging the instrument to and fro, causing friction of the mercury against the inside of the tube. The phenomenon was first noticed by Jean Picard of Paris, in 1675, and an instrument to produce this light was devised by John Bernouilli for Frederick I. of Prussia, in 1700. Using an air-pump to pro-
