olumn of mercury, one square centimetre in
section, which is supported by the atmosphere. If H cms. be the height at
0deg, and _g_ the value of gravity, the pressure is 13.596 H_g_ dynes
(13.596 being the density of mercury). At Greenwich, where _g_ = 981.17,
the standard pressure at 0deg is 1,013,800 dynes. At Paris the pressure is
1,013,600 dynes. The closeness of this unit to a mega-dyne (a million
dynes) has led to the suggestion that a mega-dyne per square centimetre
should be adopted as the standard pressure, and it has been adopted by some
modern writers on account of its convenience of calculation and
independence of locality.

[Sidenote: Barometric readings.]

The height of the barometer is expressed in English inches in England and
America, but the metric system is used in all scientific work excepting in
meteorology. In France and most European countries, the height is given in
millimetres, a millimetre being the thousandth part of a metre, which
equals 39.37079 English inches. Up to 1869 the barometer was given in
half-lines in Russia, which, equalling the twentieth of an English inch,
were readily reduced to English inches by dividing by 20. The metric
barometric scale is now used in Russia. In a few European countries the
French or Paris line, equalling 0.088814 in., is sometimes used. The
English measure of length being a standard at 62deg Fahr., the old French
measure at 61.2deg, and the metric scale at 32deg, it is necessary, before
comparing observations made with the three barometers, to reduce them to
the same temperature, so as to neutralize the inequalities arising from the
expansion of the scales by heat.

[Sidenote: Sympiezometer.]

The sympiezometer was invented in 1818 by Adie of Edinburgh. It is a
revived form of Hooke's marine barometer. It consists of a glass tube, with
a small chamber at the top and an open cistern below. The upper part of the
tube is filled with air, and the lower part and cistern with glycerin. When
atmospheric pressure is increased, the air is compressed by the rising of
the fluid; but when it is diminished the fluid falls, and the contained air
expands. To correct for the error arising from the increased pressure of
the contained air when its temperature varies, a thermometer and
sliding-scale are added, so that the instrument may be adjusted to the
temperature at each observation. It is a sensitive instrument, and well
suited for rough purposes at sea and for travel