[Sidenote: Position of barometer.]

In fixing a barometer for observation, it is indispensable that it be hung
in a perpendicular position, seeing that it is the _perpendicular distance_
between the surface of the mercury in the cistern and the top of the column
which is the true height of the barometer. The surface of the mercury
column is convex, and in noting the height of the barometer, it is not the
chord of the curve, but its tangent which is taken. This is done by setting
the straight lower edge of the vernier, an appendage with which the
barometer is furnished, as a tangent to the curve. The vernier is made to
slide up and down the scale, and by it the height of the barometer may be
read true to 0.002 or even to 0.001 in.

It is essential that the barometer is at the temperature shown by the
attached thermometer. No observation can be regarded as good if the
thermometer indicates a temperature differing from that of the whole
instrument by more than a degree. For every degree of temperature the
attached thermometer differs from the barometer, the observation will be
faulty to the extent of about 0.003 in., which in discussions of diurnal
range, &c., is a serious amount.

Before being used, barometers should be thoroughly examined as to the state
of the mercury, the size of cistern (so as to admit of low readings), and
their agreement with some known standard instrument at different points of
the scale. The pressure of the atmosphere is not expressed by the weight of
the mercury sustained in the tube by it, but by the perpendicular height of
the column. Thus, when the height of the column is 30 in., it is not said
that the atmospheric pressure is 14.7 lb on the square inch, or the weight
of the mercury filling a tube at that height whose transverse section
equals a square inch, but that it is 30 in., meaning that the pressure will
sustain a column of mercury of that height.

It is essential in gasometry to fix upon some standard pressure to which
all measurements can be reduced. The height of the standard mercury column
commonly used is 76 cms. (29.922 in.) of pure mercury at 0deg; this is near
the average height of the barometer. Since the actual _force_ exerted by
the atmosphere varies with the intensity of gravity, and therefore with the
position on the earth's surface, a place must be specified in defining the
standard pressure. This may be avoided by expressing the force as the
pressure in dynes due to a c