The barometer is designed to indicate the weight or pressure of
the air on any surface, at any particular time or place; for the
air, although invisible, is still of considerable weight, as there
are many miles of it pressing from above downwards on all parts
of everything upon the earth, and the barometer is for the purpose
of ascertaining how much this pressure amounts to. It is formed as
follows: a piece of glass tubing, about three feet long, is first
closed at one end, then turned up at the other and expanded (fig. 1);
when this tube is filled with mercury and held with the bulb
downwards, the mercury sinks in the stalk to a certain height (say
twenty-nine inches), and that height shows the weight or pressure of
the air. The reason of this will be understood by supposing a piece
of straight glass tubing, three feet long, to be closed at one end
and then filled with mercury; if the finger be placed on the end
not closed, and that end turned downwards and put into a basin of
mercury (fig. 2) before the finger is withdrawn, the fluid, if the
air exerted no pressure, would all sink down from the inside of the
tube into that in the basin, leaving a “vacuum” or empty space in the
hollow of the tube, but it is evident if the air exerted any pressure
on the surface of the mercury in the basin, this pressure would
force the mercury up the tube (for there is no opposing pressure in
an empty space), and that the mercury would rise higher and higher
the greater the the pressure. Well, then, the air really exerts this
pressure, and to such an extent as to raise the mercury somewhere
about thirty inches in height, and the pressure necessary to do
this is found by calculation to be about fifteen pounds upon every
square inch of surface. The barometer tube is divided into a scale
of inches and fractions of inches. What are called weather glasses,
are barometers having the lower part brought up by a curve, and a
small weight resting on the mercury in it, which being attached to a
corresponding weight by means of a cord running over a little wheel
or pulley fixed to hands moving round a sort of dial, turns them as
the mercury rises or sinks (fig. 3), for as the mercury falls in the
stalk it must of course rise in the short stalk of the curve; the
hands by these means are turned round, and the rise or fall of the
fluid will cause them to point to “fair,” “rain,” &c., as the case
may be, for these names are marked where a corresponding change of
the weather may so influence the weight of the air, as to raise or
depress the mercury, and so bring the hands in a position to point
to them.