The thermometer is an instrument for determining the temperature
of the air or any other fluid into which it may be introduced. The
thermometers in general use contain mercury, but some contain colored
spirit; yet, as mercury is most generally used, it will be only
necessary to say of spirit thermometers, that they act on the same
principle. A thermometer consists of a glass ball having a long thin
hollow tube rising out of it and attached to a graduated scale--the
bore or hollow of the tube is very small, scarcely sufficient to
admit a piece of sewing cotton. The ball or bulb and part of the
stalk are filled with mercury by holding a lamp to the ball till
the air is nearly all expelled by its expansion--for heat expands
air very greatly--and putting the end of the stalk into a vessel of
the fluid. When the lamp is removed the air in the bulb cools and
therefore contracts, by which means the mercury is forced up the fine
tube, very nearly filling the bulb. The bulb is then held downwards
and the mercury so heated that it expands, as did the air, till it
fills the whole of the bulb and stalk up to the very top; the top
is then melted with the blow-pipe (see “Blowpipes”), and the glass,
running together, closes up the bore at the end. As the mercury cools
it contracts, and consequently, occupying less space, falls down in
the stalk pretty close to the bulb, the space above it is therefore
empty, and forms a “vacuum.” Now, therefore, we have an instrument,
consisting of a bulb and stalk half-filled with mercury (fig. 1).
Upon any amount of heat being applied to the bulb, the mercury in it
expands, and rises in the stalk in proportion to the amount of heat
applied, or shrinks and sinks down again as it cools. The next thing
to be done is to form a “scale” by which the height of the mercury
in the stalk may indicate some known or recognised temperature.
There are three scales in use, “Fahrenheit’s,” “Reaumur’s,” and the
“centigrade.” The scale universally used in England is Fahrenheit’s,
although both this and Reaumur’s are sometimes marked on the same
thermometer (fig. 2). Fahrenheit’s scale is formed thus:--The bulb
of the thermometer is placed in boiling water, and the height to
which the mercury rises is marked by a scratch on the stalk; it is
then put into snow or ice in the act of melting, and another scratch
is made where the mercury has descended to. The space between these
two marks is divided into 180 equal parts called degrees, and these
divisions are carried upwards to nearly the end of the stalk and
downwards to near the bulb; the upper scratch, indicating the heat
of boiling water, is marked 212, and the lower one, which marks the
freezing point of water, being 180 divisions lower, will be 32; and
of course, 32 degrees lower will be 0, and is called “zero.” On the
scale of Reaumur’s thermometer the zero or point marked 0, is at the
freezing point of water, and the boiling point is marked 80 (fig. 2).
The centigrade differs from Reaumur’s only in having the space
between the boiling and freezing point of water divided into 100
parts instead of 80. What are called “register thermometers” have
two bulbs, stalks, and scales, on the same instrument (fig. 3); one
bulb is filled with mercury, and the other with colored spirit. In
each stalk a piece of enamel, about half-an-inch long and fitting
the cavity, is introduced; the one in the mercury is to register
the highest, and that in the spirit to register the lowest degree
of heat. They act in the following manner:--The spirit, being very
liquid or thin in its nature, wets the enamel and passes by it when
it rises in the stalk, so that the elevation of temperature does
not affect its position, but when the spirit sinks down it drags
the enamel with it, thus registering the lowest temperature, so that
the distance the enamel is found down the stalk indicates how low
the spirit may have descended in any particular time, say a night.
With respect to the mercury, it is not of a nature to adhere to
the enamel, and therefore instead of passing it pushes it up in the
stalk as it rises, but on descending leaves it behind, the height
at which the enamel is found up the stalk indicating the highest
point to which the mercury had risen, and consequently the highest
temperature. To adjust the instrument, a slight tap or shake will
make the index in the spirit tube fall to the surface of the spirit,
where it is held by the adhesive quality of the liquid, and by the
same process that in the mercurial stalk will fall to the surface of
the mercury, but will not penetrate it, owing to its great density.