THERMOMETRY. 
489 
and tube will be less enlarged than the volume of the mercury 
contained in them in the proportion of nearly 1 to 20 ; there-- 
fore, for the reason above stated, every elevation of tem- 
perature by which the tube and the mercury would be affected, 
will cause the column of mercury to rise in the tube, and 
every diminution of temperature will cause it to fall. 
The space through which the mercury will rise in the tube, 
by a given increase of temperature, will be greater or less 
according to the proportion which the diameter of the tube 
bears to the capacity of the bulb. The smaller the proportion 
of the one to the other, the greater will be the elevation of the 
column produced by a given increase of temperature ; for a 
given increment of temperature will produce a definite increase 
in volume in the mercury, and this increase of volume will fill 
a greater space in the tube, in proportion to the smallness 
of the bore of the tube compared with the capacity of the 
bulb. 
Such an instrument, without anything additional attached 
to it, would merely indicate, if a change of temperature took 
place, the simple fact of there being an alteration. To render 
it useful for the purposes of science, it is necessary that the 
tube should be provided with some contrivance from which 
exact numerical indications of the amount of the changes 
might be derived. A graduated scale is therefore attached 
to the thermoscopic tube previously described, and we 
then possess a thermometer of the simplest form in use. 
Of course, in the actual construction of thermometers there are 
precautions to be attended to, which need not be adverted to 
here. 
In making a thermometer, it is not indispensable that mer- 
cury should be used ; air will, in many cases, answer the pur- 
pose. From the delicacy of its indications, and the regularity 
of its expansion, air would seem to be the material best fitted 
for measuring changes of temperature, and, indeed, it was the 
substance first used. The early air-thermometer consisted of a 
simple glass tube, having attached to it at one end a bulb, the 
other dipping into some liquid contained in a vessel below ; as 
the bulb became heated, the included air expanded and forced 
down the liquid in the tube ; as it cooled, the air contracted, 
and the liquid rose ; a scale attached gave the amount of the 
corresponding variation in temperature. But the inconvenient 
size of the instrument, and the extreme delicacy of its indica- 
tions, together with its limited range, impaired its utility. It 
was also found that differences of atmospheric pressure, entirely 
independent of temperature, caused an alteration in the bulk 
of the air: these, and other circumstances combined, ultimately 
led to its falling into disuse. 
