metres, or a thermometer tube divided in degrees of
temperature, or an electrical slide-wire, it is usual to proceed by a
method of equal steps. The simplest method is that known as the method
of Gay Lussac in the calibration of mercurial thermometers or tubes of
small bore. It is essentially a method of substitution employing a
column of mercury of constant volume as the gauge for comparing the
capacities of different parts of the tube. A precisely similar method,
employing a pair of microscopes at a fixed distance apart as a
standard of length, is applicable to the calibration of a divided
scale. The interval to be calibrated is divided into a whole number of
equal steps or sections, the points of division at which the
corrections are to be determined are called _points of calibration._

_Calibration of a Mercury Thermometer_.--To facilitate description, we
will take the case of a fine-bore tube, such as that of a thermometer,
to be calibrated with a thread of mercury. The bore of such a tube
will generally vary considerably even in the best standard
instruments, the tubes of which have been specially drawn and
selected. The correction for inequality of bore may amount to a
quarter or half a degree, and is seldom less than a tenth. In ordinary
chemical thermometers it is usual to make allowance for variations of
bore in graduating the scale, but such instruments present
discontinuities of division, and cannot be used for accurate work, in
which a finely-divided scale of equal parts is essential. The
calibration of a mercury thermometer intended for work of precision is
best effected after it has been sealed. A thread of mercury of the
desired length is separated from the column. The exact adjustment of
the length of the thread requires a little manipulation. The
thermometer is inverted and tapped to make the mercury run down to the
top of the tube, thus collecting a trace of residual gas at the end of
the bulb. By quickly reversing the thermometer the bubble passes to
the neck of the bulb. If the instrument is again inverted and tapped,
the thread will probably break off at the neck of the bulb, which
should be previously cooled or warmed so as to obtain in this manner,
if possible, a thread of the desired length. If the thread so obtained
is too long or not accurate enough, it is removed to the other end of
the tube, and the bulb further warmed till