ions of thermometers would,
without special precautions, be falsified. He therefore invented a
form of aspirating thermometer, the earliest to be met with, and far
in advance of any that were subsequently used by other scientists. It
consisted of a polished tube, in which thermometers were enclosed, and
through which a stream of air was forced by bellows.

The difficulty of obtaining really accurate readings where thermometers
are being quickly transported through varying temperatures is generally
not duly appreciated. In the case of instruments carried m a balloon it
should be remembered that the balloon itself conveys, clinging about it,
no inconsiderable quantity of air, brought from other levels, while the
temperature of its own mass will be liable to affect any thermometer
in close neighbourhood. Moreover, any ordinary form of thermometer
is necessarily sluggish in action, as may be readily noticed. If, for
example, one be carried from a warm room to a cold passage, or vice
versa it will be seen that the column moves very deliberately, and quite
a long interval will elapse before it reaches its final position, the
cause being that the entire instrument, with any stand or mounting that
it may have, will have to adapt itself to the change of temperature
before a true record will be obtained. This difficulty applies
unavoidably to all thermometers in some degree, and the skill of
instrument makers has been taxed to reduce the errors to a minimum. It
is necessary, in any case, that a constant stream of surrounding
air should play upon the instrument, and though this is most readily
effected when instruments are carried aloft by kites, yet even thus it
is thought that an interval of some minutes has to elapse before any
form of thermometer will faithfully record any definite change of
temperature. It is on this account that some allowance must be made
for observations which will, in due place, be recorded of scientific
explorers; the point to be borne in mind being that, as was mentioned in
a former chapter, such observations will have to be regarded as giving
readings which are somewhat too high in ascents and too low in descents.
Two forms of thermometers at extremely simple construction, yet
possessed of great sensibility, will be discussed in later chapters.

The thermometers that Welsh used were undoubtedly far superior to any
that were devised before his time and it is much to be regretted that
they were allowed