{Table IV.)

This class of observations presents results mucli
more uniform than those taken either in Coal or
Mineral Mines, and whereas the observations in
Mines were in Palaeozoic or crystalline rocks, those in
Wells are, with few exceptions, either in secondary
or Tertiary strata, where the rocks are, as a rule, more
permeable and less disturbed.
Many of the interfering causes, difficult to eliminate
in the observations on the Mines, do not exist with
the Artesian Wells. The causes of interference in the
latter are reduced mainly to two — namely, pressure
on the instruments and convection currents. The
early experiments, where precautions were rarely
taken against pressure, are consequently unreliable.
Walferdin introduced improved and protected instru-
ments, and in some other previous cases, as in Marcet
and De la Eive's observations, protected thermometers
had been used. The need of protection against con-
vection currents had also not escaped attention, but
it was not until the later observations instituted
by the Committee of the British Association were
made, that, more efficient safeguards were intro-
duced to protect against the subtle influence of
these currents.
It is clear that we must reject all the early experi-
ments made with unprotected thermometers ; and it
is not certain whether also a large number of those
made with protected thermometers, but without



222 ON UNDERGROUND TEMPERATURES [vi

protection, or sufficient protection, against convection
currents, should not also be rejected. In large bore-
holes the disturbance from this cause is so great that
the consequences are at once sufficiently apparent to
warrant the rejection of the observations.
