Symons, F.R.S. From the circumstance that the
mud at the bottom of the bore-hole into which the
thermometer was sunk was little affected by convec-
tion currents, the results obtained show, possibly, a
near approximation to the normal temperature of the
rock. At the same time the long period that the
well had stood neglected allowed the play of those
currents in the water standing in the tube above the
mud, and this may probably have effected a slight
reduction of temperature.
2. Richmond (No. 230). — The observations here
were made by Professor Judd with standard instru-^
ments. The overflow water was in too smalL a
quantity to give the correct temperature, though
enough to check convection currents. The tempera-
ture was ascertained by letting down a thermometer

to the bottom of the well.
Q



226 ON UNDERGROUND TEMPERATURES [vi

3. The first observations at Grenelle (No. 37) were
made when the bore-hole had reached the depth of
400 metres (1,312 feet), and when no work had been
going on for some weeks. The thermometers, of
which three sets were used, were left down thirty-six
hours. They were protected against pressure, and
Arago remarks that the chalk through which they
were boring made so thick a paste filling the bore-
hole, that convection currents were hardly possible.
All three sets of instruments gave results within a
fraction of a degree to one another.
4. In the bore-hole of the well at the Ecole Mili-
taire (No. 36) the experiments were made by
Walferdin under very similar conditions.
5. Pregny near Geneva (No. 29). — The thermo-
meter was protected against pressure, and to a
certain extent against convection currents. The
bore-hole was small, and the water stood at a small
depth below the surface.
