UNDERGROUND TEMPERATURE. 
15 
from the face of the strata, of course the bulk of the air will not 
reach it, and it is therefore possible to protect the hole D, in 
which the thermometer is placed, from its effect, and when we 
remember that the top of the hole is tightly plugged with clay, 
there is not much chance of the instrument being affected. Some 
very satisfactory readings were taken in a branch of this description, 
at a depth of 1439 feet in the Speedwell Pit workings, the details 
are as follows : — 
On Saturday, August 17th, 1878, the thermometer was placed 
in hard arenaceous rock, for the usual time (Saturday, 2 p.m. till 
Monday morning early), when 69° 7 f. was read : the reading was 
confirmed on the following Saturday till Monday. In the October 
following, a seam of coal was cut, known as the “ Two feet,” and 
an observation in this, and one in the strata upon which the coal 
rests, gave the same : so that I think there can be no doubt as to 
69° 7 F. being the correct temperature for that depth. The next 
point was to ascertain the temperature at the surface, so as to get 
at the rate of increase. Various measures have been suggested, 
such as observations in wells, at stated periods, extending’ over, at 
least, twelve months, or selecting depths near the surface, so as to 
determine where the constant temperature begins. I think the 
former process is very liable to error, for two reasons — ist. Wells 
are very apt to contain organic matter, derived from the decay of 
dead leaves, and too often from various other organic contamin- 
ations, and by this decomposition, heat must be generated. 2nd. 
In deep wells, which alone would be serviceable, water, of different 
temperatures, might flow in, especially after a wet period. 
Observations have been made in various places to determine the 
difference between the temperature of the earth a few feet down, and 
the mean temperature of the air on the surface. The result at 3ft. 
below the surface, at three stations established in Edinburgh by 
Prof. Forbes, gave, after five years’ observations, an excess of 0.55 
above the mean temperature of the air. In Paris, in the year 1825, 
