1877J On Underground Temperature. 183 
flows out of the sides of the bore-hole into the imprisoned 
water is now no longer conveyed away by the currents, and 
begins to accumulate there. The rock in the neighbourhood 
of this water begins to get warmer. The region of the true 
rock-temperature approaches nearer and nearer to the bore- 
hole, until at last it reaches it, and then, and not until then, 
the imprisoned water assumes the true temperature of the 
rock. This process will require time, and that a long time. 
In the first place, on account of the great capacity of water 
for heat, it requires much more heat to warm up a volume 
of water through a given range of temperature than it 
would require to warm up an equal volume of rock. In the 
second place, it requires a long time for the heat to travel 
through the rock to reach the water. And although the 
conductivity of rock salt may be considerably higher than 
that of ordinary rock, so that the heat passes more quickly 
through it, yet there will be a compensating effeCt in the 
present instance, because, on account of the greater con- 
ductivity, the true rock temperature would not be reached 
within so small a distance from the bore-hole, so that the 
heat would have a longer distance to travel before the true 
temperature could be restored. That the geo-thermometer 
was not left in place sufficiently long for this purpose seems 
to be proved by the following instance of the extreme slow- 
ness with which the passage of heat under such circum- 
stances takes place. From comparing the British Association 
Reports for 1872 and 1873 we gather that, in the course of 
the observations made at the Artesian well of La Chapelle, 
at S. Denis, it was noticed that the temperature at the 
bottom of the hole was much affedted by the aCtion of the 
“ trepan ” or boring tool. It appears to have been expected 
that this effeCI would have passed off in a few days. 
Accordingly, a week after the tool was stopped, observations 
were taken, and there was found what was thought to be an 
unaccountably sudden increase of nearly 8° F., in 60 feet of 
descent, near the bottom of the hole. But in the following 
year a second set of observations were made “some months” 
after the boring had been suspended, and the abnormal 
increase was found to have entirely disappeared. We learn, 
then, that a week was not sufficient for the rock wall of the 
bore-hole to regain its balance of temperature after having 
been disturbed through about 7° F. How much longer was 
required we have not the means of knowing. It seems a 
necessary inference that a few hours would be quite insuffi- 
cient for a correct observation, and the time allowed at 
Sperenberg did not exceed such an interval. The conclu- 
sion must therefore follow that, on both the accounts referred 
