174 
On Underground Temperature . 
[April, 
detected in the indications given beyond 3 feet from the face 
of the roal ; and his observations were accordingly taken at 
4 feet. It is evident, as will be seen by what will be said 
hereafter, that the necessary distance must be governed by 
the difference between the temperature of the ventilating air 
and of the seam, and that the greater this difference the 
farther it would be necessary to bore into the coal to obtain 
an estimate of its true temperature. If it be attempted to 
ascertain the law of increase by means of an Artesian bore- 
hole, there are here also disturbing causes present. The 
adtion of the tool warms the rock by mechanical means, the 
fridtion or pounding adtion developing considerable heat. 
Hence while the work is in progress no reliable observations 
can be taken, except during intervals of suspension, and 
then it appears — as will be seen further on — that the inter- 
val needs to consist of weeks rather than of days. When the 
work is completed, and the bore stands nearly full of water, 
it is the temperature of the water which is obtained on 
lowering a thermometer into it, and we cannot be sure that 
that coincides with the temperature of the rock. In fade 
many reasons may be assigned why it should not do so. 
Springs may enter on one side and flow out at the other ; or 
they may rise from lower levels and flow away at higher. 
The very adt of lowering the thermometer tends to mix up 
the differently heated layers of water, and to confuse the 
result. 
A single instance will suffice to illustrate the irregularity 
of increase referred to. At Rose Bridge Colliery the tem- 
peratures were taken by drilling a hole a yard deep at the 
bottom of the shaft during the process of sinking. If the 
mean temperature of the surface there be taken at 50° F., 
the mean rate of increase for the whole depth was 1 degree 
for 55 feet. But at the successive depths of 605, 630, 663, 
671, 679, 734, 745, 761, 775, 783, 800, 806, 813 yards 
respedtively the rate appears to have been 1 degree for 70, 
25, 49, 24, no, 66, 32, 48, 51, 36, 54 feet. 
Within the last few years a very deep boring has been 
carried down at Sperenberg, near Berlin. This has reached 
the extraordinary depth of 4052 Rhenish feet, or 4172 British 
feet. A most surprising geological fadt about this boring is 
that, with the exception of the first 283 feet, which were 
carried through gypsum with some anhydrite, the remainder 
passed entirely through rock salt. This seemed to offer an 
exceptionally good opportunity for observing temperatures 
in a homogeneous rock, which might be expedted to be 
comparatively free from the sources of error arising from 
