1 877.] 
On Underground Temperature. V 1 yi 
both flesh-eaters and plant-eaters, and which may thus be 
said to dwell on the debatable line ? Hence we must con- 
sider it as not impossible that if some insectivorous bird — 
say the starling — should increase to a very great extent, and 
should become pressed for subsistence, it might perhaps be 
tempted to attack fruits and grain. 
II. ON UNDERGROUND TEMPERATURE, 
WITH A 
DISCUSSION OF THE OBSERVATIONS MADE 
AT SPERENBERG, NEAR BERLIN. 
By 0 . Fisher, Clk. M.A., F.G.S. 
t HERE is no fact more firmly established in terrestrial 
physics than that the temperature of the rocks of the 
earth’s surface increases with increasing depth. Ob- 
servations upon this subject have been made in all parts of the 
world, and the same result has been everywhere arrived at. 
Even in the frozen soil of Yakoutzk, in Siberia, this increase 
of temperature is found to prevail, although the ground is 
congealed to the whole depth penetrated.* In all mining 
operations this gradual increase of temperature becomes a 
very serious consideration, rendering human labour at great 
depths a very severe trial to the constitution of the work- 
man. And indeed it is this circumstance, more than any 
other, which fixes a practical limit to the depth at which 
mining operations are possible. It is not the raising the 
minerals from profound depths, for the resources of modern 
engineering are quite competent to overcome any difficulty 
on that score ; it is not keeping the mines clear of water, 
for they are less troubled with its influx at great than at 
moderate depths ; but it is the impossibility of furnishing 
the men with an atmosphere to breathe in below the tem- 
perature of the blood. For when the air has to be conveyed 
to long distances it acquires the temperature of the rocks, 
* The increase of temperature at Yakoutzk, although the soil is frozen even 
beyond the depth reached, proves that this freezing is owing to the present 
low mean temperature of the locality, and that it is not a residual effed of a 
former glacial and still colder period ; for if that were so the strata would be 
now warmer above than below, whereas the reverse is in fad the case. 
