172 On Underground Temperature. [April, 
and no means have been yet suggested which could furnish 
it at the atmospheric temperature unaffected — or but slightly 
so — by its long journey through the subterranean passages. 
This increase of temperature, though universal, is not 
everywhere the same. The average is about 1 degree Fahr. 
for between 50 and 60 feet of descent. Such is the result of 
very numerous observations. Some of these have been made 
by drilling holes in the rock in deep mines ; others by low- 
ering thermometers in Artesian bore-holes. A Committee 
was appointed by the British Association to report upon 
the subject, and their reports extend from the year 1869. 
Much information upon the matter may also be gathered 
from the Reports of the Parliamentary Commission upon 
the Coal Supply. 
Unquestioned as is the faCt, nevertheless the cause of this 
increase of heat has formed the ground for much specula- 
tion. The most obvious explanation of it is offered by the 
phenomena of hot springs and volcanos. These show us 
that, in some places at any rate, much higher temperatures 
exist at great depths than have ever been reached by arti- 
ficial perforations. But these phenomena might be said to 
be local, while the general slow increase of temperature we 
have been describing exists more or less at every place. Are 
these phenomena direCtly connected ? Are they indirectly 
connected ? Or, are they altogether unconnected ? Various 
answers have been returned to these questions. 
Among “ practical men” engaged in mining operations 
an opinion seems to have prevailed that the increase of 
temperature in deep mines is due to the pressure of the 
overlying strata or “ cover.” We may unhesitatingly dis- 
miss this hypothesis. Pressure by itself cannot develop 
heat. Where motion is destroyed as motion, there it is that 
it is converted into heat. Thus the motion which is 
destroyed when a hammer strikes upon an anvil will develop 
heat that can explode fulminating powder, or heat a nail 
red-hot. So, also, the bearings of a wheel become heated 
by friction, which gradually destroys its motion. But though 
mountains rise on mountains, there will be no heat produced 
unless motion of some kind is destroyed. In deep coal- 
mines an effeCt called “ creep ” is caused by the enormous 
pressure upon the sides of a passage, or upon the pillars left 
to support the roof, causing the floor of the excavation to 
swell up. In this case there is immense friction between 
the particles of the rock, were it not for which the passage 
would become instantaneously closed. It has been remarked 
that considerable heat sometimes accompanies this creep, 
