THE SUBTERRANEAN FURNACE
ET us get back to our subject. At the bottom of mines, I
told you, a high temperature prevails, which keeps up during
the whole year. There is always the same heat, winter and
summer. The deepest excavation miners have ever made is in
Bohemia. It is inaccessible to-day. Landslides have partly
filled it. At the depth of 1151 meters the thermometer
indicated a perpetual heat of forty degrees, almost the
temperature of the hottest regions in the world. And that,
mind you, in winter as well as summer. When mountainous
Bohemia was covered with ice and snow, it was only necessary
to go down to the bottom of the mine to pass from the rigors
of winter to the insupportable heat of a Senegal summer. One
shivered with cold at the entrance and stifled with heat at
"The same conditions, without exception, prevail everywhere.
The deeper one descends in the earth, the hotter one finds
the temperature. In deep mines the heat is such that the
most unobservant workman is struck by it and wonders if he
is not near some immense furnace."
"The interior of the earth is, then, really a stove?"
"Much more than a stove, as you will see. The
 name of
artesian well is given to a cylindrical hole which by means
of strong iron bars, fitted end to end, is made in the
ground until some reservoir of subterranean water, fed by
the infiltrations of neighboring streams or lakes, is
reached. The water that comes up from far under ground as
the result of such a boring reaches the surface at a
temperature equal to that of those depths; and thus we learn
about the distribution of heat in the bowels of the earth.
One of the most remarkable of these wells is that of
Grenelle, at Paris. It is 547 meters deep, and the water in
it is constantly at 28 degrees, a temperature almost as high
as that of the hottest summer days. The water of the
artesian well of Mondorf, on the frontier of France and
Luxemburg, comes from a far greater depth, 700 meters. Its
temperature is 35 degrees. Artesian wells, of which there
are at present a considerable number, illustrate the same
principle as mines: for every thirty meters of depth the
heat increases one degree."
"Then by digging wells deep enough we should at last come to
"Certainly. The difficulty is to attain the desired depth.
To reach the temperature of boiling water it would be
necessary to bore about three quarters of a league, which is
impossible. However, a number of natural springs are known
which, as they come from the ground, possess a high
temperature, sometimes reaching the boiling point. They are
called thermal springs, which means hot springs. There
prevails, then, at the depth from which they come, a heat
sufficient to make them tepid, or even
 boiling hot. The most
remarkable hot springs of France are those of Chaudes-Aigues
and Vic, in Cantal. They are almost boiling."
"Do these springs make streams that are different from
"Steaming streams, in which you can plunge an egg for a
moment and take it out cooked."
"Then there are no little fish or crabs," said Emile.
"Certainly not, my dear. You understand that if there were
any they would be cooked through and through."
"That is true."
"The little streams of boiling water in Auvergne are nothing
in comparison with what are seen in Iceland, that large
island situated at the extreme north of Europe and covered
with snow the greater part of the year. It has numbers of
springs throwing up hot water, called in that country
geysers. The most powerful, or the Great Geyser, springs
from a large basin situated on the top of a hill formed by
the smooth white incrustations deposited by the foam of the
water. The interior of this basin is funnel-shaped and
terminates in tortuous conduits penetrating to unknown
"Each eruption of this volcano of boiling water is announced
by a trembling of the earth and dull noises like distant
detonations of some subterranean artillery. Every moment the
detonations become stronger; the earth trembles, and, from
the bottom of the crater, the water rushes up in an
impetuous torrent and fills the basin, where, for a few
mo-  ments we have what looks like a boiler heated by some
In the midst of a whirlpool of steam the
water rises in a boiling flood. Suddenly the geyser musters
all its force: there is a loud explosion, and a column of
water six meters in diameter spouts upward to the height of
sixty meters, and falls again in steaming showers after
having expanded in the shape of an immense sheaf crowned
with white vapor. This formidable outburst lasts only a few
moments. Soon the liquid sheaf sinks; the water in the basin
retires, to be swallowed up in the depths of the crater, and
is replaced by a column of steam, furious and roaring, which
spouts upward with thunderous reverberations and, in its
indomitable force, hurls aloft huge masses of rock that have
fallen into the crater, or breaks them into tiny bits. The
whole neighborhood is veiled in these dense eddies of steam.
Finally calm is restored and the fury of the geyser abates,
but only to burst forth again later and repeat the same
"That must be terrible and beautiful at the same time,"
commented Emile. "No doubt you look at this furious
fountain from a long distance, so as not to be struck on the
back by boiling showers."
 "What you have just told us, Uncle," said Jules, "shows
plainly that there is great heat under ground."
"In admitting, as all these observations justify us in
doing, that the subterranean temperature increases with the
depth one degree for every thirty meters, it is estimated
that at three kilometers or three quarters of a league down,
the temperature must be that of boiling water, that is to
say 100 degrees. Five leagues down, the heat is that of
red-hot iron; at twelve leagues it is sufficient to melt all
known substances. At a greater depth the temperature,
apparently, is still higher. Accordingly we are to imagine
the earth is formed of a globe of matter liquefied by fire
and enveloped by a thin crust of solid material that is
upborne by that central ocean of melted minerals."
"You say," said Claire, "a thin crust of solid material; and
yet, according to the calculations you have just mentioned,
the thickness of the solid material must be about twelve
leagues. Under that would be the melted matter. It seems to
me twelve leagues make a good thickness, and we have nothing
to fear from the subterranean fire."
"Twelve leagues are very little in relation to the earth's
dimensions. The distance from the surface of the earth to
its center is 1600 leagues. Of this distance about twelve
leagues belong to the thickness of the solid crust, all the
rest to the molten globe. On a ball two meters in diameter
the solid crust of the earth would be represented by a
thickness of half a finger's breadth. Let us make a more
 comparison, representing the earth by an egg. Well,
the egg-shell is the solid crust of the globe; its liquid
content is the central mass in fusion."
"And we are separated from the immense subterranean furnace
only by that thin shell!" exclaimed Jules. "That is not at
"I agree, it is not without a certain emotion that one hears
for the first time what science tells us of these intimate
details of the earth's structure; one cannot think without
fear of those burning abysses that roll their waves of
melted minerals a few leagues under our feet. How can a
covering, relatively so light, resist the fluctuations of
the central liquid mass? This fragile crust, this shell of
the globe, will it not some time melt, become disjointed,
crumble, or at least move? The little it does move
makes continents tremble and the ground crack open in
"Ah!" interposed Claire, "that is the cause of earthquakes.
The liquid that is inside is stirred, and the shell moves."
"It seems to me," Jules remarked, that this shell,
comparatively so thin, ought to tremble oftener."
"Perhaps not a day passes without the solid crust of the
earth experiencing some shock, sometimes at one point,
sometimes at another, beneath the bed of the seas, as well
as under the continents. However, disastrous earthquakes are
very rare, thanks to the intervention of volcanoes.
"Volcanic orifices are, in fact, veritable safety-valves,
which put the interior of the globe in
com-  munication with
the exterior. By offering permanent vents to the
subterranean vapors that tend to liberate themselves by
overturning the earth, they render earthquakes less frequent
and less disastrous. In volcanic countries every time the
ground is shaken by strong shocks, the earthquake ceases the
moment the volcano begins to throw up its fumes and lava."
"I well remember," said Jules, "your account of the eruption
of Etna and the Catanian disaster. At first I only saw in
volcanoes terrible mountains spreading devastation around
them; now I begin to see their great use, their necessity.
Without their air-holes, the earth would seldom be still."