WHY PARLEY BELIEVES THAT THERE IS A GREAT SOURCE OF HEAT WITHIN THE
GLOBE.
If you have attended to what I have already told you, you will
have seen that there must be a close connection between the causes
of volcanoes, earthquakes, and hot springs, (if they are not all
to be ascribed to one cause,) from their always occurring in the
neighbourhood of each other. But there is something else that you ought
to notice in reference to this connection.
I said, that a volcanic eruption was almost always preceded by shakings
of the ground round the root of the mountain. These shakings are
sometimes so violent as to be dreadful earthquakes, and at other times
the earthquake will be at a long distance from the volcano.
There was once a great earthquake which kept on for some days on the
north shore of South America, and then stopped quite suddenly. It was
afterwards found out, that just at the moment it stopped, a tremendous
eruption burst forth from a volcano in one of the West-India islands,
more than 150 miles off.
You know what the safety-valve of a steam-engine is. Now it would seem
just as if volcanoes were safety-valves for the power which causes
earthquakes.
Very well.--I am now going to tell you what people have thought
this power has been owing to. Before I do so, that you may not be
disappointed, I should tell you that we know very little on the
subject; nor shall we ever know much till somebody can get down to the
centre of the earth, unless some of the little black spirits, that
the Rosicrucians called Gnomes, and fabled to live in the middle of
the globe, should be kind enough to give us some information on the
subject.
In the meantime, we can only guess; but we ought to guess as well as we
can, and see whether our guess is not much more likely to be true than
any of the others.
Volcanic bands are always near the sea-shore, and it has therefore been
generally supposed that water has something to do with their action.
If you take a mixture of sulphur and iron filings, and mix them into
a paste with water, and then bury them in the ground, after a while
they will become hot and send out a great quantity of steam. Some
people have imagined that this is the way in which volcanic activity
is produced, but no one thinks so now, and for very good reasons, for
there is nothing in the action so produced at all capable of accounting
for long continued eruptions, or for the flowing out of the lava.
Sir Humphrey Davy found out, that nearly all kinds of salt and earth,
which had been before looked upon as simple substances, contained
certain metals united to oxygen, which, (as you should know,) is one
of the parts of water, and, in the form of gas, of the air we breathe.
If you put a piece of one of these metals into water, it is in such a
hurry to join itself to the oxygen of the water, that you set it on
fire. A little piece of _potassium_, (the metal of the salt called
_potash_, or _pearlash_,) will float for a few moments on the surface
of the water, burning with a purple flame in the prettiest manner
imaginable.
[Illustration]
Sir Humphrey thought he had got the true explanation of the thing.
These wonderful metals formed the basis of every substance in lava,
and supposing them to exist in the centre of the globe, it was only
necessary for water to get to them to set them on fire, and thus to
give rise to earthquakes and volcanoes. It appeared only to be required
that a crack should be made at the bottom of the sea, and the water
flow into the beds of metal underneath, and the violent action produced
would burst through the surface, and throw out streams of melted
matter, accompanied with great clouds of steam, just as takes place in
the eruptions of Mount Vesuvius and other volcanoes.
But you would find it very difficult to think how this can account for
continual and nearly steady operations, such as have been known to
exist in the Geysers of Iceland for more than 1000 years, and in Mount
Stromboli for much longer than that, so we will try another guess.
It has been observed in descending deep mines, that the earth gets
warmer the deeper you go. The usual mode of trying it is to dig little
holes, as you go down, in the sides and bottom of the mine, and put a
thermometer into them. A great many observations of this kind, have
been made in different parts of the world, and the greatest care has
been taken to allow for all causes of irregularity, consisting in
climate and local peculiarities. Comparing these together, it has
been determined that the temperature increases about one degree every
fifteen feet of descent.
A similar conclusion has been drawn from the heat of the water of the
Artesian Wells. But, perhaps, you do not know what Artesian Wells are,
so I will tell you.
They are deep borings in the earth, out of which water rises to, or
even above, the surface of the ground. Wherever you bore, if you go
deep enough you may get plenty of water, and this water is always found
to be warmer in proportion to the depth of the well in which it rises.
They are called _Artesian Wells_, from _Artois_, a town in France,
where the first was constructed.
In a well of this kind near Rochelle, which was 316 feet deep, the
water near the surface was at a temperature of 55 degrees, and at the
bottom 60 degrees. It was afterwards sunk to the depth of 369 feet,
and the temperature at the bottom was then found to be 65 degrees.
Do you not think it most likely that the heat continues to increase
quite as fast in proportion below the deepest point that man has gone
down to? Well, if so, at the depth of some miles it must be hot enough
to melt anything, even granite, and all kinds of stones.
If we suppose, on this evidence, that the centre of the globe is
intensely heated, and that it gets gradually cooler towards the
surface, there is what at once will account for the hot springs, the
increasing heat in descending mines, the constant action of some
volcanoes, the occasional action of others, the streams of lava, and
the nature of all substances that are thrown out.
It is very natural to imagine that every now and then the action of
the great heated mass inside, would crack open the crust, and the sea
wearing away its bottom in some places as it does, may have something
to do in assisting this. Thus, new volcanoes would be formed, and
earthquakes would happen where they never happened before; and over
these immense cracks there would be a volcanic band.
Then, after a time, these cracks might be partially, or wholly, filled
up by the matter thrown into them, and the volcanoes on the surface
above may become quiet for a time.
[Illustration]