The Democratic advocate., 1877-09-08
&istellaneous.
From BaHou's Monthly Magazine.
EARTHQUAKES AND HOW CAUSED
by PROFESSOR JAMES MACINTOSH.

Earthquake action is nearly related to
volcanic, and frequently one of these na :
tural forces may pass into the other Long
continued experiments in deep mines, in
all kinds of rocks, have plainly proved that
the heat increases as we descend. This
increase of heat cannot be ascribed to the
heavier column of atmosphere which must
press down at the bottom of deep mines.
If we ascend some mountain-Side from the
sea.level, the cold increases about one de-
gree for every 300 feet. We might,
therefore, conclude that the heat in deep
mines, if it were due to atmospherical
pressure, would increase at the same rate.
1t gains, however, much more rapidly
sometimes as much as one degree for ev-
cry forty-ive feet of descent, and seldom
less than one degree for every seventy.
If the heat goes on increasing contin-
uously at the same rate, it becomes an easy
matter to calculate to what depth we should
have to 60 to reach point where all the
known solid substances are in state of
fusion. Chemists can tell US exactly what
heat is required to melt iron, copper, etc,
so that 1t would not be difficult to estimate
the depth in the earth where, if the heat
increases at the average rate of one de-
gree for about sixty feet, these well.known
substances are constantly in molten con-
dition. For various reasons Of a mathe-
matical character, it is believed that the
thickness Of the earth's crust is much
greater than it would be If the heat con.
tiDued to increase as aforementioned. 1t
is thought that as we go deeper through
the strata, the increased pressure which
ensues withstauds the tendency to molten
condition; so that at great depths the euor-
nous pressure Of the everything rocks re-
sists the tendency of the great heat to melt
them. According to the calculation that
heat increases one degree Fahrenheit every
sixty feet of descent, if there were no
counteracting agency, all solid substances
would be in molten state at the depth of
about thirty miles This is known to be
too thin crust, however, for a planet
5000 miles in diameter, and so it is usual
ly regarded as not less than 100 miles in
thickness, owing to the pressure Withstand
ing the liquefying influence of the earth $
internal heat.
The oblate shape Of the earth, and of
all the planets Of the solar system, is ex-
planned On the ground that they we're
originally in molten condition. Since
then they have lost heat and cooled down.
The external strati6ed crust, however, still
encloses the greater part of the originally
molten planet. A loss of heat is yet slowly
going on, known as the secular cooling Of
the earth" The heat from the interior
is conducted upwards through the rocky
crust, and thence radiated into space.
We have not to consider the slopness of
this operation, but the fact that it is taking
place. With loss of heat comes a shrink
ing or contraction of the earths interior.
The hardened crust everywhere envelopiug
it cannot shrink in the same degree, and
it is necessary it should be constantly fitted
to the contracting interior, otherwise there
would be broken space between it and
the crust. .t..
1t along the weakest part of the earth 5
crust that fractures usually happen. Hence
it is that earthquakes and volcanoes gen.
orally run in lines extending over enor.
nous areas of the earth's surface. The
force generated by the fracturing or fold.
ing is that usually designated as "earth.
quake shocks" MIR. Mallet has shown
that the friction which ensues during a
iracturing Of deep.seated solid rocks, aud
when the walls are rubbed against each
other, will develop heat enough to melt
rocks. In this way earthquakes pass into
volcanic action. These two forces extend
over the same areas, and are often trans.
mitted from the same spots, or foci. Vol
canic action is nearly always preceded by
earthquake shocks, whose intensity in.
creases until volcanic energy is developed,
after which the former subsidies. On Mr..
Mallet's theory, this near relation is easily
understood, and he has shown that the
amount Of heat which must be aunualIs
developed by the secular cooling Of the
earth's interior, is more than is required
70 produce all tho volcanic energy. Mr.,
Mallet, however, does not think the inte
rior OF the earth Is molten, but in a heated
solid state, the molten lava which issue,
RON the volcanic craters having beer
formed by the fusion Of the solid matter
where heat has been developed during
shrlnkage. In either case, however, wheth
er We acknowledge a molten interior or ,
highly.heated solid one, we have equal,
to explain the origin of earthquakes and
volcanoes on the theory of the secular
whole. - 2 =
The shocks prOpagated through th,
ear'h's crust will vary in their degree d
intensity in proportion to the amount ol