Saint Mary's beacon. [volume] (Leonard Town, Md.) 1867-1983, 1886-09-09
Prepared for the Beacoll
EARTHQUAKES AND HOW CAUSED.


to volcanic, and frequently one of
these natural forces may pass into the
other. Long continued experiments
in deep mines, in all kinds of rocks,
have plainly proved that the heat in
oreases as We observation-disclosures as
much as one degree for every forty
FIVE feet Of descent, and seldom less
than one degree for every seventy.
If the heat goes on increasing con.
tinuously at the same rate, it becomes
an easy matter 10 calculate to what
depth we should have to 90 to reach
a point where all known substances
are in a state Of fusion. For various
reasons of a mathematical character,
il is believed that the thickness Of
the earth's crust is much greater than
it would be if the heat continued to
increase as above mentioned. 1t is
thought that as we go deeper through
the strata, the increased pressure
which ensues withstands the tenden-
cy to a molten condition so that at
great depths the enormous pressure
of the everything rocks resists the ten
denoy Of the great heat to melt them.
Were it not for this fact, all known
solids would be in a molten state at
a depth of about thirty miles. This
is known, however, to be too thin a
crust for planet 5000 miles in diam
eter, and 30 1T is usually regarded
as not less than 100 miles in thick
ness. A loss of heat il going slowly
on, known as the secular cooling of
of the earth, " the heat being conduc-
ted through the rocky crust and
thence radiated into space. With
loss of heat comes shrinkage or con-
traction Of the earth's interior. The
hardened crustIeverywhere envelop-
ing it cannot shrink in the same de-
gree, but it must ii some manner
yield to conform to the lesser magni.
tude thus produced. When it does yield,
owing to this and other stresses, earth
qnakes are produced. Another school
Of biologists regard the interior Of the
earth, not as molten, hut as being 10
heated solid state and say, that
the lava which issues from volcanoes
is formed by the heated solid matter
being fused by the intense heat pro-
duced by friction during shrinkage.
In either case. however, whether we
acknowledge a molten interior or a
highly.heated solid one, we have
equally to explain the origin ofearth.
quakes and volcanoes on the theory
of the secular cooling and contraction
Of the earth as a whole. s
The shocks propagated through the
earths crust will vary in their degree
of intensity in proportion to the
amount of fracture or contraction. In
every case the shock is propagated in
waves, analogous to those which rip.
pie the surface of a smooth pond when
a stone has been thrown in, only yer.
iical instead of horizontal. These
are termed earthquake waves; and
when powerfully felt the surface of
the earth will actually b. thrown into
a series Of temporary undulatiors,
like that of field of wheat when
gentle breeze is passing over it. Not
unfrequently, if the surface rocks be
OF nature that does not readily trans,
init the wave, they will be rent and
assured to a considerable depth. 1t
is to the undulation produced by VIC.
client earthquake waves that 50 much
destruction is due. Houses and large
public buildings are thrown to the
ground and a "fenced city" suddenly
becomes q heap"
The power of the various agents at
work in the interior Of the earth was
shown, when during an eruption of Co.
topaxi, mass of rock weighing 300 tons
was thrown from its crater distance
OF 8 or 9 miles, No country escape.
these visitations, but in volcanic dis.
trlets they occur more frequently thar
elsewhere; and commencing in these
they have been known to pass beneath
seas and land from one hemisphere to
another until full 1 of the entire globe
has been more or less disturbed by
the movement, Such was the great
Lisbon earthquake of 1855 and to a
lesser degree that which on Tuesday,
the 8lst of August, ultimo, wrecked
oharlestown, s. g., killing 80 or 40
persons, injuring over 100 and de.
stroying 100000 worth of prop.
erty. 8Ik
The approach of earthquakes is her.
aided by several premonitory symp.
toms of an unmistakable character.
The air appears to be affected in some
respect. nerhans in ,its elentrin nnnh;.