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


to yoIeAi6t, and freunently one Of
these natural forces may pass into tie
other. Long continued experiments
in deep mines. ia all kinds Of rocks.
have plainly proved that the heat in
creases as We descend-sometimes as
rnnch as one degree for every forty
FIVE feet OF descent. and seldom less
than one degree for every seventy.
Ii the heat goes on increasing con-
tinuously at the same rate, it becomes
an easy matter to calculate to what
depth we should have to go to reach
a point where all known substances
are in q 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 wonld be il the heat continued to
increase as above mentioned. It Is
thought that as we 60 deeper through
the strata, the increased pressure
which ensues withstands the tenden-
~ TO a molten condition 80 that at
great depths the enormous pressure
of the everything rocks resists the ten
dency Of the great heat to melt them.
Wers il not for this fact, all known
solids would be in a molten state at
a depth Of about thirty miles. This
il known, however, to be too thin a
crust for a planet 5000 miles in diam
eter, and so it 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 a shrinkage or con-
traction Of the earth's interior. The
hardened crust everywhere envelop-
ing it cannot shrink in the same de-
gree, but it must in some manner
yield TO conform to the lesser magni.
tnde thus produced. When it does yield,
owing to this and other stresses, earth
1nakes are produced. Another school
of biologists regard the interior Of the
earth, not as molten, but as being 10
a 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-
dn"ed by friction during shrinkage.
In either case, however, whether we
acknowledge a molten interior or
highly-heated solid one, we have
equally to explain the origin Of earth.
qnakes and volcanoes on the theory
OF the secular cooling and contraction
Of the earth as whole.
The shocks propagated through the
ear.hs crust will vary in their degree
Of intensity in proportion to the
amornt Of fracture or contraction. Tn
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.
teal instead Of horizontal. These
are termed earthquake waves" and
when powerfully felt. the surface Of
the earth will actually be thrown into
series of temporary undulatiors,
like that Of q field OF wheat when a
gentle breeze is passing over it. Not
untreqnently, if the surface rocks be
Of a nature that does not readily trans,
mt the wave, they will be rent and
assured to a considerable depth. 1t
is to the undulation produced by vic.
oient earthquake waves that so much
destruction Is due Houses and large
public buildings are thrown to the
ground, and "fenced city suddenly
becomes heap"
The power Of the various agents at
work in the interior of the earth was
shown, when Turing an eruption of Co.
topaz, mass of rock weighing 900 tons
was thrown from :ts crater a distance
OF $ or 9 miles. No country escapes
these visitations. but in volcanic dis.
'r'"ts they occur more frequently than
elsewhere; and commencing in these
they have been known to pass beneath
seas and land from one hemisphere to
another until full 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 Slst of August, ultimo, wrecked
Charlestown, S. C., killing 80 or 40
persons, injuring over 100 and de-
stroying 100000 worth Of prop
ertv.
The approach of earthquakes is her.
aided by several premOnitory symp-
toms Of an unmistakabie character.
The air appears to be affected in some
respect, perhaps in its electric condi.
ton, and the brute animals show q
Al