Saint Mary's beacon., 1877-03-22

by violent earthquake wares that 20
much destruction - due. Houses and
large public buildings are thrown to the
groond, and i.fenced city, suddenly
'becomes . heap 49,. -T,era E.al-- 7
Earthquakes ore, Of three kinds-sa-
dalatiag. perpendienLay and korisosfal
-accoEding to the direction in which
the wave or force j. propagsted. Il y
the former 10 which We have been chief
Iy alluding. as this 10 the most freqsenA
but. - rule. also the nest haEm1Gnnk
This howeves, i aots1 1lUeys t1Ge I
As the great earthquake which destroyed
Lisbon in 1855 plainly shows, That
earthquake was Of an undalatory char
acter. The perpendicular earthquakes
are not so frequent as the last, but when
they attain any unusual development
they are more destructive. Sometimes
they have the character of an explosion,
and objects in the neighborhood are sud-
denly projected into the air, owing to
the vibrations succeeding each other
rapidly, and passing through the earth's
crust in perpendicular direction. The
earthquake which occurred at Riobam-
ba, in Ecuador. South America, in 1897,
was Of the perpendicular class. Dur
ing its occurrences, rocks, trees, togeth-
er with shattered buildings, and their
inhabitants, were projected into the air
as from the explosion Of mine. Peo-
pie were actually hurled hundreds Of
feet upwards.
Horizontal earthquakes are the rarest
in their occurrence, but they are the
most destructive Of any, as the motion
which originates them is due to a COM
bination of the two just mentioned, pro-
ducing kind Of horizontal twist. The
great Calabrian earthquake, iuvestiga-
ted by the French Commission, which
took place in 1888. was OF this kind.
In its course it destroyed two hundred
towns and villages, and killed hun
dred thousand people.
As will be understood from this clas-
sifcation, the undulatory earthquakes
are felt over the most extensive areas.
That OF Lisbon extended from Bohe mia
to the West Indies, east and west, and
from Algiers to Sweden, north and
south. The Calabrian earthquake was
confined to an area of only six hundred
square miles. We have referred to the
ripples produced by throwing a stone
into pool; the waves extend them
selves in circles, succeeding each other,
and becoming fainter further away from
the centre of disturbance, owing to the
friction of the water. This is We sim
liar TO the way in which many earth
quake shocks are propagated. They
form huge circular waves Of force, which h
traverse the rocks, and lose their vio-
lence far away from their origin, from
the resistance OF the rocks through which
they pass. hat the origin Of these
waves Is due to the weak lines in the
earth's crust, is evident from the fact
that earthquake waves usually follow
the same directions in the same dis
tricts. In Great Britain, what few
shocks are experienced are due to the
earthquake undulations travelling in a
south-westerly direction. In Scandi-
| nayia II is the same; whilst in Spain the
direction the waves take is southeast.
In the Allegheny mountains, the course
Is HOrtn-WeSteriy.
1t has been found that certain rocks
conduct away the earthquake waves
better than others, just as some metals
are better conductors of electricity or
heat than the rest. Granite and prima-
~ rocks allow the shocks TO be con-
veyed through them without much dis
turbance. On the other hand, the lat
ter tertiary strata (Which are usually
loose and uncon>olidatedO, and soft al
luvial deposits, are the worst conduc-
tors of earthquake movements, and
therefore cause the greatest destruction
to towns and villages that NAV be built
upon them. Moreover, a still greater
degree OF violence is experienced at the
junction of old primary rocks, and
where the later tertiary beds rest upon
them. The careful examination Of the
great Calabrian earthquake proved that
the greatest destruction always cccur-
red under such circumstances. This
conclusion is not without q practical
value in earthquake-visited countries,
ior 1t plainly shows which areas would
be the best tor building upon, and which
the worst. 1t is not difficult to under
stand the cause of such destruction
where the denSer rocks and the looser
strata come together. Just As eieetri-
city can be safely conducted away along
copper wiie, whilst it will manifest
itself as if II had to leap a short interval
to the end OF another wire, so would the
earthquake undulations travel at a GER
tain speed, aud with certain rhythm,
ii the rocks through which thew passed
were everywhere alike in density and
structure. AT the point W here change
takes place, as at the junction OF primary
and tertiary beds. the waves passing
through the former are suddenly altered
when they have 70 traverse the latter.
and this sudden alteration Is then DOW
eriully manifested on the surface.
The rate at which these movements
or waves are propasa,ed has been as
certained chiefly through the researches
OF Mr.. Msile,. The great earthquake
wave OF Lisbon travelled twenty miles
minute. When earthquake ioci are
situated near the sea, thea, in addition
to the destructive undula,ions which
pass through the earth's crust, NE have
subsequent inundation,, which not un-
frequently cause even more mischief
than the original shocks. THIS Vas the
case al Lisbon. aud still more recently
in Jamaica In the former place, when
the earthquake bad shaken down the
buildings, such people a. could escape
found shelter On the new quay. away