The Democratic advocate., 1877-09-08

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surface of the earth will actually be thrown
into series of temporary undulations
like that of a field Of corn when a gentle
breeze is passing over it. Not unfre-
quently, if the surface rocks be Of nature
that does not readily transmit the wave
they will be rent and 6ssured to consid.
erable depth, It is to the undulation pro-
duced by violent earthquake waves that 80
much destruction is due. Houses aud
large public buildings are thrown to the
ground, aud a "fencod city" suddenly
becomes heap"
Earthquakes are Of three kinds-undu-
latdlg, perpendicular aud horiSontat-ac-
cording to the direction in which the wave
or force is propagated. II is the former
to which we have been chiefly alludng, as
this is the most frequent, but, as a rule,
also the most harmless. This, however,
is not always the case, as the great earth
quake which destroyed Lisbon in 1855
plainly shows. That earthquake was of
an undulatory character. The perpendic-
lar 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 suddenly projected into the air, owing
to the vibrations succeeding each other
rapidly, and passing through the earth's
crust in a perpendicular direction. The
earthquake which occurred at Riobamba,
in Ecuador, South America, in 1897, was
of the perpendicular class During its
occurrences, rocks, trees, together with
shattered buildings, and their inhabitants,
were projected into the air as from the
explosion Of mine. People were actual.
ly 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 n combination of
the two just mentioned, producing kind
Of horizontal twist. The great Calabrian
earthquake, investigated by the French
Commission, which took place in 1888,
was Of this kind. In its course it de-
stroyed two hundred towns and villages,
aud killed hundred thousand people
As will be understood from this classif-
cation, the undulatory earthquakes are
felt over the most extensive areas That
of Lisbon extended from Bohemia 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 stone into pool; the waves
extend themselves in circles, succeeding
each other, and becoming fainter further
away from the centre of disturbance ow-
ing to the friction of the water. This is
very similar to the way in which many
earthquake shocks are propagated They
form huge circular waves of force, which
traverse the rocks, and lose their violence
far away from their origin, from the re-
sistance Of the rocks through which they
pass. That the origin of these waves is due
to the weak lines in the earths crust, is
evident from the fact that earthquake
waves usually follow the same directions
in the same districts. In Great Britain,
what few shocks are experienced are due
to the earthquake undulations travelling in
south-westerly direction. In Scandi-
nayia it is the same; whilst in Spain the
direction the waves take is southeast In
the Allegheny mountains, the course is
north-westerly.
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 primary rocks allow the
shocks to be conveyed through them with.
out much disturbance. On the other hand,
the latter tertiary strata (Which are usually
loose and unconsolidated, and soft alluyial
deposits, are the worst conductors of earth
quake movements, and therefore cause the
greatest destruction to towns and villages
that may be built upon them. Moreover,
a still greater degree of violence is expe.
rienced at the junction of old primary
rocks, and where the later tertiary beds
rest upon them. The carlul examination
of the great Calabrian earthquake proved
that the greatest destruction always oc-
CURED under such circumstances. This
conclusion is not without practical value
in earthquake-visited countries, for It
plainly shows which areas would be the
best for building upon, and which the
worst. 1t is not difficult to understand
the cause Of such destruction where the
denser rocks and the looser strata come
together. Just as electricity can be safely
conducted away along copper wire,
whilst 1t will manifest itself as if it had TO
leap short interval to the end of another
wire, so would the earthquake undulations
travel at certain speed, and with o GER.
tam rhythm, it the rocks through which
they passed were everywhere alike in
density and structure. Atthc point where
change takes place, as at the junction, of
primary and tertiary beds, the waves pass
ingthrough the former are suddenly altered
when they have to traverse the latter, and
this sudden alteration is then powerfully
manifested on the surface.
The rate at which these movements or
waves are propagated has been ascertained
chiefly through the researches of Mr..
Mallet' The great earthquake wave of
Lisbon travelled twenty miles a minute.
When earthquake foci arc situated near
the sea, then, in addition to the destruc.
tive undulatiOhs which pass through the
earth's crust, we have subsequent inunda-
tions, which not uufrequently cause even
more mischief than the original shocks,
This was the case at Lisbon, and still more
recently in Jamaica. In the former place.
when the earthquake bad shaken down the
buildings, such people as could escape
found shelter on the new quay, away from
the falling houses. Here they were CON
gregated in thousands, when a huge wave
of water, sixty feet high, came sweeping
in from the sea, and drowned most Of
them. These waves are caused by the
earthquake shock travelling more rapidly
through the solid crust of the earth than
through the volume of sea-water. The
first is Propagated with great rapidity; but
water is worse conductor Of the force,