EARTHQUAKES. 29

 

 

palm trees were twisted around one another like willow
wands.

There are two kinds of movement transmitted through
the crust during earthquakes: these are the earthquake
motion proper, and the motion that produces the accompanying
sounds.

58. The Velocity of Earthquake Motion varies
according to the intensity of the shock and the na-
ture of the material through which it is trans-
mitted. No average’ result can therefore be
given. Various observers have estimated it at
from 8 to 30 miles per minute.

54. The Sounds Accompanying Earthquakes
vary both in.kind and intensity. Sometimes
they resemble the hissing noises heard when red-
hot coals are thrown into water; sometimes they
are rumbling, but more frequently they are of
greater intensity, and are then comparable to
discharges of artillery or peals of thunder.

The confused roaring and rattling are probably caused

by the different rates of transmission of the sound through
the air and rocks.

55. Duration of the Shocks—When the area
of disturbance is large, shocks of varying intensity
generally follow each other at irregular intervals.
Though, in general, the violence of the shock is
soon. passed, disturbances may occur at intervals
of days, weeks, or even years.

During the earthquake in Calabria in 1783, when nearly
100,000 persons perished, the destructive vibrations lasted
scarcely two minutes, but the tremblings of the crust con-
tinued long afterward. During the earthquake at Lisbon
in 1755, when about the same number perished, the shock
which caused the greatest damage continued but five or

six seconds, while a series of terrible movements followed
one another at intervals during the space of five minutes.

56. Cause of Earthquakes.—It is generally be-
lieved that the principal cause of earthquakes is the
force produced by the contraction of a cooling crust.

During the cooling of the earth the crust con-
tinually contracts, and the pressure so produced,
slowly accumulating for years, at last rends it
in vast fissures, thus producing those violent
movements of its crust called earthquakes. If
this theory be admitted—and it is a probable one
—the earth’s crust must every now and then be
in such a strained condition that the slightest
increase of force from within, or of diminished
resistance from without, would disturb the con-
ditions of equilibrium, and thus result in an
earthquake.

57. Strain Caused by Contraction consequent on
cooling is well exhibited in the so-called “ Prince Ru-
pert’s Drops,” which are made by allowing melted glass
to fall in drops through cold water. The sudden cooling

 

of the outside produces powerful forces, which tend to
compress the drop; but, since these forces balance one
another, no movement occurs until, by breaking off the
long end of the drop, one set of forces is removed, when
the others, no longer neutralized, tear the drop into almost
countless pieces,

Similar effects are produced by unequal contraction and
expansion. Hot water poured into a tumbler will often
crack it. The crackling sound of a stovepipe when sud-
denly heated or cooled is a similar effect,

58. Other Causes of Earthquakes. — Earth-
quakes may also be occasioned by—
(1.) The sudden evolution of gases or vapors

-from the pasty interior.

This is probably the cause of many of the
slight shocks that occur in the neighborhood of
active volcanic regions.

(2.) Shocks caused by falling masses.

Those who deny ‘the existence of a pasty interior, en-
deavor to explain the production of earthquakes by the
shock caused by the occasional caving in of huge masses
of rocks, in caverns hollowed out by the action of subter-
ranean waters; or by the’gradual settling of the upturned
strata in mountainous districts. There can be no doubt
that even moderately severe shocks are caused by falling
masses; but such a force is utterly inadequate to produce
a shock like that which destroyed Lisbon, when an area
of nearly 7,500,000 square miles was shaken.

59. Periodicity of Earthquakes—It was for-
merly believed that earthquakes occurred with-
out any regularity, but by a comparison of the
times of occurrence of a great number it has been
discovered that they occur more frequently—

(1.) In winter than in summer;

(2.) At night than during the day ;

(3.) During the new and full moon, when the
attractive force of the sun and moon acts simul-
taneously on the same parts of the earth.

Earthquake shocks are more frequent in winter,
and during the night, because the cooling, and
consequent contraction, occur more rapidly at
these times, and therefore the gradually accumu-
lating force is more apt to acquire sufficient inten-
sity to rend the solid crust.

Earthquakes are more frequent during new
and full moon, because the increased force on
the earth’s crust caused by the position of the
sun and moon at these times, is then added to
the accumulated force produced by cooling.

It has been asserted that in the equatorial regions earth-
quakes are especially frequent during the setting in of
periodical winds called the monsoons, at the change of

the rainy season or during the prevalence of hurricanes,
These facts, however, are not well established.

60. Distribution of Earthquakes. — Earth-
quakes may occur in any part of the world, but