d the corresponding movements of the
rocks, by considering certain homely illustrations. He may observe how
the soil cracks as it shrinks in times of drought, the openings
closing when it rains. In a similar way the frozen earth breaks open,
sometimes with a shock which is often counted as an earthquake. Again,
the ashes in a sifter or the gravel on a sieve show how each shaking
may relieve certain tensions established by gravity, while they create
others which are in turn to be released by the next shock. An ordinary
dwelling house sways and strains with the alternations of temperature
and moisture to which it is subjected in the round of climatal
alterations. Now and then we note the movements in a cracking sound,
but by far the greater part of them escape observation.

With this sketch of the mechanism of earthquake shocks we now turn to
consider their effects upon the surface of the earth. From a
geological point of view, the most important effect of earthquake
shocks is found in the movement of rock masses down steep slopes,
which is induced by the shaking. Everywhere on the land the agents of
decay and erosion tend to bring heavy masses into position where
gravitation naturally leads to their downfall, but where they may
remain long suspended, provided they are not disturbed. Thus, wherever
there are high and steep cliffs, great falls of rock are likely to
occur when the earthquake movements traverse the under earth. In more
than one instance observers, so placed that they commanded a view of
distant mountains, have noticed the downfall of precipices in the path
of the shock before the trembling affected the ground on which they
stood. In the famous earthquake of 1783, which devastated southern
Italy, the Prince of Scylla persuaded his people to take refuge in
their boats, hoping that they might thereby escape the destruction
which threatened them on the land. No sooner were the unhappy folk on
the water than the fall of neighbouring cliffs near the sea produced a
great wave, which overwhelmed the vessels.

Where the soil lies upon steep slopes, in positions in which it has
accumulated during ages of tranquillity, a great shock is likely to
send it down into the valleys in vast landslides. Thus, in the
earthquake of 1692, the Blue Mountains of Jamaica were so violently
shaken that the soil and the forests which stood on it were
precipitated into the river beds, so that many tree-clad summits
became fields of bar