236 ECONOMIC GEOLOGY
wave traversing a plateau causes the particles to oscillate
about their original position; but at the end of the plateau
the ground is flung forward, just as a tap on one end of a
row of billiard balls sends a wave through them but only
moves the last, which is jerked forward.
Structures should be light to reduce both inertia and
momentum ; for on a sudden shock inertia tends to jerk
the lower part from under the slower upper part; while
momentum causes the upper part to continue the forward
swing after the base has begun to move back. Hence factory
chimneys are built with a broad strong base, which tapers
upward, and the upper part is composed of sheet iron, to
reduce the weight. A chimney built as a uniform cylinder
would be fractured at the base. The danger of top-heavy
chimneys was shown at the Warnambool earthquakes in
Victoria of 1903; they are built of a dune limestone which
is used in thick slabs and the weight of those at the cap
rendered the chimneys as unstable as an inverted pendulum
and many were overthrown.
The strength of a wall is reduced by the doors and windows,
for they serve like the perforations in a sheet of stamps, and
the cracks run from one to the next. A medium angle of
emergence is accompanied by cracks that radiate from the
window corners, while a horizontal emergence is marked by
vertical cracks. Vertical rows of windows are weaker than
a diagonal or quincuncial arrangement.
The arch is a dangerous structure in earthquake areas
unless embedded underground ; for during horizontal move-
ments the two sides may move in opposite directions at the
same time, and the arch be torn asunder. The Kiso Sawa
railway bridge in Japan rested on piers of masonry 26 feet
long by 10 feet thick, each supported by two circular drum
curbs, 12 feet in diameter. The bridge was destroyed by an
earthquake, as the differential movement of the two drums
tore each pier in twain; they were replaced by single drums
30 feet by 12 feet.
The reduction of the stresses due to momentum and inertia
specially concerns the design of roofs. When a building is
jerked forward the roof tends to remain behind, and the side
left unsupported may fall into the room. In Japanese
temples the roof timbers were knit together by many joints
