The natives, who have experienced many hundred earthquakes,
believe that the difference depends less upon the length or shortness of the
waves, and the slowness or rapidity of the horizontal vibrations.* than on
the uniformity of the motion in opposite directions.
[footnote] * "Tutissimum est cum vibrat crispante Aedificiorum crepitu; et
cum intumescit assurgens alternoque motu residet, innoxium et cum
concurrentia tecta contrario ictu arietant; quoniam alter motus alteri
renititur. Undantis inclinatio et fluctus more quaedam volutatio investa
est, aut cum in unam partem totus se motus impellitae -- Plin., ii., 82.
The circling rotatory commotions are the most uncommon, but, at the same
time, the most dangerous. Walls were observed to be twisted, but not thrown
down; rows of trees turned from their previous parallel direction;
p 206
and fields covered with different kinds of plants found to be displaced in
the great earthquake of Riobamba, in the province of Quito, on the 4th of
February, 1797, and in that of Calabria, between the 5th of February and the
28th of March, 1782. The phenomenon of the inversion or displacement of
fields and pieces of land, by which one is made to occupy the place of
another, is connected with a translatory motion or penetration of separate
terrestrial strata.
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