General Characters of the Disturbance. 35
(2.) Different buildings are doubtless made to oscillate for
different lengths of time by the same disturbance, the vibra-
tion period of some structures causing them more readily to
take up the earthquake movement and to oscillate longer
than other structures whose vibration period does not
harmonize so closely with the earthquake period.
(3.) The time during which a building oscillates would be
to some extent determined by its position with respect to
the direction of propagation of the seismic wave: thus a
rectangular building would probably rock for a longer time
if the wave met it "broadside" than if the direction of
propagation coincided with its greatest length. Similarly,
the duration of oscillation would be governed by the relations
of the building to other buildings, i. e., whether detached,
semi-detached, or forming one of a row of houses, &c.
(4.) The duration of the oscillation would depend upon the
nature of the underlying soil; buildings on loose formations
like gravel might be expected to swing for a shorter period
than buildings on hard elastic rock.
It would have been quite impossible in the time at our dis-
posal to have examined separately into every record respecting
the duration of the disturbance, with the object of giving
due weight to the above considerations. The estimates by
various observers, although for the reasons stated devoid of
much scientific value as indicating the actual duration,
are perhaps of sufficient interest to be given here, as fixing
some rough limits to the duration of the sensible shock.
From what has been previously said concerning the general
nature of earthquake motion, it will be understood that the
whole duration of the movement was most probably greater
than that of the so-called " shock," the preliminary and
final tremors being too small to affect the observers. The
following estimates have been selected from the whole of the
records examined:—