34 The East Anglian Earthquake.
There is every reason to believe that the earthquake with
which we are here dealing was precisely similar in character
to those frequent shakings which have been so thoroughly
studied in the Plain of Yedo. As in the case of the latter, if
our earthquake had been made to trace the story of its own
movement on a series of seismographic plates, we should no
doubt have seen the gradually commencing tremor increasing
in amplitude and complexity till the "shock" and destruction
occurred, and then again dying gradually out. Certain
observations which have been communicated, although lack-
ing the precision of instrumental records, fully bear out this
view respecting the character of the motion :—
(1.) An observer at Heybridge noticed that a T-square
hanging on a nail commenced to swing some two or three
seconds before the shock itself was felt. This indicates that
the office was in a state of gentle oscillation for two or three
seconds before the arrival of that phase of the movement of
larger amplitude which constituted the actual " shock." (See
Report under Heybridge.)
(2.) A lady in bed, at Ipswich, lying still and looking out
of window at the clouds, suddenly saw the latter apparently
thrown into a state of agitation, and immediately afterwards
heard the rumbling sound and experienced the shock. The
apparent movement of the clouds, so graphically described by
this correspondent, must be attributed to the lesser oscilla-
tions of the house which preceded the movements of greater
amplitude constituting the "shock." (See Report under
Ipswich).
With respect to the duration of the movement the esti-
mates, as might have been anticipated, differ widely, the
discrepancies being due to the following circumstances:—
(1.) The estimate depending mainly upon the judgment is
liable to " personal equation," or to subjective influence,
according to the sensitiveness of the observer; in other
words, the duration is apt to be estimated by the intensity of
the feelings, especially in the area of damage where actual
fear for the moment prevails.