General Summary. 209
is also possible, in accordance with the principles previously
stated, that the shock may have been parallel with one of the
diagonals, i.e., N. — S., S. — N., E.—W., or W. — E. These con-
clusions, vague and useless as they appear to be, are based
entirely upon the assumption that the impulse was rectilinear,
and that the movement which twisted the chimney corre-
sponded in direction with the general direction of propagation
of the disturbance. But for neither of these assumptions is
there any evidence—the motion which caused the shaft to become
dislocated and twisted may have been transverse to the line of
propagation; there may have been cm actual rotary motion of the
ground, or the displacement may have been produced by the
successive action of two shocks transverse to one another.
VIII. General Summary.
We purpose, in concluding, to give here a general summary
of the results, both positive and negative, to which we have
been led in the course of the present report, in the hope that
they may be found of use to observers in other local scientific
societies, on those happily rare occasions when opportunities
for such observations present themselves :—
1. The present earthquake is the most serious that has
happened in the British Islands for about four centuries.
The earthquake of 1580 may have approached it in
intensity, but among previous records, neglecting those
which appear to have been exaggerated, there are only
about five shocks since the beginning of the 12th
century which may have equalled or surpassed it in
the amount of damage produced.
2. The county of Essex has, since the commencement
of authentic history, been but rarely affected by
seismic disturbances, and these only of the slightest
character.
3. The sensible shock extended over an area of about
50,000 square miles.
4. The intensity of the shock was probably about one-
twentieth that of the great Lisbon earthquake of 1755.