REYNOLDS: EAST ANGLIAN EARTHQUAKE. 
219 
' Having examined the diagram traced by our recording baro- 
meter to-day (April 22nd, 1884), we find that between 9.15 and 9.30 
a.m., the ink-line thickened suddenly, whilst a general movement 
downwards is shown. A thickened line continued to be produced 
until 1.30 p.m.. when the tracing regained its usual character. 
During this period of four hours six distinct undulations of the line 
are registered, the greatest of which measures "025 in. ^Signed, 
Reynolds and Branson.' 
As Leeds is about 171 miles from the focus, this observation is 
of the greatest interest; and on communicating with Messrs. Rey- 
nolds & Branson this firm was good enough to forward the tracing, 
with an explanatory letter in which they state that the instrument 
was not observed till late in the afternoon, as they did not hear of 
the Essex earthquake before that time. It appears that the air-wave 
produced by the Krakatoa eruption of 1883 was recorded by the 
Leeds barograph, as it was by similar instruments all round the globe, 
and this led to the examination of the tracing on the present 
occasion. 
An inspection of the tracing forwarded by our correspondents 
fully bears out this statement, the ''wobbling" of the pen (or tracing 
paper) having produced a series of six indentations in the line, of 
which the maximum, and last was traced about 1 p.m. This result 
is of great importance from a seismological point of view, and we 
may take the present opportunity of pointing out its significance. 
As an earthquake-disturbance spreads outwards from its origin, 
the vibrations become longer in wave-length and period, and decrease 
in amplitude, so that the short and rapid movements which cause 
damage at the focus become slow, wave-like pulsations of the ground 
at great distances from the focus. The slow oscillations thus pro- 
duced at a distance from all great earthquake centres may be aptly 
compared to the " after-swell " observed upon a coast after a dis- 
tant storm at sea, the movements caused by distinct earthquakes, 
generally revealing themselves only by the oscillation of the water in 
ponds, lakes, etc. The effects of the great Lisbon earthquake of 
1755 upon the inland waters of this country were doubtless due to 
such a slow surging of the solid ground. That the undulations in 
