214 the East Anglian Earthquake.
'' Having examined the diagram traced by our recording
barometer 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.—Reynolds and Branson."
As Leeds is about 171 miles from the focus, this observation
is of the greatest interest; and on communicating with Messrs.
Reynolds and 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 after-
noon, 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 (see p. 27) was recorded by the
Leeds barograph, as it was by similar instruments all round
the globe,111 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 Seismo-
logical 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 produced at a
distance from all great earthquake-centres may be aptly com-
pared to the " after swell" observed upon a coast after a distant
storm at sea, the movements caused by distant earthquakes
generally revealing themselves only by the oscillation of the
water in ponds, lakes, &c, The effects of the great Lisbon
earthquake of 1755 upon the inland waters of this country
111 See papers by R. H. Scott and Gen. R. Strachey, Proc. Roy. Soc,
vol. xxxvi., pp. 139 and 143.