The only observations of any real value in determining the velocity
are those given by the stopped clock at the observatory of San
Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur,
Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon,
Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29 P.M.
respectively (Paris mean time) and the mean epicentral point as
coinciding with Alhama, the French Commission estimates roughly the
mean surface-velocity between Cadiz and Lisbon at 3.6 kms. per second,
between Cadiz and Greenwich at 4.5 kms. per second, between Cadiz and
Wilhelmshaven at 3.1 kms. per second, and between Greenwich and
Wilhelmshaven at 1.6 kms. per second. Dr. Agamennone, however,
notices that the distances from Alhama are not correctly measured, and
substitutes for the above figures 4.83, 3.43, 2.82, and 1.75 kms. per
second respectively.
These results apparently show a decrease in the velocity with the
outward spread of the earth-waves, but, as Dr. Agamennone again points
out, a comparatively small error in the time at Cadiz would neutralise
the apparent decrease. It is not to be supposed that the astronomical
clock at this observatory was wrong by more than a second or two, but
the behaviour of clocks during an earthquake is so irregular--some
stopping at once, others staggering on for some seconds before
arrest--that the Cadiz time may differ from the true time by several
seconds.
Besides this possible error, there is also considerable uncertainty in
the records from the magnetic observatories, owing to the slow rate at
which the photographic paper travels. At Parc Saint-Maur this rate is
only 10 mm. per hour, and at the other observatories about 15 mm. per
hour. Allowing, therefore, for an error of half-a-minute in the
time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and
Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and
taking the mean epicentral point as determined by the Italian
observers, Dr. Agamennone, applying the method of least squares, finds
the probable value of the velocity of propagation to be 3.15 kms. (or
nearly 2 miles) per second, with a possible error of .19 kms. per
second. This result agrees closely with the value found for the long
slow undulations of more recent earthquakes.
MISCELLANEOUS PHENOMENA.
_Connection between Geological Structure and the Intensity of the
Shock._--While a great part of the injury to buildings must be
attributed to their faulty construction, the connection between the
nature of the underlying rock and the amount of damage was very
clearly marked. Other conditions being the same, houses built on
alluvial ground suffered most of all; and the destruction was also
great in those standing on soft sedimentary rocks such as clays and
friable limestones. On the other hand, when compact limestones or
ancient schists formed the foundation-rock, the amount of damage was
conspicuously less than in other cases.
Public-domain text, read in full here on John Shaqi.
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