204 Mr R. Edmonds on Harthquakes. 
Every extraordinary agitation of the sea (unaccompanied 
with a known earthquake-shock) that I have read of, where 
the state of the weather is mentioned, has occurred during a 
thunder-storm, or at or near a minimum of the barometer ; 
whereas earthquakes appear to take place equally in all 
states of the atmosphere. It is therefore important to ascer- 
tain why such earthquakes as are known only by the extraor- 
dinary agitation of the sea which they produce should occur 
exclusively during storms, or at or near minima of the baro- 
meter. Is it because submarine shocks are always vertical, 
while those on dry land are generally horizontal? In vertical 
shocks there may be electrical discharges between the earth 
and the atmosphere which might occasion the attendant 
minima, as in the case observed by Humboldt, where “the 
mercury was precisely at its minimum height at the moment 
of the third and last shock,*”’ whilst in horizontal shocks the 
discharges may be only between differently charged portions 
of the earth without much affecting the atmosphere. 
Mr Mallet, in his first report on earthquakes, asks whether 
the reason why ducks in ponds often rush suddenly from the 
water immediately before an earthquake may not be, “ that 
with their heads immersed they are able to hear the first dis- 
tant mutterings while yet inaudible through the air?” But 
how can this be when sounds do not travel through the earth 
faster than shocks? It is true that earthquake sounds are 
often heard immediately before shocks are felt; but such sounds 
must have been produced, not by the vibrations which were 
afterwards felt, but by preceding vibrations which were not 
felt at all. The numberless rapid vibrations constituting a 
shock vary considerably in power, so that the weaker ones if 
they came first and reached no higher than the bottom of the 
pond, might have alarmed the birds before the stronger ones 
were felt on its banks. That shocks may reach ponds with- 
out being perceived by persons close by them was abundantly 
proved during the great earthquake of Lisbon. 
Humboldt, at Cumana, felt an earthquake during a thunder- 
storm at the moment of the strongest electrical explosion ; on 
the following day at the same hour was a violent gust of wind 
* Personal Narrative, vol. iii., p. 319. 
